Expression of islet inducible nitric oxide synthase and inhibition of glucose-stimulated insulin release after long-term lipid infusion in the rat is counteracted by PACAP27

Abstract

Qader SS, Jimenez-Feltström J, Ekelund M, Lundquist I, Salehi A. Expression of islet inducible nitric oxide synthase and inhibition of glucose-stimulated insulin release after long-term lipid infusion in the rat is counteracted by PACAP27. Am J Physiol Endocrinol Metab 292: E1447-E1455, 2007. First published January 30, 2007; doi:10.1152/ajpendo.00172.2006.-Chronic exposure of pancreatic islets to elevated plasma lipids (lipotoxicity) can lead to ␤-cell dysfunction, with overtime becoming irreversible. We examined, by confocal microscopy and biochemistry, whether the expression of islet inducible nitric oxide synthase (iNOS) and the concomitant inhibition of glucose-stimulated insulin release seen after lipid infusion in rats was modulated by the islet neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP)27. Lipid infusion for 8 days induced a strong expression of islet iNOS, which was mainly confined to ␤-cells and was still evident after incubating islets at 8.3 mmol/l glucose. This was accompanied by a high iNOS-derived NO generation, a decreased insulin release, and increased cyclic GMP accumulation. No iNOS expression was found in control islets. Addition of PACAP27 to incubated islets from lipid-infused rats resulted in loss of iNOS protein expression, increased cyclic AMP, decreased cyclic GMP, and suppression of the activities of neuronal constitutive (nc)NOS and iNOS and increased glucose-stimulated insulin response. These effects were reversed by the PKA inhibitor H-89. The suppression of islet iNOS expression induced by PACAP27 was not affected by the proteasome inhibitor MG-132, which by itself induced the loss of iNOS protein, making a direct proteasomal involvement less likely. Our results suggest that PACAP27 through its cyclic AMP-and PKA-stimulating capacity strongly suppresses not only ncNOS but, importantly, also the lipid-induced stimulation of iNOS expression, possibly by a nonproteasomal mechanism. Thus PACAP27 restores the impairment of glucose-stimulated insulin release and additionally might induce cytoprotection against deleterious actions of iNOS-derived NO in ␤-cells. pancreatic islets; insulin secretion; isoforms of nitric oxide synthase; pituitary adenylate cyclase-activating polypeptide 27 THE SOLUTION USED for total parenteral nutrition (TPN) contains a high amount of lipids, which results in increased plasma levels of free fatty acid (FFA), triglycerides, and cholesterol (8,31,34,36). FFA has complex effects on pancreatic ␤-cells and insulin secretion (8,9,25,31,32,34,36,45,51). Hence, FFA acutely stimulates insulin secretion from isolated pancreatic islets (9), whereas long-term incubation (24 h) of islets in the presence of FFAs negatively modulates ␤-cell function and results in a markedly decreased glucose-stimulated release of insulin (51). Indeed, it has been known for a long time that chronic elevation of plasma FFA impairs the insulin-secreting response to glucose in both humans and animals (8,31,32,34,36,45,51), and this hyperlipidemia has been suggest...

Neuronal nitric oxide synthase protects the pancreatic beta cell from glucolipotoxicity-induced endoplasmic reticulum stress and apoptosis

“…This was in marked contrast with the robust activation of iNOS by cytokines. These results are in contrast with previous suggestions that both glucose and palmitate induce iNOS in beta cell lines and rat islets [19][20][21][22]. Moreover, it was recently shown that induction of iNOS and consequently excessive generation of NO is responsible for the suppression of glucose-stimulated insulin secretion by palmitate; this was corrected by thiazolidinediones, probably through inhibition of G-protein-coupled receptor 40 (GPR40) [18].…”

“…Several studies have suggested that exposure of islets to high glucose or NEFA levels induces production of inducible nitric oxide synthase (iNOS), causing attenuation of the insulin response to glucose [18][19][20][21][22]. Thus, iNOS-dependent NO generation with subsequent induction of ER stress could be the common denominator of beta cell apoptosis induced by inflammatory cytokines and glucolipotoxicity.…”

Exendin-4 inhibits interleukin-1β-induced iNOS expression at the protein level, but not at the transcriptional and posttranscriptional levels, in RINm5F β-cells

“…However, in islet b-cells, knowledge concerning the role of ubiquitinproteasome pathway in iNOS protein degradation is very limited. Pituitary adenylate cyclase-activating polypeptide 27, a cAMP-producing peptide suppressed iNOS expression in lipid-infused rat islets, which was not reversed by the treatment of a proteasome inhibitor MG 132 (Qader et al 2007). Additionally, MG132 did not reverse the GLP-1-induced suppression of iNOS expression in diabetic GK rat islets, but instead MG132 induced loss of iNOS protein (Salehi et al 2008).…”

ADP mediates inhibition of insulin secretion by activation of P2Y13 receptors in mice

“…We found that an ADP analogue reduced cAMP levels and blocking the P2Y 13 receptor with MRS2211 restored cAMP levels in beta cells. cAMP has been shown to improve insulin release and to protect against apoptosis and cell death [24]. Interestingly, the adenosine A 1 receptor is also Gα i -coupled and also decreases cAMP levels.…”

“…The purity of each beta cell preparation was assessed by RIA measurement of insulin, glucagon, somatostatin and pancreatic polypeptide per mg protein [24]. At least 10 samples (tubes) of the cells were examined for each hormone.…”