Vascular Endothelial Cells THE INDUCTION OF 3-HYDROXY-3-METHYLGLUTARYL COENZYME A REDUCTASE ACTIVITY AND THE SUPPORT OF CELLULAR PROLIFERATION*

Human high density lipoproteins (HDLs), a heterogeneous class of particles which is mitogenic for bovine vascular endothelial cells, stimulate the 3-hydroxy-3methylglutaryl coenzyme A (HMG-CoA) reductase activity present in both actively proliferating sparse endothelial cells and in nondividing confluent cultures. Kinetics of the enzyme response to HDLs was studied in confluent cells, and the relationship between reductase activity and proliferation was examined in sparse cells. In confluent cells, HMG-CoA reductase has a stable basal activity of 1-2 pmol/min/lO‘ cells. The enzyme induction by HDLs is concentration-dependent (25-1000 pg of HDL protein/ml) and time-dependent (detectable within 3 h, rising steadily for 2-4 days). Optimal stimulation resulted in a 10-fold increase in the enzyme levels. The HDL, subfraction (density 1.130-1.21 g/ml) is a more potent stimulator than HDLz subfraction (density 1.07-1.125 g/ml). Low density lipoprotein (LDL) prevents and reverses the enzyme response to HDLs. The HMG-CoA reductase specific activity of actively dividing cells is 50-fold that of quiescent confluent cells and progressively decreases as cell density increases. Against the falling base-line activity of proliferating cells, both an HDL stimulation and an LDL inhibition of enzyme activity are seen. Cells exposed to compactin, a reductase inhibitor, stop dividing and die. Growing cells exposed to HDLs, however, are 10-fold more resistant to compactin toxicity than cells exposed to LDL. Cell proliferation thus appears to be more dependent on the activity of HMG-CoA reductase than on the availability of cholesterol (as provided by LDL). LDL does increase the rate of proliferation supported by HDLs. Similarly, HDLz (which may donate some cholesterol via apo-E, in an LDL-like manner) is a more potent mitogen, but less effective stimulator of reductase, than HDL3. These results suggest that while HDLs and LDL have opposing actions on HMG-CoA reductase, they complement each other in promoting cell growth. Further, the mitogenic effect of HDLs may be mediated by their stimulation of HMG-CoA reductase activity, which would provide cells with a critical pool of newly synthesized sterol, and/or products of the nonsterol branching isoprene pathways.