Increased neuronal nitric oxide synthase dimerisation is involved in rat and human pancreatic beta cell hyperactivity in obesity

Abstract

Aims/hypothesis Pancreatic beta cell hyperactivity is known to occur in obesity, particularly in insulin-resistant states. Our aim was to investigate whether changes in neuronal nitric oxide synthase (nNOS) function affect beta cell compensation in two relevant models: the Zucker fa/fa rats and pancreatic islets from obese humans. Methods Glucose-induced insulin response was evaluated in the isolated perfused rat pancreas and in human pancreatic islets from obese individuals. Expression of nNOS (also known as NOS1) and subcellular localisation of nNOS were studied by quantitative RT-PCR, immunoblotting, immunofluorescence and electron microscopy. Results Pancreatic beta cells from Zucker fa/fa rats and obese individuals were found to be hyper-responsive to glucose. Pharmacological blockade of nNOS was unable to modify beta cell response to glucose in fa/fa rats and in islets from obese individuals, suggesting an abnormal control of insulin secretion by the enzyme. In both cases, nNOS activity in islet cell extracts remained unchanged, despite a drastic increase in nNOS protein and an enhancement in the dimer/monomer ratio, pointing to the presence of high amounts of catalytically inactive enzyme. This relative decrease in activity could be mainly related to increases in islet asymmetric dimethyl-arginine content, an endogenous inhibitor of nNOS activity. In addition, mitochondrial nNOS level was decreased, which contrasts with a strongly increased association with insulin granules. Conclusions/interpretation Increased nNOS production and dimerisation, together with a relative decrease in catalytic activity and relocalisation, are involved in beta cell hyperactivity in insulin-resistant rats but also in human islets isolated from obese individuals.

Is there a role for neuronal nitric oxide synthase (nNOS) in cytokine toxicity to pancreatic beta cells?

“…Therefore the earlier observations together with our present data indicate that the basal nNOS expression plays a regulatory role in insulin secretion, probably by preventing possible episodes of hyperinsulinemia, but is not disadvantageous for the beta cell function or does not promote beta cell vulnerability. A decreased catalytical activity of nNOS and its relocalization have been shown to be involved in the hyperactivity of beta cells from insulinresistant rats and also from human islets isolated from obese individuals [39].…”

Metformin Ameliorates Dysfunctional Traits of Glibenclamide- and Glucose-Induced Insulin Secretion by Suppression of Imposed Overactivity of the Islet Nitric Oxide Synthase-NO System

“…Thus freshly isolated islets from both young and adult hyperglycemic ob/ob mice are almost devoid of iNOS activity and display markedly reduced ncNOS activity coincident with a high rate of insulin release and the absence of a beta-cell demise in spite of their longstanding obesity and hyperglycemia [30]. Furthermore, it was recently shown that islets from obese human individuals as well as from obese Zucker fa/fa rats displayed a drastic increase in catalytically inactive ncNOS enzyme protein coincident with an augmented glucose-stimulated insulin release [12]. These data thus strengthen our hypothesis of ncNOS-derived NO as an important negative modulator of glucose-stimulated insulin release.…”

“…The neuronal constitutive NO-synthase (ncNOS, also named nNOS or NOS1) is constitutively expressed in the beta-cells of both rodent and human islets[10–12], while the inducible NOS (iNOS, also named NOS2) is expressed mainly at high persisting glucose levels and/or after cytokine impact on the beta-cell [9,13–15]. We have shown in normal rodent islets that ncNOS-derived NO is immediately released upon glucose stimulation both in vivo and in vitro [6,7], while iNOS-derived NO is apparent first at the latter part of a 60 min exposure to an approximate glucose concentration of ≥9–10 mmol/l [7].…”

“…We have shown in normal rodent islets that ncNOS-derived NO is immediately released upon glucose stimulation both in vivo and in vitro [6,7], while iNOS-derived NO is apparent first at the latter part of a 60 min exposure to an approximate glucose concentration of ≥9–10 mmol/l [7]. Moreover, ncNOS protein is in great part associated with the insulin secretory granules and mitochondria [11,12], whereas iNOS protein is more uniformly distributed in the beta-cell cytoplasm [16]. Thus beta-cell ncNOS is conveniently located to immediately impact and regulate the secretory machinery after acute glucose stimulation, while iNOS-derived NO has been shown to be insufficient in this context [17] and more likely contributes to β-cell dysfunction during persisting hyperglycemia and/or hyperlipidemia and the development of nonimmunogenic type 2 diabetes over time [8,9,15,18–23].…”

Diet‐induced obesity in young mice: Consequences on the pancreatic intrinsic nervous system control of insulin secretion

“…In type 2 diabetes mellitus (T2DM) Chinese hamsters, reduced islet innervation was reported, as was the remodelling of the mouse islet sympathetic innervation in diabetic injuries . To date, there is no data on how nitrergic neurons are involved but abnormalities in islet NO production have been reported in T2DM Goto‐Kakizaki rats and in islets of obese Zucker fa/fa rats and humans …”