Removal of a small C-terminal region of JCV and SV40 large T antigens has differential effects on transformation

The conserved core enzymatic activities and the distinct dynamics of polyomavirus large T antigens

Several human polyomaviruses including JCV, BKV and TSV are associated with diseases, particularly in immunosuppressed patients. While the large T antigen (LT) encoded by the monkey polyomavirus SV40 is well studied, and possesses intrinsic ATPase and DNA helicase activities, the LTs of the human polyomaviruses are relatively uncharacterized. In order to evaluate whether these enzymatic activities, which are required for viral DNA replication, are conserved between polyomaviruses, we performed a comparative study using the LTs from JCV, TSV and SV40. The ATPase and DNA helicase activities and the interaction with the cellular tumor suppressor p53 were assayed for the purified Zn-ATPase domains of the three LTs. We found that all Zn-ATPases were active ATPases. The Zn-ATPase domains also functioned as DNA helicases, although the measured kinetic constants differed among the three proteins. In addition, when tested against four small molecule ATPase inhibitors, the Zn-ATPase domains of TSV was more resistant than that of SV40 and JCV. Our results show that, while LTs from JCV and TSV share the core ATPase and DNA helicase activities, they possess important functional differences that might translate into their respective abilities to infect and replicate in hosts.

Cellular transformation of mouse embryo fibroblasts in the absence of activator E2Fs

The E2F family of transcription factors, broadly divided into activator and repressor E2Fs, regulates cell cycle genes. Current models indicate that activator E2Fs are necessary for cell cycle progression and tumorigenesis and are also required to mediate transformation induced by DNA tumor viruses. E2Fs are negatively regulated by the retinoblastoma (RB) family of tumor suppressor proteins, and virus-encoded oncogenes disrupt the RB-E2F repressor complexes. This results in the release of activator E2Fs and induction of E2F-dependent genes. In agreement, expression of large tumor T antigens (TAg) encoded by polyomaviruses in mammalian cells results in increased transcriptional levels of E2F target genes. In addition, tumorigenesis induced by transgenic expression of simian virus 40 (SV40) TAg in choroid plexus or intestinal villi requires at least one activator E2F. In contrast, we show that SV40 TAg-induced transformation in mouse embryonic fibroblasts is independent of activator E2Fs. This work, coupled with recent studies showing that proliferation in stem and progenitor cells is independent of activator E2Fs, suggests the presence of parallel pathways governing cell proliferation and tumorigenesis.

IMPORTANCE

The RB-E2F pathway is altered in many cancers and is also targeted by DNA tumor viruses. Viral oncoprotein action on RBs results in the release of activator E2Fs and upregulation of E2F target genes; thus, activator E2Fs are considered essential for normal and tumorigenic cell proliferation. However, we have observed that SV40 large T antigen can induce cell proliferation and transformation in the absence of activator E2Fs. Our results also suggest that TAg action on pRBs regulates both E2F-dependent and -independent pathways that govern proliferation. Thus, specific cell proliferation pathways affected by RB alterations in cancer may be a factor in tumor behavior and response to therapy.

Polyomavirus T antigens activate an antiviral state

Ectopic expression of Simian Virus 40 (SV40) large T antigen (LT) in mouse embryonic fibroblasts (MEFs) increased levels of mRNAs encoding interferon stimulated genes (ISGs). The mechanism by which T antigen increases levels of ISGs in MEFs remains unclear. We present evidence that expression of T antigen from SV40, Human Polyomaviruses BK (BKV) or JC (JCV) upregulate production of ISGs in MEFs, and subsequently result in an antiviral state, as determined by inhibition of VSV or EMCV growth. The first 136 amino acids of LT are sufficient for these activities. Furthermore, increased ISG expression and induction of the antiviral state requires STAT1. Finally, the RB binding motif of LT is necessary for activation of STAT1. We conclude that the induction of the STAT1 mediated innate immune response in MEFs is a common feature shared by SV40, BKV and JCV.