The HXT genes (HXT1 to HXT4) of the yeast Saccharomyces cerevisiae encode hexose transporters. We found that transcription of these genes is induced 10-to 300-fold by glucose. Analysis of glucose induction of HXT gene expression revealed three types of regulation: (i) induction by glucose independent of sugar concentration (HXT3); (ii) induction by low levels of glucose and repression at high glucose concentrations (HXT2 and HXT4); and (iii) induction only at high glucose concentrations (HXT1). The lack of expression of all four HXT genes in the absence of glucose is due to a repression mechanism that requires Rgt1p and Ssn6p. GRR1 seems to encode a positive regulator of HXT expression, since grr1 mutants are defective in glucose induction of all four HXT genes. Mutations in RGT1 suppress the defect in HXT expression caused by grr1 mutations, leading us to propose that glucose induces HXT expression by activating Grr1p, which inhibits the function of the Rgt1p repressor. HXT1 expression is also induced by high glucose levels through another regulatory mechanism: rgt1 mutants still require high levels of glucose for maximal induction of HXT1 expression. The lack of induction of HXT2 and HXT4 expression on high levels of glucose is due to glucose repression: these genes become induced at high glucose concentrations in glucose repression mutants (hxk2, reg1, ssn6, tup1, or mig1). Components of the glucose repression pathway (Hxk2p and Reg1p) are also required for generation of the high-level glucose induction signal for expression of the HXT1 gene. Thus, the glucose repression and glucose induction mechanisms share some of the same components and may share the same primary signal generated from glucose.

SUMMARY Glucose, the most abundant monosaccharide in nature, is the principal carbon and energy source for nearly all cells. The first, and rate-limiting, step of glucose metabolism is its transport across the plasma membrane. In cells of many organisms glucose ensures its own efficient metabolism by serving as an environmental stimulus that regulates the quantity, types, and activity of glucose transporters, both at the transcriptional and posttranslational levels. This is most apparent in the baker’s yeast Saccharomyces cerevisiae, which has 20 genes encoding known or likely glucose transporters, each of which is known or likely to have a different affinity for glucose. The expression and function of most of these HXT genes is regulated by different levels of glucose. This review focuses on the mechanisms S. cerevisiae and a few other fungal species utilize for sensing the level of glucose and transmitting this information to the nucleus to alter HXT gene expression. One mechanism represses transcription of some HXT genes when glucose levels are high and works through the Mig1 transcriptional repressor, whose function is regulated by the Snf1-Snf4 protein kinase and Reg1-Glc7 protein phosphatase. Another pathway induces HXT expression in response to glucose and employs the Rgt1 transcriptional repressor, a ubiquitin ligase protein complex (SCFGrr1) that regulates Rgt1 function, and two glucose sensors in the membrane (Snf3 and Rgt2) that bind glucose and generate the intracellular signal to which Rgt1 responds. These two regulatory pathways collaborate with other, less well-understood, pathways to ensure that yeast cells express the glucose transporters best suited for the amount of glucose available.

Glucose is the preferred carbon source for most eukaryotic cells and has profound effects on many cellular functions. How Snf3p. We identified a dominant mutation in RGT2 that causes constitutive expression of several ILT genes, even in the absence of the inducer glucose. This same mutation introduced into SNF3 also causes glucoseindependent expression of ILT genes. Thus, the Rgt2p and Snf3p glucose transporters appear to act as glucose receptors that generate an intracellular glucose signal, suggesting that glucose signaling in yeast is a receptor-mediated process.inhibited in glucose-grown cells, leading to derepression of the HXT genes.

How eukaryotic cells sense availability of glucose, their preferred carbon and energy source, is an important, unsolved problem. Bakers' yeast (Saccharomyces cerevisiae) uses two glucose transporter homologs, Snf3 and Rgt2, as glucose sensors that generate a signal for induction of expression of genes encoding hexose transporters (HXT genes). We present evidence that these proteins generate an intracellular glucose signal without transporting glucose. The Snf3 and Rgt2 glucose sensors contain unusually long C-terminal tails that are predicted to be in the cytoplasm. These tails appear to be the signaling domains of Snf3 and Rgt2 because they are necessary for glucose signaling by Snf3 and Rgt2, and transplantation of the C-terminal tail of Snf3 onto the Hxt1 and Hxt2 glucose transporters converts them into glucose sensors that can generate a signal for glucose-induced HXT gene expression. These results support the idea that yeast senses glucose using two modified glucose transporters that serve as glucose receptors.

MicroRNAs (miRNAs) are small noncoding ribonucleotides that bind mRNAs and function mainly as translational repressors in mammals. MicroRNAs have been implicated to play a role in many diseases, including diabetes. Several reports indicate an important function for miRNAs in insulin production as well as insulin secretion. We have recently carried out a screen in the pancreatic b-cell line MIN6 to identify miRNAs with altered abundance in response to changes in glucose concentrations. This screen resulted in identification of 61 glucose-regulated miRNAs from a total of 108 miRNAs detectable in MIN6 cells. Many of the identified miRNAs, including miR-124a, miR-107, and miR-30d were up-regulated in the presence of high glucose. Only a few of the miRNAs, including miR-296, miR-484, and miR-690 were significantly down-regulated by high glucose treatment. Interestingly, we found that overexpression of miR-30d, one of the miRNAs up-regulated by glucose, increased insulin gene expression, while inhibition of miR-30d abolished glucose-stimulated insulin gene transcription. Overexpression or inhibition of miR-30d did not have any effect on insulin secretion. These data suggest that the putative target genes of miR-30d may be negative regulators of insulin gene expression.

O-linked beta-N-acetylglucosamine (O-GlcNAc) modification of nuclear and cytoplasmic proteins is important for many cellular processes, and the number of proteins that contain this modification is steadily increasing. This modification is dynamic and reversible, and in some cases competes for phosphorylation of the same residues. O-GlcNAc modification of proteins is regulated by cell cycle, nutrient metabolism, and other extracellular signals. Compared to protein phosphorylation, which is mediated by a large number of kinases, O-GlcNAc modification is catalyzed only by one enzyme called O-linked N-acetylglucosaminyl transferase or OGT. Removal of O-GlcNAc from proteins is catalyzed by the enzyme beta-N-acetylglucosaminidase (O-GlcNAcase or OGA). Altered O-linked GlcNAc modification levels contribute to the establishment of many diseases, such as cancer, diabetes, cardiovascular disease, and neurodegeneration. Many transcription factors have been shown to be modified by O-linked GlcNAc modification, which can influence their transcriptional activity, DNA binding, localization, stability, and interaction with other co-factors. This review focuses on modulation of transcription factor function by O-linked GlcNAc modification.

The RGT1 gene of Saccharomyces cerevisiae plays a central role in the glucose-induced expression of hexose transporter (HXT) genes. Genetic evidence suggests that it encodes a repressor of the HXT genes whose function is inhibited by glucose. Here, we report the isolation of RGT1 and demonstrate that it encodes a bifunctional transcription factor. Rgt1p displays three different transcriptional modes in response to glucose: (i) in the absence of glucose, it functions as a transcriptional repressor; (ii) high concentrations of glucose cause it to function as a transcriptional activator; and (iii) in cells growing on low levels of glucose, Rgt1p has a neutral role, neither repressing nor activating transcription. Glucose alters Rgt1p function through a pathway that includes two glucose sensors, Snf3p and Rgt2p, and Grr1p. The glucose transporter Snf3p, which appears to be a low-glucose sensor, is required for inhibition of Rgt1p repressor function by low levels of glucose. Rgt2p, a glucose transporter that functions as a high-glucose sensor, is required for conversion of Rgt1p into an activator by high levels of glucose. Grr1p, a component of the glucose signaling pathway, is required both for inactivation of Rgt1p repressor function by low levels of glucose and for conversion of Rgt1p into an activator at high levels of glucose. Thus, signals generated by two different glucose sensors act through Grr1p to determine Rgt1p function.Glucose is the preferred carbon and energy source for the yeast Saccharomyces cerevisiae, as well as for most mammalian cells, and has major and wide-ranging effects on gene expression. Glucose represses expression of many genes that are unnecessary for its metabolism and induces expression of many other genes required for its utilization. Among the genes whose expression is induced by glucose are several HXT genes encoding glucose transporters (5,30,31,47). Three HXT genes respond differently to different levels of glucose: HXT1 expression is induced only by high levels of glucose, whereas HXT2 and HXT4 are induced only by low concentrations of glucose.We have previously shown that glucose induction of HXT expression is due to a repression mechanism mediated by Rgt1p (31). In cells growing in the absence of glucose, Rgt1p represses HXT expression; addition of glucose causes Rgt1p function to be inhibited, leading to derepression of HXT expression. The different responses of HXT1 and HXT2 (and HXT4) to different levels of glucose are due to two additional regulatory mechanisms that act on these genes: HXT2 and HXT4 are also subject to glucose repression mediated by Mig1p, a repressor that is activated by high levels of glucose (32, 47), and HXT1 responds to an additional regulatory mechanism, whose components have not yet been identified, that requires high levels of glucose for function (31).Glucose-stimulated inhibition of Rgt1p requires Grr1p, a leucine-rich repeat-containing protein, and Snf3p, a glucose transporter thought to be involved in sensing glucose and generating an intracell...

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