Transcriptional activation of the proglucagon gene by lithium and beta-catenin in intestinal endocrine L cells

Insights on effects of Wnt pathway modulation on insulin signaling and glucose homeostasis for the treatment of type 2 diabetes mellitus: Wnt activation or Wnt inhibition?

Type 2 diabetes mellitus (T2DM) is a major worldwide chronic disease and can lead to serious diabetic complications. Despite the availability of many anti-diabetic agents in the market, they are unable to meet the long-term treatment goals. Also, they cause many side effects which justify the need for novel class of anti-diabetic drugs with newer mechanism of action. Wnt signaling is one of such novel target pathways which can be explored for metabolic disorders. Many key components of the Wnt signaling are involved in the regulation of glucose homeostasis. Polymorphism in the Transcription factor 7-like 2 (TCF7L2) gene, and mutations in the LRP5 (LDL Receptor Related Protein 5) gene lead to disturbed glucose metabolism and obesity. Despite of several years of research in this field, there is no concrete proof of concept available on whether Wnt activation or Wnt inhibition is the beneficial approach for the treatment of T2DM. Here, we have summarized the conclusions of relevant published research studies to give structured insights into possibilities to explore Wnt modulation as a novel target pathway for the treatment of T2DM. The review also highlights the present challenges and future opportunities towards the development of anti-diabetic small molecules targeting the Wnt signaling pathway.

Interactions between Intestinal Homeostasis and NAD+ Biology in Regulating Incretin Production and Postprandial Glucose Metabolism

The intestine has garnered attention as a target organ for developing new therapies for impaired glucose tolerance. The intestine, which produces incretin hormones, is the central regulator of glucose metabolism. Glucagon-like peptide-1 (GLP-1) production, which determines postprandial glucose levels, is regulated by intestinal homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT)-mediated nicotinamide adenine dinucleotide (NAD⁺) biosynthesis in major metabolic organs such as the liver, adipose tissue, and skeletal muscle plays a crucial role in obesity- and aging-associated organ derangements. Furthermore, NAMPT-mediated NAD⁺ biosynthesis in the intestines and its upstream and downstream mediators, adenosine monophosphate-activated protein kinase (AMPK) and NAD⁺-dependent deacetylase sirtuins (SIRTs), respectively, are critical for intestinal homeostasis, including gut microbiota composition and bile acid metabolism, and GLP-1 production. Thus, boosting the intestinal AMPK-NAMPT-NAD⁺-SIRT pathway to improve intestinal homeostasis, GLP-1 production, and postprandial glucose metabolism has gained significant attention as a novel strategy to improve impaired glucose tolerance. Herein, we aimed to review in detail the regulatory mechanisms and importance of intestinal NAMPT-mediated NAD⁺ biosynthesis in regulating intestinal homeostasis and GLP-1 secretion in obesity and aging. Furthermore, dietary and molecular factors regulating intestinal NAMPT-mediated NAD⁺ biosynthesis were critically explored to facilitate the development of new therapeutic strategies for postprandial glucose dysregulation.

The role of Wnt pathway in obesity induced inflammation and diabetes: a review

Diabetes has become a major killer worldwide and at present, millions are affected by it. Being a chronic disease it increases the risk of other diseases ranging from pulmonary disorders to soft tissue infections. The loss of insulin-producing capacity of the pancreatic β-cells is the main reason for the development of the disease. Obesity is a major complication that can give rise to several other diseases such as cancer, diabetes, etc. Visceral adiposity is one of the major factors that play a role in the development of insulin resistance. Obesity causes a chronic low-grade inflammation in the tissues that further increases the chances of developing diabetes. Several pathways have been associated with the development of diabetes due to inflammation caused by obesity. The Wnt pathway is one such candidate pathway that is found to have a controlling effect on the development of insulin resistance. Moreover, the pathway has also been linked to obesity and inflammation. This review aims to find a connection between obesity, inflammation, and diabetes by taking the wnt pathway as the connecting link.

Estrogen-Wnt signaling cascade regulates expression of hepatic fibroblast growth factor 21

We have generated the transgenic mouse line LTCFDN in which dominant negative TCF7L2 (TCF7L2DN) is specifically expressed in the liver during adulthood. Male but not female LTCFDN mice showed elevated hepatic and plasma triglyceride (TG) levels, indicating the existence of estrogen-β-cat/TCF signaling cascade that regulates hepatic lipid homeostasis. We show here that hepatic fibroblast growth factor 21 (FGF21) expression was reduced in male but not in female LTCFDN mice. The reduction was not associated with altered hepatic expression of peroxisome proliferator-activated receptor α (PPARα). In mouse primary hepatocytes (MPH), Wnt-3a treatment increased FGF21 expression in the presence of PPARα inhibitor. Results from our luciferase-reporter assay and chromatin immunoprecipitation suggest that evolutionarily conserved TCF binding motifs (TCFBs) on Fgf21 promoter mediate Wnt-3a-induced Fgf21 transactivation. Female mice showed reduced hepatic FGF21 production and circulating FGF21 level following ovariectomy (OVX), associated with reduced hepatic TCF expression and β-catenin S675 phosphorylation. Finally, in MPH, estradiol (E2) treatment enhanced FGF21 expression, as well as binding of TCF7L2 and ribonucleic acid (RNA) polymerase II to the Fgf21 promoter; and the enhancement can be attenuated by the G-protein-coupled estrogen receptor 1 (GPER) antagonist G15. Our observations hence indicate that hepatic FGF21 is among the effectors of the newly recognized E2-β-cat/TCF signaling cascade.NEW & NOTEWORTHY FGF21 is mainly produced in the liver. Therapeutic effect of FGF21 analogues has been demonstrated in clinical trials on reducing hyperlipidemia. We show here that Fgf21 transcription is positively regulated by Wnt pathway effector β-cat/TCF. Importantly, hepatic β-cat/TCF activity can be regulated by the female hormone estradiol, involving GPER. The investigation enriched our understanding on hepatic FGF21 hormone production, and expanded our view on metabolic functions of the Wnt pathway in the liver.

Glucagon's Metabolic Action in Health and Disease

Discovered almost simultaneously with insulin, glucagon is a pleiotropic hormone with metabolic action that goes far beyond its classical role to increase blood glucose. Albeit best known for its ability to directly act on the liver to increase de novo glucose production and to inhibit glycogen breakdown, glucagon lowers body weight by decreasing food intake and by increasing metabolic rate. Glucagon further promotes lipolysis and lipid oxidation and has positive chronotropic and inotropic effects in the heart. Interestingly, recent decades have witnessed a remarkable renaissance of glucagon's biology with the acknowledgment that glucagon has pharmacological value beyond its classical use as rescue medication to treat severe hypoglycemia. In this article, we summarize the multifaceted nature of glucagon with a special focus on its hepatic action and discuss the pharmacological potential of either agonizing or antagonizing the glucagon receptor for health and disease. © 2021 American Physiological Society. Compr Physiol 11:1759-1783, 2021.

Neuroendocrine Peptides of the Gut and Their Role in the Regulation of Food Intake

The regulation of food intake encompasses complex interplays between the gut and the brain. Among them, the gastrointestinal tract releases different peptides that communicate the metabolic state to specific nuclei in the hindbrain and the hypothalamus. The present overview gives emphasis on seven peptides that are produced by and secreted from specialized enteroendocrine cells along the gastrointestinal tract in relation with the nutritional status. These established modulators of feeding are ghrelin and nesfatin-1 secreted from gastric X/A-like cells, cholecystokinin (CCK) secreted from duodenal I-cells, glucagon-like peptide 1 (GLP-1), oxyntomodulin, and peptide YY (PYY) secreted from intestinal L-cells and uroguanylin (UGN) released from enterochromaffin (EC) cells. © 2021 American Physiological Society. Compr Physiol 11:1679-1730, 2021.

MicroRNA-194: a novel regulator of glucagon-like peptide-1 synthesis in intestinal L cells

In the status of obesity, the glucagon-like peptide-1 (GLP-1) level usually declines and results in metabolic syndrome. This study aimed to investigate the intracellular mechanism of GLP-1 synthesis in L cells from the perspective of microRNA (miRNA). In the present study, we found that GLP-1 level was down-regulated in the plasma and ileum tissues of obese mice, while the ileac miR-194 expression was up-regulated. In vitro experiments indicated that miR-194 overexpression down-regulated GLP-1 level, mRNA levels of proglucagon gene (gcg) and prohormone convertase 1/3 gene (pcsk1), and the nuclear protein level of beta-catenin (β-catenin). Further investigation confirmed that β-catenin could promote gcg transcription through binding to transcription factor 7-like 2 (TCF7L2). miR-194 suppressed gcg mRNA level via negatively regulating TCF7L2 expression. What’s more, forkhead box a1 (Foxa1) could bind to the promoter of pcsk1 and enhanced its transcription. miR-194 suppressed pcsk1 transcription through targeting Foxa1. Besides, the interference of miR-194 reduced palmitate (PA)-induced cell apoptosis and the anti-apoptosis effect of miR-194 inhibitor was abolished by TCF7L2 knockdown. Finally, in HFD-induced obese mice, the silence of miR-194 significantly elevated GLP-1 level and improved the metabolic symptoms caused by GLP-1 deficiency. To sum up, our study found that miR-194 suppressed GLP-1 synthesis in L cells via inhibiting TCF7L2-mediated gcg transcription and Foxa1-mediated pcsk1 transcription. Meanwhile, miR-194 took part in the PA-induced apoptosis of L cells.

What Is an L-Cell and How Do We Study the Secretory Mechanisms of the L-Cell?

Synthetic glucagon-like peptide-1 (GLP-1) analogues are effective anti-obesity and anti-diabetes drugs. The beneficial actions of GLP-1 go far beyond insulin secretion and appetite, and include cardiovascular benefits and possibly also beneficial effects in neurodegenerative diseases. Considerable reserves of GLP-1 are stored in intestinal endocrine cells that potentially might be mobilized by pharmacological means to improve the body's metabolic state. In recognition of this, the interest in understanding basic L-cell physiology and the mechanisms controlling GLP-1 secretion, has increased considerably. With a view to home in on what an L-cell is, we here present an overview of available data on L-cell development, L-cell peptide expression profiles, peptide production and secretory patterns of L-cells from different parts of the gut. We conclude that L-cells differ markedly depending on their anatomical location, and that the traditional definition of L-cells as a homogeneous population of cells that only produce GLP-1, GLP-2, glicentin and oxyntomodulin is no longer tenable. We suggest to sub-classify L-cells based on their differential peptide contents as well as their differential expression of nutrient sensors, which ultimately determine the secretory responses to different stimuli. A second purpose of this review is to describe and discuss the most frequently used experimental models for functional L-cell studies, highlighting their benefits and limitations. We conclude that no experimental model is perfect and that a comprehensive understanding must be built on results from a combination of models.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

What Can Diabetes-Associated Genetic Variation in TCF7L2 Teach Us About the Pathogenesis of Type 2 Diabetes?

Type 2 diabetes mellitus (T2DM) is a polygenic metabolic disorder characterized by hyperglycemia occurring as a result of impaired insulin secretion and/or insulin resistance. Among the various genetic factors associated with T2DM, a common genetic variant within the transcription factor 7-like 2 locus (TCF7L2) confers the greatest genetic risk for development of the disease. However, the mechanism(s) by which TCF7L2 predisposes to diabetes remain uncertain. Here we review the current literature pertaining to the potential mechanisms by which TCF7L2 confers risk of T2DM, using genetic variation as a probe to understand the pathogenesis of the disease.

The LIM homeodomain protein ISL1 mediates the function of TCF7L2 in pancreatic beta cells

Pancreatic β-cell Tcf7l2 deletion or its functional knockdown suggested the essential role of this Wnt pathway effector in controlling insulin secretion, glucose homeostasis and β-cell gene expression. As the LIM homeodomain protein ISL1 is a suggested Wnt pathway downstream target, we hypothesize that it mediates metabolic functions of TCF7L2. We aimed to determine the role of ISL1 in mediating the function of TCF7L2 and the incretin hormone GLP-1 in pancreatic β-cells. The effect of dominant negative TCF7L2 (TCF7L2DN) mediated Wnt pathway functional knockdown on Isl1 expression was determined in βTCFDN mouse islets and in the rat insulinoma cell line INS-1 832/13. Luciferase reporter assay and chromatin immunoprecipitation were utilized to determine whether Isl1 is a direct downstream target of Tcf7l2 TCF7L2DN adenovirus infection and siRNA-mediated Isl1 knockdown on β-cell gene expression were compared. Furthermore, Isl1 knockdown on GLP-1 stimulated β-catenin S675 phosphorylation and insulin secretion was determined. We found that TCF7L2DN repressed ISL1 levels in βTCFDN islets and the INS-1 832/13 cell line. Wnt stimulators enhanced Isl1 promoter activity and binding of TCF7L2 on Isl1 promoter. TCF7L2DN adenovirus infection and Isl1 knockdown generated similar repression on expression of β-cell genes, including the ones that encode GLUT2 and GLP-1 receptor. Either TCF7L2DN adenovirus infection or Isl1 knockdown attenuated GLP-1-stimulated β-catenin S675 phosphorylation in INS-1 832/13 cells or mouse islets and GLP-1 stimulated insulin secretion in INS-1 832/13 or MIN6 cells. Our observations support the existence of TCF7L2-ISL1 transcriptional network, and we suggest that this network also mediates β-cell function of GLP-1.

PARP-1 Inhibition Rescues Short Lifespan in Hyperglycemic C. Elegans And Improves GLP-1 Secretion in Human Cells

TCF7L2 is located at one of the most strongly associated type 2 diabetes loci reported to date. We previously reported that the most abundant member of a specific protein complex to bind across the presumed causal variant at this locus, rs7903146, was poly [ADP-ribose] polymerase type 1 (PARP-1). We analyzed the impact of PARP-1 inhibition on C. elegans health in the setting of hyperglycemia and on glucose-stimulated GLP-1 secretion in human intestinal cells. Given that high glucose concentrations progressively shorten the lifespan of C. elegans, in part by impacting key well-conserved insulin-modulated signaling pathways, we investigated the effect of PARP-1 inhibition with Olaparib on the lifespan of C. elegans nematodes under varying hyperglycemic conditions. Subsequently, we investigated whether Olaparib treatment had any effect on glucose-stimulated GLP-1 secretion in the human NCI-H716 intestinal cell line, a model system for the investigation of enteroendocrine function. Treatment with 100uM Olaparib in nematodes exposed to high concentrations of glucose led to significant lifespan rescue. The beneficial lifespan effect of Olaparib appeared to require both PARP-1 and TCF7L2, since treatment had no effect in hyperglycemic conditions in knock-out worm strains for either of these homologs. Further investigation using the NCI-H716 cells revealed that Olaparib significantly enhanced secretion of the incretin, GLP-1, plus the gene expression of TCF7L2, GCG and PC1. These data from studies in both C. elegans and a human cell line suggest that PARP-1 inhibition offers a novel therapeutic avenue to treat type 2 diabetes.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

Interactions between PPAR Gamma and the Canonical Wnt/Beta-Catenin Pathway in Type 2 Diabetes and Colon Cancer

In both colon cancer and type 2 diabetes, metabolic changes induced by upregulation of the Wnt/beta-catenin signaling and downregulation of peroxisome proliferator-activated receptor gamma (PPAR gamma) may help account for the frequent association of these two diseases. In both diseases, PPAR gamma is downregulated while the canonical Wnt/beta-catenin pathway is upregulated. In colon cancer, upregulation of the canonical Wnt system induces activation of pyruvate dehydrogenase kinase and deactivation of the pyruvate dehydrogenase complex. As a result, a large part of cytosolic pyruvate is converted into lactate through activation of lactate dehydrogenase. Lactate is extruded out of the cell by means of activation of monocarboxylate lactate transporter-1. This phenomenon is called Warburg effect. PPAR gamma agonists induce beta-catenin inhibition, while inhibition of the canonical Wnt/beta-catenin pathway activates PPAR gamma.

The incretin hormone GLP-1 and mechanisms underlying its secretion

Glucagon-like peptide-1 (GLP-1) is a cell type-specific post-translational product of proglucagon. It is encoded by the proglucagon gene and released primarily from intestinal endocrine L-cells in response to hormonal, neuronal, and nutritional stimuli. In addition to serving as an incretin in mediating the effect of meal consumption on insulin secretion, GLP-1 exerts other functions in pancreatic islets, including regulation of β-cell proliferation and protection of β-cells against metabolic stresses. Furthermore, GLP-1 exerts numerous other functions in extrapancreatic organs, whereas brain GLP-1 signaling controls satiety. Herein we review the discovery of two incretins and the development of GLP-1-based drugs. We also describe the development of cellular models for studying mechanisms underlying GLP-1 secretion over the past 30 years. However, the main content of this review is a summary of studies on the exploration of mechanisms underlying GLP-1 secretion. We not only summarize studies conducted over the past three decades on elucidating the role of nutritional components and hormonal factors in regulating GLP-1 secretion, but also present a few very recent studies showing the possible role of dietary polyphenols. Finally, the emerging role of gut microbiota in metabolic homeostasis with the potential implication on GLP-1 secretion is discussed.

Glucagon Like Peptide-1 Promotes Adipocyte Differentiation via the Wnt4 Mediated Sequestering of Beta-Catenin

Glucagon-like peptide-1 (GLP-1) plays a role in the regulation of adipogenesis; however, the precise underlying molecular mechanism has not been fully defined. Wnt was recently identified as an important regulator of adipogenesis. This study aimed to investigate the involvement of the Wnt signaling pathway in the effects of GLP-1 on adipocyte differentiation. 3T3-L1 cells were induced to differentiate. The changes in the expression levels of adipogenic transcription factors and Wnts and the phosphorylation level and subcellular localization of β-catenin were quantified after GLP-1 treatment. GLP-1 stimulated adipocyte differentiation and lipid accumulation, which were accompanied by the expression of adipocyte marker genes. The expression of Wnt4 was upregulated in the process of adipocyte differentiation, which was further enhanced by treatment with GLP-1. β-catenin, an important mediator of the Wnt pathway, was immediately dephosphorylated and translocated from cytoplasm to nucleus when differentiation was induced. In the presence of GLP-1, however, β-catenin was redirected to the cell plasma membrane leading to its decreased accumulation in the nucleus. Knockdown of Wnt4 blocked the effect of GLP-1 on the cellular localization of β-catenin and expression level of adipogenic transcription factors. Our findings showed that GLP-1 promoted adipogenesis through the modulation of the Wnt4/β-catenin signaling pathway, suggesting that the GLP-1-Wntβ-catenin system might be a new target for the treatment of metabolic disease.

Current Understanding on Role of the Wnt Signaling Pathway Effector TCF7L2 in Glucose Homeostasis

The role of the Wnt signaling pathway in metabolic homeostasis has drawn our intensive attention, especially after the genome-wide association study discovery that certain polymorphisms of its key effector TCF7L2 are strongly associated with the susceptibility to type 2 diabetes. For a decade, great efforts have been made in determining the function of TCF7L2 in various metabolic organs, which have generated both considerable achievements and disputes. In this review, I will briefly introduce the canonical Wnt signaling pathway, focusing on its effector β-catenin/TCF, including emphasizing the bidirectional feature of TCFs and β-catenin post-translational modifications. I will then summarize the observations on the association between TCF7L2 polymorphisms and type 2 diabetes risk. The main content, however, is on the intensive functional exploration of the metabolic role of TCF7L2, including the disputes generated on determining its role in the pancreas and liver with various transgenic mouse lines. Finally, I will discuss those achievements and disputes and present my future perspectives.

Improvement of blood glucose levels and obesity in mice given aronia juice by inhibition of dipeptidyl peptidase IV and α-glucosidase

Aronia berries have many potential effects on health. Previous human studies have shown that aronia juice may be useful for treatment of obesity disorders. Recently, we have reported that aronia juice has an inhibitory effect on dipeptidyl peptidase (DPP IV) activity and that the DPP IV inhibitor in aronia juice was identified as cyanidin 3,5-diglucoside. In this study, we found that body weights and blood glucose levels were reduced in diabetes model KK-Ay mice given aronia juice. We also found that weights of white adipose tissues were reduced in KK-Ay mice given aronia juice. Furthermore, levels of DPP IV activity in the serum and liver from KK-Ay mice were lower than those in the serum and liver from C57BL/6JmsSlc mice. Interestingly, although levels of DPP IV activity were not changed in the serum and liver from aronia-juice-administered KK-Ay mice, levels of DPP IV activity were increased in those from aronia-juice-administered C57BL/6JmsSlc mice. Furthermore, α-glucosidase activity was inhibited in the upper region of the small intestine from aronia-juice-administered KK-Ay mice but not in the lower region. Inhibition of α-glucosidase activity in the upper portion of the small intestine induced a reduction of glucose-dependent insulinotropic polypeptide (GIP) level. The results suggest that DPP IV activity in diabetic mice is inhibited by aronia juice, that the GIP level in the upper region of the small intestine is reduced by inhibition of α-glucosidase activity and that weights of adipose tissues are reduced by aronia juice.

Current understanding and dispute on the function of the Wnt signaling pathway effector TCF7L2 in hepatic gluconeogenesis

Approximately 10 years ago, the Wnt signaling pathway effector TCF7L2 (=TCF-4) was recognized as a type 2 diabetes (T2D) risk gene through a genome wide association study (GWAS). As the correlation between TCF7L2 polymorphisms and T2D susceptibility has been reproducibly observed by numerous follow-up investigations among different ethnic groups, great efforts have been made to explore the function of TCF7L2 in metabolic organs including the pancreas, liver and adipose tissues. Although these explorations have enriched our general knowledge on the Wnt signaling cascade in metabolic homeostasis, studies conducted to date have also generated controversial suggestions. Here I will provide a brief review on the Wnt signaling pathway as well as the milestone GWAS discovery and the follow-up studies. I will then discuss the two different opinions on the correlation between TCF7L2 variants and T2D risk, a gain-of-function event versus a loss-of-function event. This will be followed by summarizing the relevant investigations on the metabolic function of hepatic TCF7L2 and presenting our view on the discrepancy and perspectives.

Glucocorticoids decrease the production of glucagon-like peptide-1 at the transcriptional level in intestinal L-cells

Glucocorticoids are widely used as anti-inflammatory or immunosuppressive drugs, but often induce hyperglycemia as a side effect. Glucagon-like peptide-1 (GLP-1) is secreted from intestinal L cells and plays crucial roles in maintaining glucose homeostasis. However, the direct effects of glucocorticoids on the GLP-1 production pathway in L cells remain unclear. We investigated the effects of glucocorticoids on GLP-1 production in vitro and in vivo. In L cell lines, glucocorticoids decreased GLP-1 release and expression of the precursor, proglucagon, at protein and mRNA levels, which were inhibited by mifepristone. The administration of dexamethasone or budesonide to mice significantly decreased the mRNA expression of proglucagon in the ileum and partially decreased glucose-stimulated GLP-1 secretion. Compound A, a dissociated glucocorticoid receptor modulator, did not affect the expression of proglucagon in vitro. These results suggested that glucocorticoids directly reduced GLP-1 production at the transcriptional level in L cells through a glucocorticoid receptor dimerization-dependent mechanism.

Delineating the regulation of energy homeostasis using hypothalamic cell models

Attesting to its intimate peripheral connections, hypothalamic neurons integrate nutritional and hormonal cues to effectively manage energy homeostasis according to the overall status of the system. Extensive progress in the identification of essential transcriptional and post-translational mechanisms regulating the controlled expression and actions of hypothalamic neuropeptides has been identified through the use of animal and cell models. This review will introduce the basic techniques of hypothalamic investigation both in vivo and in vitro and will briefly highlight the key advantages and challenges of their use. Further emphasis will be place on the use of immortalized models of hypothalamic neurons for in vitro study of feeding regulation, with a particular focus on cell lines proving themselves most fruitful in deciphering fundamental basics of NPY/AgRP, Proglucagon, and POMC neuropeptide function.

Insulin drives glucose-dependent insulinotropic peptide expression via glucose-dependent regulation of FoxO1 and LEF1/β-catenin

Minutes after ingestion of fat or carbohydrates, vesicles stored in enteroendocrine cells release their content of incretin peptide hormones that, together with absorbed glucose, enhance insulin secretion by beta-pancreatic cells. Freshly-made incretins must therefore be packed into new vesicles in anticipation of the next meal with cells adjusting new incretin production to be proportional to the level of previous insulin release and absorbed blood glucose. Here we show that insulin stimulates the expression of the major human incretin, glucose-dependent insulinotropic peptide (GIP) in enteroendocrine cells but requires glucose to do it. Akt-dependent release of FoxO1 and glucose-dependent binding of LEF1/β-catenin mediate induction of Gip expression while insulin-induced phosphorylation of β-catenin does not alter its localization or transcriptional activity in enteroendocrine cells. Our results reveal a glucose-regulated feedback loop at the entero-insular axis, where glucose levels determine basal and insulin-induced Gip expression; GIP stimulation of insulin release, physiologically ensures a fine control of glucose homeostasis. How enteroendocrine cells adjust incretin production to replace incretin stores for future use is a key issue because GIP malfunction is linked to all forms of diabetes.

Activation of cAMP signaling attenuates impaired hepatic glucose disposal in aged male p21-activated protein kinase-1 knockout mice

p21-activated protein kinase-1 (Pak1) plays a role in insulin secretion and glucagon-like peptide-1 (GLP-1) production. Pak1(-/-) mice were found to carry a defect in ip pyruvate tolerance test (IPPTT), leading us to speculate whether Pak1 represses hepatic gluconeogenesis. We show here that the defect in IPPTT became more severe in aged Pak1(-/-) mice. In primary hepatocytes, 2,2'-dihydroxy-1,1'-dinaphthyldisulfide, a potent inhibitor of group I Paks, reduced basal glucose production (GP), attenuated forskolin- or glucagon-stimulated GP, and attenuated the stimulation of forskolin on the expression of Pck1 and G6pc. In addition, the capacity of primary hepatocytes isolated from Pak1(-/-) mice in GP at the basal level is significantly lower than that of the control littermates. These in vitro observations imply that the direct effect of Paks in hepatocytes is the stimulation of gluconeogenesis and that the impairment in IPPTT in Pak1(-/-) mice is due to the lack of Pak1 elsewhere. Consecutive ip injection of forskolin for 2 weeks increased gut proglucagon expression, associated with improved IPPTT in aged Pak1(-/-) mice and wild-type controls. In addition, administration of the DPP-IV (dipeptidyl peptidase-4) inhibitor sitagliptin for 1 week reversed the defect in IPPTT in aged Pak1(-/-) mice, associated with increased plasma GLP-1 levels. Our observations indicate a potential role of Pak1 in the gut/pancreas/liver axis in controlling glucose disposal and affirmed the therapeutic application of GLP-1 and DPP-IV inhibitors in attenuating hepatic gluconeogenesis.

p21-Activated protein kinases and their emerging roles in glucose homeostasis

p21-Activated protein kinases (PAKs) are centrally involved in a plethora of cellular processes and functions. Their function as effectors of small GTPases Rac1 and Cdc42 has been extensively studied during the past two decades, particularly in the realms of cell proliferation, apoptosis, and hence tumorigenesis, as well as cytoskeletal remodeling and related cellular events in health and disease. In recent years, a large number of studies have shed light onto the fundamental role of group I PAKs, most notably PAK1, in metabolic homeostasis. In skeletal muscle, PAK1 was shown to mediate the function of insulin on stimulating GLUT4 translocation and glucose uptake, while in pancreatic β-cells, PAK1 participates in insulin granule localization and vesicle release. Furthermore, we demonstrated that PAK1 mediates the cross talk between insulin and Wnt/β-catenin signaling pathways and hence regulates gut proglucagon gene expression and the production of the incretin hormone glucagon-like peptide-1 (GLP-1). The utilization of chemical inhibitors of PAK and the characterization of Pak1(-/-) mice enabled us to gain mechanistic insights as well as to assess the overall contribution of PAKs in metabolic homeostasis. This review summarizes our current understanding of PAKs, with an emphasis on the emerging roles of PAK1 in glucose homeostasis.

GLP-1(28-36) improves β-cell mass and glucose disposal in streptozotocin-induced diabetic mice and activates cAMP/PKA/β-catenin signaling in β-cells in vitro

Recent studies have demonstrated that the COOH-terminal fragment of the incretin hormone glucagon-like peptide-1 (GLP-1), a nonapeptide GLP-1(28-36)amide, attenuates diabetes and hepatic steatosis in diet-induced obese mice. However, the effect of this nonapeptide in pancreatic β-cells remains largely unknown. Here, we show that in a streptozotocin-induced mouse diabetes model, GLP-1(28-36)amide improved glucose disposal and increased pancreatic β-cell mass and β-cell proliferation. An in vitro investigation revealed that GLP-1(28-36)amide stimulates β-catenin (β-cat) Ser(675) phosphorylation in both the clonal INS-1 cell line and rat primary pancreatic islet cells. In INS-1 cells, the stimulation was accompanied by increased nuclear β-cat content. GLP-1(28-36)amide was also shown to increase cellular cAMP levels, PKA enzymatic activity, and cAMP response element-binding protein (CREB) and cyclic AMP-dependent transcription factor-1 (ATF-1) phosphorylation. Furthermore, GLP-1(28-36)amide treatment enhanced islet insulin secretion and increased the growth of INS-1 cells, which was associated with increased cyclin D1 expression. Finally, PKA inhibition attenuated the effect of GLP-1(28-36)amide on β-cat Ser(675) phosphorylation and cyclin D1 expression in the INS-1 cell line. We have thus revealed the beneficial effect of GLP-1(28-36)amide in pancreatic β-cells in vitro and in vivo. Our observations suggest that GLP-1(28-36)amide may exert its effect through the PKA/β-catenin signaling pathway.

Identification of a pathway by which glucose regulates β-catenin signalling via the cAMP/protein kinase A pathway in β-cell models

Pancreatic β-cells are highly responsive to changes in glucose, but the mechanisms involved are only partially understood. There is increasing evidence that the β-catenin signalling pathway plays an important role in regulating β-cell function, but the mechanisms regulating β-catenin signalling in these cells is not well understood. In the present study we show that β-catenin levels and downstream signalling are regulated by changes in glucose levels in INS-1E and β-TC6-F7 β-cell models. We found a glucose-dependent increase in levels of β-catenin in the cytoplasm and nucleus of INS-1E cells. Expression of cyclin D1 also increased with glucose and required the presence of β-catenin. This was associated with an increase in phosphorylation of β-catenin on Ser552, which is known to stabilize the molecule and increase its transcriptional activity. In a search for possible signalling intermediates we found forskolin and cell-permeable cAMP analogues recapitulated the glucose effects, suggesting a role for cAMP and PKA (cAMP-dependent protein kinase/protein kinase A) downstream of glucose. Furthermore, glucose caused sustained increases in cAMP. Two different inhibitors of adenylate cyclase and PKA signalling blocked the effects of glucose, whereas siRNA (small interfering RNA) knockdown of PKA blocked the effects of glucose on β-catenin signalling. Finally, reducing β-catenin levels with either siRNA or pyrvinium impaired glucose- and KCl-stimulated insulin secretion. Taken together the results of the present study define a pathway by which changes in glucose levels can regulate β-catenin using a mechanism which involves cAMP production and the activation of PKA. This identifies a pathway that may be important in glucose-dependent regulation of gene expression and insulin secretion in β-cells.

The Wnt signaling pathway effector TCF7L2 controls gut and brain proglucagon gene expression and glucose homeostasis

The type 2 diabetes risk gene TCF7L2 is the effector of the Wnt signaling pathway. We found previously that in gut endocrine L-cell lines, TCF7L2 controls transcription of the proglucagon gene (gcg), which encodes the incretin hormone glucagon-like peptide-1 (GLP-1). Whereas peripheral GLP-1 stimulates insulin secretion, brain GLP-1 controls energy homeostasis through yet-to-be defined mechanisms. We aim to determine the metabolic effect of a functional knockdown of TCF7L2 by generating transgenic mice that express dominant-negative TCF7L2 (TCF7L2DN) specifically in gcg-expressing cells. The gcg-TCF7L2DN transgenic mice showed reduced gcg expression in their gut and brain, but not in pancreas. Defects in glucose homeostasis were observed in these mice, associated with attenuated plasma insulin levels in response to glucose challenge. The defect in glucose disposal was exacerbated with high-fat diet. Brain Wnt activity and feeding-mediated hypothalamic AMP-activated protein kinase (AMPK) repression in these mice were impaired. Peripheral injection of the cAMP-promoting agent forskolin increased brain β-cat Ser675 phosphorylation and brain gcg expression and restored feeding-mediated hypothalamic AMPK repression. We conclude that TCF7L2 and Wnt signaling control gut and brain gcg expression and glucose homeostasis and speculate that positive cross-talk between Wnt and GLP-1/cAMP signaling is an underlying mechanism for brain GLP-1 in exerting its metabolic functions.

P21-activated protein kinase 1 (Pak1) mediates the cross talk between insulin and β-catenin on proglucagon gene expression and its ablation affects glucose homeostasis in male C57BL/6 mice

In gut endocrine L cells, the Wnt signaling pathway effector β-catenin (β-cat)/transcription factor 7-like 2 mediates the stimulatory effect of insulin on proglucagon (gcg) expression and glucagon-like peptide-1 (GLP-1) production. In several other cell lineages, insulin is able to stimulate p21-activated protein kinase 1 (Pak1). Here we determined the role of Pak1 in gcg expression and the effect of Pak1 deletion on glucose homeostasis. Insulin stimulated Pak1 activation through increasing its Thr423 phosphorylation in gut gcg-expressing cell lines, associated with increased gcg mRNA levels. This stimulation was attenuated by the Pak inhibitor 2,2'-dihydroxy-1,1'-dinaphthyldisulfide (IPA3) or dominant-negative Pak1. Both insulin and cAMP-promoting agents activated β-cat Ser675 phosphorylation, which was attenuated by IPA3 or protein kinase A inhibition, respectively. Gut gcg levels were reduced in male Pak1(-/-) mice, associated with impaired glucose tolerance after an ip or oral glucose challenge. These mice had lower circulating active GLP-1 levels after a glucose challenge as well as reduced distal ileum GLP-1 content after insulin treatment. Finally, the Pak1(-/-) mice exhibited reduced brainstem gcg level and abolished β-cat Ser675 phosphorylation in brain neurons after insulin treatment. We suggest that Pak1 mediates the cross talk between insulin and Wnt signaling pathways on gut and brain gcg expression, and its ablation impairs glucose homeostasis.

Depletion of homeodomain-interacting protein kinase 3 impairs insulin secretion and glucose tolerance in mice

AIMS/HYPOTHESIS

Insufficient insulin secretion and reduced pancreatic beta cell mass are hallmarks of type 2 diabetes. Here, we focused on a family of serine-threonine kinases known as homeodomain-interacting protein kinases (HIPKs). HIPKs are implicated in the modulation of Wnt signalling, which plays a crucial role in transcriptional activity, and in pancreas development and maintenance. The aim of the present study was to characterise the role of HIPKs in glucose metabolism.

METHODS

We used RNA interference to characterise the role of HIPKs in regulating insulin secretion and transcription activity. We conducted RT-PCR and western blot analyses to analyse the expression and abundance of HIPK genes and proteins in the islets of high-fat diet-fed mice. Glucose-induced insulin secretion and beta cell proliferation were measured in islets from Hipk3 ( -/- ) mice, which have impaired glucose tolerance owing to an insulin secretion deficiency. The abundance of pancreatic duodenal homeobox (PDX)-1 and glycogen synthase kinase (GSK)-3β phosphorylation in Hipk3 ( -/- ) islets was determined by immunohistology and western blot analyses.

RESULTS

We found that HIPKs regulate insulin secretion and transcription activity. Hipk3 expression was most significantly increased in the islets of high-fat diet-fed mice. Furthermore, glucose-induced insulin secretion and beta cell proliferation were decreased in the islets of Hipk3 ( -/- ) mice. Levels of PDX1 and GSK-3β phosphorylation were significantly decreased in Hipk3 ( -/- ) islets.

CONCLUSIONS/INTERPRETATION

Depletion of HIPK3 impairs insulin secretion and glucose tolerance. Decreased levels of HIPK3 may play a substantial role in the pathogenesis of type 2 diabetes.

The Wnt signaling pathway effector TCF7L2 is upregulated by insulin and represses hepatic gluconeogenesis

Certain single nucleotide polymorphisms (SNPs) in transcription factor 7-like 2 (TCF7L2) are strongly associated with the risk of type 2 diabetes. TCF7L2 and β-catenin (β-cat) form the bipartite transcription factor cat/TCF in stimulating Wnt target gene expression. cat/TCF may also mediate the effect of other signaling cascades, including that of cAMP and insulin in cell-type specific manners. As carriers of TCF7L2 type 2 diabetes risk SNPs demonstrated increased hepatic glucose production, we aimed to determine whether TCF7L2 expression is regulated by nutrient availability and whether TCF7L2 and Wnt regulate hepatic gluconeogenesis. We examined hepatic Wnt activity in the TOPGAL transgenic mouse, assessed hepatic TCF7L2 expression in mice upon feeding, determined the effect of insulin on TCF7L2 expression and β-cat Ser⁶⁷⁵ phosphorylation, and investigated the effect of Wnt activation and TCF7L2 knockdown on gluconeogenic gene expression and glucose production in hepatocytes. Wnt activity was observed in pericentral hepatocytes in the TOPGAL mouse, whereas TCF7L2 expression was detected in human and mouse hepatocytes. Insulin and feeding stimulated hepatic TCF7L2 expression in vitro and in vivo, respectively. In addition, insulin activated β-cat Ser⁶⁷⁵ phosphorylation. Wnt activation by intraperitoneal lithium injection repressed hepatic gluconeogenic gene expression in vivo, whereas lithium or Wnt-3a reduced gluconeogenic gene expression and glucose production in hepatic cells in vitro. Small interfering RNA-mediated TCF7L2 knockdown increased glucose production and gluconeogenic gene expression in cultured hepatocytes. These observations suggest that Wnt signaling and TCF7L2 are negative regulators of hepatic gluconeogenesis, and TCF7L2 is among the downstream effectors of insulin in hepatocytes.

New insight into the mechanisms underlying the function of the incretin hormone glucagon-like peptide-1 in pancreatic β-cells: the involvement of the Wnt signaling pathway effector β-catenin

During the past two decades, the exploration of function of two incretin hormones, namely glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP), has led to the development of two categories of novel therapeutic agents for diabetes and its complications, known as GLP-1 receptor (GLP-1R) agonists and DPP-IV inhibitors. Mechanisms underlying the function of GLP-1, however, still need to be further explored. GLP-1 not only functions as an incretin hormone in stimulating insulin secretion in response to nutritional, hormonal and neuronal stimulations, but also acts as an "insulin-like" factor in β-cell and extra-pancreatic organs. In addition to these insulinotropic and insulinomimetic effects, GLP-1 was shown to exert its protective effect in β-cell by repressing the expression of TxNIP, a mediator of glucolipotoxicity. A number of recent studies have shown that the Wnt signaling pathway effector, the bipartite transcription factor β-catenin/TCF, controls not only the production of GLP-1, but also the function of GLP-1. Furthermore, previously assumed "degradation" products of GLP-1(7-36)amide, including GLP-1(9-36)amide and GLP-1(28-36)amide, have been shown to exert beneficial effect in pancreas and extra-pancreatic tissues or cell lineages. Here we summarized our current knowledge on the metabolic, proliferative and protective effects of GLP-1(7-36)amide and its cleavage fragments, mainly focusing on pancreatic β-cells and the involvement of the Wnt signaling pathway effector β-catenin.

Insulin detemir enhances proglucagon gene expression in the intestinal L cells via stimulating β-catenin and CREB activities

Insulin therapy using insulin detemir (d-INS) has demonstrated weight-sparing effects compared with other insulin formulations. Mechanisms underlying these effects, however, remain largely unknown. Here we postulate that the intestinal tissues' selective preference allows d-INS to exert enhanced action on proglucagon (Gcg) expression and the production of glucagon-like peptide (GLP)-1, an incretin hormone possessing both glycemia-lowering and weight loss effects. To test this hypothesis, we used obese type 2 diabetic db/db mice and conducted a 14-day intervention with daily injection of a therapeutic dose of d-INS or human insulin (h-INS) in these mice. The body weight of the mice after 14-day daily injection of d-INS (5 IU/kg) was decreased significantly compared with those injected with the same dose of h-INS or saline. The weight-sparing effect of d-INS was associated with significantly elevated circulating levels of total GLP-1 and reduced food intake. Histochemistry analysis demonstrated that d-INS induced rapid phosphorylation of protein kinase B (Akt) in the gut L cells of normal mice. Western blotting showed that d-INS stimulated Akt activation in a more rapid and enhanced fashion in the mouse distal ileum compared with those by h-INS. In vitro investigation in primary fetal rat intestinal cell (FRIC) cultures showed that d-INS increased Gcg mRNA expression as determined by Northern blotting and real-time RT-PCR. Consistent with these in vivo investigations, d-INS significantly increased GLP-1 secretion in FRIC cultures. Consistently, d-INS was also shown to induce rapid phosphorylation of Akt in the clonal gut cell line GLUTag. Furthermore, d-INS increased β-catenin phosphorylation, its nuclear translocation, and enhanced cAMP response element-binding protein (CREB) phosphorylation in a phosphatidylinositol 3-kinase and/or mitogen-activated protein kinase kinase/extracellular signal-regulated kinase-sensitive manner. We suggest that the weight-sparing benefit of d-INS in mice is related to its intestinal tissues preference that leads to profound stimulation of Gcg expression and enhanced GLP-1 secretion in intestinal L cells, potentially involving the activation of insulin/β-catenin/CREB signaling pathways.

The involvement of the wnt signaling pathway and TCF7L2 in diabetes mellitus: The current understanding, dispute, and perspective

The Wnt signaling pathway was initially discovered for its role in tumorigenesis and the development of Drosophila and other eukaryotic organisms. The key effector of this pathway, the bipartite transcription factor β-cat/TCF, is formed by free β-catenin (β-cat) and a TCF protein, including TCF7L2. Extensive recent investigations have highlighted the role of the Wnt signaling pathway in metabolic homeostasis and its implication in diabetes and other metabolic diseases. Genome-wide association studies have shown that several key components of the Wnt signaling pathway are implicated in metabolic homeostasis and the development of type 2 diabetes (T2D). Despite controversial observations regarding the role of Wnt signaling in the development and function of pancreatic islets, the discovery of the association between certain single nucleotide polymorphisms of TCF7L2 and T2D susceptibility has fueled great efforts to explore the role of Wnt signaling in the function of pancreatic β-cells and glucose homeostasis. Here we have introduced our basic understanding of the canonical Wnt signaling pathway, summarized our current knowledge on its implication in metabolic homeostasis and T2D, discussed the work on TCF7L2 as a T2D susceptibility gene, and presented the controversial role of Wnt signaling and TCF7L2 in pancreatic islets as well as their potential metabolic function in other organs. We then expanded our view into the crosstalk among Wnt, insulin and FOXO signaling cascades, which further illustrates the complexity of the Wnt signaling pathway in metabolic homeostasis. Finally, we have presented our perspectives.

The role of the Wnt signaling pathway in incretin hormone production and function

Glucose metabolism is tightly controlled by multiple hormones and neurotransmitters in response to nutritional, environmental, and emotional changes. In addition to insulin and glucagon produced by pancreatic islets, two incretin hormones, namely glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP, also known as glucose-dependent insulinotropic peptide), also play important roles in blood glucose homeostasis. The incretin hormones mainly exert their regulatory effects via their corresponding receptors, which are expressed in pancreatic islets as well as many other extra-pancreatic organs. Recent studies have shown that the genes which encode these two incretin hormones can be regulated by the effectors of the Wnt signaling pathway, including TCF7L2, a transcription factor identified recently by extensive genome wide association studies as an important type 2 diabetes risk gene. Interestingly, TCF7L2 and β-catenin (β-cat), another effector of Wnt signaling pathway, may also mediate the function of the incretin hormones as well as the expression of their receptors in pancreatic β-cells. In this review, we have introduced the incretin hormones and the Wnt signaling pathway, summarized recent findings in the field, and provided our perspectives.

Exendin-4 upregulates the expression of Wnt-4, a novel regulator of pancreatic β-cell proliferation

The Wnt-signaling pathway regulates β-cell functions. It is not known how the expression of endogenous Wnt-signaling molecules is regulated in β-cells. Therefore, we investigated the effect of antidiabetic drugs and glucose on the expression of Wnt-signaling molecules in β-cells. Primary islets were isolated and cultured. The expression of Wnt-signaling molecules (Wnt-4, Wnt-10b, Frizzled-4, LRP5, TCF7L2) and TNFα was analyzed by semiquantitative PCR and Western blotting. Transient transfections were carried out and proliferation assays of INS-1 β-cells performed using [(3)H]thymidine uptake and BrdU ELISA. Insulin secretion was quantified. A knockdown (siRNA) of Wnt-4 in β-cells was carried out. Exendin-4 significantly increased the expression of Wnt-4 in β-cells on the mRNA level (2.8-fold) and the protein level (3-fold) (P < 0.001). The effect was dose dependent, with strongest stimulation at 10 nM, and it was maintained after long-term stimulation over 4 wk. Addition of exd-(9-39), a GLP-1 receptor antagonist, abolished the effect of exendin-4. Treatment with glucose, insulin, or other antidiabetic drugs had no effect on the expression of any of the examined Wnt-signaling molecules. Functionally, Wnt-4 antagonized the activation of canonical Wnt-signaling in β-cells. Wnt-4 had no effect on glucose-stimulated insulin secretion or insulin gene expression. Knocking down Wnt-4 decreased β-cell proliferation to 45% of controls (P < 0.05). In addition, Wnt-4 and exendin-4 treatment decreased the expression of TNFaα mRNA in primary β-cells. These data demonstrate that stimulation with exendin-4 increases the expression of Wnt-4 in β-cells. Wnt-4 modulates canonical Wnt signaling and acts as regulator of β-cell proliferation and inflammatory cytokine release. This suggests a novel mechanism through which GLP-1 can regulate β-cell proliferation.

Nkx6.2 synergizes with Cdx-2 in stimulating proglucagon gene expression

AIM: To investigate whether the transactivator of the proglucagon gene (Gcg), Cdx-2, synergizes with other transcription factors in stimulating Gcg expression and the trans-differentiation of Gcg-expressing cells.

METHODS: We conducted affinity chromatography to identify proteins that interact with Cdx-2, using GST-tagged Cdx-2 against cell lysates from pancreatic InR1-G9 and intestinal GLUTag cell lines. This was followed by a mass-spectrometry analysis. From a potential Cdx-2 interaction protein identified, we examined its expression in pancreatic and gut endocrine cells, confirmed its interaction with Cdx-2 by GST-pull down and determined its effect in provoking Gcg expression in cell lines that do not express endogenous Gcg.

RESULTS: We identified 18 potential Cdx-2 interacting proteins. One of them is Nkx6.2. This homeodomain (HD) protein is expressed in pancreatic α and intestinal endocrine L cells but not in insulin producing cell lines, including In111. Nkx6.2, but not Nkx6.1, was shown to interact with Cdx-2, detected by GST-pull down. Furthermore, Nkx6.2 was found to synergize with Cdx-2 in provoking Gcg expression when they were ectopically expressed in the In111 cell line. Finally, when Cdx-2 and Nkx6.2 were co-transfected into the undifferentiated rat intestinal IEC-6 cell line, it produced detectable amount of Gcg mRNA.

CONCLUSION: Cdx-2 recruits Nkx6.2 in exerting its effect in stimulating Gcg expression. Our observations further support the notion that multiple HD proteins, including Cdx-2 and Nkx6.2, are involved in the regulation of Gcg expression and the genesis of Gcg-producing cells.

PPARβ/δ activation induces enteroendocrine L cell GLP-1 production

BACKGROUND & AIMS

Glucagon-like peptide (GLP)-1, an intestinal incretin produced by L cells through proglucagon processing, is secreted after nutrient ingestion and acts on endocrine pancreas beta cells to enhance insulin secretion. Peroxisome proliferator-activated receptor (PPAR) β/δ is a nuclear receptor that improves glucose homeostasis and pancreas islet function in diabetic animal models. Here, we investigated whether PPARβ/δ activation regulates L cell GLP-1 production.

METHODS

Proglucagon regulation and GLP-1 release were evaluated in murine GLUTag and human NCI-H716 L cells and in vivo using wild-type, PPARβ/δ-null, and ob/ob C57Bl/6 mice treated with the PPARβ/δ synthetic agonists GW501516 or GW0742.

RESULTS

PPARβ/δ activation increased proglucagon expression and enhanced glucose- and bile acid-induced GLP-1 release by intestinal L cells in vitro and ex vivo in human jejunum. In vivo treatment with GW0742 increased proglucagon messenger RNA levels in the small intestine in wild-type but not in PPARβ/δ-deficient mice. Treatment of wild-type and ob/ob mice with GW501516 enhanced the increase in plasma GLP-1 level after an oral glucose load and improved glucose tolerance. Concomitantly, proglucagon and GLP-1 receptor messenger RNA levels increased in the small intestine and pancreas, respectively. Finally, PPARβ/δ agonists activate the proglucagon gene transcription by interfering with the β-catenin/TCF-4 pathway.

CONCLUSIONS

Our data show that PPARβ/δ activation potentiates GLP-1 production by the small intestine. Pharmacologic targeting of PPARβ/δ is a promising approach in the treatment of patients with type 2 diabetes mellitus, especially in combination with dipeptidyl peptidase IV inhibitors.

Therapeutic Strategies to Increase Human β-Cell Growth and Proliferation by Regulating mTOR and GSK-3/β-Catenin Pathways

This perspective delineates approaches to develop therapeutic strategies to stimulate the proliferative potential of adult human β-cells in vitro. Previous findings demonstrated that nutrients, through regulation of mTOR signaling, promote regenerative processes including DNA synthesis, cell cycle progression and β-cell proliferation in rodent islets but rarely in human islets. Recently, we discovered that regulation of the Wnt/GSK-3/β-catenin pathway by directly inhibiting GSK-3 with pharmacologic agents, in combination with nutrient activation of mTOR, was required to increase growth and proliferation in human islets. Studies also revealed that nuclear translocation of β-catenin in response to GSK-3 inhibition regulated these processes and was rapamycin sensitive, indicating a role for mTOR. Human islets displayed a high level of insulin resistance consistent with the inability of exogenous insulin to activate Akt and engage the Wnt pathway by GSK-3 inhibition. This insulin resistance in human islets is not present in rodent islets and may explain the differential requirement in human islets to inhibit GSK-3 to enhance these regenerative processes. Human islets exhibited normal insulin secretion but a loss of insulin content, which was independent of all treatment conditions. The loss of insulin content may be related to insulin resistance, the isolation process or culture conditions. In this perspective, we provide strategies to enhance the proliferative capacity of adult human β-cells and highlight important differences between human and rodent islets: the lack of a nutrient response, requirement for direct GSK-3 inhibition, insulin resistance and loss of insulin content that emphasize the physiological significance of conducting studies in human islets.

Neonatal growth and regeneration of beta-cells are regulated by the Wnt/beta-catenin signaling in normal and diabetic rats

Wnt/beta-catenin signaling is critical for a variety of fundamental cellular processes. Here, we investigated the implication of the Wnt/beta-catenin signaling in the in vivo regulation of beta-cell growth and regeneration in normal and diabetic rats. To this aim, TCF7L2, the distal effector of the canonical Wnt pathway, was knocked down in groups of normal and diabetic rats by the use of specific antisense morpholino-oligonucleotides. In other groups of diabetic rats, the Wnt/beta-catenin pathway was activated by the inhibition of its negative regulator GSK-3beta. GSK-3beta was inactivated by either LiCl or anti-GSK-3beta oligonucleotides. The beta-cell mass was evaluated by morphometry. beta-cell proliferation was assessed in vivo and in vitro by BrdU incorporation method. In vivo beta-cell neogenesis was estimated by the evaluation of PDX1-positive ductal cells and GLUT2-positive ductal cells and the number of beta cells budding from the ducts. We showed that the in vivo disruption of the canonical Wnt pathway resulted in the alteration of normal and compensatory growth of beta-cells mainly through the inhibition of beta-cell proliferation. Conversely, activation of the Wnt pathway through the inhibition of GSK-3beta had a significant stimulatory effect on beta-cell regeneration in diabetic rats. In vitro, GSK-3beta inactivation resulted in the stimulation of beta-cell proliferation. This was mediated by the stabilization of beta-catenin and the induction of cyclin D. Taken together, our results demonstrate the involvement of the canonical Wnt signaling in the neonatal regulation of normal and regenerative growth of pancreatic beta-cells. Moreover, we provide evidence that activation of this pathway by pharmacological maneuvers can efficiently improve beta-cell regeneration in diabetic rats. These findings might have potential clinical applications in the regenerative therapy of diabetes.

POU homeodomain protein Oct-1 functions as a sensor for cyclic AMP

Cyclic AMP is a fundamentally important second messenger for numerous peptide hormones and neurotransmitters that control gene expression, cell proliferation, and metabolic homeostasis. Here we show that cAMP works with the POU homeodomain protein Oct-1 to regulate gene expression in pancreatic and intestinal endocrine cells. This ubiquitously expressed transcription factor is known as a stress sensor. We found that it also functions as a repressor of Cdx-2, a proglucagon gene activator. Through a mechanism that involves the activation of exchange protein activated by cyclic AMP, elevation of cAMP leads to enhanced phosphorylation and nuclear exclusion of Oct-1 and reduced interactions between Oct-1 or nuclear co-repressors and the Cdx-2 gene promoter, detected by chromatin immunoprecipitation. In rat primary pancreatic islet cells, cAMP elevation also reduces nuclear Oct-1 content, which causes increased proglucagon and proinsulin mRNA expression. Our study therefore identifies a novel mechanism by which cAMP regulates hormone-gene expression and suggests that ubiquitously expressed Oct-1 may play a role in metabolic homeostasis by functioning as a sensor for cAMP.

New insights into the role of cAMP in the production and function of the incretin hormone glucagon-like peptide-1 (GLP-1)

The proglucagon gene (gcg) encodes both glucagon and glucagon-like peptide-1 (GLP-1), produced in pancreatic alpha cells and intestinal endocrine L cells, respectively. The incretin hormone GLP-1 stimulates insulin secretion and pro-insulin gene transcription. GLP-1 also enhances pancreatic beta-cell proliferation, inhibits cell apoptosis, and has been utilized in the trans-differentiation of insulin producing cells. A long-term effective GLP-1 receptor agonist, Byetta, has now been developed as the drug in treating type II diabetes and potentially other metabolic disorders. The expression of gcg and the production of GLP-1 can be activated by the elevation of the second messenger cyclic AMP (cAMP). Recent studies suggest that in addition to protein kinase A (PKA), exchange protein activated by cAMP (Epac), another effector of cAMP, and the crosstalk between PKA and the Wnt signaling pathway, are involved in cAMP-stimulated gcg transcription and GLP-1 production as well. Finally, functions of GLP-1 in pancreatic beta cells are also mediated by PKA, Epac, as well as the effector of the Wnt signaling pathway. Together, these novel findings bring us a new insight into the role of cAMP in the production and function of the incretin hormone GLP-1.

WNT/beta-catenin increases the production of incretins by entero-endocrine cells

AIMS/HYPOTHESIS

Glucose-dependent insulinotropic peptide (GIP) plays a pivotal role in the regulation of glucose homeostasis. Rates of diet-induced obesity, insulin resistance and type 2 diabetes are decreased when GIP signalling is disturbed in mice, suggesting that GIP plays a role in the onset of type 2 diabetes. WNT signalling is linked to type 2 diabetes and induces synthesis of the other incretin, glucagon-like peptide 1 (GLP-1). GLP-1 analogues improve treatment of type 2 diabetes patients in whom GLP-1 signalling is intact and have captured clinical attention. GIP levels are altered at the onset of type 2 diabetes and later on, while GIP signalling is impaired. Thus, GIP is not a candidate for treatment but might be an important target from a prevention perspective. Hypothesising that hypersecretion of GIP links altered WNT signalling to the onset of type 2 diabetes, we sought to determine whether WNT signalling induces GIP production by entero-endocrine cells.

METHODS

RT-PCR and chromatin immunoprecipitation (ChIP) were used to study Gip gene induction. Gip promoter elements mediating WNT/lithium induction were identified (electrophoretic mobility shift assay, co-transfection of deletion mutants, ChIP).

RESULTS

Lithium or WNT/beta-catenin signalling enhanced GIP production by entero-endocrine cells through a conserved site in the proximal Gip promoter. Lithium favours lymphoid enhancer factor-1/beta-catenin binding to Gip promoter and diminishes ChIP through T cell factor-4 and histone deacetylase 1.

CONCLUSIONS/INTERPRETATION

Lithium and WNT are incretin inducers in general. This work provides a novel link between WNT signalling, obesity and diabetes.

Pathophysiological aspects of Wnt-signaling in endocrine disease

Wnt-signaling has recently been identified as a regulator of a number of endocrine functions in health and disease in addition to its original attribution to developmental biology. Wnts are extracellular ligands on frizzled receptors and on lipoprotein receptor-related protein co-receptors. Ligand binding leads eventually to the activation of intracellular signaling cascades; based on the involvement of the transcriptional co-activator beta-catenin it can be distinguished between canonical (i.e. beta-catenin) and non-canonical Wnt-signaling. Recent studies revealed that canonical Wnt-signaling regulates the function of endocrine organs and contributes to a number of endocrine disorders. In this review, we would like to focus on a) recent mechanistic data on Wnts in pancreatic beta-cell function; b) human genetic studies on Wnt signaling in type 2 diabetes mellitus; c) crosstalk between adipocytes and endocrine cells through Wnt-signaling molecules (with a focus on the role of Wnt-signaling in adrenocortical cells).

Epac is involved in cAMP-stimulated proglucagon expression and hormone production but not hormone secretion in pancreatic alpha- and intestinal L-cell lines

Both Epac and PKA are effectors of the second messenger cAMP. Utilizing an exchange protein directly activated by cAMP (Epac) pathway-specific cAMP analog (ESCA), we previously reported that Epac signaling regulates proglucagon gene (gcg) expression in the glucagon-like peptide-1 (GLP-1)-producing intestinal endocrine L-cell lines GLUTag and STC-1. We now show that Epac-2 is also expressed in glucagon-producing pancreatic alpha-cell lines, including PKA-deficient InR1-G9 cells, and that ESCA stimulates gcg promoter and mRNA expression in the InR1-G9 cells. Using a dominant-negative Epac-2 expression plasmid (Epac-2DN), we found that Epac inhibition attenuated forskolin-stimulated gcg promoter expression in the PKA-active STC-1 cell line and blocked forskolin-stimulated gcg promoter expression in the InR1-G9 cells. Consistently, ESCA was shown to stimulate glucagon and GLP-1 production in the InR1-G9 and GLUTag cell lines, respectively. Surprisingly, ESCA treatment did not show a notable stimulation of glucagon or GLP-1 secretion from these two cell lines. This is in contrast to its ability to stimulate insulin secretion from the pancreatic INS-1 beta-cell line. Our findings suggest that Epac is selectively involved in peptide hormone secretion in pancreatic and intestinal endocrine cells and that distinct signaling cascades are involved in stimulating production vs. secretion of glucagon and GLP-1 in response to cAMP elevation.

Role of Wnt signaling in the development of type 2 diabetes

Type 2 diabetes is characterized by insulin resistance, insulin deficiency, and hyperglycemia. Susceptibility to type 2 diabetes has been linked to Wnt signaling, which plays an important role in intestinal tumorigenesis. Carriers of variants of the transcription factor 7-like 2 gene, an important component of the Wnt pathway, are at enhanced risk for developing type 2 diabetes. The modulation of proglucagon expression by Wnt activity may partially explain the link between Wnt signaling and diabetes, and one of the transcriptional and processing products of the proglucagon gene, the glucagon-like peptide-1 (GLP-1), exhibits a wide variety of antidiabetogenic activities. GLP-1 stimulates Wnt signaling in pancreatic beta cells, enhancing cell proliferation; thus, positive feedback between GLP-1 and Wnt signaling may result in increased proliferation, and suppressed apoptosis, of pancreatic cells. Since beta-cell protection is a potential treatment for type 2 diabetes, stimulation of Wnt activity may represent a valid therapeutic approach.

Glucose induces an autocrine activation of the Wnt/beta-catenin pathway in macrophage cell lines

The canonical Wnt signalling pathway acts by slowing the rate of ubiquitin-mediated beta-catenin degradation. This results in the accumulation and subsequent nuclear translocation of beta-catenin, which induces the expression of a number of genes involved in growth, differentiation and metabolism. The mechanisms regulating the Wnt signalling pathway in the physiological context is still not fully understood. In the present study we provide evidence that changes in glucose levels within the physiological range can acutely regulate the levels of beta-catenin in two macrophage cell lines (J774.2 and RAW264.7 cells). In particular we find that glucose induces these effects by promoting an autocrine activation of Wnt signalling that is mediated by the hexosamine pathway and changes in N-linked glycosylation of proteins. These studies reveal that the Wnt/beta-catenin system is a glucose-responsive signalling system and as such is likely to play a role in pathways involved in sensing changes in metabolic status.

The role of incretins in glucose homeostasis and diabetes treatment

Incretins are gut hormones that are secreted from enteroendocrine cells into the blood within minutes after eating. One of their many physiological roles is to regulate the amount of insulin that is secreted after eating. In this manner, as well as others to be described in this review, their final common raison d'être is to aid in disposal of the products of digestion. There are two incretins, known as glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1), that share many common actions in the pancreas but have distinct actions outside of the pancreas. Both incretins are rapidly deactivated by an enzyme called dipeptidyl peptidase 4 (DPP4). A lack of secretion of incretins or an increase in their clearance are not pathogenic factors in diabetes. However, in type 2 diabetes (T2DM), GIP no longer modulates glucose-dependent insulin secretion, even at supraphysiological (pharmacological) plasma levels, and therefore GIP incompetence is detrimental to beta-cell function, especially after eating. GLP-1, on the other hand, is still insulinotropic in T2DM, and this has led to the development of compounds that activate the GLP-1 receptor with a view to improving insulin secretion. Since 2005, two new classes of drugs based on incretin action have been approved for lowering blood glucose levels in T2DM: an incretin mimetic (exenatide, which is a potent long-acting agonist of the GLP-1 receptor) and an incretin enhancer (sitagliptin, which is a DPP4 inhibitor). Exenatide is injected subcutaneously twice daily and its use leads to lower blood glucose and higher insulin levels, especially in the fed state. There is glucose-dependency to its insulin secretory capacity, making it unlikely to cause low blood sugars (hypoglycemia). DPP4 inhibitors are orally active and they increase endogenous blood levels of active incretins, thus leading to prolonged incretin action. The elevated levels of GLP-1 are thought to be the mechanism underlying their blood glucose-lowering effects.