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 lipoprotein receptors for transmembrane signaling. This study has used confluent quiescent cultures of human microarteriolar VSMC to investigate the relationship between the characteristics of 125I-LDL and 125I-HDL3 binding and those of LDL- and HDL3-stimulated cell signaling. Two distinct binding sites for LDL (Kd1 approximately 2 micrograms/ml and Kd2 approximately 40 micrograms/ml) and a single class of sites for HDL3 (Kd approximately 30 micrograms/ml) were identified. The Kd1 for high affinity 125I-LDL binding in quiescent VSMC was comparable to the value for heparin-sensitive binding of 125I-LDL to apolipoprotein B/E receptors in fibroblasts (Kd approximately 1 microgram/ml). Concentrations of lipoproteins required for half-maximal stimulation (EC50) of phosphoinositide catabolism and intracellular calcium mobilization in VSMC were approximately 35 micrograms/ml for HDL3 and approximately 40 micrograms/ml for LDL. Both LDL- and HDL3-stimulated signaling responses in VSMC, as well as 125I-HDL3 binding and low affinity 125I-LDL binding to VSMC, were insensitive to heparin. Competition binding studies (with unlabeled lipoproteins at 2.5-200 micrograms/ml) showed partial displacement of 125I-LDL by HDL3 and of 125I-HDL3 by LDL, whereas complete displacement of 125I-LDL or 125I-HDL3 by their homologous lipoproteins was achieved. Thus, the binding sites for HDL3 are distinct from those for LDL. Because the response of VSMC to combinations of LDL and HDL3 was additive, LDL and HDL3 also exert their signaling effects through distinct sites. Further investigation is required to unequivocally demonstrate that the heparin-insensitive HDL3 and low affinity LDL binding sites in VSMC are those through which LDL and HDL3 stimulate transmembrane signaling.