Meta-analysis and functional effects of the SLC30A8 rs13266634 polymorphism on isolated human pancreatic islets.

BACKGROUND: The C-allele of rs13266634 located in SLC30A8 (ZNT8) has been strongly associated with decreased insulin release and with type 2 diabetes (T2D) susceptibility in some but not all studies. To shed further light on this issue, we performed a meta-analysis of the association between rs13266634 and T2D in different ethnic groups and assessed the relationships between SLC30A8 genotypes and some properties of isolated human islets. METHODS: From 32 original articles, a total of 77,234 control individuals and 44,945 subjects with T2D were studied in meta-analysis. To assess the relationships between SLC30A8 genotype and islet cell phenotype, insulin secretion in response to glucose, glucose plus arginine and glucose plus glibenclamide was determined in pancreatic islets isolated from 82 multiorgan donors genotyped for the rs13266634 polymorphism. Quantitative expression of SLC30A8, Insulin and Glucagon mRNA was also measured. RESULTS: Overall, each SLC30A8 risk allele was associated with a 14% increased risk for T2D (P=2.78 x 10(-34)). The population risk of T2D attributable to this polymorphism was estimated at 9.5% in Europeans and 8.1% in East Asians. Basal and stimulated insulin secretion from human islets as well as islet expressions of SLC30A8, Insulin and Glucagon were not affected by the presence of the polymorphism. However, SLC30A8 expression was positively correlated with Insulin (r=0.75, P=6.43 x 10(-6)) and Glucagon (r: 0.70, P=4.89 x 10(-5)) levels. CONCLUSIONS: The SLC30A8 rs13266634 polymorphism is among the most confirmed genetic markers of T2D in Europeans and East Asians. In isolated human islets, the risk C-allele does not affect ex-vivo insulin secretion and SLC30A8 expression, which is correlated with that of insulin and glucagon.

The non-peptidyl low molecular weight radical scavenger IAC protects human pancreatic islets from lipotoxicity.

BACKGROUND: Chronic exposure to high free fatty acids (FFA) can lead to irreversible damage of beta-cell accounting for impaired insulin secretion. Multiple mechanisms concur in generating the damage, but activation of oxidative stress may contribute to the final toxic effect. To better understand the phenomenon of lipotoxicity in human beta-cells, we evaluated the effects of 24-h pre-culture with 1.0 mmol/l FFA on the function, survival and mRNA expression of several enzymes involved in the generation and scavenging of reactive oxygen species (ROS). MATERIAL AND METHODS: Human islets, prepared by collagenase digestion and density gradient purification from 9 pancreases of multiorgan donors, were incubated for 24-h in the presence 1.0 mmol/l long-chain mixture (oleate:palmitate, 2:1) FFA, with or without 100 micromol/l IAC, a non-peptidyl low molecular weight radical scavenger. At the end of incubation period, insulin secretion was measured by static incubation, and mRNA expression of insulin, Cu/Zn-SOD, Mn-SOD, Catalase, Glutathione peroxidase (GSH-px) and HO-1 by quantitative Real-Time RT-PCR. Nitrotyrosine levels were determined by an ELISA technique. RESULTS: As compared to control incubation (Ctrl, no FFA), exposure to FFA was associated with impaired insulin release and reduced insulin mRNA expression. The presence of IAC in the incubation medium increased insulin release significantly and prevented changes in mRNA expression. Exposure to FFA was associated with oxidative stress as indicated by a significant accumulation of nitrotyrosine and IAC restrained such an increase. mRNA expression of Cu/Zn-SOD, Mn-SOD, Catalase, GSH-Px, and HO-1 were all modified after FFA exposure. These changes were partially prevented in the presence of IAC. CONCLUSIONS: In human islets 24-h exposure to high FFA causes oxidative stress associated with changes of several enzymes involved in ROS scavenging. These effects were prevented by the use of an antioxidant molecule.

Protective effects of St. John's wort extract and its component hyperforin against cytokine-induced cytotoxicity in a pancreatic beta-cell line.

In both type 1 and type 2 diabetes, increased production of cytokines on autoimmune or metabolic basis is supposed to trigger an inflammatory process leading to dysfunction and death of pancreatic beta-cells. Therefore, anti-inflammatory pharmacological approaches aimed at blocking cytokine signalling pathways and consequent cytotoxicity in beta-cells are highly advisable. Based on previous evidence of cytokine antagonistic effects in other cell types, we explored the protective action of Hypericum perforatum (St-John's-wort) extract and its component hyperforin against cytokine-induced functional impairment and apoptosis in the INS-1E beta-cell line, searching for the underlying mechanisms. The results showed that either St-John's-wort extract or hyperforin (at 1-3 microM) prevented cytokine-induced impairment in glucose-stimulated insulin secretion and protected cells against apoptosis in a dose-dependent fashion. Inducible-NO-synthase expression was also potently hindered by the vegetal compounds. Interestingly, cytokine-induced activations of the signal-transducer-and-activator-of-transcription-1 (STAT-1) and the nuclear-factor-kappaB (NF-kappaB) were both down-regulated by SJW extract or HPF (range 0.5-5 microM) when evaluated by electrophoretic-mobility-shift-assay. Other transcription factors (CBF-1, SP-1) were unaffected. Components of SJW extract other than HPF were much less effective in down-regulating cytokine signalling. Significantly, inhibition of cytokine-elicited STAT-1 and NF-kappaB activation was confirmed in isolated rat and human islets incubated in the presence of these vegetal compounds. In conclusion, St-John's-wort extract and hyperforin are non-peptidyl compounds which, at low concentrations, target key mechanisms of cytokine-induced beta-cell injury, thereby improving beta-cell function and survival. Thus, they are potentially valuable for the prevention or limitation of beta-cell loss in diabetes.

Reduction of oxidative stress by a new low-molecular-weight antioxidant improves metabolic alterations in a nonobese mouse diabetes model.

OBJECTIVES: We have previously established a nonobese diabetes mouse model characterized by moderate hyperglycemic levels, like those usually occurring in human type 2 diabetes. As oxidative stress is considered a major mechanism of progressive beta-cell damage in diabetes, we tested in this model the protective effects of a new low-molecular-weight antioxidant, namely, bis(1-hydroxy-2,2,6,6-tetramethyl-4-piperidinyl)decandioate dihydrochloride (IAC). METHODS: Diabetes was induced in C57Bl/6J mice by streptozotocin (STZ) and nicotinamide (NA) administration. Two weeks later, STZ-NA mice were treated for 5 weeks with different doses of IAC (15 or 30 mg/kg per day intraperitoneally) and monitored for glycemia, insulinemia, glucose tolerance, and pancreatic insulin content. RESULTS: Streptozotocin-NA mice showed moderate hyperglycemia, hypoinsulinemia, glucose intolerance, growth impairment, and markedly reduced pancreatic insulin content (22% of controls). IAC-treated STZ-NA mice showed clear-cut reduction of hyperglycemia and attenuation of glucose intolerance, associated to higher residual pancreatic insulin content with respect to untreated diabetic animals. Plasma nitrotyrosine levels (an index of oxidative stress), enhanced 3-fold in diabetic mice, were significantly reduced by IAC treatment. Significant correlations were found between plasma nitrotyrosine values and either blood glucose levels or pancreatic insulin content. CONCLUSIONS: In the STZ-NA diabetic mouse model, the new antioxidant, IAC, improves diabetic metabolic alterations, likely by counteracting beta-cell dysfunction and loss associated with oxidative stress.