BIOCHEMISTRY: A TEXTBOOK FOR UNIVERSITIES - E. S. Severin - 2004

SECTION 16. ONCOGENESIS

III. Oncogenes, Proto-oncogenes, and Tumor Suppressor Genes

For many years, it remained unclear why and how Viruses acquired genes capable of driving tumor growth. Initially, these genes were assumed to be native to the viral genome. However, in 1989, experiments involving the Hybridization of viral DNA with animal Cell DNA revealed that oncogenes are not originally intrinsic to viruses; rather, they are captured from the genomes of the host Cells they infect. Over the course of residing within viral genomes, these mammalian genes (including those of humans) undergo multiple Mutations, ultimately acquiring oncogenic properties. In some cases, tumor-causing viruses do not carry oncogenes at all; instead, the random integration of their genetic material—containing promoters within regulatory regions—into the host genome can alter the expression of neighboring host genes and trigger malignant transformation.

To distinguish normal host genes from Viral Oncogenes, the term Proto-oncogenes was introduced for the former. This group encompasses genes that encode Proteins playing a pivotal role in regulating organismal GROWTH AND DEVELOPMENT, such as growth factors (GFs), GF receptors, trans

cription factors, and proteins involved in signal Transduction.

A. Nomenclature

Oncogenes are designated by a three-letter lowercase italicized code, which typically indicates the source from which the oncogene was first isolated. For instance, the ras oncogene derives its name from a Gene first identified in rat Sarcoma (from the English rat sarcoma). Sometimes, this three-letter code is followed by a letter or num

ber. This becomes necessary when distinct oncogenes with different activities are isolated from the same source. For example, the avian erythroblastosis virus contains the erbA gene, which is a viral homolog of the thyroid hormone receptor, and the erbB gene, a homolog of the EGF receptor.

Appending a number to a gene designation often reflects the fact that these genes belong to closely related families, with the number indicating the gene's position within that family (e.g., bcl 1, bcl 2, etc.).

Viral oncogenes are designated by prefixing the three-letter name with a lowercase 'v' (for virus), yielding v-onc, whereas cellular oncogenes arising from mutations in transformed cells are designated with a lowercase 'c' (for cell), yielding c-onc.

Tumor suppressor genes, which encode proteins that inhibit Cell Growth and Division, feature an even more diverse nomenclature. Alongside two- and three-letter codes (such as the rb gene), some designations reflect the size of the encoded protein product. The p53 gene is so named because it encodes a protein with a Molecular Weight of 53 kD.

Protein gene products are usually designated in the same way as their corresponding genes, but with an initial capital letter. For example, the ras gene encodes the Ras protein, and the p53 gene encodes the P53 protein.

B. Inheritance pattern of oncogenes and tumor suppressor genes

Most tumors originate from somatic cells. Since somatic cells are diploid, they carry two alleles of each gene. If a mutation in one allele leads to impaired cellular function, the inheritance pattern is described as dominant. This mode of inheritance is characteristic of oncogenes and the p53 gene.

If a mutation in a single allele does not manifest functionally, the inheritance pattern is considered recessive. In this case, the biological effect is achieved only when both alleles are damaged. Mutations in tumor suppressor genes (with the exception of p53) follow a recessive mechanism. When the second allele in a DNA molecule is altered following the first, The Cell transitions from heterozygous to homozygous inheritance for that protein's information, a phenomenon known as loss of heterozygosity (LOH). Such genomic damage results in the synthesis of an altered and functionally inactive protein.

C. Functions of oncogenes

Studies of viral oncogenes have shown that more than 50% of them encode Tyrosine protein Kinases (tyr-PKs), while the rest carry information for various functionally active proteins, such as a truncated platelet-derived growth factor (PDGF), a truncated epidermal growth factor (EGF) and its receptor (EGFR), as well as DNA-binding, GTP-binding, and several other regulatory proteins.

Let us examine the main groups of proteins encoded by oncogenes.

Tyrosine protein kinases (tyr-PKs)

The tyr-PK group includes the erb-B oncogene of avian erythroblastosis virus, which encodes a protein identical to the β-subunit of the EGF receptor, along with homologs of platelet-derived growth factors and Insulin-like growth factor I and II receptors. The src gene, isolated from Rous sarcoma virus, encodes the pp60 protein, which exhibits tyr-PK activity. This protein phosphorylates certain glycolytic Enzymes, accelerates glucose utilization in transformed cells, disrupts contact inhibition, and stimulates cellular transformation.

In addition to oncogenes, the tyr-PK group includes several proto-oncogenes (such as the insulin receptor, EGFR, and the PDGF receptor). It should be noted that although some normal body proteins possess tyr-PK activity, The amount of phosphotyrosine in normal cells is very low (no more than 1% of all Phosphorylated Amino Acids). During tumor transformation, tyr-PK activity increases sharply, leading to an elevated proportion of phosphotyrosine within the cellular pool of protein-bound amino acids.

Ras oncogenes

Another group of oncoproteins is encoded by the ras gene family. ras proto-oncogenes carry information for a family of Ras proteins, which are small G-proteins. Similar to the G-proteins of major signaling systems, these proteins bind GTP and exhibit GTPase activity; however, unlike G-proteins that possess

an oligomeric $\alpha\beta\gamma$ Structure, Ras proteins are monomeric. They participate in the transduction of signals received by cell Membrane Receptors and, being localized on the inner leaflet of the membrane, closely interact with Phospholipids and Membrane Proteins. Ras proteins have been shown to be involved in remodeling the Cytoskeleton, regulating exocytosis and endocytosis, mediating mitogenic signals, and activating proteins required for gene METABOLISM/31.html">Transcription.

Ras oncoproteins resulting from single missense mutations in the GTP-binding domain exhibit very low GTPase activity. As a result, adenylate cyclase or phospholipase C remain in an activated state longer than usual, thereby providing a more sustained signal.

Ras oncoproteins are found in 25% of all human tumors, and significantly more often in certain types of Cancer: in 90% of pancreatic carcinomas and in more than 50% of colorectal carcinomas.

Nuclear Oncoproteins

The family of nuclear oncogenes includes the jun, fos, myc, myb, and erbA genes. Oncoproteins produced by the expression of these genes bind to specific DNA sequences and function as transcription factors.

For instance, the Jun and Fos oncoproteins form a dimer that binds to DNA, while ErbA is a modified form of the thyroid hormone receptor, which also binds to specific sequences on the DNA molecule.

The Amino Acid Sequence of the oncoprotein encoded by the v-jun gene is 80% homologous to the nuclear transcription factor AP1. When Jun and Fos proteins associate, they form a leucine zipper structure—a well-known transcription activator (see Section 1).

C. Role of Tumor Suppressors in Cellular Metabolism

The fusion of normal cells with tumor cells yields hybrid cells that generally lack malignant properties. This led to the Conclusion that normal cells contain genes whose protein products restrain the replicative potential of cells and prevent tumor development. These genes were named tumor suppressor genes, or antioncogenes. It has been established that the functions of these genes are frequently lost during malignant transformation, leading to a disruption in the control of cell proliferation.

Currently, over 10 tumor suppressor genes (such as rb1, p53, p21, p16, p15, wt1, etc.) have been described, which encode regulatory proteins that inhibit abnormal cell growth and transformation.

The rb1 gene. The product of the rb1 gene is a nuclear protein with a molecular weight of 105 kD, which participates in regulating the transition of the cell from the resting phase G₀ into the DNA Synthesis preparation phase G1, as well as the passage through the G1/S checkpoint. Like cyclin-dependent kinases (see Section 4), the Rb1 protein undergoes modifications via phosphorylation and dephosphorylation. In its dephosphorylated form, it can bind and inactivate the transcription factor E2F, which, in turn, enhances the expression of growth-stimulating Proteins and Enzymes: DNA polymerase α, MYC, CDC2, and several others (Fig. 16-8).

Class="center">Fig. 16-8. MECHANISM OF ACTION of the RB1 protein. E2F is a transcription factor that enhances the transcription of A number of proteins and enzymes regulating cell growth and division. By binding to E2F, the tumor suppressor protein Rb1 inhibits the preparation of cells for mitosis. Hyperphosphorylated and mutant forms of the Rb1 protein have no affinity for E2F and cease to inhibit cell growth.

Normally, when a cell enters the S phase and begins DNA Replication, the Rb1 protein becomes heavily phosphorylated and stops inhibiting the cell's progression through the Cell Cycle.

The p53 gene is another widely studied example of a tumor suppressor gene. This gene encodes a nuclear phosphoprotein with a molecular weight of 53 kD that prevents cells from entering the S phase, as well as preventing DNA Amplification and mutations. It is believed that the physiological function of the P53 protein is to arrest cells with DNA damage in the G₁ and G₂ phases until these lesions are repaired. If repair systems are unable to fix the defects in the Introduction/20.html">DNA Structure, this protein triggers the apoptotic pathway, destroying the damaged cell.

The human P53 protein contains 393 Amino Acids and consists of 3 domains: an N-terminal domain enriched in dicarboxylic amino acids, which regulates transcription; a central domain responsible for DNA binding; and a C-terminal domain responsible for the oligomeric STRUCTURE OF THE protein.

P53 functions as a tetramer and binds to regulatory regions of DNA. Quite a few cellular genes possess sequences capable of binding P53 and altering the expression of the corresponding genes (Fig. 16-9).

Fig. 16-9. Effect of the P53 protein on the transcription of various genes. A — main target genes whose expression is regulated by the P53 protein; B — inactivation of P53 resulting from Gene Mutations or binding to inhibitor proteins renders it unable to inhibit the transcription of these target genes.

Target genes include the gene encoding the P21 protein, an inhibitor of most cyclin-dependent kinases. P53 enhances the Transcription of the p21 gene, thereby halting cell cycle progression, growth, and division.

P53 enhances the transcription of the gadd45 gene, whose protein product stimulates repair processes. It has been shown that the expression of this gene increases significantly in cells exposed to radiation.

Two genes, bcl2 and bax, encoding proteins involved in The regulation of apoptosis, are sensitive to P53. Apoptosis is activated when P53 binds to the regulatory Regions of the bcl2 and bax genes; concomitantly, the expression of the anti-apoptotic bcl2 gene decreases, while that of the pro-apoptotic bax gene increases.

By activating key genes that execute programmed cell death, P53 accelerates the elimination of potentially dangerous cells that are damaged and prone to transformation.

P53 upregulates the expression of the gene encoding thrombospondin, a protein that inhibits Blood vessel growth within tumors (angiogenesis) and, consequently, prevents metastasis (see Section 14).

Thus, P53 functions in Tissues as the "guardian" of cellular health or the "molecular policeman."