Plasma concentrations of high density lipoproteins, (HDL3), a subclass of HDL, are relatively constant, while those of HDL2 are variable. We report that HDL2 suppress, while HDL3 stimulate 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34) activity in normal human fibroblasts. HDL3, which contained no detectable HDL2 or low density lipoproteins, stimulated 3-hydroxy-3-methylglutaryl-CoA reductase activity 2-fold from 60 to 120 pmol/mg per min. The induction, which exhibited saturation kinetics with maximum stimulation at 150 microgram HDL phospholipid/ml medium, occurred only in the late-log and stationary phases of cell growth and was abolished by 0.1 mM actinomycin D or cycloheximide.l Apoliproprotein HDL3 did not stimulate enzyme activity, whereas the total lipid extract of HDL3 was about 1.7 times more potent than were the native HDL3 in stimulating enzyme activity. HDL2 consistently suppressed 3-hydroxy-3-methylglutaryl-CoA reductase in normal fibroblasts by 20-50% at 80 microgram HDL2 protein/ml. Mixtures of HDL2 and HDL3 suppressed when HDL2 were greater than 35% of the total HDL. The suppressive effects of HDL2 were abolished by treatment with 0.1 M cyclohexanedione and restored by regeneration of arginyl residues, suggesting an apolipoprotein-mediated suppressive mechanism. The total lipid extract of HDL2 stimulated 3-hydroxy-3-methylglutaryl-CoA reductase 2-fold at 3 microgram lipid phosphorus/ml. Moreover, HDL2 and HDL3 both stimulated 3-hydroxy-3-methylglutaryl-CoA reductase activity in receptor-negative fibroblasts. HDL2 have both the ability to suppress 3-hydroxy-3-methylglutaryl-CoA reductase activity in cells which possess low density lipoproteins receptors and to activate the enzyme in receptor-negative cells. These results show that variations in culture conditions and differences in the proportions of HDL subclasses must be considered in the interpretation of studies investigating cellular responses to HDL.
Apolipoproteins Cells, Cultured