HEXIM1 modulates vascular endothelial growth factor expression and function in breast epithelial cells and mammary gland

Abstract

Recently, we found that mutation of the C-terminus of transcription factor Hexamethylene bisacetamide inducible protein 1 (HEXIM1) in mice leads to abnormalities in cardiovascular development due to aberrant vascular endothelial growth factor (VEGF) expression. HEXIM1 regulation of some genes has also been shown to be positive transcription elongation factor b (P-TEFb)-dependent. However, it is not known whether HEXIM1 regulates VEGF in the mammary gland. We demonstrate that HEXIM1 regulates estrogen-induced VEGF transcription via inhibition of Estrogen Receptor alpha recruitment to the VEGF promoter in a P-TEFb-independent manner in MCF-7 cells. Under hypoxic conditions, HEXIM1 inhibits estrogen-induced Hypoxia-inducible factor-1 alpha (HIF-1α) protein expression and recruitment of HIF-1α to the hypoxia response element in the VEGF promoter. In the mouse mammary gland, increased HEXIM1 expression decreased estrogen-driven VEGF and HIF-1α expression. Conversely, a mutation in the C-terminus of HEXIM1 (HEXIM11-312) led to increased VEGF and HIF-1α expression and vascularization in mammary glands of heterozygous HEXIM11-312 mice when compared to their wild-type littermates. Additionally, HEXIM11-312 mice have a higher incidence of carcinogen-induced mammary tumors with increased vascularization, suggesting an inhibitory role for HEXIM1 during angiogenesis. Taken together, our data provide evidence to suggest a novel role for HEXIM1 in cancer progression.

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

“…Reflecting what we observed in our previous mouse studies [16], modulation of HEXIM1 levels resulted in alterations in HIF-1 α protein levels in hypoxia-treated MCF7 cells and MDA-MB-231 cells (Figures 1A, 1B and 1C). Up-regulation of HIF-1 α protein levels was observed using two different miRNA clones (Figure 1A).…”

“…(Manassas, VA, U.S.A.) and maintained as previously described [15]. MDA-MB-231 cells were transfected with control vector or expression vector for FLAG–HEXIM1 as described previously [16]. RCC4 cells (pVHL-deficient or transfected with wild-type pVHL, see [17]) were maintained as previously described [18].…”

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

“…Recently, HEXIM1 has been shown as an inhibitor of tumor angiogenesis via down-regulation of HIF-1a protein expression and inhibition of HIF-1a-mediated transcription [17,18]. The mechanistic basis for regulation of HIF-1a by HEXIM1 has been considered that HEXIM1 down-regulates HIF-1a protein stability [19].…”

Structure and Dynamics of a Stabilized Coiled-Coil Domain in the P-TEFb Regulator Hexim1

“…25 Likewise, estrogen-induced vascular endothelial growth factor transcription is regulated through Hexim1. 26 Together with the stimulation of viral gene expression by formation of a multifunctional transcription elongation complex including Tat, 27,28 these misregulations result in a multitude of cellular effects that involve Hexim proteins in human diseases.…”