Effect of resistance exercise under conditions of reduced blood insulin on AMPKα Ser485/491 inhibitory phosphorylation and AMPK pathway activation

Time-series transcriptomics reveals distinctive mRNA expression dynamics associated with gene ontology specificity and protein expression in skeletal muscle after electrical stimulation-induced resistance exercise

Resistance exercise upregulates and downregulates the expression of a wide range of genes in skeletal muscle. However, detailed analysis of mRNA dynamics such as response rates and temporal patterns of the transcriptome after resistance exercise has not been performed. We aimed to clarify the dynamics of time-series transcriptomics after resistance exercise. We used electrical stimulation-induced muscle contraction as a resistance exercise model (5 sets × 10 times of 3 s of 100-Hz electrical stimulation) on the tibialis anterior muscle of rats and measured the transcriptome in the muscle before and at 0, 1, 3, 6, and 12 h after muscle contractions by RNA sequencing. We also examined the relationship between the parameters of mRNA dynamics and the increase in protein expression at 12 h after muscle contractions. We found that the function of the upregulated genes differed after muscle contractions depending on their response rate. Genes related to muscle differentiation and response to mechanical stimulus were enriched in the sustainedly upregulated genes. Furthermore, there was a positive correlation between the magnitude of upregulated mRNA expression and the corresponding protein expression level at 12 h after muscle contractions. Although it has been theoretically suggested, this study experimentally demonstrated that the magnitude of the mRNA response after electrical stimulation-induced resistance exercise contributes to skeletal muscle adaptation via increases in protein expression. These findings suggest that mRNA expression dynamics such as response rate, a sustained upregulated expression pattern, and the magnitude of the response contribute to mechanisms underlying adaptation to resistance exercise.

Phosphorylation of AMPKα at Ser485/491 Is Dependent on Muscle Contraction and Not Muscle-Specific IGF-I Overexpression

Glucose is an important fuel for highly active skeletal muscles. Increased adenosine monophosphate (AMP)/adenosine triphosphate (ATP) ratios during repetitive contractions trigger AMP-activated protein kinase (AMPK), indicated by phosphorylation of AMPKαThr172, which promotes glucose uptake to support heightened energy needs, but it also suppresses anabolic processes. Inhibition of AMPK can occur by protein kinase B (AKT)-mediated phosphorylation of AMPKαSer485/491, releasing its brake on growth. The influence of insulin-like growth factor I (IGF-I) on glucose uptake and its interplay with AMPK activation is not well understood. Thus, the goal of this study was to determine if increased muscle IGF-I altered AMPKα phosphorylation and activity during muscle contraction. Adult male mice harboring the rat Igf1a cDNA regulated by the fast myosin light chain promoter (mIgf1+/+) and wildtype littermates (WT) were used in the study. mIgf1+/+ mice had enhanced glucose tolerance and insulin-stimulated glucose uptake, but similar exercise capacity. Fatiguing stimulations of extensor digitorum longus (EDL) muscles resulted in upregulated AMPKα phosphorylation at both Thr172 and Ser485/491 in WT and mIgf1+/+ muscles. No differences in the phosphorylation response of the downstream AMPK target TBC1D1 were observed, but phosphorylation of raptor was significantly higher only in WT muscles. Further, total raptor content was elevated in mIgf1+/+ muscles. The results show that high muscle IGF-I can enhance glucose uptake under resting conditions; however, in contracting muscle, it is not sufficient to inhibit AMPK activity.

Ablation of Akt2 and AMPKα2 rescues high fat diet-induced obesity and hepatic steatosis through Parkin-mediated mitophagy

Given the opposing effects of Akt and AMP-activated protein kinase (AMPK) on metabolic homeostasis, this study examined the effects of deletion of Akt2 and AMPKα2 on fat diet-induced hepatic steatosis. Akt2-Ampkα2 double knockout (DKO) mice were placed on high fat diet for 5 months. Glucose metabolism, energy homeostasis, cardiac function, lipid accumulation, and hepatic steatosis were examined. DKO mice were lean without anthropometric defects. High fat intake led to adiposity and decreased respiratory exchange ratio (RER) in wild-type (WT) mice, which were ablated in DKO but not Akt2 -/- and Ampkα2 -/- mice. High fat intake increased blood and hepatic triglycerides and cholesterol, promoted hepatic steatosis and injury in WT mice. These effects were eliminated in DKO but not Akt2 -/- and Ampkα2 -/- mice. Fat diet promoted fat accumulation, and enlarged adipocyte size, the effect was negated in DKO mice. Fat intake elevated fatty acid synthase (FAS), carbohydrate-responsive element-binding protein (CHREBP), sterol regulatory element-binding protein 1 (SREBP1), peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), peroxisome proliferator-activated receptor-α (PPARα), PPARγ, stearoyl-CoA desaturase 1 (SCD-1), phosphoenolpyruvate carboxykinase (PEPCK), glucose 6-phosphatase (G6Pase), and diglyceride O-acyltransferase 1 (DGAT1), the effect was absent in DKO but not Akt2 -/- and Ampkα2 -/- mice. Fat diet dampened mitophagy, promoted inflammation and phosphorylation of forkhead box protein O1 (FoxO1) and AMPKα1 (Ser485), the effects were eradicated by DKO. Deletion of Parkin effectively nullified DKO-induced metabolic benefits against high fat intake. Liver samples from obese humans displayed lowered microtubule-associated proteins 1A/1B light chain 3B (LC3B), Pink1, Parkin, as well as enhanced phosphorylation of Akt, AMPK (Ser485), and FoxO1, which were consolidated by RNA sequencing (RNAseq) and mass spectrometry analyses from rodent and human livers. These data suggest that concurrent deletion of Akt2 and AMPKα2 offers resilience to fat diet-induced obesity and hepatic steatosis, possibly through preservation of Parkin-mediated mitophagy and lipid metabolism.

Enhanced skeletal muscle insulin sensitivity after acute resistance-type exercise is upregulated by rapamycin-sensitive mTOR complex 1 inhibition

Acute aerobic exercise (AE) increases skeletal muscle insulin sensitivity for several hours, caused by acute activation of AMP-activated protein kinase (AMPK). Acute resistance exercise (RE) also activates AMPK, possibly improving insulin-stimulated glucose uptake. However, RE-induced rapamycin-sensitive mechanistic target of rapamycin complex 1 (mTORC1) activation is higher and has a longer duration than after AE. In molecular studies, mTORC1 was shown to be upstream of insulin receptor substrate 1 (IRS-1) Ser phosphorylation residue, inducing insulin resistance. Therefore, we hypothesised that although RE increases insulin sensitivity through AMPK activation, prolonged mTORC1 activation after RE reduces RE-induced insulin sensitising effect. In this study, we used an electrical stimulation-induced RE model in rats, with rapamycin as an inhibitor of mTORC1 activation. Our results showed that RE increased insulin-stimulated glucose uptake following AMPK signal activation. However, mTORC1 activation and IRS-1 Ser632/635 and Ser612 phosphorylation were elevated 6 h after RE, with concomitant impairment of insulin-stimulated Akt signal activation. By contrast, rapamycin inhibited these prior exercise responses. Furthermore, increases in insulin-stimulated skeletal muscle glucose uptake 6 h after RE were higher in rats with rapamycin treatment than with placebo treatment. Our data suggest that mTORC1/IRS-1 signaling inhibition enhances skeletal muscle insulin-sensitising effect of RE.

A novel voluntary weightlifting model in mice promotes muscle adaptation and insulin sensitivity with simultaneous enhancement of autophagy and mTOR pathway

Our understanding of the molecular mechanisms underlying adaptations to resistance exercise remains elusive despite the significant biological and clinical relevance. We developed a novel voluntary mouse weightlifting model, which elicits squat-like activities against adjustable load during feeding, to investigate the resistance exercise-induced contractile and metabolic adaptations. RNAseq analysis revealed that a single bout of weightlifting induced significant transcriptome responses of genes that function in posttranslational modification, metabolism, and muscle differentiation in recruited skeletal muscles, which were confirmed by increased expression of fibroblast growth factor-inducible 14 (Fn14), Down syndrome critical region 1 (Dscr1) and Nuclear receptor subfamily 4, group A, member 3 (Nr4a3) genes. Long-term (8 weeks) voluntary weightlifting training resulted in significantly increases of muscle mass, protein synthesis (puromycin incorporation in SUnSET assay) and mTOR pathway protein expression (raptor, 4e-bp-1, and p70S6K proteins) along with enhanced muscle power (specific torque and contraction speed), but not endurance capacity, mitochondrial biogenesis, and fiber type transformation. Importantly, weightlifting training profound improved whole-body glucose clearance and skeletal muscle insulin sensitivity along with enhanced autophagy (increased LC3 and LC3-II/I ratio, and decreased p62/Sqstm1). These data suggest that resistance training in mice promotes muscle adaptation and insulin sensitivity with simultaneous enhancement of autophagy and mTOR pathway.

Exercise prevents the adverse effects of maternal obesity on placental vascularization and fetal growth

KEY POINTS

Maternal exercise improves the metabolic health of maternal mice challenged with a high-fat diet. Exercise intervention of obese mothers prevents fetal overgrowth. Exercise intervention reverses impaired placental vascularization in obese mice. Maternal exercise activates placental AMP-activated protein kinase, which was inhibited as a result of maternal obesity.

ABSTRACT

More than one-third of pregnant women in the USA are obese and maternal obesity (MO) negatively affects fetal development, which predisposes offspring to metabolic diseases. The placenta mediates nutrient delivery to fetuses and its function is impaired as a result of MO. Exercise ameliorates metabolic dysfunction resulting from obesity, although its effect on placental function of obese mothers has not been explored. In the present study, C57BL/6J female mice were randomly assigned into two groups fed either a control or a high-fat diet (HFD) and then the mice on each diet were further divided into two subgroups with/without exercise. In HFD-induced obese mice, daily treadmill exercise during pregnancy reduced body weight gain, lowered serum glucose and lipid concentration, and improved insulin sensitivity of maternal mice. Importantly, maternal exercise prevented fetal overgrowth (macrosomia) induced by MO. To further examine the preventive effects of exercise on fetal overgrowth, placental vascularization and nutrient transporters were analysed. Vascular density and the expression of vasculogenic factors were reduced as a result of MO but were recovered by maternal exercise. On the other hand, the contents of nutrient transporters were not substantially altered by MO or exercise, suggesting that the protective effects of exercise in MO-induced fetal overgrowth were primarily a result of the alteration of placental vascularization and improved maternal metabolism. Furthermore, exercise enhanced downstream insulin signalling and activated AMP-activated protein kinase in HFD placenta. In sum, maternal exercise prevented fetal overgrowth induced by MO, which was associated with improved maternal metabolism and placental vascularization in obese mothers with exercise.

Metformin improves the glucose homeostasis of Wuchang bream fed high-carbohydrate diets: a dynamic study

After a 12-week feeding trial, the glucose tolerance test was performed in Megalobrama amblycephala to evaluate the effects of metformin on the metabolic responses of glycolipids. Plasma insulin peaked at 2 h, then decreased to the basal value at 8-12 h post-injection. Plasma triglyceride levels and liver glycogen contents of the control group was decreased significantly during the first 2 and 1 h, respectively. Then, they returned to basal values at 12 h. During the whole sampling period, the high-carbohydrate groups had significantly higher levels of plasma metabolites and liver glycogen than those of the control group, and metformin supplementation enhanced these changes (except insulin levels). Glucose administration lowered the transcriptions of ampk α1, ampk α2, pepck, g6pase, fbpase, cpt IA and aco, the phosphorylation of Ampk α and the activities of the gluconeogenic enzymes during the first 2-4 h, while the opposite was true of glut 2, gs, gk, pk, accα and fas. High-carbohydrate diets significantly increased the transcriptions of ampk α1, ampk α2, glut 2, gs, gk, pk, accα and fas, the phosphorylation of Ampk α and the activities of the glycolytic enzymes during the whole sampling period, while the opposite was true for the remaining indicators. Furthermore, metformin significantly upregulated the aforementioned indicators (except accα and fas) and the transcriptions of cpt IA and aco. Overall, metformin benefits the glucose homeostasis of Megalobrama amblycephala fed high-carbohydrate diets through the activation of Ampk and the stimulation of glycolysis, glycogenesis and fatty acid oxidation, while depressing gluconeogenesis and lipogenesis.

Skeletal Muscle Mitochondrial Protein Synthesis and Respiration Increase With Low-Load Blood Flow Restricted as Well as High-Load Resistance Training

Purpose: It is well established that high-load resistance exercise (HLRE) can stimulate myofibrillar accretion. Additionally, recent studies suggest that HLRE can also stimulate mitochondrial biogenesis and respiratory function. However, in several clinical situations, the use of resistance exercise with high loading may not constitute a viable approach. Low-load blood flow restricted resistance exercise (BFRRE) has emerged as a time-effective low-load alternative to stimulate myofibrillar accretion. It is unknown if BFRRE can also stimulate mitochondrial biogenesis and respiratory function. If so, BFRRE could provide a feasible strategy to stimulate muscle metabolic health.

Methods: To study this, 34 healthy previously untrained individuals (24 ± 3 years) participated in BFRRE, HLRE, or non-exercise control intervention (CON) 3 times per week for 6 weeks. Skeletal muscle biopsies were collected; (1) before and after the 6-week intervention period to assess mitochondrial biogenesis and respiratory function and; (2) during recovery from single-bout exercise to assess myocellular signaling events involved in transcriptional regulation of mitochondrial biogenesis. During the 6-week intervention period, deuterium oxide (D₂O) was continuously administered to the participants to label newly synthesized skeletal muscle mitochondrial proteins. Mitochondrial respiratory function was assessed in permeabilized muscle fibers with high-resolution respirometry. Mitochondrial content was assessed with a citrate synthase activity assay. Myocellular signaling was assessed with immunoblotting.

Results: Mitochondrial protein synthesis rate was higher with BFRRE (1.19%/day) and HLRE (1.15%/day) compared to CON (0.92%/day) (P < 0.05) but similar between exercise groups. Mitochondrial respiratory function increased to similar degree with both exercise regimens and did not change with CON. For instance, coupled respiration supported by convergent electron flow from complex I and II increased 38% with BFRRE and 24% with HLRE (P < 0.01). Training did not alter citrate synthase activity compared to CON. BFRRE and HLRE elicited similar myocellular signaling responses.

Conclusion: These results support recent findings that resistance exercise can stimulate mitochondrial biogenesis and respiratory function to support healthy skeletal muscle and whole-body metabolism. Intriquingly, BFRRE produces similar mitochondrial adaptations at a markedly lower load, which entail great clinical perspective for populations in whom exercise with high loading is untenable.

AMP-activated protein kinase α1 in Megalobrama amblycephala: Molecular characterization and the transcriptional modulation by nutrient restriction and glucose and insulin loadings

This study aimed to characterize the full-length cDNA of AMP-activated protein kinase α1 (AMPKα1) from Megalobrama amblycephala and investigate the transcriptional response of this kinase to nutrient restriction and glucose and insulin loadings. The cDNA obtained was 3545-bp long with an open reading frame of 1710 bp encoding 570 amino acids. Multiple alignments and phylogenetic analyses revealed a high degree of conservation (80-100%) among most fish, retaining one kinase domain (KD), one auto-inhibitory domain (AID), one C-terminal domain (α-CTD), one regulatory-subunit-interacting motif (α-RIM), one serine/threonine-rich loop (ST loop), one α-hook, and several phosphorylation sites. AMPKα1 mRNA was predominantly expressed in white muscle, gill, and brain tissues, whereas little was expressed in the intestines. After a fasting-refeeding trial, phosphorylation and mRNA levels of AMPKα1 were significantly greater in fish fasted for 10 days, while in re-fed fish at 1 h after re-feeding, the levels of this kinase were intermediate between those of the fish in the fed and fasted groups. Further, AMPKα1 mRNA levels were quantified in the liver and muscle tissues of fish injected intraperitoneally with 1.67 g glucose per kg body weight and 0.052 mg insulin per kg body weight, respectively. Glucose and insulin administration resulted in a significant decrease in AMPKα1 expression in both tissues with minimum values attained at 2 h and 4 h after injection, respectively. Thereafter, the expression increased significantly to the basal value at 24 h after injection, except in the liver in which the maximum value was obtained at 12 h post-glucose injection. Overall, AMPKα1 of M. amblycephala was similar to that of other vertebrates, and nutrient restriction modified its phosphorylation and mRNA levels in liver and muscle tissues. Furthermore, substantial expression of this kinase was induced in both liver and muscle tissues by glucose and insulin administration.

Long-Term Chronic Intermittent Hypobaric Hypoxia Induces Glucose Transporter (GLUT4) Translocation Through AMP-Activated Protein Kinase (AMPK) in the Soleus Muscle in Lean Rats

Background: In chronic hypoxia (CH) and short-term chronic intermittent hypoxia (CIH) exposure, glycemia and insulin levels decrease and insulin sensitivity increases, which can be explained by changes in glucose transport at skeletal muscles involving GLUT1, GLUT4, Akt, and AMPK, as well as GLUT4 translocation to cell membranes. However, during long-term CIH, there is no information regarding whether these changes occur similarly or differently than in other types of hypoxia exposure. This study evaluated the levels of AMPK and Akt and the location of GLUT4 in the soleus muscles of lean rats exposed to long-term CIH, CH, and normoxia (NX) and compared the findings.

Methods: Thirty male adult rats were randomly assigned to three groups: a NX (760 Torr) group (n = 10), a CIH group (2 days hypoxia/2 days NX; n = 10) and a CH group (n = 10). Rats were exposed to hypoxia for 30 days in a hypobaric chamber set at 428 Torr (4,600 m). Feeding (10 g daily) and fasting times were accurately controlled. Measurements included food intake (every 4 days), weight, hematocrit, hemoglobin, glycemia, serum insulin (by ELISA), and insulin sensitivity at days 0 and 30. GLUT1, GLUT4, AMPK levels and Akt activation in rat soleus muscles were determined by western blot. GLUT4 translocation was measured with confocal microscopy at day 30.

Results: (1) Weight loss and increases in hematocrit and hemoglobin were found in both hypoxic groups (p < 0.05). (2) A moderate decrease in glycemia and plasma insulin was found. (3) Insulin sensitivity was greater in the CIH group (p < 0.05). (4) There were no changes in GLUT1, GLUT4 levels or in Akt activation. (5) The level of activated AMPK was increased only in the CIH group (p < 0.05). (6) Increased GLUT4 translocation to the plasma membrane of soleus muscle cells was observed in the CIH group (p < 0.05).

Conclusion: In lean rats experiencing long-term CIH, glycemia and insulin levels decrease and insulin sensitivity increases. Interestingly, there is no increase of GLUT1 or GLUT4 levels or in Akt activation. Therefore, cellular regulation of glucose seems to primarily involve GLUT4 translocation to the cell membrane in response to hypoxia-mediated AMPK activation.

Resistance training recovers attenuated APPL1 expression and improves insulin-induced Akt signal activation in skeletal muscle of type 2 diabetic rats

Adapter protein containing Pleckstrin homology (PH) domain, phosphotyrosine-binding (PTB) domain, and leucine zipper motif 1 (APPL1) has been reported as a positive regulator of insulin-stimulated Akt activation. The expression of APPL1 is reduced in skeletal muscles of type 2 diabetic (T2D) animals, implying that APPL1 may be an important factor affecting insulin sensitivity. However, the regulation of APPL1 expression and the physiological interventions modulating these effects are unclear. Accordingly, we first confirmed that APPL1 expression and insulin-induced Akt phosphorylation were significantly attenuated in skeletal muscles of T2D rats. Additionally, we found that APPL1 expression levels were significantly correlated with fasting blood glucose levels. Next, we identified important signals involved in the expression of APPL1. APPL1 mRNA expression increased upon AMP-activated protein kinase, calcium, p38 mitogen-activated protein kinase, and insulin-like growth factor-1 signal activation. Moreover, acute resistance exercise in vivo significantly activated these signaling pathways. Finally, through in vivo experiments, we found that chronic resistance training (RT) increased APPL1 expression and activated insulin-induced Akt signaling in skeletal muscles of rats with T2D. Furthermore, variations in APPL1 expression (i.e., the difference between control and RT muscles) significantly correlated with variations in insulin-stimulated Akt phosphorylation under the same conditions. Therefore, chronic RT recovered attenuated APPL1 expression and improved insulin-stimulated Akt phosphorylation in skeletal muscles of T2D rats. Accordingly, APPL1 may be a key regulator of insulin resistance in skeletal muscle, and RT may be an important physiological treatment increasing APPL1 expression, which is attenuated in T2D.

Regulatory Effects of Neuroinflammatory Responses Through Brain-Derived Neurotrophic Factor Signaling in Microglial Cells

Inhibition of microglial over-activation is an important strategy to counter balance neurodegenerative progression. We previously demonstrated that the adenosine monophosphate-activated protein kinase (AMPK) may be a therapeutic target in mediating anti-neuroinflammatory responses in microglia. Brain-derived neurotrophic factor (BDNF) is one of the major neurotrophic factors produced by astrocytes to maintain the development and survival of neurons in the brain, and have recently been shown to modulate homeostasis of neuroinflammation. Therefore, the present study focused on BDNF-mediated neuroinflammatory responses and may provide an endogenous regulation of neuroinflammation. Among the tested neuroinflammation, epigallocatechin gallate (EGCG) and minocycline exerted BDNF upregulation to inhibit COX-2 and proinflammatory mediator expressions. Furthermore, both EGCG and minocycline upregulated BDNF expression in microglia through AMPK signaling. In addition, minocycline and EGCG also increased expressions of erythropoietin (EPO) and sonic hedgehog (Shh). In the endogenous modulation of neuroinflammation, astrocyte-conditioned medium (AgCM) also decreased the expression of COX-2 and upregulated BDNF expression in microglia. The anti-inflammatory effects of BDNF were mediated through EPO/Shh in microglia. Our results indicated that the BDNF-EPO-Shh novel-signaling pathway underlies the regulation of inflammatory responses and may be regarded as a potential therapeutic target in neurodegenerative diseases. This study also reveals a better understanding of an endogenous crosstalk between astrocytes and microglia to regulate anti-inflammatory actions, which could provide a novel strategy for the treatment of neuroinflammation and neurodegenerative diseases.