Increased glycolysis in skeletal muscle coordinates with adipose tissue in systemic metabolic homeostasis

Cytosolic nucleic acid sensing and mitochondrial transcriptomic changes as early triggers of metabolic disease in db/db mice

Animal models of diabetes, such as db/db mice, are a useful tool for deciphering the genetic background of molecular changes at the initial stages of disease development. Our goal was to find early transcriptomic changes in three tissues involved in metabolism regulation in db/db mice: adipose tissue, muscle tissue and liver tissue. Nine animals (three per time point) were studied. Tissues were collected at 8, 12 and 16 weeks of age. Transcriptome-wide analysis was performed using mRNA-seq. Libraries were sequenced on NextSeq (Illumina). Differential expression (DE) analysis was performed with edgeR. The analysis of the gene expression profile shared by all three tissues revealed eight upregulated genes (Irf7, Sp100, Neb, Stat2, Oas2, Rtp4, H2-T24 and Oasl2) as early as between 8 and 12 weeks of age. The most pronounced differences were found in liver tissue: nine DE genes between 8 and 12 weeks of age (Irf7, Ly6a, Ly6g6d, H2-Dma, Pld4, Ly86, Fcer1g, Ly6e and Idi1) and five between 12 and 16 weeks of age (Irf7, Plac8, Ifi44, Xaf1 and Ly6a) (adj. p-value < 0.05). The mitochondrial transcriptomic profile also changed with time: we found two downregulated genes in mice between 8 and 12 weeks old (Ckmt2 and Cox6a2) and five DE genes between 12 and 16 weeks of age (Mavs, Tomm40L, Mtfp1, Ckmt2 and Cox6a2). The KEGG pathway analysis showed significant enrichment in pathways related to the autoimmune response and cytosolic DNA sensing. Our results suggest an important involvement of the immunological response, mainly cytosolic nucleic acid sensing, and mitochondrial signalling in the early stages of diabetes and obesity.

PHLPP1 inhibits the growth and aerobic glycolysis activity of human ovarian granular cells through inactivating AKT pathway

BACKGROUND

Polycystic ovary syndrome (PCOS) is a disorder characterized by hyperandrogenism, ovulatory dysfunction, and polycystic ovarian morphologic features, and PCOS is associated with infertility. PH domain Leucine-rich repeat Protein Phosphatase 1 (PHLPP1) has been shown to regulate AKT. The aim of present study is to investigate the role of PHLPP1 in PCOS.

METHODS

The expression levels of PHLPP1 in dihydrotestosterone (DHT)-treated human ovarian granular KGN cells were determined by qRT-PCR and Western blot. PHLPP1 was silenced or overexpressed using lentivirus. Cell proliferation was detected by CCK-8. Apoptosis and ROS generation were analyzed by flow cytometry. Glycolysis was analyzed by measuring extracellular acidification rate (ECAR).

RESULTS

DHT treatment suppressed proliferation, promoted apoptosis, enhanced ROS, and inhibited glycolysis in KGN cells. PHLPP1 silencing alleviated the DHT-induced suppression of proliferation and glycolysis, and promotion of apoptosis and ROS in KGN cells. PHLPP1 regulated cell proliferation and glycolysis in human KGN cells via the AKT signaling pathway.

CONCLUSIONS

Our results showed that PHLPP1 mediates the proliferation and aerobic glycolysis activity of human ovarian granular cells through regulating AKT signaling.

Skeletal muscle-secreted DLPC orchestrates systemic energy homeostasis by enhancing adipose browning

MyoD is a skeletal muscle-specifically expressed transcription factor and plays a critical role in regulating myogenesis during muscle development and regeneration. However, whether myofibers-expressed MyoD exerts its metabolic function in regulating whole body energy homeostasis in vivo remains largely unknown. Here, we report that genetic deletion of Myod in male mice enhances the oxidative metabolism of muscle and, intriguingly, renders the male mice resistant to high fat diet-induced obesity. By performing lipidomic analysis in muscle-conditioned medium and serum, we identify 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC) as a muscle-released lipid that is responsible for MyoD-orchestrated body energy homeostasis in male Myod KO mice. Functionally, the administration of DLPC significantly ameliorates HFD-induced obesity in male mice. Mechanistically, DLPC is found to induce white adipose browning via lipid peroxidation-mediated p38 signaling in male mice. Collectively, our findings not only uncover a novel function of MyoD in controlling systemic energy homeostasis through the muscle-derived lipokine DLPC but also suggest that the DLPC might have clinical potential for treating obesity in humans.

Fibroblast Growth Factor 21: A Fascinating Perspective on the Regulation of Muscle Metabolism

Fibroblast growth factor 21 (FGF21) plays a vital role in normal eukaryotic organism development and homeostatic metabolism under the influence of internal and external factors such as endogenous hormone changes and exogenous stimuli. Over the last few decades, comprehensive studies have revealed the key role of FGF21 in regulating many fundamental metabolic pathways, including the muscle stress response, insulin signaling transmission, and muscle development. By coordinating these metabolic pathways, FGF21 is thought to contribute to acclimating to a stressful environment and the subsequent recovery of cell and tissue homeostasis. With the emphasis on FGF21, we extensively reviewed the research findings on the production and regulation of FGF21 and its role in muscle metabolism. We also emphasize how the FGF21 metabolic networks mediate mitochondrial dysfunction, glycogen consumption, and myogenic development and investigate prospective directions for the functional exploitation of FGF21 and its downstream effectors, such as the mammalian target of rapamycin (mTOR).

The Expression of Two Distinct Sets of Glycolytic Enzymes Reveals Differential Effects of Glycolytic Reprogramming on Pancreatic Ductal Tumorigenesis in Mice

Pancreatic ductal adenocarcinoma (PDAC) is associated with enhanced aerobic glycolysis through elevated glucose uptake and the upregulated expression of genes encoding rate-limiting glycolytic enzymes. However, the direct impact of altered glycolytic pathways on pancreatic tumor progression has not been thoroughly investigated. Here, we utilized two strains of BAC transgenic mice with pancreatic expression of two distinct sets of glycolytic genes each arranged in a polycistronic fashion (PFKFB3-HK2-GLUT1 and LDHA-PDK1, respectively) to investigate the role of altered glycolysis on the development of pancreatic ductal tumor development in the Pdx1-Cre; LSL-KrasG12D mice. The overexpression of the two sets of glycolytic genes exhibited no significant effects on tumor development in the 4-5-month-old mice (the PanIN2 lesions stage). In the 9-10-month-old mice, the overexpression of PFKFB3-HK2-GLUT1 significantly accelerated PanIN3 progression, exhibiting elevated levels of ductal cell marker CK19 and tumor fibrosis. Surprisingly, the overexpression of LDHA-PDK1 significantly attenuated the progression of PanIN3 in the 9-10-month-old mice with significantly downregulated levels of CK19 and fibrosis. Therefore, distinct set of glycolytic enzymes that are involved in different glycolytic routes exhibited contrasting effects on pancreatic ductal tumor development depending on the tumor stages, providing novel insights into the complexity of the glycolytic pathway in the perspective of PDAC development and therapy.

Untargeted muscle tissue metabolites profiling in young, adult, and old rats supplemented with tocotrienol-rich fraction

The greatest significant influence on human life span and health is inevitable ageing. One of the distinguishing characteristics of ageing is the gradual decrease of muscle mass and physical function. There has been growing evidence that tocotrienol can guard against age-associated chronic diseases and metabolic disorders. This study aimed to elucidate the effects of tocotrienol-rich fraction (TRF) on muscle metabolomes and metabolic pathways in ageing Sprague Dawley (SD) rats. Three months, 9 months, and 21 months old male SD rats were divided into control and treated groups with 10 rats per group. Rats in control and treated groups were given 60 mg/kg body weight/day of palm olein and 60 mg/kg body weight/day of TRF, respectively, via oral gavage for 3 months. Muscle performance was assessed at 0 and 3 months of treatment by measuring muscle strength and function. Our results showed that TRF treatment caused a significant increase in the swimming time of the young rats. Comparison in the control groups showed that metabolites involved in lipid metabolisms such as L-palmitoyl carnitine and decanoyl carnitine were increased in ageing. In contrast, several metabolites, such as 3-phosphoglyceric acid, aspartic acid and aspartyl phenylalanine were decreased. These findings indicated that muscle metabolomes involved in lipid metabolism were upregulated in aged rats. In contrast, the metabolites involved in energy and amino acid metabolism were significantly downregulated. Comparison in the TRF-supplemented groups showed an upregulation of metabolites involved in energy and amino acid metabolism. Metabolites such as N6-methyl adenosine, spermine, phenylalanine, tryptophan, aspartic acid, histidine, and N-acetyl neuraminic acid were up-regulated, indicating promotion of amino acid synthesis and muscle regeneration. Energy metabolism was also improved in adult and old rats with TRF supplementation as indicated by the upregulation of nicotinamide adenine dinucleotide and glycerol 3-phosphate compared to the control group. In conclusion, the mechanism underlying the changes in skeletal muscle mass and functions in ageing was related to carbohydrate, lipid and amino acid metabolism. Tocotrienol supplementation showed beneficial effects in alleviating energy and amino acid synthesis that may promote the regeneration and renewal of skeletal muscle in ageing rats.

AIFM2 Is Required for High-Intensity Aerobic Exercise in Promoting Glucose Utilization

Skeletal muscle is a major regulator of glycemic control at rest, and glucose utilization increases drastically during exercise. Sustaining a high glucose utilization via glycolysis requires efficient replenishment of NAD+ in the cytosol. Apoptosis-inducing mitochondrion-associated factor 2 (AIFM2) was previously shown to be a NADH oxidoreductase domain-containing flavoprotein that promotes glycolysis for diet and cold-induced thermogenesis. Here, we find that AIFM2 is selectively and highly induced in glycolytic extensor digitorum longus (EDL) muscle during exercise. Overexpression (OE) of AIFM2 in myotubes is sufficient to elevate the NAD+-to-NADH ratio, increasing the glycolytic rate. Thus, OE of AIFM2 in skeletal muscle greatly increases exercise capacity, with increased glucose utilization. Conversely, muscle-specific Aifm2 depletion via in vivo transfection of hairpins against Aifm2 or tamoxifen-inducible haploinsufficiency of Aifm2 in muscles decreases exercise capacity and glucose utilization in mice. Moreover, muscle-specific introduction of NDE1, Saccharomyces cerevisiae external NADH dehydrogenase (NDE), ameliorates impairment in glucose utilization and exercise intolerance of the muscle-specific Aifm2 haploinsufficient mice. Together, we show a novel role for AIFM2 as a critical metabolic regulator for efficient utilization of glucose in glycolytic EDL muscles.

Dendropanax trifidus Sap-Mediated Suppression of Obese Mouse Body Weight and the Metabolic Changes Related with Estrogen Receptor Alpha and AMPK-ACC Pathways in Muscle Cells

Dendropanax trifidus (DT) is a medicinal herb native to East Asia, which has been used extensively for its therapeutic properties in traditional medicine. In this study, we examined the effects of DT sap on the regulation of body weight and muscle metabolism in mice. Obese model db/db mice were administered daily with DT sap or vehicle control over a 6-week period. The effects of DT sap on muscle metabolism were studied in C2C12 muscle cells, where glycolytic and mitochondrial respiration rates were monitored. As AMP-activated protein kinase (AMPK) is a master regulator of metabolism and plays an important function as an energy sensor in muscle tissue, signaling pathways related with AMPK were also examined. We found that DT sap inhibited body weight increase in db/db, db/+, and +/+ mice over a 6-week period, while DT sap-treated muscle cells showed increased muscle metabolism and also increased phosphorylation of AMPK and Acetyl-CoA Carboxylase (ACC). Finally, we found that DT sap, which is enriched in estrogen in our previous study, significantly activates estrogen alpha receptor in a concentration-dependent manner, which can drive the activation of AMPK signaling and may be related to the muscle metabolism and weight changes observed here.

Systems-level analysis of insulin action in mouse strains provides insight into tissue- and pathway-specific interactions that drive insulin resistance

Skeletal muscle and adipose tissue insulin resistance are major drivers of metabolic disease. To uncover pathways involved in insulin resistance, specifically in these tissues, we leveraged the metabolic diversity of different dietary exposures and discrete inbred mouse strains. This revealed that muscle insulin resistance was driven by gene-by-environment interactions and was strongly correlated with hyperinsulinemia and decreased levels of ten key glycolytic enzymes. Remarkably, there was no relationship between muscle and adipose tissue insulin action. Adipocyte size profoundly varied across strains and diets, and this was strongly correlated with adipose tissue insulin resistance. The A/J strain, in particular, exhibited marked adipocyte insulin resistance and hypertrophy despite robust muscle insulin responsiveness, challenging the role of adipocyte hypertrophy per se in systemic insulin resistance. These data demonstrate that muscle and adipose tissue insulin resistance can occur independently and underscore the need for tissue-specific interrogation to understand metabolic disease.

Increased glycolysis in skeletal muscle coordinates with adipose tissue in systemic metabolic homeostasis

Insulin‐independent glucose metabolism, including anaerobic glycolysis that is promoted in resistance training, plays critical roles in glucose disposal and systemic metabolic regulation. However, the underlying mechanisms are not completely understood. In this study, through genetically manipulating the glycolytic process by overexpressing human glucose transporter 1 (GLUT1), hexokinase 2 (HK2) and 6‐phosphofructo‐2‐kinase‐fructose‐2,6‐biphosphatase 3 (PFKFB3) in mouse skeletal muscle, we examined the impact of enhanced glycolysis in metabolic homeostasis. Enhanced glycolysis in skeletal muscle promoted accelerated glucose disposal, a lean phenotype and a high metabolic rate in mice despite attenuated lipid metabolism in muscle, even under High‐Fat diet (HFD). Further study revealed that the glucose metabolite sensor carbohydrate‐response element‐binding protein (ChREBP) was activated in the highly glycolytic muscle and stimulated the elevation of plasma fibroblast growth factor 21 (FGF21), possibly mediating enhanced lipid oxidation in adipose tissue and contributing to a systemic effect. PFKFB3 was critically involved in promoting the glucose‐sensing mechanism in myocytes. Thus, a high level of glycolysis in skeletal muscle may be intrinsically coupled to distal lipid metabolism through intracellular glucose sensing. This study provides novel insights for the benefit of resistance training and for manipulating insulin‐independent glucose metabolism.