Chemical Hybridization of Glucagon and Thyroid Hormone Optimizes Therapeutic Impact for Metabolic Disease.

Glucagon and thyroid hormone (T3) exhibit therapeutic potential for metabolic disease but also exhibit undesired effects. We achieved synergistic effects of these two hormones and mitigation of their adverse effects by engineering chemical conjugates enabling delivery of both activities within one precisely targeted molecule. Coordinated glucagon and T3 actions synergize to correct hyperlipidemia, steatohepatitis, atherosclerosis, glucose intolerance, and obesity in metabolically compromised mice. We demonstrate that each hormonal constituent mutually enriches cellular processes in hepatocytes and adipocytes via enhanced hepatic cholesterol metabolism and white fat browning. Synchronized signaling driven by glucagon and T3 reciprocally minimizes the inherent harmful effects of each hormone. Liver-directed T3 action offsets the diabetogenic liability of glucagon, and glucagon-mediated delivery spares the cardiovascular system from adverse T3 action. Our findings support the therapeutic utility of integrating these hormones into a single molecular entity that offers unique potential for treatment of obesity, type 2 diabetes, and cardiovascular disease.

Metabolic Effects of Medium-Chain Triacylglycerol Consumption are Preserved in Obesity.

Several health beneficial effects are associated with intake of medium-chain triacylglycerol (MCT), however, the underlying mechanisms are unknown. Furthermore, it remains uncertain whether the acute metabolic effects of MCT differ between lean individuals and individuals with obesity - and whether these effects are sustained following chronic intake. This study aimed to elucidate the postprandial physiological and metabolic effects of MCT before and after eight days intake compared to intake of energy-matched triacylglycerol consisting of long-chain fatty acids (LCT) using a randomized cross-over design in lean individuals (n=8) and individuals with obesity (n=8). The study revealed that consumption of MCT increased ketogenesis and metabolic rate, while lowering blood glucose levels over five hours. The hypoglycemic action of MCT intake was accompanied by a concomitant transient increase in plasma insulin and glucagon levels. Interestingly, the effects on ketogenesis, metabolic rate, and glycemia were preserved in individuals with obesity and sustained after eight days of daily supplementation. Lipidomic plasma analysis in lean individuals (n=4) showed that a part of the ingested MCT bypasses the liver and entered the systemic circulation as medium-chain fatty acids (MCFA). The findings suggest that MCFA, along with ketone bodies from the liver, may act as signaling molecules and/or substrates in the peripheral tissues, thereby contributing to the effects of MCT intake. In summary, these findings underscore the health benefits of MCT in metabolically compromised individuals after daily supplementation. Moreover, we uncover novel aspects of MCFA biology, providing insights into how these fatty acids orchestrate physiological effects in humans.

Skeletal muscle and intermuscular adipose tissue gene expression profiling identifies new biomarkers with prognostic significance for insulin resistance progression and intervention response.

AIMS/HYPOTHESIS: Although insulin resistance often leads to type 2 diabetes mellitus, its early stages are often unrecognised, thus reducing the probability of successful prevention and intervention. Moreover, treatment efficacy is affected by the genetics of the individual. We used gene expression profiles from a cross-sectional study to identify potential candidate genes for the prediction of diabetes risk and intervention response. METHODS: Using a multivariate regression model, we linked gene expression profiles of human skeletal muscle and intermuscular adipose tissue (IMAT) to fasting glucose levels and glucose infusion rate. Based on the expression patterns of the top predictive genes, we characterised and compared individual gene expression with clinical classifications using k-nearest neighbour clustering. The predictive potential of the candidate genes identified was validated using muscle gene expression data from a longitudinal intervention study. RESULTS: We found that genes with a strong association with clinical measures clustered into three distinct expression patterns. Their predictive values for insulin resistance varied substantially between skeletal muscle and IMAT. Moreover, we discovered that individual gene expression-based classifications may differ from classifications based predominantly on clinical variables, indicating that participant stratification may be imprecise if only clinical variables are used for classification. Of the 15 top candidate genes, ST3GAL2, AASS, ARF1 and the transcription factor SIN3A are novel candidates for predicting a refined diabetes risk and intervention response. CONCLUSION/INTERPRETATION: Our results confirm that disease progression and successful intervention depend on individual gene expression states. We anticipate that our findings may lead to a better understanding and prediction of individual diabetes risk and may help to develop individualised intervention strategies.

Cross-species comparison of pregnancy-induced GDF15.

Growth differentiation factor 15 (GDF15) is a stress-induced cytokine. Although the exact physiological function of GDF15 is not yet fully comprehended, the significant elevation of circulating GDF15 levels during gestation suggests a potential role for this hormone in pregnancy. This is corroborated by genetic association studies in which GDF15 and the GDF15 receptor, GDNF family receptor alpha like (GFRAL) have been linked to morning sickness and hyperemesis gravidarum (HG) in humans. Here, we studied GDF15 biology during pregnancy in mice, rats, macaques, and humans. In contrast to macaques and humans, mice and rats exhibited an underwhelming induction in plasma GDF15 levels in response to pregnancy (∼75-fold increase in macaques vs. ∼2-fold increase in rodents). The changes in circulating GDF15 levels were corroborated by the magnitude of Gdf15 mRNA and GDF15 protein expression in placentae from mice, rats, and macaques. These species-specific findings may help guide future studies focusing on GDF15 in pregnancy and on the evaluation of pharmacological strategies to interfere with GDF15-GFRAL signaling to treat severe nausea and HG.NEW & NOTEWORTHY In the present study pregnancy-induced changes in circulating growth differentiation factor 15 (GDF15) in rodents, rhesus macaques, and humans are mapped. In sum, it is demonstrated that humans and macaques exhibit a tremendous increase in placental and circulating GDF15 during pregnancy. In contrast, GDF15 is negligibly increased in pregnant mice and rats, questioning a physiological role for GDF15 in pregnancy in rodents.

Genes controlling skeletal muscle glucose uptake and their regulation by endurance and resistance exercise.

Exercise improves the insulin sensitivity of glucose uptake in skeletal muscle. Due to that, exercise has become a cornerstone treatment for type 2 diabetes mellitus (T2DM). The mechanisms by which exercise improves skeletal muscle insulin sensitivity are, however, incompletely understood. We conducted a systematic review to identify all genes whose gain or loss of function alters skeletal muscle glucose uptake. We subsequently cross-referenced these genes with recently generated data sets on exercise-induced gene expression and signaling. Our search revealed 176 muscle glucose-uptake genes, meaning that their genetic manipulation altered glucose uptake in skeletal muscle. Notably, exercise regulates the expression or phosphorylation of more than 50% of the glucose-uptake genes or their protein products. This included many genes that previously have not been associated with exercise-induced insulin sensitivity. Interestingly, endurance and resistance exercise triggered some common but mostly unique changes in expression and phosphorylation of glucose-uptake genes or their protein products. Collectively, our work provides a resource of potentially new molecular effectors that play a role in the incompletely understood regulation of muscle insulin sensitivity by exercise.

Exercise increases phosphorylation of the putative mTORC2 activity readout NDRG1 in human skeletal muscle.

In mice, exercise is suggested to activate the mechanistic target of rapamycin complex 2 (mTORC2) in skeletal muscle, and mTORC2 is required for normal muscle glucose uptake during exercise. Whether this translates to human skeletal muscle and what signaling pathways facilitate the exercise-induced mTORC2 activation is unknown. We herein tested the hypothesis that exercise increases mTORC2 activity in human skeletal muscle and investigated if ß2-adrenergic receptor (AR) activation mediates exercise-induced mTORC2 activation. We examined several mTORC2 activity readouts (p-NDRG1 Thr346, p-Akt Ser473, p-mTOR S2481, and p-Akt Thr450) in human skeletal muscle biopsies after uphill walking or cycling exercise. In mouse muscles, we assessed mTORC2 activity readouts following acute activation of muscle ß2-adrenergic or GS signaling and during in vivo and ex vivo muscle contractions. Exercise increased phosphorylation of NDRG1 Thr346 in human soleus, gastrocnemius, and vastus lateralis muscle, without changing p-Akt Ser473, p-Akt Thr450, and p-mTOR Ser2481. In mouse muscle, stimulation of ß2-adrenergic or GS signaling and ex vivo contractions failed to increase p-NDRG1 Thr346, whereas in vivo contractions were sufficient to induce p-NDRG1 Thr346. In conclusion, the mTORC2 activity readout p-NDRG1 Thr346 is a novel exercise-responsive signaling protein in human skeletal muscle. Notably, contraction-induced p-NDRG1 Thr346 appears to require a systemic factor. Unlike exercise, and in contrast to published data obtained in cultured muscles cells, stimulation of ß2-adrenergic signaling is not sufficient to trigger NDRG1 phosphorylation in mature mouse skeletal muscle.NEW & NOTEWORTHY The mTORC2 readout p-NDRG Thr346 is a novel exercise-responsive protein in human skeletal muscle. ß2-AR and GS signaling are not sufficient to induce mTORC2 signaling in adult muscle. In vivo, but not ex vivo, contraction induced p-NDRG Thr346, which indicates requirement of a systemic factor for exercise-induced mTORC2 activation.

Transcriptional programming of lipid and amino acid metabolism by the skeletal muscle circadian clock.

Circadian clocks are fundamental physiological regulators of energy homeostasis, but direct transcriptional targets of the muscle clock machinery are unknown. To understand how the muscle clock directs rhythmic metabolism, we determined genome-wide binding of the master clock regulators brain and muscle ARNT-like protein 1 (BMAL1) and REV-ERBα in murine muscles. Integrating occupancy with 24-hr gene expression and metabolomics after muscle-specific loss of BMAL1 and REV-ERBα, here we unravel novel molecular mechanisms connecting muscle clock function to daily cycles of lipid and protein metabolism. Validating BMAL1 and REV-ERBα targets using luciferase assays and in vivo rescue, we demonstrate how a major role of the muscle clock is to promote diurnal cycles of neutral lipid storage while coordinately inhibiting lipid and protein catabolism prior to awakening. This occurs by BMAL1-dependent activation of Dgat2 and REV-ERBα-dependent repression of major targets involved in lipid metabolism and protein turnover (MuRF-1, Atrogin-1). Accordingly, muscle-specific loss of BMAL1 is associated with metabolic inefficiency, impaired muscle triglyceride biosynthesis, and accumulation of bioactive lipids and amino acids. Taken together, our data provide a comprehensive overview of how genomic binding of BMAL1 and REV-ERBα is related to temporal changes in gene expression and metabolite fluctuations.

Plasma proteome profiles treatment efficacy of incretin dual agonism in diet-induced obese female and male mice.

AIMS: Unimolecular peptides targeting the receptors for glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) (GLP-1/GIP co-agonist) have been shown to outperform each single peptide in the treatment of obesity and cardiometabolic disease in preclinical and clinical trials. By combining physiological treatment endpoints with plasma proteomic profiling (PPP), we aimed to identify biomarkers to advance non-invasive metabolic monitoring of compound treatment success and exploration of ulterior treatment effects on an individual basis. MATERIALS AND METHODS: We performed metabolic phenotyping along with PPP in body weight-matched male and female diet-induced obese (DIO) mice treated for 21 days with phosphate-buffered saline, single GIP and GLP-1 mono-agonists, or a GLP-1/GIP co-agonist. RESULTS: GLP-1R/GIPR co-agonism improved obesity, glucose intolerance, non-alcoholic fatty liver disease (NAFLD) and dyslipidaemia with superior efficacy in both male and female mice compared with mono-agonist treatments. PPP revealed broader changes of plasma proteins after GLP-1/GIP co-agonist compared with mono-agonist treatments in both sexes, including established and potential novel biomarkers for systemic inflammation, NAFLD and atherosclerosis. Subtle sex-specific differences have been observed in metabolic phenotyping and PPP. CONCLUSIONS: We herein show that a recently developed unimolecular GLP-1/GIP co-agonist is more efficient in improving metabolic disease than either mono-agonist in both sexes. PPP led to the identification of a sex-independent protein panel with the potential to monitor non-invasively the treatment efficacies on metabolic function of this clinically advancing GLP-1/GIP co-agonist.

Glucometabolic consequences of acute and prolonged inhibition of fatty acid oxidation.

Excessive circulating FAs have been proposed to promote insulin resistance (IR) of glucose metabolism by increasing the oxidation of FAs over glucose. Therefore, inhibition of FA oxidation (FAOX) has been suggested to ameliorate IR. However, prolonged inhibition of FAOX would presumably cause lipid accumulation and thereby promote lipotoxicity. To understand the glycemic consequences of acute and prolonged FAOX inhibition, we treated mice with the carnitine palmitoyltransferase 1 (CPT-1) inhibitor, etomoxir (eto), in combination with short-term 45% high fat diet feeding to increase FA availability. Eto acutely increased glucose oxidation and peripheral glucose disposal, and lowered circulating glucose, but this was associated with increased circulating FAs and triacylglycerol accumulation in the liver and heart within hours. Several days of FAOX inhibition by daily eto administration induced hepatic steatosis and glucose intolerance, specific to CPT-1 inhibition by eto. Lower whole-body insulin sensitivity was accompanied by reduction in brown adipose tissue (BAT) uncoupling protein 1 (UCP1) protein content, diminished BAT glucose clearance, and increased hepatic glucose production. Collectively, these data suggest that pharmacological inhibition of FAOX is not a viable strategy to treat IR, and that sufficient rates of FAOX are required for maintaining liver and BAT metabolic function.

Pharmacological targeting of α3ß4 nicotinic receptors improves peripheral insulin sensitivity in mice with diet-induced obesity.

AIMS/HYPOTHESIS: Treatment with the α3ß4 nicotinic acetylcholine receptor (nAChR) agonist, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), improves glucose tolerance in diet-induced obese (DIO) mice, but the physiological and molecular mechanisms are unknown. METHODS: DMPP (10 mg/kg body weight, s.c.) was administered either in a single injection (acute) or daily for up to 14 days (chronic) in DIO wild-type (WT) and Chrnb4 knockout (KO) mice and glucose tolerance, tissue-specific tracer-based glucose metabolism, and insulin signalling were assessed. RESULTS: In WT mice, but not in Chrnb4 KO mice, single acute treatment with DMPP induced transient hyperglycaemia, which was accompanied by high plasma adrenaline (epinephrine) levels, upregulated hepatic gluconeogenic genes, and decreased hepatic glycogen content. In contrast to these acute effects, chronic DMPP treatment in WT mice elicited improvements in glucose tolerance already evident after three consecutive days of DMPP treatment. After seven days of DMPP treatment, glucose tolerance was markedly improved, also in comparison with mice that were pair-fed to DMPP-treated mice. The glycaemic benefit of chronic DMPP was absent in Chrnb4 KO mice. Chronic DMPP increased insulin-stimulated glucose clearance into brown adipose tissue (+69%), heart (+93%), gastrocnemius muscle (+74%) and quadriceps muscle (+59%), with no effect in white adipose tissues. After chronic DMPP treatment, plasma adrenaline levels did not increase following an injection with DMPP. In glucose-stimulated skeletal muscle, we detected a decreased phosphorylation of the inhibitory Ser640 phosphorylation site on glycogen synthase and a congruent increase in glycogen accumulation following chronic DMPP treatment. CONCLUSIONS/INTERPRETATION: Our data suggest that DMPP acutely induces adrenaline release and hepatic glycogenolysis, while chronic DMPP-mediated activation of ß4-containing nAChRs improves peripheral insulin sensitivity independently of changes in body weight via mechanisms that could involve increased non-oxidative glucose disposal into skeletal muscle.

Effect of bariatric surgery on plasma GDF15 in humans.

Bariatric surgery results in marked body weight loss and improves type 2 diabetes in most patients with obesity. The growth differentiation factor 15 (GDF15) has recently emerged as a novel satiety factor. To begin to understand whether GDF15 is involved in mediating the effects of bariatric surgery on body weight and glycemia in humans, we measured plasma GDF15 in patients with obesity ( n = 25) and in patients with obesity and diabetes ( n = 22) before and after Roux-en-Y gastric bypass (RYGB) surgery. GDF15 was increased 1 wk after RYGB compared with before surgery (689 ± 45 vs. 487 ± 28 pg/ml, P < 0.001) and GDF15 remained elevated at 3 mo (554 ± 37 pg/ml, P < 0.05), at 1 yr (566 ± 37 pg/ml, P < 0.05), and at 2.5-4 yr (630 ± 50 pg/ml, P < 0.001) after RYGB surgery. Both age and insulin sensitivity correlated with GDF15 before the surgery ( r = 0.46, P < 0.0001 and r = 0.34, P < 0.001, respectively). These correlations disappeared at 2.5-4 yr following the surgery. Conversely, weight loss magnitude correlated with GDF15, measured 2.5-4 yr postsurgery ( r = 0.21, P < 0.0055). In summary, circulating GDF15 increases and correlates with body weight loss following RYGB surgery.

Glucagon Regulation of Energy Expenditure.

Glucagon's ability to increase energy expenditure has been known for more than 60 years, yet the mechanisms underlining glucagon's thermogenic effect still remain largely elusive. Over the last years, significant efforts were directed to unravel the physiological and cellular underpinnings of how glucagon regulates energy expenditure. In this review, we summarize the current knowledge on how glucagon regulates systems metabolism with a special emphasis on its acute and chronic thermogenic effects.

Mammalian target of rapamycin complex 2 regulates muscle glucose uptake during exercise in mice.

KEY POINTS: Exercise is a potent physiological stimulus to clear blood glucose from the circulation into skeletal muscle. The mammalian target of rapamycin complex 2 (mTORC2) is an important regulator of muscle glucose uptake in response to insulin stimulation. Here we report for the first time that the activity of mTORC2 in mouse muscle increases during exercise. We further show that glucose uptake during exercise is decreased in mouse muscle that lacks mTORC2 activity. We also provide novel identifications of new mTORC2 substrates during exercise in mouse muscle. ABSTRACT: Exercise increases glucose uptake into insulin-resistant muscle. Thus, elucidating the exercise signalling network in muscle may uncover new therapeutic targets. The mammalian target of rapamycin complex 2 (mTORC2), a regulator of insulin-controlled glucose uptake, has been reported to interact with ras-related C3 botulinum toxin substrate 1 (Rac1), which plays a role in exercise-induced glucose uptake in muscle. Therefore, we tested the hypothesis that mTORC2 activity is necessary for muscle glucose uptake during treadmill exercise. We used mice that specifically lack mTORC2 signalling in muscle by deletion of the obligatory mTORC2 component Rictor (Ric mKO). Running capacity and running-induced changes in blood glucose, plasma lactate and muscle glycogen levels were similar in wild-type (Ric WT) and Ric mKO mice. At rest, muscle glucose uptake was normal, but during running muscle glucose uptake was reduced by 40% in Ric mKO mice compared to Ric WT mice. Running increased muscle phosphorylated 5' AMP-activated protein kinase (AMPK) similarly in Ric WT and Ric mKO mice, and glucose transporter type 4 (GLUT4) and hexokinase II (HKII) protein expressions were also normal in Ric mKO muscle. The mTORC2 substrate, phosphorylated protein kinase C α (PKCα), and the mTORC2 activity readout, phosphorylated N-myc downstream regulated 1 (NDRG1) protein increased with running in Ric WT mice, but were not altered by running in Ric mKO muscle. Quantitative phosphoproteomics uncovered several additional potential exercise-dependent mTORC2 substrates, including contractile proteins, kinases, transcriptional regulators, actin cytoskeleton regulators and ion-transport proteins. Our study suggests that mTORC2 is a component of the exercise signalling network that regulates muscle glucose uptake and we provide a resource of new potential members of the mTORC2 signalling network.

Rac1 governs exercise-stimulated glucose uptake in skeletal muscle through regulation of GLUT4 translocation in mice.

KEY POINT: Exercise increases skeletal muscle energy turnover and one of the important substrates for the working muscle is glucose taken up from the blood. The GTPase Rac1 can be activated by muscle contraction and has been found to be necessary for insulin-stimulated glucose uptake, although its role in exercise-stimulated glucose uptake is unknown. We show that Rac1 regulates the translocation of the glucose transporter GLUT4 to the plasma membrane in skeletal muscle during exercise. We find that Rac1 knockout mice display significantly reduced glucose uptake in skeletal muscle during exercise. ABSTRACT: Exercise increases skeletal muscle energy turnover and one of the important substrates for the working muscle is glucose taken up from the blood. Despite extensive efforts, the signalling mechanisms vital for glucose uptake during exercise are not yet fully understood, although the GTPase Rac1 is a candidate molecule. The present study investigated the role of Rac1 in muscle glucose uptake and substrate utilization during treadmill exercise in mice in vivo. Exercise-induced uptake of radiolabelled 2-deoxyglucose at 65% of maximum running capacity was blocked in soleus muscle and decreased by 80% and 60% in gastrocnemius and tibialis anterior muscles, respectively, in muscle-specific inducible Rac1 knockout (mKO) mice compared to wild-type littermates. By developing an assay to quantify endogenous GLUT4 translocation, we observed that GLUT4 content at the sarcolemma in response to exercise was reduced in Rac1 mKO muscle. Our findings implicate Rac1 as a regulatory element critical for controlling glucose uptake during exercise via regulation of GLUT4 translocation.

Regulation of autophagy in human skeletal muscle: effects of exercise, exercise training and insulin stimulation.

KEY POINTS: Regulation of autophagy in human muscle in many aspects differs from the majority of previous reports based on studies in cell systems and rodent muscle. An acute bout of exercise and insulin stimulation reduce human muscle autophagosome content. An acute bout of exercise regulates autophagy by a local contraction-induced mechanism. Exercise training increases the capacity for formation of autophagosomes in human muscle. AMPK activation during exercise seems insufficient to regulate autophagosome content in muscle, while mTORC1 signalling via ULK1 probably mediates the autophagy-inhibiting effect of insulin. Studies in rodent muscle suggest that autophagy is regulated by acute exercise, exercise training and insulin stimulation. However, little is known about the regulation of autophagy in human skeletal muscle. Here we investigate the autophagic response to acute one-legged exercise, one-legged exercise training and subsequent insulin stimulation in exercised and non-exercised human muscle. Acute one-legged exercise decreased (P<0.01) lipidation of microtubule-associated protein 1A/1B-light chain 3 (LC3) (∼ 50%) and the LC3-II/LC3-I ratio (∼ 60%) indicating that content of autophagosomes decreases with exercise in human muscle. The decrease in LC3-II/LC3-I ratio did not correlate with activation of 5'AMP activated protein kinase (AMPK) trimer complexes in human muscle. Consistently, pharmacological AMPK activation with 5-aminoimidazole-4-carboxamide riboside (AICAR) in mouse muscle did not affect the LC3-II/LC3-I ratio. Four hours after exercise, insulin further reduced (P<0.01) the LC3-II/LC3-I ratio (∼ 80%) in muscle of the exercised and non-exercised leg in humans. This coincided with increased Ser-757 phosphorylation of Unc51 like kinase 1 (ULK1), which is suggested as a mammalian target of rapamycin complex 1 (mTORC1) target. Accordingly, inhibition of mTOR signalling in mouse muscle prevented the ability of insulin to reduce the LC3-II/LC3-I ratio. In response to 3 weeks of one-legged exercise training, the LC3-II/LC3-I ratio decreased (P<0.05) in both trained and untrained muscle and this change was largely driven by an increase in LC3-I content. Taken together, acute exercise and insulin stimulation reduce muscle autophagosome content, while exercise training may increase the capacity for formation of autophagosomes in muscle. Moreover, AMPK activation during exercise may not be sufficient to regulate autophagy in muscle, while mTORC1 signalling via ULK1 probably mediates the autophagy-inhibiting effect of insulin.

PT-1 selectively activates AMPK-γ1 complexes in mouse skeletal muscle, but activates all three γ subunit complexes in cultured human cells by inhibiting the respiratory chain.

AMP-activated protein kinase (AMPK) occurs as heterotrimeric complexes in which a catalytic subunit (α1/α2) is bound to one of two ß subunits (ß1/ß2) and one of three γ subunits (γ1/γ2/γ3). The ability to selectively activate specific isoforms would be a useful research tool and a promising strategy to combat diseases such as cancer and Type 2 diabetes. We report that the AMPK activator PT-1 selectively increased the activity of γ1- but not γ3-containing complexes in incubated mouse muscle. PT-1 increased the AMPK-dependent phosphorylation of the autophagy-regulating kinase ULK1 (unc-51-like autophagy-activating kinase 1) on Ser555, but not proposed AMPK-γ3 substrates such as Ser231 on TBC1 (tre-2/USP6, BUB2, cdc16) domain family, member 1 (TBC1D1) or Ser212 on acetyl-CoA carboxylase subunit 2 (ACC2), nor did it stimulate glucose transport. Surprisingly, however, in human embryonic kidney (HEK) 293 cells expressing human γ1, γ2 or γ3, PT-1 activated all three complexes equally. We were unable to reproduce previous findings suggesting that PT-1 activates AMPK by direct binding between the kinase and auto-inhibitory domains (AIDs) of the α subunit. We show instead that PT-1 activates AMPK indirectly by inhibiting the respiratory chain and increasing cellular AMP:ATP and/or ADP:ATP ratios. Consistent with this mechanism, PT-1 failed to activate AMPK in HEK293 cells expressing an AMP-insensitive R299G mutant of AMPK-γ1. We propose that the failure of PT-1 to activate γ3-containing complexes in muscle is not an intrinsic feature of such complexes, but is because PT-1 does not increase cellular AMP:ATP ratios in the specific subcellular compartment(s) in which γ3 complexes are located.

Stretch-stimulated glucose transport in skeletal muscle is regulated by Rac1.

KEY POINTS: Rac1 regulates stretch-stimulated (i.e. mechanical stress) glucose transport in muscle. Actin depolymerization decreases stretch-induced glucose transport in skeletal muscle. Rac1 is a required part of the mechanical stress-component of the contraction-stimulus to glucose transport in skeletal muscle. ABSTRACT: An alternative to the canonical insulin signalling pathway for glucose transport is muscle contraction/exercise. Mechanical stress is an integrated part of the muscle contraction/relaxation cycle, and passive stretch stimulates muscle glucose transport. However, the signalling mechanism regulating stretch-stimulated glucose transport is not well understood. We recently reported that the actin cytoskeleton regulating GTPase, Rac1, was activated in mouse muscle in response to stretching. Rac1 is a regulator of contraction- and insulin-stimulated glucose transport, however, its role in stretch-stimulated glucose transport and signalling is unknown. We therefore investigated whether stretch-induced glucose transport in skeletal muscle required Rac1 and the actin cytoskeleton. We used muscle-specific inducible Rac1 knockout mice as well as pharmacological inhibitors of Rac1 and the actin cytoskeleton in isolated soleus and extensor digitorum longus muscles. In addition, the role of Rac1 in contraction-stimulated glucose transport during conditions without mechanical load on the muscles was evaluated in loosely hanging muscles and muscles in which cross-bridge formation was blocked by the myosin ATPase inhibitors BTS and Blebbistatin. Knockout as well as pharmacological inhibition of Rac1 reduced stretch-stimulated glucose transport by 30-50% in soleus and extensor digitorum longus muscle. The actin depolymerizing agent latrunculin B similarly decreased glucose transport in response to stretching by 40-50%. Rac1 inhibition reduced contraction-stimulated glucose transport by 30-40% in tension developing muscle but did not affect contraction-stimulated glucose transport in muscles in which force development was prevented. Our findings suggest that Rac1 and the actin cytoskeleton regulate stretch-stimulated glucose transport and that Rac1 is a required part of the mechanical stress-component of the contraction-stimulus to glucose transport in skeletal muscle.

Leukemia inhibitory factor increases glucose uptake in mouse skeletal muscle.

Members of the IL-6 family, IL-6 and ciliary neurotrophic factor (CNTF), have been shown to increase glucose uptake and fatty acid oxidation in skeletal muscle. However, the metabolic effects of another family member, leukemia inhibitory factor (LIF), are not well characterized. Effects of LIF on skeletal muscle glucose uptake and palmitate oxidation and signaling were investigated in ex vivo incubated mouse soleus and EDL muscles from muscle-specific AMPKα2 kinase-dead, muscle-specific SOCS3 knockout, and lean and high-fat-fed mice. Inhibitors were used to investigate involvement of specific signaling pathways. LIF increased muscle glucose uptake in dose (50-5,000 pM/l) and time-dependent manners with maximal effects at the 30-min time point. LIF increased Akt Ser(473) phosphorylation (P) in soleus and EDL, whereas AMPK Thr(172) P was unaffected. Incubation with parthenolide abolished LIF-induced glucose uptake and STAT3 Tyr(705) P, whereas incubation with LY-294002 and wortmannin suppressed both basal and LIF-induced glucose uptake and Akt Ser(473) P, indicating that JAK and PI 3-kinase signaling is required for LIF-stimulated glucose uptake. Incubation with rapamycin and AZD8055 indicated that mammalian target of rapamycin complex (mTORC)2, but not mTORC1, also is required for LIF-stimulated glucose uptake. In contrast to CNTF, LIF stimulation did not alter palmitate oxidation. LIF-stimulated glucose uptake was maintained in EDL from obese insulin-resistant mice, whereas soleus developed LIF resistance. Lack of SOCS3 and AMPKα2 did not affect LIF-stimulated glucose uptake. In conclusion, LIF acutely increased muscle glucose uptake by a mechanism potentially involving the PI 3-kinase/mTORC2/Akt pathway and is not impaired in EDL muscle from obese insulin-resistant mice.

Rac1--a novel regulator of contraction-stimulated glucose uptake in skeletal muscle.

Muscle contraction stimulates muscle glucose uptake by facilitating translocation of glucose transporter 4 from intracellular locations to the cell surface, which allows for diffusion of glucose into the myofibres. The intracellular mechanisms regulating this process are not well understood. The GTPase Rac1 has, until recently, been investigated only with regard to its involvement in insulin-stimulated glucose uptake. However, we recently found that Rac1 is activated during muscle contraction and exercise in mice and humans. Remarkably, Rac1 seems to be necessary for exercise and contraction-stimulated glucose uptake in skeletal muscle, because muscle-specific Rac1 knockout mice display reduced ex vivo contraction- and in vivo exercise-stimulated glucose uptake. The molecular mechanism by which Rac1 regulates glucose uptake is presently unknown. However, recent studies link Rac1 to the actin cytoskeleton, the small GTPase RalA and/or free radical production, which have previously been shown to be regulators of glucose uptake in muscle. We propose a model in which Rac1 is activated by contraction- and exercise-induced mechanical stress signals and that Rac1 in conjunction with other signalling regulates glucose uptake during muscle contraction and exercise.

GDF15 is still a mystery hormone.

Growth differentiation factor 15 (GDF15) is a member of the transforming growth factor-ß (TGF-ß) superfamily. Despite its identification over 20 years ago, the functions of GDF15 remain complex and not fully elucidated. Its concentration in plasma varies widely depending on the physiological and pathophysiological state of the organism. GDF15 has been described to regulate food intake and insulin sensitivity in rodents via the GDNF family receptor α-like (GFRAL) receptor, and to be elevated in pregnancy and many disease states and decreased in physically fit individuals. We discuss the latest developments in the regulation of GDF15 secretion and its diverse physiological effects, and touch upon possible GFRAL-independent effects of GDF15. In addition, we discuss the effects of proteins and peptides derived from the same precursor protein as GDF15.