Receptor-mediated uptake of low density lipoprotein reconstituted with 25-hydroxycholesteryl oleate suppresses 3-hydroxy-3-methylglutaryl-coenzyme A reductase and inhibits growth of human fibroblasts

The free and esterified cholesterols of plasma low density lipoprotein (LDL) were extracted with heptane and replaced with 25-hydroxycholesteryl oleate. The resulting particle, designated r-[25-HC oleate]LDL, bound to LDL receptors on human fibroblasts, was taken up by adsorptive endocytosis and was hydrolyzed in lysosomes in a manner similar to that of native LDL. The r-[25-HC oleate]LDL suppressed 3-hydroxy-3-methylglutaryl-coenzyme A reductase [mevalonate:NADP⁺ oxidoreductase (CoA-acylating), EC 1.1.1.34], the enzyme catalyzing the rate-limiting step in cholesterol biosynthesis. This suppression did not occur when lysosomal hydrolysis of r-[25-HC oleate]LDL was inhibited by chloroquine. When fibroblasts were incubated with r-[25-HC oleate]LDL in the absence of a source of cholesterol, the cells developed an abnormal morphology, their growth was inhibited, and the cells died. The toxic effects of r-[25-HC oleate]LDL were prevented when the growth medium was supplemented with cholesterol in ethanol or with mevalonate, the product of the reductase reaction. These data suggest that the toxicity of r-[25-HC oleate]LDL was due to its suppression of reductase, which in turn caused cellular cholesterol deficiency. The r-[25-HC oleate]LDL did not suppress reductase activity nor did it alter the growth or morphology of mutant fibroblasts lacking LDL receptors, which were obtained from a patient with homozygous familial hypercholesterolemia. These experiments demonstrate the feasibility of using reconstituted LDL to selectively deliver hydrophobic compounds other than typical cholesteryl esters to cells possessing LDL receptors.