The effects of BK virus large T antigen on cell cycle regulation.

Abstract

BK virus (BKV) is a member of the polyomavirus family and infects greater than 70% of the human population. BKV is a potent transforming agent in both rodent and human cells in vitro and is tumorigenic in rodents in vivo. BKV DNA has been found in several types of human tumors including brain, pancreatic islet, urinary tract, and Kaposi's sarcoma. BKV encodes a large T antigen (TAg) which shares a great deal of homology with SV40 TAg. SV40 TAg is an oncoprotein known to be responsible for the transforming functions of the virus. A characterization of the effects of BKV TAg on cell cycle regulation is in order based on the homology to SV40 TAg, the prevalence of BKV in the human population, and its association with human tumors. The experiments presented in this thesis demonstrate that although BKV TAg can interact with the retinoblastoma family (pRb) of tumor suppressor proteins, the low levels of BKV TAg present in the cell are not sufficient to bind a significant amount of these proteins. Despite the absence of complex formation, these low levels of BKV TAg can promote turnover of pRb family proteins and induce an increase in transcriptionally active E2F. Experiments using BKV TAgs with mutations in the pRb binding and the J domains show that both domains are required for the induction of E2F and degradation of pRb family proteins. Taken together, these results suggest a newly identified mechanism for E2F induction in which turnover of pRb family proteins mediated by both the pRb binding and J domains leads to an increase in free E2F in the cell. In addition, we have shown that BKV TAg can induce a semi-transformed phenotype and that only the J domain is required for this effect. The work presented in this thesis demonstrates that BKV TAg can induce E2F through previously unidentified mechanisms, and that BKV TAg can induce a semi-transformed phenotype. These results emphasize the need for further studies to assess the possible role for BKV in human cancers.