Abstract
Mevalonic acid constitutes a key intermediate in the biosynthesis of a number of isoprenoid products. These metabolites include cholesterol and sterol-derived molecules, but also a large number of non-sterol isoprene derivatives. It has been well documented that not only cholesterol, but also a non-sterol product of the mevalonate pathway is essential for normal cell growth and division. In addition, tumor cells display an increased production of mevalonate and deranged control of HMG-CoA reductase, the major rate-limiting enzyme in the mevalonate pathway. Several isoprenoid molecules have been suggested to be of importance in growth regulation.
This study was undertaken with the aim to isolate and characterize mevalonate products with important functions in cellular growth control. Initially, several different cell lines were studied with regard to their production of different isoprenoid lipids under varying growth conditions. We thereby found that a SV40-transformed cell line, 90VA-VI, exhibited an increased resistance to growth inhibition by HMG-CoA reductase inhibition that was correlated to synthesis of the long-chain polyisoprenoid alcohol dolichol. Subsequently, we studied the effects of HMG-CoA reductase inhibition and addition of dolichol in different cell lines. We found that the cells were blocked in the G1 phase of the cell cycle and that dolichol could counteract this effect. Mevalonate-derived lipids were then isolated from tumor cells and normal cells, and thereafter tested for stimulatory effects. We found that lipids corresponding to dolichol and polyprenol had a stimulative effect on cell proliferation. The identity of these lipids was established by mass spectrometry.
Protein prenylation is an important activity-regulating modification in several proteins with growth regulatory functions. Since it has been suggested that protein prenylation could occur also with long-chain isoprenoids, we also searched for covalent modification of proteins by dolichol or related lipids. We then found that proteins from several tumor cell lines were modified by dolichol-related lipids, which might be of importance for the growth-stimulatory function of dolichol in growth-inhibited cells.
