Thioredoxin-interacting protein (TXNIP) regulates critical biological processes including inflammation, stress and apoptosis. TXNIP is upregulated by glucose and is a critical mediator of hyperglycemia-induced beta-cell apoptosis in diabetes. In contrast, the saturated long-chain fatty acid palmitate, although toxic to the beta-cell, inhibits TXNIP expression. The mechanisms involved in the opposing effects of glucose and fatty acids on TXNIP expression are unknown. We found that both palmitate and oleate inhibited TXNIP in a rat beta-cell line and islets. Palmitate inhibition of TXNIP was independent of fatty acid beta-oxidation or esterification. AMP-activated protein kinase (AMPK) has an important role in cellular energy sensing and control of metabolic homeostasis; therefore we investigated its involvement in nutrient regulation of TXNIP. As expected, glucose inhibited whereas palmitate stimulated AMPK. Pharmacologic activators of AMPK mimicked fatty acids by inhibiting TXNIP. AMPK knockdown increased TXNIP expression in presence of high glucose with and without palmitate, indicating that nutrient (glucose and fatty acids) effects on TXNIP are mediated in part via modulation of AMPK activity. TXNIP is transcriptionally regulated by carbohydrate response element-binding protein (ChREBP). Palmitate inhibited glucose-stimulated ChREBP nuclear entry and recruitment to the Txnip promoter, thereby inhibiting Txnip transcription. We conclude that AMPK is an important regulator of Txnip transcription via modulation of ChREBP activity. The divergent effects of glucose and fatty acids on TXNIP expression result in part from their opposing effects on AMPK activity. In light of the important role of TXNIP in beta-cell apoptosis, its inhibition by fatty acids can be regarded as an adaptive/protective response to glucolipotoxicity. The finding that AMPK mediates nutrient regulation of TXNIP may have important implications for the pathophysiology and treatment of diabetes.

Aims/hypothesis In type 2 diabetes, glucose toxicity leads to beta cell apoptosis with decreased beta cell mass as a consequence. Thioredoxin-interacting protein (TXNIP) is a critical mediator of glucose-induced beta cell apoptosis. Since hyperglycaemia leads to elevated serum insulin, we hypothesised that insulin is involved in the regulation of TXNIP protein levels in beta cells. Methods We studied the production of TXNIP in INS-1E beta cells and in islets of Psammomys obesus, an animal model of type 2 diabetes, in response to glucose and different modulators of insulin secretion. Results TXNIP production was markedly augmented in islets from diabetic P. obesus and in beta cells exposed to high glucose concentration. In contrast, adding insulin to the culture medium or stimulating insulin secretion with different secretagogues suppressed TXNIP. Inhibition of glucose and fatty acid-stimulated insulin secretion with diazoxide increased TXNIP production in beta cells. Nitric oxide (NO), a repressor of TXNIP, enhanced insulin signal transduction, whereas inhibition of NO synthase abolished its activation, suggesting that TXNIP inhibition by NO is mediated by stimulation of insulin signalling. Treatment of beta cells chronically exposed to high glucose with insulin reduced beta cell apoptosis. Txnip knockdown mimicking the effect of insulin prevented glucose-induced beta cell apoptosis. Conclusions/interpretation Insulin is a potent repressor of TXNIP, operating a negative feedback loop that restrains the stimulation of TXNIP by chronic hyperglycaemia. Repression of TXNIP by insulin is probably an important compensatory mechanism protecting beta cells from oxidative damage and apoptosis in type 2 diabetes.

In type 2 diabetes, the β-cell is exposed to chronic hyperglycaemia, which increases its metabolic activity, with excess generation of reactive oxygen species (ROS) as a consequence. ROS accumulation induces both oxidative and endoplasmic reticulum (ER) stress, which may lead to β-cell dysfunction and apoptosis. Recent data suggest that oxidative and ER stress are interconnected, although the mechanisms involved in nutrient regulation of the different stress pathways are dissimilar. Several components of the oxidative and ER stress machineries have important roles in the physiological response to glucose and are thus necessary for normal β-cell function. Glucose stimulates signalling pathways that provide crucial messages for β-cell adaptation to metabolic stress; however, the same pathways may eventually lead to apoptosis. Dynamic, temporally fluctuating activation of stress signalling is probably required for the maintenance of β-cell survival, whereas its persistent activation results in β-cell dysfunction and apoptosis. Thus, stress signalling is a 'double-edged sword' that may promote adaptation or apoptosis according to the balance between the divergent outputs of the various pathways. Developing new strategies for β-cell protection based on inhibition of oxidative and/or ER stress requires comprehensive understanding of the switch from β-cell adaptation to β-cell apoptosis under conditions of metabolic stress, such as occurs under hyperglycaemic conditions. Keywords: β-cells, diabetes, ER stress, oxidative stress, UPR Date submitted 26 March 2010; date of final acceptance 27 April 2010 IntroductionIncreasing evidence indicates that β-cell failure is the mainstay of the development and progression of type 2 diabetes [1]. The precise mechanisms underlying the β-cell dysfunction in type 2 diabetes are not fully understood. It has been long known that insulin secretion is genetically controlled [2,3]. Genomewide linkage analysis discovered several genetic variants that are associated with increased risk of diabetes [4,5]. Intriguingly, the majority of the identified genes regulates β-cell function and probably also β-cell proliferation and apoptosis [4], supporting the hypothesis that the capacity of the β-cell to respond to prolonged increased demand, which determines the risk of developing diabetes, is under genetic control. Disparity between the demand on the β-cell, determined by the chronic nutritional load and the body's insulin sensitivity, and the capacity of the β-cell to increase insulin secretion leads to hyperglycaemia.Once hyperglycaemia develops, the β-cell is exposed to increased metabolic flux, which activates multiple pathways leading to cellular stress. This in turn may further impair β-cell function and survival, a process called glucotoxicity [6,7]. In type 2 diabetes, hyperglycaemia is commonly associated with deregulation of lipid metabolism and elevation of free fatty acids (FFA), which contributes to β-cell dysfunction (lipotoxicity) [6,8,9]. However, FFA do not adversely affect β-cell...

OBJECTIVEOveractivity of the Forkhead transcription factor FoxO1 promotes diabetic hyperglycemia, dyslipidemia, and acute-phase response, whereas suppression of FoxO1 activity by insulin may alleviate diabetes. The reported efficacy of long-chain fatty acyl (LCFA) analogs of the MEDICA series in activating AMP-activated protein kinase (AMPK) and in treating animal models of diabesity may indicate suppression of FoxO1 activity.RESEARCH DESIGN AND METHODSThe insulin-sensitizing and anti-inflammatory efficacy of a MEDICA analog has been verified in guinea pig and in human C-reactive protein (hCRP) transgenic mice, respectively. Suppression of FoxO1 transcriptional activity has been verified in the context of FoxO1- and STAT3-responsive genes and compared with suppression of FoxO1 activity by insulin and metformin.RESULTSTreatment with MEDICA analog resulted in total body sensitization to insulin, suppression of lipopolysaccharide-induced hCRP and interleukin-6–induced acute phase reactants and robust decrease in FoxO1 transcriptional activity and in coactivation of STAT3. Suppression of FoxO1 activity was accounted for by its nuclear export by MEDICA-activated AMPK, complemented by inhibition of nuclear FoxO1 transcriptional activity by MEDICA-induced C/EBPβ isoforms. Similarly, insulin treatment resulted in nuclear exclusion of FoxO1 and further suppression of its nuclear activity by insulin-induced C/EBPβ isoforms. In contrast, FoxO1 suppression by metformin was essentially accounted for by its nuclear export by metformin-activated AMPK.CONCLUSIONSSuppression of FoxO1 activity by MEDICA analogs may partly account for their antidiabetic anti-inflammatory efficacy. FoxO1 suppression by LCFA analogs may provide a molecular rational for the beneficial efficacy of carbohydrate-restricted ketogenic diets in treating diabetes.

SummaryOxygen (O2) homeostasis is important for all aerobic animals. However, the manner by which O2 sensing and homeostasis contribute to lifespan regulation is poorly understood. Here, we use the nematode Caenorhabditis elegans to address this question. We demonstrate that a loss‐of‐function mutation in the neuropeptide receptor gene npr‐1 and a deletion mutation in the atypical soluble guanylate cyclase gcy‐35 O2 sensor interact synergistically to extend worm lifespan. The function of npr‐1 and gcy‐35 in the O2‐sensing neurons AQR, PQR, and URX shortens the lifespan of the worm. By contrast, the activity of the atypical soluble guanylate cyclase O2 sensor gcy‐33 in these neurons is crucial for lifespan extension. In addition to AQR, PQR, and URX, we show that the O2‐sensing neuron BAG and the interneuron RIA are also important for the lifespan lengthening. Neuropeptide processing by the proprotein convertase EGL‐3 is essential for lifespan extension, suggesting that the synergistic effect of joint loss of function of gcy‐35 and npr‐1 is mediated through neuropeptide signal transduction. The extended lifespan is regulated by hypoxia and insulin signaling pathways, mediated by the transcription factors HIF‐1 and DAF‐16. Moreover, reactive oxygen species (ROS) appear to play an important function in lifespan lengthening. As HIF‐1 and DAF‐16 activities are modulated by ROS, we speculate that joint loss of function of gcy‐35 and npr‐1 extends lifespan through ROS signaling.