The role of HIF-1alpha in epigenetic regulation of transcription

Abstract

The oxygen level inside cells, determine the amount of HIF protein. By directly being involved in HIF protein turnover rates, through a mechanism, by which, oxygen is utilized as a co-factor, for the PHD enzymes, regulating HIF protein stability. This allows for rapid stabilization of the HIFs and subsequent gene activation during low oxygen tensions inside cells.

I investigated the role of HIF specific epigenetic effects, in cancer cell line models, as well as primary mouse CD8+ T-lymphocytes. The data in Paper I illustrates how HIF has the ability to modulate chromatin through a HIF-1α dependent chromatin remodeling event, in hypoxia responsive gene promoters. We were able to show how, during hypoxic incubations of cancer cell line models HepG2 and SK-N-Be(2)c, hypoxia target gene promoters were remodeled, removing auxiliary factors bound to DNA. Possibly giving access to transcription factor binding. We identified two separate nucleosome free regions (NFRs) in hypoxia target genes, inducible NFRs and constitutive NFRs. The latter, were characterized by no initial remodeling event during hypoxia, conversely, the inducible NFRs required HIF-1α for remodeling to occur.

In Paper II we sought to identify the isoform specific effects of HIF-1α and HIF-2α in mouse primary CD8+ T-lymphocytes. We observed that HIF-1α was necessary for proper T-cell activation and cytotoxic function, by using mouse tumor xenograft models, showing decreased tumor size control in HIF-1α knockout T-cells. In contrast, HIF-2α knockout T-cells, activated properly, and controlled tumor size as well as control cells. VEGFA, a known HIF-1α target gene, conditional knockout in T-lymphocytes, showed increased tumor growth, in xenograft studies, as well, increased chemotherapeutic response. This finding, illustrates the complex nature of the tumor microenvironment, the role of the HIFα isoforms, as well as, VEGFA ́s role in modulating tumor vessels and the ability for chemotherapeutics to be successfully delivered.

It has been shown that T-lymphocytes accumulated 2-hydroxyglutaric acid stereoisomers R/S-2HG in a HIF-1α dependent fashion. Were S-2HG, have important biological roles for CD8+ T-cell function. In Paper III we identified the epigenetic effects of S-2HG during T-cell activation ex vivo, by following epigenetic modification changes through time. Our findings, illustrate how S-2HG metabolite decreases the overall acetylation of histone tails, at the histone 3 lysine 9 mark. The acetylation of histone proteins, has been widely studied, and is associated with increases in gene transcription. The global decreases in histone acetylation, observed by exogenous administration of S-2HG, might be a plausible mechanism for how, T-lymphocytes change their epigenome during activation.