Regulation of the Histone Demethylase JMJD1A by Hypoxia-Inducible Factor 1α Enhances Hypoxic Gene Expression and Tumor Growth

Cellular hypoxia occurs when the demands of growth and metabolism of a tissue surpass the vascular oxygen supply. In response to hypoxia, cells undergo specific alterations in gene expression patterns geared to promote cell survival and maintain homeostasis. This response not only is important in normal development but also is a critical part in the progression of cancers (7). Hypoxia has been implicated in activating the metabolic shift to anaerobic glycolysis, promoting the epithelial-to-mesenchymal transition (EMT), inducing the secretion of proangiogenic factors, and remodeling the extracellular matrix. Although several transcription programs are activated in response to hypoxia, the hypoxia-inducible factors (HIFs) regulate a critical repertoire of genes, making them central regulators of the cellular response to hypoxia (10, 34). The HIFs are heterodimeric transcription factors consisting of an oxygen-sensitive alpha subunit (HIF-1α, HIF-2α, or HIF-3α) and a constitutively expressed HIF-1β subunit (also known as the arylhydrocarbon nuclear translocator [ARNT]). Under conditions where oxygen concentration is not limiting, HIF-α subunits are hydroxylated by prolyl-hydroxylases, targeting them for ubiquitin-mediated degradation by the von Hippel-Lindau tumor suppressor (VHL) (18, 19). Under hypoxic conditions, HIF-α protein is stabilized, translocates to the nucleus, dimerizes with ARNT, and binds hypoxia-responsive elements (HREs) in the regulatory regions of target genes (51). HIF-1α and HIF-2α will bind the same sequences in cells but do not have completely overlapping abilities to regulate genes (5, 17, 44). Under certain conditions, HIF-3α functions as a dominant negative, antagonizing the activity of HIF-1 and HIF-2 (32). Several hundred genes are induced in response to hypoxia, and a great deal of research has been focused on identifying direct HIF target genes (34). The massive transcriptional reorganization mediated by hypoxia and HIFs suggests that changes in histone modification would create epigenetic reinforcement of this phenotype (20). HIF-1α function has been shown to influence and be influenced by histone deacetylases (22, 33), yet comparatively little is known regarding HIF-dependent dynamics of histone methylation (8, 21). In a screen for HIF-regulated changes in gene expression, we and others have identified several Jumonji C-domain-containing histone demethylase (JHDM) promoters as direct binding targets of HIF-1α and HIF-2α (3, 43, 53, 55). Histone demethylases constitute a large and diverse family of enzymes, each with a specific ability to influence transcriptional activation or repression that is dependent on the specific histone residue targeted for demethylation (46); however, the specific roles of Jumonjis in modulating transcriptional responses to hypoxia remain unknown. Using microarray analysis and chromatin immunoprecipitation, we identified HIF targets including the adrenomedullin gene (ADM) and the growth and differentiation factor 15 gene (GDF15) and show that JMJD1A reduces histone H3K9 methylation at these promoters during hypoxia. Furthermore, we demonstrate that loss of JMJD1A is sufficient to reduce tumor growth in vivo, consistent with its role in regulating histone methylation during hypoxia. These studies identify a transcriptional regulatory circuit where induction of JMJD1A by HIF-1α acts as an epigenetic signal amplifier to enhance cellular responses to hypoxia.