Further characterization of the acetyl LDL (scavenger) receptor expressed by rabbit smooth muscle cells and fibroblasts

Abstract

We have previously demonstrated that the acetyl low density lipoprotein (LDL), or scavenger, receptor is expressed by rabbit smooth muscle cells (SMCs) and fibroblasts. Moreover, receptor activity in rabbit fibroblasts was regulated over a wide range by preincubating the cells with secretion products from human platelets or with phorbol esters. The current studies were undertaken to examine the regulation of receptor expression in rabbit SMCs, to characterize further the receptor expressed by rabbit SMCs and fibroblasts, and to determine whether incubating these cells with chemically modified lipoproteins would lead to foam cell formation. Receptor activity was increased fivefold in rabbit SMCs by preincubation with platelet secretion products and nine- to 20-fold by preincubation with phorbol esters or mezerein, a non-phorbol activator of protein kinase C. The phorbol ester-induced increase in receptor activity was due to an increased mass of scavenger receptor as determined by both ligand blots and immunoblots of membrane proteins from these cells. The immunoblotting studies suggest that the SMC scavenger receptor activity is due to type I or type II scavenger receptors. The scavenger receptor expressed by phorbol ester-treated rabbit SMCs and fibroblasts bound chemically modified LDL with an order-of-magnitude higher affinity (Kd 5.1×10-10 M) than did the scavenger receptor expressed by mouse peritoneal macrophages (Kd 4.1×10-9 M), whereas rabbit macrophages bound chemically modified LDL with intermediate affinity (Kd 1.0×10-9 M). Both the phorbol ester-stimulated rabbit fibroblasts and SMCs expressed 10-30% as many receptors per cell as did mouse peritoneal macrophages. Consistent with the difference in the number of receptors, uptake of the chemically modified LDL at 37°C by SMCs led to ∼25% as much lysosomal degradation and stimulation of cholesterol esterification as in mouse peritoneal macrophages. Incubation of phorbol ester-treated rabbit fibroblasts or SMCs with chemically modified LDL (50 μg/ml) for 24 or 48 hours, respectively, resulted in a threefold increase in total cholesterol and a 15-fold increase in cholesteryl ester within the cells. These studies suggest that the scavenger receptor-mediated uptake of modified lipoproteins may contribute to the formation of SMC foam cells in atherosclerotic lesions.