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

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.

[The multifaceted TSH receptor]

The TSH receptor is a key element of thyroid homeostasis and many gain or loss of function mutations have been described since its cloning in 1989. Behind classical and severe forms like toxic adenomas, non-autoimmune familial or sporadic hyperthyroidism or inversely complete TSH resistance syndromes, new methodological approaches allow now the functional characterization of milder clinical situations : linear regression analysis of activating mutations can differentiate mutations with moderate constitutive activity and non functional receptor's variants ; the early oligomerization of wild-type and mutated receptors in intracellular compartments explain the dominant inheritance of mild form of TSH resistance. However many aspects of TSH receptor signalization remain to be explored, for example its persistent activity after internalization, and their clinical implications to be determined. Finally, new modulators of TSH receptor with attractive therapeutic potential are being developed.