Integrin Receptor-mediated Endocytosis of Vitronectin Is Protein Kinase C-dependent (*)

Abstract

Previous studies have demonstrated that the α_vβ₅ integrin receptor functions in the endocytosis and degradation of matrix-bound vitronectin by human skin fibroblasts (Panetti, T. S., and McKeown-Longo, P. J.(1993) J. Biol. Chem. 268, 11988-11993; Panetti, T. S., and McKeown-Longo, P. J.(1993) J. Biol. Chem. 268, 11492-11495). These earlier studies demonstrated that vitronectin degradation was inhibited by either antibodies to the β₅ integrin or exogenous heparin, suggesting that both integrin receptors and cell surface heparan sulfate proteoglycans are involved in the endocytosis and degradation of vitronectin. The present study was done to define intracellular signaling pathways involved in endocytosis of vitronectin and to evaluate the relative contribution of cell surface heparan sulfate proteoglycans and the α_vβ₅ integrin in the activation of these signaling pathways. The addition of the phorbol ester phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, to monolayers of human skin fibroblasts, increased vitronectin degradation. Staurosporine and calphostin C, inhibitors of protein kinase C, blocked internalization and subsequent degradation of vitronectin, while KT5720, an inhibitor of protein kinase A, had no effect on the degradation of vitronectin. PMA was also able to reverse the inhibition of vitronectin degradation seen when cells were pretreated with heparinase or incubated with exogenous heparin. In contrast, the inhibitory effect of either RGD peptides or anti-α_vβ₅ antibodies on vitronectin degradation were not overcome by the addition of PMA. These data suggest that the internalization of vitronectin from the matrix is mediated by the α_vβ₅ integrin following activation of protein kinase C.