Fatty Acids Increase GDF15 and Reduce Food Intake Through a GFRAL Signaling Axis.

In contrast to the well-defined biological feedback loops controlling glucose, the mechanisms by which the body responds to changes in fatty acid availability are less clearly defined. Growth differentiating factor 15 (GDF15) suppresses the consumption of diets high in fat but is paradoxically increased in obese mice fed a high-fat diet. Given this interrelationship, we investigated whether diets high in fat could directly increase GDF15 independently of obesity. We found that fatty acids increase GDF15 levels dose dependently, with the greatest response observed with linolenic acid. GDF15 mRNA expression was modestly increased in the gastrointestinal tract; however, kidney GDF15 mRNA was ∼1,000-fold higher and was increased by more than threefold, with subsequent RNAscope analysis showing elevated expression within the cortex and outer medulla. Treatment of wild-type mice with linolenic acid reduced food intake and body mass; however, this effect disappeared in mice lacking the GDF15 receptor GFRAL. An equal caloric load of glucose did not suppress food intake or reduce body mass in either wild-type or GFRAL-knockout mice. These data indicate that fatty acids such as linolenic acid increase GDF15 and suppress food intake through a mechanism requiring GFRAL. These data suggest that a primary physiological function of GDF15 may be as a fatty acid sensor designed to protect cells from fatty acid overload. ARTICLE HIGHLIGHTS: The mechanisms by which the body responds to changes in fatty acid availability are less clearly defined. We investigated whether diets high in fat could directly increase growth differentiating factor 15 (GDF15) independently of obesity. Fatty acids increase GDF15 and reduce food intake through a GFRAL signaling axis. GDF15 is a sensor of fatty acids that may have important implications for explaining increased satiety after consumption of diets high in fat.

Combination of an ACLY inhibitor with a GLP-1R agonist exerts additive benefits on nonalcoholic steatohepatitis and hepatic fibrosis in mice.

Increased liver de novo lipogenesis (DNL) is a hallmark of nonalcoholic steatohepatitis (NASH). A key enzyme controlling DNL upregulated in NASH is ATP citrate lyase (ACLY). In mice, inhibition of ACLY reduces liver steatosis, ballooning, and fibrosis and inhibits activation of hepatic stellate cells. Glucagon-like peptide-1 receptor (GLP-1R) agonists lower body mass, insulin resistance, and steatosis without improving fibrosis. Here, we find that combining an inhibitor of liver ACLY, bempedoic acid, and the GLP-1R agonist liraglutide reduces liver steatosis, hepatocellular ballooning, and hepatic fibrosis in a mouse model of NASH. Liver RNA analyses revealed additive downregulation of pathways that are predictive of NASH resolution, reductions in the expression of prognostically significant genes compared with clinical NASH samples, and a predicted gene signature profile that supports fibrosis resolution. These findings support further investigation of this combinatorial therapy to treat obesity, insulin resistance, hypercholesterolemia, steatohepatitis, and fibrosis in people with NASH.

GDF15 promotes weight loss by enhancing energy expenditure in muscle.

Caloric restriction that promotes weight loss is an effective strategy for treating non-alcoholic fatty liver disease and improving insulin sensitivity in people with type 2 diabetes1. Despite its effectiveness, in most individuals, weight loss is usually not maintained partly due to physiological adaptations that suppress energy expenditure, a process known as adaptive thermogenesis, the mechanistic underpinnings of which are unclear2,3. Treatment of rodents fed a high-fat diet with recombinant growth differentiating factor 15 (GDF15) reduces obesity and improves glycaemic control through glial-cell-derived neurotrophic factor family receptor α-like (GFRAL)-dependent suppression of food intake4-7. Here we find that, in addition to suppressing appetite, GDF15 counteracts compensatory reductions in energy expenditure, eliciting greater weight loss and reductions in non-alcoholic fatty liver disease (NAFLD) compared to caloric restriction alone. This effect of GDF15 to maintain energy expenditure during calorie restriction requires a GFRAL-ß-adrenergic-dependent signalling axis that increases fatty acid oxidation and calcium futile cycling in the skeletal muscle of mice. These data indicate that therapeutic targeting of the GDF15-GFRAL pathway may be useful for maintaining energy expenditure in skeletal muscle during caloric restriction.

Inhibition of ATP-citrate lyase improves NASH, liver fibrosis, and dyslipidemia.

Elevated liver de novo lipogenesis contributes to non-alcoholic steatohepatitis (NASH) and can be inhibited by targeting acetyl-CoA carboxylase (ACC). However, hypertriglyceridemia limits the use of pharmacological ACC inhibitors as a monotherapy. ATP-citrate lyase (ACLY) generates acetyl-CoA and oxaloacetate from citrate, but whether inhibition is effective for treating NASH is unknown. Here, we characterize a new mouse model that replicates many of the pathological and molecular drivers of NASH and find that genetically inhibiting ACLY in hepatocytes reduces liver malonyl-CoA, oxaloacetate, steatosis, and ballooning as well as blood glucose, triglycerides, and cholesterol. Pharmacological inhibition of ACLY mirrors genetic inhibition but has additional positive effects on hepatic stellate cells, liver inflammation, and fibrosis. Mendelian randomization of human variants that mimic reductions in ACLY also associate with lower circulating triglycerides and biomarkers of NASH. These data indicate that inhibiting liver ACLY may be an effective approach for treatment of NASH and dyslipidemia.

Metformin-induced reductions in tumor growth involves modulation of the gut microbiome.

BACKGROUND/PURPOSE: Type 2 diabetes and obesity increase the risk of developing colorectal cancer. Metformin may reduce colorectal cancer but the mechanisms mediating this effect remain unclear. In mice and humans, a high-fat diet (HFD), obesity and metformin are known to alter the gut microbiome but whether this is important for influencing tumor growth is not known. METHODS: Mice with syngeneic MC38 colon adenocarcinomas were treated with metformin or feces obtained from control or metformin treated mice. RESULTS: We find that compared to chow-fed controls, tumor growth is increased when mice are fed a HFD and that this acceleration of tumor growth can be partially recapitulated through transfer of the fecal microbiome or in vitro treatment of cells with fecal filtrates from HFD-fed animals. Treatment of HFD-fed mice with orally ingested, but not intraperitoneally injected, metformin suppresses tumor growth and increases the expression of short-chain fatty acid (SCFA)-producing microbes Alistipes, Lachnospiraceae and Ruminococcaceae. The transfer of the gut microbiome from mice treated orally with metformin to drug naïve, conventionalized HFD-fed mice increases circulating propionate and butyrate, reduces tumor proliferation, and suppresses the expression of sterol response element binding protein (SREBP) gene targets in the tumor. CONCLUSION: These data indicate that in obese mice fed a HFD, metformin reduces tumor burden through changes in the gut microbiome.

Co-overexpression of CD36 and FABPpm increases fatty acid transport additively, not synergistically, within muscle.

We aimed to determine the combined effects of overexpressing plasma membrane fatty acid binding protein (FABPpm) and fatty acid translocase (CD36) on skeletal muscle fatty acid transport to establish if these transport proteins function collaboratively. Electrotransfection with either FABPpm or CD36 increased their protein content at the plasma membrane (+75% and +64%), increased fatty acid transport rates by +24% for FABPpm and +62% for CD36, resulting in a calculated transport efficiency of ∼0.019 and ∼0.053 per unit protein change for FABPpm and CD36, respectively. We subsequently used these data to determine if increasing both proteins additively or synergistically increased fatty acid transport. Cotransfection of FABPpm and CD36 simultaneously increased protein content in whole muscle (FABPpm, +46%; CD36, +45%) and at the sarcolemma (FABPpm, +41%; CD36, +42%), as well as fatty acid transport rates (+50%). Since the relative effects of changing FABPpm and CD36 content had been independently determined, we were able to a predict a change in fatty acid transport based on the overexpression of plasmalemmal transporters in the cotransfection experiments. This prediction yielded an increase in fatty acid transport of +0.984 and +1.722 pmol/mg prot/15 s for FABPpm and CD36, respectively, for a total increase of +2.96 pmol/mg prot/15 s. This calculated determination was remarkably consistent with the measured change in transport, namely +2.89 pmol/mg prot/15 s. Altogether, these data indicate that increasing CD36 and FABPpm alters fatty acid transport rates additively, but not synergistically, suggesting an independent mechanism of action within muscle for each transporter. This conclusion was further supported by the observation that plasmalemmal CD36 and FABPpm did not coimmunoprecipitate.

The pesticide chlorpyrifos promotes obesity by inhibiting diet-induced thermogenesis in brown adipose tissue.

Obesity results from a caloric imbalance between energy intake, absorption and expenditure. In both rodents and humans, diet-induced thermogenesis contributes to energy expenditure and involves the activation of brown adipose tissue (BAT). We hypothesize that environmental toxicants commonly used as food additives or pesticides might reduce BAT thermogenesis through suppression of uncoupling protein 1 (UCP1) and this may contribute to the development of obesity. Using a step-wise screening approach, we discover that the organophosphate insecticide chlorpyrifos suppresses UCP1 and mitochondrial respiration in BAT at concentrations as low as 1 pM. In mice housed at thermoneutrality and fed a high-fat diet, chlorpyrifos impairs BAT mitochondrial function and diet-induced thermogenesis, promoting greater obesity, non-alcoholic fatty liver disease (NAFLD) and insulin resistance. This is associated with reductions in cAMP; activation of p38MAPK and AMPK; protein kinases critical for maintaining UCP1 and mitophagy, respectively in BAT. These data indicate that the commonly used pesticide chlorpyrifos, suppresses diet-induced thermogenesis and the activation of BAT, suggesting its use may contribute to the obesity epidemic.

The SGLT2 inhibitor canagliflozin suppresses lipid synthesis and interleukin-1 beta in ApoE deficient mice.

Sodium-glucose cotransporter 2 inhibitors such as canagliflozin lower blood glucose and reduce cardiovascular events in people with type 2 diabetes through mechanisms that are not fully understood. Canagliflozin has been shown to increase the activity of the AMP-activated protein kinase (AMPK), a metabolic energy sensor important for increasing fatty acid oxidation and energy expenditure and suppressing lipogenesis and inflammation, but whether AMPK activation is important for mediating some of the beneficial metabolic effects of canagliflozin has not been determined. We, therefore, evaluated the effects of canagliflozin in female ApoE-/- and ApoE-/-AMPK ß1-/- mice fed a western diet. Canagliflozin increased fatty acid oxidation and energy expenditure and lowered adiposity, blood glucose and the respiratory exchange ratio independently of AMPK ß1. Canagliflozin also suppressed liver lipid synthesis and the expression of ATP-citrate lyase, acetyl-CoA carboxylase and sterol response element-binding protein 1c independently of AMPK ß1. Canagliflozin lowered circulating IL-1ß and studies in bone marrow-derived macrophages indicated that in contrast with the metabolic adaptations, this effect required AMPK ß1. Canagliflozin had no effect on the size of atherosclerotic plaques in either ApoE-/- and ApoE-/-AMPK ß1-/- mice. Future studies investigating whether reductions in liver lipid synthesis and macrophage IL-1ß are important for the cardioprotective effects of canagliflozin warrant further investigation.

Salicylate enhances the response of prostate cancer to radiotherapy.

BACKGROUND: Radiotherapy (RT) is a key therapeutic modality for prostate cancer (PrCa), but RT resistance necessitates dose-escalation, often causing bladder and rectal toxicity. Aspirin, a prodrug of salicylate (SAL), has been associated with improved RT response in clinical PrCa cases, but the potential mechanism mediating this effect is unknown. SAL activates the metabolic stress sensor AMP-activated protein kinase (AMPK), which inhibits de novo lipogenesis, and protein synthesis via inhibition of Acetyl-CoA Carboxylase (ACC), and the mammalian Target of Rapamycin (mTOR), respectively. RT also activates AMPK through a mechanism distinctly different from SAL. Therefore, combining these two therapies may have synergistic effects on suppressing PrCa. Here, we examined the potential of SAL to enhance the response of human PrCa cells and tumors to RT. METHODS: Androgen-insensitive (PC3) and -sensitive (LNCaP) PrCa cells were subjected to proliferation and clonogenic survival assays after treatment with clinically relevant doses of SAL and RT. Balb/c nude mice with PC3 xenografts were fed standard chow diet or chow diet supplemented with 2.5 g/kg salsalate (SAL pro-drug dimer) one week prior to a single dose of 0 or 10 Gy RT. Immunoblotting analysis of signaling events in the DNA repair and AMPK-mTOR pathways and lipogenesis were assessed in cells treated with SAL and RT. RESULTS: SAL inhibited proliferation and clonogenic survival in PrCa cells and enhanced the inhibition mediated by RT. Salsalate, added to diet, enhanced the anti-tumor effects of RT in PC3 tumor xenografts. RT activated genotoxic stress markers and the activity of mTOR pathway and AMPK and mediated inhibitory phosphorylation of ACC. Interestingly, SAL enhanced the effects of RT on AMPK and ACC but blocked markers of mTOR activation. CONCLUSIONS: Our results show that SAL can enhance RT responses in PrCa. Salsalate is a promising agent to investigate this concept in prospective clinical trials of PrCa in combination with RT.

Inhibition of Acetyl-CoA Carboxylase by Phosphorylation or the Inhibitor ND-654 Suppresses Lipogenesis and Hepatocellular Carcinoma.

The incidence of hepatocellular carcinoma (HCC) is rapidly increasing due to the prevalence of obesity and non-alcoholic fatty liver disease, but the molecular triggers that initiate disease development are not fully understood. We demonstrate that mice with targeted loss-of-function point mutations within the AMP-activated protein kinase (AMPK) phosphorylation sites on acetyl-CoA carboxylase 1 (ACC1 Ser79Ala) and ACC2 (ACC2 Ser212Ala) have increased liver de novo lipogenesis (DNL) and liver lesions. The same mutation in ACC1 also increases DNL and proliferation in human liver cancer cells. Consistent with these findings, a novel, liver-specific ACC inhibitor (ND-654) that mimics the effects of ACC phosphorylation inhibits hepatic DNL and the development of HCC, improving survival of tumor-bearing rats when used alone and in combination with the multi-kinase inhibitor sorafenib. These studies highlight the importance of DNL and dysregulation of AMPK-mediated ACC phosphorylation in accelerating HCC and the potential of ACC inhibitors for treatment.

The role of AMP-activated protein kinase in the expression of the dystrophin-associated protein complex in skeletal muscle.

Stimulation of AMPK induces the expression of dystrophin-associated protein complex (DAPC) components in skeletal muscle, whereas reductions in AMPK are associated with DAPC dysfunction. We sought to determine whether AMPK was necessary for the maintenance of DAPC expression in skeletal muscle. Fast, glycolytic extensor digitorum longus (EDL) and slow, oxidative soleus (Sol) muscles from wild-type mice and from littermates with skeletal muscle-specific knockout of the AMPK ß1 and ß2 subunits (AMPK ß1 ß2M-KO; MKO) were analyzed. DAPC mRNA and protein expression were similar between genotypes, with the exception of elevated neuronal nitric oxide synthase expression at the sarcolemma in MKO muscles. The content of transcriptional and post-transcriptional regulators of the DAPC was also not affected by the loss of AMPK. However, MyoD and myogenin expression was diminished in MKO muscles, consistent with previous reports of myopathy in these animals. Furthermore, we observed decrements in extrasynaptic utrophin expression selectively in MKO Sol muscles, likely due to the adaptive accumulation of peroxisome proliferator-activated receptor γ coactivator-1α at the sarcolemma of MKO EDL muscles. Collectively, the evidence indicates that AMPK is sufficient but not essential for the maintenance of DAPC expression in skeletal muscle, yet it is required for preserving extrasynaptic utrophin levels in slow oxidative muscles.-Dial, A. G., Rooprai, P., Lally, J. S., Bujak, A. L., Steinberg, G. R., Ljubicic, V. The role of AMP-activated protein kinase in the expression of the dystrophin-associated protein complex in skeletal muscle.

Treatment of nonalcoholic fatty liver disease: role of AMPK.

Nonalcoholic fatty liver disease (NAFLD) is a growing worldwide epidemic and an important risk factor for the development of insulin resistance, type 2 diabetes, nonalcoholic steatohepatitis (NASH), and hepatic cellular carcinoma (HCC). Despite the prevalence of NAFLD, lifestyle interventions involving exercise and weight loss are the only accepted treatments for this disease. Over the last decade, numerous experimental compounds have been shown to improve NAFLD in preclinical animal models, and many of these therapeutics have been shown to increase the activity of the cellular energy sensor AMP-activated protein kinase (AMPK). Because AMPK activity is reduced by inflammation, obesity, and diabetes, increasing AMPK activity has been viewed as a viable therapeutic strategy to improve NAFLD. In this review, we propose three primary mechanisms by which AMPK activation may improve NAFLD. In addition, we examine the mechanisms by which AMPK is activated. Finally, we identify 27 studies that have used AMPK activators to reduce NAFLD. Future considerations for studies examining the relationship between AMPK and NAFLD are highlighted.

The AMPK activator R419 improves exercise capacity and skeletal muscle insulin sensitivity in obese mice.

OBJECTIVE: Skeletal muscle AMP-activated protein kinase (AMPK) is important for regulating glucose homeostasis, mitochondrial content and exercise capacity. R419 is a mitochondrial complex-I inhibitor that has recently been shown to acutely activate AMPK in myotubes. Our main objective was to examine whether R419 treatment improves insulin sensitivity and exercise capacity in obese insulin resistant mice and whether skeletal muscle AMPK was important for mediating potential effects. METHODS: Glucose homeostasis, insulin sensitivity, exercise capacity, and electron transport chain content/activity were examined in wildtype (WT) and AMPK ß1ß2 muscle-specific null (AMPK-MKO) mice fed a high-fat diet (HFD) with or without R419 supplementation. RESULTS: There was no change in weight gain, adiposity, glucose tolerance or insulin sensitivity between HFD-fed WT and AMPK-MKO mice. In both HFD-fed WT and AMPK-MKO mice, R419 enhanced insulin tolerance, insulin-stimulated glucose disposal, skeletal muscle 2-deoxyglucose uptake, Akt phosphorylation and glucose transporter 4 (GLUT4) content independently of alterations in body mass. In WT, but not AMPK-MKO mice, R419 improved treadmill running capacity. Treatment with R419 increased muscle electron transport chain content and activity in WT mice; effects which were blunted in AMPK-MKO mice. CONCLUSIONS: Treatment of obese mice with R419 improved skeletal muscle insulin sensitivity through a mechanism that is independent of skeletal muscle AMPK. R419 also increases exercise capacity and improves mitochondrial function in obese WT mice; effects that are diminished in the absence of skeletal muscle AMPK. These findings suggest that R419 may be a promising therapy for improving whole-body glucose homeostasis and exercise capacity.

Extremely rapid increase in fatty acid transport and intramyocellular lipid accumulation but markedly delayed insulin resistance after high fat feeding in rats.

AIMS/HYPOTHESIS: The mechanisms for diet-induced intramyocellular lipid accumulation and its association with insulin resistance remain contentious. In a detailed time-course study in rats, we examined whether a high-fat diet increased intramyocellular lipid accumulation via alterations in fatty acid translocase (FAT/CD36)-mediated fatty acid transport, selected enzymes and/or fatty acid oxidation, and whether intramyocellular lipid accretion coincided with the onset of insulin resistance. METHODS: We measured, daily (on days 1-7) and/or weekly (for 6 weeks), the diet-induced changes in circulating substrates, insulin, sarcolemmal substrate transporters and transport, selected enzymes, intramyocellular lipids, mitochondrial fatty acid oxidation and basal and insulin-stimulated sarcolemmal GLUT4 and glucose transport. We also examined whether upregulating fatty acid oxidation improved glucose transport in insulin-resistant muscles. Finally, in Cd36-knockout mice, we examined the role of FAT/CD36 in intramyocellular lipid accumulation, insulin sensitivity and diet-induced glucose intolerance. RESULTS: Within 2-3 days, diet-induced increases occurred in insulin, sarcolemmal FAT/CD36 (but not fatty acid binding protein [FABPpm] or fatty acid transporter [FATP]1 or 4), fatty acid transport and intramyocellular triacylglycerol, diacylglycerol and ceramide, independent of enzymatic changes or muscle fatty acid oxidation. Diet-induced increases in mitochondria and mitochondrial fatty acid oxidation and impairments in insulin-stimulated glucose transport and GLUT4 translocation occurred much later (≥21 days). FAT/CD36 ablation impaired insulin-stimulated fatty acid transport and lipid accumulation, improved insulin sensitivity and prevented diet-induced glucose intolerance. Increasing fatty acid oxidation in insulin-resistant muscles improved glucose transport. CONCLUSIONS/INTERPRETATIONS: High-fat feeding rapidly increases intramyocellular lipids (in 2-3 days) via insulin-mediated upregulation of sarcolemmal FAT/CD36 and fatty acid transport. The 16-19 day delay in the onset of insulin resistance suggests that additional mechanisms besides intramyocellular lipids contribute to this pathology.

Skeletal muscle ACC2 S212 phosphorylation is not required for the control of fatty acid oxidation during exercise.

During submaximal exercise fatty acids are a predominant energy source for muscle contractions. An important regulator of fatty acid oxidation is acetyl-CoA carboxylase (ACC), which exists as two isoforms (ACC1 and ACC2) with ACC2 predominating in skeletal muscle. Both ACC isoforms regulate malonyl-CoA production, an allosteric inhibitor of carnitine palmitoyltransferase 1 (CPT-1); the primary enzyme controlling fatty acyl-CoA flux into mitochondria for oxidation. AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that is activated during exercise or by pharmacological agents such as metformin and AICAR. In resting muscle the activation of AMPK with AICAR leads to increased phosphorylation of ACC (S79 on ACC1 and S221 on ACC2), which reduces ACC activity and malonyl-CoA; effects associated with increased fatty acid oxidation. However, whether this pathway is vital for regulating skeletal muscle fatty acid oxidation during conditions of increased metabolic flux such as exercise/muscle contractions remains unknown. To examine this we characterized mice lacking AMPK phosphorylation sites on ACC2 (S212 in mice/S221 in humans-ACC2-knock-in [ACC2-KI]) or both ACC1 (S79) and ACC2 (S212) (ACC double knock-in [ACCD-KI]) during submaximal treadmill exercise and/or ex vivo muscle contractions. We find that surprisingly, ACC2-KI mice had normal exercise capacity and whole-body fatty acid oxidation during treadmill running despite elevated muscle ACC2 activity and malonyl-CoA. Similar results were observed in ACCD-KI mice. Fatty acid oxidation was also maintained in muscles from ACC2-KI mice contracted ex vivo. These findings indicate that pathways independent of ACC phosphorylation are important for regulating skeletal muscle fatty acid oxidation during exercise/muscle contractions.

AMPK activation of muscle autophagy prevents fasting-induced hypoglycemia and myopathy during aging.

The AMP-activated protein kinase (AMPK) activates autophagy, but its role in aging and fasting-induced muscle function has not been defined. Here we report that fasting mice lacking skeletal muscle AMPK (AMPK-MKO) results in hypoglycemia and hyperketosis. This is not due to defective fatty acid oxidation, but instead is related to a block in muscle proteolysis that leads to reduced circulating levels of alanine, an essential amino acid required for gluconeogenesis. Markers of muscle autophagy including phosphorylation of Ulk1 Ser555 and Ser757 and aggregation of RFP-LC3 puncta are impaired. Consistent with impaired autophagy, aged AMPK-MKO mice possess a significant myopathy characterized by reduced muscle function, mitochondrial disease, and accumulation of the autophagy/mitophagy proteins p62 and Parkin. These findings establish an essential requirement for skeletal muscle AMPK-mediated autophagy in preserving blood glucose levels during prolonged fasting as well as maintaining muscle integrity and mitochondrial function during aging.

Skeletal muscle AMPK is essential for the maintenance of FNDC5 expression.

Fibronectin type III domain-containing protein 5 (FNDC5) expression is controlled by the transcriptional co-activator, peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC1α). FNDC5 expression has been shown to be increased in muscle in response to endurance exercise in some but not all studies, therefore a greater understanding of the mechanisms controlling this process are needed. The AMP-activated protein kinase (AMPK) is activated by exercise in an intensity dependent manner and is an important regulator of PGC1α activity; therefore, we explored the role of AMPK in the regulation of FNDC5 using AMPK ß1ß2 double muscle-null mice (AMPK DMKO), which lack skeletal muscle AMPK activity. We found that FNDC5 expression is dramatically reduced in resting muscles of AMPK DMKO mice compared to wild-type littermates. In wild-type mice, activating phosphorylation of AMPK was elevated immediately post contraction and was abolished in muscle from AMPK DMKO mice. In contrast, PGC1α was increased in both wild-type and AMPK DMKO mice 3 h post contraction but FNDC5 protein expression was not altered. Lastly, acute or chronic activation of AMPK with the pharmacological AMPK activator AICAR did not increase PGC1α or FNDC5 expression in muscle. These data indicate that skeletal muscle AMPK is required for the maintenance of basal FNDC5 expression.

AMPK deficiency in cardiac muscle results in dilated cardiomyopathy in the absence of changes in energy metabolism.

AIMS: AMP-activated protein kinase (AMPK) is thought to be a central player in regulating myocardial metabolism and its activation has been shown to inhibit cardiac hypertrophy. Recently, mice with muscle-specific deletion of AMPK ß1/ß2 subunits (AMPKß1ß2-deficient mice, ß1ß2M-KO) have been generated and possess <10% of normal AMPK activity in muscle. However, how/if dramatic AMPK deficiency alters cardiac metabolism, function, or morphology has not been investigated. Therefore, the aim of this study was to determine whether a significant loss of AMPK activity alters cardiac function, metabolism, and hypertrophy, and whether this may play a role in the pathogenesis of heart failure. METHODS AND RESULTS: ß1ß2M-KO mice exhibit an approximate 25% reduction in systolic and diastolic function compared with wild-type (WT) littermates. Despite the well-documented role of AMPK in controlling myocardial energy metabolism, there was no difference in basal glucose and fatty acid oxidation rates between ß1ß2M-KO and WT mice. However, there was reduced AMPK-mediated phosphorylation of troponin I in ß1ß2M-KO and reduced ventricular cell shortening in the presence of low Ca(2+), which may explain the impaired cardiac function in these mice. Interestingly, ß1ß2M-KO mice did not display any signs of compensatory cardiac hypertrophy, which could be attributed to impaired activation of p38 MAPK. CONCLUSIONS: ß1ß2M-KO mice display evidence of dilated cardiomyopathy. This is the first mouse model of AMPK deficiency that demonstrates cardiac dysfunction in the absence of pathological stress and provides insights into the role of AMPK in regulating myocardial function, metabolism, hypertrophy, and the progression to heart failure.

Exercise-stimulated interleukin-15 is controlled by AMPK and regulates skin metabolism and aging.

Aging is commonly associated with a structural deterioration of skin that compromises its barrier function, healing, and susceptibility to disease. Several lines of evidence show that these changes are driven largely by impaired tissue mitochondrial metabolism. While exercise is associated with numerous health benefits, there is no evidence that it affects skin tissue or that endocrine muscle-to-skin signaling occurs. We demonstrate that endurance exercise attenuates age-associated changes to skin in humans and mice and identify exercise-induced IL-15 as a novel regulator of mitochondrial function in aging skin. We show that exercise controls IL-15 expression in part through skeletal muscle AMP-activated protein kinase (AMPK), a central regulator of metabolism, and that the elimination of muscle AMPK causes a deterioration of skin structure. Finally, we establish that daily IL-15 therapy mimics some of the anti-aging effects of exercise on muscle and skin in mice. Thus, we elucidate a mechanism by which exercise confers health benefits to skin and suggest that low-dose IL-15 therapy may prove to be a beneficial strategy to attenuate skin aging.

Early oxidative shifts in mouse skeletal muscle morphology with high-fat diet consumption do not lead to functional improvements.

Short-term consumption of a high-fat diet (HFD) can result in an oxidative shift in adult skeletal muscle. However, the impact of HFD on young, growing muscle is largely unknown. Thus, 4-week-old mice were randomly divided into sedentary HFD (60% kcal from fat), sedentary standard chow (control), or exercise-trained standard chow. Tibialis anterior (TA) and soleus muscles were examined for morphological and functional changes after 3 weeks. HFD consumption increased body and epididymal fat mass and induced whole body glucose intolerance versus control mice. Compared to controls, both HFD and exercise-trained TA muscles displayed a greater proportion of oxidative fibers and a trend for an increased succinate dehydrogenase (SDH) content. The soleus also displayed an oxidative shift with increased SDH content in HFD mice. Despite the aforementioned changes, palmitate oxidation rates were not different between groups. To determine if the adaptive changes with HFD manifest as a functional improvement, all groups performed pre- and postexperiment aerobic exercise tests. As expected, exercise-trained mice improved significantly compared to controls, however, no improvement was observed in HFD mice. Interestingly, capillary density was lower in HFD muscles; a finding which may contribute to the lack of functional differences seen with HFD despite the oxidative shift in skeletal muscle morphology. Taken together, our data demonstrate that young, growing muscle exhibits early oxidative shifts in response to a HFD, but these changes do not translate to functional benefits in palmitate oxidation, muscle fatigue resistance, or whole body exercise capacity.