An inhibitory role for polyamines in protein kinase C activation and insulin secretion in mouse pancreatic islets

Abstract

The occurrence and function of polyamines in protein kinase C activation and insulin secretion in mouse pancreatic islets were studied. Determination of polyamines in mouse islets revealed 0.9 +/- 0.3 (mean +/- S.E.M., n = 6) pmol of putrescine, 11.7 +/- 3.2 (8) pmol of spermidine and 3.7 +/- 0.6 (8) pmol of spermine per islet, corresponding to intracellular concentrations of 0.3-0.5 mM-putrescine, 3.9-5.9 mM-spermidine and 1.2-1.9 mM-spermine in mouse islets. Stimulation of insulin secretion by glucose, the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) or the sulphonylurea glibenclamide did not affect these polyamine contents. In accordance with a role for protein kinase C in insulin secretion, TPA stimulated both protein kinase C activity and insulin secretion. Stimulation of insulin secretion by TPA was dependent on a non-stimulatory concentration of glucose and was further potentiated by stimulatory concentrations of glucose, glibenclamide or 3-isobutyl-1-methylxanthine, suggesting that protein kinase C activation, Ca2+ mobilization and cyclic AMP accumulation are all needed for full secretory response of mouse islets. Spermidine (5 mM) and spermine (1.5 mM) at concentrations found in islets inhibited protein kinase C stimulated by TPA + phosphatidylserine by 55% and 45% respectively. Putrescine (0.5 mM) was without effect, but inhibited the enzyme at higher concentrations (2-10 mM). Inhibition of protein kinase C by polyamines showed competition with Ca2+, and Ca2+ influx in response to glucose or glibenclamide prevented inhibition of insulin secretion by exogenous polyamines at concentrations where they did not affect glucose oxidation. It is suggested that inhibition of protein kinase C by polyamines may be of significance for regulation of insulin secretion in vivo and that Ca2+ influx may function by displacing inhibitory polyamines bound to phosphatidylserine in membranes.

The extracellular calcium-sensing receptor on human beta-cells negatively modulates insulin secretion.

“…However, polyvalent cations, such as magnesium (Mg ), and polycationic compounds, such as polyamines, have been used in other cell systems to activate the CaR (40-42). In pancreatic islets, spermine inhibits glucoseinduced insulin-secretion (43), whereas spermidine and putrescine, which are less potent agonists of the CaR (42), had no effect on glucose-induced secretion. We have previously shown that polymyxin B, another polyamine, has a profound inhibitory effect on insulin secretion (44), whereas the inhibitory action of the divalent cation Co 2 + can be dissociated from any inhibition of Ca 2 + i n flux (45) and, thus, is consistent with activation of the CaR.…”

The effect of polyamines on voltage‐activated calcium channels in mouse neuroblastoma cells.

“…Phorbol esters, which activate protein kinase C, inhibit the L-type channel current in this preparation (Linden & Routtenberg, 1989). Putrescine is known to inhibit protein kinase C but does not affect the cyclic AMP-dependent protein kinase (Qi, Schatzman, Mazzei, Turner, Raynor, Liao & Kuo, 1983;Thams, Capito & Hedeskov, 1986;Moruzzi, Barbiroli, Monti, Tadolini, Hakim & Mezzetti, 1987). It was therefore hypothesized that the effect of putrescine might be mediated via protein kinase C.…”

Influence of polyamines on membrane functions

“…The enzyme, in various tissues, was found to be inhibited by spermine, non-competitively with respect to Ca2 and PS (Qi et al, 1983). The activation of protein kinase C, from mouse pancreatic islets, by a phorbol ester [tetradecanoylphorbol acetate (TPA)], PS and Ca2", was also inhibited by spermine or spermidine (Thams et al, 1986). This effect, which was correlated at the cellular level with a decreased insulin secretion, was attributed to a competition between polyamines and Ca'2 for the PS domain in the membrane which binds the kinase.…”