Thioredoxin-interacting protein (Txnip) gene expression: sensing oxidative phosphorylation status and glycolytic rate

Modification of the TRX2 gene dose in Saccharomyces cerevisiae affects hexokinase 2 gene regulation during wine yeast biomass production

In the industrial yeast biomass production process, cells undergo an oxidative and other stresses which worsen the quality of the produced biomass. The overexpression of the thioredoxin codifying gene TRX2 in a wine Saccharomyces cerevisiae strain increases resistance to oxidative stress and industrial biomass production yield. We observed that variations in the TRX2 gene dose in wine yeast strains are relevant to determine the fermentative capacity throughout the industrial biomass production process. So, we studied the molecular changes using a transcriptomic approach under these conditions. The results provide an overview of the different metabolic pathways affected during industrial biomass production by TRX2 gene manipulation. The oxidative stress-related genes, like those related with the glutathione metabolism, presented outstanding variations. The data also allowed us to propose new thioredoxin targets in S. cerevisiae, such as hexokinase 2, with relevance for industrial fermentation performance.

Redox redux: protecting the ischemic myocardium

Cardiac ischemia-reperfusion (I-R) injury occurs upon prompt restoration of blood flow to the ischemic myocardium after an acute myocardial infarction. Interestingly, many of the features of I-R injury are related to impaired mitochondrial signaling and mitochondrial dysfunction. Restoring cardiac energy bioavailability and reduction-oxidation (redox) signaling are therefore important in recovery after I-R injury. In this issue of the JCI, Yoshioka and colleagues describe an important and unexpected role for thioredoxin-interacting protein (TXNIP) in the control of mitochondrial respiration and cell energy metabolism. Their findings could open the door for development of TXNIP-targeted therapeutic approaches for the treatment of cardiac I-R injury.

AMP-activated protein kinase (AMPK) mediates nutrient regulation of thioredoxin-interacting protein (TXNIP) in pancreatic beta-cells

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.

Thioredoxin binding protein (TBP)-2/Txnip and α-arrestin proteins in cancer and diabetes mellitus

Thioredoxin binding protein -2/ thioredoxin interacting protein is an α-arrestin protein that has attracted much attention as a multifunctional regulator. Thioredoxin binding protein -2 expression is downregulated in tumor cells and the level of thioredoxin binding protein is correlated with clinical stage of cancer. Mice with mutations or knockout of the thioredoxin binding protein -2 gene are much more susceptible to carcinogenesis than wild-type mice, indicating a role for thioredoxin binding protein -2 in cancer suppression. Studies have also revealed roles for thioredoxin binding protein -2 in metabolic control. Enhancement of thioredoxin binding protein -2 expression causes impairment of insulin sensitivity and glucose-induced insulin secretion, and β-cell apoptosis. These changes are important characteristics of type 2 diabetes mellitus. Thioredoxin binding protein -2 regulates transcription of metabolic regulating genes. Thioredoxin binding protein -2-like inducible membrane protein/ arrestin domain containing 3 regulates endocytosis of receptors such as the β(2)-adrenergic receptor. The α-arrestin family possesses PPXY motifs and may function as an adaptor/scaffold for NEDD family ubiquitin ligases. Elucidation of the molecular mechanisms of α-arrestin proteins would provide a new pharmacological basis for developing approaches against cancer and type 2 diabetes mellitus.

Purinergic regulation of high-glucose-induced caspase-1 activation in the rat retinal Müller cell line rMC-1

Chronic activation of proinflammatory caspase-1 in the retinas of diabetic animals and patients in vivo and retinal Müller cells in vitro is well documented. In this study we characterized how elevated glucose and extracellular purines contribute to the activation of caspase-1 in a cultured rat Müller cell (rMC-1) model. The ability of high glucose (25 mM, 24 h) to activate caspase-1 was attenuated by either apyrase, which metabolizes extracellular ATP to AMP, or adenosine deaminase (ADA), which metabolizes extracellular adenosine to inosine. This suggested that autocrine stimulation of ATP-sensing P2 receptors and adenosine-sensing P1 receptors may in part mediate the response to high glucose. Exogenous ATP, 5'-N-ethylcarboxamido-adenosine (NECA), a nonselective P1 receptor agonist, or forskolin (FSK) increased caspase-1 activity in rMC-1 cells cultured in control glucose (5 mM) medium. Accumulation of active caspase-1 was also increased by dipyridamole, which suppresses adenosine reuptake. High-glucose stimulation of caspase-1 was attenuated by suramin, a nonselective P2 antagonist, or A2 adenosine receptor antagonists, but not by antagonism of P2X7 ATP-gated ion channel receptors. Although high glucose increased P2X7 mRNA, neither P2X7 protein nor function was detected in rMC-1 cells. The increased caspase-1 activity stimulated by high glucose, FSK, NECA, or ATP was correlated with increased gene expression of caspase-1 and thioredoxin-interacting-protein (TXNIP). These findings support a novel role for autocrine P1 and P2 purinergic receptors coupled to cAMP signaling cascades and transcriptional induction of caspase-1 in mediating the high-glucose-induced activation of caspase-1 and secretion of IL-1β in a cell culture model of nonhematopoietic retinal Müller cells.

Diminished AMPK signaling response to fasting in thioredoxin-interacting protein knockout mice

Thioredoxin-interacting protein (Txnip) knockout (TKO) mice exhibit impaired response to fasting. Herein, we showed that activation of adenine monophosphate-activated protein kinase and cellular AMP levels were diminished in the heart and soleus muscle but not in gastrocnemius muscle of fasting TKO mice. Similarly, glycogen content in fasted TKO mice was increased in oxidative muscles but was not different in glycolytic muscles. These data suggest Txnip deficiency has a higher impact on oxidative muscle than glycolytic muscles and provide new insights into the metabolic role of Txnip.

Hypoxia-inducible factor independent down-regulation of thioredoxin-interacting protein in hypoxia

Thioredoxin-Interacting Protein (Txnip) is an important regulator of glucose metabolism and functions by inhibiting cellular glucose uptake. The expression of the Txnip gene is sensitive to glucose availability and is negatively correlated with the glycolytic rate. Here we show that hypoxia induces a rapid decrease in Txnip mRNA and protein expression in a Hypoxia-Inducible Factor independent manner. Hypoxia caused reduced binding of the glucose responsive MondoA:Mlx transcription factor to the carbohydrate response elements (ChoREs) in the Txnip promoter. Our data suggest that hypoxia decreases MondoA:Mlx activity by increasing glycolytic flux, leading to the depletion of glycolytic intermediates which normally activate MondoA:Mlx. Hypoxia dependent Txnip down-regulation may be an important compensatory mechanism through which cancer cells adapt their metabolism to low oxygen concentrations.