Abstract
Hypoxia-inducible factor-1a (HIF-1a) plays a crucial role in cellular and tissue adaptation to low oxygen conditions. Although inflammatory stimuli such as lipopolysaccharide (LPS) also increase HIF-1a levels under normoxia, its transcriptional activity and regulatory mechanisms in this context remain unclear. To address this, we performed ChIP-seq and transcriptome analyses in murine macrophages stimulated with either LPS or hypoxia. Both stimuli stabilized HIF-1a protein, but via distinct mechanisms, hypoxia acted post-translationally, whereas LPS increased Hif-1a mRNA expression. Genome-wide HIF-1a binding was observed under both conditions; however, only hypoxia induced broad transcriptional activation of target genes, while LPS upregulated a restricted set, mostly glycolytic genes. Motif enrichment analysis revealed that hypoxia, but not LPS, promoted cooperative transcription factor engagement, including HIF-1b, ETS, and bZIP family members. Hypoxia also increased H3K27 acetylation at HIF-1a target loci, consistent with a transcriptionally permissive chromatin state. In contrast, LPS led to reduced H3K27ac at non-induced loci, suggesting epigenetic repression. Mechanistically, HIF-1a exhibited a phosphorylation-dependent band shift under hypoxia, but not LPS. Although both conditions showed comparable overall phosphorylation levels by Phos-tag analysis, only hypoxia triggered a conformational change, suggesting site-specific phosphorylation linked to transcriptional competence. These findings demonstrate that HIF-1a binding alone is insufficient for gene activation, and that phosphorylation and chromatin context determine its transcriptional output in a stimulus-dependent manner.
Yasutomi Higashikuni, M.D., Ph.D., FESC
Assistant Professor of Cardiovascular and Genetic Research
My research interests include homeostatic inflammation, RNA metabolism and modification, and synthetic biology.