Isolation and characterization of a novel transcription factor that binds to and activates insulin control element-mediated expression

Redox-sensitive up-regulation of eNOS by purple grape juice in endothelial cells: role of PI3-kinase/Akt, p38 MAPK, JNK, FoxO1 and FoxO3a

The vascular protective effect of grape-derived polyphenols has been attributable, in part, to their direct action on blood vessels by stimulating the endothelial formation of nitric oxide (NO). The aim of the present study was to determine whether Concord grape juice (CGJ), which contains high levels of polyphenols, stimulates the expression of endothelial NO synthase (eNOS) in porcine coronary artery endothelial cells and, if so, to determine the signaling pathway involved. CGJ dose- and time-dependently increased eNOS mRNA and protein levels and this effect is associated with an increased formation of NO in endothelial cells. The stimulatory effect of CGJ on eNOS mRNA is not associated with an increased eNOS mRNA stability and inhibited by antioxidants such as MnTMPyP, PEG-catalase, and catalase, and by wortmannin (an inhibitor of PI3-kinase), SB 203580 (an inhibitor of p38 MAPK), and SP 600125 (an inhibitor of JNK). Moreover, CGJ induced the formation of reactive oxygen species (ROS) in endothelial cells and this effect is inhibited by MnTMPyP, PEG-catalase, and catalase. The CGJ-induced the phosphorylation of p38 MAPK and JNK kinases is abolished by MnTMPyP. CGJ induced phosphorylation of transcription factors FoxO1 and FoxO3a, which regulate negatively eNOS expression, and this effect is prevented by MnTMPyP, PEG-catalase, wortmannin, SB203580 and SP600125. Moreover, chromatin immunoprecipitation assay indicated that the FoxO3a protein is associated with the eNOS promoter in control cells and that CGJ induced its dissociation. Thus, the present study indicates that CGJ up-regulates the expression of eNOS mRNA and protein leading to an increased formation of NO in endothelial cells. The stimulatory effect of CGJ is a redox-sensitive event involving PI3-kinase/Akt, p38 MAPK and JNK pathways, and the inactivation of the FoxO transcription factors, FoxO1 and FoxO3a, thereby preventing their repression of the eNOS gene.

Chronic hyperglycemia, independent of plasma lipid levels, is sufficient for the loss of beta-cell differentiation and secretory function in the db/db mouse model of diabetes

The beta-cell is a highly specialized cell with a unique differentiation that optimizes glucose-induced insulin secretion (GIIS). Here, we evaluated changes in gene expression that accompany beta-cell dysfunction in the db/db mouse model of type 2 diabetes. In db/db islets, mRNA levels of many genes implicated in beta-cell glucose sensing were progressively reduced with time, as were several transcription factors important for the maintenance of beta-cell differentiation. Conversely, genes normally suppressed in beta-cells, such as a variety of stress response mediators and inhibitor of differentiation/DNA binding 1, a gene capable of inhibiting differentiation, were markedly increased. We assessed whether this global alteration in the pattern of beta-cell gene expression was related more to chronic hyperglycemia or hyperlipidemia; db/db mice were treated with phlorizin, which selectively lowered plasma glucose, or bezafibrate, which selectively lowered plasma lipids. GIIS as well as the majority of the changes in gene expression were completely normalized by lowering glucose but were unaffected by lowering lipids. However, the restoration of GIIS was not accompanied by normalized uncoupling protein 2 or peroxisome proliferator-activated receptor gamma mRNA levels, which were upregulated in db/db islets. These studies demonstrate that hyperglycemia, independent of plasma lipid levels, is sufficient for the loss of beta-cell differentiation and secretory function in db/db mice.

Ca2+/calmodulin-dependent protein kinase II delta2 regulates gene expression of insulin in INS-1 rat insulinoma cells

Ca(2+)/calmodulin-dependent protein kinase II is a member of a broad family of ubiquitously expressed Ca(2+) sensing serine/threonine-kinases. Ca(2+)/calmodulin-dependent protein kinase II is highly expressed in insulin secreting cells and is associated with insulin secretory granules and has been proposed to play an important role in exocytosis or in insulin granule transport to release sites. To elucidate its function the antisense sequence of the major beta-cell subtype, Ca(2+)/calmodulin-dependent protein kinase II delta(2), was stably expressed in INS-1 rat insulinoma cells. This caused a loss of Ca(2+)/calmodulin-dependent protein kinase II delta(2) expression at the mRNA and protein level, while the expression of the 95% homologous Ca(2+)/calmodulin-dependent protein kinase II gamma and of beta-cell specific proteins such as the homeodomain factor pancreatic-duodenal homeobox factor-1 (PDX-1, also referred to as islet/duodenum homeobox-1, IDX-1, insulin promoter factor-1, IPF-1 and somatostatin transactivating factor-1, STF-1), the glucagon-like peptide-1 (GLP-1) receptor and K(ATP)-channels K(IR)6.2/SUR-1 (sulfonylurea receptor-1) was not altered. Unexpectedly, the cells showed a large reduction of insulin gene expression, which was due to reduced insulin gene transcription. Electrophoretic mobility shift assays of PDX-1 binding to the insulin promoter A1 and E2/A3A4 elements showed additional bands indicating alterations of PDX-1 complex formation. Stable over expression of Ca(2+)/calmodulin-dependent protein kinase II delta(2), by contrast, was associated with elevated expression of insulin mRNA. Therefore, we conclude that Ca(2+)/calmodulin-dependent protein kinase II delta(2) links fuel-dependent increases in intracellular Ca(2+) concentrations to transcriptional regulation of genes related to the metabolic control of insulin secretion.

Forced expression of Id-1 in the adult mouse small intestinal epithelium is associated with development of adenomas

Ids are dominant-negative helix-loop-helix (HLH) proteins that play overlapping yet distinct roles in antagonizing basic HLH transcription factors. Although Ids affect myogenesis, neurogenesis, and B-cell development, little is known about their in vivo functions in epithelia. We have examined the effects of forced expression of Id-1 in the small intestinal epithelium of adult chimeric mice. 129/Sv embryonic stem cells, transfected with DNA containing Id-1 under the control of transcriptional regulatory elements that function in all intestinal epithelial cell lineages, were introduced into C57Bl/6 (B6) blastocysts heterozygous for the ROSA26 marker. The B6 ROSA26/+ intestinal epithelium of the resulting adult chimeras produces Escherichia coli beta-galactosidase, allowing identification of this internal control cell population. Chimeras produced from nontransfected embryonic stem cells served as additional controls. Immunohistochemical studies of the control chimeras indicated that the small intestinal epithelium supports a complex pattern of endogenous Id expression. Id-1 is restricted to the cytoplasm; levels do not decrease as descendants of multipotent intestinal stem cells differentiate. Id-2 and Id-3 are only detectable in nuclei; levels increase markedly as epithelial cells differentiate. Forced expression of Id-1 in the 129/Sv epithelium results in a decline in Id-2 and Id-3 to below the limits of immunodetection. A subset of chimeric-transgenic mice lacked growth factor- and defensin-producing Paneth cells in their 129/Sv epithelium and also developed intestinal adenomas. These changes were not present in normal control chimeras. Adenomas were composed of proliferating beta-Gal-positive and -negative epithelial cells, suggesting that they arose through cooperative interactions between 129/Sv(Id-1) and B6 ROSA26/+ cells. These chimeras provide a model for studying how perturbations in Id expression affect tumorigenesis.

Glucose stimulates the activation domain potential of the PDX-1 homeodomain transcription factor

Glucose-stimulated expression of the insulin gene in beta cells is mediated by the PDX-1 transcription factor. In this report, we show that stimulation results from effects on activation and DNA-binding potential. Thus, glucose specifically stimulated expression in MIN6 beta cells from chimeras of PDX-1 and the GAL4 DNA-binding domain which spanned the N-terminal PDX-1 activation domain located between amino acids 1 to 79. GAL4:PDX activity was induced over physiological glucose concentrations and was also regulated by effectors of this response. The level of endogenous PDX-1 binding and phosphorylation were also induced under these conditions. We discuss how changes in PDX-1 phosphorylation may influence activity in glucose-treated beta cells.

Analysis of the role of E2A-encoded proteins in insulin gene transcription

Pancreatic beta-cell type-specific transcription of the insulin gene is mediated, in part, by factors in the basic helix-loop-helix (bHLH) family that act on a site within the insulin enhancer, termed the E1-box. Expression from this element is regulated by a heteromeric protein complex containing ubiquitous (i.e. the E2A- and HEB-encoded proteins) and islet-enriched members of the bHLH family. Recent studies indicate that the E2A- and HEB-encoded proteins contain a transactivation domain, termed AD2, that functions more efficiently in transfected beta-cell lines. In the present report, we extend this observation by demonstrating that expression of full-length E2A proteins (E47, E12, and E2/5) activates insulin E element-directed transcription in a beta-cell line-selective manner. Stimulation required functional interactions with other key insulin gene transcription factors, including its islet bHLH partner as well as those that act on the RIPE3b1 and RIPE3a2 elements of the insulin gene enhancer. The conserved AD2 domain in the E2A proteins was essential in this process. The effect of the E2A- and HEB-encoded proteins on insulin gene expression was also analyzed in mice lacking a functional E2A or HEB gene. There was no apparent difference in insulin production between wild type, heterozygote, and homozygous mutant E2A or HEB mice. These results suggest that neither the E2A- or HEB-encoded proteins are essential for insulin transcription and that one factor can substitute for the other to impart normal insulin E1 activator function in mutant animals.

Glucose response elements in a gene that codes for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

We have shown previously that rat hepatoma FTO-2B cells express two mRNAs, called F (fetal) and L (liver), from distinct promoters of the same gene coding for 6-phosphofructo-2-kinase (PFK-2). This enzyme catalyzes the synthesis of fructose 2,6-bisphosphate, an allosteric stimulator of glycolysis. We have now found that glucose, as well as lactate and pyruvate, increases the concentration of the F and L mRNAs. The effect of glucose was mimicked by xylitol, a precursor of xylulose 5-phosphate, and hence of intermediates of the pentose phosphate and glycolytic pathways, and was inhibited by okadaic acid, an inhibitor of protein phosphatases. Transfection experiments showed that the F promoter region is a target of the glucose effect, with glucose stimulating F promoter activity in a way probably similar to mitogens. Another region of the gene, located between the F and L promoters, also behaved as a glucose-sensitive element. This region corresponds to a cluster of DNase I-hypersensitive sites that were induced in chromatin following glucose treatment. The sequence organization of this region is very similar to the functional architecture of the glucose-sensitive insulin gene promoter. We propose a model for the concerted regulation by glucose metabolites of three pathways that lead to increased PFK-2 activity.