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Cell Cycle‐Specific Requirement for Mevalonate, but Not for Cholesterol, for DNA Synthesis in Glial Primary Cultures

The requirement for the sterol biosynthetic pathway for the occurrence of DNA synthesis in glial cells and, in particular, the relative roles of cholesterol and of mevalonate have been studied. Primary cultures of developing glial cells were synchronized by reducing the content of fetal calf serum (FCS) in the culture medium from 10% to 0.1% (vol/ vol) for 48 h between days 4 and 6 in culture. Reversal of the resulting quiescent state by the return of the cultures to 10% serum caused after 24 h a marked increase in DNA synthesis, and this increase was prevented by the simultaneous addition of mevinolin, a specific inhibitor of the sterol biosynthetic pathway at the 3‐hydroxy‐3‐methylglutaryl coenzyme A reductase step, at the time of serum repletion. A dose‐dependent reversal of the mevinolin inhibition of DNA synthesis occurred with simultaneous addition of mevalonate to the culture medium. The induction of DNA synthesis by serum repletion, its inhibition by mevinolin, and the reversal of the inhibition by mevalonate were unaffected by a 95% reduction in exogenous cholesterol produced by utilization of lipoprotein‐poor serum (LPPS) rather than FCS. Similarly, return of quiescent cultures to 10% LPPS containing mevinolin and sufficient low‐density lipoprotein (LDL) to raise the cholesterol concentration 80‐fold failed to restore DNA synthesis. In addition, reversal of the mevinolin inhibition of DNA synthesis by mevalonate occurred despite the continuous presence of mevinolin if mevalonate was added as late as 12 h after serum repletion, but not if added after 16 h or more. This temporal aspect of the requirement for mevalonate was unaffected by the presence of a 90‐fold excess of cholesterol, provided as LDL. Thus, these data establish a requirement for mevalonate or for a presumably nonsterol derivative of mevalonate for the occurrence of DNA synthesis in synchronized glial primary cultures, and demonstrate that this requirement is expressed at a specific time in the cell cycle.