Cholesterol and mevalonic acid modulation in cell metabolism and multiplication.
Cholesterol in animals is a major structural component of cell membranes. It may therefore play a functional role in the modulation of cell osmolarity, the process of pinocytosis and the activities of membrane-associated proteins such as ionic pumps, immune responses, etc. A major relationship exists between the cell-growth processes and the cholesterol biosynthetic pathway. The cholesterol needed for new membranes may be derived either from endogenous synthesis or from exogenous sources, principally plasma low-density-lipoproteins (LDL) which enter the cells by receptor-mediated endocytosis. Both these pathways are enhanced in rapidly growing cells. Conversely, if synthesis is inhibited and no exogenous cholesterol is available, cell growth is blocked. The 3-hydroxy-3-methylglutaryl CoA (HMGCoA) reductase (the rate-limiting reaction in cholesterol biosynthesis) is the enzyme which catalyzes the conversion of HMGCoA to mevalonic acid. It has been suggested that mevalonate may play an important role in cell proliferation. All cells need at least two products synthesized from mevalonate in order to proliferate, and the only one yet identified is cholesterol. Other melavonate-derived potential candidates as cell-cycle and cell-survival products include the dolichols ubiquinone side chains, isopentenyladenosine derivatives, etc. Furthermore, it has recently been shown that membrane association appears to be an important function in mevalonate-derive modifications of several important proteins such as cellular membrane G proteins, those coded for by oncogenes (ras proteins) and lamins (nuclear proteins). In recent years the development of cholesterol-synthesis-inhibiting drugs, for lowering plasma cholesterol levels has mainly been centred on the control of HMGCoA reductase activity (vastatins). However, because mevalonic acid is the precursor of numerous metabolites, any reduction of such activity may potentiate pleiotropic effects. Vastatins are now, therefore, receiving increased attention as potential pharmacological tools for the control of abnormal cell growth in pathological situations, i.e. tumours and vascular smooth muscle cell proliferation under atherogenic conditions. In our laboratories, we have demonstrated that simvastatin can prevent arterial myocyte proliferation both in vivo and in vitro. Simvastatin can also inhibit in vitro the rate of human glioma cell growth, since it shows a strong synergistic inhibitory effect on cell proliferation when used in association with anticancer agents such as Carmustine or beta-interferon. Both simvastatin-induced cell growth inhibition and the synergy observed with these drugs can be completely reversed by incubating cells with mevalonate. This shows that the effect of simvastatin of cell proliferation is due to its specific inhibitory activity on intracellular mevalonate synthesis.