Insulin at the intersection of thermoregulation and glucose homeostasis.

Mammals are protected from changes in environmental temperature by altering energetic processes that modify heat production. Insulin is the dominant stimulus of glucose uptake and metabolism, which are fundamental for thermogenic processes. The purpose of this work was to determine the interaction of ambient temperature induced changes in energy expenditure (EE) on the insulin sensitivity of glucose fluxes. Short-term and adaptive responses to thermoneutral temperature (TN, ∼28 °C) and room (laboratory) temperature (RT, ∼22 °C) were studied in mice. This range of temperature does not cause detectable changes in circulating catecholamines or shivering and postabsorptive glucose homeostasis is maintained. We tested the hypothesis that a decrease in EE that occurs with TN causes insulin resistance and that this reduction in insulin action and EE is reversed upon short term (<12h) transition to RT. Insulin-stimulated glucose disposal (Rd) and tissue-specific glucose metabolic index were assessed combining isotopic tracers with hyperinsulinemic-euglycemic clamps. EE and insulin-stimulated Rd are both decreased (∼50%) in TN-adapted vs RT-adapted mice. When RT-adapted mice are switched to TN, EE rapidly decreases and Rd is reduced by ∼50%. TN-adapted mice switched to RT exhibit a rapid increase in EE, but whole-body insulin-stimulated Rd remains at the low rates of TN-adapted mice. In contrast, whole body glycolytic flux rose with EE. This higher EE occurs without increasing glucose uptake from the blood, but rather by diverting glucose from glucose storage to glycolysis. In addition to adaptations in insulin action, 'insulin-independent' glucose uptake in brown fat is exquisitely sensitive to thermoregulation. These results show that insulin action adjusts to non-stressful changes in ambient temperature to contribute to the support of body temperature homeostasis without compromising glucose homeostasis.

Repression of latent NF-κB enhancers by PDX1 regulates ß cell functional heterogeneity.

Interactions between lineage-determining and activity-dependent transcription factors determine single-cell identity and function within multicellular tissues through incompletely known mechanisms. By assembling a single-cell atlas of chromatin state within human islets, we identified ß cell subtypes governed by either high or low activity of the lineage-determining factor pancreatic duodenal homeobox-1 (PDX1). ß cells with reduced PDX1 activity displayed increased chromatin accessibility at latent nuclear factor κB (NF-κB) enhancers. Pdx1 hypomorphic mice exhibited de-repression of NF-κB and impaired glucose tolerance at night. Three-dimensional analyses in tandem with chromatin immunoprecipitation (ChIP) sequencing revealed that PDX1 silences NF-κB at circadian and inflammatory enhancers through long-range chromatin contacts involving SIN3A. Conversely, Bmal1 ablation in ß cells disrupted genome-wide PDX1 and NF-κB DNA binding. Finally, antagonizing the interleukin (IL)-1ß receptor, an NF-κB target, improved insulin secretion in Pdx1 hypomorphic islets. Our studies reveal functional subtypes of single ß cells defined by a gradient in PDX1 activity and identify NF-κB as a target for insulinotropic therapy.

Insulin at the Intersection of Thermoregulation and Glucose Homeostasis.

Mammals are protected from changes in environmental temperature by altering energetic processes that modify heat production. Insulin is the dominant stimulus of glucose uptake and metabolism, which are fundamental for thermogenic processes. The purpose of this work was to determine the interaction of ambient temperature induced changes in energy expenditure (EE) on the insulin sensitivity of glucose fluxes. Short-term and adaptive responses to thermoneutral temperature (TN, ~28°C) and room (laboratory) temperature (RT, ~22°C) were studied in mice. This range of temperature does not cause detectable changes in circulating catecholamines or shivering and postabsorptive glucose homeostasis is maintained. We tested the hypothesis that a decrease in EE that occurs with TN causes insulin resistance and that this reduction in insulin action and EE is reversed upon short term (<12h) transition to RT. Insulin-stimulated glucose disposal (Rd) and tissue specific glucose uptake were assessed combining isotopic tracers with hyperinsulinemic-euglycemic clamps. EE and insulin-stimulated Rd are both decreased (~50%) in TN-adapted vs RT-adapted mice. When RT-adapted mice are switched to TN, EE rapidly decreases and Rd is reduced by ~50%. TN-adapted mice switched to RT exhibit a rapid increase in EE, but whole body insulin-stimulated Rd remains at the low rates of TN-adapted mice. In contrast, whole body glycolytic flux rose with EE. This higher EE occurs without increasing glucose uptake from the blood, but rather by diverting glucose from glucose storage to glycolysis. In addition to adaptations in insulin action, 'insulin-independent' glucose uptake in brown fat is exquisitely sensitive to thermoregulation. These results show that insulin action adjusts to non-stressful changes in ambient temperature to contribute to the support of body temperature homeostasis without compromising glucose homeostasis.

Gßγ-SNAP25 exocytotic brake removal enhances insulin action, promotes adipocyte browning, and protects against diet-induced obesity.

Negative regulation of exocytosis from secretory cells is accomplished through inhibitory signals from Gi/o GPCRs by Gßγ subunit inhibition of 2 mechanisms: decreased calcium entry and direct interaction of Gßγ with soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) plasma membrane fusion machinery. Previously, we disabled the second mechanism with a SNAP25 truncation (SNAP25Δ3) that decreased Gßγ affinity for the SNARE complex, leaving exocytotic fusion and modulation of calcium entry intact and removing GPCR-Gßγ inhibition of SNARE-mediated exocytosis. Here, we report substantial metabolic benefit in mice carrying this mutation. Snap25Δ3/Δ3 mice exhibited enhanced insulin sensitivity and beiging of white fat. Metabolic protection was amplified in Snap25Δ3/Δ3 mice challenged with a high-fat diet. Glucose homeostasis, whole-body insulin action, and insulin-mediated glucose uptake into white adipose tissue were improved along with resistance to diet-induced obesity. Metabolic protection in Snap25Δ3/Δ3 mice occurred without compromising the physiological response to fasting or cold. All metabolic phenotypes were reversed at thermoneutrality, suggesting that basal autonomic activity was required. Direct electrode stimulation of sympathetic neuron exocytosis from Snap25Δ3/Δ3 inguinal adipose depots resulted in enhanced and prolonged norepinephrine release. Thus, the Gßγ-SNARE interaction represents a cellular mechanism that deserves further exploration as an additional avenue for combating metabolic disease.

Direct and systemic actions of growth hormone receptor (GHR)-signaling on hepatic glycolysis, de novo lipogenesis and insulin sensitivity, associated with steatosis.

BACKGROUND: Evidence is accumulating that growth hormone (GH) protects against the development of steatosis and progression of non-alcoholic fatty liver disease (NAFLD). GH may control steatosis indirectly by altering systemic insulin sensitivity and substrate delivery to the liver and/or by the direct actions of GH on hepatocyte function. APPROACH: To better define the hepatocyte-specific role of GH receptor (GHR) signaling on regulating steatosis, we used a mouse model with adult-onset, hepatocyte-specific GHR knockdown (aHepGHRkd). To prevent the reduction in circulating insulin-like growth factor 1 (IGF1) and the subsequent increase in GH observed after aHepGHRkd, subsets of aHepGHRkd mice were treated with adeno-associated viral vectors (AAV) driving hepatocyte-specific expression of IGF1 or a constitutively active form of STAT5b (STAT5bCA). The impact of hepatocyte-specific modulation of GHR, IGF1 and STAT5b on carbohydrate and lipid metabolism was studied across multiple nutritional states and in the context of hyperinsulinemic:euglycemic clamps. RESULTS: Chow-fed male aHepGHRkd mice developed steatosis associated with an increase in hepatic glucokinase (GCK) and ketohexokinase (KHK) expression and de novo lipogenesis (DNL) rate, in the post-absorptive state and in response to refeeding after an overnight fast. The aHepGHRkd-associated increase in hepatic KHK, but not GCK and steatosis, was dependent on hepatocyte expression of carbohydrate response element binding protein (ChREBP), in re-fed mice. Interestingly, under clamp conditions, aHepGHRkd also increased the rate of DNL and expression of GCK and KHK, but impaired insulin-mediated suppression of hepatic glucose production, without altering plasma NEFA levels. These effects were normalized with AAV-mediated hepatocyte expression of IGF1 or STAT5bCA. Comparison of the impact of AAV-mediated hepatocyte IGF1 versus STAT5bCA in aHepGHRkd mice across multiple nutritional states, indicated the restorative actions of IGF1 are indirect, by improving systemic insulin sensitivity, independent of changes in the liver transcriptome. In contrast, the actions of STAT5b are due to the combined effects of raising IGF1 and direct alterations in the hepatocyte gene program that may involve suppression of BCL6 and FOXO1 activity. However, the direct and IGF1-dependent actions of STAT5b cannot fully account for enhanced GCK activity and lipogenic gene expression observed after aHepGHRkd, suggesting other GHR-mediated signals are involved. CONCLUSION: These studies demonstrate hepatocyte GHR-signaling controls hepatic glycolysis, DNL, steatosis and hepatic insulin sensitivity indirectly (via IGF1) and directly (via STAT5b). The relative contribution of these indirect and direct actions of GH on hepatocytes is modified by insulin and nutrient availability. These results improve our understanding of the physiologic actions of GH on regulating adult metabolism to protect against NAFLD progression.

Congenital adiponectin deficiency mitigates high-fat-diet-induced obesity in gonadally intact male and female, but not in ovariectomized mice.

Epidemiological literature indicates that women are less susceptible to type II diabetes (T2D) than males. The general consensus is that estrogen is protective, whereas its deficiency in post-menopause is associated with adiposity and impaired insulin sensitivity. However, epidemiological data suggests that males are more prone to developing T2D, and at a lower BMI, compared to females during post-menopausal years; suggesting that another factor, other than estrogen, protects females. We proposed to determine if adiponectin (APN) serves as this protective factor. An initial experiment was performed in which gonadally intact male and female mice were fed either a purified low-fat diet (LFD) or high-fat diet (HFD) (40% kcals from fat) for 16 weeks. An additional group of HFD ovariectomy (OVX) mice were included to assess estrogen deficiency's impact on obesity. Body composition, adipose tissue inflammation, ectopic lipid accumulation as well as glucose metabolism and insulin resistance were assessed. In corroboration with previous data, estrogen deficiency (OVX) exacerbated HFD-induced obesity in female mice. However, despite a higher body fat percentage and a similar degree of hepatic and skeletal muscle lipid accumulation, female OVX HFD-fed mice exhibited enhanced insulin sensitivity relative to HFD-fed males. Therefore, a subsequent HFD experiment was performed utilizing male and female (both gonadally intact and OVX) APN deficient mice (APN-/-) and wildtype littermates to determine if APN is the factor which protects OVX females from the similar degree of metabolic dysfunction as males in the setting of obesity. Indirect calorimetry was used to determine observed phenotype differences. APN deficiency limited adiposity and mitigated HFD-induced insulin resistance and adipose tissue inflammation in gonadally intact male and female, but not in OVX mice. Using indirect calorimetry, we uncovered that slight, but non-statistically significant differences in food intake and energy expenditure leading to a net difference in energy balance likely explain the reduced body weight exhibited by male APN-deficient mice. In conclusion, congenital APN deficiency is protective against obesity development in gonadally intact mice, however, in the setting of estrogen deficiency (OVX) this is not true. These findings suggest that gonadal status dictates the protective effects of congenital APN deficiency in the setting of HFD-induced obesity.

Peptide-YY3-36/glucagon-like peptide-1 combination treatment of obese diabetic mice improves insulin sensitivity associated with recovered pancreatic ß-cell function and synergistic activation of discrete hypothalamic and brainstem neuronal circuitries.

OBJECTIVE: Obesity-linked type 2 diabetes (T2D) is a worldwide health concern and many novel approaches are being considered for its treatment and subsequent prevention of serious comorbidities. Co-administration of glucagon like peptide 1 (GLP-1) and peptide YY3-36 (PYY3-36) renders a synergistic decrease in energy intake in obese men. However, mechanistic details of the synergy between these peptide agonists and their effects on metabolic homeostasis remain relatively scarce. METHODS: In this study, we utilized long-acting analogues of GLP-1 and PYY3-36 (via Fc-peptide conjugation) to better characterize the synergistic pharmacological benefits of their co-administration on body weight and glycaemic regulation in obese and diabetic mouse models. Hyperinsulinemic-euglycemic clamps were used to measure weight-independent effects of Fc-PYY3-36 + Fc-GLP-1 on insulin action. Fluorescent light sheet microscopy analysis of whole brain was performed to assess activation of brain regions. RESULTS: Co-administration of long-acting Fc-IgG/peptide conjugates of Fc-GLP-1 and Fc-PYY3-36 (specific for PYY receptor-2 (Y2R)) resulted in profound weight loss, restored glucose homeostasis, and recovered endogenous ß-cell function in two mouse models of obese T2D. Hyperinsulinemic-euglycemic clamps in C57BLKS/J db/db and diet-induced obese Y2R-deficient (Y2RKO) mice indicated Y2R is required for a weight-independent improvement in peripheral insulin sensitivity and enhanced hepatic glycogenesis. Brain cFos staining demonstrated distinct temporal activation of regions of the hypothalamus and hindbrain following Fc-PYY3-36 + Fc-GLP-1R agonist administration. CONCLUSIONS: These results reveal a therapeutic approach for obesity/T2D that improved insulin sensitivity and restored endogenous ß-cell function. These data also highlight the potential association between the gut-brain axis in control of metabolic homeostasis.

Time-optimized feeding is beneficial without enforced fasting.

Time-restricted feeding (TRF) studies underscore that when food is consumed during the daily cycle is important for weight gain/loss because the circadian clock rhythmically modulates metabolism. However, the interpretation of previous TRF studies has been confounded by study designs that introduced an extended period of enforced fasting. We introduce a novel time-optimized feeding (TOF) regimen that disentangles the effects of phase-dependent feeding from the effects of enforced fasting in mice, as well as providing a laboratory feeding protocol that more closely reflects the eating patterns of humans who usually have 24 hour access to food. Moreover, we test whether a sudden switch from ad libitum food access to TRF evokes a corticosterone (stress) response. Our data indicate that the timing of high-fat feeding under TOF allows most of the benefit of TRF without obligatory fasting or evoking a stress response. This benefit occurs through stable temporal coupling of carbohydrate/lipid oxidation with feeding. These results highlight that timing the ingestion of calorically dense foods to optimized daily phases will enhance lipid oxidation and thereby limit fat accumulation.

Effects of a novel polyphenol-rich plant extract on body composition, inflammation, insulin sensitivity, and glucose homeostasis in obese mice.

BACKGROUND/OBJECTIVES: The worldwide prevalence of obesity, metabolic syndrome and type 2 diabetes (T2D) is reaching epidemic proportions that urge the development of new management strategies. Totum-63 is a novel, plant-based polyphenol-rich active principle that has been shown to reduce body weight, fasting glycemia, glucose intolerance, and fatty liver index in obese subjects with prediabetes. Here, we investigated the effects and underlying mechanism(s) of Totum-63 on metabolic homeostasis in insulin-resistant obese mice. METHODS: Male C57Bl6/J mice were fed a high-fat diet for 12 weeks followed by supplementation with Totum-63 for 4 weeks. The effects on whole-body energy and metabolic homeostasis, as well as on tissue-specific inflammation and insulin sensitivity were assessed using a variety of immunometabolic phenotyping tools. RESULTS: Totum-63 decreased body weight and fat mass in obese mice, without affecting lean mass, food intake and locomotor activity, and increased fecal energy excretion and whole-body fatty acid oxidation. Totum-63 reduced fasting plasma glucose, insulin and leptin levels, and improved whole-body insulin sensitivity and peripheral glucose uptake. The expression of insulin receptor ß and the insulin-induced phosphorylation of Akt/PKB were increased in liver, skeletal muscle, white adipose tissue (WAT) and brown adipose tissue (BAT). Hepatic steatosis was also decreased by Totum-63 and associated with a lower expression of genes involved in fatty acid uptake, de novo lipogenesis, inflammation, and fibrosis. Furthermore, a significant reduction in pro-inflammatory macrophages was also observed in epidydimal WAT. Finally, a potent decrease in BAT mass associated with enhanced tissue expression of thermogenic genes was found, suggesting BAT activation by Totum-63. CONCLUSIONS: Our results show that Totum-63 reduces inflammation and improves insulin sensitivity and glucose homeostasis in obese mice through pleiotropic effects on various metabolic organs. Altogether, plant-derived Totum-63 might constitute a promising novel nutritional supplement for alleviating metabolic dysfunctions in obese people with or without T2D.

G6PC2 confers protection against hypoglycemia upon ketogenic diet feeding and prolonged fasting.

OBJECTIVE: G6PC2 is predominantly expressed in pancreatic islet beta cells. G6PC2 hydrolyzes glucose-6-phosphate to glucose and inorganic phosphate, thereby creating a futile substrate cycle that opposes the action of glucokinase. This substrate cycle determines the sensitivity of glucose-stimulated insulin secretion to glucose and hence regulates fasting blood glucose (FBG) but not fasting plasma insulin (FPI) levels. Our objective was to explore the physiological benefit this cycle confers. METHODS: We investigated the response of wild type (WT) and G6pc2 knockout (KO) mice to changes in nutrition. RESULTS: Pancreatic G6pc2 expression was little changed by ketogenic diet feeding but was inhibited by 24 hr fasting and strongly induced by high fat feeding. When challenged with either a ketogenic diet or 24 hr fasting, blood glucose fell to 70 mg/dl or less in G6pc2 KO but not WT mice, suggesting that G6PC2 may have evolved, in part, to prevent hypoglycemia. Prolonged ketogenic diet feeding reduced the effect of G6pc2 deletion on FBG. The hyperglycemia associated with high fat feeding was partially blunted in G6pc2 KO mice, suggesting that under these conditions the presence of G6PC2 is detrimental. As expected, FPI changed but did not differ between WT and KO mice in response to fasting, ketogenic and high fat feeding. CONCLUSIONS: Since elevated FBG levels are associated with increased risk for cardiovascular-associated mortality (CAM), these studies suggest that, while G6PC2 inhibitors would be useful for lowering FBG and the risk of CAM, partial inhibition will be important to avoid the risk of hypoglycemia.

Resolution of NASH and hepatic fibrosis by the GLP-1R/GcgR dual-agonist Cotadutide via modulating mitochondrial function and lipogenesis.

Non-alcoholic fatty liver disease and steatohepatitis are highly associated with obesity and type 2 diabetes mellitus. Cotadutide, a GLP-1R/GcgR agonist, was shown to reduce blood glycemia, body weight and hepatic steatosis in patients with T2DM. Here, we demonstrate that the effects of Cotadutide to reduce body weight, food intake and improve glucose control are predominantly mediated through the GLP-1 signaling, while, its action on the liver to reduce lipid content, drive glycogen flux and improve mitochondrial turnover and function are directly mediated through Gcg signaling. This was confirmed by the identification of phosphorylation sites on key lipogenic and glucose metabolism enzymes in liver of mice treated with Cotadutide. Complementary metabolomic and transcriptomic analyses implicated lipogenic, fibrotic and inflammatory pathways, which are consistent with a unique therapeutic contribution of GcgR agonism by Cotadutide in vivo. Significantly, Cotadutide also alleviated fibrosis to a greater extent than Liraglutide or Obeticholic acid (OCA), despite adjusting dose to achieve similar weight loss in 2 preclinical mouse models of NASH. Thus Cotadutide, via direct hepatic (GcgR) and extra-hepatic (GLP-1R) effects, exerts multi-factorial improvement in liver function and is a promising therapeutic option for the treatment of steatohepatitis.

Potential positive and negative consequences of ZnT8 inhibition.

SLC30A8 encodes the zinc transporter ZnT8. SLC30A8 haploinsufficiency protects against type 2 diabetes (T2D), suggesting that ZnT8 inhibitors may prevent T2D. We show here that, while adult chow fed Slc30a8 haploinsufficient and knockout (KO) mice have normal glucose tolerance, they are protected against diet-induced obesity (DIO), resulting in improved glucose tolerance. We hypothesize that this protection against DIO may represent one mechanism whereby SLC30A8 haploinsufficiency protects against T2D in humans and that, while SLC30A8 is predominantly expressed in pancreatic islet beta cells, this may involve a role for ZnT8 in extra-pancreatic tissues. Consistent with this latter concept we show in humans, using electronic health record-derived phenotype analyses, that the 'C' allele of the non-synonymous rs13266634 SNP, which confers a gain of ZnT8 function, is associated not only with increased T2D risk and blood glucose, but also with increased risk for hemolytic anemia and decreased mean corpuscular hemoglobin (MCH). In Slc30a8 KO mice, MCH was unchanged but reticulocytes, platelets and lymphocytes were elevated. Both young and adult Slc30a8 KO mice exhibit a delayed rise in insulin after glucose injection, but only the former exhibit increased basal insulin clearance and impaired glucose tolerance. Young Slc30a8 KO mice also exhibit elevated pancreatic G6pc2 gene expression, potentially mediated by decreased islet zinc levels. These data indicate that the absence of ZnT8 results in a transient impairment in some aspects of metabolism during development. These observations in humans and mice suggest the potential for negative effects associated with T2D prevention using ZnT8 inhibitors.

A big-data approach to understanding metabolic rate and response to obesity in laboratory mice.

Maintaining a healthy body weight requires an exquisite balance between energy intake and energy expenditure. To understand the genetic and environmental factors that contribute to the regulation of body weight, an important first step is to establish the normal range of metabolic values and primary sources contributing to variability. Energy metabolism is measured by powerful and sensitive indirect calorimetry devices. Analysis of nearly 10,000 wild-type mice from two large-scale experiments revealed that the largest variation in energy expenditure is due to body composition, ambient temperature, and institutional site of experimentation. We also analyze variation in 2329 knockout strains and establish a reference for the magnitude of metabolic changes. Based on these findings, we provide suggestions for how best to design and conduct energy balance experiments in rodents. These recommendations will move us closer to the goal of a centralized physiological repository to foster transparency, rigor and reproducibility in metabolic physiology experimentation.

Maintaining a healthy weight requires the body to balance energy intake and expenditure. The body converts food to energy through a process called energy metabolism. Genetic and environmental factors can affect energy metabolism and energy balance contributing to conditions like obesity. To better understand metabolism, scientists often study mice in laboratories, but mice from different laboratories appear to convert food to energy at different rates. This makes it hard to determine what is 'normal' for mouse metabolism. These discrepancies could be due to small differences between how mice are kept in different laboratories. For example, the temperatures of the mouse cages or how active the mice are might differ depending on the laboratory. Identifying the effects of such differences is essential, but it requires looking at data from hundreds of mice. Corrigan et al. examined data from more than 30,000 mice at laboratories around the world to show that room temperatures and the amount of muscle and fat in a mouse's body have the biggest influence on energy balance. These two factors affected the metabolism of both typical mice and mice with mutations that affect their energy balance. These results suggest that it is important for scientists to report factors like room temperatures, the body make-up of the mice, or the animals' activity levels in metabolism studies. This can help scientists compare results and repeat experiments, which could speed up research into mouse metabolism. Corrigan et al. also found that other unknown factors also affect mouse metabolism in different laboratories. Further studies are needed to identify these factors.

Dapagliflozin Does Not Directly Affect Human α or ß Cells.

Selective inhibitors of sodium glucose cotransporter-2 (SGLT2) are widely used for the treatment of type 2 diabetes and act primarily to lower blood glucose by preventing glucose reabsorption in the kidney. However, it is controversial whether these agents also act on the pancreatic islet, specifically the α cell, to increase glucagon secretion. To determine the effects of SGLT2 on human islets, we analyzed SGLT2 expression and hormone secretion by human islets treated with the SGLT2 inhibitor dapagliflozin (DAPA) in vitro and in vivo. Compared to the human kidney, SLC5A2 transcript expression was 1600-fold lower in human islets and SGLT2 protein was not detected. In vitro, DAPA treatment had no effect on glucagon or insulin secretion by human islets at either high or low glucose concentrations. In mice bearing transplanted human islets, 1 and 4 weeks of DAPA treatment did not alter fasting blood glucose, human insulin, and total glucagon levels. Upon glucose stimulation, DAPA treatment led to lower blood glucose levels and proportionally lower human insulin levels, irrespective of treatment duration. In contrast, after glucose stimulation, total glucagon was increased after 1 week of DAPA treatment but normalized after 4 weeks of treatment. Furthermore, the human islet grafts showed no effects of DAPA treatment on hormone content, endocrine cell proliferation or apoptosis, or amyloid deposition. These data indicate that DAPA does not directly affect the human pancreatic islet, but rather suggest an indirect effect where lower blood glucose leads to reduced insulin secretion and a transient increase in glucagon secretion.

Pancreatic islet beta cell-specific deletion of G6pc2 reduces fasting blood glucose.

The G6PC1, G6PC2 and G6PC3 genes encode distinct glucose-6-phosphatase catalytic subunit (G6PC) isoforms. In mice, germline deletion of G6pc2 lowers fasting blood glucose (FBG) without affecting fasting plasma insulin (FPI) while, in isolated islets, glucose-6-phosphatase activity and glucose cycling are abolished and glucose-stimulated insulin secretion (GSIS) is enhanced at submaximal but not high glucose. These observations are all consistent with a model in which G6PC2 regulates the sensitivity of GSIS to glucose by opposing the action of glucokinase. G6PC2 is highly expressed in human and mouse islet beta cells however, various studies have shown trace G6PC2 expression in multiple tissues raising the possibility that G6PC2 also affects FBG through non-islet cell actions. Using real-time PCR we show here that expression of G6pc1 and/or G6pc3 are much greater than G6pc2 in peripheral tissues, whereas G6pc2 expression is much higher than G6pc3 in both pancreas and islets with G6pc1 expression not detected. In adult mice, beta cell-specific deletion of G6pc2 was sufficient to reduce FBG without changing FPI. In addition, electronic health record-derived phenotype analyses showed no association between G6PC2 expression and phenotypes clearly unrelated to islet function in humans. Finally, we show that germline G6pc2 deletion enhances glycolysis in mouse islets and that glucose cycling can also be detected in human islets. These observations are all consistent with a mechanism by which G6PC2 action in islets is sufficient to regulate the sensitivity of GSIS to glucose and hence influence FBG without affecting FPI.

Eating breakfast and avoiding late-evening snacking sustains lipid oxidation.

Circadian (daily) regulation of metabolic pathways implies that food may be metabolized differentially over the daily cycle. To test that hypothesis, we monitored the metabolism of older subjects in a whole-room respiratory chamber over two separate 56-h sessions in a random crossover design. In one session, one of the 3 daily meals was presented as breakfast, whereas in the other session, a nutritionally equivalent meal was presented as a late-evening snack. The duration of the overnight fast was the same for both sessions. Whereas the two sessions did not differ in overall energy expenditure, the respiratory exchange ratio (RER) was different during sleep between the two sessions. Unexpectedly, this difference in RER due to daily meal timing was not due to daily differences in physical activity, sleep disruption, or core body temperature (CBT). Rather, we found that the daily timing of nutrient availability coupled with daily/circadian control of metabolism drives a switch in substrate preference such that the late-evening Snack Session resulted in significantly lower lipid oxidation (LO) compared to the Breakfast Session. Therefore, the timing of meals during the day/night cycle affects how ingested food is oxidized or stored in humans, with important implications for optimal eating habits.

Liver-derived FGF21 is essential for full adaptation to ketogenic diet but does not regulate glucose homeostasis.

BACKGROUND: Fibroblast growth factor 21 (FGF21) is expressed in several metabolically active tissues, including liver, fat, and acinar pancreas, and has pleiotropic effects on metabolic homeostasis. The dominant source of FGF21 in the circulation is the liver. OBJECTIVE AND METHODS: To analyze the physiological functions of hepatic FGF21, we generated a hepatocyte-specific knockout model (LKO) by mating albumin-Cre mice with FGF21 flox/flox (fl/fl) mice and challenged it with different nutritional models. RESULTS: Mice fed a ketogenic diet typically show increased energy expenditure; this effect was attenuated in LKO mice. LKO on KD also developed hepatic pathology and altered hepatic lipid homeostasis. When evaluated using hyperinsulinemic-euglycemic clamps, glucose infusion rates, hepatic glucose production, and glucose uptake were similar between fl/fl and LKO DIO mice. CONCLUSIONS: We conclude that liver-derived FGF21 is important for complete adaptation to ketosis but has a more limited role in the regulation of glycemic homeostasis.

Perfusion controls muscle glucose uptake by altering the rate of glucose dispersion in vivo.

These studies test, using intravital microscopy (IVM), the hypotheses that perfusion effects on insulin-stimulated muscle glucose uptake (MGU) are 1) capillary recruitment independent and 2) mediated through the dispersion of glucose rather than insulin. For experiment 1, capillary perfusion was visualized before and after intravenous insulin. No capillary recruitment was observed. For experiment 2, mice were treated with vasoactive compounds (sodium nitroprusside, hyaluronidase, and lipopolysaccharide), and dispersion of fluorophores approximating insulin size (10-kDa dextran) and glucose (2-NBDG) was measured using IVM. Subsequently, insulin and 2[14C]deoxyglucose were injected and muscle phospho-2[14C]deoxyglucose (2[C14]DG) accumulation was used as an index of MGU. Flow velocity and 2-NBDG dispersion, but not perfused surface area or 10-kDa dextran dispersion, predicted phospho-2[14C]DG accumulation. For experiment 3, microspheres of the same size and number as are used for contrast-enhanced ultrasound (CEU) studies of capillary recruitment were visualized using IVM. Due to their low concentration, microspheres were present in only a small fraction of blood-perfused capillaries. Microsphere-perfused blood volume correlated to flow velocity. These findings suggest that 1) flow velocity rather than capillary recruitment controls microvascular contributions to MGU, 2) glucose dispersion is more predictive of MGU than dispersion of insulin-sized molecules, and 3) CEU measures regional flow velocity rather than capillary recruitment.