Vn. Bochkov et al., CHARACTERISTICS OF LOW AND HIGH-DENSITY-LIPOPROTEIN BINDING AND LIPOPROTEIN-INDUCED SIGNALING IN QUIESCENT HUMAN VASCULAR SMOOTH-MUSCLE CELLS, Molecular pharmacology, 45(2), 1994, pp. 262-270
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.