EPIDERMAL GROWTH-FACTOR AND PLATELET-DERIVED GROWTH FACTOR-BB INDUCE A STABLE INCREASE IN THE ACTIVITY OF LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN IN VASCULAR SMOOTH-MUSCLE CELLS BY ALTERING RECEPTOR DISTRIBUTION AND RECYCLING

Low density lipoprotein receptor-related protein (LRP) is a multifunct ional receptor, expressed by vascular smooth muscle cells (VSMCs) in n ormal arteries and in atherosclerotic lesions. In this investigation, we demonstrate a novel mechanism for the regulation of LRP activity in cultured rat aortic VSMCs. Cells that were treated with platelet-deri ved growth factor-BB (PDGF-BB) or epidermal growth factor (EGF) for 24 h bound increased amounts of the LRP ligand, activated alpha(2)-macro globulin (alpha(2)M), at 4 degrees C. The B-max for activated alpha(2) M was increased from 56 +/- 5 to 178 +/- 24 and 143 +/- 11 fmol/mg cel l protein by PDGF-BB and EGF, respectively, while the K-D was unchange d. Northern and Western blot analyses demonstrated that neither PDGF-B B nor EGF increase LRP mRNA or protein levels. Instead, LRP was redist ributed to the cell surface and remained localized primarily in coated pits, as determined by surface protein biotinylation, affinity labeli ng, and immunoelectron microscopy studies. The increase in cell-surfac e LRP was partially explained by a 50% decrease in receptor endocytosi s rate; however, at 37 degrees C, PDGF-BB- and EGF-treated VSMCs still bound/internalized increased amounts of activated alpha(2)M and subse quently released increased amounts of trichloroacetic acid-soluble rad ioactivity. The cytokine-induced shifts in LRP subcellular distributio n were stable when VSMCs were challenged with a saturating concentrati on of ligand and then incubated, in the absence of cytokine, for 2.5 h at 37 degrees C. Regulation of LRP distribution and activity may be a n important aspect of the VSMC response to the atherogenic cytokines, PDGF-BB and EGF.