The bidirectional relationship between AMPK pathway activation and myokine secretion in skeletal muscle: How it affects energy metabolism

Myokines are peptides and proteins secreted by skeletal muscle cells, into the interstitium, or in the blood. Their regulation may be dependent or independent of muscle contraction to induce a variety of metabolic effects. Numerous myokines have been implicated in influencing energy metabolism via AMP-activated protein kinase (AMPK) signalling. As AMPK is centrally involved in glucose and lipid metabolism, it is important to understand how myokines influence its signalling, and vice versa. Such insight will better elucidate the mechanism of metabolic regulation during exercise and at rest. This review encompasses the latest research conducted on the relationship between AMPK signalling and myokines within skeletal muscles via autocrine or paracrine signalling.

AMPKα2 Deficiency Does Not Affect the Exercise-Induced Improvements in Glucose Tolerance and Metabolic Disorders in Mice Fed a High-Fat Diet

Exercise can improve obesity and metabolic disorders in mice fed a high-fat diet (HFD), but the role of AMPKα2 in the process remains unclear. The aim of this study was to investigate the role of AMPKα2 in the exercise-induced improvements in glucose tolerance and metabolic turnover in obesity mice. Male wild-type mice (n=12) and AMPKα2 knockout (AMPKα2 KO) mice (n=12) were fed a HFD for 16 wk and were then randomly divided into four groups: WT HFD group (WT HF), AMPKα2 KO HFD group (AMPKα2 KO HF), WT HFD exercise group (WT HE), and AMPK HFD exercise group (AMPKα2 KO HE). The HF groups continue to be fed a HFD from 16 wk to 24 wk, and the HE groups were fed a HFD and performed exercise training. After 8 wk of exercise, all mice were placed in an energy metabolism chamber to test their metabolic turnover, include locomotor activity, food intake, oxygen consumption (VO₂), carbon dioxide production (VCO₂), energy expenditure (EE) and respiratory exchange ratio (RER), over a period of 3 d. Exercise improved glucose tolerance, VO₂, VCO₂ and EE in mice fed a HFD (p<0.05). The VO₂, VCO₂ and EE in AMPKα2 KO HE group were lower than these in WT HE group (p<0.05). Our findings revealed exercise improved glucose tolerance and metabolic disorders in C57 and AMPKα2 KO mice fed a HFD. AMPKα2 is not essential for exercise-induced improvements in glucose tolerance and metabolic disorders.

AMPK in skeletal muscle function and metabolism

Skeletal muscle possesses a remarkable ability to adapt to various physiologic conditions. AMPK is a sensor of intracellular energy status that maintains energy stores by fine-tuning anabolic and catabolic pathways. AMPK’s role as an energy sensor is particularly critical in tissues displaying highly changeable energy turnover. Due to the drastic changes in energy demand that occur between the resting and exercising state, skeletal muscle is one such tissue. Here, we review the complex regulation of AMPK in skeletal muscle and its consequences on metabolism (e.g., substrate uptake, oxidation, and storage as well as mitochondrial function of skeletal muscle fibers). We focus on the role of AMPK in skeletal muscle during exercise and in exercise recovery. We also address adaptations to exercise training, including skeletal muscle plasticity, highlighting novel concepts and future perspectives that need to be investigated. Furthermore, we discuss the possible role of AMPK as a therapeutic target as well as different AMPK activators and their potential for future drug development.—Kjøbsted, R., Hingst, J. R., Fentz, J., Foretz, M., Sanz, M.-N., Pehmøller, C., Shum, M., Marette, A., Mounier, R., Treebak, J. T., Wojtaszewski, J. F. P., Viollet, B., Lantier, L. AMPK in skeletal muscle function and metabolism.

IL-15 Activates the Jak3/STAT3 Signaling Pathway to Mediate Glucose Uptake in Skeletal Muscle Cells

Myokines are specialized cytokines that are secreted from skeletal muscle (SKM) in response to metabolic stimuli, such as exercise. Interleukin-15 (IL-15) is a myokine with potential to reduce obesity and increase lean mass through induction of metabolic processes. It has been previously shown that IL-15 acts to increase glucose uptake in SKM cells. However, the downstream signals orchestrating the link between IL-15 signaling and glucose uptake have not been fully explored. Here we employed the mouse SKM C2C12 cell line to examine potential downstream targets of IL-15-induced alterations in glucose uptake. Following differentiation, C2C12 cells were treated overnight with 100 ng/ml of IL-15. Activation of factors associated with glucose metabolism (Akt and AMPK) and known downstream targets of IL-15 (Jak1, Jak3, STAT3, and STAT5) were assessed with IL-15 stimulation. IL-15 stimulated glucose uptake and GLUT4 translocation to the plasma membrane. IL-15 treatment had no effect on phospho-Akt, phospho-Akt substrates, phospho-AMPK, phospho-Jak1, or phospho-STAT5. However, with IL-15, phospho-Jak3 and phospho-STAT3 levels were increased along with increased interaction of Jak3 and STAT3. Additionally, IL-15 induced a translocation of phospho-STAT3 from the cytoplasm to the nucleus. We have evidence that a mediator of glucose uptake, HIF1α, expression was dependent on IL-15 induced STAT3 activation. Finally, upon inhibition of STAT3 the positive effects of IL-15 on glucose uptake and GLUT4 translocation were abolished. Taken together, we provide evidence for a novel signaling pathway for IL-15 acting through Jak3/STAT3 to regulate glucose metabolism.

Adverse effects of AMP-activated protein kinase alpha2-subunit deletion and high-fat diet on heart function and ischemic tolerance in aged female mice

AMP-activated protein kinase (AMPK) plays a role in metabolic regulation under stress conditions, and inadequate AMPK signaling may be also involved in aging process. The aim was to find out whether AMPK alpha2-subunit deletion affects heart function and ischemic tolerance of adult and aged mice. AMPK alpha2(-/-) (KO) and wild type (WT) female mice were compared at the age of 6 and 18 months. KO mice exhibited subtle myocardial AMPK alpha2-subunit protein level, but no difference in AMPK alpha1-subunit was detected between the strains. Both alpha1- and alpha2-subunits of AMPK and their phosphorylation decreased with advanced age. Left ventricular fractional shortening was lower in KO than in WT mice of both age groups and this difference was maintained after high-fat feeding. Infarct size induced by global ischemia/reperfusion of isolated hearts was similar in both strains at 6 months of age. Aged WT but not KO mice exhibited improved ischemic tolerance compared with the younger group. High-fat feeding for 6 months during aging abolished the infarct size-reduction in WT without affecting KO animals; nevertheless, the extent of injury remained larger in KO mice. The results demonstrate that adverse effects of AMPK alpha2-subunit deletion and high-fat feeding on heart function and myocardial ischemic tolerance in aged female mice are not additive.

Amelioration of palmitate-induced metabolic dysfunction in L6 muscle cells expressing low levels of receptor-interacting protein 140

We have shown that reduced expression of receptor-interacting protein 140 (RIP140) alters the regulation of fatty-acid (FA) oxidation in muscle. To determine whether a high level of FA availability alters the effects of RIP140 on metabolic regulation, L6 myotubes were transfected with or without RNA interference oligonucleotide sequences to reduce RIP140 expression, and then incubated with high levels of palmitic acid, with or without insulin. High levels of palmitate reduced basal (53%-58%) and insulin-treated (24%-44%) FA uptake and oxidation, and increased basal glucose uptake (88%). In cells incubated with high levels of palmitate, low RIP140 increased basal FA uptake and insulin-treated FA oxidation and glucose uptake, and decreased basal glucose uptake and insulin-treated FA uptake. Under basal conditions, low RIP140 increased the mRNA content of FAT/CD36 (159%) and COX4 (61%), as well as the protein content of Nur77 (68%), whereas the mRNA expression of FGF21 (50%) was decreased, as was the protein content of CPT1b (35%) and FGF21 (44%). Under insulin-treated conditions, low RIP140 expression increased the mRNA content of MCAD (84%) and Nur77 (84%), as well as the protein content of Nur77 (23%). Thus, a low level of RIP140 restores the rates of FA uptake in the basal state, in part via a reduction in upstream insulin signaling. Our data also indicate that the protein expression of Nur77 may be modulated by RIP140 when muscle cells are metabolically challenged by high levels of palmitate.

AMPK-α2 is involved in exercise training-induced adaptations in insulin-stimulated metabolism in skeletal muscle following high-fat diet

AMP-activated protein kinase (AMPK) has been studied extensively and postulated to be a target for the treatment and/or prevention of metabolic disorders such as insulin resistance. Exercise training has been deemed a beneficial treatment for obesity and insulin resistance. Furthermore, exercise is a feasible method to combat high-fat diet (HFD)-induced alterations in insulin sensitivity. The purpose of this study was to determine whether AMPK-α2 activity is required to gain beneficial effects of exercise training with high-fat feeding. Wild-type (WT) and AMPK-α2 dominant-negative (DN) male mice were fed standard diet (SD), underwent voluntary wheel running (TR), fed HFD, or trained with HFD (TR + HFD). By week 6, TR, irrespective of genotype, decreased blood glucose and increased citrate synthase activity in both diet groups and decreased insulin levels in HFD groups. Hindlimb perfusions were performed, and, in WT mice with SD, TR increased insulin-mediated palmitate uptake (76.7%) and oxidation (>2-fold). These training-induced changes were not observed in the DN mice. With HFD, TR decreased palmitate oxidation (61–64%) in both WT and DN and increased palmitate uptake (112%) in the WT with no effects on palmitate uptake in the DN. With SD, TR increased ERK1/2 and JNK1/2 phosphorylation, regardless of genotype. With HFD, TR reduced JNK1/2 phosphorylation, regardless of genotype, carnitine palmitoyltransferase 1 expression in WT, and CD36 expression in both DN and WT. These data suggest that low AMPK-α2 signaling disrupts, in part, the exercise training-induced adaptations in insulin-stimulated metabolism in skeletal muscle following HFD.

Nesfatin-1 stimulates fatty-acid oxidation by activating AMP-activated protein kinase in STZ-induced type 2 diabetic mice

Nesfatin-1 is an anorexigenic peptide involved in energy homeostasis. Recently, nesfatin-1 was reported to decrease blood glucose level and improve insulin sensitivity in high-fat diet-fed rats. However, little information is known about the influence of nesfatin-1 on lipid metabolism either in physiological or diabetic condition. This study undertook whether nesfatin-1 was involved in the pathophysiology in Streptozotocin-induced type 2 diabetic mice (T2DM), which was induced by a combination of high-calorie diet and two low-doses Streptozotocin. We observed that plasma nesfatin-1 was significantly increased while expression of nesfatin-1 neurons were decreased in hypothalamus in diabetes group compared to only high-calorie diet control group; intravenous injection of nesfatin-1 decreased 0–1h, 0–2h, 0–3h cumulative food intake in T2DM, but 0–24h total food intake had no difference between groups. Body weight and plasma FFA were normalized after nesfatin-1(10 µg/Kg) administration for 6 days. These results suggested that nesfatin-1 improved lipid disorder in T2DM. It was found that blood glucose and insulin resistance coefficient decreased with treatment of nesfatin-1 (both in 1 µg/Kg and 10 µg/Kg doses) in diabetes mice. For further understanding the role of nesfatin-1 on lipid metabolism, we detected p-AMPK and p-ACC of skeletal muscle in T2DM using western blotting. The expression of p-AMPK and p-ACC increased when nesfatin-1 was given with doses 1 µg/Kg but not in doses 10 µg/Kg. Taken together, nesfatin-1 participated in the development of T2DM and stimulated free fatty acid utilization via AMPK-ACC pathway in skeletal muscle in T2DM.

Common mechanisms of dysfunctional adipose tissue and obesity-related cancers

The relation between cancer and metabolic disorders was recognized several decades ago, but the underlying mechanisms involved in cancer development and progression remain obscure. In the last years, many groups have been studying systemic adipose tissue markers in cancer patients. However, few consistent results were obtained. On the other hand, several studies revealed many aspects of adipose tissue physiology in obesity. Nowadays, it is recognized that excessive lipid uptake in adipocytes leads to hypertrophy and consequently to metabolic dysregulation, hypoxia, inflammation, impaired adipocytokine expression and angiogenesis, insulin resistance and macrophage recruitment. In obese patients, tumours commonly colocalize with excessive adipose tissue accumulation, and most of the features of hypertrophic adipose tissue are observed in cancer patients, namely breast and colon. This review aimed to summarize pathological adipose tissue alterations that may contribute to cancer aetiology and development.