DNA Tumor Viruses
DNA viruses are less closely associated with cancer than are the retroviruses because their infection does not usually yield tumors. Except for papillomaviruses, which cause warts, DNA viruses cannot readily be found in natural tumors. Papovaviruses, including SV4O, and the very closely related polyoma virus, as well as adenoviruses and herpes viruses, do have the capability of transforming cells in culture so that they lose their density-dependent growth regulation. These trans-formed cells can then be injected into animals, where they will cause tumors.
Transformation of mouse cells by SV4O and of hamster cells by polyoma virus involves T proteins that are called T antigens. Normally three are found. They are called small, middle, and large T, for their respective sizes. The messenger for these three different proteins origi-nates from one gene as a result of alternative splicing of the mRNA. The three proteins were identified immunologically in the same way the retroviral oncogene proteins were identified. Antibodies against these proteins are induced in response to the presence of the virus.
The antibodies obtained from animals carrying SV4O- and polyoma-induced tumors have been used to show that the T antigen genes are expressed early after infection and that the T antigens bind to proteins in the nucleus, to DNA, and to other cellular structures. The presence of one of the T antigens alone is usually insufficient to transform cells, suggesting that in general the process of transformation requires a number of steps. This conclusion is in accord with the fact that the NIH 3T3 cell foci-inducing DNAs isolated from tumors are not capable of transforming primary cell lines.
The activity of the T antigens has been a subject of great interest, but the low levels at which they are synthesized in vivo has greatly hampered their study. They were therefore perfect targets for genetic engineering. Indeed, by properly fusing the DNA encoding the different proteins to suitable ribosome-binding sequences, these proteins have been hy-persynthesized in bacteria, and the proteins can be used for biochemical studies. They also serve as a marker for expression of a promoter in an organism. A promoter and its associated regulatory sequences can be fused to the T antigen gene and inserted into the chromosome of an animal. Expression of the T antigen gene frequently gives rise to tumors that are confined to the tissues in which the gene is expressed.
Figure 23.10 Products of adenovirus E1a and E1b bind multiple cellularproteins.
Adenoviruses and papillomaviruses also possess genes responsible for transformation of cells. Like the SV40 virus, their oncogenes possess no cellular counterparts. Two transcripts are synthesized from the E1a and E1b genes of adenovirus (Fig. 23.10). Through splicing and protein processing, each gives rise to four or five protein products. Some regulate other genes, and two bind to cellular proteins important in transformation. The adenovirus E1a binds to a protein discussed below called Rb for retinoblastoma. The adenovirus E1b protein binds to a cellular protein of 53,000 molecular weight called p53. One of the SV40 T antigens also binds to this same protein, as does a protein encoded by papillomaviruses. Papillomaviruses also encode a protein which binds to Rb. The fact that different tumor viruses synthesize proteins that inactivate Rb and p53 indicates that these two cellular proteins are important for normal cell growth. Indeed, the next section describes experiments leading to the same conclusion.
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