Protective Role of Taurine against Arsenic-Induced Mitochondria-Dependent Hepatic Apoptosis via the Inhibition of PKCδ-JNK Pathway

BACKGROUND: Oxidative stress-mediated hepatotoxic effect of arsenic (As) is mainly due to the depletion of glutathione (GSH) in liver. Taurine, on the other hand, enhances intracellular production of GSH. Little is known about the mechanism of the beneficial role of taurine in As-induced hepatic pathophysiology. Therefore, in the present study we investigated its beneficial role in As-induced hepatic cell death via mitochondria-mediated pathway. METHODOLOGY/PRINCIPAL FINDINGS: Rats were exposed to NaAsO(2) (2 mg/kg body weight for 6 months) and the hepatic tissue was used for oxidative stress measurements. In addition, the pathophysiologic effect of NaAsO(2) (10 microM) on hepatocytes was evaluated by determining cell viability, mitochondrial membrane potential and ROS generation. As caused mitochondrial injury by increased oxidative stress and reciprocal regulation of Bcl-2, Bcl-xL/Bad, Bax, Bim in association with increased level of Apaf-1, activation of caspase 9/3, cleavage of PARP protein and ultimately led to apoptotic cell death. In addition, As markedly increased JNK and p38 phosphorylation with minimal disturbance of ERK. Pre-exposure of hepatocytes to a JNK inhibitor SP600125 prevented As-induced caspase-3 activation, ROS production and loss in cell viability. Pre-exposure of hepatocytes to a p38 inhibitor SB2035, on the other hand, had practically no effect on these events. Besides, As activated PKCdelta and pre-treatment of hepatocytes with its inhibitor, rottlerin, suppressed the activation of JNK indicating that PKCdelta is involved in As-induced JNK activation and mitochondrial dependent apoptosis. Oral administration of taurine (50 mg/kg body weight for 2 weeks) both pre and post to NaAsO(2) exposure or incubation of the hepatocytes with taurine (25 mM) were found to be effective in counteracting As-induced oxidative stress and apoptosis. CONCLUSIONS/SIGNIFICANCE: Results indicate that taurine treatment improved As-induced hepatic damages by inhibiting PKCdelta-JNK signalling pathways. Therefore taurine supplementation could provide a new approach for the reduction of hepatic complication due to arsenic poisoning

Increased nicotinamide nucleotide transhydrogenase levels predispose to insulin hypersecretion in a mouse strain susceptible to diabetes

Aims/hypothesis Insulin hypersecretion may be an independent predictor of progression to type 2 diabetes. Identifying genes affecting insulin hypersecretion are important in understanding disease progression. We have previously shown that diabetes-susceptible DBA/2 mice congenitally display high insulin secretion. We studied this model to map and identify the gene(s) responsible for this trait.Methods Intravenous glucose tolerance tests followed by a genome-wide scan were performed on 171 (C57BL/6 &times; DBA/2) &times; C57BL/6 backcross mice.Results A quantitative trait locus, designated hyperinsulin production-1 (Hip1), was mapped with a logarithm of odds score of 7.7 to a region on chromosome 13. Production of congenic mice confirmed that Hip1 influenced the insulin hypersecretion trait. By studying appropriate recombinant inbred mouse strains, the Hip1 locus was further localised to a 2 Mb interval, which contained only nine genes. Expression analysis showed that the only gene differentially expressed in islets isolated from the parental strains was Nnt, which encodes the mitochondrial proton pump, nicotinamide nucleotide transhydrogenase (NNT). We also found in five mouse strains a positive correlation (r 2 &thinsp;=&thinsp;0.90, p&thinsp;&lt;&thinsp;0.01) between NNT activity and first-phase insulin secretion, emphasising the importance of this enzyme in beta cell function. Furthermore, of these five strains, only those with high NNT activity are known to exhibit severe diabetes after becoming obese.Conclusions/interpretation Insulin hypersecretion is associated with increased Nnt expression. We suggest that NNT must play an important role in beta cell function and that its effect on the high insulin secretory capacity of the DBA/2 mouse may predispose beta cells of these mice to failure.<br /

The influence of genetic background on the induction of oxidative stress and impaired insulin secretion in mouse islets

Aims/hypothesis We determined whether high-glucose-induced beta cell dysfunction is associated with oxidative stress in the DBA/2 mouse, a mouse strain susceptible to islet failure.Materials and methods Glucose- and non-glucose-mediated insulin secretion from the islets of DBA/2 and control C57BL/6 mice was determined following a 48-h exposure to high glucose. Flux via the hexosamine biosynthesis pathway was assessed by determining O-glycosylated protein levels. Oxidative stress was determined by measuring hydrogen peroxide levels and the expression of anti-oxidant enzymes.Results Exposure to high glucose levels impaired glucose-stimulated insulin secretion in DBA/2 islets but not C57BL/6 islets, and this was associated with reduced islet insulin content and lower ATP levels than in C57BL/6 islets. Exposure of islets to glucosamine for 48 h mimicked the effects of high glucose on insulin secretion in the DBA/2 islets. High glucose exposure elevated O-glycosylated proteins; however, this occurred in islets from both strains, excluding a role for O-glycosylation in the impairment of DBA/2 insulin secretion. Additionally, both glucosamine and high glucose caused an increase in hydrogen peroxide in DBA/2 islets but not in C57BL/6 islets, an effect prevented by the antioxidant N-acetyl-l-cysteine. Interestingly, while glutathione peroxidase and catalase expression was comparable between the two strains, the antioxidant enzyme manganese superoxide dismutase, which converts superoxide to hydrogen peroxide, was increased in DBA/2 islets, possibly explaining the increase in hydrogen peroxide levels.Conclusions/interpretation Chronic high glucose culture caused an impairment in glucose-stimulated insulin secretion in DBA/2 islets, which have a genetic predisposition to failure, and this may be the result of oxidative stress.<br /