The dual role of mevalonate in the cell cycle

It is well established that either exogenous or endogenous cholesterol is required for both cell growth and proliferation. This laboratory has recently discovered that, in baby hamster kidney-21 cells, independent of its role as a cholesterol precursor, mevalonic acid plays an essential role in S phase DNA replication. It was later shown that isopentenyl adenine, a known product of mevalonate in prokaryotes and lower eukaryotes, is 100 to 200 times more effective than mevalonate in restoring DNA replication in cells in which mevalonic acid synthesis is blocked with the beta-hydroxy-beta-methylglutaryl-CoA reductase inhibitor, compactin. The present study was designed to determine the relationship in the cell cycle between the known requirement for cholesterol and the newly discovered effect of mevalonic acid and isopentenyl adenine on S phase DNA synthesis. Employing cells arrested by serum depletion, it was shown that the cholesterol requirement is limited to the early and mid-G1 phases, whereas the isopentenyl effect is required at the late G1-S interphase of the cell cycle. The evidence supporting these conclusions involves: first, in serum-arrested cells blocked early in G1 by compactin, only the combination of cholesterol added in early G1 and either mevalonate or isopentenyl adenine in late G1 permitted progression through the G1 and S phase DNA synthesis. Neither isopentenyl adenine added early in G1 nor cholesterol in late G1 was capable of restoring DNA synthesis in this system. Second, in accord with the above formulation, inhibition of cholesterol synthesis with the oxidosqualene cyclase inhibitor, dl-4,4,10 beta-trimethyl-trans-decal-3 beta-ol, affected only the early G1 phase of the cell cycle, but had no late G1 effect on DNA replication.