Simian Virus 40 Infection Triggers a Balanced Network That Includes Apoptotic, Survival, and Stress Pathways

Abstract

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 (PLC␥). 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 PLC␥ 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.

Murine Polyomavirus Cell Surface Receptors Activate Distinct Signaling Pathways Required for Infection

“…Many polyomaviruses activate signaling during virus entry (7, 9, 36), yet it is unclear whether the signaling pathways required for infection are conserved across species. For example, EGFR activation is required for JCPyV infection (9), and we found that the EGFR was also activated by MuPyV.…”

“…S3D in the supplemental material). SV40, a monkey polyomavirus, requires caspase activation during entry (36). We tested whether caspases were functioning during MuPyV infection by using a caspase inhibitor, Z-VAD-FMK.…”

“…Signaling at the cell surface appears to be a critically conserved step in PyV entry, although different PyV species may utilize distinct signaling pathways for infection (5–7, 9, 36). For example, JCPyV infection of human glial cells requires activation of the EGFR and the MAPK pathway (9), whereas MuPyV also activates EGFR and MAPK but this activation is not required for infection (Fig.…”

“…S1E and S3A in the supplemental material). SV40 also induces phosphorylation of AKT after virus binding, but unlike MuPyV, activation of PI3K does not appear to be required for SV40 infection (36). Differences in signaling between PyVs may be due to the distinct cell surface receptors found on the host cells for these viruses.…”

Old foes, new understandings: nuclear entry of small non-enveloped DNA viruses

“…Moreover, western blot showed fluctuations in the levels of lamin A/C in SV40 infected cells, which are prevented by a caspase-6 inhibitor [53 ]. However, unlike parvovirus infection, caspase-6 is activated during early infection [54], and a caspase-6 inhibitor prevents the NE disruptions and viral protein expression [53 ,54]. Thus, parvoviruses and polyomaviruses disrupt the NE and the underlying nuclear lamina to enter the nucleus by a caspase-dependent mechanism, but the molecular details of this mechanism are different: while parvoviruses use 62 Virus structure and expression SV40 enters the nucleus either through the NPC or through direct penetration of the nuclear envelope from the ER.…”