Whole-body Glucose Research Articles

Sclerostin Loop3 Participates in Whole-body Lipid and Glucose Metabolism Impairment Effects of Sclerostin

Sclerostin Loop3 Participates in Whole-body Lipid and Glucose Metabolism Impairment Effects of Sclerostin

A homeostatic metabolic role for estrogen receptor α in the subfornical organ

Approximately 70% of American adults are considered overweight or obese putting them at increased risk for metabolic syndrome, which includes a constellation of abnormalities that can result in cardiovascular disease, type II diabetes, and fatty liver. Sexual dimorphism in metabolic syndrome is clearly established. Adult men display increased prevalence of metabolic syndrome comorbidities when compared to fertile women. However, post-menopause, the incidence of metabolic syndrome in women significantly increases. It is widely proposed that the increased post-menopausal incidence of metabolic syndrome is due, in part, to estrogen deficiency. Within the brain, estrogen regulates metabolism primarily through actions on estrogen receptor alpha (ERα). While a major emphasis has been placed on hypothalamic nuclei, whether there is a distributed neural network of ERα that works through autonomic and endocrine actions to influence metabolism remains incompletely defined. We have recently shown a critical role for the subfornical organ (SFO), a circumventricular region that lies outside the blood barrier, in the regulation of peripheral metabolism in males. However, whether this brain nucleus and further estrogen signaling within this region is involved the control of metabolism in females is unknown. Taken together, we hypothesized that ERα in the SFO may regulate peripheral metabolism in female mice during obesity. To investigate this, 6 wk old female ESRα flox mice received targeted SFO microinjections of AAV-Cre-GFP and titer matched control AAV-LacZ-GFP. Following 2 wks of viral incorporation, mice were fed a high fat diet (HFD; 60% kcal from fat) for 10 weeks. Indirect calorimetry measurements indicated that SFO ERα removal resulted inhyperphagic behavior during HFD feeding (Cumulative food intake: 27.4±2.7 g vs 37.5±3.3 g, GFP vs. Cre, n=7-8, p<0.05) that was counterbalanced by increased energy expenditure (0.46±0.02 vs. 0.51±0.02 kcal/hr, GFP vs. Cre, n=7-8, p<0.05) resulting in no influence on body weight (ΔBody weight: 18.9±3.2 vs. 20.3±1.6 g, GFP vs. Cre, n=4, p>0.05) or adiposity. Interestingly, SFO ERα knockdown exacerbated hepatic steatosis during diet-induced obesity (Oil Red staining, 8.8±1.9 vs 18.8±2.8 au x 107, GFP vs. Cre, n=7-8, p<0.05), although this was in contrast to improved whole-body glucose tolerance in response to an i.p. glucose load (6.1±0.2 vs 4.7±0.5 AUC x 104, GFP vs. Cre, n=5-6, p<0.05) and upregulation of skeletal muscle long-chain fatty acid b-oxidation genes (e.g. Cpt1: 2.3±0.7 Cre fold GFP, n=4-5, p<0.05). Finally, deletion of SFO ERα was associated with a reduction in circulating estrogen during obesity (80.0±4.3 vs 45.2±12.5 pg/mL, GFP vs. Cre, n=3-4, p=0.06). Collectively, these findings indicate that SFO ERα may differentially regulate target organs during obesity and points to a complex set of interactions whereby SFO estrogen signaling can both positively and negatively regulate peripheral metabolism. R01DK117007, R01HL141393, F31 NIDDK12829388 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Effect of β-Caryophyllene on oxidative stress, glucose metabolism in the skeletal muscle of high fat diet and fructose-induced type-2 diabetic adult male rats.

Skeletal muscle is where the majority of insulin-stimulated whole-body glucose elimination takes place under normal circumstances. A significant risk factor for metabolic diseases is high dietary fat consumption, which also increases stored fat mass. Natural solutions with anti-diabetic effectiveness and fewer side effects are becoming more popular as a result of the conventional pharmacological treatments' numerous negative side effects and high rates of secondary failure. Cannabis and a variety of culinary herbs and spices may include the naturally occurring sequiterpene β-caryophyllene. Among other things, it has antioxidant, anti-inflammatory, and anti-lipidemic properties. However, it is not yet known how β-caryophyllene affects the uptake and oxidation of glucose. Determining if β -caryophyllene has anti-diabetic properties in type-2 diabetes brought on by a high-fat diet was the objective of the current investigation. A sufficient dose of β-caryophyllene (200 mg/kg b.w.t., orally for 30 days) was given to type-2 diabetic rats fed a high-fat diet and given fructose as an inducer of diabetes to assess its anti-diabetic activity. The treatment of diabetes-induced rats with β -Caryophyllene restored the altered levels of blood glucose, serum insulin as well as the lipid parameters, oxidative stress markers, antioxidant enzymes. Our findings show that β-caryophyllene improves glycemia control by enhancing glucose absorption and oxidation in the skeletal muscle of type-2 diabetic rats. From the present findings, it is evident that β -caryophyllene can be used as an anti-diabetic drug.

Pancreas-derived DPP4 is not essential for glucose homeostasis under metabolic stress

Mice systemically lacking dipeptidyl peptidase-4 (DPP4) have improved islet health, glucoregulation, and reduced obesity with high-fat diet (HFD) feeding compared to wild-type mice. Some, but not all, of this improvement can be linked to the loss of DPP4 in endothelial cells (ECs), pointing to the contribution of non-EC types. The importance of intra-islet signaling mediated by α to β cell communication is becoming increasingly clear; thus, our objective was to determine if β cell DPP4 regulates insulin secretion and glucose tolerance in HFD-fed mice by regulating the local concentrations of insulinotropic peptides. Using β cell double incretin receptor knockout mice, β cell- and pancreas-specific Dpp4-/- mice, we reveal that β cell incretin receptors are necessary for DPP4 inhibitor effects. However, although β cell DPP4 modestly contributes to high glucose (16.7mM)-stimulated insulin secretion in isolated islets, it does not regulate whole-body glucose homeostasis.

Systemic depletion of WWP1 improves insulin sensitivity and lowers triglyceride content in the liver of obese mice.

Obesity is a metabolic disorder associated with many diseases. WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) is a HECT-type E3 ubiquitin ligase involved in several diseases. Recently, we found that the level of WWP1 is increased in white adipose tissue in a mouse model of obesity and that obese Wwp1 knockout (KO) mice exhibit improved whole-body glucose metabolism. Here, to determine which insulin-sensitive tissues contribute to this phenotype, we investigated the levels of several insulin signaling markers in white adipose tissue, liver, and skeletal muscle of Wwp1 KO mice, which were fed a normal or high-fat diet and transiently treated with insulin. In obese Wwp1 KO mice, phosphorylated Akt levels were increased in the liver but not in white adipose tissue or skeletal muscle. Moreover, the weight and triglyceride content of the liver of obese Wwp1 KO mice were decreased. These results suggest that systemic deletion of WWP1 improves glucose metabolism via enhanced hepatic insulin signaling and suppressed hepatic fat accumulation. In summary, WWP1 participates in obesity-related metabolic dysfunction and pathologies related to hepatic steatosis via suppressed insulin signaling.

Astaxanthin, a natural antioxidant, lowers cholesterol and markers of cardiovascular risk in individuals with prediabetes and dyslipidaemia.

To determine the effects of astaxanthin treatment on lipids, cardiovascular disease (CVD) markers, glucose tolerance, insulin action and inflammation in individuals with prediabetes and dyslipidaemia. Adult participants with dyslipidaemia and prediabetes (n = 34) underwent baseline blood draw, an oral glucose tolerance test and a one-step hyperinsulinaemic-euglycaemic clamp. They were then randomized (n = 22 treated, 12 placebo) to receive astaxanthin 12 mg daily or placebo for 24 weeks. Baseline studies were repeated after 12 and 24 weeks of therapy. After 24 weeks, astaxanthin treatment significantly decreased low-density lipoprotein (-0.33 ± 0.11 mM) and total cholesterol (-0.30 ± 0.14 mM) (both P < .05). Astaxanthin also reduced levels of the CVD risk markers fibrinogen (-473 ± 210 ng/mL), L-selectin (-0.08 ± 0.03 ng/mL) and fetuin-A (-10.3 ± 3.6 ng/mL) (all P < .05). While the effects of astaxanthin treatment did not reach statistical significance, there were trends toward improvements in the primary outcome measure, insulin-stimulated, whole-body glucose disposal (+0.52 ± 0.37 mg/m2 /min, P = .078), as well as fasting [insulin] (-5.6 ± 8.4 pM, P = .097) and HOMA2-IR (-0.31 ± 0.16, P = .060), suggesting improved insulin action. No consistent significant differences from baseline were observed for any of these outcomes in the placebo group. Astaxanthin was safe and well tolerated with no clinically significant adverse events. Although the primary endpoint did not meet the prespecified significance level, these data suggest that astaxanthin is a safe over-the-counter supplement that improves lipid profiles and markers of CVD risk in individuals with prediabetes and dyslipidaemia.

Ormdl3 regulation of specific ceramides is dispensable for mouse β-cell function and glucose homeostasis under obesogenic conditions.

Chronic elevation of sphingolipids contributes to β-cell failure. ORMDL3 has been identified as a key regulator of sphingolipid homeostasis, however, its function in pancreatic β-cell pathophysiology remains unclear. Here, we generated a mouse model lacking Ormdl3 within pancreatic β-cells (Ormdl3 β-/-). We show that loss of β-cell Ormdl3 does not alter glucose tolerance, insulin sensitivity, insulin secretion, islet morphology, or cellular ceramide levels on standard chow diet. When challenged with a high fat diet, while Ormdl3 β-/- mice did not exhibit any alteration in metabolic parameters or islet architecture, lipidomics analysis revealed significantly higher levels of very long chain ceramides in their islets. Taken together, our results reveal that loss of Ormdl3 alone is not sufficient to impinge upon β-cell function or whole-body glucose and insulin homeostasis, however, β-cell-specific loss of Ormdl3 does significantly alter levels of specific sphingolipid species in islets upon high fat feeding.

Effects of short-term endurance and strength exercise in the molecular regulation of skeletal muscle in hyperinsulinemic and hyperglycemic Slc2a4+/- mice.

Intriguingly, hyperinsulinemia, and hyperglycemia can predispose insulin resistance, obesity, and type 2 diabetes, leading to metabolic disturbances. Conversely, physical exercise stimulates skeletal muscle glucose uptake, improving whole-body glucose homeostasis. Therefore, we investigated the impact of short-term physical activity in a mouse model (Slc2a4+/-) that spontaneously develops hyperinsulinemia and hyperglycemia even when fed on a chow diet. Slc2a4+/- mice were used, that performed 5days of endurance or strength exercise training. Further analysis included physiological tests (GTT and ITT), skeletal muscle glucose uptake, skeletal muscle RNA-sequencing, mitochondrial function, and experiments with C2C12 cell line. When Slc2a4+/- mice were submitted to the endurance or strength training protocol, improvements were observed in the skeletal muscle glucose uptake and glucose metabolism, associated with broad transcriptomic modulation, that was, in part, related to mitochondrial adaptations. The endurance training, but not the strength protocol, was effective in improving skeletal muscle mitochondrial activity and unfolded protein response markers (UPRmt). Moreover, experiments with C2C12 cells indicated that insulin or glucose levels could contribute to these mitochondrial adaptations in skeletal muscle. Both short-term exercise protocols were efficient in whole-body glucose homeostasis and insulin resistance. While endurance exercise plays an important role in transcriptome and mitochondrial activity, strength exercise mostly affects post-translational mechanisms and protein synthesis in skeletal muscle. Thus, the performance of both types of physical exercise proved to be a very effective way to mitigate the impacts of hyperglycemia and hyperinsulinemia in the Slc2a4+/- mouse model.

Undercarboxylated osteocalcin and ibandronate combination ameliorates hindlimb immobilization-induced muscle wasting.

Immobilization leads to muscle wasting and insulin resistance, particularly during ageing. It has been suggested that undercarboxylated osteocalcin (ucOC) improves muscle mass and glucose metabolism. Bisphosphonates, an anti-osteoporosis treatment, might protect muscle wasting independent of ucOC. We hypothesize that the combination of ucOC and ibandronate (IBN) treatments has superior protective effects against immobilization-induced muscle wasting and insulin resistance than either treatment alone. C57BL/6J mice were hindlimb-immobilized for two weeks, with injections of vehicle, ucOC (90ng/g daily) and/or IBN (2μg/g weekly). Insulin/oral glucose tolerance tests (ITT/OGTT) were performed. Immediately after immobilization, muscles (extensor digitorum longus (EDL), soleus, tibialis anterior, gastrocnemius and quadriceps) were isolated and measured for muscle mass. Insulin-stimulated glucose uptake (EDL and soleus) was examined. Phosphorylation/expression of proteins in anabolic/catabolic pathways were examined in quadriceps. Primary human myotubes derived from older adult muscle biopsies were treated with ucOC and/or IBN, then signalling proteins were analysed. Combined treatment, but not individual treatments, significantly increased the muscle weight/body weight ratio in immobilized soleus (31.7%; P=0.013) and quadriceps (20.0%; P=0.0008) muscles, concomitant with elevated p-Akt (S473)/Akt ratio (P=0.0047). Combined treatment also enhanced whole-body glucose tolerance (16.6%; P=0.0011). In human myotubes, combined treatment stimulated greater activation of ERK1/2 (P=0.0067 and 0.0072) and mTOR (P=0.036), and led to a lesser expression of Fbx32 (P=0.049) and MuRF1 (P=0.048) than individual treatments. These findings suggest a potential therapeutic role for the ucOC and bisphosphonates combination in protecting against muscle wasting induced by immobilization and ageing. KEY POINTS: It has been suggested that undercarboxylated osteocalcin (ucOC) improves muscle mass and glucose metabolism. Bisphosphonates, an anti-osteoporosis treatment, might protect against muscle wasting independent of ucOC. The combination treatment of ucOC and ibandronate was shown to exert a greater therapeutic effect against immobilization-induced muscle wasting, and led to greater activation of anabolic pathway and less expression of catabolic signalling proteins in myotubes derived from older adults, compared with individual treatments. The combination treatment was found to improve whole-body glucose tolerance. Our findings suggest a potential therapeutic role for the ucOC and bisphosphonates combination in protecting against muscle wasting induced by immobilization and ageing.

Aiding Cancer's "Sweet Tooth": Role of Hexokinases in Metabolic Reprogramming.

Hexokinases (HKs) convert hexose sugars to hexose-6-phosphate, thus trapping them inside cells to meet the synthetic and energetic demands. HKs participate in various standard and altered physiological processes, including cancer, primarily through the reprogramming of cellular metabolism. Four canonical HKs have been identified with different expression patterns across tissues. HKs 1-3 play a role in glucose utilization, whereas HK 4 (glucokinase, GCK) also acts as a glucose sensor. Recently, a novel fifth HK, hexokinase domain containing 1 (HKDC1), has been identified, which plays a role in whole-body glucose utilization and insulin sensitivity. Beyond the metabolic functions, HKDC1 is differentially expressed in many forms of human cancer. This review focuses on the role of HKs, particularly HKDC1, in metabolic reprogramming and cancer progression.

Tilorone increases glucose uptake in vivo and in skeletal muscle cells by enhancing Akt2/AS160 signaling and glucose transporter levels.

Skeletal muscle plays a major role in whole-body glucose metabolism. Insulin resistance in skeletal muscle is characterized by decreased insulin-stimulated glucose uptake resulting from impaired intracellular trafficking and decreased glucose transporter 4 (GLUT4) expression. In this study, we illustrated that tilorone, a low-molecular-weight antiviral agent, improves glucose uptake in vitro and in vivo. Tilorone increased bone morphogenetic protein (BMP) signaling in C2C12 myoblasts, the transcription of multiple BMPs (BMP2, BMP4, BMP7, and BMP14), Smad4 expression, and the phosphorylation of BMP-mediated Smad1/5/8. The activation of Akt2/AS160(TBC1D4) signaling, the critical regulator of GLUT4 translocation, was also increased, as well as the levels of GLUT4 and GLUT1, leading to enhanced uptake of the radioactively labeled glucose analog 18 F-fluoro-2-deoxyglucose (18 FDG). However, this excess glucose content did not result in increased ATP formation by mitochondrial respiration; both basal and ATP-linked respiration were diminished, thereby contributing to the induction of AMPK. In differentiated myotubes, AS160 phosphorylation and 18 FDG uptake also increased. Moreover, tilorone administration further increased insulin-stimulated phosphorylation of Akt2 and glucose uptake of myotubes indicating an insulin-sensitizing effect. Importantly, during in vivo experiments, the systemic administration of tilorone resulted in increased 18 FDG uptake of skeletal muscle, liver, and adipose tissue in C57BL/6 mice. Our results provide new perspectives for the treatment of type 2 diabetes, which has a limited number of treatments that regulate protein expression or translocation.

Deuterium body array for the simultaneous measurement of hepatic and renal glucose metabolism and gastric emptying with dynamic 3D deuterium metabolic imaging at 7 T.

Deuterium metabolic imaging (DMI) is a novel noninvasive method to assess tissue metabolism and organ (patho)physiology in vivo using deuterated substrates, such as [6,6'-2 H2 ]-glucose. The liver and kidneys play a central role in whole-body glucose homeostasis, and in type 2 diabetes, both hepatic and renal glucose metabolism are dysregulated. Diabetes is also associated with gastric emptying abnormalities. In this study, we developed a four-channel 2 H transmit/receive body array coil for DMI in the human abdomen at 7T and assessed its performance. In addition, the feasibility of simultaneously measuring gastric emptying, and hepatic and renal glucose uptake and metabolism with dynamic 3D DMI upon administration of deuterated glucose, was investigated. Simulated and measured B1 + patterns were in good agreement. The intrasession variability of the natural abundance deuterated water signal in the liver and right kidney, measured in nine healthy volunteers, was 5.6% ± 0.9% and 4.9% ± 0.7%, respectively. Dynamic 3D DMI scans with oral administration of [6,6'-2 H2 ]-glucose showed similar kinetics of deuterated glucose appearance and disappearance in the liver and kidney. The measured gastric emptying half time was 80 ± 10min, which is in good agreement with scintigraphy measurements. In conclusion, DMI with oral administration of [6,6'-2 H2 ]-glucose enables simultaneous assessment of gastric emptying and liver and kidney glucose uptake and metabolism. When applied in patients with diabetes, this approach may advance our understanding of the interplay between disturbances in liver and kidney glucose uptake and metabolism and gastric emptying, at a detail that cannot be achieved by any other method.

Lack of adipocyte IP3R1 reduces diet-induced obesity and greatly improves whole-body glucose homeostasis

The normal function of skeletal muscle and adipose tissue ensures whole-body glucose homeostasis. Ca2+ release channel inositol 1,4,5-trisphosphate receptor 1 (IP3R1) plays a vital role in regulating diet-induced obesity and disorders, but its functions in peripheral tissue regulating glucose homeostasis remain unexplored. In this study, mice with Ip3r1 specific knockout in skeletal muscle or adipocytes were used for investigating the mediatory role of IP3R1 on whole-body glucose homeostasis under normal or high-fat diet. We reported that IP3R1 expression levels were increased in the white adipose tissue and skeletal muscle of diet-induced obese mice. Ip3r1 knockout in skeletal muscle improved glucose tolerance and insulin sensitivity of mice on a normal chow diet, but worsened insulin resistance in diet-induced obese mice. These changes were associated with the reduced muscle weight and compromised Akt signaling activation. Importantly, Ip3r1 deletion in adipocytes protected mice from diet-induced obesity and glucose intolerance, mainly due to the enhanced lipolysis and AMPK signaling pathway in the visceral fat. In conclusion, our study demonstrates that IP3R1 in skeletal muscle and adipocytes exerts divergent effects on systemic glucose homeostasis, and characterizes adipocyte IP3R1 as a promising target for treating obesity and type 2 diabetes.

Abstract 25: Long-Term Trends of BMI During Childhood and Adolescence Modify the Association of DNA Methylation Change at TXNIP With Glucose Change During Midlife

Background: Thioredoxin-interacting protein (TXNIP) is a central master regulator of whole-body glucose homeostasis in humans; and DNA methylation (DNAm) at TXNIP has been identified to be related to diabetes. It remains unknown whether dynamic changes in DNAm at TXNIP are associated with glycemic changes; and whether early-life adiposity changes modify such association. Hypothesis: We hypothesized that dynamic changes in DNAm at TXNIP are associated with glycemic changes in midlife, and early-life adiposity changes may modify such association. Methods: A total of 594 Bogalusa Heart Study participants who had DNAm measurements at two time points in midlife were included in this study. Of them, 353 participants had at least 4 examinations of BMI during childhood and adolescence (aged 4–19 years). The area under the curve (AUC) was calculated as a measure of long-term burden (total AUC) and trends (incremental AUC) of BMI during childhood and adolescence. Results: We found 25.3% of participants showed greater than 5% DNAm change at TXNIP during a mean of 6.2 years follow-up. Increase in DNAm at TXNIP was significantly associated with decrease in FPG independent of cell-type composition and other covariates ( P &lt;0.001). Moreover, we found the increasing trend of BMI during childhood and adolescence significantly modified the above dynamic association. Each 1% increase in DNAm at TXNIP was associated with 2.90 (0.77) mg/dl decrease in FPG among participants with the highest tertile of incremental AUC of BMI, 0.96 (0.38) mg/dl decrease among those with middle tertile, whereas no significant association was observed among participants with the lowest tertile ( P -interaction=0.003). Conclusions: Our results indicate that changes in DNAm at TXNIP are significantly associated with dynamics in FPG in midlife, and such association is modified by BMI changes during childhood and adolescence.

Nutrient Uptake by the Hindlimb of Growing Pigs Treated with Porcine Somatotropin and Insulin

The shift in nutrient partitioning induced by porcine somatotropin (pST) is accompanied by a decrease in insulin sensitivity for whole-body glucose uptake. The relative contribution of metabolic changes in the hindlimb was investigated in eight pigs (55 kg) that had received recombinant pST (120 μg/kg) or excipient (control) for 7 d. Uptake of metabolites by the hindlimb was measured under basal conditions and during hyperinsulinemic-euglycemic clamps at low [14 ng/(kg·min)] and high [360 ng/(kg·min)] insulin infusion rates. Dextrose infusion rate required to maintain euglycemia was used as an index of whole-body glucose uptake in response to exogenous insulin. Effects of pST on hindlimb and whole-body glucose uptake were evident only at physiological levels of insulin (basal and low insulin infusion rate). During the low rate of insulin infusion, dextrose infusion rate was 79% lower for pST-treated pigs and glucose uptake by the hindlimb was 59% lower compared with control pigs. The decrease in glucose uptake by the hindlimb was entirely accounted for by the estimated reduction in glucose utilization by adipose tissue of the hindlimb. Glucose:oxygen quotients were reduced during basal (57%) and low insulin infusion (63%) with pST treatment, indicating a change in the pattern of substrate utilization. This is consistent with the concept that pST directs nutrients away from adipose and towards muscle growth by altering the response of tissues to homeostatic signals such as insulin.

E-DES-PROT: A novel computational model to describe the effects of amino acids and protein on postprandial glucose and insulin dynamics in humans

Current computational models of whole-body glucose homeostasis describe physiological processes by which insulin regulates circulating glucose concentrations. While these models perform well in response to oral glucose challenges, interaction with other nutrients that impact postprandial glucose metabolism, such as amino acids (AAs), is not considered. Here, we developed a computational model of the human glucose-insulin system, which incorporates the effects of AAs on insulin secretion and hepatic glucose production. This model was applied to postprandial glucose and insulin time-series data following different AA challenges (with and without co-ingestion of glucose), dried milk protein ingredients, and dairy products. Our findings demonstrate that this model allows accurate description of postprandial glucose and insulin dynamics and provides insight into the physiological processes underlying meal responses. This model may facilitate the development of computational models that describe glucose homeostasis following the intake of multiple macronutrients, while capturing relevant features of an individual's metabolic health.

The impact of physical inactivity on glucose homeostasis when diet is adjusted to maintain energy balance in healthy, young males

It is unclear if dietary adjustments to maintain energy balance during reduced physical activity can offset inactivity-induced reductions in insulin sensitivity and glucose disposal to produce normal daily glucose concentrations and meal responses. Therefore, the aim of the present study was to examine the impact of long-term physical inactivity (60 days of bed rest) on daily glycemia when in energy balance. Interstitial glucose concentrations were measured using Continuous Glucose Monitoring Systems (CGMS) for 5 days before and towards the end of bed rest in 20 healthy, young males (Age: 34±8 years; BMI: 23.5±1.8kg/m2). Energy intake was reduced during bed rest to match energy expenditure, but the types of foods and timing of meals was maintained. Fasting venous glucose and insulin concentrations were determined, as well as the change in whole-body glucose disposal using a hyperinsulinemic-euglycemic clamp (HIEC). Following long-term bed rest, fasting plasma insulin concentration increased 40% (p=0.004) and glucose disposal during the HIEC decreased 24% (p<0.001). Interstitial daily glucose total area under the curve (tAUC) from pre-to post-bed rest increased on average by 6% (p=0.041), despite a 20 and 25% reduction in total caloric and carbohydrate intake, respectively. The nocturnal period (00:00-06:00) showed the greatest change to glycemia with glucose tAUC for this period increasing by 9% (p=0.005). CGMS measures of daily glycemic variability (SD, J-Index, M-value and MAG) were not changed during bed rest. Reduced physical activity (bed rest) increases glycemia even when daily energy intake is reduced to maintain energy balance. However, the disturbance to daily glucose homeostasis was much more modest than the reduced capacity to dispose of glucose, and glycemic variability was not negatively affected by bed rest, likely due to positive mitigating effects from the contemporaneous reduction in dietary energy and carbohydrate intake. NCT03594799 (registered July 20, 2018) (https://clinicaltrials.gov/ct2/show/NCT03594799).

Cold exposure alters lipid metabolism of skeletal muscle through HIF-1α-induced mitophagy

BackgroundIn addition to its contractile properties and role in movement, skeletal muscle plays an important function in regulating whole-body glucose and lipid metabolism. A central component of such regulation is mitochondria, whose quality and function are essential in maintaining proper metabolic homeostasis, with defects in processes such as autophagy and mitophagy involved in mitochondria quality control impairing skeletal muscle mass and function, and potentially leading to a number of associated diseases. Cold exposure has been reported to markedly induce metabolic remodeling and enhance insulin sensitivity in the whole body by regulating mitochondrial biogenesis. However, changes in lipid metabolism and lipidomic profiles in skeletal muscle in response to cold exposure are unclear. Here, we generated lipidomic or transcriptome profiles of mouse skeletal muscle following cold induction, to dissect the molecular mechanisms regulating lipid metabolism upon acute cold treatment.ResultsOur results indicated that short-term cold exposure (3 days) can lead to a significant increase in intramuscular fat deposition. Lipidomic analyses revealed that a cold challenge altered the overall lipid composition by increasing the content of triglyceride (TG), lysophosphatidylcholine (LPC), and lysophosphatidylethanolamine (LPE), while decreasing sphingomyelin (SM), validating lipid remodeling during the cold environment. In addition, RNA-seq and qPCR analysis showed that cold exposure promoted the expression of genes related to lipolysis and fatty acid biosynthesis. These marked changes in metabolic effects were associated with mitophagy and muscle signaling pathways, which were accompanied by increased TG deposition and impaired fatty acid oxidation. Mechanistically, HIF-1α signaling was highly activated in response to the cold challenge, which may contribute to intramuscular fat deposition and enhanced mitophagy in a cold environment.ConclusionsOverall, our data revealed the adaptive changes of skeletal muscle associated with lipidomic and transcriptomic profiles upon cold exposure. We described the significant alterations in the composition of specific lipid species and expression of genes involved in glucose and fatty acid metabolism. Cold-mediated mitophagy may play a critical role in modulating lipid metabolism in skeletal muscle, which is precisely regulated by HIF-1α signaling.

Protein kinase D1 (Prkd1) deletion in brown adipose tissue leads to altered myogenic gene expression after cold exposure, while thermogenesis remains intact.

Brown adipose tissue (BAT) has in recent times been rediscovered in adult humans, and together with work from preclinical models, has shown to have the potential of providing a variety of positive metabolic benefits. These include lower plasma glucose, improved insulin sensitivity, and reduced susceptibility to obesity and its comorbidities. As such, its continued study could offer insights to therapeutically modulate this tissue to improve metabolic health. It has been reported that adipose-specific deletion of the gene for protein kinase D1 (Prkd1) in mice enhances mitochondrial respiration and improves whole-body glucose homeostasis. We sought to determine whether these effects were mediated specifically through brown adipocytes using a Prkd1 brown adipose tissue (BAT) Ucp1-Cre-specific knockout mouse model, Prkd1BKO . We unexpectedly observed that upon both cold exposure and β3 -AR agonist administration, Prkd1 loss in BAT did not alter canonical thermogenic gene expression or adipocyte morphology. We took an unbiased approach to assess whether other signaling pathways were affected. RNA from cold-exposed mice was subjected to RNA-Seq analysis. These studies revealed that myogenic gene expression is altered in Prkd1BKO BAT after both acute and extended cold exposure. Given that brown adipocytes and skeletal myocytes share a common precursor cell lineage expressing myogenic factor 5 (Myf5), these data suggest that loss of Prkd1 in BAT may alter the biology of mature brown adipocytes and preadipocytes in this depot. The data presented herein clarify the role of Prkd1 in BAT thermogenesis and present new avenues for the further study of Prkd1 function in BAT.

Loss of skeletal muscle estrogen-related receptors leads to severe exercise intolerance

ObjectiveSkeletal muscle oxidative capacity is central to physical activity, exercise capacity and whole-body metabolism. The three estrogen-related receptors (ERRs) are regulators of oxidative metabolism in many cell types, yet their roles in skeletal muscle remain unclear. The main aim of this study was to compare the relative contributions of ERRs to oxidative capacity in glycolytic and oxidative muscle, and to determine defects associated with loss of skeletal muscle ERR function. MethodsWe assessed ERR expression, generated mice lacking one or two ERRs specifically in skeletal muscle and compared the effects of ERR loss on the transcriptomes of EDL (predominantly glycolytic) and soleus (oxidative) muscles. We also determined the consequences of the loss of ERRs for exercise capacity and energy metabolism in mice with the most severe loss of ERR activity. ResultsERRs were induced in human skeletal muscle in response to an exercise bout. Mice lacking both ERRα and ERRγ (ERRα/γ dmKO) had the broadest and most dramatic disruption in skeletal muscle gene expression. The most affected pathway was “mitochondrial function”, in particular Oxphos and TCA cycle genes, and transcriptional defects were more pronounced in the glycolytic EDL than the oxidative soleus. Mice lacking ERRβ and ERRγ, the two isoforms expressed highly in oxidative muscles, also exhibited defects in lipid and branch chain amino acid metabolism genes, specifically in the soleus. The pronounced disruption of oxidative metabolism in ERRα/γ dmKO mice led to pale muscles, decreased oxidative capacity, histochemical patterns reminiscent of minicore myopathies, and severe exercise intolerance, with the dmKO mice unable to switch to lipid utilization upon running. ERRα/γ dmKO mice showed no defects in whole-body glucose and energy homeostasis. ConclusionsOur findings define gene expression programs in skeletal muscle that depend on different combinations of ERRs, and establish a central role for ERRs in skeletal muscle oxidative metabolism and exercise capacity. Our data reveal a high degree of functional redundancy among muscle ERR isoforms for the protection of oxidative capacity, and show that ERR isoform-specific phenotypes are driven in part, but not exclusively, by their relative levels in different muscles.