Signal Regulatory Protein α Ligation Induces Macrophage Nitric Oxide Production through JAK/STAT- and Phosphatidylinositol 3-Kinase/Rac1/NAPDH Oxidase/H2O2-Dependent Pathways

Abstract

Signal regulatory protein α (SIRPα) is a glycoprotein receptor that recruits and signals via the tyrosine phosphatases SHP-1 and SHP-2. In macrophages SIRPα can negatively regulate the phagocytosis of host cells and the production of tumor necrosis factor alpha. Here we provide evidence that SIRPα can also stimulate macrophage activities, in particular the production of nitric oxide (NO) and reactive oxygen species. Ligation of SIRPα by antibodies or soluble CD47 triggers inducible nitric oxide synthase expression and production of NO. This was not caused by blocking negative-regulatory SIRPα-CD47 interactions. SIRPα-induced NO production was prevented by inhibition of the tyrosine kinase JAK2. JAK2 was found to associate with SIRPα in macrophages, particularly after SIRPα ligation, and SIRPα stimulation resulted in JAK2 and STAT1 tyrosine phosphorylation. Furthermore, SIRPα-induced NO production required the generation of hydrogen peroxide (H₂O₂) by a NADPH oxidase (NOX) and the phosphatidylinositol 3-kinase (PI3-K)-dependent activation of Rac1, an intrinsic NOX component. Finally, SIRPα ligation promoted SHP-1 and SHP-2 recruitment, which was both JAK2 and PI3-K dependent. These findings demonstrate that SIRPα ligation induces macrophage NO production through the cooperative action of JAK/STAT and PI3-K/Rac1/NOX/H₂O₂ signaling pathways. Therefore, we propose that SIRPα is able to function as an activating receptor.