Control of cholesterol biosynthesis in Schwann cells

Cholesterol accounts for over one-fourth of total myelin lipids. We found that, during development of the rat sciatic nerve, expression of mRNA for hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in cholesterol biosynthesis, was up-regulated in parallel with mRNA for P0, the major structural protein of PNS myelin, and with ceramide galactosyltransferase (CGT), the rate-limiting enzyme in cerebroside biosynthesis. To help establish the nature of this coordinate regulation of myelin-related genes, we examined their steady-state mRNA levels in cultured primary Schwann cells. We also assayed synthesis of cholesterol and cerebroside to distinguish how much control of synthetic activity for these two myelin lipids involved mRNA levels for HMG-CoA reductase and CGT, and how much involved post-mRNA control mechanisms. Addition of forskolin to cells cultured in media supplemented with normal calf serum resulted in up-regulation of P0 and CGT mRNA expression and cerebroside synthesis, without corresponding increases in HMG-CoA reductase mRNA or cholesterol synthesis. Cholesterol synthesis increased approximately threefold in Schwann cells cultured with lipoprotein-deficient serum, without any increase in HMG-CoA reductase mRNA. Furthermore, addition of either serum lipoproteins or 25-hydroxycholesterol decreased cholesterol synthesis without altering HMG-CoA reductase mRNA levels. We conclude that, as in other tissues, cholesterol synthesis in Schwann cells is regulated primarily by intracellular sterol levels. Much of this regulation occurs at posttranscriptional levels. Thus, the in vivo coordinate up-regulation of HMG-CoA reductase gene expression in myelinating Schwann cells is secondary to intracellular depletion of cholesterol, as it is compartmentalized within the myelin. It is probably not due to coordinate control at the level of mRNA expression.