Lack of O-GlcNAcylation enhances exercise-dependent glucose utilization potentially through AMP-activated protein kinase activation in skeletal muscle.

O-GlcNAcylation is a post-translational modification that is characterized by the addition of N-acetylglucosamine (GlcNAc) to proteins by O-GlcNAc transferase (Ogt). The degree of O-GlcNAcylation is thought to be associated with glucotoxicity and diabetic complications, because GlcNAc is produced by a branch of the glycolytic pathway. However, its role in skeletal muscle has not been fully elucidated. In this study, we created skeletal muscle-specific Ogt knockout (Ogt-MKO) mice and analyzed their glucose metabolism. During an intraperitoneal glucose tolerance test, blood glucose was slightly lower in Ogt-MKO mice than in control Ogt-flox mice. High fat diet-induced obesity and insulin resistance were reversed in Ogt-MKO mice. In addition, 12-month-old Ogt-MKO mice had lower adipose and body mass. A single bout of exercise significantly reduced blood glucose in Ogt-MKO mice, probably because of higher AMP-activated protein kinase α (AMPKα) protein expression. Furthermore, intraperitoneal injection of 5-aminoimidazole-4-carboxamide ribonucleotide, an AMPK activator, resulted in a more marked decrease in blood glucose levels in Ogt-MKO mice than in controls. Finally, Ogt knockdown by siRNA in C2C12 myotubes significantly increased protein expression of AMPKα, glucose uptake and oxidation. In conclusion, loss of O-GlcNAcylation facilitates glucose utilization in skeletal muscle, potentially through AMPK activation. The inhibition of O-GlcNAcylation in skeletal muscle may have an anti-diabetic effect, through an enhancement of glucose utilization during exercise.

Free essential amino acid feeding improves endurance during resistance training via DRP1-dependent mitochondrial remodelling.

BACKGROUND: Loss of muscle strength and endurance with aging or in various conditions negatively affects quality of life. Resistance exercise training (RET) is the most powerful means to improve muscle mass and strength, but it does not generally lead to improvements in endurance capacity. Free essential amino acids (EAAs) act as precursors and stimuli for synthesis of both mitochondrial and myofibrillar proteins that could potentially confer endurance and strength gains. Thus, we hypothesized that daily consumption of a dietary supplement of nine free EAAs with RET improves endurance in addition to the strength gains by RET. METHODS: Male C57BL6J mice (9 weeks old) were assigned to control (CON), EAA, RET (ladder climbing, 3 times a week), or combined treatment of EAA and RET (EAA + RET) groups. Physical functions focusing on strength or endurance were assessed before and after the interventions. Several analyses were performed to gain better insight into the mechanisms by which muscle function was improved. We determined cumulative rates of myofibrillar and mitochondrial protein synthesis using 2H2O labelling and mass spectrometry; assessed ex vivo contractile properties and in vitro mitochondrial function, evaluated neuromuscular junction (NMJ) stability, and assessed implicated molecular singling pathways. Furthermore, whole-body and muscle insulin sensitivity along with glucose metabolism, were evaluated using a hyperinsulinaemic-euglycaemic clamp. RESULTS: EAA + RET increased muscle mass (10%, P < 0.05) and strength (6%, P < 0.05) more than RET alone, due to an enhanced rate of integrated muscle protein synthesis (19%, P < 0.05) with concomitant activation of Akt1/mTORC1 signalling. Muscle quality (muscle strength normalized to mass) was improved by RET (i.e., RET and EAA + RET) compared with sedentary groups (10%, P < 0.05), which was associated with increased AchR cluster size and MuSK activation (P < 0.05). EAA + RET also increased endurance capacity more than RET alone (26%, P < 0.05) by increasing both mitochondrial protein synthesis (53%, P < 0.05) and DRP1 activation (P < 0.05). Maximal respiratory capacity increased (P < 0.05) through activation of the mTORC1-DRP1 signalling axis. These favourable effects were accompanied by an improvement in basal glucose metabolism (i.e., blood glucose concentrations and endogenous glucose production vs. CON, P < 0.05). CONCLUSIONS: Combined treatment with balanced free EAAs and RET may effectively promote endurance capacity as well as muscle strength through increased muscle protein synthesis, improved NMJ stability, and enhanced mitochondrial dynamics via mTORC1-DRP1 axis activation, ultimately leading to improved basal glucose metabolism.

Insulin Resistance: From Mechanisms to Therapeutic Strategies.

Insulin resistance is the pivotal pathogenic component of many metabolic diseases, including type 2 diabetes mellitus, and is defined as a state of reduced responsiveness of insulin-targeting tissues to physiological levels of insulin. Although the underlying mechanism of insulin resistance is not fully understood, several credible theories have been proposed. In this review, we summarize the functions of insulin in glucose metabolism in typical metabolic tissues and describe the mechanisms proposed to underlie insulin resistance, that is, ectopic lipid accumulation in liver and skeletal muscle, endoplasmic reticulum stress, and inflammation. In addition, we suggest potential therapeutic strategies for addressing insulin resistance.

Bone morphogenic protein 9 is a novel thermogenic hepatokine secreted in response to cold exposure.

OBJECTIVE: Maintaining a constant core body temperature is essential to homeothermic vertebrate survival. Adaptive thermogenesis in brown adipose tissue and skeletal muscle is the primary mechanism of adjustment to an external stimulus such as cold exposure. Recently, several reports have revealed that the liver can play a role as a metabolic hub during adaptive thermogenesis. In this study, we suggest that the liver plays a novel role in secreting thermogenic factors in adaptive thermogenesis. Bone morphogenetic protein 9 (BMP9) is a hepatokine that regulates many biological processes, including osteogenesis, chondrogenesis, hematopoiesis, and angiogenesis. Previously, BMP9 was suggested to affect preadipocyte proliferation and differentiation. However, the conditions and mechanisms underlying hepatic expression and secretion and adipose tissue browning of BMP9 remain largely unknown. In this study, we investigated the physiological conditions for secretion and the regulatory mechanism of hepatic Bmp9 expression and the molecular mechanism by which BMP9 induces thermogenic gene program activation in adipose tissue. Here, we also present the pharmacological effects of BMP9 on a high-fat-induced obese mouse model. METHODS: To investigate the adaptive thermogenic role of BMP9 in vivo, we challenged mice with cold temperature exposure for 3 weeks and then examined the BMP9 plasma concentration and hepatic expression level. The cellular mechanism of hepatic Bmp9 expression under cold exposure was explored through promoter analysis. To identify the role of BMP9 in the differentiation of brown and beige adipocytes, we treated pluripotent stem cells and inguinal white adipose tissue (iWAT)-derived stromal-vascular (SV) cells with BMP9, and brown adipogenesis was monitored by examining thermogenic gene expression and signaling pathways. Furthermore, to evaluate the effect of BMP9 on diet-induced obesity, changes in body composition and glucose tolerance were analyzed in mice administered recombinant BMP9 (rBMP9) for 8 weeks. RESULTS: Hepatic Bmp9 expression and plasma levels in mice were significantly increased after 3 weeks of cold exposure. Bmp9 mRNA expression in the liver was regulated by transcriptional activation induced by cAMP response-element binding protein (CREB) and CREB-binding protein (CBP) on the Bmp9 promoter. Treatment with BMP9 promoted the differentiation of multipotent stem cells and iWAT-derived SV cells into beige adipocytes, as indicated by the increased expression of brown adipocyte and mitochondrial biogenesis markers. Notably, activation of the mothers against decapentaplegic homolog 1 (Smad1) and p44/p42 mitogen-activated protein kinase (MAPK) pathways was required for the induction of uncoupling protein 1 (UCP1) and peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC1α) expression in BMP9-induced differentiation of SVs into beige adipocytes. The administration of rBMP9 in vivo also induced browning markers in white adipose tissue. In high-fat diet-induced obese mice, rBMP9 administration conferred protection against obesity and enhanced glucose tolerance. CONCLUSIONS: BMP9 is a hepatokine regulated by cold-activated CREB and CBP and enhances glucose and fat metabolism by promoting the activation of the thermogenic gene program in adipocytes. These data implicate BMP9 as a potential pharmacological tool for protecting against obesity and type 2 diabetes.

Coenzyme Q10 decreases basic fibroblast growth factor (bFGF)-induced angiogenesis by blocking ERK activation.

Coenzyme Q10 (CoQ10) is an essential factor of the mitochondrial respiratory chain and has effective antioxidant properties. Therefore, CoQ10 has been used in a variety of clinical applications and used as a nutritional supplement to treat several human diseases. Here, we tested the effects of CoQ10 on angiogenesis stimulated by basic fibroblast growth factor (bFGF). CoQ10 significantly inhibited bFGF-induced angiogenesis in a mouse Matrigel plug and the sprouting of endothelial cells in rat aortic rings. In addition, CoQ10 decreased the ability of tube formation, migration, and invasion in endothelial cells. When CoQ10 was used to inhibit angiogenesis in endothelial cells, the expression of vascular endothelial growth factor (VEGF) and the phosphorylation of ERK were decreased. Taken together, these results indicate that CoQ10 is able to act as an antiangiogenic regulator, and its inhibitory activity is mediated by blocking an ERK-dependent pathway. This study suggests that CoQ10 may be used a therapeutic agent to decrease neovascularization in several diseases, including solid tumors.

Ablation of USP21 in skeletal muscle promotes oxidative fibre phenotype, inhibiting obesity and type 2 diabetes.

BACKGROUND: Skeletal muscle as a metabolic consumer determines systemic energy homeostasis by regulating myofibre type conversion and muscle mass control. Perturbation of the skeletal muscle metabolism elevates the risk of a variety of diseases including metabolic disorders. However, the regulatory pathways and molecules are not completely understood. The discovery of relevant responsible molecules and the associated network could be an attractive strategy to overcome diseases associated with muscle problems. METHODS: An initial screening using quantitative trait locus analysis enabled us to extract a set of genes including ubiquitin-specific proteases21 (USP21) (r = 0.738; P = 0.004) as potential targets associated with fasting blood glucose content. Given tight regulation of the ubiquitination status of proteins in muscle, we focused on USP21 and generated whole-body (KO) and skeletal muscle-specific USP21 knockout (MKO) mice. Transcriptomics, proteomics, and lipidomics assays in combination with various in vivo and in vitro experiments were performed to understand the functions of USP21 and underlying mechanisms. A high-fat diet (60%)-fed mouse model and diabetic patient-derived samples were utilized to assess the effects of USP21 on energy metabolism in skeletal muscle. RESULTS: USP21 was highly expressed in both human and mouse skeletal muscle, and controlled skeletal muscle oxidative capacity and fuel consumption. USP21-KO or USP21-MKO significantly promoted oxidative fibre type changes (Δ36.6% or Δ47.2%), muscle mass increase (Δ13.8% to Δ22.8%), and energy expenditure through mitochondrial biogenesis, fatty acid oxidation, and UCP2/3 induction (P < 0.05 or P < 0.01). Consistently, cold exposure repressed USP21 expression in mouse skeletal muscle (Δ55.3%), whereas loss of USP21 increased thermogenesis (+1.37°C or +0.84°C; P < 0.01). Mechanistically, USP21 deubiquitinated DNA-PKcs and ACLY, which led to AMPK inhibition. Consequently, USP21 ablation diminished diet-induced obesity (WT vs. USP21-KO, Δ8.02 g, 17.1%, P < 0.01; litter vs. USP21-MKO, Δ3.48 g, 7.7%, P < 0.05) and insulin resistance. These findings were corroborated in a skeletal muscle-specific gene KO mouse model. USP21 was induced in skeletal muscle of a diabetic patient (1.94-fold), which was reciprocally changed to p-AMPK (0.30-fold). CONCLUSIONS: The outcomes of this research provide novel information as to how USP21 in skeletal muscle contributes to systemic energy homeostasis, demonstrating USP21 as a key molecule in the regulation of myofibre type switch, muscle mass control, mitochondrial function, and heat generation and, thus, implicating the potential of this molecule and its downstream substrates network as targets for the treatment and/or prevention of muscle dysfunction and the associated metabolic diseases.

TFEB-GDF15 axis protects against obesity and insulin resistance as a lysosomal stress response.

TFEB, a key regulator of lysosomal biogenesis and autophagy, is induced not only by nutritional deficiency but also by organelle stress. Here, we find that Tfeb and its downstream genes are upregulated together with lipofuscin accumulation in adipose tissue macrophages (ATMs) of obese mice or humans, suggestive of obesity-associated lysosomal dysfunction/stress in ATMs. Macrophage-specific TFEB-overexpressing mice display complete abrogation of diet-induced obesity, adipose tissue inflammation and insulin resistance, which is independent of autophagy, but dependent on TFEB-induced GDF15 expression. Palmitic acid induces Gdf15 expression through lysosomal Ca2+-mediated TFEB nuclear translocation in response to lysosomal stress. In contrast, mice fed a high-fat diet with macrophage-specific Tfeb deletion show aggravated adipose tissue inflammation and insulin resistance, accompanied by reduced GDF15 level. Finally, we observe activation of TFEB-GDF15 in ATMs of obese humans as a consequence of lysosomal stress. These findings highlight the importance of the TFEB-GDF15 axis as a lysosomal stress response in obesity or metabolic syndrome and as a promising therapeutic target for treatment of these conditions.

Thromboxane A(2) increases endothelial permeability through upregulation of interleukin-8.

Thromboxane A(2) (TXA(2)), a major prostanoid formed from prostaglandin H(2) by thromboxane synthase, is involved in the pathogenesis of a variety of vascular diseases. In this study, we report that TXA(2) mimetic U46619 significantly increases the endothelial permeability both in vitro and in vivo. U46619 enhanced the expression and secretion of interleukin-8 (IL-8), a major inducer of vascular permeability, in endothelial cells. Promoter analysis showed that the U46619-induced expression of IL-8 was mainly regulated by nuclear factor-kappaB (NF-kappaB). U46619 induced the activation of NF-kappaB through IkappaB kinase (IKK) activation, IkappaB phosphorylation and NF-kappaB nuclear translocation. Furthermore, the inhibition of IL-8 or blockade of the IL-8 receptor attenuated the U46619-induced endothelial cell permeability by modulating the cell-cell junctions. Overall, these results suggest that U46619 promotes vascular permeability through the production of IL-8 via NF-kappaB activation in endothelial cells.

Melatonin suppresses tumor angiogenesis by inhibiting HIF-1alpha stabilization under hypoxia.

Angiogenesis is an important mediator of tumor progression. As tumors expand, diffusion distances from the existing vascular supply increases, resulting in hypoxia in the cancer cells. Sustained expansion of a tumor mass requires new blood vessel formation to provide rapidly proliferating tumor cells with an adequate supply of oxygen and nutrients. The key regulator of hypoxia-induced angiogenesis is the transcription factor known as hypoxia-inducible factor (HIF)-1. HIF-1alpha is stabilized by hypoxia-induced reactive oxygen species (ROS) and enhances the expression of several types of hypoxic genes, including that of the angiogenic activator known as vascular endothelial cell growth factor (VEGF). In this study, we found that melatonin, a small lipophilic molecule secreted primarily by the pineal gland, destabilizes hypoxia-induced HIF-1alpha protein levels in the HCT116 human colon cancer cell line. This destabilization of HIF-1alpha resulted from the antioxidant activity of melatonin against ROS induced by hypoxia. Moreover, under hypoxia, melatonin suppressed HIF-1 transcriptional activity, leading to a decrease in VEGF expression. Melatonin also blocked in vitro tube formation and invasion and migration of human umbilical vein endothelial cells induced by hypoxia-stimulated conditioned media of HCT116 cells. These findings suggest that melatonin could play a pivotal role in tumor suppression via inhibition of HIF-1-mediated angiogenesis.

The essential role of fructose-1,6-bisphosphatase 2 enzyme in thermal homeostasis upon cold stress.

Skeletal muscle is a major organ for glucose disposal and thermogenesis. While hepatic fructose-1,6-bisphosphatase is well known as a key enzyme for gluconeogenesis, the role of muscle fructose-1,6-bisphosphatase 2 (Fbp2) in glucose disposal and thermogenesis is unknown. Here, using Fbp2 knockout (KO) mice, we assessed the physiological role of Fbp2 in energy and glucose metabolism and thermogenesis. In vivo assessments of energy metabolism, glucose metabolism, and thermogenesis were performed by indirect calorimetry, hyperinsulinemic-euglycemic clamp, and cold challenge studies, respectively. Under both feeding and fasting conditions, Fbp2 KO mice showed similar phenotypes regarding energy and glucose metabolism compared to wild-type (WT) mice. However, Fbp2 KO mice were severely intolerant to cold challenge under fasting conditions. Mechanistically, the cold-induced intramuscular conversion of lactate to glycogen (glyconeogenesis) is completely abolished in the KO muscle, which leads to a lack of glycogen source for thermogenesis in Fbp2 KO mice. The cold-intolerant phenotype of KO mice disappeared after feeding, and the KO mice were equally as cold tolerant as the WT mice and survived during the cold challenge for three weeks. Taken together, these data demonstrate that Fbp2 is essential for muscle thermogenesis by replenishing the intramuscular glycogen pool through glyconeogenesis when the exogenous glucose source is limited. These data imply the physiological importance of Fbp2 in thermal homeostasis and suggest a potential novel therapy targeted to increase glycogen replenishment upon cold stress.

Grasp55-/- mice display impaired fat absorption and resistance to high-fat diet-induced obesity.

The Golgi apparatus plays a central role in the intracellular transport of macromolecules. However, molecular mechanisms of Golgi-mediated lipid transport remain poorly understood. Here, we show that genetic inactivation of the Golgi-resident protein GRASP55 in mice reduces whole-body fat mass via impaired intestinal fat absorption and evokes resistance to high-fat diet induced body weight gain. Mechanistic analyses reveal that GRASP55 participates in the Golgi-mediated lipid droplet (LD) targeting of some LD-associated lipases, such as ATGL and MGL, which is required for sustained lipid supply for chylomicron assembly and secretion. Consequently, GRASP55 deficiency leads to reduced chylomicron secretion and abnormally large LD formation in intestinal epithelial cells upon exogenous lipid challenge. Notably, deletion of dGrasp in Drosophila causes similar defects of lipid accumulation in the midgut. These results highlight the importance of the Golgi complex in cellular lipid regulation, which is evolutionary conserved, and uncover potential therapeutic targets for obesity-associated diseases.

Overproduction of inter-α-trypsin inhibitor heavy chain 1 after loss of Gα13 in liver exacerbates systemic insulin resistance in mice.

The impact of liver disease on whole-body glucose homeostasis is largely attributed to dysregulated release of secretory proteins in response to metabolic stress. The molecular cues linking liver to whole-body glucose metabolism remain elusive. We found that expression of G protein α-13 (Gα13) was decreased in the liver of mice and humans with diabetes. Liver-specific deletion of the Gna13 gene in mice resulted in systemic glucose intolerance. Comparative secretome analysis identified inter-α-trypsin inhibitor heavy chain 1 (ITIH1) as a protein secreted by liver that was responsible for systemic insulin resistance in Gna13-deficient mice. Liver expression of ITIH1 positively correlated with surrogate markers for diabetes in patients with impaired glucose tolerance or overt diabetes. Mechanistically, a decrease in hepatic Gα13 caused ITIH1 oversecretion by liver through induction of O-GlcNAc transferase expression, facilitating ITIH1 deposition on the hyaluronan surrounding mouse adipose tissue and skeletal muscle. Neutralization of secreted ITIH1 ameliorated glucose intolerance in obese mice. Our findings demonstrate systemic insulin resistance in mice resulting from liver-secreted ITIH1 downstream of Gα13 and its reversal by ITIH1 neutralization.

KR-31831, benzopyran derivative, inhibits VEGF-induced angiogenesis of HUVECs through suppressing KDR expression.

Angiogenesis is important in the development and progression of cancer, therefore the therapeutic approach based on anti-angiogenesis may represent a promising therapeutic option. KR-31831 is a novel anti-ischemic agent. Previously, we reported the anti-angiogenic activity of KR-31831. In the present study we investigated the molecular mechanisms underlying anti-angiogenic activity of KR-31831. We show that KR-31831 inhibits vascular endothelial growth factor (VEGF)-induced proliferation and tube formation via release of intracellular Ca2+ and phosphorylation of extra-cellular regulated kinase 1/2 (Erk 1/2) in human umbilical vein endothelial cells (HUVECs). Moreover, the expression of VEGF receptor 2 (VEGFR2, known as Flk-1 or KDR) was reduced by the treatment of KR-31831. These results suggest that KR-31831 may have inhibitory effects on tumor angiogenesis through down-regulation of KDR expression.

Mitochondrial dysfunction suppresses p53 expression via calcium-mediated nuclear factor-kB signaling in HCT116 human colorectal carcinoma cells.

Mitochondrial DNA (mtDNA) mutations are often observed in various cancer types. Although the correlation between mitochondrial dysfunction and cancer malignancy has been demonstrated by several studies, further research is required to elucidate the molecular mechanisms underlying accelerated tumor development and progression due to mitochondrial mutations. We generated an mtDNA-depleted cell line, ρ°, via long-term ethidium bromide treatment to define the molecular mechanisms of tumor malignancy induced by mitochondrial dysfunction. Mitochondrial dysfunction in ρ° cells reduced drug-induced cell death and decreased the expression of pro-apoptotic proteins including p53. The p53 expression was reduced by activation of nuclear factor-κB that depended on elevated levels of free calcium in HCT116/ρ° cells. Overall, these data provide a novel mechanism for tumor development and drug resistance due to mitochondrial dysfunction. [BMB Reports 2018; 51(6): 296-301].

Eosinophils support adipocyte maturation and promote glucose tolerance in obesity.

Accumulating data have indicated a fundamental role of eosinophils in regulating adipose tissue homeostasis. Here, we performed whole-genome RNA sequencing of the small intestinal tract, which suggested the presence of impaired lipid metabolism in eosinophil-deficient ΔdblGATA mice. ΔdblGATA mice fed a high-fat diet (HFD) showed reduced body fat mass, impaired enlargement of adipocytes, decreased expression of adipogenic genes, and developed glucose intolerance. HFD induced accumulation of eosinophils in the perigonadal white adipose tissue. Concordantly, adipocyte-differentiated 3T3-L1 cells promoted the migration of eosinophils through the expression of CCL11 (eotaxin-1) and likely promoted their survival through the expression of interleukin (IL)-3, IL-5, and granulocyte-macrophage colony-stimulating factor. HFD-fed ΔdblGATA mice showed increased infiltration of macrophages, CD4+ T-cells, and B-cells, increased expression of interferon-γ, and decreased expression of IL-4 and IL-13 in white adipose tissue. Interferon-γ treatment significantly decreased lipid deposition in adipocyte-differentiated 3T3-L1 cells, while IL-4 treatment promoted lipid accumulation. Notably, HFD-fed ΔdblGATA mice showed increased lipid storage in the liver as compared with wild-type mice. We propose that obesity promotes the infiltration of eosinophils into adipose tissue that subsequently contribute to the metabolic homeostasis by promoting adipocyte maturation.

Pivotal Role of O-GlcNAc Modification in Cold-Induced Thermogenesis by Brown Adipose Tissue Through Mitochondrial Biogenesis.

Adipose tissues considerably influence metabolic homeostasis, and both white (WAT) and brown (BAT) adipose tissue play significant roles in lipid and glucose metabolism. O-linked N-acetylglucosamine (O-GlcNAc) modification is characterized by the addition of N-acetylglucosamine to various proteins by O-GlcNAc transferase (Ogt), subsequently modulating various cellular processes. However, little is known about the role of O-GlcNAc modification in adipose tissues. Here, we report the critical role of O-GlcNAc modification in cold-induced thermogenesis. Deletion of Ogt in WAT and BAT using adiponectin promoter-driven Cre recombinase resulted in severe cold intolerance with decreased uncoupling protein 1 (Ucp1) expression. Furthermore, Ogt deletion led to decreased mitochondrial protein expression in conjunction with decreased peroxisome proliferator-activated receptor γ coactivator 1-α protein expression. This phenotype was further confirmed by deletion of Ogt in BAT using Ucp1 promoter-driven Cre recombinase, suggesting that O-GlcNAc modification in BAT is responsible for cold-induced thermogenesis. Hypothermia was significant under fasting conditions. This effect was mitigated after normal diet consumption but not after consumption of a fatty acid-rich ketogenic diet lacking carbohydrates, suggesting impaired diet-induced thermogenesis, particularly by fat. In conclusion, O-GlcNAc modification is essential for cold-induced thermogenesis and mitochondrial biogenesis in BAT. Glucose flux into BAT may be a signal to maintain BAT physiological responses.

Gα13 ablation reprograms myofibers to oxidative phenotype and enhances whole-body metabolism.

Skeletal muscle is a key organ in energy homeostasis owing to its high requirement for nutrients. Heterotrimeric G proteins converge signals from cell-surface receptors to potentiate or blunt responses against environmental changes. Here, we show that muscle-specific ablation of Gα13 in mice promotes reprogramming of myofibers to the oxidative type, with resultant increases in mitochondrial biogenesis and cellular respiration. Mechanistically, Gα13 and its downstream effector RhoA suppressed nuclear factor of activated T cells 1 (NFATc1), a chief regulator of myofiber conversion, by increasing Rho-associated kinase 2-mediated (Rock2-mediated) phosphorylation at Ser243. Ser243 phosphorylation of NFATc1 was reduced after exercise, but was higher in obese animals. Consequently, Gα13 ablation in muscles enhanced whole-body energy metabolism and increased insulin sensitivity, thus affording protection from diet-induced obesity and hepatic steatosis. Our results define Gα13 as a switch regulator of myofiber reprogramming, implying that modulations of Gα13 and its downstream effectors in skeletal muscle are a potential therapeutic approach to treating metabolic diseases.

Adipocyte-Specific Deficiency of De Novo Sphingolipid Biosynthesis Leads to Lipodystrophy and Insulin Resistance.

Sphingolipids have been implicated in the etiology of chronic metabolic diseases. Here, we investigated whether sphingolipid biosynthesis is associated with the development of adipose tissues and metabolic diseases. SPTLC2, a subunit of serine palmitoyltransferase, was transcriptionally upregulated in the adipose tissues of obese mice and in differentiating adipocytes. Adipocyte-specific SPTLC2-deficient (aSPTLC2 KO) mice had markedly reduced adipose tissue mass. Fatty acids that were destined for the adipose tissue were instead shunted to liver and caused hepatosteatosis. This impaired fat distribution caused systemic insulin resistance and hyperglycemia, indicating severe lipodystrophy. Mechanistically, sphingosine 1-phosphate (S1P) was reduced in the adipose tissues of aSPTLC2 KO mice, and this inhibited adipocyte proliferation and differentiation via the downregulation of S1P receptor 1 and decreased activity of the peroxisome proliferator-activator receptor γ. In addition, downregulation of SREBP (sterol regulatory element-binding protein)-1c prevented adipogenesis of aSPTLC2 KO adipocytes. Collectively, our observations suggest that the tight regulation of de novo sphingolipid biosynthesis and S1P signaling plays an important role in adipogenesis and hepatosteatosis.

Corrigendum: Mitochondrial ATP transporter depletion protects mice against liver steatosis and insulin resistance.

[This corrects the article DOI: 10.1038/ncomms14477.].

Mitochondrial ATP transporter depletion protects mice against liver steatosis and insulin resistance.

Non-alcoholic fatty liver disease (NAFLD) is a common metabolic disorder in obese individuals. Adenine nucleotide translocase (ANT) exchanges ADP/ATP through the mitochondrial inner membrane, and Ant2 is the predominant isoform expressed in the liver. Here we demonstrate that targeted disruption of Ant2 in mouse liver enhances uncoupled respiration without damaging mitochondrial integrity and liver functions. Interestingly, liver specific Ant2 knockout mice are leaner and resistant to hepatic steatosis, obesity and insulin resistance under a lipogenic diet. Protection against fatty liver is partially recapitulated by the systemic administration of low-dose carboxyatractyloside, a specific inhibitor of ANT. Targeted manipulation of hepatic mitochondrial metabolism, particularly through inhibition of ANT, may represent an alternative approach in NAFLD and obesity treatment.