MODULATION OF INSULIN-SECRETION FROM NORMAL RAT ISLETS BY INHIBITORS OF THE POSTTRANSLATIONAL MODIFICATIONS OF GTP-BINDING PROTEINS

Many GTP-binding proteins (GBPs) are modified by mevalonic acid (MVA)-dependent isoprenylation, carboxyl methylation or palmitoylation. The effects of inhibitors of these processes on insulin release were studied. Intact pancreatic islets were shown to synthesize and metabolize MVA and to prenylate several candidate proteins. Culture with lovastatin (to inhibit synthesis of endogenous MVA) caused the accumulation in the cytosol of low-M(r) GBPs (labelled by the [alpha-P-32]GTP overlay technique), suggesting a disturbance of membrane association. Concomitantly, lovastatin pretreatment reduced glucose-induced insulin release by about 50 %; co-provision of 100-200 muM MVA totally prevented this effect. Perillic acid, a purported inhibitor of the prenylation of small GBPs, also markedly reduced glucose-induced insulin secretion. Furthermore, both N-acetyl-S-trans, trans-farnesyl-L-cysteine (AFC), which inhibited the baselabile carboxyl methylation of GBPs in islets or in transformed beta-cells, and cerulenic acid, an inhibitor of protein palmitoylation, also reduced nutrient-induced secretion; an inactive analogue of AFC (which did not inhibit carboxyl methylation in islets) had no effect on secretion. In contrast with nutrients, the effects of agonists that induce secretion by directly activating distal components in signal transduction (such as a phorbol ester or mastoparan) were either unaffected or enhanced by lovastatin or AFC. These data are compatible with the hypothesis that post-translational modifications are required for one or more stimulatory GBPs to promote proximal step(s) in fuel-induced insulin secretion, whereas one or more inhibitory GBPs might reduce secretion at a more distal locus.