Adenoviruses are a large group of viruses that have been isolated from a wide range mammals and birds (for a review, see Schenk, 1996). Adenoviruses that infect humans can cause mild res- piratory distress, conjunctivitis, or gastroenteritis. In general, adenovirus infection is relatively innocuous in humans. It does not lead to cancer. Adenoviruses have double-stranded linear DNA genomes of approximately 36 kb. Human cell cultures are permissive for human adenovirus rep-lication. Productive infection results in virus multiplication, the death of the host cell, and spread of the virus. Rodent cells are not permissive for human adenovirus replication; instead, human adenovirus infection of rodent cells can result in cell transformation, albeit at a low frequency, one in 105 cells. Also, infection of newborn hamsters with some human adenoviruses leads to the development of tumors (Trentin et al., 1962). The study of the mechanism of adenovirus- induced cell transformation has helped to uncover the action of the class of cellular genes called tumor suppressor genes (see above and Chapter 5). In the transformed rodent cell, a piece of adenovirus DNA is always found integrated into the host DNA (Figure 4.9). In contrast to retro- viruses, adenoviruses do not have a required integration step in their replication cycle. In a cell that is not permissive for adenovirus replication, the viral DNA can be integrated into one of the host chromosomes. This process is carried out by host enzymes and occurs infrequently. The integration is haphazard; usually incomplete copies of the viral DNA are present. Common to all transformed cells is the presence and expression of two adenovirus genes, E1A and E1B (Doerfler,
1968; Gallimore et al., 1974; Sharp et al., 1974). Expression of the E1A and E1B genes is re- quired and is sufficient to transform rodent cells (Graham et al., 1975). Conversely, adenoviruses with certain mutations within E1A or E1B are not transforming. Each gene encodes several polypeptides. In a productive infection, E1A and E1B proteins are required for viral replication. E1A encodes two related proteins of 289 and 243 amino acids, while E1B codes for a 19- and 55-kDa protein. The 289-amino acid E1A protein is required for transformation and binds to several cellular proteins, including the retinoblastoma gene product (Rb). The ability of E1A to bind to Rb coincides with the transforming activity of adenoviruses (Whyte et al., 1988). In binding to Rb, E1A inactivates its function. Rb is an example of a protein with tumor suppressor activity. (Tumor suppressor proteins function to prevent the cell from growing out of control and becoming cancerous.) Two of the ways that tumor suppressors work are (1) by regulating pro- gression through the cell cycle and (2) by regulating induction of programmed cell death or apoptosis. By inactivating Rb function, E1A contributes to the cancer phenotype. Similarly, the 55-kDa E1B protein inactivates a second tumor suppressor gene product, the p53 protein (Sar-
Figure 4.9 In adenovirus-transformed rodent cells, a common piece of viral genome is found integrated into cellular DNA. The adenovirus genome is a linear double-stranded DNA of approximately 35 kb. When a rodent cell is transformed by adenovirus, a portion of the viral DNA is found integrated into the cellular DNA. The exact boundaries of the fragment that is integrated in each transformed clone differ, but contain a common set of adenovirus sequences. The integrated DNA contains the adenovirus E1A and E1B genes, which each express two mRNAs and two proteins. Transformation of the cell is dependent on expression of the E1A and E1B polypeptides. (Adapted from Cooper, 1995, with permission of author and publisher.)
now et al., 1982). In lytically infected human cells, E1A proteins function to cause quiescent cells to enter S phase of the cell cycle, and E1B proteins block apoptosis. These changes make the cell a better host for adenovirus replication, which culminates in death of the cell. In the nonpermissive rodent cell, these same activities of E1A and E1B contribute to oncogenic trans- formation. E1A and E1B can be considered oncogenes. In addition to their role in virally in- duced cancers, tumor suppressor genes, such as Rb and p53, can play a crucial role in other nonviral human cancers (Chapter 5). In contrast to oncogenes of transducing retroviruses, the oncogenes of adenoviruses are not derived from cellular proto-oncogenes. In addition, adeno- viral oncogenes are required for viral replication, whereas retroviral oncogenes are not. These themes in adenovirus-induced transformation are also seen in other DNA tumor viruses, namely, the SV40 virus and papillomaviruses.