Abstract 8401: Switching of Hif-α Isoforms is Critical in the Resolution of Inflammation

Abstract

Hypoxia is a pathological condition in which the tissue is deprived of adequate oxygen supply. It occurs in myocardial ischemia, cardiac hypertrophy and atherosclerosis, and accelerates the inflammatory processes in cardiac tissues. Each cell exerts its own responses to hypoxia, and most of them are mediated through a pair of transcription factors, hypoxia inducible factors- 1α (HIF- 1α) and HIF- 2α. Macrophages, key mediators of inflammation, are activated and accumulate in hypoxic area. Macrophages are currently classified into M1 (pro-inflammatory) and M2 (anti-inflammatory). However, the molecular mechanisms of M1 and M2-specific activations remain elusive. Here we revealed M1 and M2 specific functions of HIF-1α and HIF-2α, which play essential roles in the initiation and resolution of inflammation. HIF-1α was acutely induced in M1 activation, whereas HIF-2α was exclusively expressed in M2, but not in M1 macrophages. Th1 cytokines such as IFNγ induced HIF-1α, but strongly suppressed HIF-2α. In contrast, Th2 cytokine, including IL-4 increased HIF-2α level. We identified inducible NO synthase (iNOS, a M1 marker gene) as a HIF-1α target, and arginase1 (arg1, a M2 marker gene) as a HIF-2α target gene, respectively. These two enzymes compete for the common substrate, l-arginine, and inhibit each other's function. HIF-1α elicits NO production in M1, however, HIF-2α suppresses NO production through arginase1 induction in M2 macrophages. The balance between HIF-1α and HIF-2α determine that of iNOS/arg1, resulting in the regulation of NO production. To verify the roles of HIF-α in vivo, we generated macrophage specific HIF-1α knockout mice (mHIF-1α KO) and HIF-2α knockout (mHIF-2α KO). Intraperitoneal injection of lipopolysaccaride (LPS) induces plasma NO rises acutely (6hr), and it normalizes within 24 hours. In mHIF-1α KO mice, peak NO rise was attenuated. In contrast, mHIF-2α KO mice exhibited delayed recovery of NO level after LPS injection. Those results implied the essential role of HIF-1α in initiating NO production, whereas HIF-2α is critically important in the resolution. The switching of HIF-α isoforms could account for the functional diversity between M1 and M2, and contribute to the initiation and resolution of the inflammation.