Chemokine-Coupled beta 2Integrin-Dependent mRNA Stabilization in Macrophages Modulates VEGF-A Protein Levels During Ischemia-Induced Arteriogenesis

VEGF-A directs both developmental and inducible arteriogenesis through the maturation of smaller vessels and the recruitment of collateral networks. Our lab has demonstrated that integrin-induced mRNA stability plays an important role in macrophage production of VEGF-A protein through translocation and activation of the mRNA stabilizing protein, HuR. It is well known that macrophages may serve as a source of VEGF-A; however, it remains unclear whether macrophages are the principle source of VEGF-A during ischemia-induced arteriogenesis and whether this integrin signaling mechanism modulates VEGF-A protein levels in ischemic muscle tissue in vivo. Here we demonstrate that chemokine-coupled β2 integrin adhesion signaling results in the formation of a complex between the low molecular weight G-protein, Rac2, and the heavy chain of Nonmuscle Myosin IIA (MyH9), leading to translocation of the message stabilizing protein, HuR, and the subsequent prolongation of VEGF-A mRNA half-life. Macrophage-specific gene deletion of MyH9 reduces chemokine-coupled β2 integrin adhesion-stimulated HuR translocation and consequent stabilization of VEGF-A message. Moreover, during hindlimb ischemia, there is global loss of VEGF-A protein in the arteriogenic muscle tissue along with decreased perfusion recovery secondary to decreased small vessel arteriogenesis in the macrophage-specific MyH9-/- mice. These defects appear specific to the induced inflammatory response as these mice demonstrate adequate capillary angiogenic, arteriogenic and arteriolar collateral development at baseline. This chemokine-coupled, adhesion-based signaling pathway provides a unique, rapid molecular switch in gene expression whereby inflammatory monocyte-derived macrophages become the key in vivo source of VEGF-A protein during ischemia-induced arteriogenesis. We anticipate this pathway to be a novel target for the therapeutic induction or inhibition of VEGF-A-mediated arteriogenesis where clinically relevant.