Pro-inflammatory cytokines activate hypoxia-inducible factor 3α via epigenetic changes in mesenchymal stromal/stem cells

Human mesenchymal stromal/stem cells (hMSCs) emerged as a promising therapeutic tool for ischemic disorders, due to their ability to regenerate damaged tissues, promote angiogenesis and reduce inflammation, leading to encouraging, but still limited results. The outcomes in clinical trials exploring hMSC therapy are influenced by low cell retention and survival in affected tissues, partially influenced by lesion's microenvironment, where low oxygen conditions (i.e. hypoxia) and inflammation coexist. Hypoxia and inflammation are pathophysiological stresses, sharing common activators, such as hypoxia-inducible factors (HIFs) and NF-kappa B. HIF1 alpha and HIF2 alpha respond essentially to hypoxia, activating pathways involved in tissue repair. Little is known about the regulation of HIF3 alpha. Here we investigated the role of HIF3 alpha in vitro and in vivo. Human MSCs expressed HIF3 alpha, differentially regulated by pro-inflammatory cytokines in an oxygen-independent manner, a novel and still uncharacterized mechanism, where NF-kappa B is critical for its expression. We investigated if epigenetic modifications are involved in HIF3 alpha expression by methylation-specific PCR and histone modifications. Robust hypermethylation of histone H3 was observed across HIF3A locus driven by pro-inflammatory cytokines. Experiments in a murine model of arteriotomy highlighted the activation of Hif3 alpha expression in infiltrated inflammatory cells, suggesting a new role for Hif3 alpha in inflammation in vivo.