Protein acylation in the inhibition of insulin secretion by norepinephrine, somatostatin, galanin, and PGE₂

Abstract

The major physiological inhibitors of insulin secretion, norepinephrine, somatostatin, galanin, and prostaglandin E 2, act via specific receptors that activate pertussis toxin (PTX)-sensitive G proteins. Four inhibitory mechanisms are known: 1) activation of ATP-sensitive K channels and repolarization of the ␤-cell; 2) inhibition of L-type Ca 2ϩ channels; 3) decreased activity of adenylyl cyclase; and 4) inhibition of exocytosis at a "distal" site in stimulus-secretion coupling. We have examined the underlying mechanisms of inhibition at this distal site. In rat pancreatic islets, 2-bromopalmitate, cerulenin, and polyunsaturated fatty acids, all of which suppress protein acyltransferase activity, blocked the distal inhibitory effects of norepinephrine in a concentration-dependent manner. In contrast, control compounds such as palmitate, 16-hydroxypalmitate, and etomoxir, which do not block protein acylation, had no effect. Furthermore, 2-bromopalmitate also blocked the distal inhibitory actions of somatostatin, galanin, and prostaglandin E 2. Importantly, neither 2-bromopalmitate nor cerulenin affected the action of norepinephrine to decrease cAMP production. We also examined the effects of norepinephrine, 2-bromopalmitate, and cerulenin on palmitate metabolism. Palmitate oxidation and its incorporation into lipids seemed not to contribute to the effects of 2-bromopalmitate and cerulenin on norepinephrine action. These data suggest that protein acylation mediates the distal inhibitory effect on insulin secretion. We propose that the inhibitors of insulin secretion, acting via PTX-sensitive G proteins, activate a specific protein acyltransferase, causing the acylation of a protein or proteins critical to exocytosis. This particular acylation and subsequent disruption of the essential and precise interactions involved in core complex formation would block exocytosis. rat pancreatic islets; ␤-cell; signaling; pertussis toxin; G proteins INSULIN SECRETION is a complex process regulated by nutrients, such as glucose and amino acids, and hormonal and neural factors that provide stimulatory and inhibitory influences on the pancreatic ␤-cells. The major physiological inhibitors, such as norepinephrine (NE), somatostatin, prostaglandin E 2 (PGE 2 ), and galanin, can inhibit insulin secretion that is stimulated by all of the nutrient and modulatory pathways. They interact with specific G protein-linked receptors at the plasma membrane, which activate pertussis toxin (PTX)-sensitive G i and G o proteins that mediate the inhibition of insulin secretion (17,27,30).The inhibitory pathways act in contradictory fashion on many of the same sites as the stimulatory pathways (30). For example, 1) inhibitors activate the ATP-sensitive K (K ATP ) channel, repolarize or hyperpolarize the cell membrane, and thus inhibit the action of glucose that depolarizes the membrane via closure of this channel; 2) inhibitors decrease the activity of the Ltype Ca 2ϩ channel by two means, indirectly by the activation of the K ATP channel and hyperp...

Combined galanin with insulin improves insulin sensitivity of diabetic rat muscles

“…After administration of insulin, the galanin mRNA levels in adrenals were maximally increased at 4 h, remained at maximal elevation for at least 48 h, and returned to baseline levels in 6 days (Anouar & Eiden 1995). Whereas an infusion of galanin, as mentioned above, caused a rapid, reversible, and dose-dependent reduction in basal insulin secretion from pancreatic islets via pertussis-toxinsensitive G proteins (Cheng et al 2003, Manabe et al 2003. Interestingly, the injection of galanin has an inhibitory effect on glucose-stimulated, but not L-arginineor potassium-stimulated, insulin release in animals (Yoshimura et al 1989) but fails to modify insulin secretion in man (Ghigo et al 1992).…”

“…of petussis-toxin-sensitive inhibitory GTP-binding regulatory protein (Cheng et al 2003, Manabe et al 2003. In the galanin-knockout mice, the initial short inhibition of insulin secretion was impaired, followed by an augmentation of insulin secretion when sympathetic and parasympathetic branches were chemically activated (Ahrén et al 2004).…”

The inhibitors of protein acylation, cerulenin and tunicamycin, increase voltage-dependent Ca2+ currents in the insulin-secreting INS 832/13 cell

“…It should also be noted that the direct application of long chain acyl CoA amplified the secretory response to Ca 2+ when capacitance was used as a measure of exocytosis and that the amplification was blocked by cerulenin [16]. Interestingly, there is evidence that protein acylation may be involved in the inhibition of insulin secretion by the physiological inhibitors norepinephrine, somatostatin, galanin and PGE 2 [20]. This suggests that in the same way, for example, that signaling through protein phosphorylation can have stimulatory or inhibitory effects within the cell depending upon their targets, so protein acylation may do likewise.…”

“…As protein acylation is a reversible process with dynamic cycles of acylation and deacylation [8,14] it is capable of playing a major role in signal transduction. It has been suggested that protein acylation plays a signaling role in the control of glucose-stimulated insulin secretion [15][16][17][18][19] and in the action of inhibitors of secretion such as norepinephrine [20]. In view of the importance of Ca 2+ -channels in the stimulation of insulin secretion [21][22][23][24][25][26] and the observation that Ca 2+ channels in bovine chromaffin cells are acylated [9], we studied the effects of two inhibitors of protein acylation, cerulenin and tunicamycin, on the activity of L-type Ca 2+ -channels in the β-cell line INS 832/13.…”

Modulation of Insulin Secretion by Fatty Acyl Analogs

“…The amplifying incretin-like activity of LCFA has been proposed to reflect increased levels of endogenous LCFA-CoA in excess of those maintained by inhibition of ␤-oxidation by glucose-derived malonylCoA (3). LCFA-CoAs were assumed to directly modulate effector systems involved in GSIS (e.g., VGCC [11], protein kinase C [PKC] [12], exocytosis proteins [13]) or to serve as a precursor of respective signaling lipids (e.g., diacylglycerol, phosphatidylinositol 4,5-bisphosphate) (3). The incretin-like activity of LCFA-CoA has been proposed to be further complemented by activation of G-protein coupled receptor 40 (GPR40) by the free-acid form of LCFA (14 -16), resulting in Ca ϩ2 mobilization, PKC activation, cAMP-dependent protein kinase activation (4), and inhibition of voltage-gated outward K ϩ currents (17).…”