Abstract
By phosphorylating elongation factors and the C-terminal domain of RNA polymerase II, the positive transcription elongation factor b (P-TEFb) is the critical kinase for transcription elongation and co-transcriptional processing of eukaryotic genes. It exists in inactive small nuclear ribonucleoprotein (7SK snRNP) and active (free P-TEFb) complexes in cells. The P-TEFb equilibrium determines the state of cellular activation, proliferation, and differentiation. Free P-TEFb, which is required for growth, can be recruited to RNA polymerase II via transcription factors, BRD4, or the super elongation complex (SEC). UV light, various signaling cascades, transcriptional blockade, or compounds such as hexamethylene bisacetamide (HMBA), suberoylanilide hydroxamic acid (SAHA), and other histone deacetylase inhibitors lead to a rapid release of free P-TEFb, followed by its reassembly into the 7SK snRNP. As a consequence, transcription of HEXIM1, a critical 7SK snRNP subunit, and HIV is induced. In this study, we found that a bromodomain and extra-terminal (BET) bromodomain inhibitor, JQ1, which inhibits BRD4 by blocking its association with chromatin, also leads to the rapid release of free P-TEFb from the 7SK snRNP. Indeed, JQ1 transiently increased levels of free P-TEFb and BRD4·P-TEFb and SEC·P-TEFb complexes in cells. As a consequence, the levels of HEXIM1 and HIV proteins rose. Importantly, the knockdown of ELL2, a subunit of the SEC, blocked the ability of JQ1 to increase HIV transcription. Finally, the effects of JQ1 and HMBA or SAHA on the P-TEFb equilibrium were cooperative. We conclude that HMBA, SAHA, and JQ1 affect transcription elongation by a similar and convergent mechanism.
Highlights
positive transcription elongation factor b (P-TEFb) regulates transcription elongation, cell growth, and proliferation
We studied the effects of hexamethylene bisacetamide (HMBA) and the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) on HIV transcription in transformed and primary cells, with special emphasis on proviral latency and reactivation [15,16,17]. Both compounds released P-TEFb from the 7SK small nuclear ribonucleoprotein (snRNP) and potently induced HIV gene expression when the viral LTR was not occluded by transcriptional interference [15,16,17]
These results indicate that the bromodomain and extra-terminal (BET) bromodomain inhibition by JQ1 activates HIV transcription, which is reminiscent of other HIV activators such as SAHA and HMBA [15,16,17]
Summary
P-TEFb regulates transcription elongation, cell growth, and proliferation. Results: BET bromodomain inhibition by JQ1 transiently releases free P-TEFb from the inactive 7SK snRNP, activating HEXIM1 and HIV transcription. By phosphorylating elongation factors and the C-terminal domain of RNA polymerase II, the positive transcription elongation factor b (P-TEFb) is the critical kinase for transcription elongation and co-transcriptional processing of eukaryotic genes It exists in inactive small nuclear ribonucleoprotein (7SK snRNP) and active (free P-TEFb) complexes in cells. We studied the effects of hexamethylene bisacetamide (HMBA) and the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) on HIV transcription in transformed and primary cells, with special emphasis on proviral latency and reactivation [15,16,17] Both compounds released P-TEFb from the 7SK snRNP and potently induced HIV gene expression when the viral LTR was not occluded by transcriptional interference (integration into active genes) [15,16,17]. Its effects on HIV transcription could be inhibited by depletion of ELL2, a subunit of the SEC [20]
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