Role of Protein Prenylation in Cellular Proliferation

Prenylation is a post-translational, covalent modification of a protein by the attachment of a lipophilic isoprenoid group, linked by a thioether bond to a cysteine at or near to the carboxyl terminus of the protein. Ras is one of the important proteins which undergoes such modification at the C-terminus CAAX box motif (where C=Cys, A=aliphatic amino acid, X=usually Ser or Met) by a 15C isoprenoid moiety, farnesyl, and is mediated by the enzyme, farnesyltransferase. Oncogenic forms of Ras have been shown to lose their transforming activity when farnesylation is prevent. Given that Ras proteins are implicated in a majority of human pancreatic and colorectal cancers, blocking Ras farnesylation would be a good strategy for developing new anti- cancer treatments. Benzodiazepine peptidomimetics, which mimic a natural dipeptide turn of the two aliphatic residues of the CAAX box, are effective inhibitors of farnesyltransferase, and normalizes the morphology of Rat1 cells transformed with H-ras (v12). The purpose of this study was to further characterize the potency, toxicity, and cellular uptake of the various benzodiazepine analogs using the Met18b-2 cell line as a model. Another aspect of the study was to investigate the reversion of compactin-induced cell rounding with prenyl alcohols. Since compactin-induced cell rounding was known to be reversed by mevalonate, the possibility of such reversion was investigated with prenyl alcohols to demonstrate that the control of cell shape is mediated by nonsterol products of the mevalonate pathway, and in turn demonstrate a salvage pathway for the prenyl alcohols. The results provide preliminary evidence for a salvage pathway, where the alcohols recycle back to the mono-and di-phosphates. The activity of the prenyl kinases, which phosphorylate the corresponding prenyl alcohols to the phosphates were also assayed. Consequently, these data support the possibility of the control of cellular growth and cell shape maintenance by prenylated proteins