Intestinal-derived FGF15 protects against deleterious effects of vertical sleeve gastrectomy in mice

Bariatric surgeries such as the Vertical Sleeve Gastrectomy (VSG) are invasive but provide the most effective improvements in obesity and Type 2 diabetes. We hypothesized a potential role for the gut hormone Fibroblast-Growth Factor 15/19 which is increased after VSG and pharmacologically can improve energy homeostasis and glucose handling. We generated intestinal-specific FGF15 knockout (FGF15INT-KO) mice which were maintained on high-fat diet. FGF15INT-KO mice lost more weight after VSG as a result of increased lean tissue loss. FGF15INT-KO mice also lost more bone density and bone marrow adipose tissue after VSG. The effect of VSG to improve glucose tolerance was also absent in FGF15INT-KO. VSG resulted in increased plasma bile acid levels but were considerably higher in VSG-FGF15INT-KO mice. These data point to an important role after VSG for intestinal FGF15 to protect the organism from deleterious effects of VSG potentially by limiting the increase in circulating bile acids.http://deepblue.lib.umich.edu/bitstream/2027.42/169579/2/s41467-021-24914-y.pdfAccepted versio

Loss of mTORC1 signalling impairs β-cell homeostasis and insulin processing

Deregulation of mTOR complex 1 (mTORC1) signalling increases the risk for metabolic diseases, including type 2 diabetes. Here we show that β-cell-specific loss of mTORC1 causes diabetes and β-cell failure due to defects in proliferation, autophagy, apoptosis and insulin secretion by using mice with conditional (βraKO) and inducible (MIP-βraKO(f/f)) raptor deletion. Through genetic reconstitution of mTORC1 downstream targets, we identify mTORC1/S6K pathway as the mechanism by which mTORC1 regulates β-cell apoptosis, size and autophagy, whereas mTORC1/4E-BP2-eIF4E pathway regulates β-cell proliferation. Restoration of both pathways partially recovers β-cell mass and hyperglycaemia. This study also demonstrates a central role of mTORC1 in controlling insulin processing by regulating cap-dependent translation of carboxypeptidase E in a 4EBP2/eIF4E-dependent manner. Rapamycin treatment decreases CPE expression and insulin secretion in mice and human islets. We suggest an important role of mTORC1 in β-cells and identify downstream pathways driving β-cell mass, function and insulin processing

Loss of mTORC1 signaling alters pancreatic α cell mass and impairs glucagon secretion

Glucagon plays a major role in the regulation of glucose homeostasis during fed and fasting states. However, the mechanisms responsible for the regulation of pancreatic α cell mass and function are not completely understood. In the current study, we identified mTOR complex 1 (mTORC1) as a major regulator of α cell mass and glucagon secretion. Using mice with tissue-specific deletion of the mTORC1 regulator Raptor in α cells (αRaptorKO), we showed that mTORC1 signaling is dispensable for α cell development, but essential for α cell maturation during the transition from a milk-based diet to a chow-based diet after weaning. Moreover, inhibition of mTORC1 signaling in αRaptorKO mice and in WT animals exposed to chronic rapamycin administration decreased glucagon content and glucagon secretion. In αRaptorKO mice, impaired glucagon secretion occurred in response to different secretagogues and was mediated by alterations in KATP channel subunit expression and activity. Additionally, our data identify the mTORC1/FoxA2 axis as a link between mTORC1 and transcriptional regulation of key genes responsible for α cell function. Thus, our results reveal a potential function of mTORC1 in nutrient-dependent regulation of glucagon secretion and identify a role for mTORC1 in controlling α cell-mass maintenance

The Role of Nutrient Signaling in the Regulation of Pancreatic -cell Mass and Glucagon Secretion.

Increased α-cell mass and aberrant glucagon response play major roles in the pathogenesis and complications associated with diabetes. In these studies, we determined that mTOR Complex 1 (mTORC1) is a major regulator of α-cell mass and glucagon secretion. Using mice deficient of raptor exclusively in α-cells we revealed that mTORC1 signaling promotes glucagon secretion during fasting by positively regulating KATP channels and glucagon expression. A novel mTORC1/FoxA2 axis provided a link between mTORC1 and transcriptional regulation of key genes responsible for α-cell function. Furthermore, we also found that mTORC1 signaling positively modulates the maintenance of islet α-cells by an autophagy-mediated process. In proceeding studies, we demonstrated that activation of mTORC1 signaling in α-cells is sufficient to induce α-cell mass expansion and hyperglucagonemia as observed in diabetic patients. We increased mTORC1 activity in α-cells by deletion of TSC2, a negative mTORC1 regulator. Our results revealed that gain of mTORC1 signaling in α-cells leads to increased fed and fasting glucagon levels and increased α-cell mass. Despite hyperglucagonemia, these mice were normoglycemic and had improved glucose tolerance and decreased hepatic glucose production. The results of these studies support a role for increased mTORC1 signaling in promoting the increase in α-cell mass and glucagon secretion observed in diabetic patients. Overall, our results revealed a novel function of mTORC1 in nutrient-dependent regulation of glucagon secretion and support a role for mTORC1 in controlling α-cell mass and function in patients treated with rapamycin analogs.PhDCellular and Molecular BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/133487/1/nibozad_1.pd

1812-P: Glucagon Resistance and Decreased Susceptibility to Diabetes in a Model of Chronic Hyperglucagonemia

Elevation of glucagon levels and increase in α-cell mass are associated to states of hyperglycemia in diabetes. However, little is known about the mechanisms that control glucagon secretion and α-cell mass expansion in normal or diabetogenic conditions. The current studies investigated the effects of activation of nutrient signaling by conditional deletion of the mTORC1 inhibitor, TSC2, in α-cells (αTSC2KO), we showed that activation of mTORC1 signaling is sufficient to induce chronic hyperglucagonemia as a result of α-cell proliferation, cell size and mass expansion. Hyperglucagonemia in αTSC2KO was associated to increase in glucagon content, enhanced glucagon secretion and defective adaptation to fasting. This model allowed us to identify the beneficial effects of chronic hyperglucagonemia in glucose homeostasis by inducing insulin secretion and resistance to glucagon in the liver. Liver glucagon resistance in αTSC2KO mice was characterized by reduced expression of the glucagon receptor (GCGR), phosphoenolpyruvate carboxykinase (PEPCK) and genes involved in amino acid metabolism and urea production. Surprisingly, hyperglucagonemia in αTSC2KO mice was associated to improved glucose levels in models of Streptozotocin (STZ)-induced β-cell destruction and high fat diet-induced glucose intolerance. Contrary to our current understanding of glucagon action, these studies demonstrate that endogenous chronic hyperglucagonemia improve glucose homeostasis by augmenting insulin secretion and by inducing glucagon resistance in the liver. These novel in vivo findings support the concept that enhancing glucagon action could be used as an alternative strategy to treat hyperglycemia in diabetes. Disclosure C. Lubaczeuski: None. N. Bozadjieva: None. M. Blandino-Rosano: None. E. Bernal-Mizrachi: None. Funding U.S. Department of Veterans Affairs (IBX002728A); Diabetes Research Connectio

Are elevated systemic bile acids involved in the pathophysiology of sarcopenia and liver injury following gastric bypass?

Bariatric surgery is currently the most effective treatment for sustained weight loss in severe obesity. However, recent data describe the development of liver damage and in particular massive steatosis and cholangitis in some patients, for which certain pathophysiological mechanisms are suggested such as bacterial overgrowth, malabsorption or sarcopenia. We describe the case of a patient presenting with a new liver dysfunction 6 years after a gastric bypass. The work-up revealed sarcopenic obesity characterised by low muscle mass and low muscle function as well as elevated fasting bile acids, severe liver steatosis and cholangitis. The pathophysiology of this disease is complex and multifactorial but could include bile acid toxicity. Bile acids are increased in cases of liver steatosis, but also in cases of gastric bypass and malnutrition. In our opinion, they may contribute to the loss of muscle mass and the vicious circle observed in this situation. Treatment with enteral feeding, intravenous albumin supplementation and diuretics reversed the liver dysfunction and the patient was discharged from hospital

Glucagon Resistance and Decreased Susceptibility to Diabetes in a Model of Chronic Hyperglucagonemia

Elevation of glucagon levels and increase in α-cell mass are associated with states of hyperglycemia in diabetes. Our previous studies have highlighted the role of nutrient signaling via mTOR complex 1 (mTORC1) regulation that controls glucagon secretion and α-cell mass. In the current studies we investigated the effects of activation of nutrient signaling by conditional deletion of the mTORC1 inhibitor, TSC2, in α-cells (αTSC2 ). We showed that activation of mTORC1 signaling is sufficient to induce chronic hyperglucagonemia as a result of α-cell proliferation, cell size, and mass expansion. Hyperglucagonemia in αTSC2 was associated with an increase in glucagon content and enhanced glucagon secretion. This model allowed us to identify the effects of chronic hyperglucagonemia on glucose homeostasis by inducing insulin secretion and resistance to glucagon in the liver. Liver glucagon resistance in αTSC2 mice was characterized by reduced expression of the glucagon receptor (GCGR), PEPCK, and genes involved in amino acid metabolism and urea production. Glucagon resistance in αTSC2 mice was associated with improved glucose levels in streptozotocin-induced β-cell destruction and high-fat diet-induced glucose intolerance. These studies demonstrate that chronic hyperglucagonemia can improve glucose homeostasis by inducing glucagon resistance in the liver

Overexpression of Kinase-Dead mTOR Impairs Glucose Homeostasis by Regulating Insulin Secretion and Not β-Cell Mass

Regulation of glucose homeostasis by insulin depends on β-cell growth and function. Nutrients and growth factor stimuli converge on the conserved protein kinase mechanistic target of rapamycin (mTOR), existing in two complexes, mTORC1 and mTORC2. To understand the functional relevance of mTOR enzymatic activity in β-cell development and glucose homeostasis, we generated mice overexpressing either one or two copies of a kinase-dead mTOR mutant (KD-mTOR) transgene exclusively in β-cells. We examined glucose homeostasis and β-cell function of these mice fed a control chow or high-fat diet. Mice with two copies of the transgene [RIPCre;KD-mTOR (Homozygous)] develop glucose intolerance due to a defect in β-cell function without alterations in β-cell mass with control chow. Islets from RIPCre;KD-mTOR (Homozygous) mice showed reduced mTORC1 and mTORC2 signaling along with transcripts and protein levels of Pdx-1. Islets with reduced mTORC2 signaling in their β-cells (RIPCre;Rictor ) also showed reduced Pdx-1. When challenged with a high-fat diet, mice carrying one copy of KD-mTOR mutant transgene developed glucose intolerance and β-cell insulin secretion defect but showed no changes in β-cell mass. These findings suggest that the mTOR-mediated signaling pathway is not essential to β-cell growth but is involved in regulating β-cell function in normal and diabetogenic conditions