CancerCancer

The proliferation, differentiation, and survival of individual cells in multicellular organisms are carefully regulated.

This regulation is lost in cancer cells, which grow and divide in an uncontrolled manner, ultimately spreading throughout the body and interfering with the functions of normal tissues and organsand interfering with the functions of normal tissues and organs

Types of CancerTumor – any abnormal proliferation of cellsTumor any abnormal proliferation of cellsA benign tumor – a tumor that remains confined to its original location, neither invatingsurrounding normal tissue nor spreading to distant body sitesA malignant tumor (=cancer) – invades surrounding normal tissue and spreads throughout the body via circulatory or lymphatic system (metastasis)body via circulatory or lymphatic system (metastasis)Classification according to the types of cell from which they ariseCarcinoma – malignancies of epithelial cells (include ~90% of human cancers)Sarcoma – solid tumors of connective tissues such as muscle, and, cartilage and fibrous tissueLymphomas – arise from the blood-forming cells and from cells of the immune systemy p o as a se o t e b ood o g ce s a d o ce s o t e u e syste

The development of Cancer

The development of cancer is a multistep process in which cells gradually become malignant through a progressive series of alterations A dramatic increase of cancer incidence with age suggests that most cancers develop as a consequence of multiple abnormalities which accumulate over periods of many years

Tumor initiation – a genetic alteration leading to abnormal proliferation of a single cellCell proliferation leads to the outgrowth of a population of clonally derived tumor cells

Tumor progression – additional mutation occur within cells of the tumor populationTumor progression additional mutation occur within cells of the tumor populationsome of these mutations confer a selective advantage to the cell, such as more rapid growth

Causes of Cancer

Carcinogen – substances that cause cancer

1. Damage of DNA, which induces mutationssolar ultraviolet radiation – the major cause of skin cancercarcinogenic chemicals in tobacco smoke (benzopyrene, dimethylnitrosamine, nickel compounds)- the undisputed cause of 80-90% of lung cancers- the oral cavity, pharynx, larynx, esophagus and other sites

aflatoxin

2. Abnormally stimulating cell proliferation (these types of carcinogens are also referred to tumor promoters)Hormones (especially estrogen)- endometrial cancer (자궁내막암): long-term post-menopausal estrogen replacement therapy with high dosesof estrogen substantially increase the

k f d lrisk of endometrial cancer

3. Virushepatitis B & C virus – liver cancerh ill i i lhuman papillomavirus – cervical cancer

Properties of cancer cells1 Density dependent inhibition of cell proliferation1. Density-dependent inhibition of cell proliferation2. Many cancers cells have reduced requirements

for extracellular growth factors- autocrine growth stimulationabnormalities in intracellular signaling system- abnormalities in intracellular signaling system(unregulated activity of growth factor receptorsor other signaling proteins e.g. Ras, other proteinkinases)

3. Cancer cells are less stringently regulated than normalcells by cell-cell or cell-matrix interactions- for example, loss of E-cadherin is important in thedevelopment of carcinomas

- contact inhibition4 I i i d t t i ll t4. In invasion and metastasis, cancer cells secrete

collagenase in order to penetrate through basal laminae and invade underlying connective tissue

5. Cancer cells secrete growth factors that promote the fo mation of ne blood essels (angiogenesis)the formation of new blood vessels (angiogenesis)

6. Most cancer cells fail to differentiateleukemia provide a particularly good example.- the differentiation of leukemic cells is blocked at early stages of maturation consistent with their continued proliferationof maturation, consistent with their continued proliferation

7. Many cancer cells fail to undergo apoptosis and therefore exhibit extended life spans

- The survival of normal cells are dependent on growth factors or interactions with ECMor interactions with ECMNormal cells undergo apoptosis following DNA damageBut, cancer cells lose these properties, which plays a major role in the unrelenting growth of cance cells in an animal

Tumor viruses

The chronic infections by hepatitis virus B and C are associated with a high risk of liver cancer, which eventually develops in 10-20% of people chronically infected with hepatitis B and 5% of y p p p y ppeople with hepatitis C.

SV40 T antigen protein binds to and inactivates the host cell tumor suppressor proteins Rb and p53, which are key regulators of cell proliferation and survivalp y g p

Two of Papillomavirus genes (E6 and E7) and two of adenovirus gene (E1A and E1B) transform cells. Like SV40 T antigen, these proteins induce transformation by interacting with and thus inhibiting Rb and p53. g p

OncogenesOncogenes

Cancer results from alterations in critical regulatory genes that control cellg y g

proliferation, differentiation, and survival. Studies of tumor viruses revealed that

specific genes (called oncogenes) are capable of inducing cell transformation,

thereby providing the first insights into the molecular basis of cancer.

However, the majority (~80%) of human cancers are not induced by viruses and

apparently arise from other causes such as radiation and chemical carcinogenapparently arise from other causes, such as radiation and chemical carcinogen.

Therefore, in terms of our overall understanding of cancer, it has been critically

important that studies of viral oncogenes also lead to the identification of cellular g

oncogenes, which are involved in the development of non-virus-induced cancers.

Retroviral oncogene

The genome of some viruses contain a oncogene

that are responsible for host cell transformation,

irrespective of viral genome replication and viral p g p

particle production

- Src oncogene from RSV was first protein-

t i ki t b id tifi dtyrosine kinase to be identified

Proto-oncogenes

The normal cell genes from which the retroviral oncogenes originated are called proto oncogenesThe normal cell genes from which the retroviral oncogenes originated are called proto-oncogenesProto-oncogenes are important cell regulatory genes, in many cases encoding proteins that function in the signal pathways controlling cell proliferation- Viral oncogenes originated from host proto-oncogenes through a virus-host recombination event

Oncogenes / proto oncogenes- Oncogenes / proto-oncogenes1. the viral oncogene is transcribed under the control of viral promoters

abnormally high expression or expression in inappropriate cell types2. the viral oncogene is frequently encode proteins that differ in structure and function from normal

homologshomologs The viral Raf: its N-terminal regulatory domain is deleted, instead replaced by viral Gag gene

As a result, the Raf kinase in the C-terminus is constitutively active, causing cell transformation

Viral Ras is point muatated (amino acid substitution)Viral Ras is point-muatated (amino acid substitution)

Oncogenes in human cancers

Viral oncogenes induce cancers. Then, non-virus-induced cancer?Cellular oncogenes are generated by mutations or DNA rearrangements of proto-oncogenes

during tumor development

◆ Ras oncogenesRas oncogenes generated from their

proto-oncogenes by point mutations resulting in single amino acidresulting in single amino acid substitutions at critical points These mutations maintain the Ras proteins constitutively in the active GTP-bound yconformation

In large part, this effect is a result of nullifying the response of oncogenic Ras to GAP (GTPase-activating protein)

Three closely related members of the ras oncogene family (rasH rasK rasN)ras oncogene family (rasH, rasK, rasN) encountered in human tumors

20 % of all human malignancies, about 50% of colon and 25% of lung gcarcinomas !!

◆ Oncogenesis by abnormalities in chromosome structure including translocation, duplication, deletionsc-myc (c-myc gene encodes a transcription factor normally induced in response to growth factor stimulation)- human Burkitt’s lymphomas (malignancies in antibody-producing B cells)- c-myc proto-oncogene is translocated from chromosome 8 to the immunoglubulin heavy-chain

locus on chromosome 14, resulting in abnormal c-myc expressionabl (a protein-tyrosine kinase)Translocation leads to abl-bcr fusion, in which abl protein kinase activity is unregulated

N-myc (a homolog of c-myc), erbB-2 (a receptor protein-tyrosine kinase)n-myc gene is amplified into multicopies in neuroblastomaAmplification of erbB-2 is related to progression of breast and ovarian carcinomas

c-mycabl

Functions of oncogene products

Oncogenes involved in ERK

PDGF is converted to an oncogene in some human leukemia- PDGF is fused to Tel in its amino-terminus by a chromosome translocation, leading to constitutive dimerization

signaling pathway

ERK pathway ultimately leads to the phosphorylation of transcription factors and alterations in gene expessionMany oncogenes encode transcription regulatory proteins that are normally y g p g y p yinduced in response to growth factor stimulationFos and Jun are proto-oncogenes which activate transcription of a variety of genes including cyclin D1

The gene encoding cyclin D1 is a proto-oncogene which can be activated as aThe gene encoding cyclin D1 is a proto-oncogene, which can be activated as a oncogene (called CCND1) by chromosomal translocation or gene amplification

Wnt and β catenin genes can act as oncogenesWnt and β-catenin genes can act as oncogenes

- β-catenin in Wnt pathway is converted to an oncogene (CTNNB1)- β-catenin is mutated thus stabilized, leading to an abnormal stimulation of gene transcription including c myc or cyclin D1stimulation of gene transcription including c-myc or cyclin D1

- The abnormality of Wnt pathway is frequently found in colon cancers

Tumor suppressors● Tumor suppressors represent the opposite side of cell growth control, normally acting to inhibit cell● Tumor suppressors represent the opposite side of cell growth control, normally acting to inhibit cell proliferation and tumor development● In many tumors, these genes are lost or inactivated

Retinoblastoma (Rb)Retinoblastoma (Rb)- This gene is first identified to be lost in retinoblastoma, a rare childhood eye tumor- Development of retinoblastoma requires loss of both of the functional copies of Rb genes on homologous chromosomes of a normal diploid gene

- The deletion of Rb alleles are frequently found in retinoblastomae de et o o b a e es a e eque t y ou d et ob asto a- It has been established that Rb is lost or inactivated in a variety of tumors - Rb is a key target for the oncogenes of DNA tumor viruses, which inhibits its activity

Wnt signaling

DNA repairDNA repair

Inhibits cell cycle progression

Inhibits cell cycle progression

apoptosis

TGF- signaling

Functions of Tumor suppressor gene products

PTEN is a lipid phosphatase that dephosphorylates the 3

position of phosphatidylinositides such as PIP3

antagonization of PI 3 kinase and Akt (both can act as

oncogenes by promoting cell survival

Smoothened is an oncogene in basal cell carcinomas,

whereas Patched (the negative regulator of Smoothened)

is a tumor suppressor

WT1 is frequently inactivated in Wilms’ tumors (a

childhood kidney tumor). The WT1 protein is a repressor

that appears to suppress transcription of a number of

growth factor-inducible genes – one of the targets of WT1

is the gene that encoding insulin-like growth factor II

(which is overexpressed in Wilms’ tumors and may act as

i h fan autocrine growth factor.

Two other tumor suppressor genes, Smad2 and Smad4,

d t i ti f t th t ti t d b TGFencode transcription factors that are activated by TGF-.

TGF receptor gene is also a tumor suppressor gene.

The products of the Rb and INK4 tumor suppressor genes regulate cell cycle progression at the same point as that affected by cyclin D1 and Cdk4 (both of which can act asaffected by cyclin D1 and Cdk4 (both of which can act as oncogenes)

The p53 gene product regulates both cell cycle progression and apoptosis DNA damage leads to rapid induction of p53and apoptosis. DNA damage leads to rapid induction of p53, which activates transcription of both proapoptotic and cell cycle inhibitory genesCells lacking p53 fail to undergo apoptosis in response to agents that damage DNA (including radiation and many ofagents that damage DNA (including radiation and many of the drugs used in cancer chemotherapy), growth factor deprivation, or oxygen deprivation These effects of p53 on cell survival are thought to account for the high frequency of p53 mutations in human cancersp53 mutations in human cancersLoss of p53 prevents this damage-induced cell cycle arrest, leading to increased mutation frequencies and a general instability of the cell genome Such genetic instability if a common property of cancer cells and contributes to furthercommon property of cancer cells and contributes to further alteration in oncogenes and tumor suppressor genes during tumor progression

The products of the BRCA1 and 2 genes are involved inThe products of the BRCA1 and 2 genes are involved in checkpoint control of cell cycle progression and repair of breaks in DNA The mutations in genes in this type lead to a high frequency of mutations (stability genes)ATM gene the mismatch repair genes and the nucleotideATM gene, the mismatch repair genes, and the nucleotide excision repair genes are also stability genes

Roles of oncogenes and tumor suppressor genes in tumor development

The role of multiple genetic defects in the case of colon carcinoma

1 Early adenoma an early event in tumor development1. Early adenoma - an early event in tumor development

Inactivation of APC (Wnt signaling) results in abnormal colon cell

proliferation (mutations in patients with familial adenomatous

polyposis or with noninherited colon carcinomas)polyposis or with noninherited colon carcinomas)

In some cases, activation of Wnt signaling results from mutations

in the gene encoding β-catenin

Wnt signaling is a major pathway responsible for proliferationWnt signaling is a major pathway responsible for proliferation

of colon epithelial stem cells

2. Late adenoma

Activation of rasK or B-raf oncogene leads to stimulation of ERK g

signaling

Mutations affecting TGF-β signaling occur relatively early in the

development of colon cancersp

- Tβ RII (TGF receptor gene), smad2 or Smad4

3. Carcinoma

inactivation of p53 tumor suppressor gene

Molecular approaches to cancer treatment

Prevention and early detection

- Early premalignant stages of colon cancer (adenoma) are completely curable

- The cure rate for early carcinomas remaining localized to their site of origin

~90%

- Survival rate drop to about 50% for patients whose cancers have spread to

adjacent tissues and lymph nodes

- Metastatic colon cancer less than 10%

Detection of inherited cancer susceptibilities by identification of mutations

in tumor suppressor genes, oncogenes, or stability genes

F i t th t i h it d tibilit iFor instance, the most common inherited cancer susceptibility is

hereditary nonpolyposis colon cancers (15% of colon cancer), BRCA1 and

BRCA2 tumor suppressor genes (5% of all breast cancers)

Individuals with such inherited susceptibility genes could be appropriately

advised to avoid exposure to relevant carcinogens and carefully monitored

to detect tumors at early stages that are more readily treatedto detect tumors at early stages that are more readily treated.

Molecular diagnosis and treatmentThe most critical question is whether the discovery of oncogenes and tumor suppressor gene willThe most critical question is whether the discovery of oncogenes and tumor suppressor gene will allow the development of new drugs that act selectively against cancer cells?

기존 항암제의 부작용 – 1) most of the drugs currently used in cancer treatment either damage DNA or inhibit DNA replication thus these are toxic to particularly rapidly growing normal cellsor inhibit DNA replication thus these are toxic to particularly rapidly growing normal cells (hematopoietic stem cells, epithelial cells or gastrointestinal tract, hair follicle cells)2) proto-oncogene products in normal cells

해 결 책해 결 책- drugs that inhibits tumor growth by interfering with angiogenesis (blood vessel formation)

tumor cells secrete a number of growth factors including VEGF for growth of new blood vessels into the tumors angiogenesis inhibitor, a monoclonal antibody against VEGF was approved- selective oncogene targeted drug or therapyg g g py문제점 – proto-oncogene products in normal cells예) PML/RARα in acute promyelocytic leukemia – high doses of retinoic acid combined with

standard chemotherapy

Sequestering VEGF (vascular endothelial growth factor) With Novel Decoy Receptors

1. Anti-VEGF antibody sequesters VEGF, preventing it from binding to VEGF receptor

A modified strategy for sequestering VEGFTrap fusion proteins (solubilized VEGF-R),Extracellular domains of VEGF receptor is

linked to a portion of human immunoglobulin (which increases the stability of the solubilizedVEGF-R)

3. Inhibitors against the protein kinase domain

2. Anti-VEGFR antibody binds to VEGF receptor, preventing it from binding VEGF

Herceptin, a monoclonal antibody against the ErbB-2 oncogene proteinThe ErbB-2 protein is overexpressed in about 30% of breast cancers as a result of amplification of

the genethe geneThe monoclonal antibody herceptin binds to the extracellular domain of ErbB-2 thus inhibits the

growth stimulating signaling pathway from the ebB-2 reduce tumor growth and prolong patient survival for metastatic breast cancersErbitux, a monoclonal antibody against the EGF receptor (the erbB oncogene product) for the Erbitux, a monoclonal antibody against the EGF receptor (the erbB oncogene product) for the treatment of advanced colorectal cancer

Small molecule inhibitors- a potent and specific inhibitor of the Bcr/Abl protein kinase, STI-571 (Gleevec)p p / p , ( )

this compound effectively blocks proliferation of chronic myeloid leukemia cellsSTI-571 also a potent inhibitor of the PDGF receptor and Kit protein-tyrosine kinaseeffective therapy for tumors in which the genes encoding protein kinases are mutationally

activated Kit is activated as an oncogene by point mutations resulting in constitutive protein kinase

activity in approximately 90% of gastrointestinal stromal tumorsPDGF receptor is also mutationally activated in many of the gastrointestinal stromal tumors

- A small molecule inhibitor of the EGF receptor (Gefitinib)p ( )EGF receptors are overexpressed in most lung cancersGefitinib is dramatically effective to a subset of lung cancers in which point mutations resulted

in constitutive activation of the EGF receptor tyrosine kinase

an activated oncogene becomes a major driving force in the tumor cell, such that other signaling pathways in the tumor cell become secondary in importance – the proliferation and survival of a tumor cell may become dependent on continuing activity of the oncogene, whereas normal cells have alternative signaling pathways that can compensate if any one pathway is blocked

A wide variety of drugs targeted against both oncogene proteins (B-raf, PI-3kinase, and Akt) and downstream components of oncogenic signaling pathways (MEK and mTOR) are being tested

Th ti it f t i ki h h bi di it f ATP Th ti ti it

Imatinib [STI-571 (Gleevec)]

The active sites of tyrosine kinases each have a binding site for ATP. The enzymatic activity catalyzed by a tyrosine kinase is the transfer of the terminal phosphate from ATP to tyrosineresidues on its substrates, a process known as protein tyrosine phosphorylation. Imatinib works by binding to the ATP binding site of bcr-abl and inhibiting the enzyme activity of the prt i titi lotein competitively.

Imatinib is quite selective for bcr-abl – it does also inhibit other targets mentioned above (c-kit and PDGF-R), but no other known tyrosine kinases. Imatinib also inhibits the abl protein of non-cancer cells but cells normally have additional redundant tyrosine kinases which allo

th t ti t f ti if bl t i ki i i hibit dw them to continue to function even if abl tyrosine kinase is inhibited.

Gefitinib inhibits EGFR tyrosine kinase by binding to the adenosine triphosphate (ATP)-binding site of the enzyme. Thus the function oftriphosphate (ATP) binding site of the enzyme. Thus the function of the EGFR tyrosine kinase in activating the Ras signal transduction cascade is inhibited, and malignant cells are inhibited.

Herceptin a monoclonal antibody against the ErbB 2 oncogene proteinHerceptin, a monoclonal antibody against the ErbB-2 oncogene proteinThe ErbB-2 protein is overexpressed in about 30% of breast cancers as a result of amplification of the

geneThe monoclonal antibody binds to the extracellular domain of ErbB-2 thus inhibits the growth

stimulating signaling pathway from the ebB 2 reduce tumor growth and prolong patient survival forstimulating signaling pathway from the ebB-2 reduce tumor growth and prolong patient survival for metastatic breast cancers

Erbitux, a monoclonal antibody against the EGF receptor (the erbB oncogene product) for the treatment of advanced colorectal cancertreatment of advanced colorectal cancer