Identification of a novel antiapoptotic functional domain in simian virus 40 large T antigen

Abstract

The ability of DNA tumor virus proteins to trigger apoptosis in mammalian cells is well established. For example, transgenic expression of a simian virus 40 (SV40) T-antigen N-terminal fragment (N-termTag) is known to induce apoptosis in choroid plexus epithelial cells. SV40 T-antigen-induced apoptosis has generally been considered to be a p53-dependent event because cell death in the brain is greatly diminished in a p53 ؊/؊ background strain and is abrogated by expression of wild-type (p53-binding) SV40 T antigen. We now show that while N-termTags triggered apoptosis in rat embryo fibroblasts cultured in low serum, expression of full-length T antigens unable to bind p53 [mut (p53؊) Tags] protected against apoptosis without causing transformation. One domain essential for blocking apoptosis by T antigen was mapped to amino acids 525 to 541. This domain has >60% homology with a domain of adenovirus type 5 E1B 19K required to prevent E1A-induced apoptosis. In the context of both wild-type T antigen and mut (p53؊) Tags, mutation of two conserved amino acids in this region eliminated T antigen's antiapoptotic activity in REF-52 cells. These data suggest that SV40 T antigen contains a novel functional domain involved in preventing apoptosis independently of inactivation of p53.

Induction of hepatocyte proliferation and death by modulation of T-Antigen expression

“…Whether this represents direct activation of the apoptotic machinery by TAg or is simply a general apoptotic response to aberrant hepatocyte proliferation is unknown. There is considerable evidence for both positive and negative modulation of apoptotic sensitivity by the transforming proteins encoded by SV40 and other DNA tumor viruses (Conzen et al, 1997;Fearnhead et al, 1998;Phillips et al, 1999;Kohlhoff et al, 2000;Rodier et al, 2000;Tsai et al, 2000;Cole and Tevethia, 2002). These proteins can sensitize cells to apoptosis mediated by both the death-receptor (extrinsic) pathway (Phillips et al, 1999) via activation of caspase 8 (Sartorius et al, 2001), and the intrinsic pathway (Cole and Tevethia, 2002;Fearnhead et al, 1998) via an Apaf-1/cytochromec/caspase-9-dependent mechanism (Li et al, 1997;Zou et al, 1999).…”

“…Although many of the mechanisms involved in mediating TAg's stimulation of both the cell-cycle and the transcriptional machinery have been elucidated (Moens et al, 1997;Saenz-Robles et al, 2001), its modulation of apoptosis is less clear. TAg can either enhance (Tzeng et al, 1998;Kohlhoff et al, 2000;Cole and Tevethia, 2002) or inhibit (Rouquet et al, 1995;Conzen et al, 1997;Tsai et al, 2000) apoptosis, indicating that its ability to modify apoptotic sensitivity is complex and may depend on the initiating death stimulus, cellular context and the apoptotic pathway(s) utilized by specific cell types.…”