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 cellular responses during wound repair and tissue remodeling. Thus, we sought to determine whether A549 lung epithelial type Ii-like cells would endocytose 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 beta3 but not alpha5 beta1 integrin receptors. Soluble FBC added to confluent monolayers of A549 cells was not endocytosed. Unlike the uptake of the extracellular matrix glycoproteins vitronectin and thrombospondin by other cell 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 partial colocalization with FBC in endocytic vesicles, suggesting non-clathrin-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 actin polymerization inhibited integrin-mediated cell spreading, which contributed to an overall reduction in FBC clearance that was most likely due to reduced cell migration and associated pericellular proteolysis. Trasylol inhibition of extracellular plasmin activity did not inhibit endocytosis of FBC. The endocytosed FBC was degraded to trichloroacetic acid-soluble fragments that showed an electrophoretic pattern distinctly different from plasmin-degraded FBC. Together, these results suggest that endocytosis of matrix-associated FBC by alveolar epithelial cells may be involved in the processes of alveolar tissue repair and matrix remodeling.