Novel Mechanisms of E2F Induction by BK Virus Large-T Antigen: Requirement of Both the pRb-Binding and the J Domains

Abstract

E2F activity is regulated in part by the retinoblastoma family of tumor suppressor proteins. Viral oncoproteins, such as simian virus 40 (SV40) large-T antigen (TAg), adenovirus E1A, and human papillomavirus E7, can disrupt the regulation of cellular proliferation by binding to pRb family members and dissociating E2F-pRb family protein complexes. BK virus (BKV), which infects a large percentage of the human population and has been associated with a variety of human tumors, encodes a TAg homologous to SV40 TAg. It has been shown that BKV TAg, when expressed at low levels, does not detectably bind to pRb family members, yet it induces a serum-independent phenotype and causes a decrease in the overall levels of pRb family proteins. The experiments presented in this report show that, despite the lack of TAg-pRb interactions, BKV TAg can induce transcriptionally active E2F and that this induction does in fact require an intact pRb-binding domain as well as an intact J domain. In addition, E2F-pRb family member complexes can be detected in both BKV and SV40 TAg-expressing cells. These results suggest the presence of alternate cellular mechanisms for the release of E2F in addition to the well-established model for TAg-pRb interactions. These results also emphasize a role for BKV TAg in the deregulation of cellular proliferation, which may ultimately contribute to neoplasia.Cell cycle progression is a tightly controlled process that involves the interactions of a complex network of proteins, including the members of the retinoblastoma family of tumor suppressor proteins and the E2F family of transcription factors. The retinoblastoma family includes the retinoblastoma susceptibility protein, pRb, and the related proteins p107 and p130. Overexpression of any of these three proteins can induce growth arrest, implicating this family of proteins as negative regulators of cell growth (32,70,92,96,99). Overexpression of E2F can induce quiescent cells to enter the S phase, implicating this family of proteins as positive regulators of cell growth (48). Further experiments have shown that ectopic expression of E2F in cells arrested by overexpression of pRb is sufficient to relieve the growth arrest phenotype (78, 99). In addition, E2F-pRb complexes have been shown to bind to and repress E2F-responsive promoters (see reference 94 for a review). The cell must therefore maintain a delicate balance between the active forms of these and other cell cycle regulators in order to maintain proliferative control. When pRb, E2F, and other key regulatory proteins are mutated or when their expression is altered, regulation is lost and cell proliferation can proceed unchecked. Evidence supporting the importance of these proteins in maintaining cell growth control comes from the discovery that many human cancers in addition to retinoblastoma have inactivating mutations in the retinoblastoma susceptibility gene, RB1, or in other genes whose products are involved in the pRb pathway (36,43,44,56,57,59,91).The current model for pRb regulation of the...

Retinoblastoma family proteins as key targets of the small DNA virus oncoproteins

“…In the intact cell, low levels of BKV LT-Ag, even if undetectable in their binding with the RB proteins, induce a serum-independent phenotype and downregulation of all the three RB family proteins as well as the release of transcriptionally functional E2F, fully dependent upon the presence of intact LxCxE motif and J domain in the BKV LT-Ag molecule. This suggests a different, but not well characterized yet, mechanism regulating the BKV LT-Ag-mediated E2F release (Harris et al, 1998b).…”

“…Indeed, similarly to JCV, BKV infects a large percentage of individuals and becomes clinically evident mainly in immunodeficient or immunosuppressed patients (Wang et al, 2004). Equally, BKV DNA has been detected in human tumors, not only of the CNS but also of the pancreas and the urinary tract (Harris et al, 1998a;Croul et al, 2003;Fioriti et al, 2005). BK virus LT-Ag is composed of 695 amino acids and is structurally similar to SV40 and JCV LT-Ag (Figure 1).…”

Detection and expression of human BK virus sequences in neoplastic prostate tissues

“…The early proteins, encoded by alternatively spliced mRNAs, are the large tumor antigen (TAg), the main regulatory protein, and the supporting small tumor antigen. TAg aids in the oncogenic transformation of cells through its ability to bind and inactivate the products of tumor suppressor genes, including p53 and (Nakashatri et al, 1988;Dyson et al, 1989Dyson et al, , 1990Kang and Folk, 1992;Simmons, 1995;Staib et al, 1996;Shivakumar and Das, 1996;Harris et al, 1996Harris et al, , 1998a. TAg also regulates viral DNA replication and gene expression, and coregulates activated and basal cellular transcription factors (Moens and Rekvig, 2001).…”

Molecular basis for the actions of Hsp90 inhibitors and cancer therapy

“…The viral oncogenic partner proteins of pRB, including human papilloma-coded protein E7, adenovirus E1A-encoded protein and SV40 large T antigen associated with pRB, deregulate the cell cycle progression and the cellular proliferation of neoplasia. 10 These viral oncogenic partner proteins of pRB preferentially bind to underphosphorylated gene products of the retinoblastoma family gene products. The association of these viral oncogenic partner proteins Hypoxia-inducible factor-1a, a family of proteins responsible for cellular oxygen deficiency SV40 large T antigen A potent oncogene able to transform many cell types, binds to the viral replication origin and promotes the SV40 replication SV40 small of pRB causes the dissociation of the pRB/E2Fcomplex, and the released E2F upregulates a large number of genes essential for cell cycle progression.…”