Inhibition of the nuclear factor-κB pathway prevents beta cell failure and diet induced diabetes in Psammomys obesus

The significance of glutaredoxins for diabetes mellitus and its complications

Diabetes mellitus is a non-communicable metabolic disease hallmarked by chronic hyperglycemia caused by beta-cell failure. Diabetic complications affect the vasculature and result in macro- and microangiopathies, which account for a significantly increased morbidity and mortality. The rising incidence and prevalence of diabetes is a major global health burden. There are no feasible strategies for beta-cell preservation available in daily clinical practice. Therefore, patients rely on antidiabetic drugs or the application of exogenous insulin. Glutaredoxins (Grxs) are ubiquitously expressed and highly conserved members of the thioredoxin family of proteins. They have specific functions in redox-mediated signal transduction, iron homeostasis and biosynthesis of iron-sulfur (FeS) proteins, and the regulation of cell proliferation, survival, and function. The involvement of Grxs in chronic diseases has been a topic of research for several decades, suggesting them as therapeutic targets. Little is known about their role in diabetes and its complications. Therefore, this review summarizes the available literature on the significance of Grxs in diabetes and its complications. In conclusion, Grxs are differentially expressed in the endocrine pancreas and in tissues affected by diabetic complications, such as the heart, the kidneys, the eye, and the vasculature. They are involved in several pathways essential for insulin signaling, metabolic inflammation, glucose and fatty acid uptake and processing, cell survival, and iron and mitochondrial metabolism. Most studies describe significant changes in glutaredoxin expression and/or activity in response to the diabetic metabolism. In general, mitigated levels of Grxs are associated with oxidative distress, cell damage, and even cell death. The induced overexpression is considered a potential part of the cellular stress-response, counteracting oxidative distress and exerting beneficial impact on cell function such as insulin secretion, cytokine expression, and enzyme activity.

Glucolipotoxic conditions induce β-cell iron import, cytosolic ROS formation and apoptosis

Type 2 diabetes (T2D) arises when the pancreatic beta-cell fails to compensate for increased insulin needs due to insulin resistance. Glucolipotoxicity (GLT) has been proposed to induce beta-cell dysfunction in T2D by formation of reactive oxygen species (ROS). Here, we examined if modeling glucolipotoxic conditions by high glucose-high free fatty acid (FFA) exposure (GLT) regulates beta-cell iron transport, by increasing the cytosolic labile iron pool (LIP). In isolated mouse islets, the GLT-induced increase in the LIP catalyzed cytosolic ROS formation and induced apoptosis. We show that GLT-induced ROS production is regulated by an increased LIP associated with elevated expression of genes regulating iron import. Using pharmacological and transgenic approaches, we show that iron reduction and decreased iron import protects from GLT-induced ROS production, prevents impairment of the mitochondrial membrane potential (MMP) and inhibits apoptosis. This study identifies a novel pathway underlying GLT-induced apoptosis involving increased iron import, generation of hydroxyl radicals from hydrogen peroxide through the Fenton reaction in the cytosolic compartment associated with dissipation of the MMP and beta-cell apoptosis.

Eicosapentaenoic acid modulates fatty acid metabolism and inflammation in Psammomys obesus

The desert gerbil, Psammomys obesus, is a unique polygenic animal model of metabolic syndrome (insulin resistance, obesity and type 2 diabetes), and these pathological conditions resemble to those in human beings. In this study, the animals were fed ad libitum either a natural diet (ND) which contained desertic halophile plants or a standard laboratory diet (STD) or a diet which contained eicosapentaenoic acid (EPA), hence, termed as EPA diet (EPAD). In EPAD, 50% of total lipid content was replaced by EPA oil. By employing real-time PCR, we assessed liver expression of key genes involved in fatty acid metabolism such as PPAR-α, SREBP-1c, LXR-α and CHREBP. We also studied the expression of two inflammatory genes, i.e., TNF-α and IL-1β, in liver and adipose tissue of these animals. The STD, considered to be a high caloric diet for this animal, triggered insulin resistance and high lipid levels, along with high hepatic SREBP-1c, LXR-α and CHREBP mRNA expression. TNF-α and IL-1β mRNA were also high in liver of STD fed animals. Feeding EPAD improved plasma glucose, insulin and triacylglycerol levels along with hepatic lipid composition. These observations suggest that EPA exerts beneficial effects in P. obesus.

Transcriptional and translational regulation of cytokine signaling in inflammatory β-cell dysfunction and apoptosis

Disease is conventionally viewed as the chaotic inappropriate outcome of deranged tissue function resulting from aberrancies in cellular processes. Yet the patho-biology of cellular dysfunction and death encompasses a coordinated network no less sophisticated and regulated than maintenance of homeostatic balance. Cellular demise is far from passive subordination to stress but requires controlled coordination of energy-requiring activities including gene transcription and protein translation that determine the graded transition between defensive mechanisms, cell cycle regulation, dedifferentiation and ultimately to the activation of death programmes. In fact, most stressors stimulate both homeostasis and regeneration on one hand and impairment and destruction on the other, depending on the ambient circumstances. Here we illustrate this bimodal ambiguity in cell response by reviewing recent progress in our understanding of how the pancreatic β cell copes with inflammatory stress by changing gene transcription and protein translation by the differential and interconnected action of reactive oxygen and nitric oxide species, microRNAs and posttranslational protein modifications.

Divalent metal transporter 1 regulates iron-mediated ROS and pancreatic β cell fate in response to cytokines

Reactive oxygen species (ROS) contribute to target-cell damage in inflammatory and iron-overload diseases. Little is known about iron transport regulation during inflammatory attack. Through a combination of in vitro and in vivo studies, we show that the proinflammatory cytokine IL-1β induces divalent metal transporter 1 (DMT1) expression correlating with increased β cell iron content and ROS production. Iron chelation and siRNA and genetic knockdown of DMT1 expression reduce cytokine-induced ROS formation and cell death. Glucose-stimulated insulin secretion in the absence of cytokines in Dmt1 knockout islets is defective, highlighting a physiological role of iron and ROS in the regulation of insulin secretion. Dmt1 knockout mice are protected against multiple low-dose streptozotocin and high-fat diet-induced glucose intolerance, models of type 1 and type 2 diabetes, respectively. Thus, β cells become prone to ROS-mediated inflammatory damage via aberrant cellular iron metabolism, a finding with potential general cellular implications.

Prunella vulgaris aqueous extract attenuates IL-1β-induced apoptosis and NF-κB activation in INS-1 cells

We previously reported that Prunella vulgaris aqueous extract (PVAE) promotes hepatic glycogen synthesis and decreases postprandial hyperglycemia in ICR mice. Inflammatory cytokines play a critical role in the pathogenesis of diabetes. This study was designed to examine whether PVAE has a protective effect on IL-1β-induced apoptosis in INS-1 cells. INS-1 pancreatic β cells were plated at 2×10(6)/ml and treated with PVAE (100 µg/ml) 30 min before the cells were challenged with IL-1β (10 ng/ml). Untreated INS-1 cells served as control. INS-1 cell cytotoxicity was examined by MTT and lactate dehydrogenase (LDH) activity assays. Caspase-3 activity and activation of the apoptotic signaling pathway were analyzed by western blotting. NF-κB binding activity was examined by EMSA. The levels of inflammatory cytokines in the supernatant were measured by ELISA. IL-1β treatment significantly induced INS-1 cell death by 49.2%, increased LDH activity by 1.5-fold and caspase-3 activity by 7.6-fold, respectively, compared with control cells. However, PVAE administration significantly prevented IL-1β-increased INS-1 cell death and LDH activity and attenuated IL-1β-increased caspase-3 activity. Western blot data showed that PVAE also significantly attenuated IL-1β-increased Fas, FasL and phospho-JNK levels in the INS-1 cells. In addition, PVAE treatment significantly attenuated IL-1β-increased NF-κB binding activity and prevented IL-1β-increased TNF-α and IL-6 expression in INS-1 cells. Our data suggest that PVAE has a protective effect on IL-1β-induced INS-1 cell apoptosis. PVAE also attenuates IL-1β-increased NF-κB binding activity and inflammatory cytokine expression in INS-1 cells. PVAE may have a benefit for type I diabetic patients.