RAT FIBRONECTINS: BIOSYNTHESIS AND CELL SURFACE INTERACTIONS

Evidence is summarized that cellular and plasma fibronectins are two distinct molecular species with different molecular weights and different biological activities. Because of plasma fibronectin\u27s involvement with blood coagulation, wound healing, and the reticuloendothelial system, an understanding of its biosynthesis and function is important. This thesis presents the results of experiments designed to determine the role of the liver in the synthesis of plasma fibronectin. Described are the isolation of plasma fibronectins from several species and evidence that the liver is capable of synthesizing fibronectin in vitro using a liver slice system as well as the route of synthesis and intracellular transport of fibronectin by rat liver in vivo. Immunoprecipitated fibronectin reached peak specific activity post injection of (\u273)H-leucine at 30 min in the endoplasmic reticulum, 45 min in the Golgi apparatus, and at 60 min in the plasma membrane fraction. Plasma membranes isolated from hepatic tumors in which higher gangliosides were depleted bound 43 to 75% less (\u27125)I-fibronectin than did plasma membranes from control livers. The binding of fibronectins to gangliosides was tested directly using an enzyme-linked immuno absorbent assay (ELISA) where gangliosides were immobilized to polystyrene tubes and relative binding of fibronectin was estimated by alkaline phosphatase activity of conjugated second antibody. Above a critical ganglioside concentration, the order of efficiency of the gangliosides to bind fibronectin was G(,T)(TURNEQ)G(,D1b)(TURNEQ)G(,D1a) \u3e G(,M1) \u3e\u3e G(,M2)(TURNEQ)G(,D3)(TURNEQ)G(,M3). The findings are consistent with low affinity binding of fibronectins to gangliosides, including the same gangliosides depleted from cell surfaces during tumorigenesis in the rat