Induction of endothelial cell chemotaxis by sphingosine phosphate and stabilization of endothelial monolayer barrier function by lysophosphatidic acid, potential mediators of hematopoietic angiogenesis

Angiogenesis,the formation of new blood vessels, is an important component of restoration of hematopoiesis after BMT, but the mediators involved in he matopoietic angiogenesis have not been identified. We examined the influenc e of the lipid growth factors, phosphatidic acid (PA), lysophosphatidic aci d (LPA), and sphingosine 1-phosphate (S1P), on several angiogenic propertie s of endothelial cells, including migration and stabilization of vascular b arrier integrity. In a previous study, PA was found to disrupt the permeabi lity of established endothelial monolayers, an early event in the angiogeni c response that liberates cells for subsequent mobilization. In the present study, both PA and LPA weakly induced the chemotactic migration of endothe lial cells from an established monolayer. The chemotactic response induced by PA and LPA was similar in intensity to that observed with optimal levels of the known protein endothelial cell chemoattractants, basic fibroblast g rowth factor(bFGF) and vascular endothelial growth factor (VEGF). A markedl y greater chemotactic response was effected by nanomolar concentrations of S1P, indicating that this platelet-derived factor plays an important role i n a key aspect of angiogenesis, chemotactic migration of endothelial cells, The chemotactic response to S1P was completely inhibited by preincubation of endothelial cells with antisense oligonucleotides to the high-affinity S 1P receptor, Edg-1. In addition, chemotaxis of endothelial cells to S1P was inhibited by preincubation of cells with specific inhibitors of tryosine k inases, but inhibitors of phosphatidylinositol 3' kinase had little effect. Finally, LPA effectively stabilized endothelial monolayer barrier function , a late event in angiogenesis. Thus, the phospholipid growth factors, PA, S1P, and LPA, display divergent and potent effects on angiogenic properties of endothelial cells and giogenic differentiation of endothelial cells pot entially act in tandem to effectively induce neovascularization. These medi ators may thus exert important roles in restoration of hematopoiesis, as th ey facilitate blood vessel formation at sites of transplanted stem cells, a llowing the progeny of engrafted progenitors to move from marrow sinusoids to the peripheral vasculature.