7.Kuliah Dasar Molekuler Kanker Part 2

8/10/2019 7.Kuliah Dasar Molekuler Kanker Part 2

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Brian Wasita,dr.,Ph.D

Bagian Patologi Anatomi FK UNS


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A fundamental property of cancer cells is their ability to evade the

apoptotic cellular death program.

Apoptosis can be initiated through the extrinsic or intrinsic pathways

Both pathways result in the activation of a proteolytic cascade of

caspases that destroys the cell.

Mitochondrial outer membrane permeabilization is regulated by the

balance between pro-apoptotic (e.g., BAX, BAK) and anti-apoptotic

molecules (BCL2, BCL-XL).

BH-3-only molecules activate apoptosis by tilting the balance in favor

of the pro-apoptotic molecules


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In cancer, oncogenes overcome these protective cellular responses by taking

advantage of cooperating, additional mutations in apoptosis signaling molecules,

resulting in the abnormal proliferation and survival/antiapoptosis of the tumor cell.

Mechanism of evasion from apoptosis:

1. Reduced levels of CD95 may render the tumor cells less susceptible to

apoptosis by Fas ligand (FasL).

2. High levels of FLIP, a protein that can bind death-inducing signaling complex

and prevent activation of caspase 8.


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3. Overexpression of the BCL2 protein. This in turn increases the BCL2/BCL-

XL buffer, protecting lymphocytes from apoptosis and allowing them to

survive for long periods. Approximately 85% of B-cell lymphomas of the

follicular type carry a characteristic t(14;18) (q32;q21) translocation.

4. p53 is an important pro-apoptotic gene that induces apoptosis in cells that

are unable to repair DNA damage.The actions ofp53are mediated in part

by transcriptional activation of BAX, but there are other connections as

well between p53 and the apoptotic machinery.


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In normal cells, which lack expression of telomerase, the shortenedtelomeres generated by cell division eventually activate cell cycle

checkpoints, leading to senescence and placing a limit on the

number of divisions a cell may undergo.

In cells that have disabled checkpoints, DNA repair pathways are

inappropriately activated by shortened telomeres, leading to massive

chromosomal instability and mitotic crisis. Tumor cells reactivatetelomerase, thus staving off mitotic catastrophe and achieving

immortality.

4. Limitless Replicative Potential


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Most normal human cells have a capacity of 60 to 70 doublings. After

this, the cells lose the capacity to divide and enter senescence. This

phenomenon has been ascribed to progressive shortening of

telomeresat the ends of chromosomes.

Short telomeres seem to be recognized by the DNA repair machinery

as double-stranded DNA breaks, and this leads to cell cycle arrest

mediated byp53and RB.


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Cells in which the checkpoints are disabled by p53 or RB

mutations, the nonhomologous end-joining pathway is activated as

a last-ditch effort to save the cell, joining the shortened ends of two

chromosomes.

This inappropriately activated repair system results in dicentric

chromosomes that are pulled apart at anaphase, resulting in new

double-stranded DNA breaks.

The resulting genomic instability from the repeated bridge-fusion-

breakage cycles eventually produces mitotic catastrophe,

characterized by massive cell death. If during crisis a cell manages to reactivate telomerase, the bridge-

fusion-breakage cycles cease and the cell is able to avoid death.


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Telomerase, active in normal stem cells, is normally absent from, or at very

low levels in, most somatic cells.

By contrast, in 85% to 95% of cancers telomere is preserved due to up-

regulation of the enzyme telomerase.

A few tumors use other mechanisms, termed alternative lengthening of

telomeres, which probably depend on DNA recombination.


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Vascularization of tumors is essential for their growth and is controlled

by the balance between angiogenic and anti-angiogenic factors that areproduced by tumor and stromal cells.

Hypoxia triggers angiogenesis through the actions of HIF1.

VHL acts as a tumor suppressor gene because of its ability to degrade

HIF1and thus prevent angiogenesis.

Inheritance of germ-line mutations of this gene causes VHL syndrome,

characterized by the development of a variety of tumors. Many other factors regulate angiogenesis; for example, p53 induces

synthesis of the angiogenesis inhibitor thrombospondin-1.

5. Development of Sustained Angiogenesis


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Tumor angiogenesis is controlled by the balance between angiogenic

factors and factors that inhibit angiogenesis

The molecular basis of the angiogenic switch involves increased

production of angiogenic factors and/or loss of angiogenesis inhibitors.

These factors may be produced directly by the tumor cells themselves or

by inflammatory cells (e.g., macrophages) or other stromal cells

associated with the tumors.

The angiogenic switch is controlled by several physiologic stimuli, such as

hypoxia. Relative lack of oxygen stimulates production of a variety of pro-

angiogenic cytokines, such as vascular endothelial growth factor (VEGF),

through activation of hypoxia-induced factor-1 (HIF1), an oxygen-

sensitive transcription factor


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Both pro- and anti-angiogenic factors are regulated by many other

genes frequently mutated in cancer.

For example, in normal cells, p53 can stimulate expression of anti-

angiogenic molecules, such as thrombospondin-1, and repress

expression of pro-angiogenic molecules, such as VEGF.

Thus, loss of p53 in tumor cells not only removes the cell cycle

checkpoints listed above, but also provides a more permissive

environment for angiogenesis.

The transcription of VEGF is also influenced by signals from the RAS-

MAP kinase pathway, and mutations of RAS or MYC up-regulate the

production of VEGF.


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6. Invasion and Metastasis


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Steps of Invasion :

1. Detachment of tumor cells from each otherE-cadherin function is lost in almost all epithelial cancers, either by

mutational inactivation of E-cadherin genes, by activation of -catenin

genes, or by inappropriate expression of the SNAIL and TWIST

transcription factors, which suppress E-cadherin expression.2. Degradation of ECM

Tumor cells may either secrete proteolytic enzymes themselves or

induce stromal cells (e.g., fibroblasts and inflammatory cells) to

elaborate proteases. Multiple different families of proteases, such as

matrix metalloproteinases (MMPs), cathepsin D, and urokinase

plasminogen activator. Overexpression of MMPs and other proteases

have been reported for many tumors


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(Wasita et al, 2009)


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3. Attachment to novel ECM components

Cleavage of the basement membrane proteins collagen IV and

laminin by MMP-2 or MMP-9 generates novel sites that bind to

receptors on tumor cells and stimulate migration.4. Migration of tumor cells

Migration is a complex, multistep process that involves many

families of receptors and signaling proteins that eventually impinge

on the actin cytoskeleton. Such movement seems to be potentiated

and directed by tumor cell-derived cytokines, such as autocrine

motility factors.In addition, cleavage products of matrix components (e.g.,

collagen, laminin) and some growth factors (e.g., insulin-like growth

factors I and II) have chemotactic activity for tumor cells. Stromal

cells also produce paracrine effectors of cell motility, such as

hepatocyte growth factor/scatter factor (HGF/SCF), which bind to

receptors on tumor cells.


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Metastatic cascade:

1. Invasion of ECM and vascular dissemination

In the bloodstream, some tumor cells form emboli by aggregating andadhering to circulating leukocytes, particularly platelets; aggregated tumor cells

are thus afforded some protection from the antitumor host effector cells.

Most tumor cells, circulate as single cells.

Extravasation of free tumor cells or tumor emboli involves adhesion to the

vascular endothelium, followed by egress through the basement membrane into

the organ parenchyma by mechanisms similar to those involved in invasion.2.Homing of tumor cells

The site of extravasation and the organ distribution of metastases

generally can be predicted by the location of the primary tumor and its vascular

or lymphatic drainage.

Many tumors arrest in the first capillary bed they encounter (lung and liver, most

commonly).

Some tumors show organ tropism, probably due to expression of adhesion or

chemokine receptors whose ligands are expressed by the metastatic site

preferentially on the endothelium of target organs


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.

Another mechanism of site-specific homing involves chemokines and

their receptors. Chemokines participate in directed movement (chemotaxis) of

leukocytes, and it seems that cancer cells use similar tricks to home in

on specific tissues.

Human breast cancer cells express high levels of the chemokine

receptors CXCR4 and CCR7. The ligands for these receptors (i.e.,

chemokines CXCL12 and CCL21) are highly expressed only in those

organs where breast cancer cells metastasize.

After extravasation, tumor cells are dependent on a receptive stroma for

growth. Thus, tumors may fail to metastasize to certain target tissues

because they present a nonpermissive growth environment. Despite the

foregoing considerations, the precise localization of metastases cannotbe predicted with any form of cancer


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Candidates metastasis oncogenes are SNAIL and TWIST, which

encode transcription factors whose primary function is to promote aprocess called epithelial-to-mesenchymal transition (EMT).

In EMT, carcinoma cells down-regulate certain epithelial markers (e.g.,

E-cadherin) and up-regulate certain mesenchymal markers (e.g.,

vimentin and smooth muscle actin).

These changes are believed to favor the development of a

promigratory phenotype that is essential for metastasis.

Loss of E-cadherin expression seems to be a key event in EMT, and

SNAIL and TWIST are transcriptional repressors that promote EMT by

down-regulating E-cadherin expression.

EMT has been documented mainly in breast cancers

Molecular Genetics of Metastasis


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Individuals with inherited mutations of genes involved in DNA repair systems

are at a greatly increased risk of developing cancer.

Example:

1. Patients with HNPCC syndrome have defects in the mismatch repair system

and develop carcinomas of the colon.

2. Patients with xeroderma pigmentosum have a defect in the nucleotide

excision repair pathway and are at increased risk for the development of

cancers of the skin exposed to UV light, because of an inability to repair

pyrimidine dimers.

3. Patient with familial breast cancers have mutation in BRCA1 and BRCA2,

which involved in DNA repair.

7. Genomic Instability-Enabler of Malignancy


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7.Kuliah Dasar Molekuler Kanker Part 2

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