Targeting Proteasomes with Naturally Occurring Compounds in Cancer Treatment

Current Cancer Drug Targets, 2011, 11, ???-??? 1

1568-0096/11 $58.00+.00 © 2011 Bentham Science Publishers Ltd.

Targeting Proteasomes with Naturally Occurring Compounds in Cancer

Treatment

V. Cecarini#, M. Cuccioloni

#, M. Mozzicafreddo

#, L. Bonfili

#, M. Angeletti and A.M. Eleuteri*

University of Camerino - School of Biosciences and Biotechnology, Via Gentile III da Varano, Camerino, Italy

Abstract: Aberrant cellular proliferation and compromised apoptotic pathways are hallmarks of cancer aggressiveness,

and in this framework, the role of protein degradation machineries have been extensively dissected.

Among proteases, the proteasome is unequivocally central in the intracellular regulation of both these processes, thus sev-

eral proteasome-directed therapies have been investigated, aiming at controlling its activity and possibly restoring normal

cell functions. Numerous studies reported proteasome inhibitors (both synthetic and natural occurring) to potently and se-

lectively induce apoptosis in many types of cancer cells. In this review, we discuss recent advances in proteasomal modu-

lation by some natural occurring polyphenols, globally providing evidence of the proteasome role as therapeutic target in

cancer treatment.

Keywords: Apoptosis, cancer, polyphenols, proteasome.

INTRODUCTION

Tumorigenesis is characterized by an uncontrolled cell proliferation and by aberrations and defections in the apop-totic pathway, these processes being mutually linked to the transcription, translation and degradation of regulatory pro-teins. In this context, lysosomes and the proteasomal system are the major mechanisms used by cells in the regulation of intracellular protein turnover and stability, being deputed to the degradation of extracellular and transmembrane proteins, and intracellular proteins, respectively.

In the effort of developing new anti-cancer compounds, several epidemiological evidences revealed a promising cor- relation between a polyphenol-rich diet and a significant decrease in the occurrence of malignancies. In detail, polyphenols were tested for their anti-proliferative and cytotoxic effects, and more specifically for their pro-apoptotic activities exerted both in several cancer cells lines and in animal tumor models.

Herein, we will focus our attention on the direct effects of natural polyphenols from different sources on proteasome, dissecting their contribution in proteasome-mediated apopto-sis.

The Proteasome and its Role in Cancer

The proteasomal system mediates most of the intracellu-lar proteolytic processes, being implicated in the regulation of signal transduction, immune response, carcinogenesis, cell division, growth and differentiation, in the DNA repair, morphogenesis of neuronal networks, biogenesis of organ-elles in the apoptotic process [1-3]. The main particle of the system is the 20S proteasome, a large, barrel-shaped organi-zation with a molecular weight of about 700 kDa. It

*Address correspondence to this author at the School of Biosciences and

Biotechnology, University of Camerino, Via Gentile III da Varano,

Camerino, 62032 (MC), Italy; Tel: +39 0737 403267; Fax: +30 0737

403290; E-mail: [email protected] #These authors contributed equally to this work.

constitutes up to 1% of the total cellular proteins, and nu-merous studies have suggested that its structure and biogene-sis are highly conserved from yeast to mammals [4, 5]. The 20S core consists of two external rings, each containing seven different -subunits, and two internal rings, each con-taining seven different -subunits, with a definitive configu-ration 1-7 1-7 1-7 1-7 [4, 6]. The -rings define the main in-ternal chamber of the complex and carry the catalytic activ-ity, while the outer -rings regulate the substrate entry into the catalytic chamber and the binding of different regulatory proteins. Two outer cavities (named the “antechambers”) are formed together by one and one ring [4]. Eukaryotic 20S proteasomes display three well-described proteolytic com-ponents: a chymotrypsin-like activity (ChT-L, which cleaves after hydrophobic residues), a trypsin-like activity (T-L, which cleaves after basic residues) and a peptidylglutamyl peptide-hydrolyzing activity (PGPH, which cleaves after acidic residues) [4]. However, two other catalytic activities have been described in eukaryotes: the branched chain amino acid preferring activity (BrAAP) and the small neutral amino acid preferring activity (SNAAP) [7]. Mutagenesis studies allowed the characterization of three subunits responsible for the described cleavages, namely 1, 2 and 5. Subunit 1 is associated with the PGPH activity and also possesses a lim-ited BrAAP activity. Subunit 2 carries the trypsin-like ac-tivity and subunit 5 accounts for the ChT-L activity. How-ever, it has been shown that subunit 5 also has the tendency to cleave after small neutral and branched side chains; there-fore, SNAAP and BrAAP activity can additionally be as-signed to this subunit [8]. Studies on the way the protea-some-mediated degradation occurs revealed that the nucleo-philic attack is mediated by the N-terminal threonine of the three catalytic subunits [9]. Upon -interferon exposure, as well as treatment with tumour necrosis factor alpha (TNF- ) or lipopolysaccharides (LPS), in mammalian proteasomes the three constitutive proteolytic subunits 1, 2 and 5 are replaced by homologous immune-subunits, 1i, 2i and 5i [10-12]. The new-assembled structure is known as 20S im-munoproteasome and is mostly involved in the antigen pres-entation process and in the immune response mediated by cytotoxic T lymphocytes (CTLs) [13]. The inducible

https://www.researchgate.net/publication/13753860_The_Proteasome_Paradigm_of_a_Self-Compartmentalizing_Protease?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==

https://www.researchgate.net/publication/14129164_Groll_M_et_al_Structure_of_20S_proteasome_from_yeast_at_24_A_resolution_Nature_386_463-471?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==

2 Current Cancer Drug Targets, 2011, Vol. 11, No. 3 Cecarini et al.

subunits are associated to the catalytic components as fol-lows: 5i to the ChT-L and BrAAP; 2i to the T-L, and 1i to the PGPH in the immunoproteasome. Regarding the pro-teolytic abilities of the immunoproteasome, it shows a re-duced activity for cleavage after acidic amino acids, whereas the ChT-L and BrAAP activities are significantly enhanced [5]. From the interaction of the regulatory particle 19S with each of the two external rings of the 20S proteasome a large complex, the so-called 26S proteasome (molecular weight 2 MDa), is generated. The 26S proteasome complex is the main proteolytic component of the ubiquitin pathway and is responsible for the ATP-dependent degradation of ubiquitin-tagged substrates [1, 14].

Interestingly, among the three major activities of the pro-teasome, the inhibition of the ChT-L has been associated with the induction of apoptotic pathways in cancer cells, suggesting that the tumor cells survival may be linked to this proteasomal component [15, 16].

The key role the proteasome plays in the onset and pro-gression of cancer is due to its intervention in the degrada-tion of oncogenes and tumour suppressor gene products, transcription factors, and other signaling molecules (Fig. (1)). The inhibitor of nuclear factor B (NF B; I B), the tumour suppressor p53, the cyclin-dependent kinase inhibi-tors p21 and p27 and the pro-apoptotic protein Bax are all proteasome substrates [17]. Briefly, proteasome inhibition stabilizes cytoplasmic I B and blocks NF B nuclear translo-cation favouring apoptotic pathways in cancer cells. A de-crease in the NF B-mediated transcription blocks tumour cell survival, proliferation, invasion and metastasis, and an-giogenesis [17]. Regarding p53 protein, stress conditions that promote proteasome inhibition stop p53 degradation, rapidly

rising its levels [18]. The accumulation of p53 triggers cellu-lar responses that involve apoptosis, cell-cycle arrest, DNA repair, differentiation, and senescence [19, 20]. p53 pro-motes apoptosis through the induction of the pro-apoptotic protein Bax (also known as a proteasomal substrate) and the inhibition of anti-apoptotic proteins Bcl-2 and Bcl-xL in mi-tochondria, thus triggering cytochrome c release, caspases activation and, ultimately, cell death [21]. Finally, protea-some-mediated accumulation of p21 and p27, is involved in cell cycle progression, contributing to cell-cycle arrest of tumour cells and eventual apoptosis [17].

All these properties make the proteasome a highly attrac-tive target for a chemotherapeutic approach mainly for can-cer therapy. In particular, because proteasome inhibitors are considered very effective and selective for the proteasome, their application has been extensively documented.

The Role of Polyphenols in Proteasome Inhibition and

Cancer Prevention

As a general rule, proteases inhibitors are short peptides linked to a functional group, generally positioned at the C-terminus, that interacts with the catalytic residue forming reversible or irreversible covalent adducts. The peptide por-tion specifically associates with the enzyme substrate bind-ing pocket in the active site [22].

Almost all the synthetic and natural proteasome inhibi-tors act predominantly on the ChT-L activity showing weaker effects on the T-L and PGPH components [22]. In fact, while inactivation of the T-L or PGPH sites does not reduce the rates of protein degradation, inhibition of the ChT-L component causes a large decline in protein break-

Fig. (1). Schematic representation of apoptotic pathways triggered by natural polyphenols-mediated proteasome inhibition. The inhibition of

the proteasomal system upon treatment with polyphenols determines the accumulation of proteasome substrates such as I B, p53, p21 and

p27, evolving in the activation of apoptotic cell death mechanisms.


Proteasome and Polyphenols in Cancer Treatment Current Cancer Drug Targets, 2011, Vol. 11, No. 3 3

down [22]. Proteasome inhibitors can be divided into two classes: reversible inhibitors, including peptide aldehydes (MG132) and peptide boronates (MG262, PS341 or Borte-zomib), and irreversible inhibitors, including lactacystin and derivatives, peptide vinyl sulfones (YLVS) and peptide ep-oxyketones (epoxomicin) [22]. Bortezomib was the first in-hibitor to enter clinical trials, particularly for the treatment of multiple myeloma and mantle cell lymphoma and it has been studied as both a single agent and in combination with other anti-myeloma compounds [23]. It induces proteasome inhibi-tion specifically and reversibly binding to the N-terminal threonine in the active site of the 20S proteolytic core [23]. Bortezomib prevalently acts by blocking proteasomal degra-dation of I B, thus diminishing NF B activity and stabiliz-ing p53, p21, p27, and Bax thus enhancing treatment re-sponses and reversing chemoresistance [24].

Despite their efficacy, currently used inhibitors, includ-ing bortezomib, may display serious side effects and partial cell resistance and for this reason the design of new, more specific and potent drugs is an extremely attractive field.

In this scenario, the finding that natural proteasome in-hibitors, such as polyphenols, are able to selectively induce apoptosis in tumour cells opened the way to their application as potential drugs in human disease therapy and increasing evidence suggests that many dietary factors, used alone or in combination with traditional chemotherapeutic agents, can be useful in the prevention and even in the treatment of can-cer.

Polyphenols are important components of human diet and

can be found in fruit, vegetables, flowers, seeds, sprouts and

beverages, providing them with much of their flavour and

colour. The best-known biological effects include cancer

prevention, inhibition of bone resorption, hormonal and car-

dioprotective action, antibacterial and antiviral properties,

and antioxidant activity [25]. They can be divided into dif-

ferent classes on the basis of their chemical structures, but

the presence of at least one aromatic ring with one or more

hydroxyl groups is a common hallmark. Phenolic acids, fla-

vonoids,

and tannins are the

most common polyphenol

groups and they are further divided into subclasses. Phenolic

acids, such as hydroxycinnamic acids and hydroxybenzoic

acids, have a simple structure with a single aromatic ring.

Flavonoids include anthocyanidins, flavonols, flavones,

flavanones, catechins and isoflavonoids. Finally, tannins can

be divided into condensed tannins, proanthocyanidins, and

hydrolysable tannins, gallo- and ellagitannins [26, 27].

Recently, great attention has been paid towards the green and black tea content in polyphenolic compounds especially for their properties to reduce the risk of a variety of diseases [28]. In fact, it has been reported that the consumption of green tea is associated to a balanced controlled diet that can improve the overall anti-oxidative status and protect against oxidative damage in humans [29]. Tea polyphenols include catechins, flavones, anthocyanins and phenolic acid. Cate-chins are the main components, with a content > 80% [30]. (-)-Epigallocatechin-3-gallate (EGCG) and other tea poly-phenols are potential chemopreventative agents, able to modulate multiple intracellular pathways, including the pro-teasomal complex (for a detailed review of the effects of tea

polyphenols on the proteasomal system, see Yang et al. arti-cle in the same issue).

Besides the role in cancer prevention of tea-derived poly-phenols, several natural occurring compounds from other sources have been characterized for their ability to affect proteasome functionality, thus being involved in cancer progression. The focus of this review is therefore to summa-rize and discuss the recent findings on this field of investiga-tion.

Effects of Isolated Polyphenols on Proteasome Function-

ality

A wide number of studies reported on the effects of natu-ral occurring polyphenols on the proteasomal system and, as intimately correlated, on the activation of apoptotic cell death pathways. A list of such compounds and of their re-lated effects on cell lines are provided in Tables 1 and 2.

Curcumin is a natural polyphenolic compound extracted from the spice turmeric (Curcuma longa), which has been reported to have anti-inflammatory, antioxidant and anti-proliferative properties [31-33]. It has been shown that it possesses the ability to modulate multiple cellular machiner-ies, among them the ubiquitin proteasome system. Milacic et al. demonstrated a potent curcumin-mediated inhibition of the ChT-L activity (IC50=1.85 mol/L) in purified rabbit 20S proteasome. Inhibition of proteasome activity by curcumin was also evidenced in human colon cancer HCT-116 and SW480 cell lines leading to the accumulation of ubiquiti-nated proteins and several proteasome targets, and, conse-quently, triggering apoptosis [34]. Furthermore, curcumin treatment of HCT-116 colon tumour-bearing ICR SCID mice resulted in decreased tumour growth, associated with protea-some inhibition, proliferation suppression, and apoptosis induction in tumour tissues. This study shows that protea-some inhibition and the consequent induction of apoptosis and reduction of tumour proliferation could be one of the mechanisms for the chemopreventive and/or therapeutic roles of curcumin in human colon cancer [34]. Another re-search group observed in Neuro2a cells treated with curcu-min a dose dependent inhibition of proteasome activities mediated by a direct effect on the 20S core catalytic compo-nent [35, 36]. Furthermore, curcumin treatment of human epidermal keratinocytes increased the ChT-L activity at low doses (up to 1 mol/L), whereas higher concentrations of curcumin (10 mol/L) caused a 46% decrease in proteasome activity [37]. HeLa cells treated with 30 mol/L curcumin showed a reduction of almost 30% in the ChT-L, T-L and PGPH components of the 20S proteasome, with a marked accumulation of ubiquitin–protein conjugates. A stronger effect was observed in purified 20S proteasomes where the ChT-L, T-L and PGPH hydrolytic activities presented an inhibition > 90% in the presence of curcumin [38]. Interest-ingly, curcumin fluoro and copper complexes displayed a potent inhibitory activity against the proteasome, with the difluoro compound CDF being able to induce a higher inhi-bition of cell growth in both colon and pancreatic cancer cells with respect to the parental curcumin, encouraging fur-ther development for establishing its role as a chemopreven-tative and/or therapeutic agent against cancers [39].

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Table I. List of Polyphenols with Related Effects on Proteasome Functionality

COMPOUND CHEMICAL STRUCTURE NATURAL SOURCES EFFECT ON PROTEASOME

ACTIVITY

5,6,3',4'-tetrahydroxy-

7-methoxyflavone

O

OO

OH

OH

HO

OH

Anisomeles ovate, Syzy-

gium aromaticum

Inhibition of ChT-L (IC50=14

mol/L), PGPH (IC50=5.4 mol/L)

and T-L (IC50=24 mol/L) activities

of 26S proteasome from pig red

blood cell [44].

5,6,4'-trihydroxy-7,3'-

dimethoxyflavone

O

O

O

O

OH

HO

OH

Thymus satureiodes,

Thymbra spieata, Popu-

lus angustifolia, Lantana

montevidensis

Inhibition of ChT-L (IC50=

29 mol/L) and PGPH

(IC50=40.7 mol/L) activities of 26S

proteasome from pig red blood cell

[44].

6,7,4'-

trihydroxyisoflavone

O

O

OH

HO

HO

Glycine max, degrada-

tion product of glycitein

and daidzein in bacteria

and humans

Inhibition of ChT-L (IC50=113.1

mol/L) activity of 26S proteasome

from pig red blood cell [44].

Apigenin-6-hydroxy

OOH

HO O

OH

HO

Erigeron breviscapus,

Sorbaria sorbifolia,

Scutellaria lateriflora



and T-L (IC50=54.7 mol/L) activi-

ties of 26S proteasome from pig red

blood cell [44].

Apigenin-6-hydroxy-

7-O- -D-glucoside

OOH

O

OH

HO

HO

HO

OH

OH

O

O

Thymus serpyllum


mol/L), and PGPH (IC50=64.9

mol/L) activities of 26S protea-

some from pig red blood cell [44].

Cyanidin

OH

OH

HO O+

OH

OH

Vaccinium species,

Morus species, Vitis

rotundifolia, Brassica

oleracea, Viola sororia

Inhibition of proteasomal ChT-L

activity in HL-60 cells (IC50=18.4

mol/L) [50].

Cyanin

HO O+

OH

OH

O

OH

OH

OH

O

OH

O

OH

OH

HO

O

OH

Vaccinium species,





activity in HL-60 cells (IC50=11

mol/L) [50].


(Table 1). Contd.....


ACTIVITY

Delphinidin

OH

OH

HO O+

OH

OH

OH

Vaccinium species,






mol/L) [50].

Glycitein

O

H3CO

OHO

HO

Glycine max



and T-L (IC50=17.1 mol/L) activi-

ties of 26S proteasome from pig red

blood cell [44].

Ideain

OH

HO O+

OH

OH

OH

OH

OH

HO

O

O

Vaccinium species,






mol/L) [50].

Kaempferidinidin

OH

OH

HO O+

OCH3

Vaccinium species,



oleracea, Viola sororia,



mol/L) [50].

Malvidin

OH

OH

HO O+

O

OH

O

Vaccinium species,






mol/L) [50].

Malvidin-3-

galactoside

OH

HO O+

O

OH

O

OH

OH

OH

HO

O

O

Vaccinium species,






mol/L) [50].




ACTIVITY

Malvin

HO O+

O

OH

O

O

O

OH

OH

HO

O

OH

OH

OH

OH

O

HO

Vaccinium species,






mol/L) [50].

Myrtillin O

OH

HO O+

OH

OH

OH

OH

OH

OH

O

HO

Vaccinium species,






mol/L) [50].

Oenin O

OH

HO O+

OCH3

OH

OCH3

OH

OH

OH

O

HO

Vaccinium species,






mol/L) [50].

Pelargonidin

OH

OH

HO O+

OH

Vaccinium species,






mol/L) [50].

Pelargonin

O

HO O+

OH

OH

OH

HO

O

OH O

OH

OH

OH

O

OH

Vaccinium species,






mol/L) [50].




ACTIVITY

Peonidin

OH

HO O+

OH

OH

OCH3

Vaccinium species,






mol/L) [50].

Peonidin-3-glucoside

OH

HO O+

OH

O

OH

OH

OH

O

OH

Vaccinium species,






mol/L) [50].

Petunidin

OH

HO O+

OH

OH

OH

OCH3

Vaccinium species,






mol/L) [50].

Rutin OH

OOH

HO O

OH

OH

OH

OH

OH

OH

O

O

OH

OH

HO

O

Fagopyrum esculentum,

Rheum Species, Aspara-

gus officinalis, Citrus

Species, Morus Species,

Ruta graveolens


mol/L), and PGPH

(IC50=57 mol/L) activities of 26S

proteasome from pig red blood cell

[44].

Sumatranoside O

OH

OH

HO

O

OH

NH

HO

OH

O

OH

Ormosia sumatrana

Inhibition of ChT-L activity of 20S

proteasome from HL-60 human

leukemic cell line (IC50 =30

mol/L) [89].

Witharfarin A O

OHO

CH3

O

OH

Withania somnifera

Inhibition of ChT-L activity of 20S

proteasome from rabbit (IC50=4.5

mol/L) [90].


Table 2. List of Polyphenols with Related Effects on Cell Survival and Proteasome Functionality

COMPOUND CHEMICAL STRUCTURE NATURAL SOURCES

EFFECT ON

PROTEASOME

ACTIVITY

EFFECT ON

CELL LINES

Apigenin

OOH

HO O

OH

Lantana montevidensis, Adinandra

nitida, Cajanus cajan, Mentha

spicata, Rumex acetosella, Nastur-

tium officinale, Tanacetum Parthe-

nium, Turnera aphrodisiaca,

Gmelina arborea, Chamaemelum

nobile, Apium graveolens, Anethum

graveolens

Inhibition of ChT-L

activity of purified 20S

proteasome (IC50 = 1.8

mol/L) and of 26S

proteasome in intact

leukaemia Jurkat T

cells (IC50=1 mol/L)

[40].

Accumulation of

Bax and I B in

Jurkat T cells, and

activation of

caspase-3 and

cleavage of PARP

in Jurkat T cells

[40].

Capsaicin O

HN

OH

O

Capsicum frutescens

No direct effect on the

ChT-L protease activity

of 20S proteasome

[91].

Accumulation of

ubiquitinated

proteins and other

target substrates of

proteasome (p53,

Bax and p27);

induction of 50%

cell death at 300

mol/L, and 50%

cell viability at

600 mol/L;

induction of the

intrinsic pathway

of apoptosis

involving mito-

chondria and

induces neurite

outgrowth [91].

Celastrol

HO

O

COOH

H

Tripterygium wilfordii

Inhibition of ChT-L

activity of 20S protea-

some from rabbit

(IC50=2.5 mol/L) [92].

Suppression of

human prostate

cancer growth in

nude mice [92].

Chlorogenic

acid O

O

HO

OH

HO COOH

OH

OH

Anethum graveolens

Inhibition of TPA-

induced T-L activity of

mouse epidermal 20S

proteasome [70].

Significant de-

crease in TPA-

induced NF- B

activation in

mouse epidermis;

inhibition of the

TPA-stimulated

p65-DNA binding;

reduction of NF-

B p65 subunit

translocation from

cytosol to the

nucleus and en-

hanced the reten-

tion of I B in the

cytosol; inhibition

of the I B-

kinase and COX-2

activities; no

effect on TPA-

stimulated degra-

dation of I B-

[70].




EFFECT ON

PROTEASOME

ACTIVITY

EFFECT ON

CELL LINES

Chrysin

OOH

HO O

Passiflora caerulea, Radix Scutel-

laria

40-50% inhibition of

ChT-L activity at 20

mol/L, 40-50% inhibi-

tion of T-L activity at

20 mol/L, and 90-92%

inhibition of PGPH

activity at 20 mol/L in

proteasome extract

from HepG2, A549,

HL60, and HCT-116

cell lines [42].

Dose-dependent

cell death in

HepG2, A549,

HL60, and HCT-

116 cell lines

treated with

chrisin (IC50=20-

26 mol/L) [42].

Curcumin

O OH

HO OH

OCH3H3CO

Curcuma longa

Possible inhibition of

proteasome activity,

facilitating sorting of

ubiquitinated proteins

into the exosomes [93].

Attenuation of the

increase ChT-L activity

induced by proteolysis-

inducing factor in

murine myotubes [94].

Strong inhibition of

ChT-L, T-L and PGPH

activities in both Neuro

2a and HeLa cells [35].

20–70% reduction of

ChT-L activity of the

proteasome at 1–10 μM

on purified rabbit 20S

proteasome. 80%

inhibition of ChT-L

activity by fluorine

substituted analogs on

purified rabbit 20S

proteasome [39].

Accumulation of

proteasome target

proteins through

mitochondrial

pathways [35];

induction of

apoptosis in hu-

man colon cancer

HCT-116 and

SW480 cells.

Decreased tumour

growth, prolifera-

tion suppression,

and apoptosis

induction in tu-

mour tissues

(HCT-116 colon

tumour–bearing

ICR SCID mice)

[34].

Attenuation of

tumour exosome-

mediated immu-

nosuppression of

NK cell activation

and tumour cyto-

toxicity [93].

Amplification of

antitumour activ-

ity of celecoxib in

colourectal cancer

cells [95].

Enhancement of

the antitumour

activities of cis-

platin, doxorubi-

cin, and taxol in

different cancer

cells [96-98].

Attenuation of

total protein

degradation in

murine myotubes

at 50 mol/L [94].

Stronger inhibition

growth of cancer

cell lines and

induction apop-

totic cell death by

some fluorine

substituted ana-

logs with respect

to curcumin [39].




EFFECT ON

PROTEASOME

ACTIVITY

EFFECT ON CELL

LINES

Fisetin

OH

O

HO O

OH

OH

Fragaria species, Acacia greg-

gii, Acacia berlandier, Rhus

cotinus, Butea frondosa,

Gleditschia triacanthos, Que-

bracho colourado

Increase of ChT-L

activity in protea-

some from cortical

neurons [99].

Early initiation of chro-

mosome segregation and

exit from mitosis without

normal cytokinesis.

Rapid compromission of

microtubule drug-induced

mitotic block in a protea-

some-dependent manner

in several human cell

lines [100].

Genistein

OOH

HO O

OH

Lupinus perennis, Vicia faba,

Glycine max, Pueraria lobata,

Psoralea lanceolata

Inhibition of ChT-L

activity in 20S

purified proteasome

(IC50 =26 mol/L).

Inhibition of ChT-L

proteasomal activity

in human prostate

cancer LNCaP (IC50

= 70 mol/L) and

breast cancer MCF-7

cells (IC50=65

mol/L) in cell free

assay [101].

Inhibition of proliferation

and induction of apopto-

sis in CaCo HT-29 cells

[102].

Suppressession of prolif-

eration and MET onco-

gene expression and

induces EGR-1 tumour

suppressor expression in

immortalized human

breast epithelial cells

[103].

Induction of a time

dependent accumulation

of p27Kip1 and increased

expression of p27, IkB-

and Bax both in LNCaP

and MCF-7 cancer cells.

Increased levels of ubiq-

uitinated proteins [101].

Kaempferol

OH

OOH

HO O

OH

Anethum graveolens, Brassica

oleracea, Delphinium sta-

phisagria, Hamamelis virgini-

ana, hybrid citrus, Malus do-

mestica

Inhibition of ChT-L

activity of purified

20S proteasome

(IC50 = 10.5

mol/L.) and of 26S

proteasome in intact

leukaemia Jurkat T

cells (IC50 = 11

mol/L) [40].

Accumulation of Bax and

I B- in Jurkat T cells,

and activation of caspase-

3 and cleavage of PARP

in Jurkat T cells [40].

Promotion of proteasome

mediated degradation of

surviving. Sensitization

of some glioma cells to

TRAIL-mediated apopto-

sis [46].

Luteolin

OOH

HO O

OH

OH

Anethum graveolens, Apium

graveolens, Thymus vulgaris,

Taraxacum erythrospermum,

Capsicum annuum, Perilla

frutescens, Daucus carota

Inhibition of ChT-L

(IC50 = 9 - 13

mol/L) and T-L

(IC50 = 12-15

mol/L) activities,

88-90% inhibition of

PGPH activity at 20

mol/L in protea-

some extract from

HepG2, A549,

HL60, and HCT-116

cell lines [42].

Dose-dependent cell

death in HepG2, A549,

HL60, and HCT-116 cell

lines treated with luteolin

(IC50 = 9.65-12.5 mol/L)

[42].

Myricetin

OOH

HO O

OH

OH

OH

OH

Anethum graveolens, Juglans

major, Hibiscus cannabinus,

Ribes nigrum, Trifoluim repens,

Ampelopsis grossedentata,

Vaccinium Oxycoccos

Inhibition of ChT-L


20S proteasome

(IC50=10 mol/L)

and of 26S protea-

some in intact leu-

kaemia Jurkat T cells

(IC50=12 mol/L)

[40].









EFFECT ON

PROTEASOME

ACTIVITY

EFFECT ON CELL

LINES

Quercetin

OOH

HO O

OH

OH

OH

Anethum graveolens, Capparis

spinosa, Levisticum officinale,

Malus domestica, Allium cepa,

Vitis vinifera, Citrus reticulate,

Solanum lycopersicum, Prunus

avium, Rubus species, Vac-

cinium vitis-idaea, Hippophae

rhamnoides

Inhibition of ChT-L


20S proteasome

(IC50 = 3.5 mol/L)

and of 26S protea-

some in intact leu-

kaemia Jurkat T cells

(IC50 = 2 mol/L)

[40].

Inhibition of ChT-L

(IC50 = 38.5 mol/L),

PGPH (I IC50 = 71.8

mol/L) and T-L

(IC50 = 79.9 mol/L)

activities of 26S

proteasome from pig

red blood cell [44].






Decrease in level of Her-

2/neu protein; inhibition

of the downstream sur-

vival PI3K-Akt signaling

pathway in Her-2/ neu-

overexpressing breast

cancer SK-Br3 cells.;

induction of polyubiquit-

ination of Her-2/neu

[104].

Proteasome mediate

decrease in oncogenic

Ras protein [105].

Recover of tumour necro-

sis factor-related apopto-

sis-inducing ligand

(TRIAL) sensitivity in

various hepatoma cells

via Sp1-mediated DR5

up-regulation and protea-

some-mediated down-

regulation of c-FLIP

[106].

Resveratrol

OH

HO

OH

Vitis rotundifolia

Possible inhibition of

proteasome activity

[107].

Attenuated of in-

crease ChT-L activ-

ity induced by prote-

olysis-inducing

factor in murine

myotubes [94].

Suppression of IL-1 -

induced NF-kB depend-

ent expression of apopto-

sis-related gene products,

likely to occur through

the inhibition of protea-

some activity [107].

Multiple level antitumour

effects in different cell

lines [108].

Attenuation of total

protein degradation in

murine myotubes at 50

mol/L. Significant

attenuation of weight loss

and protein degradation

in skeletal muscle Sig-

nificant reduction in NF-

B DNA binding activity

[94].

Silibinin

HO

O OH

OHO

O

O

OH

O

HO

Silybum marianum

Down-regulation of the

antiapoptotic proteins

FLIPL, FLIPS, and

survivin via proteasome-

mediated degradation

[48].

https://www.researchgate.net/publication/5245396_Resveratrol_suppresses_interleukin-1b-induced_inflammatory_signaling_and_apoptosis_in_human_articular_chondrocytes_Potential_for_use_as_a_novel_nutraceutical_for_the_treatment_of_osteoarthritis?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==


https://www.researchgate.net/publication/7169222_Resveratrol_as_a_Chemopreventive_Agent_A_Promising_Molecule_for_Fighting_Cancer?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==




EFFECT ON

PROTEASOME

ACTIVITY

EFFECT ON CELL

LINES

Tannic acid

O

OO

O

O

O

O

OHHO

O

OH

OH

OH

O

O

OH

OH

O

HO OH

OH

O

O

O OH

OH

HO

HO OH

O

O

O

HO OH

HO

HO

HO

O

O

O

HO

HO

OHHO

HO

O

Quercus robur, Juglans major,

Swietenia mahagoni, caesalpina

spinosa

Potent and specific

inhibition of the

ChT-L activity in

purified 20S protea-

some (IC50=0.06

g/mL), 26S protea-

some of Jurkat T-cell

extracts, and 26S

proteasome of living

Jurkat cells [69].

Inhibition of TPA-

induced T-L activity

of mouse epidermal

20S proteasome [70].

Accumulation of the

cyclin-dependent kinase

inhibitor p27Kip1 and the

proapoptotic protein Bax,

followed by growth arrest

in G1 and induction of

apoptotic cell death [69].

Decrease in TPA-induced

NF- B activation in

mouse epidermis; inhibi-

tion of the TPA-

stimulated p65-DNA

binding; reduction of NF-

B p65 subunit transloca-

tion from cytosol to the

nucleus and enhanced the

retention of I B in the

cytosol. Inhibition of the

I B kinase and COX-2

activities [70].

Four additional flavonoids were investigated for their proteasome-inhibitory properties in vitro and in cultured leukaemia cells in a paper from Chen et al. Quercetin,

kaempferol and myricetin, flavonoids primarily found in grapes, and apigenin, abundant in celery seed and chamo-mile flowers, were used in this study. It was reported a more potent effect of apigenin and quercetin with respect to kaempferol and myricetin in inhibiting the ChT-L activity of the proteasomal system in both models. This inhibition was shown to trigger the accumulation of ubiquitinated Bax and IkB- , inducing the activation of caspase-3 and the cleavage of poly (ADP-ribose) polymerase (PARP) [40]. In a previous paper, the authors demonstrated the ability of apigenin to act as well in cultured breast cancer cells and in breast cancer xenografts, favouring proteasome inhibition, apoptotic cell death and anticancer activities [41]. Another work demon-strated the selectivity of apigenin action in inhibiting protea-some functionality, with a strong effect on the ChT L and T L catalytic activities and a weaker effect on the PGPH component [42]. Additional data on the quercetin-proteasome interaction were obtained by Dosenko et al. that performed experiments on purified 20S proteasomes, show-ing that this polyphenol inhibits three of the proteasomal peptidase activities, in particular the ChT-L component, in a dose-dependent manner, having comparable affinity with respect to a specific proteasome inhibitor. Similarly, quer-

cetin inhibited the activities of the 26S proteasome in a car-diomyocytes culture [43].

As for apigenin, also the 6-hydroxy- and 6-hydroxy-7-

O-beta-d-glucoside derivatives were shown to inhibit 26S proteasome components in pig red blood cells [44]. Further-more, apigenin was shown to inhibit the growth of MDAMB-453 HER2/neu-overexpressing breast cancer cells and to induce apoptosis in these cells [45]. These data are of particular interest considering that the manipulation of HER2/neu may be of significant value in the treatment of breast cancer. The authors demonstrated that HER2/neu was degraded in those cells through a mechanism favoured by

apigenin, involving an increased ubiquitination of the recep-tor and the consequent proteasomal degradation. Depleting HER2/neu protein, apigenin suppressed the HER2/HER3 signal and the initiation of the anti-apoptotic PI3K/Akt pathway, thus contributing to the activation of cell death mechanisms. These data indicate an additional pathway for the apigenin-mediated killing of cancer cells that does not involve proteasome inhibition and suggest an important role for this low toxic compound as a chemopreventative and therapeutic agent against HER2/neu overexpressing breast cancers [45]. Also kaempferol and quercetin were reported to cause an apoptosis-inducing mechanism not directly re-lated to proteasome inhibition. In fact, they were able to promote cell death in glioma cells by favouring the down-regulation of survivin, a member of the IAP (inhibitor of apoptosis proteins) family of anti-apoptotic proteins [46, 47].

A similar mechanism was observed upon treatment of glioma cells with subtoxic doses of silibinin, a polyphenolic flavonoid isolated from the seeds

of milk thistle. Interest-

ingly, combination of silibinin with tumour necrosis factor–related apoptosis-inducing

ligand (TRAIL) induced apoptosis

in TRAIL-resistant glioma cells, but not in human astrocytes.

Silibinin promoted a sensitizing effect on TRAIL-induced

apoptosis due to the modulation of multiple components in the

death receptor–mediated apoptotic signaling pathway,

including the proteasome-mediated down-regulation of FLIP

(an inhibitor of caspase-8) and survivin [48]. It is therefore

evident that the induction of programmed cell death by poly-phenols involves the proteasomal system regulating it at dif-ferent levels, both directly inhibiting the complex and fa-vouring the selective degradation of apoptosis-related pro-teins.

A paper published by Chang analyzed the effects of nu-merous flavonoids, especially the 5,6,4'-trihydroxy-7,3'-

dimethoxyflavone, 5,6,3',4'-tetrahydroxy-7-methoxy-fla-vone and 6,7,4'-trihydroxyisoflavone, towards the ChT-L, T-L and PGPH activities of the 26S proteasome. The func-tionality of the complex measured in pig red blood cells re-


sulted compromised suggesting that both the 6-hydroxy and 7-methoxy positions of the flavone may play an important role in targeting 26S activity [44]. Similarly, 5,6,3',4'-tetrahydroxy-7-methoxyflavone showed the highest inhibi-tory effects on 26S proteasome activities when compared to the other flavonoids [49].

Another important class of flavonoids is that of anthocyanins, contained in red/purplish fruit and vegetables with a role as free radical scavengers and pigments contributing to the colour of the flowers. A recent study addressed the ability of anthocyanins and their aglycons, the anthocyanidins, to induce proteasome inhibition, suggesting it as a further mechanism by which they may exert their known anti-carcinogenic, anti-oxidative, anti-inflammatory and neuroprotective activities [50-56]. HL-60 cells treated with 19 substances showed an inhibition of the ChT-L activity with anthocyanins and their aglycons, presenting IC50 values ranging from 7.8 mol/L (kaempferidinidin and

pelargonidin) to 32.4 mol/L (delphinidin) [50].

Hypericin is the active compound of St. John’s wort (Hypericum perforatum) and exhibits potent pharmacologi-cal effects including induction of apoptosis, inhibition of protein kinase C, and inhibition of the activation of NF B. Pajonk et al. demonstrated a direct inhibitory effect of hy-pericin on the ChT-L, T-L and PGPH components of the 20S and 26S proteasomes, suggesting that this could be one of the mechanisms through which hypericin activates apoptotic pathways in cancer cells [57].

Luteolin is another important flavonoid present in a vari-ety of edible plants often found in leaves and exhibits a wide spectrum of properties, including anticancer properties [58, 59]. Previous papers have shown that luteolin is able to in-duce cell cycle arrest or apoptosis in various human cancer cells [60-62]. Together with apigenin and chrysin, luteolin inhibited the ChT L and T L catalytic activities in a dose-dependent manner, with a weaker effect on the PGPH com-ponent, suggesting that proteasome inhibition could be the mechanism involved in apoptosis activation [42].

Oleuropein, the major constituent of Olea europea leaf extract, olive oil and olives, was reported to enhance protea-some activity in vitro more strongly than other known chemical activators, possibly through conformational changes in the proteasome. The same study reported that the continuous treatment of early-passage human embryonic fibroblasts with oleuropein decreased the intracellular levels of reactive oxygen species, reduced the amount of oxidized proteins through increased proteasome-mediated degradation rates and retained proteasome function during replicative senescence [63].

The anti-carcinogenic effect of resveratrol, a polypheno-lic compound mainly found in wine, grape skins and pea-nuts, has been elucidated in numerous in vivo and in vitro studies [64]. Resveratrol enhanced the radiosensitivity of human non-small cell lung cancer NCI-H838 cells with the simultaneous NF B inhibition and S-phase arrest [65]. Un-fortunately, it is unclear whether the observed inhibition of NF B involves the proteasome as a potential mechanism of action of resveratrol. Interestingly, proteasome has been in-volved in the anti-amyloidogenic activity of resveratrol, par-

ticularly in the promotion of the intracellular degradation of amyloid- [66].

In a study from Pettinari et al. the IC50 values of a wide number of polyphenols, testing the four catalytic components of both the constitutive and immunoproteasome, were meas-ured [67]. The same group then conducted an in silico study where the same polyphenols were independently docked onto each catalytic subunit of both the proteasomes. The presence of a specific binding site for monomeric polyphenol compounds on subunits was evidenced: in fact, all poly-phenols bind to the same catalytic sites, supporting their role as competitive inhibitors of distinct proteasomal activities [68].

As aforementioned, tannins are plant-derived polyphe-nolic compounds with varying molecular masses, further classified into two main groups, hydrolysable and condensed tannins. A potent and specific inhibition of the ChT-L activ-ity in purified 20S proteasome and in 26S proteasome of Jurkat T-cell extracts and of living Jurkat cells was observed with the activation of apoptotic pathways through the accu-mulation of the cyclin-dependent kinase inhibitor p27 and the proapoptotic protein Bax [69]. Another paper reported on the significant decrease in the TPA-induced NF B activation in mouse epidermis with chlorogenic acid and tannic acid affecting the key events of initiation and promotion of car-cinogenesis. The authors observed the inhibition of the TPA-stimulated p65-DNA binding, the reduction of NF B p65 subunit translocation from cytosol to the nucleus, an en-hanced retention of I B in the cytosol, the inhibition of the I B kinase and COX-2 activities and the reduction of the TPA-induced ChT-L activity of 20S proteasomes [70].

Effects of Whole Natural Extracts on Proteasome Func-

tionality

A number of evidences highlighted the ability of several fruits and vegetables whole extracts to reduce cancer risk by inhibiting proteasome functionality. In an interesting paper by Chen et al. extracts of apple, grape, strawberry, onion,

tomato and celery were investigated for their proteasome-inhibitory and apoptosis-inducing abilities in human leukae-mia Jurkat T cells, evidencing that, upon 1 hour incubation of cell lysates with 1-10% (v/v) of these extracts, apple, grape, onion and strawberry extracts potently inhibited ChT-L activity in a concentration-dependent manner, although green tea extract resulted more potent. 3-48 hours treatment of intact Jurkat T cells with 5% of each extracts demon-strated that proteasome functionality impairment was associ-ated with the induction of apoptosis, as demonstrated by the accumulation of ubiquitinated proteins, the activation of caspase-3/-7 and the increased cleavage of PARP. Apple and grape were more potent than tomato and celery, but less po-tent than green tea extract in this cellular model, while strawberry extract was inactive [71]. Moreover, morphologi-cal changes in prostate cancer cells and not in normal cells were induced by green tea, apple and grape extracts and, more weakly by onion, tomato and celery, suggesting that these whole extracts selectively affected the viability of tu-mour cells [71].

A Musaceas plant extract, rich in tannic acid, tannin complex and other polyphenols, is a recognized anticancer


agent. A partially purified fraction of the extract has been demonstrated to selectively inhibit proteasome ChT-L com-ponent in human leukemic Jurkat T cells and in simian virus 40 (SV40)-transformed fibroblasts, with consequent accumu-lation of ubiquitinated proteins and of p27 protein leading to apoptosis, whereas normal cells were resistant to the treat-ments [72]. Proteasome inhibition, associated with the induc-tion of apoptosis by the same fraction of Musaceas plant extract was also observed in human prostate and breast can-cer cells upon 12 and 48 hours treatment, suggesting that the anticancer activity of the Musaceas plant extract can be me-diated by such pathways [72].

Recently the effects of wheat sprout hydroalcoholic ex-

tracts on 20S proteasome functionality have been investi-gated, since wheat sprouts contain a very high level of or-ganic phosphates and a powerful cocktail of different mole-cules such as enzymes, reducing glycosides and polyphenols. In vitro ChT-L, T-L, PGPH and BrAAP components were all inhibited by the whole extract, with the BrAAP component being the most affected. Purified constitutive proteasomes and the immunoproteasome were differently affected by the extract depending on the proteasome subunit composition. Proteasome inhibition was also observed in human colon carcinoma cells, with 40% and 30% inhibition of ChT-L and T-L activities upon 48 hours treatment with the extract [73]. A polyphenol and a protein fractions were isolated from wheat sprout extract demonstrating that in vitro the protein component was a more rapid and potent effector of the con-stitutive 20S proteasome. The polyphenol fraction was able to inhibit the immunoproteasome functionality at low con-centrations (1 mg GAE/mL polyphenols). Interestingly wheat sprout extract components, separately administered, triggered the apoptotic pathway in human cervical carcinoma cells and in plasmacytoma cells, through the selective inhibi-tion of tumour cell proteasome, whereas a counterpart of normal cells was resistant to the treatments [74].

Extra virgin olive oil is particularly rich in polyphenols because it is obtained from the olive fruit (Olea Europea L.) solely by mechanical means without further treatments. Phe-nolic fractions directly extracted from extra virgin olive oil have been demonstrated to inhibit HER2 activity in cultured human breast cancer cell lines by promoting the proteasomal degradation of the HER2 protein itself, thus affecting tumour cell proliferation and survival [75].

Polyphenols Adverse Effects in Myeloma Treatment

Despite of these very promising data regarding polyphe-nols in cancer treatment, an increasing number of papers have recently focused their attention on the potential con-trasting effects of such compounds towards proteasome in-hibitors action. Actually, similar results were previously ob-tained with other antioxidants such as edaravone and tiron that were able to specifically inhibit the cytotoxic effects of bortezomib and of aldehyde-based proteasome inhibitors in endometrial carcinoma cells. Furthermore, similar to tiron, vitamin C inhibited cell death by blocking the ability of bortezomib to inhibit the proteasome [76]. In addition, an-other study showed that vitamin C significantly reduced the activity of bortezomib treatment in vivo, advising that pa-

tients receiving treatment with bortezomib should avoid tak-ing vitamin C dietary supplements [77].

Some flavonoids, especially quercetin, were able to in-hibit bortezomib-induced apoptosis in malignant B-cell lines and primary chronic lymphoid leukaemia cells [78]. Myricetin showed a similar effect to quercetin, but kaempferol and apigenin did not. This singular effect of quercetin was explained through the formation of a complex with bortezomib [78]. Such data indicate that caution is needed in giving dietary advice to patients treated with these specific drugs.

Another paper was aimed to determine if, in multiple myeloma cell lines and primary myeloma cells from patients, known polyphenols, such as quercetin dehydrate, EGCG, caffeic acid, gallic acid and tannic acid, could synergistically enhance the potency of bortezomib [79]. Surprisingly, upon the co-treatment, the authors observed that polyphenols pos-sessed antagonistic effects on bortezomib-induced apoptosis. They obtained evidence through 11apx nuclear magnetic resonance spectroscopy that this effect is due to a direct in-teraction between the two molecules. In details, the vicinal diol in the polyphenols interacts with the boronic acid of bortezomib and converts the active triangular boronic acid of bortezomib to an inactive tetrahedral boronate [79]. Both the structure and the concentration of polyphenols control the equilibrium of this conversion which is finally responsible for abolishing the antimyeloma activity of bortezomib [79]. These results further confirm that attention must be paid in the intake of natural polyphenols in food or vitamin supple-ments during bortezomib treatment. In addition, various green tea constituents, in particular EGCG and other poly-phenols with 1,2-benzenediol moieties, were shown to effec-tively prevent apoptosis induced by bortezomib both in vitro and in vivo, suggesting that the consumption of green tea products may be contraindicated during bortezomib-based therapies [80].

A possible alternative strategy to overcome these side ef-fects could be the ad hoc synthesis (or the identification) of new proteasome inhibitors, whose administration is com-patible with a regular and healthy consumption of polyphe-nols. Besides, it could be worthy also to assess if any of the currently available proteasome inhibitors can avoid the inter-ference due to the co-administration of polyphenols.

Unfortunately, both these possibilities are an arduous goal to achieve since extensive and careful studies would be advisable in order to evaluate and exclude possible interac-tions with polyphenolic compounds. Nevertheless, although speculative at this stage, we believe that particular attention should be paid on the results obtained testing both the in vitro and in vivo effects of two other proteasome inhibitors, both lacking the boronic group of bortezomib which is tar-geted by polyphenols, namely NPI-0052 [81, 82] and disul-firam [83-86]. In particular, previous studies on NPI-0052 elucidated its structure, mechanisms of action and effects on the single proteasome components, describing dissimilarities with respect to bortezomib and underlying its ability to pre-vail over the problems related to bortezomib resistance and side effects [81, 87, 88]. In the view of these encouraging results, this molecule entered phase I clinical trials in pa-

Max

Barra

Max

Testo sostitutivo

11(superscript)B


tients with relapsed and relapsed/refractory multiple mye-loma.

CONCLUSIONS

The data discussed in the present work underline the im-portance of naturally occurring compounds in human diet in view of their ability to act as chemopreventative agents. Such therapeutic potential is principally due to their capacity to modulate proteasomes functionality, triggering those mechanisms that finally drive the cell towards apoptosis. In fact, taking into account the numerous processes in which the proteasome is involved, its modulation represents an in-teresting target for designing and developing novel antican-cer drugs.

Interestingly, the modulation of the proteasome does not uniquely mean inhibition of the system. In fact, giving that some of the considered polyphenols exert their pro-apoptotic activity stimulating the degradation of substrates that act as inhibitors of programmed cell death, it is evident that protea-some regulation occurs at different levels. These data further confirm the extreme complexity of the pathways in which the proteasome is implicated, thus causing difficulties in the selection of the correct therapeutic strategy.

In addition, despite the promising opportunities displayed by polyphenols, it is important to point out that caution needs to be paid in recommending a diet rich in such com-pounds, especially for those subjects affected by particular categories of cancer, including myeloma. Actually, the treatment with bortezomib and the simultaneous intake of polyphenols was shown to reverse the effects of the drug, preventing malignant cells from undergoing apoptosis. While it is unquestionable that polyphenols represent an in-triguing subject for future research and for the production of new therapeutic approaches in cancer treatment, at the same time it is necessary to correctly inform people about the re-lated risks and to acquire additional data about the potential adverse effects. As previously mentioned, the identification of proteasome inhibitors compatible with the assumption of such important compounds could be a feasible way to over-come this problem.

ABBREVIATIONS

ChT-L = chymotrypsin-like activity

T-L = trypsin-like activity

PGPH = peptidylglutamyl peptide-hydrolyzing activity

BrAAP = branched chain amino acid preferring activity

SNAAP = small neutral amino acid preferring activity

NF B = nuclear factor B

EGCG = (-)-Epigallocatechin-3-gallate

PARP = Poly (ADP-ribose) polymerase

REFERENCES

[1] Ciechanover, A. The ubiquitin-proteasome pathway: on protein

death and cell life. EMBO J. 1998, 17(24), 7151-7160.

[2] Ciechanover, A. Intracellular protein degradation: from a vague

idea thru the lysosome and the ubiquitin-proteasome system and

onto human diseases and drug targeting. Cell Death Differ. 2005,

12(9), 1178-1190.

[3] Jung, T.; Catalgol, B.; Grune, T. The proteasomal system. Mol.

Aspects Med. 2009, 30(4), 191-296.

[4] Baumeister, W.; Walz, J.; Zuhl, F.; Seemuller, E. The proteasome:

paradigm of a self-compartmentalizing protease. Cell 1998, 92(3),

367-380.

[5] Groll, M.; Ditzel, L.; Lowe, J.; Stock, D.; Bochtler, M.; Bartunik,

H. D.; Huber, R. Structure of 20S proteasome from yeast at 2.4 A

resolution. Nature 1997, 386(6624), 463-471.

[6] Lowe, J.; Stock, D.; Jap, B.; Zwickl, P.; Baumeister, W.; Huber, R.

Crystal structure of the 20S proteasome from the archaeon T. aci-

dophilum at 3.4 A resolution. Science 1995, 268(5210), 533-539.

[7] Orlowski, M.; Cardozo, C.; Michaud, C. Evidence for the presence

of five distinct proteolytic components in the pituitary multicata-

lytic proteinase complex. Properties of two components cleaving

bonds on the carboxyl side of branched chain and small neutral

amino acids. Biochemistry 1993, 32(6), 1563-1572.

[8] Groll, M.; Bochtler, M.; Brandstetter, H.; Clausen, T.; Huber, R.

Molecular machines for protein degradation. ChemBioChem 2005,

6(2), 222-256.

[9] Seemuller, E.; Lupas, A.; Stock, D.; Lowe, J.; Huber, R.; Baumei-

ster, W. Proteasome from Thermoplasma acidophilum: a threonine

protease. Science 1995, 268(5210), 579-582.

[10] Aki, M.; Shimbara, N.; Takashina, M.; Akiyama, K.; Kagawa, S.;

Tamura, T.; Tanahashi, N.; Yoshimura, T.; Tanaka, K.; Ichihara, A.

Interferon-gamma induces different subunit organizations and func-

tional diversity of proteasomes. J. Biochem. 1994, 115(2), 257-269.

[11] Llovera, M.; Garcia-Martinez, C.; Agell, N.; Lopez-Soriano, F. J.;

Argiles, J. M. TNF can directly induce the expression of ubiquitin-

dependent proteolytic system in rat soleus muscles. Biochem. Bio-

phys. Res. Commun. 1997, 230(2), 238-241.

[12] Nelson, J. E.; Loukissa, A.; Altschuller-Felberg, C.; Monaco, J. J.;

Fallon, J. T.; Cardozo, C. Up-regulation of the proteasome subunit

LMP7 in tissues of endotoxemic rats. J. Lab. Clin. Med. 2000,

135(4), 324-331.

[13] Fehling, H. J.; Swat, W.; Laplace, C.; Kuhn, R.; Rajewsky, K.;

Muller, U.; von Boehmer, H. MHC class I expression in mice lack-

ing the proteasome subunit LMP-7. Science 1994, 265(5176),

1234-1237.

[14] Ciechanover, A.; Orian, A.; Schwartz, A. L. The ubiquitin-

mediated proteolytic pathway: mode of action and clinical implica-

tions. J. Cell. Biochem. Suppl. 2000, 34, 40-51.

[15] An, B.; Goldfarb, R. H.; Siman, R.; Dou, Q. P. Novel dipeptidyl

proteasome inhibitors overcome Bcl-2 protective function and se-

lectively accumulate the cyclin-dependent kinase inhibitor p27 and

induce apoptosis in transformed, but not normal, human fibroblasts.

Cell Death Differ. 1998, 5(12), 1062-1075.

[16] Lopes, U. G.; Erhardt, P.; Yao, R.; Cooper, G. M. p53-dependent

induction of apoptosis by proteasome inhibitors. J. Biol. Chem.

1997, 272(20), 12893-12896.

[17] Voorhees, P. M.; Dees, E. C.; O'Neil, B.; Orlowski, R. Z. The

proteasome as a target for cancer therapy. Clin. Cancer Res. 2003,

9(17), 6316-6325.

[18] Wojcik, C. Regulation of apoptosis by the ubiquitin and protea-

some pathway. J. Cell. Mol. Med. 2002, 6(1), 25-48.

[19] Chene, P. Inhibiting the p53-MDM2 interaction: an important

target for cancer therapy. Nat. Rev. Cancer 2003, 3(2), 102-109.

[20] Concannon, C. G.; Koehler, B. F.; Reimertz, C.; Murphy, B. M.;

Bonner, C.; Thurow, N.; Ward, M. W.; Villunger, A.; Strasser, A.;

Kogel, D.; Prehn, J. H. Apoptosis induced by proteasome inhibition

in cancer cells: predominant role of the p53/PUMA pathway. On-

cogene 2007, 26(12), 1681-1692.

[21] Sheikh, M. S.; Fornace, A. J., Jr. Role of p53 family members in

apoptosis. J. Cell. Physiol. 2000, 182(2), 171-181.

[22] Kisselev, A. F.; Goldberg, A. L. Proteasome inhibitors: from re-

search tools to drug candidates. Chem. Biol. 2001, 8(8), 739-758.

[23] Kropff, M.; Bisping, G.; Wenning, D.; Berdel, W. E.; Kienast, J.

Proteasome inhibition in multiple myeloma. Eur. J. Cancer 2006,

42(11), 1623-1639.












https://www.researchgate.net/publication/12537800_Up-regulation_of_the_proteasome_subunit_LMP7_in_tissues_of_endotoxemic_rats?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==






[24] Ludwig, H.; Khayat, D.; Giaccone, G.; Facon, T. Proteasome inhi-

bition and its clinical prospects in the treatment of hematologic and

solid malignancies. Cancer 2005, 104(9), 1794-1807.

[25] Bonfili, L.; Cecarini, V.; Amici, M.; Cuccioloni, M.; Angeletti, M.;

Keller, J. N.; Eleuteri, A. M. Natural polyphenols as proteasome

modulators and their role as anti-cancer compounds. FEBS J. 2008,

275(22), 5512-5526.

[26] Ovaskainen, M. L.; Torronen, R.; Koponen, J. M.; Sinkko, H.;

Hellstrom, J.; Reinivuo, H.; Mattila, P. Dietary intake and major

food sources of polyphenols in Finnish adults. J. Nutr. 2008,

138(3), 562-566.

[27] Arts, I. C.; Hollman, P. C. Polyphenols and disease risk in epide-

miologic studies. Am. J. Clin. Nutr. 2005, 81 (1 Suppl), 317S-325S.

[28] Mukhtar, H.; Ahmad, N. Tea polyphenols: prevention of cancer

and optimizing health. Am. J. Clin. Nutr. 2000, 71(6 Suppl),

1698S-1702S; discussion 1703S-1694S.

[29] Erba, D.; Riso, P.; Bordoni, A.; Foti, P.; Biagi, P. L.; Testolin, G.

Effectiveness of moderate green tea consumption on antioxidative

status and plasma lipid profile in humans. J. Nutr. Biochem. 2005,

16(3), 144-149.

[30] Dalluge, J. J.; Nelson, B. C.; Thomas, J. B.; Sander, L. C. Selection

of column and gradient elution system for the separation of cate-

chins in green tea using high-performance liquid chromatography.

J. Chromatogr. A 1998, 793(2), 265-274.

[31] Jin, C. Y.; Lee, J. D.; Park, C.; Choi, Y. H.; Kim, G. Y. Curcumin

attenuates the release of pro-inflammatory cytokines in lipopoly-

saccharide-stimulated BV2 microglia. Acta Pharmacol. Sin. 2007,

28(10), 1645-1651.

[32] Kunnumakkara, A. B.; Anand, P.; Aggarwal, B. B. Curcumin in-

hibits proliferation, invasion, angiogenesis and metastasis of differ-

ent cancers through interaction with multiple cell signaling pro-

teins. Cancer Lett. 2008, 269(2), 199-225.

[33] Khan, N.; Afaq, F.; Mukhtar, H. Cancer chemoprevention through

dietary antioxidants: progress and promise. Antioxid. Redox Signal.

2008, 10(3), 475-510.

[34] Milacic, V.; Banerjee, S.; Landis-Piwowar, K. R.; Sarkar, F. H.;

Majumdar, A. P.; Dou, Q. P. Curcumin inhibits the proteasome ac-

tivity in human colon cancer cells in vitro and in vivo. Cancer Res.

2008, 68(18), 7283-7292.

[35] Jana, N. R.; Dikshit, P.; Goswami, A.; Nukina, N. Inhibition of

proteasomal function by curcumin induces apoptosis through mito-

chondrial pathway. J. Biol. Chem. 2004, 279(12), 11680-11685.

[36] Dikshit, P.; Goswami, A.; Mishra, A.; Nukina, N.; Jana, N. R.

Curcumin enhances the polyglutamine-expanded truncated N-

terminal huntingtin-induced cell death by promoting proteasomal

malfunction. Biochem. Biophys. Res. Commun. 2006, 342(4), 1323-

1328.

[37] Ali, R. E.; Rattan, S. I. Curcumin's biphasic hormetic response on

proteasome activity and heat-shock protein synthesis in human

keratinocytes. Ann. N. Y. Acad. Sci. 2006, 1067, 394-399.

[38] Si, X.; Wang, Y.; Wong, J.; Zhang, J.; McManus, B. M.; Luo, H.

Dysregulation of the ubiquitin-proteasome system by curcumin

suppresses coxsackievirus B3 replication. J. Virol. 2007, 81(7),

3142-3150.

[39] Padhye, S.; Yang, H.; Jamadar, A.; Cui, Q. C.; Chavan, D.;

Dominiak, K.; McKinney, J.; Banerjee, S.; Dou, Q. P.; Sarkar, F.

H. New difluoro Knoevenagel condensates of curcumin, their

Schiff bases and copper complexes as proteasome inhibitors and

apoptosis inducers in cancer cells. Pharm. Res. 2009, 26(8), 1874-

1880.

[40] Chen, D.; Daniel, K. G.; Chen, M. S.; Kuhn, D. J.; Landis-

Piwowar, K. R.; Dou, Q. P. Dietary flavonoids as proteasome in-

hibitors and apoptosis inducers in human leukemia cells. Biochem.

Pharmacol. 2005, 69(10), 1421-1432.

[41] Chen, D.; Landis-Piwowar, K. R.; Chen, M. S.; Dou, Q. P. Inhibi-

tion of proteasome activity by the dietary flavonoid apigenin is as-

sociated with growth inhibition in cultured breast cancer cells and

xenografts. Breast Cancer Res. 2007, 9(6), R80.

[42] Wu, Y. X.; Fang, X. Apigenin, chrysin, and luteolin selectively

inhibit chymotrypsin-like and trypsin-like proteasome catalytic ac-

tivities in tumor cells. Planta Med. 2010, 76(2), 128-132.

[43] Dosenko, V. E.; Nagibin, V. S.; Tumanovskaia, L. V.; Zagorii,

VIu; Moibenko, A. A. [The influence of quercetin on the activity of

purified 20S, 26S proteasome and proteasomal activity in isolated

cardiomyocytes]. Biomed. Khim. 2006, 52(2), 138-145.

[44] Chang, T. L. Inhibitory Effect of Flavonoids on 26S Proteasome

Activity. J. Agric. Food. Chem. 2009, 57, 9706–9715.

[45] Way, T. D.; Kao, M. C.; Lin, J. K. Apigenin induces apoptosis

through proteasomal degradation of HER2/neu in HER2/neu-

overexpressing breast cancer cells via the phosphatidylinositol 3-

kinase/Akt-dependent pathway. J. Biol. Chem. 2004, 279(6), 4479-

4489.

[46] Siegelin, M. D.; Reuss, D. E.; Habel, A.; Herold-Mende, C.; von

Deimling, A. The flavonoid kaempferol sensitizes human glioma

cells to TRAIL-mediated apoptosis by proteasomal degradation of

survivin. Mol. Cancer Ther. 2008, 7(11), 3566-3574.

[47] Siegelin, M. D.; Reuss, D. E.; Habel, A.; Rami, A.; von Deimling,

A. Quercetin promotes degradation of survivin and thereby en-

hances death-receptor-mediated apoptosis in glioma cells. Neuro

Oncol. 2009, 11(2), 122-131.

[48] Son, Y. G.; Kim, E. H.; Kim, J. Y.; Kim, S. U.; Kwon, T. K.;

Yoon, A. R.; Yun, C. O.; Choi, K. S. Silibinin sensitizes human

glioma cells to TRAIL-mediated apoptosis via DR5 up-regulation

and down-regulation of c-FLIP and survivin. Cancer Res. 2007,

67(17), 8274-8284.

[49] Chang, T. L.; Ding, H. Y.; Teng, K. N.; Fang, Y. C. 5,6,3',4'-

Tetrahydroxy-7-methoxyflavone as a Novel Potential Proteasome

Inhibitor. Planta Med. 2010, 76(10), 987-94.

[50] Dreiseitel, A.; Schreier, P.; Oehme, A.; Locher, S.; Rogler, G.;

Piberger, H.; Hajak, G.; Sand, P. G. Inhibition of proteasome activ-

ity by anthocyanins and anthocyanidins. Biochem. Biophys. Res.

Commun. 2008, 372(1), 57-61.

[51] Zheng, W.; Wang, S. Y. Oxygen radical absorbing capacity of

phenolics in blueberries, cranberries, chokeberries, and lingonber-

ries. J. Agric. Food. Chem. 2003, 51(2), 502-509.

[52] Wang, H.; Nair, M. G.; Strasburg, G. M.; Chang, Y. C.; Booren, A.

M.; Gray, J. I.; DeWitt, D. L. Antioxidant and antiinflammatory ac-

tivities of anthocyanins and their aglycon, cyanidin, from tart cher-

ries. J. Nat. Prod. 1999, 62(2), 294-296.

[53] Hou, D. X. Potential mechanisms of cancer chemoprevention by

anthocyanins. Curr. Mol. Med. 2003, 3(2), 149-159.

[54] Sweeney, M. I.; Kalt, W.; MacKinnon, S. L.; Ashby, J.; Gottschall-

Pass, K. T. Feeding rats diets enriched in lowbush blueberries for

six weeks decreases ischemia-induced brain damage. Nutr. Neuro-

sci. 2002, 5(6), 427-431.

[55] Tarozzi, A.; Morroni, F.; Hrelia, S.; Angeloni, C.; Marchesi, A.;

Cantelli-Forti, G.; Hrelia, P. Neuroprotective effects of anthocyan-

ins and their in vivo metabolites in SH-SY5Y cells. Neurosci. Lett.

2007, 424(1), 36-40.

[56] Kim, H. G.; Ju, M. S.; Shim, J. S.; Kim, M. C.; Lee, S. H.; Huh, Y.;

Kim, S. Y.; Oh, M. S. Mulberry fruit protects dopaminergic neu-

rons in toxin-induced Parkinson's disease models. Br. J. Nutr.

2010, 1-9.

[57] Pajonk, F.; Scholber, J.; Fiebich, B. Hypericin-an inhibitor of pro-

teasome function. Cancer Chemother. Pharmacol. 2005, 55(5),

439-446.

[58] Manju, V.; Nalini, N. Protective role of luteolin in 1,2-

dimethylhydrazine induced experimental colon carcinogenesis. Cell

Biochem. Funct. 2007, 25(2), 189-194.

[59] Bagli, E.; Stefaniotou, M.; Morbidelli, L.; Ziche, M.; Psillas, K.;

Murphy, C.; Fotsis, T. Luteolin inhibits vascular endothelial growth

factor-induced angiogenesis; inhibition of endothelial cell survival

and proliferation by targeting phosphatidylinositol 3'-kinase activ-

ity. Cancer Res. 2004, 64(21), 7936-7946.

[60] Leung, H. W.; Wu, C. H.; Lin, C. H.; Lee, H. Z. Luteolin induced

DNA damage leading to human lung squamous carcinoma CH27

cell apoptosis. Eur. J. Pharmacol. 2005, 508(1-3), 77-83.

[61] Selvendiran, K.; Koga, H.; Ueno, T.; Yoshida, T.; Maeyama, M.;

Torimura, T.; Yano, H.; Kojiro, M.; Sata, M. Luteolin promotes

degradation in signal transducer and activator of transcription 3 in

human hepatoma cells: an implication for the antitumor potential of

flavonoids. Cancer Res. 2006, 66(9), 4826-4834.

[62] Shi, R. X.; Ong, C. N.; Shen, H. M. Luteolin sensitizes tumor ne-

crosis factor-alpha-induced apoptosis in human tumor cells. Onco-

gene 2004, 23(46), 7712-7721.

[63] Katsiki, M.; Chondrogianni, N.; Chinou, I.; Rivett, A. J.; Gonos, E.

S. The olive constituent oleuropein exhibits proteasome stimulatory






https://www.researchgate.net/publication/6980726_Curcumin's_Biphasic_Hormetic_Response_on_Proteasome_Activity_and_Heat-Shock_Protein_Synthesis_in_Human_Keratinocytes?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==




https://www.researchgate.net/publication/8378449_Luteolin_sensitizes_tumor_necrosis_factor-a-induced_apoptosis_in_human_tumor_cells?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==




https://www.researchgate.net/publication/6934399_Protective_role_of_luteolin_in_12-dimethylhydrazine_induced_experimental_colon_carcinogenesis?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==







https://www.researchgate.net/publication/7585975_Proteasome_inhibition_and_its_clinical_prospects_in_the_treatment_of_hematologic_and_solid_malignancies?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==




https://www.researchgate.net/publication/41620590_Mulberry_fruit_protects_dopaminergic_neurons_in_toxin-induced_Parkinson's_disease_models?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==






properties in vitro and confers life span extension of human embry-

onic fibroblasts. Rejuvenation Res. 2007, 10(2), 157-172.

[64] Liu, E. H.; Qi, L. W.; Wu, Q.; Peng, Y. B.; Li, P. Anticancer

Agents Derived from Natural Products. Mini Rev. Med. Chem.

2009, 9(13), 1547-1555.

[65] Liao, H. F.; Kuo, C. D.; Yang, Y. C.; Lin, C. P.; Tai, H. C.; Chen,

Y. Y.; Chen, Y. J. Resveratrol enhances radiosensitivity of human

non-small cell lung cancer NCI-H838 cells accompanied by inhibi-

tion of nuclear factor-kappa B activation. J. Radiat. Res. (Tokyo)

2005, 46(4), 387-393.

[66] Marambaud, P.; Zhao, H.; Davies, P. Resveratrol promotes clear-

ance of Alzheimer's disease amyloid-beta peptides. J. Biol. Chem.

2005, 280(45), 37377-37382.

[67] Pettinari, A.; Amici, M.; Cuccioloni, M.; Angeletti, M.; Fioretti, E.;

Eleuteri, A. M. Effect of polyphenolic compounds on the prote-

olytic activities of constitutive and immuno-proteasomes. Antioxid.

Redox Signal. 2006, 8(1-2), 121-129.

[68] Mozzicafreddo, M.; Cuccioloni, M.; Cecarini, V.; Eleuteri, A. M.;

Angeletti, M. Homology modeling and docking analysis of the in-

teraction between polyphenols and mammalian 20S proteasomes. J.

Chem. Inf. Model. 2009, 49(2), 401-409.

[69] Nam, S.; Smith, D. M.; Dou, Q. P. Tannic acid potently inhibits

tumor cell proteasome activity, increases p27 and Bax expression,

and induces G1 arrest and apoptosis. Cancer Epidemiol. Biomark-

ers Prev. 2001, 10(10), 1083-1088.

[70] Cichocki, M.; Blumczynska, J.; Baer-Dubowska, W. Naturally

occurring phenolic acids inhibit 12-O-tetradecanoylphorbol-13-

acetate induced NF-kappaB, iNOS and COX-2 activation in mouse

epidermis. Toxicology 2010, 268(1-2), 118-124.

[71] Chen, M. S.; Chen, D.; Dou, Q. P. Inhibition of proteasome activity

by various fruits and vegetables is associated with cancer cell

death. In Vivo 2004, 18(1), 73-80.

[72] Kazi, A.; Urbizu, D. A.; Kuhn, D. J.; Acebo, A. L.; Jackson, E. R.;

Greenfelder, G. P.; Kumar, N. B.; Dou, Q. P. A natural musaceas

plant extract inhibits proteasome activity and induces apoptosis

selectively in human tumor and transformed, but not normal and

non-transformed, cells. Int. J. Mol. Med. 2003, 12(6), 879-887.

[73] Amici, M.; Bonfili, L.; Spina, M.; Cecarini, V.; Calzuola, I.; Mar-

sili, V.; Angeletti, M.; Fioretti, E.; Tacconi, R.; Gianfranceschi, G.

L.; Eleuteri, A. M. Wheat sprout extract induces changes on 20S

proteasomes functionality. Biochimie 2008, 90(5), 790-801.

[74] Bonfili, L.; Amici, M.; Cecarini, V.; Cuccioloni, M.; Tacconi, R.;

Angeletti, M.; Fioretti, E.; Keller, J. N.; Eleuteri, A. M. Wheat

sprout extract-induced apoptosis in human cancer cells by protea-

somes modulation. Biochimie 2009, 91(9), 1131-1144.

[75] Menendez, J. A.; Vazquez-Martin, A.; Garcia-Villalba, R.; Car-

rasco-Pancorbo, A.; Oliveras-Ferraros, C.; Fernandez-Gutierrez,

A.; Segura-Carretero, A. tabAnti-HER2 (erbB-2) oncogene effects

of phenolic compounds directly isolated from commercial Extra-

Virgin Olive Oil (EVOO). BMC Cancer 2008, 8, 377.

[76] Llobet, D.; Eritja, N.; Encinas, M.; Sorolla, A.; Yeramian, A.;

Schoenenberger, J. A.; Llombart-Cussac, A.; Marti, R. M.; Matias-

Guiu, X.; Dolcet, X. Antioxidants block proteasome inhibitor func-

tion in endometrial carcinoma cells. Anticancer Drugs 2008, 19(2),

115-124.

[77] Perrone, G.; Hideshima, T.; Ikeda, H.; Okawa, Y.; Calabrese, E.;

Gorgun, G.; Santo, L.; Cirstea, D.; Raje, N.; Chauhan, D.; Bacca-

rani, M.; Cavo, M.; Anderson, K. C. Ascorbic acid inhibits antitu-

mor activity of bortezomib in vivo. Leukemia 2009, 23(9), 1679-

1686.

[78] Liu, F. T.; Agrawal, S. G.; Movasaghi, Z.; Wyatt, P. B.; Rehman, I.

U.; Gribben, J. G.; Newland, A. C.; Jia, L. Dietary flavonoids in-

hibit the anticancer effects of the proteasome inhibitor bortezomib.

Blood 2008, 112(9), 3835-3846.

[79] Kim, T. Y.; Park, J.; Oh, B.; Min, H. J.; Jeong, T. S.; Lee, J. H.;

Suh, C.; Cheong, J. W.; Kim, H. J.; Yoon, S. S.; Park, S. B.; Lee,

D. S. Natural polyphenols antagonize the antimyeloma activity of

proteasome inhibitor bortezomib by direct chemical interaction. Br.

J. Haematol. 2009, 146(3), 270-281.

[80] Golden, E. B.; Lam, P. Y.; Kardosh, A.; Gaffney, K. J.; Cadenas,

E.; Louie, S. G.; Petasis, N. A.; Chen, T. C.; Schonthal, A. H.

Green tea polyphenols block the anticancer effects of bortezomib

and other boronic acid-based proteasome inhibitors. Blood 2009,

113(23), 5927-5937.

[81] Chauhan, D.; Hideshima, T.; Anderson, K. C. A novel proteasome

inhibitor NPI-0052 as an anticancer therapy. Br. J. Cancer 2006,

95(8), 961-965.

[82] Singh, A. V.; Palladino, M. A.; Lloyd, G. K.; Potts, B. C.;

Chauhan, D.; Anderson, K. C. Pharmacodynamic and efficacy

studies of the novel proteasome inhibitor NPI-0052 (marizomib) in

a human plasmacytoma xenograft murine model. Br. J. Haematol.

2010, 149(4), 550-559.

[83] Wang, W.; McLeod, H. L.; Cassidy, J. Disulfiram-mediated inhibi-

tion of NF-kappaB activity enhances cytotoxicity of 5-fluorouracil

in human colorectal cancer cell lines. Int. J. Cancer 2003, 104(4),

504-511.

[84] Brar, S. S.; Grigg, C.; Wilson, K. S.; Holder, W. D., Jr.; Dreau, D.;

Austin, C.; Foster, M.; Ghio, A. J.; Whorton, A. R.; Stowell, G. W.;

Whittall, L. B.; Whittle, R. R.; White, D. P.; Kennedy, T. P. Disul-

firam inhibits activating transcription factor/cyclic AMP-

responsive element binding protein and human melanoma growth

in a metal-dependent manner in vitro, in mice and in a patient with

metastatic disease. Mol. Cancer Ther. 2004, 3(9), 1049-1060.

[85] Cen, D.; Gonzalez, R. I.; Buckmeier, J. A.; Kahlon, R. S.; To-

hidian, N. B.; Meyskens, F. L., Jr. Disulfiram induces apoptosis in

human melanoma cells: a redox-related process. Mol. Cancer Ther.

2002, 1(3), 197-204.

[86] Shian, S. G.; Kao, Y. R.; Wu, F. Y.; Wu, C. W. Inhibition of inva-

sion and angiogenesis by zinc-chelating agent disulfiram. Mol.

Pharmacol. 2003, 64(5), 1076-1084.

[87] Chauhan, D.; Catley, L.; Li, G.; Podar, K.; Hideshima, T.; Ve-

lankar, M.; Mitsiades, C.; Mitsiades, N.; Yasui, H.; Letai, A.; Ovaa,

H.; Berkers, C.; Nicholson, B.; Chao, T. H.; Neuteboom, S. T.;

Richardson, P.; Palladino, M. A.; Anderson, K. C. A novel orally

active proteasome inhibitor induces apoptosis in multiple myeloma

cells with mechanisms distinct from Bortezomib. Cancer Cell.

2005, 8(5), 407-419.

[88] Groll, M.; Huber, R.; Potts, B. C. Crystal structures of Salinospo-

ramide A (NPI-0052) and B (NPI-0047) in complex with the 20S

proteasome reveal important consequences of beta-lactone ring

opening and a mechanism for irreversible binding. J. Am. Chem.

Soc. 2006, 128(15), 5136-5141.

[89] Su, B. N.; Hwang, B. Y.; Chai, H.; Carcache-Blanco, E. J.; Kar-

dono, L. B.; Afriastini, J. J.; Riswan, S.; Wild, R.; Laing, N.;

Farnsworth, N. R.; Cordell, G. A.; Swanson, S. M.; Kinghorn, A.

D. Activity-guided fractionation of the leaves of Ormosia suma-

trana using a proteasome inhibition assay. J. Nat. Prod. 2004,

67(11), 1911-1914.

[90] Yang, H.; Shi, G.; Dou, Q. P. The tumor proteasome is a primary

target for the natural anticancer compound Withaferin A isolated

from "Indian winter cherry". Mol. Pharmacol. 2007, 71(2), 426-

437.

[91] Maity, R.; Sharma, J.; Jana, N. R. Capsaicin induces apoptosis

through ubiquitin-proteasome system dysfunction. J. Cell. Bio-

chem. 2010, 109(5), 933-942

[92] Yang, H.; Chen, D.; Cui, Q. C.; Yuan, X.; Dou, Q. P. Celastrol, a

triterpene extracted from the Chinese "Thunder of God Vine," is a

potent proteasome inhibitor and suppresses human prostate cancer

growth in nude mice. Cancer Res. 2006, 66(9), 4758-4765.

[93] Zhang, H. G.; Kim, H.; Liu, C.; Yu, S.; Wang, J.; Grizzle, W. E.;

Kimberly, R. P.; Barnes, S. Curcumin reverses breast tumor

exosomes mediated immune suppression of NK cell tumor cytotox-

icity. Biochim. Biophys. Acta 2007, 1773(7), 1116-1123.

[94] Wyke, S. M.; Russell, S. T.; Tisdale, M. J. Induction of proteasome

expression in skeletal muscle is attenuated by inhibitors of NF-

kappaB activation. Br. J. Cancer 2004, 91(9), 1742-1750.

[95] Lev-Ari, S.; Strier, L.; Kazanov, D.; Madar-Shapiro, L.; Dvory-

Sobol, H.; Pinchuk, I.; Marian, B.; Lichtenberg, D.; Arber, N.

Celecoxib and curcumin synergistically inhibit the growth of colo-

rectal cancer cells. Clin. Cancer Res. 2005, 11(18), 6738-6744.

[96] Bava, S. V.; Puliappadamba, V. T.; Deepti, A.; Nair, A.; Karunaga-

ran, D.; Anto, R. J. Sensitization of taxol-induced apoptosis by cur-

cumin involves down-regulation of nuclear factor-kappaB and the

serine/threonine kinase Akt and is independent of tubulin polym-

erization. J. Biol. Chem. 2005, 280(8), 6301-6308.

[97] Notarbartolo, M.; Poma, P.; Perri, D.; Dusonchet, L.; Cervello, M.;

D'Alessandro, N. Antitumor effects of curcumin, alone or in com-

bination with cisplatin or doxorubicin, on human hepatic cancer

https://www.researchgate.net/publication/7376336_Resveratrol_Enhances_Radiosensitivity_of_Human_Non-Small_Cell_Lung_Cancer_NCI-H838_Cells_Accompanied_by_Inhibition_of_Nuclear_Factor-Kappa_B_Activation?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==









https://www.researchgate.net/publication/51407705_Dietary_flavonoids_inhibit_the_anticancer_effects_of_the_proteasome_inhibitor_bortezomib?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==





https://www.researchgate.net/publication/5331934_A_natural_musaceas_plant_extract_inhibits_proteasome_activity_and_induces_apoptosis_selectively_in_human_tumor_and_transformed_but_not_normal_and_non-transformed_cells?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==






https://www.researchgate.net/publication/8357378_Inhibition_of_proteasome_activity_by_various_fruits_and_vegetables_is_associated_with_cancer_cell_death?el=1_x_8&enrichId=rgreq-bf39d94c-7409-4d8e-b783-e9778c19d91c&enrichSource=Y292ZXJQYWdlOzIzMzYzMDg4OTtBUzoxNTA2NzcyMDQ1MDg2NzNAMTQxMjkzNTY1MzI2NA==





cells. Analysis of their possible relationship to changes in NF-kB

activation levels and in IAP gene expression. Cancer Lett. 2005,

224(1), 53-65.

[98] Chirnomas, D.; Taniguchi, T.; de la Vega, M.; Vaidya, A. P.;

Vasserman, M.; Hartman, A. R.; Kennedy, R.; Foster, R.; Ma-

honey, J.; Seiden, M. V.; D'Andrea, A. D. Chemosensitization to

cisplatin by inhibitors of the Fanconi anemia/BRCA pathway. Mol.

Cancer Ther. 2006, 5(4), 952-961.

[99] Maher, P. The flavonoid fisetin promotes nerve cell survival from

trophic factor withdrawal by enhancement of proteasome activity.

Arch. Biochem. Biophys. 2008, 476(2), 139-144.

[100] Salmela, A. L.; Pouwels, J.; Varis, A.; Kukkonen, A. M.;

Toivonen, P.; Halonen, P. K.; Perala, M.; Kallioniemi, O.; Gorb-

sky, G. J.; Kallio, M. J. Dietary flavonoid fisetin induces a forced

exit from mitosis by targeting the mitotic spindle checkpoint. Car-

cinogenesis 2009, 30(6), 1032-1040.

[101] Kazi, A.; Daniel, K. G.; Smith, D. M.; Kumar, N. B.; Dou, Q. P.

Inhibition of the proteasome activity, a novel mechanism associ-

ated with the tumor cell apoptosis-inducing ability of genistein.

Biochem. Pharmacol. 2003, 66(6), 965-976.

[102] Yu, Z.; Li, W.; Liu, F. Inhibition of proliferation and induction of

apoptosis by genistein in colon cancer HT-29 cells. Cancer Lett.

2004, 215(2), 159-166.

[103] Singletary, K.; Ellington, A. Genistein suppresses proliferation and

MET oncogene expression and induces EGR-1 tumor suppressor

expression in immortalized human breast epithelial cells. Antican-

cer Res. 2006, 26(2A), 1039-1048.

[104] Jeong, J. H.; An, J. Y.; Kwon, Y. T.; Li, L. Y.; Lee, Y. J. Quer-

cetin-induced ubiquitination and down-regulation of Her-2/neu. J.

Cell. Biochem. 2008, 105(2), 585-595.

[105] Psahoulia, F. H.; Moumtzi, S.; Roberts, M. L.; Sasazuki, T.;

Shirasawa, S.; Pintzas, A. Quercetin mediates preferential degrada-

tion of oncogenic Ras and causes autophagy in Ha-RAS-

transformed human colon cells. Carcinogenesis 2007, 28(5), 1021-

1031.

[106] Kim, J. Y.; Kim, E. H.; Park, S. S.; Lim, J. H.; Kwon, T. K.; Choi,

K. S. Quercetin sensitizes human hepatoma cells to TRAIL-

induced apoptosis via Sp1-mediated DR5 up-regulation and protea-

some-mediated c-FLIPS down-regulation. J. Cell. Biochem. 2008,

105(6), 1386-1398.

[107] Shakibaei, M.; Csaki, C.; Nebrich, S.; Mobasheri, A. Resveratrol

suppresses interleukin-1beta-induced inflammatory signaling and

apoptosis in human articular chondrocytes: potential for use as a

novel nutraceutical for the treatment of osteoarthritis. Biochem.

Pharmacol. 2008, 76(11), 1426-1439.

[108] Delmas, D.; Lancon, A.; Colin, D.; Jannin, B.; Latruffe, N. Res-

veratrol as a chemopreventive agent: a promising molecule for

fighting cancer. Curr. Drug Targets 2006, 7(4), 423-442.

Received: September 20, 2010 Revised: December 30, 2010 Accepted: December 30, 2010

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