Regulation of pancreas duodenum homeobox-1 expression by early growth response-1

NEUROD1 reinforces endocrine cell fate acquisition in pancreatic development

NEUROD1 is a transcription factor that helps maintain a mature phenotype of pancreatic β cells. Disruption of Neurod1 during pancreatic development causes severe neonatal diabetes; however, the exact role of NEUROD1 in the differentiation programs of endocrine cells is unknown. Here, we report a crucial role of the NEUROD1 regulatory network in endocrine lineage commitment and differentiation. Mechanistically, transcriptome and chromatin landscape analyses demonstrate that Neurod1 inactivation triggers a downregulation of endocrine differentiation transcription factors and upregulation of non-endocrine genes within the Neurod1-deficient endocrine cell population, disturbing endocrine identity acquisition. Neurod1 deficiency altered the H3K27me3 histone modification pattern in promoter regions of differentially expressed genes, which resulted in gene regulatory network changes in the differentiation pathway of endocrine cells, compromising endocrine cell potential, differentiation, and functional properties.

Inferring regulators of cell identity in the human adult pancreas

Cellular identity during development is under the control of transcription factors that form gene regulatory networks. However, the transcription factors and gene regulatory networks underlying cellular identity in the human adult pancreas remain largely unexplored. Here, we integrate multiple single-cell RNA-sequencing datasets of the human adult pancreas, totaling 7393 cells, and comprehensively reconstruct gene regulatory networks. We show that a network of 142 transcription factors forms distinct regulatory modules that characterize pancreatic cell types. We present evidence that our approach identifies regulators of cell identity and cell states in the human adult pancreas. We predict that HEYL, BHLHE41 and JUND are active in acinar, beta and alpha cells, respectively, and show that these proteins are present in the human adult pancreas as well as in human induced pluripotent stem cell (hiPSC)-derived islet cells. Using single-cell transcriptomics, we found that JUND represses beta cell genes in hiPSC-alpha cells. BHLHE41 depletion induced apoptosis in primary pancreatic islets. The comprehensive gene regulatory network atlas can be explored interactively online. We anticipate our analysis to be the starting point for a more sophisticated dissection of how transcription factors regulate cell identity and cell states in the human adult pancreas.

Glucose Homeostasis and Pancreatic Islet Size Are Regulated by the Transcription Factors Elk-1 and Egr-1 and the Protein Phosphatase Calcineurin

Pancreatic β-cells synthesize and secrete insulin. A key feature of diabetes mellitus is the loss of these cells. A decrease in the number of β-cells results in decreased biosynthesis of insulin. Increasing the number of β-cells should restore adequate insulin biosynthesis leading to adequate insulin secretion. Therefore, identifying proteins that regulate the number of β-cells is a high priority in diabetes research. In this review article, we summerize the results of three sophisticated transgenic mouse models showing that the transcription factors Elk-1 and Egr-1 and the Ca²⁺/calmodulin-regulated protein phosphatase calcineurin control the formation of sufficiently large pancreatic islets. Impairment of the biological activity of Egr-1 and Elk-1 in pancreatic β-cells leads to glucose intolerance and dysregulation of glucose homeostasis, the process that maintains glucose concentration in the blood within a narrow range. Transgenic mice expressing an activated calcineurin mutant also had smaller islets and showed hyperglycemia. Calcineurin induces dephosphorylation of Elk-1 which subsequently impairs Egr-1 biosynthesis and the biological functions of Elk-1 and Egr-1 to regulate islet size and glucose homeostasis.

Pdx1 Is Transcriptionally Regulated by EGR-1 during Nitric Oxide-Induced Endoderm Differentiation of Mouse Embryonic Stem Cells

The transcription factor, early growth response-1 (EGR-1), is involved in the regulation of cell differentiation, proliferation, and apoptosis in response to different stimuli. EGR-1 is described to be involved in pancreatic endoderm differentiation, but the regulatory mechanisms controlling its action are not fully elucidated. Our previous investigation reported that exposure of mouse embryonic stem cells (mESCs) to the chemical nitric oxide (NO) donor diethylenetriamine nitric oxide adduct (DETA-NO) induces the expression of early differentiation genes such as pancreatic and duodenal homeobox 1 (Pdx1). We have also evidenced that Pdx1 expression is associated with the release of polycomb repressive complex 2 (PRC2) and P300 from the Pdx1 promoter; these events were accompanied by epigenetic changes to histones and site-specific changes in the DNA methylation. Here, we investigate the role of EGR-1 on Pdx1 regulation in mESCs. This study reveals that EGR-1 plays a negative role in Pdx1 expression and shows that the binding capacity of EGR-1 to the Pdx1 promoter depends on the methylation level of its DNA binding site and its acetylation state. These results suggest that targeting EGR-1 at early differentiation stages might be relevant for directing pluripotent cells into Pdx1-dependent cell lineages.

Cell therapy research for Diabetes: Pancreatic β cell differentiation from pluripotent stem cells

Human pluripotent stem cells (PSCs), both embryonic and induced pluripotent stem cells (iPSCs), have been differentiated into pancreatic β isletsin vitrofor more than a decade. The idea is to get enough β cells for cell transplantation for diabetics. Finding a standard cell therapy for diabetes is essential because of the logarithmic increase in the global population of people with diabetes and the insufficient availability of the human cadaveric pancreas. Moreover, with better insights into developmental biology, thein vitroβ cell differentiation protocols have depended on thein vivoβ cell organogenesis. Various protocols for pancreatic β cell differentiation have been developed. Such protocols are based on the modulation of cell signalling pathways with growth factors, small molecules, RNAi approaches, directed differentiation using transcription factors, genome editing. Growth factor free differentiation protocols, epigenetic modulations, 3D differentiation approaches, and encapsulation strategies have also been reported for better glycemic control and endocrine modulations. Here, we have reviewed various aforementionedin vitroβ cell differentiation protocols from human PSCs, their respective comparisons, challenges, past, present, and future. The literature has been reviewed primarily from PubMed from the year 2000 till date using the mentioned keywords.

Glucose stimulates microRNA-199 expression in murine pancreatic β-cells

MicroRNA 199 (miR-199) negatively impacts pancreatic β-cell function and its expression is highly increased in islets from diabetic mice as well as in plasma of diabetic patients. Here we investigated how miR-199 expression is regulated in β-cells by assessing expression of miR-199 precursors (primiR-199a1, primiR-199a2, and primiR-199b) and mature miR-199 (miR-199-3p and miR-199-5p) and promoter transcriptional activity assays in mouse islets and mouse insulinoma cells (MIN6) under different stimuli. We found that mouse islets equally express miR-199-3p and miR-199-5p. However, the primiRNA expression levels differed; although primiR-199a1 expression was about 30% greater than that of primiR-199a2, primiR-199b is barely detected in islets. We observed a 2-fold increase in primiR-199a1 and primiR-199a2 mRNA levels in mouse islets cultured in 10 mm glucose compared with 5.5 mm glucose. Similar responses to glucose were observed in MIN6 cells. Exposure to 30 mm KCl to induce membrane depolarization and calcium influx increased expression of primiR-199a2 but not of primiR-199a1 in MIN6 cells, indicating that calcium influx was involved. Transcriptional activity studies in MIN6 cells also revealed that primiR-199a2 promoter activity was enhanced by glucose and reduced by 2-deoxy-D-glucose-induced starvation. KCl and the potassium channel blocker tolbutamide also stimulated primiR-199a2 promoter activity. Calcium channel blockade by nifedipine reduced primiR-199a2 promoter activity in MIN6 cells, and diazoxide-mediated calcium influx inhibition blunted glucose up-regulation of miR-199-3p in islets. In conclusion, we uncover that glucose acutely up-regulates miR-199 family expression in β-cells. Glucose metabolism and calcium influx are involved in primiR-199a2 expression but not primiR-199a1 expression.

Role of Egr1 on Pancreatic Endoderm Differentiation

The low efficiency of in vitro differentiation of human embryonic stem cells (hESCs) or human-induced pluripotent stem cells (iPSCs) into insulin-producing cells is a crucial hurdle for the clinical implementation of human pluripotent stem cells (PSCs). Our previous investigation into the key factors for the differentiation of PSCs into insulin-producing cells suggested that the expression of GATA binding protein 6 (GATA6) and Gremlin 1 (GREM1) and inhibition of early growth response protein 1 (Egr1) may be important factors. In this study, we investigated the role of Egr1 in pancreas development. The transfection of small interfering RNA (siRNA) of Egr1 in the early phase induced the differentiation of iPSCs derived from fibroblasts (FiPSCs) into pancreatic endoderm and insulin-producing cells. In contrast, the downregulation of Egr1 in the late phase suppressed the differentiation of FiPSCs into pancreatic endoderm and insulin-producing cells. In addition, the overexpression of Egr1 suppressed the differentiation of iPSCs derived from pancreatic cells into pancreatic endoderm and insulin-producing cells. These data suggest that the downregulation of Egr1 in the early phase can efficiently induce the differentiation of iPSCs into insulin-producing cells.

DT(270-326) , a Truncated Diphtheria Toxin, Increases Blood-Tumor Barrier Permeability by Upregulating the Expression of Caveolin-1

AIM

The nontoxic mutant of diphtheria toxin (DT) has been demonstrated to act as a receptor-specific carrier protein to delivery drug into brain. Recent research showed that the truncated "receptorless" DT was still capable of being internalized into cells. This study investigated the effects and potential mechanisms of DT(270-326) , a truncated "receptorless" DT, on the permeability of the blood-tumor barrier (BTB).

METHODS

BTB and GECs were subjected to DT(270-326) treatment. HRP flux assays, immunofluorescent, co-immunoprecipitation, Western blot, CCK-8, and Flow cytometry analysis were used to evaluate the effects of DT(270-326) administration.

RESULTS

Our results revealed that 5 μM of DT(270-326) significantly increased the permeability of BTBin vitro, which reached its peak at 6 h. The permeability was reduced by pretreatment with filipinIII. DT(270-326) co-localized and interacted with caveolin-1 via its caveolin-binding motif. The mRNA and protein expression levels of caveolin-1 were identical with the changes of BTB permeability. The upregulated expression of caveolin-1 was associated with Src kinase-dependent tyrosine phosphorylation of caveolin-1, which subsequently induced phosphorylation and inactivation of the transcription factor Egr-1. The combination of DT(270-326) with doxorubicin significantly enhanced the loss of cell viability and apoptosis of U87 glioma cells in contrast to doxorubicin alone.

CONCLUSIONS

DT(270-326) might provide a novel strategy to increase the delivery of macromolecular therapeutic agents across the BTB.

EGR-1/Bax pathway plays a role in vitamin E δ-tocotrienol-induced apoptosis in pancreatic cancer cells

The anticancer activity of δ-tocotrienol, a bioactive vitamin E present in whole grain cereals, annatto beans and palm fruit, is strongly dependent on its effect on the induction of apoptosis. δ-Tocotrienol-induced apoptosis is associated with consistent induction in the expression of the proapoptotic protein Bcl-2-associated X protein (Bax). The molecular mechanism by which δ-tocotrienol regulates Bax expression is unknown. We carried out a DNA microarray study that identified δ-tocotrienol induction of the zinc finger transcription factor EGR-1 in pancreatic cancer cells. Here, we provide evidence linking δ-tocotrienol-induced apoptosis in pancreatic cancer cells to EGR-1 regulation of Bax expression. Forced expression of EGR-1 induces Bax expression and apoptosis in pancreatic cancer cells. In contrast, knockdown of δ-tocotrienol-induced EGR-1 by small interfering RNA attenuated δ-tocotrienol-induced Bax expression and reduced δ-tocotrienol-induced apoptosis. Further analyses showed that de novo protein synthesis was not required for δ-tocotrienol-induced EGR-1 expression, suggesting a direct effect of δ-tocotrienol on EGR-1 expression. Furthermore, a chromatin immunoprecipitation assay demonstrated that EGR-1 binds to the Bax gene promoter. Finally, δ-tocotrienol treatment induced Bax expression and activated EGR-1 in the pancreatic neoplastic cells of the PDX-Cre Kras genetically engineered model of pancreatic cancer. Our study provides the first evidence for EGR-1 as a direct target of vitamin E δ-tocotrienol, suggesting that EGR-1 may act as a proapoptotic factor in pancreatic cancer cells via induction of Bax.

Activation and inhibition of transient receptor potential TRPM3-induced gene transcription

BACKGROUND AND PURPOSE

Transient receptor potential-3 (TRPM3) channels function as Ca2+ permeable cation channels. While the natural ligands for these channels are still unknown, several compounds have been described that either activate or inhibit TRPM3 channel activity. experimental approach: We assessed TRPM3-mediated gene transcription, which relies on the induction of intracellular signalling to the nucleus following activation of TRPM3 channels. Activator protein-1 (AP-1) and Egr-1-responsive reporter genes were integrated into the chromatin of the cells. This strategy enabled us to analyse gene transcription of the AP-1 and Egr-1-responsive reporter genes that were packed into an ordered chromatin structure.

KEY RESULTS

The neurosteroid pregnenolone sulfate strikingly up-regulated AP-1 and Egr-1 transcriptional activity, while nifedipine and D-erythro-sphingosine, also putative activators of TRPM3 channels, exhibited either no or TRPM3-independent effects on gene transcription. In addition, pregnenolone sulfate robustly enhanced the transcriptional activation potential of the ternary complex factor Elk-1. Pregnenolone sulfate-induced activation of gene transcription was blocked by treatment with mefenamic acid and, to a lesser extent, by the polyphenol naringenin. In contrast, progesterone, pregnenolone and rosiglitazone reduced AP-1 activity in the cells, but had no inhibitory effect on Egr-1 activity in pregnenolone sulfate-stimulated cells.

CONCLUSION AND IMPLICATIONS

Pregnenolone sulfate is a powerful activator of TRPM3-mediated gene transcription, while transcription is completely inhibited by mefenamic acid in cells expressing activated TRPM3 channels. Both compounds are valuable tools for further investigating the biological functions of TRPM3 channels.

Potential Factors for the Differentiation of ESCs/iPSCs Into Insulin-Producing Cells

The low efficiency of in vitro differentiation of human embryonic stem cells (ESCs) or human induced pluripotent stem cells (iPSCs) into insulin-producing cells thus creates a crucial hurdle for the clinical implementation of human pluripotent stem cells (PSCs). In this study, we investigated the key factors for the differentiation of PSCs into insulin-producing cells. We obtained microarray data of HUES8 and HUES6 from two GeneChips (GPL3921: Affymetrix HT Human Genome U133A Array, GPL570: Affymetrix Human Genome U133 Plus 2.0 Array) in a database of GEO (NCBI), since HUES8 can differentiate into pancreatic cells, while HUES6 hardly demonstrates any differentiation at all. The genes with more than fourfold higher expressions in HUES8 compared to HUES6 included RPS4Y1, DDX3Y, EIF1AY, GREM1, GATA6, and NLGN4Y. Since there were four genes, RPS4Y1, DDX3Y, EIF1AY, and NLGN4Y, on the Y chromosome and HUES8 was a male cell line and HUES6 was a female cell line, we excluded these genes in this study. On the other hand, genes with more than fourfold higher expressions in HUES6 compared to HUES8 included NLRP2, EGR1, and SMC3. We next compared iPSCs derived from pancreatic cells (PiPSCs) and iPSCs derived from fibroblasts (FiPSCs). PiPSCs differentiated into insulin-producing cells more easily than FiPSCs because of their epigenetic memory. The gene expressions of GREM1, GATA6, NLRP2, EGR1, and SMC3 in PiPSCs and FiPSCs were also investigated. The expression level of GREM1 and GATA6 in PiPSCs were higher than in FiPSCs. On the other hand, EGR1, which was lower in HUES8 than in HUES6, was predictably lower in PiPSCs than FiPSCs, while NLRP2 and SMC3 were higher in PiPSCs than FiPSCs. These data suggest that the expression of GATA6 and GREM1 and the inhibition of EGR1 may be important factors for the differentiation of PSCs into insulin-producing cells.

Expression, signaling and function of Egr transcription factors in pancreatic β-cells and insulin-responsive tissues

Egr-1 and the related zinc finger transcription factors Egr-2, Egr-3, and Egr-4 are stimulated by many extracellular signaling molecules and represent a convergence point for intracellular signaling cascades. Egr-1 expression is induced in insulinoma cells and pancreatic β-cells following stimulation with either glucose, or pregnenolone sulfate. Moreover, stimulation of Gαq and Gαs-coupled receptors enhances EGR-1 gene transcription. Functional studies revealed that Egr transcription factors control insulin biosynthesis via regulation of Pdx-1 expression. Glucose homeostasis and pancreatic islet size are regulated by Egr transcription factors, indicating that these proteins control central physiological parameters regulated by pancreatic β-cells. In addition, Egr-1 is an integral part of the insulin receptor signaling cascade in insulin-responsive tissues and influences insulin resistance.

Signal transduction via TRPM3 channels in pancreatic β-cells

Transient receptor potential melastatin 3 (TRPM3) channels are non-selective cation channels that are expressed in insulinoma cells and pancreatic β-cells. Stimulation of TRPM3 with the neurosteroid pregnenolone sulfate induces an intracellular signaling cascade, involving a rise in intracellular Ca(2)(+) concentration, activation of the protein kinases Raf and ERK, and a change in the gene expression pattern of the cells. In particular, biosynthesis of insulin is altered following activation of TRPM3 by pregnenolone sulfate. Moreover, a direct effect of TRPM3 stimulation on insulin secretion has been reported. The fact that stimulation of TRPM3 induces a signaling cascade that is very similar to the signaling cascade induced by glucose in β-cells suggests that TRPM3 may influence main functions of pancreatic β-cells. The view that TRPM3 represents an ionotropic steroid receptor of pancreatic β-cells linking insulin release with steroid hormone signaling is discussed.

Disease progression mediated by egr-1 associated signaling in response to oxidative stress

When cellular reducing enzymes fail to shield the cell from increased amounts of reactive oxygen species (ROS), oxidative stress arises. The redox state is misbalanced, DNA and proteins are damaged and cellular transcription networks are activated. This condition can lead to the initiation and/or to the progression of atherosclerosis, tumors or pulmonary hypertension; diseases that are decisively furthered by the presence of oxidizing agents. Redox sensitive genes, like the zinc finger transcription factor early growth response 1 (Egr-1), play a pivotal role in the pathophysiology of these diseases. Apart from inducing apoptosis, signaling partners like the MEK/ERK pathway or the protein kinase C (PKC) can activate salvage programs such as cell proliferation that do not ameliorate, but rather worsen their outcome. Here, we review the currently available data on Egr-1 related signal transduction cascades in response to oxidative stress in the progression of epidemiologically significant diseases. Knowing the molecular pathways behind the pathology will greatly enhance our ability to identify possible targets for the development of new therapeutic strategies.

Critical role of Egr transcription factors in regulating insulin biosynthesis, blood glucose homeostasis, and islet size

Expression of early growth response protein (Egr)-1, a protein of the Egr family of zinc finger transcription factors, is stimulated in glucose-treated pancreatic β-cells and insulinoma cells. The purpose of this study was to elucidate the role of Egr transcription factors in pancreatic β-cells in vivo. To overcome the problem associated with redundancy of functions between Egr proteins, conditional transgenic mice were generated expressing a dominant-negative mutant of Egr-1 in pancreatic β-cells. The Egr-1 mutant interferes with DNA binding of all Egr proteins and thus impairs the biological functions of the entire Egr family. Expression of the Egr-1 mutant reduced expression of TGFβ and basic fibroblast growth factor, known target genes of Egr-1, whereas the expression of Egr-1, Egr-3, Ets-like gene-1 (Elk-1), and specificity protein-3 was not changed in the presence of the Egr-1 mutant. Expression of the homeobox protein pancreas duodenum homeobox-1, a major regulator of insulin biosynthesis, was reduced in islets expressing the Egr-1 mutant. Accordingly, insulin mRNA and protein levels were reduced by 75 or 25%, respectively, whereas expression of glucagon and somatostatin was not altered after expression of the Egr-1 mutant in β-cells. Glucose tolerance tests revealed that transgenic mice expressing the Egr-1 mutant in pancreatic β-cells displayed impaired glucose tolerance. In addition, increased caspase-3/7 activity was detected as a result of transgene expression, leading to a 20% decrease of the size of the islets. These results show that Egr proteins play an important role in controlling insulin biosynthesis, glucose homeostasis, and islet size of pancreatic β-cells in vivo.

Signal transduction of pregnenolone sulfate in insulinoma cells: activation of Egr-1 expression involving TRPM3, voltage-gated calcium channels, ERK, and ternary complex factors

The neurosteroid pregnenolone sulfate acts on the nervous system by modifying neurotransmission and receptor functions, thus influencing synaptic strength, neuronal survival, and neurogenesis. Here we show that pregnenolone sulfate induces a signaling cascade in insulinoma cells leading to enhanced expression of the zinc finger transcription factor Egr-1 and Egr-1-responsive target genes. Pharmacological and genetic experiments revealed that influx of Ca(2+) ions via transient receptor potential M3 and voltage-gated Ca(2+) channels, elevation of the cytosolic Ca(2+) level, and activation of ERK are essential for connecting pregnenolone sulfate stimulation with enhanced Egr-1 biosynthesis. Expression of a dominant-negative mutant of Elk-1, a key regulator of gene transcription driven by a serum response element, attenuated Egr-1 expression following stimulation, indicating that Elk-1 or related ternary complex factors connect the transcription of the Egr-1 gene with the pregnenolone sulfate-induced intracellular signaling cascade elicited by the initial influx of Ca(2+). The newly synthesized Egr-1 was biologically active and bound under physiological conditions to the regulatory regions of the Pdx-1, Synapsin I, and Chromogranin B genes. Pdx-1 is a major regulator of insulin gene transcription. Accordingly, elevated insulin promoter activity and increased mRNA levels of insulin could be detected in pregnenolone sulfate-stimulated insulinoma cells. Likewise, the biosynthesis of synapsin I, a synaptic vesicle protein that is found at secretory granules in insulinoma cells, was stimulated in pregnenolone sulfate-treated INS-1 cells. Together, these data show that pregnenolone sulfate induces a signaling cascade in insulinoma cells that is very similar to the signaling cascade induced by glucose in β-cells.

Transcriptional regulation of glucose sensors in pancreatic β-cells and liver: an update

Pancreatic β-cells and the liver play a key role in glucose homeostasis. After a meal or in a state of hyperglycemia, glucose is transported into the β-cells or hepatocytes where it is metabolized. In the β-cells, glucose is metabolized to increase the ATP:ADP ratio, resulting in the secretion of insulin stored in the vesicle. In the hepatocytes, glucose is metabolized to CO(2), fatty acids or stored as glycogen. In these cells, solute carrier family 2 (SLC2A2) and glucokinase play a key role in sensing and uptaking glucose. Dysfunction of these proteins results in the hyperglycemia which is one of the characteristics of type 2 diabetes mellitus (T2DM). Thus, studies on the molecular mechanisms of their transcriptional regulations are important in understanding pathogenesis and combating T2DM. In this paper, we will review a recent update on the progress of gene regulation of glucose sensors in the liver and β-cells.

Insulin expressed from endogenously active glucose-responsive EGR1 promoter in bone marrow mesenchymal stromal cells as diabetes therapy

Advances in islet transplantation have encouraged efforts to create alternative insulin-secreting cells that overcome limitations associated with current therapies. We have recently demonstrated durable correction of murine and porcine diabetes by syngeneic and autologous implantation, respectively, of primary hepatocytes non-virally modified with a glucose-responsive promoter-regulated insulin transgene. As surgical procurement of hepatocytes may be clinically unappealing, we here describe primary bone marrow-derived mesenchymal stromal cells (BMMSC) as alternative insulin-secreting bioimplants. BMMSC are abundant and less invasively procured for clinical autologous transplantation. Electroporation achieved high transgene transfection efficiencies in human BMMSC (HBMMSC) and porcine BMMSC (PBMMSC). We transcriptomically identified an HBMMSC glucose-responsive promoter, EGR1. This endogenously active promoter drove rapid glucose-induced transgene secretions in BMMSC with near-physiological characteristics during static and kinetic induction assays simulating normal human islets. Preparatory to preclinical transplantation, PBMMSC transfected with the circular insulin transgene vector or stably integrated with the linearized vector were evaluated by intrahepatic or intraperitoneal xenotransplantation in streptozotocin-diabetic and non-diabetic NOD-SCID mice. Hyperglycemia, glucose tolerance and body weight were corrected in a dose-responsive manner. Hypoglycemia was not observed even in identically implanted non-diabetic mice. These results establish human EGR1 promoter-insulin construct-modified BMMSC as safe and efficient insulin-secreting bioimplants for diabetes treatment.

Glucose-responsive gene expression system for gene therapy

Regulation of gene expression by glucose is an important mechanism for mammals in adapting to their nutritional environment. Glucose, the primary fuel for most cells, modulates gene expression that is crucial in the cellular adaptation to glycemic variation. Transcription of the genes for insulin and glycolytic and lipogenic enzymes is stimulated by glucose in pancreatic beta-cells and liver. Recent findings further support the key role of the carbohydrate-responsive element binding protein in the regulation of glycolytic and lipogenic genes by glucose and dietary carbohydrates. Herein, we review the transcriptional regulation of glucose-responsive genes, and recent advances in the gene therapy using glucose-responsive gene expression for diabetes.

Calcium influx into MIN6 insulinoma cells induces expression of Egr-1 involving extracellular signal-regulated protein kinase and the transcription factors Elk-1 and CREB

Glucose induces many changes in the transcriptional pattern of beta-cells derived from the endocrine pancreas. The zinc finger protein Egr-1 belongs to the transcription factors that are activated in glucose-treated beta-cells. Egr-1 expression is additionally induced by treatment of MIN6 pancreatic beta-cells with tolbutamide, a compound that triggers a closure of ATP-dependent potassium channels, K(ATP), in the plasma membrane or by KCl that depolarizes the cell membrane. Stimulation with glucose, tolbutamide or KCl induces a Ca2+ influx into the beta-cells via L-type Ca2+ channels. Accordingly, incubation of the cells with the L-type Ca2+ channel blocker nifedipine or the acetoxymethylester of the cytosolic Ca2+ chelator BAPTA prevented Egr-1 expression. Moreover, diacylgycerol-dependent protein kinase C isoenzymes and activation of extracellular signal-regulated protein kinase (ERK) are required for glucose-, tolbutamide- and KCl-induced Egr-1 expression. The signaling cascade was blocked by MAP kinase phosphatase-1 (MKP-1) overexpression that dephosphorylated ERK in the nucleus. Stimulation of beta-cells by glucose, tolbutamide and KCl induced the phosphorylation of the transcription factors Elk-1 and CREB. ChIP experiments revealed that phosphorylated Elk-1 and CREB bound under physiological conditions to the Egr-1 gene. Lentiviral-mediated expression of dominant-negative mutants of Elk-1 or CREB interfered with glucose-, tolbutamide- and KCl-induced upregulation of Egr-1 biosynthesis. Together, these data indicate that stimulus-induced transcription of the Egr-1 gene in beta-cells requires combinatorial regulation by Elk-1 and CREB following activation of ERK. The newly synthesized Egr-1 is biologically active and binds under physiological conditions to the genes encoding basic fibroblast growth factor, tumor necrosis factor alpha, transforming growth factor beta and PTEN.

Preferential reduction of beta cells derived from Pax6-MafB pathway in MafB deficient mice

During pancreatic development insulin(+) cells co-express the transcription factors MafB and Pax6, and transition from a MafA(-) to MafA(+) state. To examine the role of Pax6 and MafB in the development of beta-cells, we analyzed embryonic pancreata from Pax6- and MafB-deficient mice. Pax6 deficiency, as manifest in the Pax6(Sey-Neu) allele, reduced not only the number of cells expressing insulin or glucagon, but also the number of MafB, PDX-1 and MafA expressing cells. We show that MafB can directly activate expression of insulin and glucagon, and a MafB protein engineered to contain N248S mutation in the MafB (kr(ENU)) results in significantly reduced activation. Furthermore, pancreata from MafB deficient (kr(ENU)/kr(ENU)) mice exhibited reduced number of cells expressing insulin, glucagon, PDX-1 and MafA, with only a minor reduction in MafB expressing cells. MafB deficiency does not affect endocrine specification but does affect the lineage commitment of the endocrine cells and their maturation. Similar to Pax6 deficient mice, MafB deficient mice showed reductions both in insulin and glucagon expressing cells and in the ability of MafB and PDX-1 expressing cells to activate expression of these hormones. However, MafB deficient mice exhibited no effect on Pax6 expression. These results suggest that MafB may function as a downstream mediator of Pax6 in regulating the specification of insulin and glucagon expressing cells. Interestingly, the remaining insulin(+) cells in these knockouts preferentially express Hb9, suggesting the existence of an alternate pathway for the generation of insulin expressing cells, even in the absence of Pax6 and MafB function. Thus, Pax6 acts upstream of MafB, which in turn may trigger the expression of insulin and regulate the PDX-1 and MafA expression required for beta-cell maturation.

Regulation of gene expression by glucose

PURPOSE OF THE REVIEW

In addition to its metabolic function, glucose modulates gene expression which is crucial in adapting cells to variations in glycaemia. We summarize recent advances in our understanding of regulation of gene expression by glucose.

RECENT FINDINGS

In-vivo and in-vitro experiments demonstrated that glucose regulates the transcription of genes encoding not only lipogenic and glycolytic enzymes but also proteins involved in global cell functions. The molecular mechanisms have begun to be elucidated, and the transcription factor carbohydrate responsive element-binding protein has emerged as a key actor, at least in liver. More recently, other candidates have been proposed, such as liver X receptors. In pathological situations, altered glycaemic control, as observed in diabetes mellitus, is associated with increased risk for microvascular and macrovascular complications. Recent findings suggest that changes in gene expression occurring in response to hyperglycaemia represent a novel component of glucotoxicity.

SUMMARY

Until recently, the direct transcriptional effects of glucose were underestimated, and insulin was considered to be the major regulator of gene expression in response to glycaemic variation. The recent discovery and characterization of transcription factors mediating the glucose response demonstrate that glucose, like fatty acids and other key nutrients, can directly control gene expression.