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Foamy viruses belong to the Spumaretrovirinae subfamily member of the Retroviridae family and produce nonpathogenic infection to hosts in the natural conditions. However, infections of foamy viruses can dramatically cause severe cytopathic effects in vitro. To date, the exact molecular mechanism has remained unclear which implied the tremendous importance of virus-host cell immune reactions. In this study, we found that the transactivator Tas in two foamy viruses isolated from Old World Monkey (OWM) induced obvious inhibition of cell proliferation via the upregulation of Foxo3a expression. It was mediated by the generation of ROS and the initiation of ER stress, and ultimately, the mitochondrial apoptosis pathway was triggered. Notably, PFV Tas contributed to the accumulation of G0/G1 phase cycle arrest induced by the activation of the p53 signaling pathway and the nuclear transportation of HDAC4 via upregulating PPM1E expression. Together, these results demonstrated the different survival strategies by which foamy virus can hijack host cell cytokines and regulate virus-host cell interactions.

Background: Foamy virus, which belong to the Spumaretrovirinae subfamily of Retroviridae, bridge the gap between Orthoretrovirinae and Hepadnaviridae, and display a long co-evolution with their hosts. Like other retroviruses, FVs encode a transactivator, Tas, which governs the levels of viral transcripts initiated through binding to the conserved promoters in 5’ long terminal repeat (LTR) and a unique internal promoter (IP). Unlike the other retrovirus, HIV, foamy viruses induced significant cytopathic effects in vitro, but has no significant disease association at the infection level in vivo. The characterization of regulator Tas in the Paradox of nonpathogenicity and cytopathic effects was still unknown.Results: Foamy virus separated from different hosts could form three groups which paralleled with the worldwide distribution of hosts due to geographical isolation. Although the physicochemical properties of different Tas were mainly in line with each other, the conserved motifs analyses still suggested divergences in protein functions. The proteins identified to interact with Tas of PFV, SFVora and SFVagm displayed that the three kinds of foamy viruses regulated different signal pathways and impacted virus-host immune interaction. Interestingly, predictions of interactional factors based on protein sequence showed USP7, a kind of deubiquitinating enzyme, could binding directly to Tas which led to its ubiquitin-dependent proteasome degradation. And this results also implyed that fomay virus could hijacked cytokine USP7 to stabilize the transcriptional activator Tas by de-ubiquitylation and thereby regulate the viral life cycle.Conclusion: Our experiments help to understand the survival strategy of foamy viruses in their hosts.