The Expression of Human Endogenous Retroviruses is modulated by the Tat protein of HIV-1.

Abstract

Human Endogenous Retroviruses (HERVs) make up 8% of the human genome. Expression of HERV-K (HML-2), the family of HERVs that most recently entered the genome, is tightly regulated but becomes markedly increased following infection with HIV-1. To better understand the mechanisms involved in this activation, we explored the role of the HIV-1 Tat protein in inducing expression of these endogenous retroviral genes. Tat is a transcriptional activator that can affect both viral and cellular genes in addition to being secreted from infected cells and taken up by neighboring cells. Administration of Tat by transfection of an expression vector or by addition of recombinant protein caused a marked increase in HERV-K (HML-2) Gag expression in Jurkat T cells and in primary lymphocytes. Transcript levels from the HERV-K (HML-2) rec and np9 oncogenes were also increased. Luciferase reporter gene assays demonstrated that the effect of Tat on HERV-K (HML-2) expression occurred at the level of the transcriptional promoter, with a contribution from the transcription factors NF-kappaB and NF-AT as shown by chromatin immunoprecipitation assays, mutational analysis of the HERV-K (HML-2) LTR, and treatment with agents that inhibit NF-kappaB or NF-AT activation. Additionally, through next generation sequencing technology and subsequent qRT-PCR validation analyses, we observed that Tat treatment activates the expression of 24 unique viruses out of the 91 annotated HERV-K (HML-2) proviruses, while it silences 28. Interestingly, some activated proviruses lack 5’-LTRs, or have partial 5’-LTRs missing promoter and enhancer sequences. This observation points towards alternative mechanisms of HERV-K (HML-2) provirus expression, such as antisense transcription, for some of the proviruses. Additionally, most Tat-activated and silenced proviruses are Type 2, coding for mostly Rec, with a few activated Type 1 proviruses potentially coding for Np9. These data are consistent with the dual effect Tat has been shown to have on some cellular genes, for which it behaves as both an activator and a suppressor, as well as further demonstrate the complexity of the genome-wide regulation of HERV-K (HML-2) expression. HIV-1 Tat, thus, plays an important role in activating expression of HERV-K (HML-2) in the setting of HIV-1 infection.