Glucose, the most abundant carbon and energy source, regulates the expression of genes required for its own efficient metabolism. In the yeast Saccharomyces cerevisiae, glucose induces the expression of the hexose transporter (HXT) genes by modulating the activity of the transcription factor Rgt1 that functions as a repressor when glucose is absent. However, in the presence of high concentrations of glucose, Rgt1 is converted from a repressor to an activator and is required for maximal induction of HXT1 gene expression. We report that Rgt1 binds to the HXT1 promoter only in the absence of glucose, suggesting that Rgt1 increases HXT1 gene expression at high levels of glucose by an indirect mechanism. It is likely that Rgt1 stimulates the expression of an activator of the HXT1 gene at high concentrations of glucose. In addition, we demonstrate that Rgt1 becomes hyperphosphorylated in response to high glucose levels and that this phosphorylation event is required for Rgt1 to activate transcription. Furthermore, Rgt1 lacks the glucose-mediated phosphorylation in the snf3 rgt2 and grr1 mutants, which are defective in glucose induction of HXT gene expression. In these mutants, Rgt1 behaves as a constitutive repressor independent of the carbon source. We conclude that phosphorylation of Rgt1 in response to glucose is required to abolish the Rgt1-mediated repression of the HXT genes and to convert Rgt1 from a transcriptional repressor to an activator.The yeast Saccharomyes cerevisiae uses glucose as its preferred carbon and energy source. Glucose not only represses the expression of genes that are required for the metabolism of alternate sugars but also induces the transcription of genes that are essential for its own efficient utilization (1-3). Among the genes that are induced by glucose are the members of the HXT gene family, which encode glucose transporters. Glucose induces the expression of the HXT1-HXT4 genes by 10 -300-fold (4, 5).Several components of the glucose induction pathway required for HXT gene expression have been identified, including the glucose sensors Snf3 and Rgt2, that are responsible for sensing extracellular glucose and generating the intracellular signal (6, 7). A strain mutated for both sensors (snf3 rgt2 double mutant) is completely defective in glucose induction of the HXT gene expression (7). Another component that is absolutely essential for glucose induction of the HXT gene expression is the ubiquitin ligase Grr1 (5, 8). Two homologous proteins, Std1 and Mth1, have been shown to negatively regulate HXT gene expression and to interact with the carboxyl-terminal tails of the Snf3 and Rgt2 sensors (9 -11). Repression of HXT gene expression in the absence of glucose is abolished in a std1 mth1 double mutant (9, 11).The target of the glucose induction signal is the Cys 6 DNAbinding protein Rgt1, which belongs to the family of the Gal4 transcription factors (12). In the absence of glucose, Rgt1 represses HXT gene expression, whereas at high concentrations of glucose Rgt1 is required for maxima...

In the yeast Saccharomyces cerevisiae, glucose induces expression of the hexose transporter ( HXT) genes by inhibiting the repressor function of the transcription factor Rgt1. We have previously shown that Rgt1 binds to the HXT gene promoters only in the absence of glucose. In the presence of glucose, Rgt1 becomes phosphorylated and is unable to bind to the HXT promoters and repress their transcription. We report that Rgt1 interacts with Std1 and Mth1 in a yeast two-hybrid assay and co-immunoprecipitates with both proteins in vivo only when glucose is absent. In addition, we demonstrate that repression of HXT gene expression by Rgt1 is abolished in the std1 mth1 double mutant. While Rgt1 is normally phosphorylated only in the presence of high concentrations of glucose, it is constitutively modified in the std1 mth1 double mutant. Based on these data, we conclude that, in the absence of glucose, Rgt1 associates with Std1 and Mth1 to repress HXT gene expression.

Several antioxidants and agents having similar antioxidant effects are known to exert beneficial effects in ameliorating the injurious effects of hyperglycemia on liver in different diabetic in vitro and in vivo models. The review deals with some of the agents which have been shown to exert protective effects on liver against hyperglycemic insult and the various mechanisms involved. The different classes of agents which protect the diabetic liver or decrease the severity of hyperglycemia mediated injury include flavonoids, catechins, and other polyphenolic compounds, curcumin and its derivatives, certain vitamins, hormones and drugs, trace elements, prototypical antioxidants and amino acids. Some of the pronounced changes mediated by the antioxidants in liver exposed to hyperglycemia include decreased oxidative stress, and alterations in carbohydrate and lipid metabolism. Other mechanisms through which the agents ameliorate hyperglycemia mediated liver injury include decrease in oxidative DNA and protein damage, restoration of mitochondrial structural and functional integrity, decrease in inflammation and improved insulin signaling. Thus, antioxidants may prove to be an important mode of defense in maintaining normal hepatic functions in diabetes.

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Myo-inositol is an important constituent of membrane phospholipids and is a precursor for the phosphoinositide signaling pathway. It is synthesized from glucose 6-phosphate by myo-inositol-3-phosphate synthase (IP synthase), a homotrimer composed of a 68-kDa polypeptide in most mammalian tissues. It is a putative target for mood-stabilizing drugs such as lithium and valproate. Here, we show that the rat gene (Isyna1) encoding this enzyme generates a number of alternatively spliced transcripts in addition to the fully spliced form that encodes the 68-kDa subunit (the alpha isoform). Specifically, we identify a small 16-kDa subunit (the gamma(c) isoform) derived by an intron retention mechanism and provide evidence for its existence in rat tissues. The gamma(c) isoform is highly conserved in mammals, but it lacks the catalytic domain while retaining the NAD(+) binding domain. Both alpha and gamma(c) isoforms are predominantly expressed in many rat tissues and display apparent stoichiometry in purified enzyme preparations. An IP synthase polyclonal antibody not only detects the alpha and gamma(c) isoforms but also several other isoforms in pancreas, intestine, and testis suggesting that the holoenzyme is composed of unique subunits in various tissues. Interestingly, the alpha isoform is not expressed in the intestine. IP synthase activity assays using purified alpha and gamma(c) isoforms indicate that the latter negatively modulates alpha isoform activity, possibly by competing for NAD(+) molecules. Our findings have important ramifications for understanding the mood stabilization process and suggest that inositol biosynthesis is a highly regulated and dynamic process.