Dolichol-like lipids with stimulatory effect on DNA synthesis: substrates for protein dolichylation?

Substantial evidence has suggested that a nonsterol product of mevalonic acid (MVA) is essential for the initiation of DNA synthesis in mammalian cells. Several possible isoprenoid candidates have been suggested, but the identity of this compound still remains unknown. In this study we have isolated and purified MVA products from SV40-transformed human fibroblasts and identified fractions with a growth-stimulatory effect. The cells were labelled with [14C]MVA in the presence of inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. After lipid extraction, the [14C]MVA-labelled lipids were subjected to high performance liquid chromatography and size-exclusion chromatography, and the effect of the fractionated eluate on the DNA synthesis of arrested MVA-depleted target cells was tested. Thereby we found a fraction of [14C]MVA-labelled lipids with a substantial stimulatory effect on DNA synthesis. The chromatographic behavior suggested that the growth-stimulating fractions contained dolichol-20. This was confirmed by mass spectrometric analysis. Similar results were obtained when lipids from hepatocellular carcinoma cells and a sample from breast tumor were isolated and analyzed by the same procedure. The mechanisms by which these compounds induce DNA synthesis are unknown. Recent data obtained in our laboratory have provided evidence that dolichyl groups are covalently linked to tumor cell proteins, which implicates a new biological function for long-chain polyisoprenoid alcohols (Hjertman et al. [1997] FEBS Lett 416:235-238). In this study we demonstrate that tumor cells containing dolichol-like growth-stimulatory lipids also contained dolichylated proteins. This raises the question whether the growth-stimulatory dolichol-like lipids serve as substrates for the dolichylation reaction.