Role of erbB2 in breast cancerchemosensitivityDihua Yu* and Mien-Chie Hung

SummaryThe erbB2 gene, which encodes a transmembranegrowth factor receptor, is overexpressed in approxi-mately 30% of breast cancers. Overexpressing this genemakes breast cancers resistant to certain chemother-apeutic agents. In this article, we review what is knownabout ErbB2-mediated chemoresistance and the con-troversies surrounding it. We also examine the anti-apoptotic function of erbB2 as one of the molecularmechanisms of ErbB2-mediated Taxol resistance anddescribe several emerging strategies for overcomingintrinsic ErbB2-mediated chemoresistance. Finally, wediscuss future avenues for studies of chemosensitivityin ErbB2-overexpressing breast cancers that may leadto the development of effective biology-based treatmentstrategies. BioEssays 22:673±680, 2000. ß 2000 JohnWiley & Sons, Inc.

Introduction

The erbB2 gene (also known as HER-2, neu, and NGL)

encodes a 185-kDa transmembrane glycoprotein (ErbB2),

that belongs to the epidermal growth factor receptor (EGFR)

family.(1,2) Similar to EGFR, the ErbB2 growth factor receptor

has intrinsic tyrosine-kinase activities.(3±5) Although the

ligand of the ErbB2 receptor has not been clearly defined,

a Neu-activating factor (NAF) has been found that binds to

the Neu receptor and to an intramembrane modulator of the

ErbB2 receptor (named ASGP2) that potentiates neuregulin-

1 signaling.(6,7) It is well known that the ErbB2 receptor is the

preferred heterodimerization partner of all ErbB receptors

(EGFR, ErbB3 and ErbB4), and that it can mediate signal

transduction of all ErbB-receptor-family members when they

bind to their cognate ligands.(8±10) Therefore, the ErbB2

receptor can mediate the lateral signal transduction of all

ErbB receptors,(8±10) and hence can be involved in the

regulation of a variety of vital functions controlled by any of

the ErbB-receptor-family members, including cell growth,

differentiation, and apoptosis.

The erbB2 gene plays an important role in human malign-

ancies. It is amplified and/or overexpressed in approximately

30% of human breast carcinomas(11,12) and in many other

types of human malignancies.(13) People with erbB2-over-

expressing tumors have a significantly lower overall survival

rate and a shorter time to relapse than do patients whose

tumors did not overexpress erbB2. For example, in studies of

patients with ovarian cancer overexpressing erbB2, all

patients died soon after diagnosis,(12,14) whereas some

ovarian-cancer patients whose tumors did not overexpress

erbB2 lived for much longer periods after diagnosis. Studies

in patients with node-positive breast cancer have also

revealed a prognostic correlation between the number of

lymph-node metastases and erbB2 overexpression.(12,15±17)

Consistent with this clinical correlation, we have observed

that enhanced erbB2 expression in human ovarian cancer

cells, leads to more aggressive malignant behavior.(18) Over-

expression of erbB2 can also enhance the intrinsic meta-

static potential of breast cancer cells.(19)

Overwhelming evidence from multiple studies indicates

that overexpression of the ErbB2 receptor plays an important

role in cancer development and progression.(13) The ErbB2

receptor is thus an excellent target for novel cancer therapies

and, as a result, there have been many attempts to develop

erbB2-targeting cancer therapies.

One successful example of this is the recombinant

humanized anti-ErbB2 antibody Herceptin which binds to

the extracellular domain of ErbB2. This monoclonal antibody

has a cytostatic effect in erbB2-overexpressing human

breast tumor xenografts.(20) Moreover, in both Phase II and

Phase III clinical trials, the antibody has shown remarkable

therapeutic effects in patients with erbB2-overexpressing

breast cancers when given in combination with chemother-

apeutic agents.(21,22) Another erbB2-targeting therapy has

been developed as a result of studies showing that the

adenovirus 5 E1A gene suppresses the growth of erbB2-

overexpressing cancer cells by repressing erbB2 transcrip-

tion.(23) Subsequently, both liposome- and adenovirus-

mediated E1A gene transfer were observed to inhibit the

proliferation of erbB2-overexpressing ovarian cancer cells

injected into the peritoneal cavity of mice as well as the

proliferation of erbB2-overexpressing breast cancer cells

inoculated in the mammary fat pads of mice.(24±26) E1A gene

BioEssays 22:673±680, ß 2000 John Wiley & Sons, Inc. BioEssays 22.7 673

Department of Surgical Oncology, Department of Cancer Biology, The

University of Texas M. D. Anderson Cancer Center, Houston, Texas.

Funding agencies: NIH: Grant numbers: CA60488 (D.Y.), CA58880

(M.C.H.) and CA60856 (M.C.H.). USAMRMC: Grant numbers:

DAMD17-98-8338 (D.Y.) and DAMD17-99-9271 (D.Y.). M. D. Ander-

son Breast Cancer Research Program Fund.

*Correspondence to: Dr. Dihua Yu, Department of Surgical Oncology,

Department of Cancer Biology, The University of Texas M. D.

Anderson Cancer Center, Houston, TX 77030.

E-mail: dyu@notes.mdacc.tmc.edu

Challenges

transfer was also associated with prolonged survival in mice

compared with that in untreated controls.(24±26) A Phase I

clinical trial conducted in patients with metastatic breast or

epithelial ovarian cancers that overexpressed erbB2 showed

the efficacy of E1A gene transduction into tumor cells and

the subsequent downregulation of erbB2 expression in tumor

cells.(27)

Other efforts to develop erbB2-targeting cancer therapies

have yielded promising agents, such as anti-ErbB2 intra-

cellular single-chain antibodies(28) and tyrosine-kinase in-

hibitors that specifically inhibit the ErbB2 tyrosine-kinase

activities.(29)

Because all these novel erbB2-targeting cancer therapies

are still in development, conventional chemotherapy is still

the main treatment for breast cancers that overexpress

erbB2, especially in patients who are unlikely to respond to

hormone therapy, or are clearly hormone refractory or who

have rapidly progressive visceral disease.(30)

ErbB2 and chemosensitivity

The efficacy of chemotherapy in controlling breast cancers is

determined mostly by the susceptibility of the cancer cells to

the killing effects of the various anticancer agents. Although

this is controversial (see below), the erbB2 gene might affect

the sensitivity of breast cancer cells to several anticancer

agents.

Clinical observationsA clinical study conducted in 1992 found that erbB2-

overexpressing breast tumors are less responsive to

adjuvant chemotherapy regimens consisting of cyclopho-

sphamide, methotrexate and 5-fluorouracil (CMF) than are

tumors that express a normal amount of erbB2.(31) Another

study showed that overexpression of the erbB2 gene might

have predictive value in patients with advanced breast

cancers receiving epirubicin therapy. In particular, the

response rate to epirubicin therapy in patients with erbB2-

overexpressing tumors was 32% compared with 65% in

patients with non-erbB2-overexpressing tumors (p�0.0058).(32) Interestingly, patients with node-positive early-

stage breast tumors that overexpressed erbB2 benefited

from higher doses of chemotherapy, whereas patients with

early-stage disease not exhibiting erbB2 overexpression did

not.(33) This suggests a relationship between chemosensi-

tivity and erbB2 expression in breast cancers. Meanwhile,

another study showed that the status of erbB2 expression

was useful for predicting survival time only in patients

receiving adjuvant therapy, further suggesting that erbB2

may be a marker of drug resistance.(34) In addition, several

studies have shown that the serum level of the ErbB2 protein

can also predict the responsiveness of breast cancers to

adjuvant treatment.(35,36) For example, a study of 80 patients

with metastatic breast cancers showed that elevated ErbB2

serum protein levels are associated with a lower rate of

response (including complete remission, partial remission

and no change) to first-line chemotherapy than are normal

ErbB2 levels (29% vs. 59%, p < 0.01).(37)

There have been several exciting reports of enhanced

chemosensitivity in patients receiving recombinant huma-

nized anti-ErbB2 monoclonal antibody,(21,22) a finding that

supports a role for the erbB2 gene in chemoresistance.

For example, in a Phase II study, recombinant humanized

anti-ErbB2 monoclonal antibody plus cisplatin were admin-

istered to patients with erbB2-overexpressing metastatic

breast cancer refractory to chemotherapy. This led to

objective clinical response rates higher than those seen in

patients receiving either cisplatin or the anti-ErbB2 antibody

alone.(21)

In general, clinical investigations have shown that breast

tumors overexpressing the erbB2 gene are less responsive

to treatment than those expressing a normal amount,

suggesting that the erbB2 expression level can be used as

a marker for response to chemotherapy in patients with

breast cancer.

Laboratory findingsData from laboratory studies have provided more direct

evidence that erbB2 overexpression can render breast

cancer cells resistant to certain chemotherapeutic agents.

Using a panel of established human breast cancer cell lines

that express different levels of erbB2,(38) higher levels of

erbB2 expression in these cell lines were correlated with

increased resistance to palclitaxel (Taxol) and docetaxel

(Taxotere). Conversely, N29 monoclonal antibodies (which

can specifically downregulate ErbB2) sensitized the erbB2-

overexpressing breast cancer cells to Taxol.(38) The collec-

tive findings from this study indicated that overexpression of

erbB2 renders these breast cancer cells more resistant to

Taxol. In another study, MDA-MB-435 human breast cancer

cells were stably transfected with the human erbB2 cDNA.

These cells showed increased expression of erbB2 and no

change in p170mdr1 expression but were more resistant to

Taxol and Taxotere than their parental cells.(39) This leads

to the conclusion that overexpression of erbB2 can lead to

intrinsic Taxol resistance independent of mdr-1-based multi-

drug-resistance mechanisms.(39) These findings are sup-

ported by another study in which Herceptin enhanced the

antitumor activity of Taxol and doxorubicin against erbB2-

overexpressing human breast cancer xenografts.(40) Most

importantly, the notion that overexpressing erbB2 confers

Taxol resistance on the breast cancer cells derived from

these laboratory studies was further supported by the results

of a Phase III clinical trial. This trial showed that the Taxol

response rate in patients with erbB2-overexpressing breast

cancers was significantly higher in patients receiving both

Taxol and Herceptin.(22)

Challenges

674 BioEssays 22.7

Several earlier studies also showed that ErbB2 antibodies

work synergistically with other chemotherapeutic drugs (such

as cisplatin) in inhibiting breast and ovarian cancer cell

growth in vitro(41±43) and in vivo.(44) Moreover, treating

erbB2-overexpressing human breast tumor xenografts with a

combination of cisplatin and Herceptin also blocked the

repair of cisplatin-damaged DNA, thus eliciting antibody-

enhanced chemosensitivity. Taken together, these labora-

tory findings clearly indicated that erbB2 overexpression is

linked to resistance to particular chemotherapeutic agents.

The existing controversy

Despite the above supporting evidence, the role of erbB2 in

modulating chemosensitivity remains controversial. One

source of controversy was a report of 81 patients with

primary breast carcinoma treated with adjuvant chemother-

apy and followed up for a median of 5 years.(45) In this

study, patients' responses to adjuvant chemotherapy did not

significantly correlate with erbB2 expression, and/or p53

expression as shown by immunohistochemical analysis of

the ErbB2 and p53 proteins.(45) Similarly, neither erbB2 or

p53 expression was significantly associated with tumor

response to neoadjuvant chemo-radiotherapy in a series of

329 patients with breast cancer (stage T2/T3-N0N1b-M0)

treated either with the combination of fluorouracil, Adria-

mycin and cyclophosphamide (FAC) or with radiotherapy

followed by surgery.(20) In yet another study, erbB2-over-

expressing breast cancers responded better to doxorubicin

than did breast cancers expressing low levels of erbB2.(46) In

another study of 127 breast cancer patients who received a

fluorouracil, epirubicin, and cyclophosphamide (FEC) regi-

men either once a week or once every four weeks, erbB2

gene amplification and receptor-protein expression were not

found to predict the response.(47)

These seemingly contradictory clinical observations may

be partly explained by intrinsic differences in the design of

the clinical studies. We believe that numerous factors may

affect the outcome of investigations into the role of erbB2 in

modulating chemosensitivity in complicated clinical study

settings. Some of these factors include: the timing of

treatment (neoadjuvant or adjuvant);, the type of regimen

(e.g. FAC, CMF, FEC); the status of treatment in patients

(previously treated or untreated); patient age (younger or

older); the menopausal status of the patient; the ethnic

background of the patient; the estrogen-receptor status of

the tumor (positive or negative); the presence or absence of

other genetic alterations that may interact with the ErbB2

receptor; and, very importantly, the way in which the erbB2

expression level was determined and defined. Amplification

and overexpression of erbB2 can be detected by fluorecence

in situ hybridization (FISH), immunohistochemistry (IHC) and

enzyme-linked immunosorbent assay (ELISA) on tumor-

tissue samples.(48) However, the lack of standardization of

the assay, the subjectivity of the assay carrier, the limi-

tations of the techniques, differences in antibodies or DNA

probes used and differences in tissue treatment have

resulted in discordance in determining erbB2 expression

levels. Thus, these may lead to discordance in the outcome

of investigations regarding the role of erbB2 in modulating

chemosensitivity.

Interestingly, the controversy over the role of erbB2 in

chemosensitivity is not limited to complicated clinical

settings. For example, in a laboratory study in which

erbB2-transfected breast and ovarian cancer cells were

used to determine the effect of erbB2 overexpression on

chemosensitivity, erbB2 overexpression was found to be

insufficient to induce intrinsic, pleiomorphic resistance to

drugs including Taxol.(49) Although this carefully designed

study involved the use of a comprehensive series of erbB2-

transfected breast cancer cells, including erbB2-transfected

MDA-MB-435 cells, it apparently yielded results that dis-

agreed with an earlier study showing that erbB2 can confer

Taxol resistance in erbB2-transfected MDA-MB-435 breast

cancer cells.(39) The discrepancy may be partly explained by

differences in the erbB2 expression levels between the

erbB2-transfected MDA-MB-435 cells used in the two

studies. In particular, the erbB2 transfectants that showed

Taxol resistance expressed very high levels of ErbB2

protein, similar to those expressed in the SKBR3 breast

cancer cell line.(39) In the later study,(49) however, the erbB2

transfectants that showed no Taxol resistance produced less

than one-third of the level of ErbB2 protein produced by

SKBR3 cells. On the basis of the data from these two

independent studies, we suggest that erbB2 overexpression

must reach a threshold level in breast cancer cells before

they become resistant to Taxol. This would provide a

reasonable explanation for the discrepancy between the

two studies. It is further plausible that not all of the erbB2-

overexpressing breast cancers in patients enrolled in the

Herceptin trial expressed erbB2 at levels sufficient to confer

Taxol resistance (i.e. above threshold levels). Therefore, by

downregulating erbB2, Herceptin may only increase Taxol

sensitivity in patients whose breast tumors overexpress

erbB2 at above-threshold levels.

Molecular mechanisms of resistance

Partly because of the controversy just discussed and partly

because of the complexity of the problem, our understanding

of the molecular mechanisms of erbB2-mediated chemore-

sistance is limited. It is generally believed that breast cancer

cells overexpressing erbB2 are intrinsically resistant to DNA-

damaging agents such as cisplatin as the result of an altered

cell-cycle checkpoint, altered DNA repair mechanisms and

altered apoptosis responses.(44,50)

In elucidating the molecular mechanisms underlying

erbB2-mediated Taxol resistance in erbB2-transfected

Challenges

BioEssays 22.7 675

MDA-MB-435 breast cancer cells (435.eB cells), our data

has led to a model for Taxol-induced apoptosis and ErbB2-

mediated antiapoptosis in breast cancer cells (Fig. 1). This

model shows Taxol activating p34Cdc2 kinase in MDA-MB-

435 breast cancer cells, which is needed to induce

apoptosis. On the other hand, overexpression of erbB2 in

435.eB cells upregulates the cyclin-dependent kinase (cdk)

inhibitor p21Cip1, which in turn inhibits the Taxol-mediated

activation of p34Cdc2 kinase and thus impedes Taxol-induced

apoptosis.(51) This model synthesizes the critical role of the

G2-M cdk p34Cdc2, the newly recognized function of p21Cip1

as a mammalian (human breast cancer) cell G2-M cdk

inhibitor, and the effects of the overproduction of the receptor

tyrosine kinase ErbB2. It also provides a molecular basis for

Taxol-induced apoptosis and ErbB2-mediated antiapoptosis,

and for the Taxol resistance seen in breast cancers that

overexpress erbB2.

We believe that there may be other molecular mechan-

isms underlying erbB2-mediated Taxol resistance that

deserve further investigation. Moreover, the molecular

mechanisms for erbB2-mediated resistance to different

drugs could be divergent and complex. It is clear that more

questions remain to be answered.

Possible ways to overcome

ErbB2-mediated chemoresistance

The development of new therapies that overcome the

intrinsic chemoresistance in erbB2-overexpressing breast

cancers remains a high priority. Our knowledge of molecular

biology and oncology will lead the way to the development

of new strategies that not only inhibit further progression

of erbB2-overexpressing breast cancers but perhaps even

circumvent the mechanisms of chemoresistance. In the past

ten years, several exciting strides have been made in this

area (Table 1) and these are discussed below.

Reversal of ErbB2-mediated Taxolresistance by HerceptinHerceptin is a humanized antibody that binds to the

extracellular domain of ErbB2 and can downregulate the

expression of cell-surface erbB2 protein.(52) Herceptin has

demonstrated tumor inhibitory and chemosensitizing effects

for Taxol and several chemotherapeutic agents in pre-

clinical studies and in Phase II and Phase III clinical

trials.(21,22,40,44,53) These results represent an excellent

example of antireceptor antibody-mediated chemosensitiza-

tion. One of the remaining questions, however, is this: how

does Herceptin sensitize erbB2-overexpressing breast

Figure 1. A model for the induction of apoptosis in breast

cancer cells by Taxol and for the inhibition of Taxol-inducedapoptosis by ErbB2 overproduction. Taxol induces apoptosis

in MDA-MB-435 breast cancer cells by activating p34Cdc2

kinase. By contrast, overproduction of ErbB2 in MDA-MB-435

breast cancer cells transfected with erbB2 upregulatesp21Cip1, which in turn inhibits Taxol-mediated activation of

p34Cdc2 kinase and thereby impedes Taxol-induced apoptosis.

Table 1. Possible ways to overcome ErbB2-mediated chemoresistance

ErbB2-targeting agents ErbB2-targeting mechanisms Chemosensitization efficacy

Anti-ErbB2 receptor antibody Binds to the extracellular domain of ErbB2 and Demonstrated in animal and in Phase II and III clinical

(e.g. Herceptin1) downregulates the cell surface ErbB2 trials

Adenovirus type 5 E1A proteins Transcriptionally repress erbB2 gene expression Demonstrated in animal. Will be tested in phase II

clinical trials

ErbB2-specific tyrosine kinase inhibitors Inhibits the tyrosine kinase activities of the ErbB2 Demonstrated in laboratory studies

(e.g. emodin) receptor tyrosine kinase

Anti-ErbB2 intracellular single-chain Eradicate ErbB2 from the cell surface membrane Demonstrated in laboratory studies

antibodies by ectopic localization of the ErbB2 protein to the

endoplasmic reticulum

Retinoic acid Downregulation of erbB2 mRNA and protein Demonstrated in laboratory studies

expression

erbB2 antisense oligonucleotides and Block erbB2 transcription To be tested

antisense RNA

Challenges

676 BioEssays 22.7

cancer cells to Taxol? Based on our understanding of the

mechanisms of erbB2-mediated Taxol resistance, we sug-

gest that Herceptin sensitizes erbB2-overexpressing breast

cancer cells to Taxol-induced apoptosis by repressing

erbB2-mediated upregulation of p21Cip1, allowing the effi-

cient activation of p34Cdc2 kinase by Taxol and hence

apoptosis. We are currently testing this hypothesis both in

vitro and in vivo.

Sensitization of erbB2-overexpressing humanbreast cancer cells to Taxol by adenovirustype 5 E1A proteinThe E1A proteins of adenovirus type 5 have been shown to

repress erbB2 expression at the transcriptional level and to

inhibit erbB2-overexpressing tumor growth and dissemina-

tion in animal models.(23,24,54,55) Later, an adenoviral vector

was used to transfer the E1A gene into two human breast

cancer cell lines that overexpress erbB2 (MDA-MB-453 and

MDA-MB-361). After E1A delivery, erbB2 expression levels

were reduced and cells were treated with Taxol. Cell

proliferation assays and soft-agar colony-formation assays

showed that the combination of E1A and Taxol has a

synergistic growth-inhibitory effect on the cancer cells.(56)

The data revealed by these studies indicated that E1A can

sensitize erbB2-overexpressing breast cancer cells to Taxol

through E1A-mediated erbB2 repression. This finding has

important clinical implications for the development of a novel

therapeutic strategy that combines Taxol chemotherapy and

E1A gene therapy for erbB2-overexpressing breast cancers.

Currently, a Phase II clinical trial has been approved to

determine the therapeutic efficacy of combining Taxol and

E1A for erbB2-overexpressing breast cancers. We have also

successfully generated several E1A mutants that can inhibit

erbB2 expression and transformation.(57) A particular ad-

vantage of these E1A mutants is that they do not contain the

CR2 domain, which is crucial for binding the RB protein and

for cell immortalization, and hence may be more selective

tumor suppressors and safer therapeutic agents for E1A

gene therapy either alone or in combination with Taxol.

erbB2-specific tyrosine-kinase inhibitorsas chemosensitization agentsOverexpression of the ErbB2 receptor can lead to the

increased tyrosine-kinase activity that is critical for the

activation of its signal-transduction pathways and subse-

quent biological functions, including chemoresistance.

Therefore, another way to block the erbB2-mediated

chemoresistance is to develop inhibitors of the ErbB2

tyrosine-kinase activity. In an effort to accomplish this, our

group found that emodin (3-methyl-1,6,8-trihydroxyanthra-

quinone), a tyrosine-kinase inhibitor, suppresses the autop-

hosphorylation and transphosphorylation activities of ErbB2

tyrosine-kinase. This results in reduced tyrosine hypopho-

sphorylation of ErbB2 in erbB2-overexpressing breast cancer

cells.(29) Emodin also sensitized erbB2-overexpressing

breast cancer cells to Taxol through the specific inhibition

of ErbB2 tyrosine-kinase activity.(58) Similarly, curcumin, a

natural compound present in turmeric, reduced the tyrosine-

kinase activity of ErbB2 by depleting ErbB2 and also inhibited

growth of erbB2-overexpressing breast cancer cell lines.(59)

These promising laboratory findings call for clinical validation

of the efficacy of tyrosine-kinase inhibitors in the treatment

of erbB2-overexpressing breast cancers, either alone or in

combination with other chemotherapeutic agents.

The potential of anti-ErbB2 intracellularsingle-chain antibodies for chemosensitizationSeveral remarkable efforts have been made to downregulate

the expression of the erbB2 receptor or to eradicate it from

the cell surface membrane using gene constructs encoding

anti-ErbB2 intracellular single-chain antibodies.(60±62) In one

approach, recombinant adenoviruses encoding the endo-

plasmic-reticulum-directed anti-ErbB2 intracellular single-

chain antibodies (sFv) were markedly cytotoxic (60±95% of

cells) in erbB2-overexpressing human breast cancer cells.(63)

These sFv also induced apoptosis in erbB2-overexpressing

cells that was secondary to the intracellular-antibody-

mediated ectopic localization of the ErbB2 protein.(28) In

addition, anti-ErbB2 single-chain antibodies sensitized

erbB2-overexpressing breast cancer cells to cisplatin.(62) It

will be intriguing to find out whether anti-ErbB2 single-chain

antibodies produce such an effect in erbB2-overexpressing

and chemoresistant tumors in patients through the synergis-

tic induction of apoptosis after DNA damage produced

by chemotherapeutic agents. Such a clinical study will

provide critical data on the efficacy of using the novel gene

therapy approach to introduce anti-ErbB2 single-chain

antibodies into tumor cells to overcome erbB2-mediated

chemoresistance.

Chemosensitizing effects of retinoic acid inerbB2-overexpressing breast cancer cellsRetinoic acid has shown preventive and even therapeutic

effects in several laboratory and clinical studies. It also

induces apoptosis in various cancer types. Interestingly, all-

trans retinoic acid (ATRA) and fenretinide (4-HPR) have

been shown to downregulate ErbB2 protein and mRNA in

erbB2-overexpressing SK-BR-3 and BT-474 breast cancer

cells as well as in MCF-7 cells which produce lower levels of

ErbB2. Moreover, 4-HPR plus cisplatin has been shown to be

more effective in inhibiting the growth of erbB2-overexpres-

sing cells than either agent alone.(64) On the basis of these

collective findings, combination therapy consisting of retinoic

acid and chemotherapeutic agents offers an attractive

approach for the treatment of erbB2-overexpressing breast

cancers.

Challenges

BioEssays 22.7 677

Several of the above approaches have already shown

promise in overcoming erbB2-mediated chemoresistance in

either laboratory or clinical studies. Other novel approaches

are also under development. For example, antisense

oligonucleotides and antisense RNAs, which induce apopto-

sis after DNA damage by lowering erbB2 expression, may

also be explored for their ability to chemosensitize erbB2-

overexpressing breast cancer cells.(65) Additionally, attempts

are being made to modulate existing chemotherapeutic

agents so that they can overcome resistance.(30) These

combined efforts will spawn improved therapies for patients

with erbB2-overexpressing breast tumors.

Several critical issues for

future investigations

The role of ErbB2 in chemoresistance is very complex, as it

may be cell, tumor, drug, timing, regimen, population, or age

specific. To understand more comprehensively the role of

ErbB2 in breast cancer chemosensitivity and thereby

develop more-effective treatments for patients with erbB2-

overexpressing tumors, scientists and physicians need to

team up to explore the following, and other, related issues.

Role of erbB2 in resistance toother chemotherapeutic agentsCurrently available data from most of the studies that have

been performed indicate that erbB2 renders breast cancers

resistant to DNA-damaging agents such as cisplatin(50) and

the microtubule-stabilizing agents Taxol and Taxotere.(38,39)

As part of the effort to understand better the role of erbB2 in

chemosensitivity, it is imperative to determine whether erbB2

overexpression also renders breast cancers resistant to

other chemotherapeutic agents frequently used to treat such

patients. This should be investigated (a) in carefully designed

laboratory studies using erbB2-overexpressing breast cancer

cell lines that express erbB2 at levels similar to those seen in

primary breast tumors and (b) in well-controlled and defined

large clinical trials that can adequately assess the impact of

the various factors important for a patient's chemoresponse.

Other possible mechanisms forerbB2-mediated chemoresistanceNumerous mechanisms of drug resistance in various tumor

cells are well established including enhanced drug metabo-

lism, altered drug accumulation, drug target amplification,

repair of damaged targets and apoptosis resistance.(66) In

breast cancer cells, we know that the overexpression of

erbB2 can block Taxol-induced apoptosis by upregulating

p21Cip1, which in turn inhibits p34Cdc2 kinase which is

required for apoptosis. It has also been reported that drug-

induced apoptosis depends on a balance between cell-cycle

checkpoints and DNA-repair mechanisms. In one study, for

example, blockade of erbB2 signaling using ErbB2 antago-

nists, dominant negative mutants, or chemical inhibitors of

ErbB2 tyrosine kinase induced cell-cycle arrest, inhibited

DNA repair, and promoted apoptosis, and thereby sensitized

erbB2-overexpressing cells to drug killing.(50) Signaling of the

ErbB2-receptor tyrosine kinase might thus play a critical role

in ErbB2-mediated chemoresistance. However, the down-

stream signal-transduction cascades by which ErbB2 affects

chemosensitivity are much less well understood. Most

ErbB2-overproducing breast cancers also express other

ErbB receptors, which can form heterodimers with ErbB2.

An interconnected network of ErbB signaling pathways may

thus be activated that determines tumor cell response to

chemotherapy stress. It is therefore important to perform in-

depth investigations into the role of ErbB2-receptor signaling

in the regulation of stress-responsive genes. In addition,

better understanding of other possible mechanisms under-

lying ErbB2-mediated chemoresistance is also critical to the

development of better alternative therapeutic strategies to

overcome intrinsic chemoresistance.

Tailoring therapeutic strategies toindividual patientsAs we try to understand more completely the molecular

biology of erbB2-overexpressing breast cancers and to

assess more accurately the sensitivity of individual patient

tumors to particular therapies, it is important for future

research efforts to focus on identifying other genetic

alterations that interact with erbB2 and contribute to ErbB2-

mediated chemoresistance. This information can then be

used to develop therapeutic strategies whose designs are

based on the biology of erbB2-overexpressing cancers and

any other genetic alterations. For example, activation of

ErbB2 or one of the other members of the ErbB receptor

family by their cognate ligand might increase sensitivity to

doxorubicin by upregulating topo isomerase II a.(67) This

observation suggested the use of receptor+ligand expres-

sion to direct the planning of patient-specific therapy (e.g.

doxorubicin versus alkylator-based or Taxol-based regi-

mens) and also the use of biological agents (e.g. anti-ErbB2

antibodies) in combination with certain chemotherapeutic

agents (e.g. Taxol) to enhance the response to treatment in

breast cancer patients. In addition, as erbB2 overexpression

has been linked to resistance to particular chemotherapeutic

agents and the ErbB2 receptor can be targeted by biological

therapies (Table 1), clinicians will have options that will help

them to tailor therapeutic strategies to maximize the thera-

peutic benefit for individual patients.

Conclusions

Chemotherapeutic drug resistance is a major clinical

problem and a source of treatment failure in cancer patients.

An increased understanding of the role of erbB2 in

modulating chemosensitivity in breast cancers and of the

Challenges

678 BioEssays 22.7

underlying molecular mechanisms will have a tremendous

impact on the development of breast cancer therapies. We

have provided an overview of various aspects of the role of

erbB2 in breast cancer chemosensitivity and the controver-

sies surrounding the issue, together with explanations that

might resolve the debate. We have also touched on the

chemosensitization of erbB2-overexpressing breast cancers

and future directions of research that may lead to the

development of more effective therapies for erbB2-over-

expressing breast cancers. We believe that the persistent

study of the biological bases of ErbB2-modulated chemo-

sensitivity will bring unprecedented clinical benefits to

patients with erbB2-overexpressing breast cancers.

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