Thrombospondin‐1 (TSP1) Impairs Vasorelaxation via Signal Regulatory Protein (SIRP)‐α‐Mediated Activation of NADPH Oxidase 1 (NOX1)

Abstract

The secreted matricellular protein TSP1 acutely controls vascular tone by inhibiting relaxation of vascular smooth muscle cells (VSMC). However, the exact mechanism involved remains unknown. TSP1 is the soluble ligand for the cell surface receptor CD47, whereas CD47 has been reported to be a ligand for signal inhibitor regulatory protein SIRP‐α. Both CD47 and SIRP‐α are expressed in VSMC and SIRP‐α was associated with reactive oxygen species (ROS) production in macrophages. This suggested that TSP1, via SIRP‐α, inhibits vasodilation by promoting ROS production in VSMC. We herein show that TSP1 binds to SIRP‐α and activates its intracellular signal transducer Src homology 2 domain‐containing tyrosine phosphatase‐1 (SHP‐1). Knocking down CD47 or antibody blockade of CD47 in VSMC does not alter TSP1‐mediated activation of SIRP‐α. TSP1 phosphorylates the p47phox subunit of Nox1 and increases O2•− production in VSMC, which is decreased following siRNA silencing of SIRP‐α. In functional bioassays TSP1, in a SIRP‐α‐dependent manner, inhibits vasorelaxation through Nox1‐stimulated O2•− production. These data identify the TSP1‐SIRP‐α signaling axis as a novel activator of Nox1‐derived O2•− in VSMC for inhibition of arterial vasodilation, and further suggest that targeting SIRP‐α may offer therapeutic potential to mitigate pathologic ROS and enhance tissue blood flow in cardiovascular disease.