Modulation of insulin secretion from normal rat islets by inhibitors of the post-translational modifications of GTP-binding proteins

Abstract

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-32P]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 microM 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 base-labile 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.

Métabolisme du cholestérol et function β-cellulaire

“…Pourquoi n'y a-t-il pas alors plus d'exemples citant l'efficacité des statines dans la prévention du DT2 ? Trois études indépendantes ont montré qu'un traitement d'îlots isolés de rat ou de cellules MIN-6 par des statines lipophiles (simvastatine, atorvastatine et lovastatine) est toxique sur la cellule , alors que la pravastatine, hydrophile, est sans effet [28][29][30]. Des études interventionnelles chez l'homme, ciblant spécifiquement le rôle des statines sur la fonction  cellulaire, seront nécessaires pour déterminer la nature exacte de ce lien.…”

Characterization of prenylated protein methyltransferase in Leishmania

“…Carboxymethylation is implicated in cell activation [1,[7][8][9]11,13,30,31] and GTP[S], the non-hydrolysable analogue of guanosine triphosphate, stimulates the methylation of some small G-proteins [1,3,13]. Therefore the possibility that this enzyme was regulated by GTP or by a signalling pathway such as Ca# + , cAMP or a protein-kinase-C-mediated process was examined.…”

“…It has been reported that carboxymethylation is involved in the activation of platelets, neutrophils and macrophages [1,7,11] as well as that of normal pancreatic rat islets [13,14] and a pancreatic β-cell line [8]. It is also implicated in the regulation of intracellular pH homeostasis [15].…”

“…It is also implicated in the regulation of intracellular pH homeostasis [15]. The inhibition of protein carboxymethyl- ation alters chemotactic responses [1,7] and insulin secretion [8,13]. Both heterotrimeric and small G-proteins have been implicated in these events [1,7].…”

Store-activated Ca2+ inflow in Xenopus laevis oocytes: inhibition by primaquine and evaluation of the role of membrane fusion

“…No detectable inhibition of Ins(1,4,5)P $ F-stimulated Ca# + inflow was observed when oocytes were preincubated for : 27 h with pertussis toxin, which ADP-ribosylates G i , G o and G t (transducin) [35] ; 24 h with lovastatin, which inhibits the isoprenylation of some monomeric G-proteins [36,37] ; or 5 h with Brefeldin A, an inhibitor of membrane fusion events involving Arf proteins [38] (results not shown). The peptide Arf-1 (2-17), which inhibits the functions of Arf proteins, including the budding of transport vesicles involved in protein trafficking [39], also had no effect on store-activated Ca# + inflow (results not shown).…”