CHARACTERISTICS OF LOW AND HIGH-DENSITY-LIPOPROTEIN BINDING AND LIPOPROTEIN-INDUCED SIGNALING IN QUIESCENT HUMAN VASCULAR SMOOTH-MUSCLE CELLS

Low density lipoprotein (LDL) and high density lipoprotein (HDL) have been shown to stimulate signal transduction events in a number of cell types, including cultured vascular smooth muscle cells (VSMC), but it is not known whether these events are mediated through distinct lipop rotein receptors for transmembrane signaling. This study has used conf luent quiescent cultures of human microarteriolar VSMC to investigate the relationship between the characteristics of I-125-LDL and I-125-HD L(3) binding and those of LDL- and HDL(3)-stimulated cell signaling. T wo distinct binding sites for LDL (K-d1 approximate to 2 mu g/ml and K -d2 approximate to 40 mu g/ml) and a single class of sites for HDL(3) (K-d approximate to 30 mu g/ml) were identified. The K-d1 for high aff inity I-125-LDL binding in quiescent VSMC was comparable to the value for heparin-sensitive binding of I-125-LDL to apolipoprotein B/E recep tors in fibroblasts (K-d approximate to 1 mu g/ml). Concentrations of lipoproteins required for half-maximal stimulation (EC(50)) of phospho inositide catabolism and intracellular calcium mobilization in VSMC we re approximate to 35 mu g/ml for HDL(3) and approximate to 40 mu g/ml for LDL. Both LDL- and HDL(3)-stimulated signaling responses in VSMC, as well as I-125-HDL(3) binding and low affinity I-125-LDL binding to VSMC, were insensitive to heparin. Competition binding studies (with u nlabeled lipoproteins at 2.5-200 mu g/ ml) showed partial displacement of I-125-LDL by HDL(3) and of I-125-HDL(3) by LDL, whereas complete d isplacement of I-125-LDL or I-125-HDL(3) by their homologous lipoprote ins was achieved. Thus, the binding sites for HDL(3) are distinct from those for LDL. Because the response of VSMC to combinations of LDL an d HDL(3) was additive, LDL and HDL(3) also exert their signaling effec ts through distinct sites. Further investigation is required to unequi vocally demonstrate that the heparin-insensitive HDL(3) and low affini ty LDL binding sites in VSMC are those through which LDL and HDL(3) st imulate transmembrane signaling.