Geranylgeraniol Overcomes the Block of Cell Proliferation by Lovastatin in C6 Glioma Cells

: It is well documented that 3‐hydroxy‐3‐methylglutaryl‐CoA reductase inhibitors prevent cultured mammalian cells from progressing through the cell cycle, suggesting a critical role for a mevalonate‐derived product. Recently, it has been shown that free geranylgeraniol (GG‐OH) and farnesol (F‐OH) can be utilized by C6 glioma cells for protein isoprenylation. The ability of GG‐OH and F‐OH to restore protein geranylgeranylation or farnesylation selectively has enabled us to examine the possibility that mevalonate is essential for cell proliferation because it is a precursor of farnesyl pyrophosphate or geranylgeranyl pyrophosphate, the isoprenyl donors involved in the post‐translational modification of key regulatory proteins. In this study we report that GG‐OH, as well as mevalonate, overcomes the arrest of cell proliferation of C6 glioma cells treated with lovastatin, as assessed by increased cell numbers and a stimulation in [3H]thymidine incorporation. The increase in cell number and [3H]thymidine incorporation were significantly lower when F‐OH was added. Under these conditions [3H]mevalonate and [3H]GG‐OH are actively incorporated into a set of isoprenylated proteins in the size range of small, GTP‐binding proteins (19–27 kDa) and a polypeptide with the molecular size (46 kDa) of the smaller isoform of 2′,3′‐cyclic nucleotide 3′‐phosphodiesterase. Analysis of the proteins metabolically labeled by [3H]mevalonate and [3H]GG‐OH reveals the presence of labeled proteins containing geranylgeranylated cysteinyl residues. Consistent with geranylgeranylated proteins playing a critical role in the entry of C6 cells into the cell cycle, a (phosphonoacetamido)oxy derivative of GG‐OH, a drug previously shown to interfere with protein geranylgeranylation, prevented the increase in cell number when mevalonate or GG‐OH was added to lovastatin‐treated cells. These results strongly suggest that geranylgeranylated proteins are essential for progression of C6 cells into the S phase of the cell cycle and provide the first evidence that the “salvage” pathway for the utilization of the free isoprenols is physiologically significant in the CNS.