Macrophage Hif-2 Alpha Regulates Tumor-Suppressive Spint1 In The Tumor Microenvironment

In solid tumors, tumor-associated macrophages (TAMs) commonly accumulate within hypoxic areas. Adaptations to such environments evoke transcriptional changes by the hypoxia-inducible factors (HIFs). While HIF-1 alpha is ubiquitously expressed, HIF-2 alpha appears tissue-specific with consequences of HIF-2 alpha expression in TAMs only being poorly characterized. An E0771 allograft breast tumor model revealed faster tumor growth in myeloid HIF-2 alpha knockout (HIF-2 alpha(LysM-/-)) compared with wildtype (wt) mice. In an RNA-sequencing approach of FACS sorted wt and HIF-2 alpha(LysM-/-) TAMs, serine protease inhibitor, Kunitz type-1 (Spint1) emerged as a promising candidate for HIF-2 alpha-dependent regulation. We validated reduced Spint1 messenger RNA expression and concomitant Spint1 protein secretion under hypoxia in HIF-2 alpha-deficient bone marrow-derived macrophages (BMDMs) compared with wt BMDMs. In line with the physiological function of Spint1 as an inhibitor of hepatocyte growth factor (HGF) activation, supernatants of hypoxic HIF-2 alpha knockout BMDMs, not containing Spint1, were able to release proliferative properties of inactive pro-HGF on breast tumor cells. In contrast, hypoxic wt BMDM supernatants containing abundant Spint1 amounts failed to do so. We propose that Spint1 contributes to the tumor-suppressive function of HIF-2 alpha in TAMs in breast tumor development.