Vascular smooth muscle cell growth arrest on blockade of thrombospondin-1 requires p21(Cip1/WAF1)

Abnormal proliferation of vascular smooth muscle cells (VSMCs) is thought t o play an important role in the pathogenesis of atherosclerosis and resteno sis. Previous studies have implicated the extracellular matrix protein thro mbospondin-l (TSP1) in mitogen-dependent proliferation of VSMCs. In this st udy, we investigated the molecular mechanisms involved in TSP1-mediated reg ulation of VSMC growth. Neutralizing A4.1 anti-TSP1 antibody inhibited the activity of the G(1)/S cyclin-dependent kinase 2 (cdka2) and blocked the in duction of S-phase entry, which normally occurs in serum-stimulated VSMCs. This growth-inhibitory effect was associated with a marked induction of p21 (Cip1/WAF1) (p21) expression in A4.1-treated VSMCs. Moreover, addition of A 4.1 antibody to VSMCs markedly increased the level of p21 bound to cdk2. Th us growth arrest on antibody blockade of TSP1 may be mediated by the cdk in hibitory protein p21. Consistent with this notion, anti TSP1 antibody inhib ited [H-3]-thymidine incorporation in wild-type but not in p21-deficient; m ouse embryonic fibroblasts (MEFs). Together, these data suggest that p21 pl ays an important role in TSP1-mediated control of cellular proliferation.