Histone acetyltransferase 1 (HAT1) acetylates hypoxia-inducible factor 2 alpha (HIF2A) to execute hypoxia response

Hypoxic response to low oxygen levels is characteristic of most solid cancers. Hypoxia-inducible factors (HIFs) regulate cellular metabolism, survival, proliferation, and cancer stem cell growth during hypoxia. The genome-wide analysis identified HAT1, a type B histone acetyltransferase, as an upregulated and essential gene in glioblastoma (GBM). GSEA analysis of differentially regulated genes in HAT1 silenced cells identified significant depletion of “hypoxia” gene sets. Hypoxia conditions induced HIF2A, not HIF1A protein levels in glioma cells in a HAT1-dependent manner. HAT1 and HIF2A interacted with each other and occupied the promoter of VEGFA, a bonafide HIF1A/HIF2A target. Acetylation of K512 and K596 residues by HAT1 is essential for HIF2A stabilization under normoxia and hypoxia as HIF2A carrying acetylation mimic mutations at either of these residues (H512Q or K596Q) showed stable expression in HAT1 silenced cells under normoxia and hypoxia conditions. Finally, we demonstrate that the HAT1-HIF2A axis is essential for hypoxia-promoted cancer stem cell maintenance and reprogramming. Thus, our study identifies that the HAT1-dependent acetylation of HIF2A is vital to executing the hypoxia-induced cell survival and cancer stem cell growth, therefore proposing the HAT1-HIF2A axis as a potential therapeutic target.     

Zinc induces the accumulation of hypoxia-inducible factor (HIF)-1alpha, but inhibits the nuclear translocation of HIF-1beta, causing HIF-1 inactivation

The replacement of heme iron by cobalt or nickel in a putative oxygen sensor is supposed to reduce oxygen binding to the heme protein, resulting in HIF-1 activation and erythropoietin (EPO) induction. According to this hypothesis, zinc might be another example of a transition metal which is capable of stimulating EPO production. By substituting for heme iron, zinc protoporphyrin IX is produced, which has a known low oxygen affinity. However, it has been reported that zinc fails to induce EPO in normoxia, and that it suppresses EPO production in hypoxic cells. This unexpected effect of zinc on EPO production is not understood. In this study, we found that zinc induced the accumulation and nuclear translocation of hypoxia-inducible factor (HIF)-1alpha but inhibited the nuclear translocation of HIF-1beta, which inactivated HIF-1 and suppressed EPO mRNA induction in hypoxic cells.     

Hypoxia-inducible factor 1alpha (HIF-1alpha)-mediated hypoxia increases BACE1 expression and beta-amyloid generation

The incidence of Alzheimer disease (AD) and vascular dementia is greatly increased following cerebral ischemia and stroke in which hypoxic conditions occur in affected brain areas. beta-Amyloid peptide (Abeta), which is derived from the beta-amyloid precursor protein (APP) by sequential proteolytic cleavages from beta-secretase (BACE1) and presenilin-1 (PS1)/gamma-secretase, is widely believed to trigger a cascade of pathological events culminating in AD and vascular dementia. However, a direct molecular link between hypoxic insults and APP processing has yet to be established. Here, we demonstrate that acute hypoxia increases the expression and the enzymatic activity of BACE1 by up-regulating the level of BACE1 mRNA, resulting in increases in the APP C-terminal fragment-beta (betaCTF) and Abeta. Hypoxia has no effect on the level of PS1, APP, and tumor necrosis factor-alpha-converting enzyme (TACE, an enzyme known to cleave APP at the alpha-secretase cleavage site). Sequence analysis, mutagenesis, and gel shift studies revealed binding of HIF-1 to the BACE1 promoter. Overexpression of HIF-1alpha increases BACE1 mRNA and protein level, whereas down-regulation of HIF-1alpha reduced the level of BACE1. Hypoxic treatment fails to further potentiate the stimulatory effect of HIF-1alpha overexpression on BACE1 expression, suggesting that hypoxic induction of BACE1 expression is primarily mediated by HIF-1alpha. Finally, we observed significant reduction in BACE1 protein levels in the hippocampus and the cortex of HIF-1alpha conditional knock-out mice. Our results demonstrate an important role for hypoxia/HIF-1alpha in modulating the amyloidogenic processing of APP and provide a molecular mechanism for increased incidence of AD following cerebral ischemic and stroke injuries.