Publications by authors named "Xiemin Cao"

22 Publications

  • Page 1 of 1

TCF7 is not essential for glucose homeostasis in mice.

Mol Metab 2021 Mar 16:101213. Epub 2021 Mar 16.

From the Department of Medicine, Lunenfeld-Tanenbaum Research Institute, Mt. Sinai Hospital, University of Toronto, Toronto, ON, Canada, M5G1X5. Electronic address:

Objective: Glucose-dependent insulinotropic polypeptide (GIP) and Glucagon-like peptide-1(GLP-1) are incretin hormones that exert overlapping yet distinct actions on islet β-cells. We recently observed that GIP, but not GLP-1, upregulated islet expression of Transcription Factor 7 (TCF7), a gene expressed in immune cells and associated with the risk of developing type 1 diabetes. TCF7 has also been associated with control of glucose homeostasis in the liver. Here we studied the relative metabolic importance of TCF7 expression in hepatocytes vs. islet β-cells in mice.

Methods: Tcf7 expression was selectively inactivated in adult mouse hepatocytes using adenoviral Cre expression and targeted in β-cells using two different lines of insulin promoter-Cre mice. Glucose homeostasis, plasma insulin and triglyceride responses, islet histology, hepatic and islet gene expression and body weight gain were evaluated in mice fed regular chow or high fat diets. Tcf7 expression within pancreatic islets and immune cells was evaluated using published single cell RNA-seq (scRNA-seq) data, and in islet RNA from immunodeficient Rag2Il2rg mice.

Results: Reduction of hepatocyte Tcf7 expression did not impair glucose homeostasis, lipid tolerance or hepatic gene expression profiles linked to control of metabolic or immune pathways. Similarly, oral and intraperitoneal glucose tolerance, plasma insulin responses, islet histology, body weight gain, and insulin tolerance were not different in mice with targeted recombination of Tcf7 in insulin-positive β-cells. Surprisingly, islet Tcf7 mRNA transcripts were not reduced in total islet RNA containing endocrine and associated non-endocrine cell types from Tcf7 mice, despite Cre-mediated recombination of islet genomic DNA. Furthermore, glucose tolerance was normal in whole body Tcf7 mice. Analysis of scRNA-seq datasets localized pancreatic Tcf7 expression to islet progenitors during development, and immune cells, but not within differentiated islet β-cells or endocrine lineages within mature islets. Moreover, the expression of Tcf7 was extremely low in islet RNA from Rag2Il2rg mice and consistent with expression within immune cells, Tcf7 was highly correlated with levels of Cd3g mRNA transcripts in RNA from wild type mouse islets.

Conclusion: These findings demonstrate that Tcf7 expression is not a critical determinant of glucose homeostasis in mice. Moreover, detection of Tcf7 expression within islet mRNA is attributable to expression of Tcf7 RNA in islet-associated immune cells, and not islet β-cells.
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http://dx.doi.org/10.1016/j.molmet.2021.101213DOI Listing
March 2021

Plasma levels of DPP4 activity and sDPP4 are dissociated from inflammation in mice and humans.

Nat Commun 2020 07 28;11(1):3766. Epub 2020 Jul 28.

Department of Medicine, Lunenfeld-Tanenbaum Research Institute, Mt. Sinai Hospital, Toronto, ON, Canada.

Dipeptidyl peptidase-4 (DPP4) modulates inflammation by enzymatic cleavage of immunoregulatory peptides and through its soluble form (sDPP4) that directly engages immune cells. Here we examine whether reduction of DPP4 activity alters inflammation. Prolonged DPP4 inhibition increases plasma levels of sDPP4, and induces sDPP4 expression in lymphocyte-enriched organs in mice. Bone marrow transplantation experiments identify hematopoietic cells as the predominant source of plasma sDPP4 following catalytic DPP4 inhibition. Surprisingly, systemic DPP4 inhibition increases plasma levels of inflammatory markers in regular chow-fed but not in high fat-fed mice. Plasma levels of sDPP4 and biomarkers of inflammation are lower in metformin-treated subjects with type 2 diabetes (T2D) and cardiovascular disease, yet exhibit considerable inter-individual variation. Sitagliptin therapy for 12 months reduces DPP4 activity yet does not increase markers of inflammation or levels of sDPP4. Collectively our findings dissociate levels of DPP4 enzyme activity, sDPP4 and biomarkers of inflammation in mice and humans.
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http://dx.doi.org/10.1038/s41467-020-17556-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7387453PMC
July 2020

Hematopoietic cell- versus enterocyte-derived dipeptidyl peptidase-4 differentially regulates triglyceride excursion in mice.

JCI Insight 2020 08 20;5(16). Epub 2020 Aug 20.

Lunenfeld-Tanenbaum Research Institute, Department of Medicine, Mt. Sinai Hospital, Toronto, Ontario, Canada.

Postprandial triglycerides (TGs) are elevated in people with type 2 diabetes (T2D). Glucose-lowering agents, such as glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors, also reduce postprandial TG excursion. Although the glucose-lowering mechanisms of DPP-4 have been extensively studied, how the reduction of DPP-4 activity improves lipid tolerance remains unclear. Here, we demonstrate that gut-selective and systemic inhibition of DPP-4 activity reduces postprandial TG excursion in young mice. Genetic inactivation of Dpp4 simultaneously within endothelial cells and hematopoietic cells using Tie2-Cre reduced intestinal lipoprotein secretion under regular chow diet conditions. Bone marrow transplantation revealed a key role for hematopoietic cells in modulation of lipid responses arising from genetic reduction of DPP-4 activity. Unexpectedly, deletion of Dpp4 in enterocytes increased TG excursion in high-fat diet-fed (HFD-fed) mice. Moreover, chemical reduction of DPP-4 activity and increased levels of GLP-1 were uncoupled from TG excursion in older or HFD-fed mice, yet lipid tolerance remained improved in older Dpp4-/- and Dpp4EC-/- mice. Taken together, this study defines roles for specific DPP-4 compartments, age, and diet as modifiers of DPP-4 activity linked to control of gut lipid metabolism.
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http://dx.doi.org/10.1172/jci.insight.140418DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7455127PMC
August 2020

Physiological roles of the GIP receptor in murine brown adipose tissue.

Mol Metab 2019 10 10;28:14-25. Epub 2019 Aug 10.

Lunenfeld-Tanenbaum Research Institute, Mt. Sinai Hospital, Toronto, Ontario, Canada; Department of Medicine, University of Toronto, Toronto, Ontario, Canada. Electronic address:

Objective: Glucose-dependent insulinotropic polypeptide (GIP) is secreted from the gut in response to nutrient ingestion and promotes meal-dependent insulin secretion and lipid metabolism. Loss or attenuation of GIP receptor (GIPR) action leads to resistance to diet-induced obesity through incompletely understood mechanisms. The GIPR is expressed in white adipose tissue; however, its putative role in brown adipose tissue (BAT) has not been explored.

Methods: We investigated the role of the GIPR in BAT cells in vitro and in BAT-specific (Gipr) knockout mice with selective elimination of the Gipr within the Myf5 expression domain. We analyzed body weight, adiposity, glucose homeostasis, insulin and lipid tolerance, energy expenditure, food intake, body temperature, and iBAT oxygen consumption ex vivo. High-fat diet (HFD)-fed Gipr mice were studied at room temperature (21 °C), 4 °C, and 30 °C ambient temperatures.

Results: The mouse Gipr gene is expressed in BAT, and GIP directly increased Il6 mRNA and IL-6 secretion in BAT cells. Additionally, levels of thermogenic, lipid and inflammation mRNA transcripts were altered in BAT cells transfected with Gipr siRNA. Body weight gain, energy expenditure, and glucose and insulin tolerance were normal in HFD-fed Gipr mice housed at room temperature. However, Gipr mice exhibited higher body temperatures during an acute cold challenge and a lower respiratory exchange ratio and impaired lipid tolerance at 21 °C. In contrast, body weight was lower and iBAT oxygen consumption was higher in HFD-fed mice housed at 4 °C but not at 30 °C.

Conclusions: The BAT GIPR is linked to the control of metabolic gene expression, fuel utilization, and oxygen consumption. However, the selective loss of the GIPR within BAT is insufficient to recapitulate the findings of decreased weight gain and resistance to obesity arising in experimental models with systemic disruption of GIP action.
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http://dx.doi.org/10.1016/j.molmet.2019.08.006DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6822254PMC
October 2019

Distinct Neural Sites of GLP-1R Expression Mediate Physiological versus Pharmacological Control of Incretin Action.

Cell Rep 2019 06;27(11):3371-3384.e3

Lunenfeld-Tanenbaum Research Institute, Mt. Sinai Hospital, Toronto, ON M5G 1X5, Canada; Department of Medicine, University of Toronto, Toronto, ON M5S 2J7, Canada. Electronic address:

Glucagon-like peptide 1 (GLP-1) receptors are widely distributed throughout the nervous system, enabling physiological and pharmacological control of glucose and energy homeostasis. Here we elucidated the importance of Glp1r expression within cellular domains targeted by expression of Wnt1-Cre2 or Phox2b-Cre. Widespread loss of neural Glp1r in Glp1r mice had no effect on basal food intake, gastric emptying, and glucose homeostasis. However, the glucoregulatory actions of GLP-1R agonists, but not gut-selective DPP-4 inhibition, were preserved in Glp1r mice. Unexpectedly, selective reduction of Glp1r expression within neurons targeted by Phox2b-Cre impaired glucose homeostasis and gastric emptying and attenuated the extent of weight loss achieved with sustained GLP-1R agonism. Collectively, these studies identify discrete neural domains of Glp1r expression mediating GLP-1-regulated control of metabolism and the gut-brain axis and reveal the unexpected importance of neuronal Phox2b cells expressing GLP-1R for physiological regulation of gastric emptying, islet hormone responses, and glucose homeostasis.
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http://dx.doi.org/10.1016/j.celrep.2019.05.055DOI Listing
June 2019

The brown adipose tissue glucagon receptor is functional but not essential for control of energy homeostasis in mice.

Mol Metab 2019 04 5;22:37-48. Epub 2019 Feb 5.

Lunenfeld-Tanenbaum Research Institute, Mt. Sinai Hospital, Toronto, Ontario, Canada; Department of Medicine, University of Toronto, Toronto, Ontario, Canada. Electronic address:

Objective: Administration of glucagon (GCG) or GCG-containing co-agonists reduces body weight and increases energy expenditure. These actions appear to be transduced by multiple direct and indirect GCG receptor (GCGR)-dependent mechanisms. Although the canonical GCGR is expressed in brown adipose tissue (BAT) the importance of BAT GCGR activity for the physiological control of body weight, or the response to GCG agonism, has not been defined.

Methods: We studied the mechanisms linking GCG action to acute increases in oxygen consumption using wildtype (WT), Ucp1 and Fgf21 mice. The importance of basal GCGR expression within the Myf5 domain for control of body weight, adiposity, glucose and lipid metabolism, food intake, and energy expenditure was examined in Gcgr mice housed at room temperature or 4 °C, fed a regular chow diet (RCD) or after a prolonged exposure to high fat diet (HFD).

Results: Acute GCG administration induced lipolysis and increased the expression of thermogenic genes in BAT cells, whereas knockdown of Gcgr reduced expression of genes related to thermogenesis. GCG increased energy expenditure (measured by oxygen consumption) both in vivo in WT mice and ex vivo in BAT and liver explants. GCG also increased acute energy expenditure in Ucp1 mice, but these actions were partially blunted in Ffg21 mice. However, acute GCG administration also robustly increased oxygen consumption in Gcgr mice. Moreover, body weight, glycemia, lipid metabolism, body temperature, food intake, activity, energy expenditure and adipose tissue gene expression profiles were normal in Gcgr mice, either on RCD or HFD, whether studied at room temperature, or chronically housed at 4 °C.

Conclusions: Exogenous GCG increases oxygen consumption in mice, also evident both in liver and BAT explants ex vivo, through UCP1-independent, FGF21-dependent pathways. Nevertheless, GCGR signaling within BAT is not physiologically essential for control of body weight, whole body energy expenditure, glucose homeostasis, or the adaptive metabolic response to cold or prolonged exposure to an energy dense diet.
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http://dx.doi.org/10.1016/j.molmet.2019.01.011DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6437632PMC
April 2019

Circulating Levels of Soluble Dipeptidyl Peptidase-4 Are Dissociated from Inflammation and Induced by Enzymatic DPP4 Inhibition.

Cell Metab 2019 02 1;29(2):320-334.e5. Epub 2018 Nov 1.

Lunenfeld-Tanenbaum Research Institute, Mt. Sinai Hospital, LTRI, 600 University Avenue TCP5-1004, Toronto, ON M5G 1X5, Canada; Department of Medicine, University of Toronto, Toronto, ON M5S 2J7, Canada. Electronic address:

Dipeptidyl peptidase-4 (DPP-4) controls glucose homeostasis through enzymatic termination of incretin action. We report that plasma DPP-4 activity correlates with body weight and fat mass, but not glucose control, in mice. Genetic disruption of adipocyte Dpp4 expression reduced plasma DPP-4 activity in older mice but did not perturb incretin levels or glucose homeostasis. Knockdown of hepatocyte Dpp4 completely abrogated the obesity-associated increase in plasma DPP-4 activity, reduced liver cytokine expression, and partially attenuated inflammation in adipose tissue without changes in incretin levels or glucose homeostasis. In contrast, circulating levels of soluble DPP4 (sDPP4) were dissociated from inflammation in mice with endothelial-selective or global genetic inactivation of Dpp4. Remarkably, inhibition of DPP-4 enzymatic activity upregulated circulating levels of sDPP4 originating from endothelial or hematopoietic cells without inducing systemic or localized inflammation. Collectively, these findings reveal unexpected complexity in regulation of soluble versus enzymatic DPP-4 and control of inflammation and glucose homeostasis.
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http://dx.doi.org/10.1016/j.cmet.2018.10.001DOI Listing
February 2019

GLP-1 Receptor Expression Within the Human Heart.

Endocrinology 2018 04;159(4):1570-1584

Department of Medicine, Lunenfeld-Tanenbaum Research Institute, Mt. Sinai Hospital, Toronto, Ontario, Canada.

Glucagonlike peptide-1 receptor (GLP-1R) agonists, which are used to treat type 2 diabetes and obesity, reduce the rates of myocardial infarction and cardiovascular death. GLP-1R has been localized to the human sinoatrial node; however, its expression in ventricular tissue remains uncertain. Here we studied GLP-1R expression in the human heart using GLP-1R-directed antisera, quantitative polymerase chain reaction (PCR), reverse transcription PCR to detect full-length messenger RNA (mRNA) transcripts, and in situ hybridization (ISH). GLP1R mRNA transcripts, encompassing the entire open reading frame, were detected in all four cardiac chambers from 15 hearts at levels approximating those detected in human pancreas. In contrast, cardiac GLP2R expression was relatively lower, and cardiac GCGR expression was sporadic and not detected in the left ventricle. GLP1R mRNA transcripts were not detected in RNA from human cardiac fibroblasts, coronary artery endothelial, or vascular smooth muscle cells. Human Brunner glands and pancreatic islets exhibited GLP-1R immunopositivity and abundant expression of GLP1R mRNA transcripts by ISH. GLP1R transcripts were also detected by ISH in human cardiac sinoatrial node tissue. However, definitive cellular localization of GLP1R mRNA transcripts or immunoreactive GLP-1R protein within human cardiomyocytes or cardiac blood vessels remained elusive. Moreover, validated GLP-1R antisera lacked sufficient sensitivity to detect expression of the endogenous islet or cardiac GLP-1R by Western blotting. Hence, although human cardiac ventricles express the GLP1R, the identity of one or more ventricular cell type(s) that express a translated GLP1R protein requires further clarification with highly sensitive methods of detection.
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http://dx.doi.org/10.1210/en.2018-00004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5939638PMC
April 2018

Inactivation of the Glucose-Dependent Insulinotropic Polypeptide Receptor Improves Outcomes following Experimental Myocardial Infarction.

Cell Metab 2018 02 21;27(2):450-460.e6. Epub 2017 Dec 21.

Lunenfeld Tanenbaum Research Institute, Mount Sinai Hospital, University of Toronto, Toronto, ON M5G 1X5, Canada. Electronic address:

Incretin hormones exert pleiotropic metabolic actions beyond the pancreas. Although the heart expresses both incretin receptors, the cardiac biology of GIP receptor (GIPR) action remains incompletely understood. Here we show that GIPR agonism did not impair the response to cardiac ischemia. In contrast, genetic elimination of the Gipr reduced myocardial infarction (MI)-induced ventricular injury and enhanced survival associated with reduced hormone sensitive lipase (HSL) phosphorylation; it also increased myocardial triacylglycerol (TAG) stores. Conversely, direct GIPR agonism in the isolated heart reduced myocardial TAG stores and increased fatty acid oxidation. The cardioprotective phenotype in Gipr mice was partially reversed by pharmacological activation or genetic overexpression of HSL. Selective Gipr inactivation in cardiomyocytes phenocopied Gipr mice, resulting in improved survival and reduced adverse remodeling following experimental MI. Hence, the cardiomyocyte GIPR regulates fatty acid metabolism and the adaptive response to ischemic cardiac injury. These findings have translational relevance for developing GIPR-based therapeutics.
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http://dx.doi.org/10.1016/j.cmet.2017.11.003DOI Listing
February 2018

The autonomic nervous system and cardiac GLP-1 receptors control heart rate in mice.

Mol Metab 2017 11 1;6(11):1339-1349. Epub 2017 Sep 1.

Department of Medicine, Lunenfeld-Tanenbaum Research Institute, Mt. Sinai Hospital, University of Toronto, Canada. Electronic address:

Objectives: Glucagon-like peptide-1 (GLP-1) is secreted from enteroendocrine cells and exerts a broad number of metabolic actions through activation of a single GLP-1 receptor (GLP-1R). The cardiovascular actions of GLP-1 have garnered increasing attention as GLP-1R agonists are used to treat human subjects with diabetes and obesity that may be at increased risk for development of heart disease. Here we studied mechanisms linking GLP-1R activation to control of heart rate (HR) in mice.

Methods: The actions of GLP-1R agonists were examined on the control of HR in wild type mice (WT) and in mice with cardiomyocyte-selective disruption of the GLP-1R (Glp1r). Complimentary studies examined the effects of GLP-1R agonists in mice co-administered propranolol or atropine. The direct effects of GLP-1R agonism on HR and ventricular developed pressure were examined in isolated perfused mouse hearts ex vivo, and atrial depolarization was quantified in mouse hearts following direct application of liraglutide to perfused atrial preparations ex vivo.

Results: Doses of liraglutide and lixisenatide that were equipotent for acute glucose control rapidly increased HR in WT and Glp1r mice in vivo. The actions of liraglutide to increase HR were more sustained relative to lixisenatide, and diminished in Glp1r mice. The acute chronotropic actions of GLP-1R agonists were attenuated by propranolol but not atropine. Neither native GLP-1 nor lixisenatide increased HR or developed pressure in perfused hearts ex vivo. Moreover, liraglutide had no direct effect on sinoatrial node firing rate in mouse atrial preparations ex vivo. Despite co-localization of HCN4 and GLP-1R in primate hearts, HCN4-directed Cre expression did not attenuate levels of Glp1r mRNA transcripts, but did reduce atrial Gcgr expression in the mouse heart.

Conclusions: GLP-1R agonists increase HR through multiple mechanisms, including regulation of autonomic nervous system function, and activation of the atrial GLP-1R. Surprisingly, the isolated atrial GLP-1R does not transduce a direct chronotropic effect following exposure to GLP-1R agonists in the intact heart, or isolated atrium, ex vivo. Hence, cardiac GLP-1R circuits controlling HR require neural inputs and do not function in a heart-autonomous manner.
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http://dx.doi.org/10.1016/j.molmet.2017.08.010DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5681270PMC
November 2017

β-Cell Inactivation of Unmasks Incretin Dependence of GPR119-Mediated Glucoregulation.

Diabetes 2017 06 2;66(6):1626-1635. Epub 2017 Mar 2.

Lunenfeld-Tanenbaum Research Institute, Department of Medicine, Mount Sinai Hospital, University of Toronto, Toronto, Ontario, Canada

GPR119 was originally identified as an orphan β-cell receptor; however, subsequent studies demonstrated that GPR119 also regulates β-cell function indirectly through incretin hormone secretion. We assessed the importance of GPR119 for β-cell function in mice and in newly generated mice. mice displayed normal body weight and glucose tolerance on a regular chow (RC) diet. After high-fat feeding, mice exhibited reduced fat mass, decreased levels of circulating adipokines, improved insulin sensitivity, and better glucose tolerance. Unexpectedly, oral and intraperitoneal glucose tolerance and the insulin response to glycemic challenge were not perturbed in mice on RC and high-fat diets. Moreover, islets from and mice exhibited normal insulin responses to glucose and β-cell secretagogues. Furthermore, the selective GPR119 agonist AR231453 failed to directly enhance insulin secretion from perifused islets. In contrast, AR231453 increased plasma glucagon-like peptide 1 (GLP-1) and insulin levels and improved glucose tolerance in wild-type and mice. These findings demonstrate that β-cell GPR119 expression is dispensable for the physiological control of insulin secretion and the pharmacological response to GPR119 agonism, findings that may inform the lack of robust efficacy in clinical programs assessing GPR119 agonists for the therapy of type 2 diabetes.
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http://dx.doi.org/10.2337/db17-0017DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5860191PMC
June 2017

GLP-1R agonists promote normal and neoplastic intestinal growth through mechanisms requiring Fgf7.

Cell Metab 2015 Mar;21(3):379-91

Department of Medicine, Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital, Toronto, ON M5G1X5, Canada. Electronic address:

Glucagon-like peptide-1 (GLP-1) secreted from enteroendocrine L cells promotes nutrient disposal via the incretin effect. However, the majority of L cells are localized to the distal gut, suggesting additional biological roles for GLP-1. Here, we demonstrate that GLP-1 receptor (GLP-1R) signaling controls mucosal expansion of the small bowel (SB) and colon. These actions did not require the epidermal growth factor (EGF) or intestinal epithelial insulin-like growth factor (IGF1) receptors but were absent in Glp1r(-/-) mice. Polyp number and size were increased in SB of exendin-4-treated Apc(Min/+) mice, whereas polyp number was reduced in SB and colon of Glp1r(-/-):Apc(Min/+) mice. Exendin-4 increased fibroblast growth factor 7 (Fgf7) expression in colonic polyps of Apc(Min/+) mice and failed to increase intestinal growth in mice lacking Fgf7. Exogenous exendin-4 and Fgf7 regulated an overlapping set of genes important for intestinal growth. Thus, gain and loss of GLP-1R signaling regulates gut growth and intestinal tumorigenesis.
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http://dx.doi.org/10.1016/j.cmet.2015.02.005DOI Listing
March 2015

GLP-1R Agonists Modulate Enteric Immune Responses Through the Intestinal Intraepithelial Lymphocyte GLP-1R.

Diabetes 2015 Jul 3;64(7):2537-49. Epub 2015 Mar 3.

Department of Medicine, Mount Sinai Hospital, Lunenfeld-Tanenbaum Research Institute, Toronto, ON, Canada

Obesity and diabetes are characterized by increased inflammation reflecting disordered control of innate immunity. We reveal a local intestinal intraepithelial lymphocyte (IEL)-GLP-1 receptor (GLP-1R) signaling network that controls mucosal immune responses. Glp1r expression was enriched in intestinal IEL preparations and copurified with markers of Tαβ and Tγδ IELs, the two main subsets of intestinal IELs. Exendin-4 increased cAMP accumulation in purified IELs and reduced the production of cytokines from activated IELs but not from splenocytes ex vivo. These actions were mimicked by forskolin, absent in IELs from Glp1r(-/-) mice, and attenuated by the GLP-1R agonist exendin (9-39) consistent with a GLP-1R-dependent mechanism of action. Furthermore, Glp1r(-/-) mice exhibited dysregulated intestinal gene expression, an abnormal representation of microbial species in feces, and enhanced sensitivity to intestinal injury following administration of dextran sodium sulfate. Bone marrow transplantation using wild-type C57BL/6 donors normalized expression of multiple genes regulating immune function and epithelial integrity in Glp1r(-/-) recipient mice, whereas acute exendin-4 administration robustly induced the expression of genes encoding cytokines and chemokines in normal and injured intestine. Taken together, these findings define a local enteroendocrine-IEL axis linking energy availability, host microbial responses, and mucosal integrity to the control of innate immunity.
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http://dx.doi.org/10.2337/db14-1577DOI Listing
July 2015

Glucagon-like peptide-1 receptor agonists increase pancreatic mass by induction of protein synthesis.

Diabetes 2015 Mar 2;64(3):1046-56. Epub 2014 Oct 2.

Department of Medicine, Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital, Toronto, Ontario, Canada

Glucagon-like peptide-1 (GLP-1) controls glucose homeostasis by regulating secretion of insulin and glucagon through a single GLP-1 receptor (GLP-1R). GLP-1R agonists also increase pancreatic weight in some preclinical studies through poorly understood mechanisms. Here we demonstrate that the increase in pancreatic weight following activation of GLP-1R signaling in mice reflects an increase in acinar cell mass, without changes in ductal compartments or β-cell mass. GLP-1R agonists did not increase pancreatic DNA content or the number of Ki67(+) cells in the exocrine compartment; however, pancreatic protein content was increased in mice treated with exendin-4 or liraglutide. The increased pancreatic mass and protein content was independent of cholecystokinin receptors, associated with a rapid increase in S6 phosphorylation, and mediated through the GLP-1R. Rapamycin abrogated the GLP-1R-dependent increase in pancreatic mass but had no effect on the robust induction of Reg3α and Reg3β gene expression. Mass spectrometry analysis identified GLP-1R-dependent upregulation of Reg family members, as well as proteins important for translation and export, including Fam129a, eIF4a1, Wars, and Dmbt1. Hence, pharmacological GLP-1R activation induces protein synthesis, leading to increased pancreatic mass, independent of changes in DNA content or cell proliferation in mice.
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http://dx.doi.org/10.2337/db14-0883DOI Listing
March 2015

Inactivation of the cardiomyocyte glucagon-like peptide-1 receptor (GLP-1R) unmasks cardiomyocyte-independent GLP-1R-mediated cardioprotection.

Mol Metab 2014 Aug 9;3(5):507-17. Epub 2014 May 9.

Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital, Toronto, Canada.

GLP-1R agonists improve outcomes in ischemic heart disease. Here we studied GLP-1R-dependent adaptive and cardioprotective responses to ventricular injury. Glp1r (-/-) hearts exhibited chamber-specific differences in gene expression, but normal mortality and left ventricular (LV) remodeling after myocardial infarction (MI) or experimental doxorubicin-induced cardiomyopathy. Selective disruption of the cardiomyocyte GLP-1R in Glp1r (CM-/-) mice produced no differences in survival or LV remodeling following LAD coronary artery occlusion. Unexpectedly, the GLP-1R agonist liraglutide still produced robust cardioprotection and increased survival in Glp1r (CM-/-) mice following LAD coronary artery occlusion. Although liraglutide increased heart rate (HR) in Glp1r (CM-/-) mice, basal HR was significantly lower in Glp1r (CM-/-) mice. Hence, endogenous cardiomyocyte GLP-1R activity is not required for adaptive responses to ischemic or cardiomyopathic injury, and is dispensable for GLP-1R agonist-induced cardioprotection or enhanced chronotropic activity. However the cardiomyocyte GLP-1R is essential for the control of HR in mice.
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http://dx.doi.org/10.1016/j.molmet.2014.04.009DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4099509PMC
August 2014

GLP-1 receptor activation indirectly reduces hepatic lipid accumulation but does not attenuate development of atherosclerosis in diabetic male ApoE(-/-) mice.

Endocrinology 2013 Jan 26;154(1):127-39. Epub 2012 Nov 26.

Departments of Medicine, Samuel Lunenfeld Research Institute, Mt. Sinai Hospital, Toronto Ontario Canada M5G 1X5.

Glucagon-like peptide-1 receptor (GLP-1R) agonists reduce lipid accumulation in peripheral tissues, attenuating atherosclerosis and hepatic steatosis in preclinical studies. We examined whether GLP-1R activation decreases atherosclerosis progression in high-fat diet-fed male ApoE(-/-) mice after administration of streptozotocin and treatment with the long-acting GLP-1R agonist taspoglutide administered once monthly vs. metformin in the drinking water for 12 wk. Taspoglutide did not reduce plaque area or lipid content in the aortic arch or abdominal aorta, and no significant change in aortic macrophage accumulation was detected after taspoglutide or metformin. In contrast, hepatic triglyceride levels were significantly reduced in livers from taspoglutide-treated mice. Both peripheral and intracerebroventricular administration of exendin-4 rapidly decreased plasma triglyceride levels in fasted mice, and taspoglutide therapy in ApoE(-/-) mice modulated the expression of hepatic genes controlling fatty acid uptake and oxidation. We were unable to detect expression of the entire Glp1r coding sequence in macrophages isolated from ApoE(-/-), C57BL/6, and IL10(-/-) mice. Similarly, Glp1r mRNA transcripts were not detected in RNA from isolated murine hepatocytes. Using Western blotting and tissue extracts from Glp1r(+/+) and Glp1r(-/-) mice, and cells transfected with a tagged murine GLP-1R cDNA, we could not validate the sensitivity and specificity of three different GLP-1R antisera commonly used for the detection of GLP-1R protein. Taken together, these findings illustrate divergent actions of GLP-1R agonists on atherosclerosis progression and accumulation of ectopic lipid in ApoE(-/-) mice and highlight the importance of indirect GLP-1R actions for the control of hepatic lipid accumulation.
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http://dx.doi.org/10.1210/en.2012-1937DOI Listing
January 2013

Activation of enteroendocrine membrane progesterone receptors promotes incretin secretion and improves glucose tolerance in mice.

Diabetes 2013 Jan 29;62(1):283-90. Epub 2012 Aug 29.

Department of Medicine, Institute of Medical Sciences, Samuel Lunenfeld Research Institute, University of Toronto, Toronto, Ontario, Canada.

Glucagon-like peptide-1 (GLP-1) secretion is classically regulated by ingested nutrients. To identify novel molecular targets controlling incretin secretion, we analyzed enteroendocrine cell pathways important for hormone biosynthesis and secretion. We demonstrate that progesterone increases GLP-1 secretion and extracellular signal-related kinase 1/2 (ERK1/2) phosphorylation in enteroendocrine GLUTag cells via mechanisms sensitive to the mitogen-activated protein kinase inhibitor U0126. The stimulatory effects of progesterone (P4) or the synthetic progestin R5020 on ERK1/2 phosphorylation were independent of the classical progesterone receptor antagonist RU486. Furthermore, a cell-impermeable BSA-progesterone conjugate rapidly increased ERK1/2 phosphorylation and GLP-1 secretion. Knockdown of the membrane progesterone receptors Paqr5 or Paqr7 in GLUTag cells eliminated the stimulatory effect of R5020 and progesterone on GLP-1 secretion. Enteral progesterone administration increased plasma levels of GLP-1, glucose-dependent insulinotropic polypeptide (GIP), and insulin, and improved oral glucose tolerance in an RU486-insensitve manner in mice: however, systemic progesterone exposure did not improve glucose homeostasis. Unexpectedly, the glucoregulatory actions of enteral progesterone did not require classical incretin receptor signaling and were preserved in Glp1r(-/-) and Glp1r(-/-):Gipr(-/-) mice. Intestine-restricted activation of membrane progesterone receptors may represent a novel approach for stimulation of incretin hormone secretion and control of glucose homeostasis.
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http://dx.doi.org/10.2337/db12-0601DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3526055PMC
January 2013

An albumin-exendin-4 conjugate engages central and peripheral circuits regulating murine energy and glucose homeostasis.

Gastroenterology 2008 Apr 11;134(4):1137-47. Epub 2008 Jan 11.

Department of Medicine, Mt. Sinai Hospital, Toronto, Ontario, Canada.

Background & Aims: Glucagon-like peptide-1 (GLP-1) regulates glucose homeostasis through multiple mechanisms including direct actions on the endocrine pancreas and indirect activation of central nervous system circuits regulating gastric emptying, satiety, and body weight. Because native GLP-1 is rapidly degraded, there is considerable interest in development of more potent GLP-1 receptor (GLP-1R) agonists with sustained activity; however, the extent to which much larger GLP-1R agonists will mimic some or all of the actions of smaller peptides remains uncertain.

Methods: We studied the actions of CJC-1134-PC, a recombinant human serum albumin-exendin-4 conjugated protein, at the GLP-1R using heterologous cells expressing the GLP-1R in vitro and both wild-type and Glp1r(-/-) mice in vivo.

Results: CJC-1134-PC activated GLP-1R-dependent signaling in baby hamster kidney-GLP-1R cells and acutely lowered blood glucose in wild-type but not in Glp1r(-/-) mice. Moreover, acute administration of CJC-1134-PC rapidly activated c-Fos expression in multiple regions of the central nervous system, acutely inhibited gastric emptying, and produced sustained inhibition of food intake in a GLP-1R-dependent manner. Furthermore, chronic daily treatment of high-fat diet-fed wild-type mice with CJC-1134-PC for 4 weeks led to improved glucose tolerance, increased levels of glucose-stimulated insulin, decreased HbA1c, and weight loss associated with decreased hepatic triglyceride content.

Conclusions: These findings illustrate that a high-molecular-weight exendin-4-albumin conjugate retains the ability to mimic a full spectrum of GLP-1R-dependent actions, including activation of central nervous system circuits regulating gastric emptying, food intake, and body weight.
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http://dx.doi.org/10.1053/j.gastro.2008.01.017DOI Listing
April 2008

beta-Cell Pdx1 expression is essential for the glucoregulatory, proliferative, and cytoprotective actions of glucagon-like peptide-1.

Diabetes 2005 Feb;54(2):482-91

Department of Medicine, University of Toronto, Banting and Best Diabetes Centre, Toronto General Hospital, 200 Elizabeth St., MBRW 4R402-2, Toronto, Canada M5G 2C4.

Glucagon-like peptide-1 (GLP-1) regulates energy intake, gastrointestinal motility, and nutrient disposal. The relative importance of the islet beta-cell for GLP-1 actions remains unclear. We determined the role of the islet beta-cell and the pancreatic duodenal homeobox-1 (Pdx1) transcription factor for GLP-1 receptor (GLP-1R)-dependent actions through analysis of mice with beta-cell-specific inactivation of the Pdx1 gene (beta-cell(Pdx1-/-) mice). The GLP-1R agonist exendin-4 (Ex-4) reduced glycemic excursion following intraperitoneal (i.p.) glucose challenge in control littermates (beta-cell(Pdx1+/+) mice) but not in beta-cell(Pdx1-/-) mice. Similarly, Ex-4 failed to increase levels of plasma insulin, pancreatic insulin content, and pancreatic insulin mRNA transcripts in beta-cell(Pdx1-/-) mice. Furthermore, Ex-4 significantly increased beta-cell proliferation and reduced beta-cell apoptosis in beta-cell(Pdx1+/+) mice but not in beta-cell(Pdx1-/-) mice. Moreover, Ex-4 increased the levels of insulin and amylin mRNA transcripts and augmented glucose-stimulated insulin secretion in islets from beta-cell(Pdx1+/+) mice but not in beta-cell(Pdx1-/-) islets. Surprisingly, Ex-4 failed to reduce levels of plasma glucagon in beta-cell(Pdx1-/-) mice. These findings demonstrate that Pdx1 expression is essential for integrating GLP-1R-dependent signals regulating alpha-cell glucagon secretion and for the growth, differentiated function, and survival of islet beta-cells.
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http://dx.doi.org/10.2337/diabetes.54.2.482DOI Listing
February 2005

Pdx-1 is not sufficient for repression of proglucagon gene transcription in islet or enteroendocrine cells.

Endocrinology 2005 Jan 7;146(1):441-9. Epub 2004 Oct 7.

Department of Medicine, Toronto General Hospital, Banting and Best Diabetes Centre, University of Toronto, Toronto, Ontario, Canada.

Pdx-1 plays a key role in the development of the pancreas and the control of islet gene transcription and has also been proposed as a dominant regulator of the alpha- vs. beta-cell phenotype via extinction of proglucagon expression. To ascertain the relationship between Pdx-1 and proglucagon gene expression, we examined the effect of enhanced pdx-1 expression on proglucagon gene expression in murine islet alphaTC-1 and GLUTag enteroendocrine cells. Although adenoviral transduction increased the levels of pdx-1 mRNA transcripts and nuclear Pdx-1 protein, overexpression of pdx-1 did not repress endogenous proglucagon gene expression in alphaTC-1 or GLUTag cells or murine islets. Immunohistochemical analysis of cells transduced with Ad-pdx-1 demonstrated multiple individual islet or enteroendocrine cells exhibiting both nuclear Pdx-1 and cytoplasmic glucagon-like peptide-1 immunopositivity. The failure of pdx-1 to inhibit endogenous proglucagon gene expression was not attributable to defects in Pdx-1 nuclear translocation or DNA binding as demonstrated using Western blotting and EMSA analyses. Furthermore, Ad-pdx-1 transduction did not repress proglucagon promoter activity in alphaTC-1 or GLUTag cells. Taken together, these findings demonstrate that pdx-1 alone is not sufficient for specification of the hormonal phenotype or extinction of proglucagon gene expression in islet or enteroendocrine cells.
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http://dx.doi.org/10.1210/en.2004-0495DOI Listing
January 2005

Aberrant regulation of human intestinal proglucagon gene expression in the NCI-H716 cell line.

Endocrinology 2003 May;144(5):2025-33

Banting and Best Diabetes Centre, Toronto General Hospital, University of Toronto, Toronto, Canada M5G 2C4.

Despite interest in understanding glucagon-like peptide-1 (GLP-1) production, the factors important for GLP-1 biosynthesis remain poorly understood. We examined control of human proglucagon gene expression in NCI-H716 cells, a cell line that secretes GLP-1 in a regulated manner. Insulin, phorbol myristate acetate, or forskolin, known regulators of rodent proglucagon gene expression, had no effect, whereas sodium butyrate decreased levels of NCI-H716 proglucagon mRNA transcripts. The inhibitory effect of sodium butyrate was mimicked by trichostatin A but was not detected with sodium acetate or isobutyrate. The actions of butyrate were not diminished by the ERK1/2 inhibitor PD98059, p38 inhibitor SB203580, or soluble guanylate cyclase inhibitor LY83583 or following treatment of cells with KT5823, a selective inhibitor of cGMP-dependent protein kinase. NCI-H716 cells expressed multiple proglucagon gene transcription factors including isl-1, pax-6, pax-2, cdx-2/3, pax-4, hepatocyte nuclear factor (HNF)-3 alpha, HNF-3beta, HNF-3 gamma, and Nkx2.2. Nevertheless, the butyrate-dependent inhibition of proglucagon gene expression was not associated with coordinate changes in transcription factor expression and both the human and rat transfected proglucagon promoters were transcriptionally inactive in NCI-H716 cells. Hence, NCI-H716 cells may not be a physiologically optimal model for studies of human enteroendocrine proglucagon gene transcription.
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http://dx.doi.org/10.1210/en.2002-0049DOI Listing
May 2003

Cellular context of coregulator and adaptor proteins regulates human adenovirus 5 early region 1A-dependent gene activation by the thyroid hormone receptor.

Mol Endocrinol 2003 Jun 13;17(6):1095-105. Epub 2003 Mar 13.

Samuel Lunenfeld Research Institute of Mount Sinai Hospital, Endocrine Division, University of Toronto Medical School, Toronto, Ontario, Canada M5G 1X5.

In mammalian cells, the human adenovirus type 5 early region 1A (E1A) oncoprotein functions as a thyroid hormone (TH)-dependent activator of the thyroid hormone receptor (TR). Interestingly, in the cellular context of the yeast Saccharomyces cerevisiae, E1A acts as a TR-specific constitutive coactivator that is down-regulated by TH. TH reduces the interaction of E1A with the TR in yeast but not HeLa cells. The N-terminal 82 amino acids of E1A are sufficient for coactivation in yeast and residues 4-29 are essential. In yeast, expression of the nuclear receptor corepressor (N-CoR) could down-regulate constitutive transcriptional activation of the TR by E1A, whereas expression of the glucocorticoid receptor interacting protein 1 (GRIP-1) coactivator reconstituted the E1A-induced pattern of enhanced TH-dependent gene activation by TR observed in mammalian cells. We further show that the mating type switching gene (SWI)/sucrose nonfermenting (SNF) gene chromatin remodeling complex is required for both TH/GRIP-1- and E1A-dependent coactivator function, whereas the general control nonrepressed protein (GCN5)/alteration/deficiency in activation protein (ADA2) components of the SPT, ADA, GCN5, acetylation (SAGA) transcriptional adaptor complex are required for TH/GRIP-1, but not E1A-dependent activation of the TR. Taken together, these studies demonstrate that the novel TR-specific coactivator function of E1A in yeast depends on the SWI/SNF chromatin remodeling complex and can be further influenced by changes in the cellular complement of transcriptional coregulatory proteins.
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http://dx.doi.org/10.1210/me.2002-0294DOI Listing
June 2003