HEXIM1 down-regulates hypoxia-inducible factor-1α protein stability

Abstract

We have previously reported on the inhibition of HIF-1α (hypoxia-inducible factor α)-regulated pathways by HEXIM1 [HMBA (hexamethylene-bis-acetamide)-inducible protein 1]. Disruption of HEXIM1 activity in a knock-in mouse model expressing a mutant HEXIM1 protein resulted in increased susceptibility to the development of mammary tumours, partly by up-regulation of VEGF (vascular endothelial growth factor) expression, HIF-1α expression and aberrant vascularization. We now report on the mechanistic basis for HEXIM1 regulation of HIF-1α. We observed direct interaction between HIF-1α and HEXIM1, and HEXIM1 up-regulated hydroxylation of HIF-1α, resulting in the induction of the interaction of HIF-1α with pVHL (von Hippel–Lindau protein) and ubiquitination of HIF-1α. The up-regulation of hydroxylation involves HEXIM1-mediated induction of PHD3 (prolyl hydroxylase 3) expression and interaction of PHD3 with HIF-1α. Acetylation of HIF-1α has been proposed to result in increased interaction of HIF-1α with pVHL and induced pVHL-mediated ubiquitination, which leads to the proteasomal degradation of HIF-1α. HEXIM1 also attenuated the interaction of HIF-1α with HDAC1 (histone deacetylase 1), resulting in acetylation of HIF-1α. The consequence of HEXIM1 down-regulation of HIF-1α protein expression is attenuated expression of HIF-1α target genes in addition to VEGF and inhibition of HIF-1α-regulated cell invasion.

Induction of HEXIM1 activities by HMBA derivative 4a1: Functional consequences and mechanism

“…These studies should also provide mechanistic basis for the phenotypic effects of 4a1. We focused on direct targets of HEXIM1 that we have identified [9, 10, 13, 27]. 4a1 and HMBA regulated expression of cyclin D1, c-Myc, HIF-1α, and KDM5B, (Figures 5A and 5B), which we have previously determined to be directly regulated by HEXIM1 [9, 13, 32], and play critical roles in mammary tumorigenesis, angiogenesis, and metastasis.…”

“…Following blasticidin selection, cells expressing the highest level of GFP were flow-sorted and expanded. MDA-MB-231 cells were transfected with control or expression vector for Flag-tagged HEXIM1 as previously described [27]. …”

Stress from Nucleotide Depletion Activates the Transcriptional Regulator HEXIM1 to Suppress Melanoma

“…HMBA induces differentiation in a variety of cancer cell lines and primary human cancer cell cultures (Marks and Rifkind, 1989). HEXIM1 upregulation is correlated with the transition of cancer cells from proliferation to differentiation, and has been studied in breast cancer (Ketchart et al, 2013; Wittmann et al, 2003, 2005; Yeh et al, 2013) and prostate cancer (Mascareno et al, 2012). 7SK snRNP member LARP7 is mutated in gastric cancer (He et al, 2008; Mori et al, 2002).…”

Down-regulation of hypoxia-inducible factor-1 alpha and vascular endothelial growth factor by HEXIM1 attenuates myocardial angiogenesis in hypoxic mice

“…In line with our results, the levels of HIF-1a protein was reported to be increased in HEXIM1+/À heterozygous hearts subjected ischaemic stress and that down-regulation of HIF-1a protein by HEXIM1 allows not only for inhibition of VEGF-regulated angiogenesis, but also for inhibition of migration and invasion of breast cancer cells [18,31]. Recently, HEXIM1 was shown to down-regulate HIF-1a protein stability by direct interaction with HIF-1a and up-regulation of hydroxylation of HIF-1a, resulting in the induction of the interaction of HIF-1a with pVHL (von Hippel-Lindau protein) and ubiquitination of HIF-1a [19]. On the other hand, HEXIM1 was suggested to interact with HIF-1a and increase HIF-1a protein expression in H9C2 cells [20].…”

“…The mechanistic basis for regulation of HIF-1a by HEXIM1 has been considered that HEXIM1 down-regulates HIF-1a protein stability [19]. On the other hand, HEXIM1 re-expression in the heart results in the induction of angiogenesis that allows physiological hypertrophy via activation of several transcription factors including HIF-1a [20].…”