Acute hypoxia affects P-TEFb through HDAC3 and HEXIM1-dependent mechanism to promote gene-specific transcriptional repression.

Hypoxia is associated with a variety of physiological and pathological conditions and elicits specific transcriptional responses. The elongation competence of RNA Polymerase II is regulated by the positive transcription elongation factor b (P-TEFb)-dependent phosphorylation of Ser2 residues on its C-terminal domain. Here, we report that hypoxia inhibits transcription at the level of elongation. The mechanism involves enhanced formation of inactive complex of P-TEFb with its inhibitor HEXIM1 in an HDAC3-dependent manner. Microarray transcriptome profiling of hypoxia primary response genes identified ∼79% of these genes being HEXIM1-dependent. Hypoxic repression of P-TEFb was associated with reduced acetylation of its Cdk9 and Cyclin T1 subunits. Hypoxia caused nuclear translocation and co-localization of the Cdk9 and HDAC3/N-CoR repressor complex. We demonstrated that the described mechanism is involved in hypoxic repression of the monocyte chemoattractant protein-1 (MCP-1) gene. Thus, HEXIM1 and HDAC-dependent deacetylation of Cdk9 and Cyclin T1 in response to hypoxia signalling alters the P-TEFb functional equilibrium, resulting in repression of transcription.

Protein kinase CK2 is a key activator of histone deacetylase in hypoxia-associated tumors.

Increasing evidence points to a link between histone deacetylases (HDACs) and tumorigenesis. Although several HDAC inhibitors have been tested in clinical trials for cancer therapies, the mechanisms of HDAC activation in tumors remain unknown. In this study, we investigated the pathway of HDAC activation in the context of hypoxia and inflammation, common features of solid tumors. In HeLa cells, hypoxia was a more potent activator of HDAC than IL-1beta. As HDAC protein expression did not change during treatment, we hypothesized that hypoxia regulated HDAC activity through post-translational modification. We observed that hypoxia induced HDAC1 and HDAC2 protein phosphorylation both in the presence and absence of IL-1beta. Using TBB, an inhibitor of protein kinase CK2, we showed that CK2 was required for hypoxia-induced HDAC activation. We also observed that CK2 activity was induced by hypoxia but not by IL-1beta alone. While CK2beta subunits were retained in the cytoplasm upon hypoxic treatment, CK2alpha and CK2alpha' subunits were shuttled to the nucleus, where HDAC1 and HDAC2 are predominantly localized. Knockdown of catalytic and regulatory subunits of CK2 revealed that formation of heterotetramic complex was not required for HDAC phosphorylation. von Hippel-Lindau protein (pVHL) inactivation and hypoxia inducible factor-1alpha (HIF-1alpha) activation are associated with tumor growth and vasculogenesis. Use of Apicidin (an HDAC inhibitor) and TBB revealed that CK2-dependent HDAC activation contributed to pVHL downregulation and HIF-1alpha stabilization under hypoxia. Our findings that CK2 may be a key mediator for HDAC activation under hypoxia support the future application of CK2 inhibitors in cancer therapy.