Phosphatidylinositol 3-kinase and focal adhesion kinase are early signals in the growth factor-like responses to thrombospondin-1 seen in human vascular smooth muscle.

Thrombospondin-1 (TSP-1) is a matricellular protein that is expressed in negligible amounts in normal blood vessels but is markedly upregulated in vascular injury. Although TSP-1 can act as a pleiotropic regulator for human vascular smooth muscle cells (HVSMCs), the intracellular signaling pathways stimulated by this protein remain obscure. In cultured HVSMCs derived from saphenous vein, TSP-1 induces tyrosine phosphorylation of a number of cellular proteins, with a complex temporal pattern of activation. Immunoprecipitation techniques have identified the early tyrosine-phosphorylated signals as being the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI 3-K) and focal adhesion kinase (FAK). Tyrosine phosphorylation of the p85 subunit of PI 3-K showed a biphasic response to TSP-1 stimulation, which corresponded to a biphasic activation of the lipid kinase. Treatment with both wortmannin and LY294002 inhibited PI 3-K activity of HVSMCs but did not affect tyrosine phosphorylation of the p85 regulatory subunit. TSP-1-stimulated FAK phosphorylation, however, was substantially reduced by these inhibitors, as was the TSP-1-induced chemotaxis of these cells. These results suggest that activation of PI 3-K is an early signal induced by TSP-1 and is critical for chemotaxis. Activation of this kinase precedes and may occur upstream from FAK phosphorylation, although the nature of the interaction between these 2 enzymes remains obscure.