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The rate of cholesterol synthesis from [14C]acetate was low in circulating blood lymphocytes freshly isolated from 17 normal subjects and 4 subjects with homozygous FH. On the other hand, the rate of cholesterol synthesis was two to fourfold above normal in freshly isolated lymphocytes from two subjects with abetalipoproteinemia. When the lymphocytes from subjects with all three genotypes were incubated for 48-72 h in the absence of lipoproteins, the rate of cholesterol synthesis increased by 5-15-fold. The subsequent addition of plasma LDL, but not HDL, rapidly suppressed cholesterol synthesis in the lymphocytes from normal subjects. In contrast, lymphocytes from the FH homozygotes, which have been shown previously to be deficient in cell surface LDL receptors, were resistant to LDL-mediated suppression of cholesterol synthesis. In addition to its ability to suppress cholesterol synthesis after it had been elevated by incubation of the cells in the absence of lipoproteins, LDL was able to suppress the induction of the enhanced rate of sterol synthesis when added to normal lymphocytes immediately after their isolation from the bloodstream. In contrast to the former action of LDL, the latter action of LDL-i.e., the suppression of induction of sterol synthesis-also occurred to a limited extent in lymphocytes from FH homozygotes. However, the FH lymphocytes, but not the normal cells, could be made resistant to this action of LDL by inclusion in the incubation medium of lipoprotein-deficient serum (30 percent, vol/vol) plus HDL (1 mg protein/ml). Considered together with previous data demonstrating a deficiency of LDL receptors in freshly isolated lymphocytes from FH homozygotes, the current studies provide evidence in support of the hypothesis that the interaction of plasma LDL with its cell surface receptor serves to regulate cholesterol synthesis in human lymphocytes.
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