Simian virus 40 infection triggers a balanced network that includes apoptotic, survival, and stress pathways

The infection process by simian virus 40 (SV40) and entry of its genome into nondividing cells are only partly understood. Infection begins by binding to GM1 receptors at the cell surface, cellular entry via caveolar invaginations, and trafficking to the endoplasmic reticulum, where the virus disassembles. To gain a deeper insight into the contribution of host functions to this process, we studied cellular signaling elicited by the infecting virus. Signaling proteins were detected by Western blotting and immunofluorescence staining. The study was assisted by a preliminary proteomic screen. The contribution of signaling proteins to the infection process was evaluated using specific inhibitors. We found that CV-1 cells respond to SV40 infection by activating poly(ADP-ribose) polymerase 1 (PARP-1)-mediated apoptotic signaling, which is arrested by the Akt-1 survival pathway and stress response. A single key regulator orchestrating the three pathways is phospholipase C-gamma (PLCgamma). The counteracting apoptotic and survival pathways are robustly balanced as the infected cells neither undergo apoptosis nor proliferate. Surprisingly, we have found that the apoptotic pathway, including activation of PARP-1 and caspases, is absolutely required for the infection to proceed. Thus, SV40 hijacks the host defense to promote its infection. Activities of PLCgamma and Akt-1 are also required, and their inhibition abrogates the infection. Notably, this signaling network is activated hours before T antigen is expressed. Experiments with recombinant empty capsids, devoid of DNA, indicated that the major capsid protein VP1 alone triggers this early signaling network. The emerging robust signaling network reflects a delicate evolutionary balance between attack and defense in the host-virus relationship.