regulation of carcinogenicity

Regulation of cellular growth- Development of carcinogenicity

Contents:
• What is cancer
• What is Carcinogen and carcinogenicity
• Regulation of cancer
• Cancer and cell cycle
• Cancer and programmed cell death
• Genetic basis for cancer
• Oncogenes
• Proto- oncogenes
• Mutant cellular oncogenes and cancer
• Tumor suppresser genes
• Knudson’s two hit hypothesis
• Conclusion

What is cancer?
Cancer is uncontrolled, abnormal proliferation of any cell type.

Without regulation, cancer cells divide ceaselessly, piling up on top of each other to form tumors.
When cells detach form tumor and invade surrounding tissues, the tumor is malignant. When the cells do not invade surrounding tissues, the tumor is benign.

What is carcinogen and carcinogenicity?
The agents that can irreversibly transform normal cells into cancerous cells are called carcinogens. E.g. radiation, mutagenic chemicals, and certain types of viruses

Carcinogenicity- Ability of carcinogen to induce cancer is called carcinogenicity.


The abiding characteristic of all cancer cells is that their growth is unregulated. When normal cells are cultured in vitro, they form a single layer- a monolayer- on the surface of the culture medium. Cancer cells, by contrast, overgrow each other, piling up on the surface of the culture medium to form masses. This unregulated pileup occurs because cancer cells do not respond to the chemical signals that inhibit cell division and because they can not form stable associations with their neighbors.

The external abnormalities that are apparent in a culture of cancer cells are correlated with profound intracellular abnormalities. Cancer cells often have a disorganized cytoskeleton, they may synthesize unusual proteins and display them on their surfaces and they frequently have abnormal chromosome numbers- that is they are aneuploid.

Regulation of carcinogenicity- carcinogenicity is regulated at several levels viz.

• Cancer and cell cycle
• Cancer and programmed cell death
• Genetic basis for cancer
• Oncogenes
• Proto- oncogenes
• Mutant cellular oncogenes and cancer
• Tumor suppresser genes
• Knudson’s two hit hypothesis

Cancer and cell cycle-
The cell cycle consists of periods of growth, DNA synthesis and division. The length of cell cycle and the duration of each of its components are controlled by external and internal chemical signals. The transition from each phase of the cycle requires the integration of specific signals and precise responses to these signals. If the signals are incorrectly sensed or if the cell is not properly respond, the cell could become cancerous.
The current view of cell cycle control is that transitions between different phases of cell cycle (G1, S, G2 and M) are regulated at “checkpoints.” A checkpoint is a mechanism that halts progression through the cycle until a critical process such as DNA synthesis is completed or until damaged DNA is repaired. The molecular machinery that operates each checkpoint is complex. Two types of proteins are known to play especially critical roles: the cyclins and the cyclin dependant kinases, often abbreviated as CDKs. Complexes formed between cyclins and CDKs cause the cell cycle to progress.

The CDKs are the catalytically active components of the cell cycling mechanisms. These proteins regulate the activities of other proteins by transferring phosphate groups to them. However, the phosphorylation activity of the CDKs depends on the presence of the cyclins. The cyclins enable the CDKs to carry out their function by forming cyclin / CDK complexes. When the cyclins are absent, these complexes can not form, and CDKs are inactive. Cell cycling therefore requires the alternate formation and degradation of cyclin/ CDK complexes.

One of the most important cell cycle checkpoints, called START, is in mid G1. The cell receives both external and internal signals at this checkpoints to determine when it is appropriate to move into S phase. This checkpoint is regulated by D type cyclins in conjunction with CDK4. If a cell driven past the START checkpoint, it becomes committed to another round of DNA replication. Inhibitory proteins with the capability of sensing problems in late G1 phase such as low level of nutrients or DNA damage, can put a brake on the cyclin/ CDK complex and prevent the cell from entering the S phase. In the absence of such problems, the cyclin D/ CDK4 complex drives the cell through the end of G1 phase and into S phase, there by initiating the DNA replication that is a prelude to cell division.

In tumor cells, the checkpoints in the cell cycle are typically deregulated. This deregulation is due to genetic defects in the machinery that alternately raise and lower the abundance of cyclin/ CDK complexes. E.g the genes encoding the cyclins or CDKs may be mutated, or the genes encoding the proteins that respond to specific cyclin/ CDK complexes or that regulate the abundance of these complexes may be mutated. Many different types of genetic defects can deregulate the cell cycle, with the ultimate consequence that the cells may become cancerous.

Cells in which START checkpoint is dysfunctional are especially prone to become cancerous. The START checkpoint controls entry into the S phase of cell cycle. If DNA within a cell is damaged, it is important that entry into S phase be delayed to allow for the damaged DNA to be repaired. Otherwise, the damaged DNA will be replicated and transmitted to all cell’s descendents. Normal cells are programmed to pause at the START checkpoint to ensure that repair is completed before DNA replication commences. By contrast, cells in which the START checkpoints is dysfunctional move into S phase without repairing damaged DNA. Over a series of cell cycles, mutations that result from the replication of unrepaired DNA may accumulate and cause further deregulation of cell cycle. A clone of cells with a dysfunctional START checkpoint may therefore become aggressively cancerous.

Cancer and programmed cell death-
Every cancer involves accumulation of unwanted cells. In many animals, superfluous cells can be disposed of by mechanisms that are programmed into cells them selves. This programmed cell death was originally discovered in studies wit nematode Caenorhabitidis elegans. This tiny roundworm loses some of the cells that accumulate during the 10 or so cycles of division that occur during its development from a fertilized egg. Genetic analyses by Robert Horvitz demonstrated that the loss of these cells does not occur in certain mutants of C. Elegans. Thus cell death is part of normal developmental program in this animal- and in others too, e.g during development of hands and feet of many vertebrates, the cells that lie between the developing digits must die; if they do not, the digits remain fused. Programmed cell death is therefore a fundamental phenomenon among animals. Without it, the formation and function of organs would be impaired by cells that simply ‘get in the way.’

Incomplete differentiation in two toes (syndactyly) due to lack of apoptosis

Programmed cell death is also important in preventing the occurrence of cancers. If a cell with an abnormal ability to replicate is killed, it can not multiply to form tumor. Thus, programmed cell death is an important check against renegade cells that could otherwise proliferate uncontrollably in an organism.

Programmed cell death is called apoptosis from greek roots meaning “falling away.” The events that trigger cell death are only partially understood. A family of proteolytic enzymes called caspases play a crucial role in the cell death phenomenon. The caspases remove small part of other proteins by cleaving peptide bonds. Through this enzymatic trimming, the target proteins are inactivated. The caspases attack many different kinds of proteins, including the lamins, which make up the inner lining of nuclear envelope, and several components of cytoskeleton. The collective impact of these proteolytic cleavage is that cells in which it occurs lose their integrity; their chromatin becomes fragmented, blebs of cytoplasm form at their surfaces, and they begin to shrink. Cells undergoing this kind if disintegration are usually engulfed by Phagocytosis, and are then destroyed. If the apoptotic mechanism has been impaired or inactivated, a cell that should otherwise be killed can survive and proliferate. Such a cell has the potential to form a clone that could become cancerous if it acquire the ability to divide uncontrollably.

Genetic basis for cancer- There are strong evidence that the underlying causes of cancer are genetic.
1. It was known that cancerous state is clonally inherited. When cancer cells are grown in culture, their descendents are all cancerous. Therefore, cancerous condition is transmitted from each cell to its daughter cells at the time of division.
2. It was known that certain types of viruses ca induce the formation of tumors in experimental animals. The induction of cancer by viruses implies that the protein encoded by viral genes are involved in production of the cancerous state.
3. Cancer can be induced by agents capable of causing mutations. Mutagenic chemicals and ionizing radiations had been shown to induce tumors in experimental animals.
4. It was known that certain types of cancer run in families. In particular susceptibility to retinoblastoma, a rare cancer of eye, is inherited as dominant conditions, albeit with incomplete penetrance and variable expressivity.
So all these shows that special types of cancer is inherited- it seemed plausible that all cancer types might have their genetic basis.

In 1980s when molecular techniques were first used to study cancer cells, researchers discovered that the cancerous state is indeed, traceable to genetic defects. Typically, however not one but several such defects are required to convert a normal cell to cancerous cell.

Researchers identified two broad classes of genes that when mutated, can contribute to the development of a cancerous state.
• In one of these class mutated genes actively promote cell division
• In other class mutant genes fail to express cell division.
Genes in first class are called oncogenes from greek word tumor
In second class are called tumor suppressor genes.

Oncogenes- An oncogene is a gene that, when mutated or expressed at high levels, helps turn a normal cell into a tumor cell.
Many abnormal cells normally undergo a programmed form of death (apoptosis). Activated oncogenes can cause those cells to survive and proliferate instead. Most oncogenes require an additional step, such as mutations in another gene, or environmental factors, such as viral infection, to cause cancer. Since the 1970s, dozens of oncogenes have been identified in human cancer. Many cancer drugs target those DNA sequences and their products.

History-The first oncogene was discovered in 1970 and was termed src (pronounced sarc as in sarcoma). Src was in fact first discovered as an oncogene in a chicken retrovirus. Experiments performed by Dr G. Steve Martin of the University of California, Berkeley demonstrated that the SRC was indeed the oncogene of the virus.
In 1976 Drs. J. Michael Bishop and Harold E. Varmus of the University of California, San Francisco demonstrated that oncogenes were defective proto-oncogenes, found in many organisms including humans. For this discovery Bishop and Varmus were awarded the Nobel Prize in 1989.

Proto- oncogenes- A proto-oncogene is a normal gene that can become an oncogene due to mutations or increased expression. The resultant protein may be termed an oncoprotein. Proto-oncogenes code for proteins that help to regulate cell growth and differentiation. Proto-oncogenes are often involved in signal transduction and execution of mitogenic signals, usually through their protein products. Upon activation, a proto-oncogene (or its product) becomes a tumor-inducing agent, an oncogene. Examples of proto-oncogenes include RAS, WNT, MYC, ERK, and TRK.

Activation-The proto-oncogene can become an oncogene by a relatively small modification of its original function. There are three basic activation types:
• A mutation within a proto-oncogene can cause a change in the protein structure, causing
o An increase in protein (enzyme) activity
o A loss of regulation
• An increase in protein concentration, caused by
o An increase of protein expression (through misregulation)
o An increase of protein (mRNA) stability, prolonging its existence and thus its activity in the cell
o A gene duplication (one type of chromosome abnormality), resulting in an increased amount of protein in the cell
• A chromosomal translocation (another type of chromosome abnormality), causing
o An increased gene expression in the wrong cell type or at wrong times
o The expression of a constitutively active hybrid protein. This type of aberration in a dividing stem cell in the bone marrow leads to adult leukemia
The expression of oncogenes can be regulated by microRNAs (miRNAs), small RNAs 21-25 nucleotides in length that control gene expression by downregulating them Mutations in such microRNAs (known as oncomirs) can lead to activation of oncogenes. Antisense messenger RNAs could theoretically be used to block the effects of oncogenes.

Conversion of proto-oncogenes-There are two mechanisms by which proto-oncogenes can be converted to cellular oncogenes:

Quantitative: Tumor formation is induced by an increase in the absolute number of proto-oncogene products or by its production in inappropriate cell types.
Qualitative: Conversion from proto-oncogene to transforming gene (c-onc) with changes in the nucleotide sequence which are responsible for the acquisition of the new properties

Tumor suppressor gene- Tumor suppressor gene, or anti-oncogene, is a gene that protects a cell from one step on the path to cancer. When this gene is mutated to cause a loss or reduction in its function, the cell can progress to cancer, usually in combination with other genetic changes.

Two-hit hypothesis-Unlike oncogenes, tumor suppressor genes generally follow the 'two-hit hypothesis', which implies that both alleles that code for a particular gene must be affected before an effect is manifested. This is due to the fact that if only one allele for the gene is damaged, the second can still produce the correct protein. In other words, mutant tumor suppressors alleles are usually recessive whereas mutant oncogene alleles are typically dominant. The two-hit hypothesis was first proposed by A.G. Knudson for cases of retinoblastoma. Knudson observed that the age of onset of retinoblastoma followed 2nd order kinetics, implying that two independent genetic events were necessary. He recognized that this was consistent with a recessive mutation involving a single gene, but requiring biallelic mutation. Oncogene mutations, in contrast, generally involve a single allele because they are gain of function mutations. There are notable exceptions to the 'two-hit' rule for tumor suppressors, such as certain mutations in the p53 gene product. P53 mutations can function as a 'dominant negative', meaning that a mutated p53 protein can prevent the function of normal protein from the un-mutated allele. Other tumor-suppressor genes that are exceptions to the 'two-hit' rule are those which exhibit haploinsufficiency. An example of this is the p27Kip1 cell-cycle inhibitor, in which mutation of a single allele causes increased carcinogen susceptibility.

Examples-The first tumor-suppressor protein discovered was the Retinoblastoma protein (prb) in human retinoblastoma; however, recent evidence has also implicated prb as a tumor-survival factor.

Another important tumor suppressor is the p53 tumor-suppressor protein encoded by the TP53 gene. Homozygous loss of p53 is found in 70% of colon cancers, 30–50% of breast cancers, and 50% of lung cancers. Mutated p53 is also involved in the pathophysiology of leukemias, lymphomas, sarcomas, and neurogenic tumors. Abnormalities of the p53 gene can be inherited in Li-Fraumeni syndrome (LFS), which increases the risk of developing various types of cancers.
PTEN acts by opposing the action of PI3K, which is essential for anti-apoptotic, pro-tumorogenic Akt activation.
Other examples of tumor suppressors include APC, CD95, ST5, ST7, and ST14.
Functions-
Tumor-suppressor genes, or more precisely, the proteins for which they code, either have a dampening or repressive effect on the regulation of the cell cycle or promote apoptosis, and sometimes do both. The functions of tumor-suppressor proteins fall into several categories including the following:
1. Repression of genes that are essential for the continuing of the cell cycle. If these genes are not expressed, the cell cycle will not continue, effectively inhibiting cell division.
2. Coupling the cell cycle to DNA damage. As long as there is damaged DNA in the cell, it should not divide. If the damage can be repaired, the cell cycle can continue.
3. If the damage cannot be repaired, the cell should initiate apoptosis (programmed cell death) to remove the threat it poses for the greater good of the organism.
Some proteins involved in cell adhesion prevent tumor cells from dispersing, block loss of contact inhibition, and inhibit metastasis. These proteins are known as metastasis suppressors.

Conclusion-

Cancer results from breakdown of the regulatory mechanisms that govern from normal cell behavior. The proliferation, differentiation and survival of individual cells in multicellular organisms are carefully regulated to meet the needs of the organism as a whole. This regulation is lost in cancer cells, which grow and divide in an uncontrolled manner, ultimately spreading throughout body and interfering with the function of normal tissues and organs.

Because cancer results from defects in fundamental cell regulatory mechanisms, it is a disease that ultimately has to be understood at the molecular and cellular levels.