Integrin alpha v beta 3-mediated endocytosis of immobilized fibrinogen by A549 lung alveolar epithelial cells

Fibrinogen (FBG), together with its polymerized form fibrin, modulates cell ular responses during wound repair and tissue remodeling. Thus, we sought t o determine whether A549 lung epithelial type Ii-like cells would endocytos e insoluble, surface-bound FBC as a potential mechanism of alveolar matrix remodeling. Surface-bound FBC was endocytosed into either lysosomes or late endosomes by A549 cells through arg-gly-asp-dependent binding to alphav be ta3 but not alpha5 beta1 integrin receptors. Soluble FBC added to confluent monolayers of A549 cells was not endocytosed. Unlike the uptake of the ext racellular matrix glycoproteins vitronectin and thrombospondin by other cel l types, endocytosis of FBC by A549 cells was neither inhibited by heparin nor dependent on binding to cell-surface heparan sulfate proteoglycans. FBC did not colocalize with endocytosed transferrin, whereas dextran showed pa rtial colocalization with FBC in endocytic vesicles, suggesting non-clathri n-mediated endocytosis. Inhibition of actin filament polymerization blocked endocytosis of both dextran and FBC but not transferrin, providing further support that FBC is endocytosed via a nonclathrin pathway. Disruption of a ctin polymerization inhibited integrin-mediated cell spreading, which contr ibuted to an overall reduction in FBC clearance that was most likely due to reduced cell migration and associated pericellular proteolysis. Trasylol i nhibition of extracellular plasmin activity did not inhibit endocytosis of FBC. The endocytosed FBC was degraded to trichloroacetic acid-soluble fragm ents that showed an electrophoretic pattern distinctly different from plasm in-degraded FBC. Together, these results suggest that endocytosis of matrix -associated FBC by alveolar epithelial cells may be involved in the process es of alveolar tissue repair and matrix remodeling.