Role of biosynthesis of cholesterol and isoprenoid derivatives in regulation of G1 progression and cell proliferation of 3T6 cells.

The growth of 3T6-cells was rapidly decreased, although not completely stopped, as a consequence of treatment of the cell cultures with serum-free medium. By analyzing the cell cycle traverse it was found that the decrease in growth rate was attributable to a 6-8-h delay in progression through the proximal part of G1. These effects of serum depletion on cell-cycle traverse and cell proliferation were compared with simultaneous effects on the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase and the biosynthesis of cholesterol and the two isoprenoid derivatives coenzyme Q and dolichol. The activity of HMG CoA reductase was unaffected during the first 8 h of serum depletion and depressed by approximately 50% during the next 16 h. In contrast, the rate of coenzyme Q and dolichol synthesis, as related to cholesterol synthesis, was substantially decreased, and within 4 h after shift to serum-free medium both were reduced by 70-80%. As distinguished from dolichol synthesis, the synthesis of coenzyme Q was even decreased following exposures of the cell cultures to cholesterol-poor serum. This indicates that the rate of coenzyme Q synthesis is dependent on the concentration of serum cholesterol, whereas rate of dolichol synthesis is stimulated by some other serum factor(s). In addition, supplementation with dolichol to serum-depleted cells partially normalized G1 traverse and DNA synthesis, whereas cholesterol or coenzyme Q was ineffective. Taken together, the results suggest that a certain level of de novo synthesis of dolichol is required to maintain normal cell-cycle traverse and growth of 3T6 cells.