Featured ArticlesVolume 19 | January 2019
|SGLT1 in pancreatic α cells regulates glucagon secretion Pancreatic α cells secrete glucagon, which controls blood glucose levels. In patients with type 1 or type 2 diabetes, hyperglycemia is often associated with hyperglucagonemia. However, the mechanism underlying excessive glucagon secretion is not fully understood. A recent report described the expression of sodium glucose cotransporters (SGLT) 1 and 2 in α cells. Suga et al. suggest that a novel mechanism regulates glucagon secretion through SGLT1 in α cells. Their findings might have implications for the classification and selection of SGLT2 inhibitors in the treatment of diabetes.|
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Objectives: It is controversial whether sodium glucose transporter (SGLT) 2 inhibitors increase glucagon secretion via direct inhibition of SGLT2 in pancreatic α cells. The role of SGLT1 in α cells is also unclear. We aimed to elucidate these points that are important not only for basic research but also for clinical insight.
Methods: Plasma glucagon levels were assessed in the high-fat, high-sucrose diet (HFHSD) fed C57BL/6J mice treated with dapagliflozin or canagliflozin. RT-PCR, RNA sequence, and immunohistochemistry were conducted to test the expression of SGLT1 and SGLT2 in α cells. We also used αTC1 cells and mouse islets to investigate the molecular mechanism by which SGLT1 modulates glucagon secretion.
Results: Dapagliflozin, but not canagliflozin, increased plasma glucagon levels in HFHSD fed mice. SGLT1 and glucose transporter 1 (GLUT1), but not SGLT2, were expressed in αTC1 cells, mouse islets and human islets. A glucose clamp study revealed that the plasma glucagon increase associated with dapagliflozin could be explained as a response to acute declines in blood glucose. Canagliflozin suppressed glucagon secretion by inhibiting SGLT1 in α cells; consequently, plasma glucagon did not increase with canagliflozin, even though blood glucose declined. SGLT1 effect on glucagon secretion depended on glucose transport, but not glucose metabolism. Islets from HFHSD and db/db mice displayed higher SGLT1 mRNA levels and lower GLUT1 mRNA levels than the islets from control mice. These expression levels were associated with higher glucagon secretion. Furthermore, SGLT1 inhibitor and siRNA against SGLT1 suppressed glucagon secretion in isolated islets.
Conclusions: These data suggested that a novel mechanism regulated glucagon secretion through SGLT1 in α cells. This finding possibly explained the distinct effects of dapagliflozin and canagliflozin on plasma glucagon levels in mice.[Hide abstract]
|Diacylglycerol kinase is a regulator of insulin secretionObesity is a complex disorder involving many genetic and environmental factors that are required to maintain energy homeostasis. Trinh et al. performed a large-scale in vivo screen for obesity-associated genes in Drosophila neurons. They identified 24 genes, including genes that have been previously associated with energy homeostasis in flies. One of the genes identified was Diacylglycerol kinase (Dgk), the homologues of which have been associated with obesity in human populations and have been shown to regulate insulin secretion in vitro. The authors performed the first functional study of Dgk in flies and showed that it is involved in lipid and carbohydrate metabolism.|
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Objective: Obesity is a complex disorder involving many genetic and environmental factors that are required to maintain energy homeostasis. While studies in human populations have led to significant progress in the generation of an obesity gene map and broadened our understanding of the genetic basis of common obesity, there is still a large portion of heritability and etiology that remains unknown. Here, we have used the genetically tractable fruit fly, Drosophila melanogaster, to identify genes/pathways that function in the nervous system to regulate energy balance.
Methods: We performed an in vivo RNAi screen in Drosophila neurons and assayed for obese or lean phenotypes by measuring changes in levels of stored fats (in the form of triacylglycerides or TAG). Three rounds of screening were performed to verify the reproducibility and specificity of the adiposity phenotypes. Genes that produced >25% increase in TAG (206 in total) underwent a second round of screening to verify their effect on TAG levels by retesting the same RNAi line to validate the phenotype. All remaining hits were screened a third time by testing the TAG levels of additional RNAi lines against the genes of interest to rule out any off-target effects.
Results: We identified 24 genes including 20 genes that have not been previously associated with energy homeostasis. One identified hit, Diacylglycerol kinase (Dgk), has mammalian homologues that have been implicated in genome-wide association studies for metabolic defects. Downregulation of neuronal Dgk levels increases TAG and carbohydrate levels and these phenotypes can be recapitulated by reducing Dgk levels specifically within the insulin-producing cells that secrete Drosophila insulin-like peptides (dILPs). Conversely, overexpression of kinase-dead Dgk, but not wild-type, decreased circulating dILP2 and dILP5 levels resulting in lower insulin signalling activity. Despite having higher circulating dILP levels, Dgk RNAi flies have decreased pathway activity suggesting that they are insulin-resistant.
Conclusion: Altogether, we have identified several genes that act within the CNS to regulate energy homeostasis. One of these, Dgk, acts within the insulin-producing cells to regulate the secretion of dILPs and energy homeostasis in Drosophila.[Hide abstract]
|Moderate weight loss attenuates endoplasmic reticulum stress and mitochondrial dysfunction Endoplasmic reticulum (ER) stress and mitochondrial dysfunction seem to play a role in obesity and might be interesting as therapeutic targets. López-Domènech and colleagues assessed the effect of moderate weight loss in obese individuals on ER and mitochondrial function. Interestingly, the improvement in systemic inflammation and glucose tolerance was mirrored by an attenuation of chronic ER stress and mitochondrial dysfunction and was accompanied by enhanced expression of chaperones and antioxidants.|
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Objective: In obese patients undergoing caloric restriction, there are several potential mechanisms involved in the improvement of metabolic outcomes. The present study further explores whether caloric restriction can modulate endoplasmic reticulum (ER) stress and mitochondrial function, as both are known to be mechanisms underlying inflammation and insulin resistance (IR) during obesity.
Methods: A total of 64 obese patients with BMI ≥35 kg/m2 underwent a dietary program consisting of 6 weeks of a very-low-calorie diet followed by 18 weeks of low-calorie diet. We evaluated changes in the metabolic and inflammatory markers -TNFα, hsCRP, complement component 3 (C3c), and retinol binding protein 4 (RBP4)-, in the ER stress markers and modulators -eIF2α-P, sXBP1, ATF6, JNK-P, CHOP, GRP78, and SIRT1-, and in mitochondrial function parameters -mitochondrial reactive oxygen species (mROS), glutathione peroxidase 1 (GPX1), cytosolic Ca2+, and mitochondrial membrane potential.
Results: The dietary intervention produced an 8.85% weight loss associated with enhanced insulin sensitivity, a less marked atherogenic lipid profile, and a decrease in systemic inflammation (TNFα, hsCRP) and adipokine levels (RBP4 and C3c). Chronic ER stress was significantly reduced (ATF6-CHOP, JNK-P) and expression levels of SIRT1 and GRP78 – a Ca2+-dependent chaperone – were increased and accompanied by the restoration of Ca2+ depots. Furthermore, mROS production and mitochondrial membrane potential improvement were associated with the up-regulation of the antioxidant enzyme GPX1.
Conclusions: Our data provide evidence that moderate weight loss attenuates systemic inflammation and IR and promotes the amelioration of ER stress and mitochondrial dysfunction, increasing the expression of chaperones, SIRT1 and antioxidant GPX1.[Hide abstract]
|Tannic acid, a histone acetyltransferase inhibitor, prevents non-alcoholic fatty liver disease Non-alcoholic fatty liver disease (NAFLD) development is influenced by environmental as well as genetic factors. The environment influences gene expression by altering histone acetylation patterns in the chromatin, which is regulated by histone acetyltransferases (HATs) and deacetylases. Chung et al. identified tannic acid as a novel HAT inhibitor and demonstrated that tannic acid prevented the development of NAFLD via its HAT inhibition activity, both in vitro and in vivo.
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Objective: We examined the potential of tannic acid (TA) as a novel histone acetyltransferase inhibitor (HATi) and demonstrated that TA prevents non-alcoholic fatty liver disease (NAFLD) by inhibiting HAT activity.
Methods: The anti-HAT activity of TA was examined using HAT activity assays. An in vitro NAFLD model was generated by treating HepG2 cells with oleic and palmitic acids. Male C57BL/6J mice were fed a control diet (CD) or Western diet (WD) with or without supplementation with either 1% or 3% TA (w/w) for 12 weeks. Finally, the possibility of interacting p300 and TA was simulated.
Results: TA suppressed HAT activity both in vitro and in vivo. Interestingly, TA abrogated occupancy of p300 on the sterol regulatory element in the fatty acid synthase and ATP-citrate lyase promoters, eventually inducing hypoacetylation of H3K9 and H3K36. Furthermore, TA decreased acetylation at lysine residues 9 and 36 of histone H3 protein and that of total proteins. Consequently, TA decreased the mRNA expression of lipogenesis-related genes and attenuated lipid accumulation in vivo. We observed that NAFLD features, including body weight, liver mass, fat mass, and lipid profile in serum, were improved by TA supplementation in vivo. Finally, we demonstrated the possibility that TA directly binds to p300 through docking simulation between ligand and protein.
Conclusions: Our findings demonstrate that TA, a novel HATi, has potential application for the prevention of NAFLD.[Hide abstract]
|GPR142 controls metabolism through regulation of pancreatic and gut hormonesGPR142 is a G protein coupled receptor that is almost exclusively expressed in the islets of the pancreas. Our knowledge about its physiological role and pharmacological potential is very limited. Rudenko and colleagues discovered that GPR142 efficiently stimulates pancreatic glucagon secretion and found that the effect of GPR142 agonism on glucose disposal is a balance between insulin-mediated glucose uptake in fat and muscle and glucagon-mediated glucose production in the liver. Surprisingly, GPR142 agonism also increased energy expenditure and improved insulin sensitivity.
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Objectives: GPR142, which is highly expressed in pancreatic islets, has recently been deorphanized as a receptor for aromatic amino acids; however, its physiological role and pharmacological potential is unclear.
Methods and results: We find that GPR142 is expressed not only in β- but also in α-cells of the islets as well as in enteroendocrine cells, and we confirm that GPR142 is a highly selective sensor of essential aromatic amino acids, in particular Trp and oligopeptides with N-terminal Trp. GPR142 knock-out mice displayed a very limited metabolic phenotype but demonstrated that L-Trp induced secretion of pancreatic and gut hormones is mediated through GPR142 but that the receptor is not required for protein-induced hormone secretion. A synthetic GPR142 agonist stimulated insulin and glucagon as well as GIP, CCK, and GLP-1 secretion. In particular, GIP secretion was sensitive to oral administration of the GPR142 agonist an effect which in contrast to the other hormones was blocked by protein load. Oral administration of the GPR142 agonist increased [3H]-2-deoxyglucose uptake in muscle and fat depots mediated through insulin action while it lowered liver glycogen conceivably mediated through glucagon, and, consequently, it did not lower total blood glucose. Nevertheless, acute administration of the GPR142 agonist strongly improved oral glucose tolerance in both lean and obese mice as well as Zucker fatty rat. Six weeks in-feed chronic treatment with the GPR142 agonist did not affect body weight in DIO mice, but increased energy expenditure and carbohydrate utilization, lowered basal glucose, and improved insulin sensitivity.
Conclusions: GPR142 functions as a sensor of aromatic amino acids, controlling GIP but also CCK and GLP-1 as well as insulin and glucagon in the pancreas. GPR142 agonists could have novel interesting potential in modifying metabolism through a balanced action of gut hormones as well as both insulin and glucagon.[Hide abstract]
|Fibroblast activation protein is dispensable for glucose homeostasis and body weight Fibroblast activation protein (FAP) has over 50% sequence identity with dipeptidyl peptidase-4 (DPP-4). The therapeutic success of DPP-4 inhibitors for the treatment of type 2 diabetes has stimulated interest in the metabolic actions of FAP. Panaro and colleagues found that Fap-/- mice did not exhibit improved glucose control or resistance to weight gain after prolonged high fat feeding. Their findings indicate that murine FAP is not a critical determinant of islet function, glucose homeostasis, weight gain, and the metabolic response to high fat feeding in mice.
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Objective: Fibroblast Activation Protein (FAP), an enzyme structurally related to dipeptidyl peptidase-4 (DPP-4), has garnered interest as a potential metabolic drug target due to its ability to cleave and inactivate FGF-21 as well as other peptide substrates. Here we investigated the metabolic importance of FAP for control of body weight and glucose homeostasis in regular chow-fed and high fat diet-fed mice.
Methods: FAP enzyme activity was transiently attenuated using a highly-specific inhibitor CPD60 and permanently ablated by genetic inactivation of the mouse Fap gene. We also assessed the FAP-dependence of CPD60 and talabostat (Val-boroPro), a chemical inhibitor reportedly targeting both FAP and dipeptidyl peptidase-4
Results: CPD60 robustly inhibited plasma FAP activity with no effect on DPP-4 activity. Fap gene disruption was confirmed by assessment of genomic DNA, and loss of FAP enzyme activity in plasma and tissues. CPD60 did not improve lipid tolerance but modestly improved acute oral and intraperitoneal glucose tolerance in a FAP-dependent manner. Genetic inactivation of Fap did not improve glucose or lipid tolerance nor confer resistance to weight gain in male or female Fap−/− mice fed regular chow or high-fat diets. Moreover, talabostat markedly improved glucose homeostasis in a FAP- and FGF-21-independent, DPP-4 dependent manner.
Conclusions: Although pharmacological FAP inhibition improves glucose tolerance, the absence of a metabolic phenotype in Fap−/−mice suggest that endogenous FAP is dispensable for the regulation of murine glucose homeostasis and body weight. These findings highlight the importance of characterizing the specificity and actions of FAP inhibitors in different species and raise important questions about the feasibility of mouse models for targeting FAP as a treatment for diabetes and related metabolic disorders.[Hide abstract]
|CPT1C in the hypothalamus is necessary for brown fat thermogenesis activationCarnitine palmitoyltransferase 1 isoform C (CPT1C) is located in the endoplasmic reticulum of hypothalamic neurons. It is able to bind malonyl-CoA, suggesting that it could act as a sensor of this lipid intermediary. CPT1C knockout (KO) mice are more prone to become obese when chronically fed a high fat diet with a reduced peripheral fatty acid oxidation. Rodríguez-Rodríguez et al. show that the obese phenotype and metabolic inflexibility that characterizes CPT1C KO mice is related to an impaired brown adipose tissue thermogenesis. Thus, their data uncover CPT1C as a key factor in the control of brown fat thermogenesis.|
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Objective: Carnitine palmitoyltransferase 1C (CPT1C) is implicated in central regulation of energy homeostasis. Our aim was to investigate whether CPT1C in the ventromedial nucleus of the hypothalamus (VMH) is involved in the activation of brown adipose tissue (BAT) thermogenesis in the early stages of diet-induced obesity.
Methods: CPT1C KO and wild type (WT) mice were exposed to short-term high-fat (HF) diet feeding or to intracerebroventricular leptin administration and BAT thermogenesis activation was evaluated. Body weight, adiposity, food intake, and leptinemia were also assayed.
Results: Under 7 days of HF diet, WT mice showed a maximum activation peak of BAT thermogenesis that counteracted obesity development, whereas this activation was impaired in CPT1C KO mice. KO animals evidenced higher body weight, adiposity, hyperleptinemia, ER stress, and disrupted hypothalamic leptin signaling. Leptin-induced BAT thermogenesis was abolished in KO mice. These results indicate an earlier onset leptin resistance in CPT1C KO mice. Since AMPK in the VMH is crucial in the regulation of BAT thermogenesis, we analyzed if CPT1C was a downstream factor of this pathway. Genetic inactivation of AMPK within the VMH was unable to induce BAT thermogenesis and body weight loss in KO mice, indicating that CPT1C is likely downstream AMPK in the central mechanism modulating thermogenesis within the VMH. Quite opposite, the expression of CPT1C in the VMH restored the phenotype.
Conclusions: CPT1C is necessary for the activation of BAT thermogenesis driven by leptin, HF diet exposure, and AMPK inhibition within the VMH. This study underscores the importance of CPT1C in the activation of BAT thermogenesis to counteract diet-induced obesity.[Hide abstract]
|Function of the extracellular matrix proteoglycan Lumican in obesity and glucose homeostasisAdipose tissue expansion in the face of caloric excess initiates processes including angiogenesis, inflammation, and extracellular matrix (ECM) remodeling. The small leucine-rich proteoglycan Lumican, a component of the ECM, binds collagen and is associated with repair processes in collagen-rich connective tissues. Wolff et al. explored the impact of Lumican loss and over-expression on metabolism and adipose tissue. They observed a sex-specific effect on fat accumulation in female Lumican knockout mice that coincided with increased inflammation and insulin resistance. Likewise, Lumican overexpression in the visceral fat and liver improved glucose clearance and insulin sensitivity.|
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Objective: Extracellular matrix remodeling is required for adipose expansion under increased caloric intake. In turn, inhibited expandability due to aberrant collagen deposition promotes insulin resistance and progression towards the metabolic syndrome. An emerging role for the small leucine-rich proteoglycan Lumican in metabolically driven nonalcoholic fatty liver disease sparks an interest in further understanding its role in diet-induced obesity and metabolic complications.
Methods: Whole body ablation of Lumican (Lum−/−) gene and adeno-associated virus-mediated over-expression were used in combination with control or high fat diet to assess energy balance, glucose homeostasis as well as adipose tissue health and remodeling.
Results: Lumican was found to be particularly enriched in the stromal cells isolated from murine gonadal white adipose tissue. Likewise murine and human visceral fat showed a robust increase in Lumican as compared to fat from the subcutaneous depot. Lumican null female mice exhibited moderately increased fat mass, decreased insulin sensitivity and increased liver triglycerides in a diet-dependent manner. These changes coincided with inflammation in adipose tissue and no overt effects in adipose expandability, i.e. adipocyte formation and hypertrophy. Lumican over-expression in visceral fat and liver resulted in improved insulin sensitivity and glucose clearance.
Conclusions: These data indicate that Lumican may represent a functional link between the extracellular matrix, glucose homeostasis, and features of the metabolic syndrome.[Hide abstract]