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 multifunctional receptor, expressed by vascular smooth muscle cells (VSMCs) in normal 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-derived growth factor-BB (PDGF-BB) or epidermal growth factor (EGF) for 24 h bound increased amounts of the LRP ligand, activated alpha(2)-macroglobulin (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 cell protein by PDGF-BB and EGF, respectively, while the K-D was unchanged. Northern and Western blot analyses demonstrated that neither PDGF-BB nor EGF increase LRP mRNA or protein levels. Instead, LRP was redistributed to the cell surface and remained localized primarily in coated pits, as determined by surface protein biotinylation, affinity labeling, and immunoelectron microscopy studies. The increase in cell-surface LRP was partially explained by a 50% decrease in receptor endocytosis rate; however, at 37 degrees C, PDGF-BB- and EGF-treated VSMCs still bound/internalized increased amounts of activated alpha(2)M and subsequently released increased amounts of trichloroacetic acid-soluble radioactivity. The cytokine-induced shifts in LRP subcellular distribution were stable when VSMCs were challenged with a saturating concentration 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 an important aspect of the VSMC response to the atherogenic cytokines, PDGF-BB and EGF.