Jembrana disease virus Tat can regulate human immunodeficiency virus (HIV) long terminal repeat-directed gene expression and can substitute for HIV Tat in viral replication

Jembrana disease virus (JDV) is a bovine lentivirus genetically similar to bovine immunodeficiency virus; it causes an acute and sometimes fatal disease in infected animals. This virus carries a very potent Tat that can strongly activate not only its own long terminal repeat (LTR) but also the human immunodeficiency virus (HIV) LTR. In contrast, HIV Tat cannot reciprocally activate the JDV LTR (H. Chen, G. E. Wilcox, G. Kertayadnya, and C. Wood, J. Virol. 73:658-666, 1999). This indicates that in transactivation JDV Tat may utilize a mechanism similar to but not the same as that of the HIV Tat. To further study the similarity of JDV and HIV tat in transactivation, we first tested the responses of a series of HIV LTR mutants to the JDV Tat. Cross-transactivation of HIV LTR by JDV Tat was impaired by mutations that disrupted the HIV type 1 transactivation response element (TAR) RNA stem-loop structure. Our results demonstrated that JDV Tat, like HIV Tat, transactivated the HIV LTR at least partially in a TAR-dependent manner. However, the sequence in the loop region of TAR was not as critical for the function of JDV Tat as it was for HIV Tat. The competitive inhibition of Tat-induced transactivation by the truncated JDV or HIV Tat, which consisted only of the activation domain, suggested that similar cellular factors were involved in both JDV and HIV Tat-induced transactivation. Based on the one-round transfection assay with HIV tat mutant proviruses, the cotransfected JDV tat plasmid can functionally complement the HIV tat defect. To further characterize the effect of JDV Tat on HIV, a stable chimeric HIV carrying the JDV tat gene was generated. This chimeric HIV replicated in a T-cell line, C8166, and in peripheral blood mononuclear cells, which suggested that JDV Tat can functionally substitute for HIV Tat. Further characterization of this chimeric virus will help to elucidate how JDV Tat functions and to explain the differences between HIV and JDV Tat transactivation.     

Epigenetic silencing of HIV-1 by the histone H3 lysine 27 methyltransferase enhancer of Zeste 2

Latent HIV proviruses are silenced as the result of deacetylation and methylation of histones located at the viral long terminal repeat (LTR). Inhibition of histone deacetylases (HDACs) leads to the reemergence of HIV-1 from latency, but the contribution of histone lysine methyltransferases (HKMTs) to maintaining HIV latency remains uncertain. Chromatin immunoprecipitation experiments using latently infected Jurkat T-cell lines demonstrated that the HKMT enhancer of Zeste 2 (EZH2) was present at high levels at the LTR of silenced HIV proviruses and was rapidly displaced following proviral reactivation. Knockdown of EZH2, a key component of the Polycomb repressive complex 2 (PRC2) silencing machinery, and the enzyme which is required for trimethyl histone lysine 27 (H3K27me3) synthesis induced up to 40% of the latent HIV proviruses. In contrast, there was less than 5% induction of latent proviruses following knockdown of SUV39H1, which is required for H3K9me3 synthesis. Knockdown of EZH2 also sensitized latent proviruses to external stimuli, such as T-cell receptor stimulation, and slowed the reversion of reactivated proviruses to latency. Similarly, cell populations that responded poorly to external stimuli carried HIV proviruses that were enriched in H3K27me3 and relatively depleted in H3K9me3. Treating latently infected cells with the HKMT inhibitor 3-deazaneplanocin A, which targets EZH2, led to the reactivation of silenced proviruses, whereas chaetocin and BIX01294 showed only minimal reactivation activities. These findings suggest that PRC2-mediated silencing is an important feature of HIV latency and that inhibitors of histone methylation may play a useful role in induction strategies designed to eradicate latent HIV pools.