Role of HIV-1 Vpr in regulation of expression of unintegrated viral DNA
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Vpr, one of the four accessory proteins in HIV-1, is expressed in all primate lentiviruses, and has been shown to facilitate viral replication through degradation of various host factors in a DCAFCrl4 E3 ubiquitin ligase dependent manner and induce a G2/M cell cycle arrest. But the underlying mechanism of Vpr-dependent replication enhancement has remained unclear. Previous studies from our laboratory and in the literature have demonstrated the ability of HIV-1 Vpr to mediate enhanced gene expression from the viral long terminal repeat (LTR) in a DCAFCrl4-dependent manner. In recently published studies, Vpx, a homolog of Vpr, expressed by HIV-2/SIVmac/SIVsm lineage of lentiviruses has been shown to enhance proviral gene expression through degradation of components in Human Silencing Hub (HUSH) complex by hijacking the DCAF1 mediated ubiquitin proteasome pathway. The aim of this study is to determine if HIV-1 Vpr has the same role as Vpx in promoting viral expression through degradation of HUSH complex components. I specifically focused on the role of Vpr in mediating enhanced viral gene expression from unintegrated viral DNA. Unintegrated viral DNA can be present as 1-LTR or 2-LTR circles when viral cDNA integration is blocked by use of integrase inhibitors, such as Raltegravir, a commonly used drug in HIV treatment regimens. To assess Vpr’s role, I created HeLa cell lines in which expression of HUSH components, including the upstream SETDB1 protein, was diminished by use of targeted shRNAs. After assessing the knockdown efficiency using RT-qPCR, the cell lines are infected with luciferase expressing Vpr null (ΔVpr) or Wildtype (WT) HIV-1 that are normal or defective for viral DNA integration capability. Measurement of luciferase activity in infected cell lysates provided a quantitative measure of expression from viral DNA. Expression from unintegrated viral DNA was attenuated in the absence of Vpr compared to WT HIV-1. Furthermore, knockdown of HUSH complex subunits expression did not rescue luciferase reporter gene expression from unintegrated viral DNA in Vpr-null virus infections, suggesting that Vpr-mediated enhancement of viral gene expression from unintegrated viral DNA is not dependent on targeting HUSH complex functions. Future studies will be needed to fully understand the mechanism by which Vpr facilitates viral gene expression from unintegrated viral DNA.