Thrombospondin-1 Inhibition of Vascular Smooth Muscle Cell Responses Occurs via Modulation of Both cAMP and cGMP

Nitric oxide (NO) drives pro-survival responses in vascular cells and limits platelet adhesion, enhancing blood flow and minimizing thrombosis. The matricellular protein thrombospondin-1 (TSP1), through interaction with its receptor CD47, inhibits soluble guanylyl cyclase (sGC) activation by NO in vascular cells. In vascular smooth muscle cells (VSMCs) both intracellular cGMP and cAMP regulate adhesion, contractility, proliferation, and migration. cGMP can regulate cAMP through feedback control of hydrolysis. Inhibition of the cAMP phosphodiesterase-4 selectively interfered with the ability of exogenous TSP1 to block NO-driven VSMC adhesion but not cGMP accumulation, suggesting that cAMP also contributes to VSMC regulation by TSP1. Inhibition of phosphodiesterase-4 was sufficient to elevate cAMP levels, and inhibiting guanylyl cyclase or phosphodiesterase-3, or adding exogenous TSP1 reversed this increase in cAMP. Thus, TSP1 regulates VSMC cAMP levels in part via cGMP-dependent inhibition of phosphodiesterase-3. Additionally basal cAMP levels were consistently elevated in both VSMCs and skeletal muscle from TSP1 null mice, and treating null cells with exogenous TSP1 suppressed cAMP levels to those of wild type cells. TSP1 inhibited both forskolin and isoprotereno