วั�ตถุ�ประสงค์� สามารถุอธิ�บายสามารถุอธิ�บาย multi-step tumorigenesis multi-step tumorigenesis ได้�ได้� สามารถุอธิ�บายบทบาทของ สามารถุอธิ�บายบทบาทของ tumor promoter tumor promoter ในการเร��มต�นการในการเร��มต�นการ

เก�ด้มะเร�งเก�ด้มะเร�ง

9. การเร��มต�นการเก�ด้มะเร�ง(Carcinogenesis)

เน !อหา Multi-step tumor formationMulti-step tumor formation Cancer development and Darwinian evolutionCancer development and Darwinian evolution NonmutagenicNonmutagenic agents and tumorigenesisagents and tumorigenesis Chronic Inflammation and tumorigenesisChronic Inflammation and tumorigenesis

9. 9. CarcinogenesisCarcinogenesis

Carcinogenesis is the process by which normal cells are transformed into cancer cells.

9.1 Most human cancers develop over many decades of time

The formation of tumor is a complex process that usually proceeds over a period of decades.

9.2 Histopathology provides evidence of multi-step tumor formation most clearly in the epithelia of the intestine ( the intestinal epithelial cells form a layer that is only one cell thick in many places).

normal colonic crypts (20x)

small tubular adenoma (4x)

large tubular adenoma (1x)

Stalk attachingHead of polypTo wall of colon

head

same tubular adenoma (20x)

liver metastases (4x)

invasive carcinoma (20x)

villous adenoma (4x)

early adenomatous crypt (20x)

carcinoma grows directly from an adenomatous polyp.

Multi-step tumorigenesis (carcinogenesis) in a variety of organ sites.

CIS = Carcinoma in situCIN = Cervical intraepithelial neoplasia

DCIS = Ductal Carcinoma in situPIN = Prostatic intraepithelial neoplasia

9.3 Colonic growths accumulate genetic alterations as tumor progression proceeds

Many of the steps of tumor progression are driven by genetic alterations accumulated in the genomes of developing tumor cells.

Loss of APC function (or the functionally equivalent gain of -catenin function) represents a starting point that is common to almost all human colon carcinomas.

Normalepithelium

APC

TSG = tumor suppressor gene

9.4 Cancer development seems to follow the rules of Darwinian evolution

FIRST CLONAL EXPANSION

SECOND CLONAL EXPANSION

THIRD CLONAL EXPANSION

FOURTH CLONAL EXPANSION

Darwinian evolution involves expansion of organisms that are endowed with advantageous genotypes and thus phenotypes; a similar scheme seems to describe how tumor progression occurs.

9.5 Tumor stem cells further complicate the Darwinian model of clonal succession and tumor progression

“Progenitor”

9.6 A linear path of clonal succession oversimplifies the reality of cancer

The rate of generation of new mutant alleles may exceed the rate at which Darwinian selection eliminates less-fit clones.----> the tumor mass becomes composed of an increasing number of distinct subclones.----> the genotypes of tumors of the same type that arise in different patients are likely to be markedly different from on another.

9.7 The Darwinian model of tumor development is difficult to validate experimentally

Analyses of the genomes of cells at different states of tumor progression are unlikely to converge on the critical genetic changes that are responsible for many clonal successions.----> inactivation of tumor suppressor gene by the epigenetic process of gene silencing via promoter methylation is difficult to identified.----> the kinetics of each step of the multi-step progression are extremely difficult to measure.9.8 Multiple lines of evidence reveal that normal cells are resistant to transformation by a single mutated gene Single mutations are necessary but not sufficient for the development of cancers.----> primary cells transfected with ras oncogene.

----> multiple changes seem to be required in order for cell to reach a tumorigenic state.

9.9 Transformation usually requires collaboration between two or more mutant genes ras-like oncogenes encode Ras-like oncoproteins that are components of the cytoplasmic mitogenic signaling cascade. myc-like oncogenes encode Myc-like oncoproteins that perturb in various ways the cell cycle control machinery, which operates in the nucleus.

ras + SV40 large Tras + E1AerbB + erbATGF- + myc

V-sea + v-ski

Bcl-2 + myc

ras + myc

raf + myc

src + myc

ras was able to elicit anchorage independence, a rounded, refractile appearance in the phase microscope, and loss of contact inhibition. myc helped the cells to become immortalized and reduced somewhat their dependence on growth factors.

Cell proliferation and cell survival are governed by a number (two or more) of distinct regulatory circuits, all of which must be perturbed before the cell will become tumorigenic.

9.10 Transgenic mice provides models of oncogene collaboration and multi-step cell transformation

Insert mutant, activated oncogene into a germ line of laboratory mouse. Confine the expression of this oncogene to a small subset of tissues in the mouse.

Mammary glands showed minimal morphologic changes in the case of myc transgene.

Mammary glands showed hyperplasia in the case of ras transgene.

The presence of a single oncogene within a normal cell in living tissue is not sufficient to transform this cell into a tumor cell.

9.11 Human cells are constructed to be highly resistant to immortalization and transformation Primary rodent cells become transformed in vitro following the introduction of pairs of oncogenes (such as myc and ras), while such pairs of introduced oncogenes consistently fail to yield tumorigenic human cells. Experimentally, five distinct cellular regulatory circuits need to be altered before human cells can grow as tumor cells.

+ Rac1

It is unknown whether these five pathways are required for the experimental transformation of all human cell types, and whether deregulation of all of these five pathways occurs in spontaneously arising human tumors.

9.12 Nonmutagenic agents, including those favoring cell proliferation, make important contributions to tumorigenesis Nonmutagenic (nongenotoxic) carcinogens induce skin cancers in mice

DMBA = initiating agent (initiator) ---> potent mutagen

TPA = promoting agent (promoter)

DMBA=7,12-dimethylbenz[a]anthracene

TPA= tetradecanoylphorbol-13-acetatePMA= phorbol-12-myristate-13-acetate

first painting with initiator

second painting with initiator

Scheme of initiation and promotion of epidermal carcinomas in mice. Tumor promoters like TPA

do not directly affect the genomes of cells, however, they can function as important agents in driving forward multi-step tumorigenesis. TPA mimics DAG (diacylglycerol) to activate Protein kinase C (PKC ).= potent stimulator of cell proliferation

Halt

Genes and proteins involved in mouse skin carcinogenesis.

first painting with initiator

second painting with initiator

9.13 Toxic and mitogenic agents can act as human tumor promoters Alcohol is a toxic agent acting as a tumor promoter for cancers of the mouth and throat (head-and-neck cancers). (initiator = mutagenic carcinogens in cigarette smoke)

Estrogen (and perhaps other hormones such as progesterone and even prolactin) is a mitogenic agent acting as a tumor promoter for breast cancers (mammary epithelial cell = MECs). (initiator = metabolites of estrogen)9.14 Chronic inflammation often serves to promote tumor progression in mice and humans The development of adenomas and carcinomas in the colon is strongly dependent on chronic inflammation.

Hepatocellular carcinomas are associated with chronic HBV or HCV infection and accompanying inflammation of the liver. ----> HBV acts as a tumor promoter and functions synergistically with aflatoxin-B1, a highly mutagenic compound that is made by Aspergillus fungi.

9.15 Inflammation-dependent tumor promotion operates through defined signaling pathway

Chronic liver inflammation acts via NF-B to induce hepatocellular carcinomas.

9.15 Tumor promotion is likely to be critical determinant of the rate of tumor progression in many human tissues.

Known or suspected human promoters and their sites of action

1) Promoter can stimulate the clonal expansions.

2) Promoter favoring cell proliferation are indirectly mutagenic.

3) Repeated cycles of growth and division lead to progressive shortening of telomeric DNA .