Release of Positive Transcription Elongation Factor b (P-TEFb) from 7SK Small Nuclear Ribonucleoprotein (snRNP) Activates Hexamethylene Bisacetamide-inducible Protein (HEXIM1) Transcription

Abstract

Background: HEXIM1 inhibits P-TEFb in the 7SK snRNP. Results: The critical promoter driving HEXIM1 expression was identified and was found to respond to levels of free P-TEFb. Conclusion: HEXIM1 expression levels rise to compensate for excessive P-TEFb activity. Significance: Increased levels of free P-TEFb lead to induction of HEXIM1 to maintain transcriptional homeostasis.

Visualization of Positive Transcription Elongation Factor b (P-TEFb) Activation in Living Cells

“…As shown Fig. 5C, AzaC stimulated luciferase activity by 5-fold, which is similar to levels achieved by HMBA (37). These results were consistent with AzaC-increased BiFC signals in the V-PAC assay (Fig.…”

“…Components of the 7SK snRNP also play critical roles in gene expression during stem cell differentiation and in cancer (43)(44)(45)(46)(47)(48)(49). Moreover, increased P-TEFb activity and the subsequent synthesis of its inhibitor HEXIM1 represent critical common steps for many anti-cancer and anti-inflammatory drugs (27,37,50). Importantly, it is this reassembly of the 7SK snRNP following the increased synthesis of HEXIM1 that causes proliferating cells to differentiate and rapidly dividing cells to arrest and undergo apoptosis.…”

“…The effects of AzaC on P-TEFb-dependent transcription were tested further by reporter assays. Previously, we demonstrated that the HEXIM1 promoter is most responsive to active free P-TEFb; the P-TEFb-responsive region contains 104 nucleotides upstream of the transcription start site (37). This HEXIM1 promoter, HexP(104) was linked to the luciferase reporter gene and introduced stably into HeLa cells to establish HeLa/HexP.Luc cells.…”

Transcriptional interference by RNA polymerase III affects expression of the Polr3e gene

“…1A; see also Canella et al 2012). ChIP-seq data from HeLa cells (Liu et al 2014) show DSIF accumulation near both the TSS and the MIR but NELF accumulation only near the TSS, arguing against Pol II accumulation at the MIR resulting from a second, unannotated TSS in this region (Fig. 2).…”

Molecular mechanisms of HIV latency

“…RNA-associated HEXIMs then bind and inhibit P-TEFb in the 7SK snRNP. Notably, the rapid release of P-TEFb from the 7SK snRNP also increases the synthesis of HEXIM1, which reassembles the 7SK snRNP and sequesters free P-TEFb (64). The ratio of free and bound P-TEFb (the P-TEFb equilibrium) then determines the state of cell growth, proliferation, and differentiation (65).…”