The Low Density Lipoprotein Receptor-related Protein Mediates Fibronectin Catabolism and Inhibits Fibronectin Accumulation on Cell Surfaces

Abstract

Low density lipoprotein receptor-related protein (LRP) is a member of the low density lipoprotein receptor family, which functions as an endocytic receptor for diverse ligands. In this study, we demonstrate that murine embryonic fibroblasts (MEF-2 cells) and 13-5-1 Chinese hamster ovary cells, which are LRP-deficient, accumulate greatly increased levels of cell-surface fibronectin (Fn), compared with LRP-expressing MEF-1 and CHO-K1 cells. Increased Fn was also detected in conditioned medium from LRP-deficient MEF-2 cells; however, biosynthesis of Fn by MEF-1 and MEF-2 cells was not significantly different. When LRP-deficient cells were dissociated from monolayer culture, increased levels of Fn remained with the cells, as determined by cell-surface protein biotinylation, suggesting an intimate relationship with cell surface-binding sites. The LRP antagonist, receptor-associated protein (RAP), promoted Fn accumulation in association with MEF-1 cells, whereas expression of full-length LRP in MEF-2 cells substantially decreased Fn accumulation, confirming the role of LRP in this process. Purified LRP bound directly to immobilized Fn, and this interaction was inhibited by RAP. Furthermore, MEF-1 cells degraded (125)I-Fn at an increased rate, compared with MEF-2 cells. (125)I-Fn degradation by MEF-1 cells was inhibited by RAP. These results demonstrate that LRP functions as a catabolic receptor for Fn. The function of LRP in Fn degradation and the ability of LRP to regulate levels of other plasma membrane proteins represent possible mechanisms whereby LRP prevents Fn accumulation on cell surfaces.