Protein kinase B/Akt participates in GLUT4 translocation by insulin in L6 myoblasts

Aerobic exercise attenuates high-fat diet-induced glycometabolism impairments in skeletal muscle of rat: role of EGR-1/PTP1B signaling pathway

OBJECTIVE

Impaired skeletal muscle glycogen synthesis contributes to insulin resistance (IR). Aerobic exercise reported to ameliorate IR by augmenting insulin signaling, however the detailed mechanism behind this improvement remains unclear. This study investigated whether aerobic exercise enhances glycogen anabolism and insulin sensitivity via EGR-1/PTP1B signaling pathway in skeletal muscle of rats.

METHODS

Sprague-Dawley rats fed a high-fat diet (HFD), and performed treadmill exercise training for 6-week. Oral glucose tolerance test was conducted to confirm the IR. Periodic Acid-Schiff (PAS) staining and anthrone colorimetry were used to assess the skeletal muscle glycogen. RT-qPCR, western blot, and immunofluorescence were used to detect the EGR-1/PTP1B pathway and associated signaling molecules.

RESULTS

We found that exercise training significantly decreased blood glucose, insulin, and homeostasis model assessment for IR (HOMA-IR) against HFD-induced elevation. Decreased muscle glycogen content due to HFD was significantly restored after exercise training. Exercise training promoted mRNA expressions of Irs1, Akt, and Glut4, while inhibited Gsk-3β expression against HFD. Next, the decreased IRS1 (phosphorylated/total), AKT (phosphorylated/total), and GLUT4, and increased GSK-3β proteins with HFD were significantly reversed by exercise. Furthermore, HFD-induced overexpression of EGR-1 and PTP1B evidenced by mRNA, protein, and immunofluorescence intensity, were substantially inhibited by exercise, which may contribute to promote insulin sensitivity and glycogen anabolism.

CONCLUSIONS

Aerobic exercise training promotes insulin sensitivity and skeletal muscle glycogen synthesis in HFD-fed rats. The beneficial effects of exercise might be mediated by EGR-1/PTP1B signaling pathway in skeletal muscle, however further studies are necessary to confirm this mechanism.

Irisin: A Multifaceted Hormone Bridging Exercise and Disease Pathophysiology

The fibronectin domain-containing protein 5 (FNDC5), or irisin, is an adipo-myokine hormone produced during exercise, which shows therapeutic potential for conditions like metabolic disorders, osteoporosis, sarcopenia, obesity, type 2 diabetes, and neurodegenerative diseases, including Alzheimer's disease (AD). This review explores its potential across various pathophysiological processes that are often considered independent. Elevated in healthy states but reduced in diseases, irisin improves muscle-adipose communication, insulin sensitivity, and metabolic balance by enhancing mitochondrial function and reducing oxidative stress. It promotes osteogenesis and mitigates bone loss in osteoporosis and sarcopenia. Irisin exhibits anti-inflammatory effects by inhibiting NF-κB signaling and countering insulin resistance. In the brain, it reduces amyloid-β toxicity, inflammation, and oxidative stress, enhancing brain-derived neurotrophic factor (BDNF) signaling, which improves cognition and synaptic health in AD models. It also regulates dopamine pathways, potentially alleviating neuropsychiatric symptoms like depression and apathy. By linking physical activity to systemic health, irisin emphasizes its role in the muscle-bone-brain axis. Its multifaceted benefits highlight its potential as a therapeutic target for AD and related disorders, with applications in prevention, in treatment, and as a complement to exercise strategies.

Unveiling the Mechanism of Protective Effects of Tanshinone as a New Fighter Against Cardiovascular Diseases: A Systematic Review

Tanshinone, a natural compound found in the roots of Salvia miltiorrhiza, has been shown to possess various pharmacological properties, including anti-inflammatory, antioxidant, and cardiovascular protective effects. This article aims to review the literature on the cardiovascular protective effects of tanshinone and its underlying mechanisms. Tanshinone has been demonstrated to improve cardiac function, reduce oxidative stress, and inhibit inflammation in various animal models of cardiovascular diseases. Additionally, it has been shown to regulate multiple signaling pathways involved in the pathogenesis of cardiovascular diseases, such as the PI3K/AKT, MAPK, and NF-κB pathways. Clinical studies have also suggested that tanshinone may have therapeutic potential for treating cardiovascular diseases. In conclusion, tanshinone has emerged as a promising natural compound with significant cardiovascular protective effects, and further research is warranted to explore its potential clinical applications.

An antifibrotic compound that ameliorates hyperglycaemia and fat accumulation in cell and HFD mouse models

AIMS/HYPOTHESIS

Appropriate management of blood glucose levels and the prevention of complications are important in the treatment of diabetes. We have previously reported on a compound named HPH-15 that is not only antifibrotic but also AMP-activated protein kinase (AMPK)-activating. In this study, we evaluated whether HPH-15 is useful as a therapeutic medication for diabetes.

METHODS

We examined the effects of HPH-15 on AMPK activation, glucose uptake, fat accumulation and lactic acid production in L6-GLUT4, HepG2 and 3T3-L1 cells, as a model of muscle, liver and fat tissue, respectively. Additionally, we investigated the glucose-lowering, fat-accumulation-suppressing, antifibrotic and AMPK-activating effect of HPH-15 in mice fed a high-fat diet (HFD).

RESULTS

HPH-15 at a concentration of 10 µmol/l increased AMPK activation, glucose uptake and membrane translocation of GLUT4 in each cell model to the same extent as metformin at 2 mmol/l. The production of lactic acid (which causes lactic acidosis) in HPH-15-treated cells was equal to or less than that observed in metformin-treated cells. In HFD-fed mice, HPH-15 lowered blood glucose from 11.1±0.3 mmol/l to 8.2±0.4 mmol/l (10 mg/kg) and 7.9±0.4 mmol/l (100 mg/kg) and improved insulin resistance. The HPH-15 (10 mg/kg) group showed the same level of AMPK activation as the metformin (300 mg/kg) group in all organs. The HPH-15-treated HFD-fed mice also showed suppression of fat accumulation and fibrosis in the liver and fat tissue; these effects were more significant than those obtained with metformin. Mice treated with high doses of HPH-15 also exhibited a 44% reduction in subcutaneous fat.

CONCLUSIONS/INTERPRETATION

HPH-15 activated AMPK at lower concentrations than metformin in vitro and in vivo and improved blood glucose levels and insulin resistance in vivo. In addition, HPH-15 was more effective than metformin at ameliorating fatty liver and adipocyte hypertrophy in HFD-fed mice. HPH-15 could be effective in preventing fatty liver, a common complication in diabetic individuals. Additionally, in contrast to metformin, high doses of HPH-15 reduced subcutaneous fat in HFD-fed mice. Presumably, HPH-15 has a stronger inhibitory effect on fat accumulation and fibrosis than metformin, accounting for the reduction of subcutaneous fat. Therefore, HPH-15 is potentially a glucose-lowering medication that can lower blood glucose, inhibit fat accumulation and ameliorate liver fibrosis.

Phosphorylation Codes in IRS-1 and IRS-2 Are Associated with the Activation/Inhibition of Insulin Canonical Signaling Pathways

Insulin receptor substrates 1 and 2 (IRS-1 and IRS-2) are signaling adaptor proteins that participate in canonical pathways, where insulin cascade activation occurs, as well as in non-canonical pathways, in which phosphorylation of substrates is carried out by a diverse array of receptors including integrins, cytokines, steroid hormones, and others. IRS proteins are subject to a spectrum of post-translational modifications essential for their activation, encompassing phosphorylation events in distinct tyrosine, serine, and threonine residues. Tyrosine residue phosphorylation is intricately linked to the activation of the insulin receptor cascade and its interaction with SH2 domains within a spectrum of proteins, including PI3K. Conversely, serine residue phosphorylation assumes a different function, serving to attenuate the effects of insulin. In this review, we have identified over 50 serine residues within IRS-1 that have been reported to undergo phosphorylation orchestrated by a spectrum of kinases, thereby engendering the activation or inhibition of different signaling pathways. Furthermore, we delineate the phosphorylation of over 10 distinct tyrosine residues at IRS-1 or IRS-2 in response to insulin, a process essential for signal transduction and the subsequent activation of PI3K.

An ADAM17 selective inhibitor promotes glucose uptake by activating AMPK

AMPK activation promotes glucose and lipid metabolism. Here, we found that our previously reported ADAM17 inhibitor SN-4 activates AMPK and promotes membrane translocation and sugar uptake of GLUT4. AMPK inhibitor dorsomorphin reversed this effect of SN-4, confirming that the effect is mediated by AMPK activation. In addition, SN-4 inhibited lipid accumulation in HepG2 under high glucose conditions by promoting lipid metabolism and inhibiting lipid synthesis. Although lactic acidosis is a serious side effect of biguanides such as metformin, SN-4 did not affect lactate production. Furthermore, SN-4 was confirmed to inhibit the release of TNF-α, a causative agent of insulin resistance, from adipocytes. In diabetes treatment, it is important to not only regulate blood sugar levels but also prevent complications. Our findings reveal the therapeutic potential of SN-4 as a new antidiabetic drug that can also help prevent future complications.

Palmitate-induced insulin resistance causes actin filament stiffness and GLUT4 mis-sorting without altered Akt signalling

Skeletal muscle insulin resistance, a major contributor to type 2 diabetes, is linked to the consumption of saturated fats. This insulin resistance arises from failure of insulin-induced translocation of glucose transporter type 4 (GLUT4; also known as SLC2A4) to the plasma membrane to facilitate glucose uptake into muscle. The mechanisms of defective GLUT4 translocation are poorly understood, limiting development of insulin-sensitizing therapies targeting muscle glucose uptake. Although many studies have identified early insulin signalling defects and suggest that they are responsible for insulin resistance, their cause-effect has been debated. Here, we find that the saturated fat palmitate (PA) causes insulin resistance owing to failure of GLUT4 translocation in skeletal muscle myoblasts and myotubes without impairing signalling to Akt2 or AS160 (also known as TBC1D4). Instead, PA altered two basal-state events: (1) the intracellular localization of GLUT4 and its sorting towards a perinuclear storage compartment, and (2) actin filament stiffness, which prevents Rac1-dependent actin remodelling. These defects were triggered by distinct mechanisms, respectively protein palmitoylation and endoplasmic reticulum (ER) stress. Our findings highlight that saturated fats elicit muscle cell-autonomous dysregulation of the basal-state machinery required for GLUT4 translocation, which 'primes' cells for insulin resistance.

Liver acts as a metabolic gate for the traumatic brain injury pathology: Protective action of thyroid hormone

Clinical evidence indicates that injury to the brain elicits systemic metabolic disturbances that contributes to the brain pathology. Since dietary fructose is metabolized in the liver, we explored mechanisms by which traumatic brain injury (TBI) and dietary fructose influence liver function and their possible repercussions to brain. Consumption of fructose contributed to the detrimental effects of TBI on liver operation, in terms of glucose and lipid metabolism, de novo lipogenesis, lipid peroxidation. Thyroid hormone (T4) is metabolized in the liver and found that T4 supply improved lipid metabolism by reducing de novo lipogenesis, lipid accumulation, lipogenic enzymes (ACC, AceCS1, FAS), lipid peroxidation in liver in response to fructose and fructose-TBI. T4 supply also helped to normalize glucose metabolism and improve insulin sensitivity. Furthermore, T4 counteracted elevations of the pro-inflammatory cytokines, Tnfα and Mcp-1 after TBI and/or fructose intake in liver and circulation. T4 also exerted an effect on isolated primary hepatocytes by potentiating phosphorylation of AMPKα and AKT substrate, AS160, leading to increased glucose uptake. In addition, T4 restored the metabolism of DHA in the liver disrupted by TBI and fructose, adding important information to optimize the action of DHA in therapeutics. The overall evidence seems to indicate that the liver works as a gate for the regulation of the effects of brain injury and foods on brain pathologies.

Biochemical and functional characterization of skeletal muscle cells differentiated from tonsil-derived mesenchymal stem cells

INTRODUCTION/AIMS

Human tonsils are a readily accessible source of stem cells for the potential treatment of skeletal muscle disorders. We reported previously that tonsil-derived mesenchymal stem cells (TMSCs) can differentiate into skeletal muscle cells (SKMCs), which renders TMSCs promising candidates for cell therapy for skeletal muscle disorders. However, the functional properties of the myocytes differentiated from mesenchymal stem cells have not been clearly evaluated. In this study we investigated whether myocytes differentiated from TMSCs (skeletal muscle cells derived from tonsil mesenchymal stem cells [TMSC-SKMCs]) exhibit the functional characteristics of SKMCs.

METHODS

To test the insulin reactivity of TMSC-SKMCs, the expression of glucose transporter 4 (GLUT4) and phosphatidylinositol 3-kinase/Akt was analyzed after the cells were treated for 30 minutes with 100 nmol/L insulin in normal or high-glucose medium. We also examined whether these cells formed a neuromuscular junction (NMJ) when cocultured with motor neurons, and whether they were stimulated by electrical signals using whole-cell patch clamping.

RESULTS

Skeletal muscle cells derived from tonsil mesenchymal stem cells expressed SKMC markers, such as MYOD, MYH3, MYH8, TNNI1, and TTN, at high levels, and exhibited a multinucleated cell morphology and a myotube-like shape. The expression of the acetylcholine receptor and GLUT4 was confirmed in TMSC-SKMCs. In addition, these cells exhibited insulin-mediated glucose uptake, NMJ formation, and transient changes in cell membrane action potential, all of which are representative functions of human SKMCs.

DISCUSSION

Tonsil-derived mesenchymal stem cells can be functionally differentiated into SKMCs and may have potential for clinical application for the treatment of skeletal muscle disorders.

Effect of RONS-Induced Intracellular Redox Homeostasis in 6-NBDG/Glucose Uptake in C2C12 Myotubes and Single Isolated Skeletal Muscle Fibres

The glucose uptake in skeletal muscle is essential to produce energy through ATP, which is needed by this organ to maintain vital functions. The impairment of glucose uptake compromises the metabolism and function of skeletal muscle and other organs and is a feature of diabetes, obesity, and ageing. There is a need for research to uncover the mechanisms involved in the impairment of glucose uptake in skeletal muscle. In this study, we adapted, developed, optimised, and validated a methodology based on the fluorescence glucose analogue 6-NBDG, combined with a quantitative fluorescence microscopy image analysis, to determine the glucose uptake in two models of skeletal muscle cells: C2C12 myotubes and single fibres isolated from muscle. It was proposed that reactive oxygen and nitrogen species (RONS) and redox homeostasis play an important role in the modulation of intracellular redox signalling pathways associated with glucose uptake. In this study, we prove that the prooxidative intracellular redox environment under oxidative eustress produced by RONS such as hydrogen peroxide and nitric oxide improves glucose uptake in skeletal muscle cells. However, when oxidation is excessive, oxidative distress occurs, and cellular viability is compromised, although there might be an increase in the glucose uptake. Based on the results of this study, the determination of 6-NBDG/glucose uptake in myotubes and skeletal muscle cells is feasible, validated, and will contribute to improve future research.

Reactive Oxygen and Nitrogen Species (RONS) and Cytokines-Myokines Involved in Glucose Uptake and Insulin Resistance in Skeletal Muscle

Insulin resistance onset in skeletal muscle is characterized by the impairment of insulin signaling, which reduces the internalization of glucose, known as glucose uptake, into the cell. Therefore, there is a deficit of intracellular glucose, which is the main source for energy production in the cell. This may compromise cellular viability and functions, leading to pathological dysfunction. Skeletal muscle fibers continuously generate reactive oxygen and nitrogen species (RONS). An excess of RONS produces oxidative distress, which may evoke cellular damage and dysfunction. However, a moderate level of RONS, which is called oxidative eustress, is critical to maintain, modulate and regulate cellular functions through reversible interactions between RONS and the components of cellular signaling pathways that control those functions, such as the facilitation of glucose uptake. The skeletal muscle releases peptides called myokines that may have endocrine and paracrine effects. Some myokines bind to specific receptors in skeletal muscle fibers and might interact with cellular signaling pathways, such as PI3K/Akt and AMPK, and facilitate glucose uptake. In addition, there are cytokines, which are peptides produced by non-skeletal muscle cells, that bind to receptors at the plasma membrane of skeletal muscle cells and interact with the cellular signaling pathways, facilitating glucose uptake. RONS, myokines and cytokines might be acting on the same signaling pathways that facilitate glucose uptake in skeletal muscle. However, the experimental studies are limited and scarce. The aim of this review is to highlight the current knowledge regarding the role of RONS, myokines and cytokines as potential signals that facilitate glucose uptake in skeletal muscle. In addition, we encourage researchers in the field to lead and undertake investigations to uncover the fundamentals of glucose uptake evoked by RONS, myokines, and cytokines.

GSK3α phosphorylates dynamin-2 to promote GLUT4 endocytosis in muscle cells

Insulin-stimulated translocation of glucose transporter 4 (GLUT4) to plasma membrane of skeletal muscle is critical for postprandial glucose uptake; however, whether the internalization of GLUT4 is also regulated by insulin signaling remains unclear. Here, we discover that the activity of dynamin-2 (Dyn2) in catalyzing GLUT4 endocytosis is negatively regulated by insulin signaling in muscle cells. Mechanistically, the fission activity of Dyn2 is inhibited by binding with the SH3 domain of Bin1. In the absence of insulin, GSK3α phosphorylates Dyn2 to relieve the inhibition of Bin1 and promotes endocytosis. Conversely, insulin signaling inactivates GSK3α and leads to attenuated GLUT4 internalization. Furthermore, the isoform-specific pharmacological inhibition of GSK3α significantly improves insulin sensitivity and glucose tolerance in diet-induced insulin-resistant mice. Together, we identify a new role of GSK3α in insulin-stimulated glucose disposal by regulating Dyn2-mediated GLUT4 endocytosis in muscle cells. These results highlight the isoform-specific function of GSK3α on membrane trafficking and its potential as a therapeutic target for metabolic disorders.

Involvement of the extracellular matrix and integrin signalling proteins in skeletal muscle glucose uptake

Whole-body euglycaemia is partly maintained by two cellular processes that encourage glucose uptake in skeletal muscle, the insulin- and contraction-stimulated pathways, with research suggesting convergence between these two processes. The normal structural integrity of the skeletal muscle requires an intact actin cytoskeleton as well as integrin-associated proteins, and thus those structures are likely fundamental for effective glucose uptake in skeletal muscle. In contrast, excessive extracellular matrix (ECM) remodelling and integrin expression in skeletal muscle may contribute to insulin resistance owing to an increased physical barrier causing reduced nutrient and hormonal flux. This review explores the role of the ECM and the actin cytoskeleton in insulin- and contraction-mediated glucose uptake in skeletal muscle. This is a clinically important area of research given that defects in the structural integrity of the ECM and integrin-associated proteins may contribute to loss of muscle function and decreased glucose uptake in type 2 diabetes.

Effects of Acute Muscle Contraction on the Key Molecules in Insulin and Akt Signaling in Skeletal Muscle in Health and in Insulin Resistant States

Insulin signaling plays a key role in glucose uptake, glycogen synthesis, and protein and lipid synthesis. In insulin-resistant states like obesity and type 2 diabetes mellitus, these processes are dysregulated. Regular physical exercise is a potential therapeutic strategy against insulin resistance, as an acute bout of exercise increases glucose disposal during the activity and for hours into recovery. Chronic exercise increases the activation of proteins involved in insulin signaling and increases glucose transport, even in insulin resistant states. Here, we will focus on the effect of acute exercise on insulin signaling and protein kinase B (Akt) pathways. Activation of proximal proteins involved in insulin signaling (insulin receptor, insulin receptor substrate-1 (IRS-1), phosphoinoside-3 kinase (PI3K)) are unchanged in response to acute exercise/contraction, while activation of Akt and of its substrates, TBC1 domain family 1 (TBC1D1), and TBC domain family 4 (TBC1D4) increases in response to such exercise/contraction. A wide array of Akt substrates is also regulated by exercise. Additionally, AMP-activated protein kinase (AMPK) seems to be a main mediator of the benefits of exercise on skeletal muscle. Questions persist on how mTORC1 and AMPK, two opposing regulators, are both upregulated after an acute bout of exercise.

Post-translational Modifications: The Signals at the Intersection of Exercise, Glucose Uptake, and Insulin Sensitivity

Diabetes is a global epidemic, of which type 2 diabetes makes up the majority of cases. Nonetheless, for some individuals, type 2 diabetes is eminently preventable and treatable via lifestyle interventions. Glucose uptake into skeletal muscle increases during and in recovery from exercise, with exercise effective at controlling glucose homeostasis in individuals with type 2 diabetes. Furthermore, acute and chronic exercise sensitizes skeletal muscle to insulin. A complex network of signals converge and interact to regulate glucose metabolism and insulin sensitivity in response to exercise. Numerous forms of post-translational modifications (eg, phosphorylation, ubiquitination, acetylation, ribosylation, and more) are regulated by exercise. Here we review the current state of the art of the role of post-translational modifications in transducing exercise-induced signals to modulate glucose uptake and insulin sensitivity within skeletal muscle. Furthermore, we consider emerging evidence for noncanonical signaling in the control of glucose homeostasis and the potential for regulation by exercise. While exercise is clearly an effective intervention to reduce glycemia and improve insulin sensitivity, the insulin- and exercise-sensitive signaling networks orchestrating this biology are not fully clarified. Elucidation of the complex proteome-wide interactions between post-translational modifications and the associated functional implications will identify mechanisms by which exercise regulates glucose homeostasis and insulin sensitivity. In doing so, this knowledge should illuminate novel therapeutic targets to enhance insulin sensitivity for the clinical management of type 2 diabetes.

Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

Insulin-stimulated glucose uptake in skeletal muscle is of fundamental importance to prevent postprandial hyperglycemia, and long-term deficits in insulin-stimulated glucose uptake underlie insulin resistance and type 2 diabetes. Skeletal muscle is responsible for ~80% of the peripheral glucose uptake from circulation via the insulin-responsive glucose transporter GLUT4. GLUT4 is mainly sequestered in intracellular GLUT4 storage vesicles in the basal state. In response to insulin, the GLUT4 storage vesicles rapidly translocate to the plasma membrane, where they undergo vesicle docking, priming, and fusion via the high-affinity interactions among the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) exocytosis proteins and their regulators. Numerous studies have elucidated that GLUT4 translocation is defective in insulin resistance and type 2 diabetes. Emerging evidence also links defects in several SNAREs and SNARE regulatory proteins to insulin resistance and type 2 diabetes in rodents and humans. Therefore, we highlight the latest research on the role of SNAREs and their regulatory proteins in insulin-stimulated GLUT4 translocation in skeletal muscle. Subsequently, we discuss the novel emerging role of SNARE proteins as interaction partners in pathways not typically thought to involve SNAREs and how these atypical functions reveal novel therapeutic targets for combating peripheral insulin resistance and diabetes.

Trans-ferulic acid attenuates hyperglycemia-induced oxidative stress and modulates glucose metabolism by activating AMPK signaling pathway in vitro

Adenosine monophosphate-activated protein kinase (AMPK) is a potent metabolic regulator and an attractive target for antidiabetic activators. Here we report for the first that, trans-ferulic acid (TFA) is a potent dietary bioactive molecule of hydroxycinnamic acid derivative for the activation of AMPK with a maximum increase in phosphorylation (2.71/2.67 ± 0.10; p < .001 vs. high glucose [HG] control) in hyperglycemia-induced human liver cells (HepG2) and rat skeletal muscle cells (L6), where HG suppresses the AMPK pathway. It was also observed that TFA increased activation of AMPK in a dose- and time-dependent manner and also increased the phosphorylation of acetyl-CoA carboxylase (ACC), suggesting that it may promotes fatty acid oxidation; however, pretreatment with compound C reversed the effect. In addition, TFA reduced the level of intracellular reactive oxygen species (ROS) and nitric oxide (NO) induced by hyperglycemia and subsequently increased the level of glutathione. Interestingly, TFA also upregulated the glucose transporters, GLUT2 and GLUT4, and inhibited c-Jun N-terminal protein kinase (JNK1/2) by decreasing the phosphorylation level in tested cells under HG condition. Our studies provide critical insights into the mechanism of action of TFA as a potential natural activator of AMPK under hyperglycemia. PRACTICAL APPLICATIONS: Hydroxycinnamic acid derivatives possess various pharmacological properties and are found to be one of the most ubiquitous groups of plant metabolites in almost all dietary sources. However, the tissue-specific role and its mechanism under hyperglycemic condition remain largely unknown. The present study showed that TFA is a potent activator of AMPK under HG condition and it could be used as a therapeutic agent against hyperglycemia in type 2 diabetes.

New Intracellular Peptide Derived from Hemoglobin Alpha Chain Induces Glucose Uptake and Reduces Blood Glycemia

Intracellular peptides were shown to derive from proteasomal degradation of proteins from mammalian and yeast cells, being suggested to play distinctive roles both inside and outside these cells. Here, the role of intracellular peptides previously identified from skeletal muscle and adipose tissues of C57BL6/N wild type (WT) and neurolysin knockout mice were investigated. In differentiated C2C12 mouse skeletal muscle cells, some of these intracellular peptides like insulin activated the expression of several genes related to muscle contraction and gluconeogenesis. One of these peptides, LASVSTVLTSKYR (Ric4; 600 µg/kg), administrated either intraperitoneally or orally in WT mice, decreased glycemia. Neither insulin (10 nM) nor Ric4 (100 µM) induced glucose uptake in adipose tissue explants obtained from conditional knockout mice depleted of insulin receptor. Ric4 (100 µM) similarly to insulin (100 nM) induced Glut4 translocation to the plasma membrane of C2C12 differentiated cells, and increased GLUT4 mRNA levels in epididymal adipose tissue of WT mice. Ric4 (100 µM) increased both Erk and Akt phosphorylation in C2C12, as well as in epididymal adipose tissue from WT mice; Erk, but not Akt phosphorylation was activated by Ric4 in tibial skeletal muscle from WT mice. Ric4 is rapidly degraded in vitro by WT liver and kidney crude extracts, such a response that is largely reduced by structural modifications such as N-terminal acetylation, C-terminal amidation, and substitution of Leu8 for DLeu8 (Ac-LASVSTV[DLeu]TSKYR-NH2; Ric4-16). Ric4-16, among several Ric4 derivatives, efficiently induced glucose uptake in differentiated C2C12 cells. Among six Ric4-derivatives evaluated in vivo, Ac-LASVSTVLTSKYR-NH2 (Ric4-2; 600 µg/kg) and Ac-LASVSTV[DLeu]TSKYR (Ric4-15; 600 µg/kg) administrated orally efficiently reduced glycemia in a glucose tolerance test in WT mice. The potential clinical application of Ric4 and Ric4-derivatives deserves further attention.

Proto-oncoprotein Zbtb7c and SIRT1 repression: implications in high-fat diet-induced and age-dependent obesity

Zbtb7c is a proto-oncoprotein that controls the cell cycle and glucose, glutamate, and lipid metabolism. Zbtb7c expression is increased in the liver and white adipose tissues of aging or high-fat diet-fed mice. Knockout or knockdown of Zbtb7c gene expression inhibits the adipocyte differentiation of 3T3-L1 cells and decreases adipose tissue mass in aging mice. We found that Zbtb7c was a potent transcriptional repressor of SIRT1 and that SIRT1 was derepressed in various tissues of Zbtb7c-KO mice. Mechanistically, Zbtb7c interacted with p53 and bound to the proximal promoter p53RE1 and p53RE2 to repress the SIRT1 gene, in which p53RE2 was particularly critical. Zbtb7c induced p53 to interact with the corepressor mSin3A-HADC1 complex at p53RE. By repressing the SIRT1 gene, Zbtb7c increased the acetylation of Pgc-1α and Pparγ, which resulted in repression or activation of Pgc-1α or Pparγ target genes involved in lipid metabolism. Our study provides a molecular target that can overexpress SIRT1 protein in the liver, pancreas, and adipose tissues, which can be beneficial in the treatment of diabetes, obesity, longevity, etc.

Targeting a regulatory DNA sequence linked to the repression of a critical enzyme during metabolic diseases could prove valuable for future therapies. The SIRT1 enzyme is involved in metabolic processes and stress resistance, and its dysregulation is linked to obesity and diabetes development. SIRT1 expression also decreases with aging and stress, but the precise regulation mechanisms are unclear. In experiments on aging mice and mice fed a high-fat diet, Man-Wook Hur at Yonsei University in Seoul, South Korea, and co-workers demonstrated that SIRT1 expression is repressed by a protein called Zbtb7c, which is highly expressed in fat and liver tissues. Aging mice without the Zbtb7c-encoding gene had less fatty tissue than controls. Zbtb7c represses the SIRT1 gene by interacting with protein p53. A sequence critical to this repression mechanism may provide a therapeutic target.

The many actions of insulin in skeletal muscle, the paramount tissue determining glycemia

As the principal tissue for insulin-stimulated glucose disposal, skeletal muscle is a primary driver of whole-body glycemic control. Skeletal muscle also uniquely responds to muscle contraction or exercise with increased sensitivity to subsequent insulin stimulation. Insulin's dominating control of glucose metabolism is orchestrated by complex and highly regulated signaling cascades that elicit diverse and unique effects on skeletal muscle. We discuss the discoveries that have led to our current understanding of how insulin promotes glucose uptake in muscle. We also touch upon insulin access to muscle, and insulin signaling toward glycogen, lipid, and protein metabolism. We draw from human and rodent studies in vivo, isolated muscle preparations, and muscle cell cultures to home in on the molecular, biophysical, and structural elements mediating these responses. Finally, we offer some perspective on molecular defects that potentially underlie the failure of muscle to take up glucose efficiently during obesity and type 2 diabetes.

Complexin-2 redistributes to the membrane of muscle cells in response to insulin and contributes to GLUT4 translocation

Insulin stimulates glucose uptake in muscle cells by rapidly redistributing vesicles containing GLUT4 glucose transporters from intracellular compartments to the plasma membrane (PM). GLUT4 vesicle fusion requires the formation of SNARE complexes between vesicular VAMP and PM syntaxin4 and SNAP23. SNARE accessory proteins usually regulate vesicle fusion processes. Complexins aide in neuro-secretory vesicle-membrane fusion by stabilizing trans-SNARE complexes but their participation in GLUT4 vesicle fusion is unknown. We report that complexin-2 is expressed and homogeneously distributed in L6 rat skeletal muscle cells. Upon insulin stimulation, a cohort of complexin-2 redistributes to the PM. Complexin-2 knockdown markedly inhibited GLUT4 translocation without affecting proximal insulin signalling of Akt/PKB phosphorylation and actin fiber remodelling. Similarly, complexin-2 overexpression decreased maximal GLUT4 translocation suggesting that the concentration of complexin-2 is finely tuned to vesicle fusion. These findings reveal an insulin-dependent regulation of GLUT4 insertion into the PM involving complexin-2.

PTEN inhibitor bpV(HOpic) confers protection against ionizing radiation

Exposure to Ionizing radiation (IR) poses a severe threat to human health. Therefore, there is an urgent need to develop potent and safe radioprotective agents for radio-nuclear emergencies. Phosphatidylinositol-3-kinase (PI3K) mediates its cytoprotective signaling against IR by phosphorylating membrane phospholipids to phosphatidylinositol 3,4,5 triphosphate, PIP3, that serve as a docking site for AKT. Phosphatase and Tensin Homolog on chromosome 10 (PTEN) antagonizes PI3K activity by dephosphorylating PIP3, thus suppressing PI3K/AKT signaling that could prevent IR induced cytotoxicity. The current study was undertaken to investigate the radioprotective potential of PTEN inhibitor (PTENi), bpV(HOpic). The cell cytotoxicity, proliferation index, and clonogenic survival assays were performed for assessing the radioprotective potential of bpV(HOpic). A safe dose of bpV(HOpic) was shown to be radioprotective in three radiosensitive tissue origin cells. Further, bpV(HOpic) significantly reduced the IR-induced apoptosis and associated pro-death signaling. A faster and better DNA repair kinetics was also observed in bpV(HOpic) pretreated cells exposed to IR. Additionally, bpV(HOpic) decreased the IR-induced oxidative stress and significantly enhanced the antioxidant defense mechanism in cells. The radioprotective effect of bpV(HOpic) was found to be AKT dependant and primarily regulated by the enhanced glycolysis and associated signaling. Furthermore, this in-vitro observation was verified in-vivo, where administration of bpV(HOpic) in C57BL/6 mice resulted in AKT activation and conferred survival advantage against IR-induced mortality. These results imply that bpV(HOpic) ameliorates IR-induced oxidative stress and cell death by inducing AKT signaling mediated antioxidant defense system and DNA repair pathways, thus strengthening its potential to be used as a radiation countermeasure.

Tmod3 Phosphorylation Mediates AMPK-Dependent GLUT4 Plasma Membrane Insertion in Myoblasts

Insulin and muscle contractions mediate glucose transporter 4 (GLUT4) translocation and insertion into the plasma membrane (PM) for glucose uptake in skeletal muscles. Muscle contraction results in AMPK activation, which promotes GLUT4 translocation and PM insertion. However, little is known regarding AMPK effectors that directly regulate GLUT4 translocation. We aim to identify novel AMPK effectors in the regulation of GLUT4 translocation. We performed biochemical, molecular biology and fluorescent microscopy imaging experiments using gain- and loss-of-function mutants of tropomodulin 3 (Tmod3). Here we report Tmod3, an actin filament capping protein, as a novel AMPK substrate and an essential mediator of AMPK-dependent GLUT4 translocation and glucose uptake in myoblasts. Furthermore, Tmod3 plays a key role in AMPK-induced F-actin remodeling and GLUT4 insertion into the PM. Our study defines Tmod3 as a key AMPK effector in the regulation of GLUT4 insertion into the PM and glucose uptake in muscle cells, and offers new mechanistic insights into the regulation of glucose homeostasis.

Anti-diabetic effects of Grifola frondosa bioactive compound and its related molecular signaling pathways in palmitate-induced C2C12 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Grifola frondosa (GF), a high value medicinal mushroom, is popularly consumed as traditional medicines and health foods in China and Japan. It is a herbal medicine traditionally used for treating inflammation, cancer and diabetes.

AIM OF THE STUDY

This study aimed to examine the anti-diabetic effects of a GF bioactive compound ergosterol peroxide (EPO), and its mechanism(s) of action in palmitate (PA)-induced C2C12 cells.

MATERIALS AND METHODS

EPO was isolated and purified from GF fruiting bodies, and used to test for anti-diabetic activity in PA-induced murine C2C12 skeletal muscle cells through measuring glucose uptake, intracellular ROS production, and expressions of MAPKs, IRS-1, PI3K, Akt and GLUT-4 proteins.

RESULTS

EPO significantly up-regulated glucose absorption and increased cell growth. At 5 μM, EPO significantly enhanced glucose uptake and decreased ROS formation, as well as up-regulated the expression of IRS-1, p-IRS-1, PI3K, Akt, p-Akt, and GLUT-4 proteins in PA-induced cells, while their p-JNK and p-p38 expression were down-regulated. GLUT-4 siRNA treatment effectively down-regulated the EPO-induced absorption of glucose and inhibited the expression of GLUT-4.

CONCLUSION

These results suggest that the anti-diabetic effect of GF was from its bioactive compound EPO through the inhibition of ROS production, up-regulation of glucose absorption, and modulation of PI3K/Akt, MAPKs and GLUT-4 signaling transduction pathways.

β-catenin regulates muscle glucose transport via actin remodelling and M-cadherin binding

Objective

Skeletal muscle glucose disposal following a meal is mediated through insulin-stimulated movement of the GLUT4-containing vesicles to the cell surface. The highly conserved scaffold-protein β-catenin is an emerging regulator of vesicle trafficking in other tissues. Here, we investigated the involvement of β-catenin in skeletal muscle insulin-stimulated glucose transport.

Methods

Glucose homeostasis and transport was investigated in inducible muscle specific β-catenin knockout (BCAT-mKO) mice. The effect of β-catenin deletion and mutation of β-catenin serine 552 on signal transduction, glucose uptake and protein–protein interactions were determined in L6-G4-myc cells, and β-catenin insulin-responsive binding partners were identified via immunoprecipitation coupled to label-free proteomics.

Results

Skeletal muscle specific deletion of β-catenin impaired whole-body insulin sensitivity and insulin-stimulated glucose uptake into muscle independent of canonical Wnt signalling. In response to insulin, β-catenin was phosphorylated at serine 552 in an Akt-dependent manner, and in L6-G4-myc cells, mutation of β-catenin^S552 impaired insulin-induced actin-polymerisation, resulting in attenuated insulin-induced glucose transport and GLUT4 translocation. β-catenin was found to interact with M-cadherin in an insulin-dependent β-catenin^S552-phosphorylation dependent manner, and loss of M-cadherin in L6-G4-myc cells attenuated insulin-induced actin-polymerisation and glucose transport.

Conclusions

Our data suggest that β-catenin is a novel mediator of glucose transport in skeletal muscle and may contribute to insulin-induced actin-cytoskeleton remodelling to support GLUT4 translocation.

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Deletion of β-catenin from the muscles of adult mice attenuates skeletal muscle glucose uptake.

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Insulin stimulates phosphorylation of β-catenin^S552 by a mechanism involving Akt, and this is required for insulin's effects on both GLUT4 trafficking and actin remodelling.

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Insulin promotes β-catenin/M-cadherin binding, to support cortical actin remodelling associated with GLUT4 translocation.

From insulin to Akt: Time delays and dominant processes

Akt/PKB regulates numerous processes in the mammalian cell, including cell survival and proliferation, and glucose uptake in response to insulin. Abnormalities in Akt signalling are linked to the development of Type 2 diabetes, cardio-vascular disease, and cancer. In the absence of insulin, Akt is predominantly found in the inactive state in the cytosol. Following insulin stimulation, Akt translocates to the plasma membrane, docks, and is phosphorylated to take on the active conformation. In turn, the activated Akt travels to and phosphorylates its many downstream substrates. Although crucial to the activation process, the translocation of Akt from the cytosol to the plasma membrane is currently not well understood. Here we detail the parameter optimisation of a mathematical model of Akt translocation to experimental data. We have quantified the time delay between the application of insulin and the downstream Akt translocation response, indicating the constraints on the timing of the intermediate processes. A delay of approximately 0.4 min prior to the Akt response was determined for the application of 1 nM insulin to cells in the basal state, whereas it was found that a further transition from physiological insulin to higher stimuli did not incur a delay. Furthermore, our investigation indicates that the dominant processes regulating the appearance of Akt at the plasma membrane differ with the insulin level. For physiological insulin, the rate limiting step was the release of Akt to the plasma membrane in response to the insulin signal. In contrast, at high insulin levels, regulation of the recycling of Akt from the plasma membrane to the cytosol was also required.

Role of Skeletal Muscle in Insulin Resistance and Glucose Uptake

The skeletal muscle is the largest organ in the body, by mass. It is also the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation. Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in exercise and metabolic disease. In this article, we give an overview of the importance of skeletal muscle in metabolism, describing its role in glucose uptake and the diseases that are associated with skeletal muscle metabolic dysregulation. We focus on the role of skeletal muscle in peripheral insulin resistance and the potential for skeletal muscle-targeted therapeutics to combat insulin resistance and diabetes, as well as other metabolic diseases like aging and obesity. In particular, we outline the possibilities and pitfalls of the quest for exercise mimetics, which are intended to target the molecular mechanisms underlying the beneficial effects of exercise on metabolic disease. We also provide a description of the molecular mechanisms that regulate skeletal muscle glucose uptake, including a focus on the SNARE proteins, which are essential regulators of glucose transport into the skeletal muscle. © 2020 American Physiological Society. Compr Physiol 10:785-809, 2020.

Statin Treatment-Induced Development of Type 2 Diabetes: From Clinical Evidence to Mechanistic Insights

Statins are the gold-standard treatment for the prevention of primary and secondary cardiovascular disease, which is the leading cause of mortality worldwide. Despite the safety and relative tolerability of statins, observational studies, clinical trials and meta-analyses indicate an increased risk of developing new-onset type 2 diabetes mellitus (T2DM) after long-term statin treatment. It has been shown that statins can impair insulin sensitivity and secretion by pancreatic β-cells and increase insulin resistance in peripheral tissues. The mechanisms involved in these processes include, among others, impaired Ca²⁺ signaling in pancreatic β-cells, down-regulation of GLUT-4 in adipocytes and compromised insulin signaling. In addition, it has also been described that statins’ impact on epigenetics may also contribute to statin-induced T2DM via differential expression of microRNAs. This review focuses on the evidence and mechanisms by which statin therapy is associated with the development of T2DM. This review describes the multifactorial combination of effects that most likely contributes to the diabetogenic effects of statins. Clinically, these findings should encourage clinicians to consider diabetes monitoring in patients receiving statin therapy in order to ensure early diagnosis and appropriate management.

Building GLUT4 Vesicles: CHC22 Clathrin's Human Touch

Insulin stimulates glucose transport by triggering regulated delivery of intracellular vesicles containing the GLUT4 glucose transporter to the plasma membrane. This process is defective in diseases such as type 2 diabetes (T2DM). While studies in rodent cells have been invaluable in understanding GLUT4 traffic, evolutionary plasticity must be considered when extrapolating these findings to humans. Recent work has identified species-specific distinctions in GLUT4 traffic, notably the participation of a novel clathrin isoform, CHC22, in humans but not rodents. Here, we discuss GLUT4 sorting in different species and how studies of CHC22 have identified new routes for GLUT4 trafficking. We further consider how different sorting-protein complexes relate to these routes and discuss other implications of these pathways in cell biology and disease.

Zengye Decoction Ameliorates Insulin Resistance by Promoting Glucose Uptake

The incidence of type 2 diabetes mellitus (T2DM) has been increasing in recent years and has become a serious threat to human health. Zengye Decoction (ZYD), a well-known traditional Chinese medicinal formula, has been used in the treatment of T2DM with yin asthenia and extreme heat since Qing Dynasty. However, the characteristics of antidiabetic activities of ZYD have not been fully elucidated. In our study, high-fat diet and streptozotocin were used to establish the T2DM rat model. After 3 weeks of treatment with ZYD, the fasting blood glucose (FBG), oral glucose tolerance, the fasting serum insulin concentration, insulin sensitivity index (ISI), serum lipid profiles, and pancreas histopathology were measured. Then, under circumstance of insulin-resistant glucose consumption, 2-(N-(7-nitrobenz-2-oxa-l,3-diazol-4-yl) amino)-2-deoxyglucose (2-NBDG) uptake and glycogen content in C2C12 myotubes, 3T3-L1 adipocytes, and HepG2 cells were determined, respectively. Finally, the expressions of key targets in the insulin signaling pathway were measured to explain the potential mechanism underlying these activities. After administration with ZYD, a notable reduction in FBG levels, oral glucose tolerance test-area under the curve, blood lipid metabolism, and ISI values were observed compared with the diabetic control group. Moreover, ZYD restored the damaged islet cells in T2DM rats. Significant increases in glucose consumption, glucose uptake, glycogen content, expression of glucose transporter type 4, and the ratio of p-Akt/Akt were observed in the ZYD groups. According to the above results, ZYD exhibited glucose disposal, including glucose consumption, glucose uptake, and glycogen content and promoted the Akt signal pathway, which indicates that ZYD exerts significant hypoglycemic effect in T2DM.

Neferine Promotes GLUT4 Expression and Fusion With the Plasma Membrane to Induce Glucose Uptake in L6 Cells

Glucose transporter 4 (GLUT4) is involved in regulating glucose uptake in striated muscle, liver, and adipose tissue. Neferine is a dibenzyl isoquinoline alkaloid derived from dietary lotus seeds and has multiple pharmacological effects. Therefore, this study investigated neferine’s role in glucose translocation to cell surface, glucose uptake, and GLUT4 expression. In our study, neferine upregulated GLUT4 expression, induced GLUT4 plasma membrane fusion, increased intracellular Ca²⁺, promoted glucose uptake, and alleviated insulin resistance in L6 cells. Furthermore, neferine significantly activated phosphorylation of AMP-activated protein kinase (AMPK) and protein kinase C (PKC). AMPK and PKC inhibitors blocked neferine-induced GLUT4 expression and increased intracellular Ca²⁺. While neferine-induced GLUT4 expression and intracellular Ca²⁺ were inhibited by G protein and PLC inhibitors, only intracellular Ca²⁺ was inhibited by inositol trisphosphate receptor (IP₃R) inhibitors. Thus, neferine promoted GLUT4 expression via the G protein-PLC-PKC and AMPK pathways, inducing GLUT4 plasma membrane fusion and subsequent glucose uptake and increasing intracellular Ca²⁺ through the G protein-PLC-IP₃-IP₃R pathway. Treatment with 0 mM extracellular Ca²⁺ + Ca²⁺ chelator did not inhibit neferine-induced GLUT4 expression but blocked neferine-induced GLUT4 plasma membrane fusion and glucose uptake, suggesting the latter two are Ca²⁺-dependent. Therefore, we conclude that neferine is a potential treatment for type 2 diabetes.

Simulation of gastric bypass effects on glucose metabolism and non-alcoholic fatty liver disease with the Sleeveballoon device

Background

Gastric bypass surgery is a very effective treatment of obesity and type 2 diabetes. However, very few eligible patients are offered surgery. Some patients also prefer less invasive approaches.

We aimed to study the effects of the Sleeveballoon – a new device combining an intragastric balloon with a connecting sleeve, which covers the duodenal and proximal jejunal mucosa – on insulin sensitivity, glycemic control, body weight and body fat distribution.

Methods

We compared the effects of Sleeveballoon, Roux-en-Y Gastric-Bypass (RYGB) and sham-operation in 30 high-fat diet (HFD) fed Wistar rats. Whole body and hepatic insulin sensitivity and insulin signaling were studied. Transthoracic echocardiography was performed using a Vevo 2100 system (FUJIFILM VisualSonics Inc., Canada). Gastric emptying was measured using gastrografin.

Findings

Hepatic (P = .023) and whole-body (P = .011) insulin sensitivity improved in the Sleeveballoon and RYGB groups compared with sham-operated rats. Body weight reduced in both Sleeveballoon and RYGB groups in comparison to the sham-operated group (503.1 ± 8.9 vs. 614.4 ± 20.6 g, P = .006 and 490.0 ± 17.7 vs. 614.4 ± 20.6 g, P = .006, respectively). Ectopic fat deposition was drastically reduced while glycogen content was increased in both liver and skeletal muscle. Gastric emptying (T_1/2) was longer (157.7 ± 29.2 min, P = .007) in the Sleeveballoon than in sham-operated rats (97.1 ± 26.3 min), but shorter in RYGB (3.5 ± 1.1 min, P < .0001). Cardiac function was better in Sleeveballoon and RYGB versus sham-operated rats.

Interpretation

The Sleeveballoon reduces peripheral and hepatic insulin resistance, glycaemia, body weight and ectopic fat deposition to a similar level as RYGB, although the contribution of gastric emptying to blood glucose reduction is different.

Akt is a critical node of acute myocardial insulin resistance and cardiac dysfunction after cardiopulmonary bypass

AIMS

Acute myocardial insulin resistance is an independent risk factor for patients who undergo cardiac surgery with cardiopulmonary bypass (CPB). However, the underlying mechanism of insulin resistance during CPB has not been fully investigated.

MATERIALS AND METHODS

To explore the role of myocardial insulin resistance on the cardiac function and its underlying mechanism, CPB operation and pharmacological intervention were applied in mini pigs, and myocardial insulin signaling, glucose uptake, ATP production and cardiac function were examined.

KEY FINDINGS

Our data showed that CPB elicited not only hyperglycemia and hyperinsulinemia, but also inactivated Akt, and impaired the transposition of membrane glucose transporter-4 (GLUT-4), reduced glucose uptake and ATP production in the myocardium as well, which in turn was accompanied with cardiac dysfunction. Meanwhile, linear correlations were established among reduced myocardial glucose uptake, ATP production, and depressed cardiac systolic or diastolic function. Reactivation of Akt by SC79, an Akt agonist, partially alleviated myocardial insulin resistance and restored post CPB cardiac function via augmenting myocardial glucose uptake and ATP production.

SIGNIFICANCE

These findings revealed that acute myocardial insulin resistance due to inactivation of Akt played a key role in cardiac dysfunction post CPB via suppressing glucose metabolism related energy supply.

Klotho induces insulin resistance possibly through interference with GLUT4 translocation and activation of Akt, GSK3β, and PFKfβ3 in 3T3-L1 adipocyte cells

Klotho is considered as an anti-aging factor inducing insulin resistance and involved in type 2 diabetes. However, mechanisms by which klotho induces insulin resistance remain to be understood. Thus, in this study, we aimed to evaluate possible interference points of klotho with insulin signaling pathways in 3T3-L1 adipocyte cells by focusing on phosphorylation levels of Akt, GSK3β, PFK-fβ3, and GLUT4 translocation. Differentiation of 3T3-L1 cells to the adipocyte-like cells were performed using specific differentiation kit and confirmed by mRNA expression assay of PPARγ using qRT-PCR, and Sudan black staining of lipid droplets. Then cells were co-treated with klotho and insulin. Expression and translocation of GLUT4 mRNA were evaluated using qRT-PCR and Alexa flour 488 conjugated GLUT4 antibody, respectively. P-Akt/Akt, p-GSK3β/GSK3β, and p-PFKfβ3/PFKfβ3 ratios were determined in insulin and klotho/insulin treated cells using western blot. Our result indicated that GLUT4 expression were decreased to 0.72 ± 0.16 fold in insulin treated cells, however it was calculated 1.12 ± 0.25 fold in klotho/insulin treated cells. In addition, klotho prevented GLUT4 membrane translocation by 27.2% in comparison with insulin-treated cells (P < 0.05). Interestingly, in insulin/klotho co-treated cells, phospho-levels of Akt, GSK3β, and PFKfβ3 proteins was decreased to 2.34 ± 0.14, 2.29 ± 0.63, and 1.95 ± 0.37 fold in comparison with the insulin cells, (P < 0.05). In conclusion, our study indicated that klotho induces insulin resistance in adipocytes possibly through prevention of GLUT4 translocation, and interfere with phosphorylation of Akt, GSK3β, and PFKf3β intracellular signaling mediators by insulin.

High-throughput Exploration of the Network Dependent on AKT1 in Mouse Ovarian Granulosa Cells

The PI3K/AKT signaling pathway is known to regulate a broad range of cellular processes, and it is often altered in several types of cancers. Recently, somatic AKT1 mutations leading to a strong activation of this kinase have been reported in juvenile granulosa cell tumors. However, the molecular role of AKT1 in the supporting cell lineage of the ovary is still poorly understood. To get insights into its function in such cells, we depleted Akt1 in murine primary granulosa cells and assessed the molecular consequences at both the transcript and protein levels. We were able to corroborate the involvement of AKT1 in the regulation of metabolism, apoptosis, cell cycle, or cytoskeleton dynamics in this ovarian cell type. Consistently, we showed in established granulosa cells that depletion of Akt1 provoked altered directional persistent migration and increased its velocity. This study also allowed us to put forward new direct and indirect targets of the kinase. Indeed, a series of proteins involved in intracellular transport and mitochondrial physiology were significantly affected by Akt1 depletion. Using in silico analyses, we also propose a set of kinases and transcription factors that can mediate the action of AKT1 on the deregulated transcripts and proteins. Taken altogether, our results provide a resource of direct and indirect AKT1 targets in granulosa cells and may help understand its roles in this ovarian cell type.

Metabolic and Epigenetic Action Mechanisms of Antidiabetic Medicinal Plants

Diabetes is a predominant metabolic disease nowadays due to the off-beam lifestyle of diet and reduced physical activity. Complications of the illness include the gene-environment interactions and the downstream genetic and epigenetic consequences, e.g., cardiovascular diseases, tumor progression, retinopathy, nephropathy, neuropathy, polydipsia, polyphagia, polyuria, and weight loss. This review sheds the light on the mechanistic insights of antidiabetic medicinal plants in targeting key organs and tissues involved in regulating blood glucose homeostasis including the pancreas, liver, muscles, adipose tissues, and glucose absorption in the intestine. Diabetes is also involved in modulating major epigenetic pathways such as DNA methylation and histone modification. In this respect, we will discuss the phytochemicals as current and future epigenetic drugs in the treatment of diabetes. In addition, several proteins are common targets for the treatment of diabetes. Some phytochemicals are expected to directly interact with these targets. We lastly uncover modeling studies that predict such plausible interactions. In conclusion, this review article presents the mechanistic insight of phytochemicals in the treatment of diabetes by combining both the cellular systems biology and molecular modeling.

Association of Delta-6-Desaturase Expression with Aggressiveness of Cancer, Diabetes Mellitus, and Multiple Sclerosis: A Narrative Review

Background: The phosphatidylinositol 3-kinase/ protein kinase B /mammalian target of rapamycin (PI3K/Akt/ mTOR) signaling regulates multiple cellular processes and organizes cell proliferation, survival, and differentiation with the available nutrients, in particular, fatty acids. Polyunsaturated fatty acids (PUFAs) are cytotoxic to cancer cells and play a critical role in the treatment of multiple sclerosis (MS) and diabetes mellitus (DM). PUFAs are produced in the body by desaturases and elongases from dietary essential fatty acids (EFAs), primarily involving delta-6-desaturase (D6D). D6D is a rate-limiting enzyme for maintaining many aspects of lipid homeostasis and normal health. D6D is important to recognize the mechanisms that regulate the expression of this enzyme in humans. A lower level of D6D was seen in breast tumors compared to normal tissues. Interestingly, the elevated serum level of D6D was seen in MS and DM, which explains the critical role of D6D in inflammatory diseases. Methods: We searched databases of PubMed, Web of Science (WOS), Google Scholar, Scopus and related studies by predefined eligibility criteria. We assessed their quality and extracted data. Results: Regarding the mTOR signaling pathway, there is remarkable contributions of many inflammatory diseases to attention to common metabolic pathways are depicted. Of course, we need to have the insights into each disorder and their pathological process. The first step in balancing the intake of EFAs is to prevent the disruption of metabolism and expression of the D6D enzyme. Conclusions: The ω6 and ω3 pathways are two major pathways in the biosynthesis of PUFAs. In both of these, D6D is a vital bifunctional enzyme desaturating linoleic acid or alpha-linolenic acid. Therefore, if ω6 and ω3 EFAs are given together in a ratio of 2: 1, the D6D expression will be down-regulated and normalized.

High-throughput Exploration of the Network Dependent on AKT1 in Mouse Ovarian Granulosa Cells

The PI3K/AKT signaling pathway is known to regulate a broad range of cellular processes, and it is often altered in several types of cancers. Recently, somatic AKT1 mutations leading to a strong activation of this kinase have been reported in juvenile granulosa cell tumors. However, the molecular role of AKT1 in the supporting cell lineage of the ovary is still poorly understood. To get insights into its function in such cells, we depleted Akt1 in murine primary granulosa cells and assessed the molecular consequences at both the transcript and protein levels. We were able to corroborate the involvement of AKT1 in the regulation of metabolism, apoptosis, cell cycle, or cytoskeleton dynamics in this ovarian cell type. Consistently, we showed in established granulosa cells that depletion of Akt1 provoked altered directional persistent migration and increased its velocity. This study also allowed us to put forward new direct and indirect targets of the kinase. Indeed, a series of proteins involved in intracellular transport and mitochondrial physiology were significantly affected by Akt1 depletion. Using in silico analyses, we also propose a set of kinases and transcription factors that can mediate the action of AKT1 on the deregulated transcripts and proteins. Taken altogether, our results provide a resource of direct and indirect AKT1 targets in granulosa cells and may help understand its roles in this ovarian cell type.

Mechanisms of insulin resistance by simvastatin in C2C12 myotubes and in mouse skeletal muscle

Statins inhibit cholesterol biosynthesis and lower serum LDL-cholesterol levels. They are generally well tolerated, but can cause insulin resistance in patients. Therefore, we investigated the mechanisms underlying the statin-induced insulin resistance. We used mice and C2C12 myotubes (murine cell line): mice (n = 10) were treated with oral simvastatin (5 mg/kg/day) or water (control) for 21 days and C2C12 cells were exposed to 10 μM simvastatin for 24 h. After intraperitoneal glucose application (2 g/kg), simvastatin-treated mice had higher glucose but equal insulin plasma concentrations than controls and lower glucose transport into skeletal muscle. Similarly, glucose uptake by C2C12 myotubes exposed to 10 μM simvastatin for 24 h was impaired compared to control cells. In simvastatin-treated C2C12 myotubes, mRNA and protein expression of the insulin receptor (IR) β-chain was increased, but the phosphorylation (Tyr1361) was impaired. Simvastatin decreased numerically Akt/PKB Thr308 phosphorylation (via insulin signaling pathway) and significantly Akt/PKB Ser473 phosphorylation (via mTORC2), which was explained by impaired phosphorylation of mTOR Ser2448. Reduced phosphorylation of Akt/PKB impaired downstream phosphorylation of GSK3β, leading to impaired translocation of GLUT4 into plasma membranes of C2C12 myotubes. In contrast, reduced phosphorylation of AS160 could be excluded as a reason for impaired GLUT4 translocation. In conclusion, simvastatin caused insulin resistance in mice and impaired glucose uptake in C2C12 myotubes. The findings in myotubes can be explained by diminished activation of Akt/PKB by mTORC2 and downstream effects on GSK3β, impairing the translocation of GLUT4 and the uptake of glucose.

Acute treadmill exercise discriminately improves the skeletal muscle insulin-stimulated growth signaling responses in mice lacking REDD1

A loss of the regulated in development and DNA damage 1 (REDD1) hyperactivates mechanistic Target of Rapamycin Complex 1 (mTORC1) reducing insulin-stimulated insulin signaling, which could provide insight into mechanisms of insulin resistance. Although aerobic exercise acutely inhibits mTORC1 signaling, improvements in insulin-stimulated signaling are exhibited. The goal of this study was to determine if a single bout of treadmill exercise was sufficient to improve insulin signaling in mice lacking REDD1. REDD1 wildtype (WT) and REDD1 knockout (KO) mice were acutely exercised on a treadmill (30 min, 20 m/min, 5% grade). A within animal noninsulin-to-insulin-stimulated percent change in skeletal muscle insulin-stimulated kinases (IRS-1, ERK1/2, Akt), growth signaling activation (4E-BP1, S6K1), and markers of growth repression (REDD1, AMPK, FOXO1/3A) was examined, following no exercise control or an acute bout of exercise. Unlike REDD1 KO mice, REDD1 WT mice exhibited an increase (P < 0.05) in REDD1 following treadmill exercise. However, both REDD1 WT and KO mice exhibited an increase (P < 0.05) AMPK phosphorylation, and a subsequent reduction (P < 0.05) in mTORC1 signaling after the exercise bout versus nonexercising WT or KO mice. Exercise increased (P < 0.05) the noninsulin-to-insulin-stimulated percent change phosphorylation of mTORC1, ERK1/2, IRS-1, and Akt on S473 in REDD1 KO mice when compared to nonexercised KO mice. However, there was no change in the noninsulin-to-insulin-stimulated percent change activation of Akt on T308 and FOXO1/3A in the KO when compared to WT or KO mouse muscle after exercise. Our data show that a bout of treadmill exercise discriminately improves insulin-stimulated signaling in the absence of REDD1.

The Dietary Supplement Chondroitin-4-Sulfate Exhibits Oncogene-Specific Pro-tumor Effects on BRAF V600E Melanoma Cells

Dietary supplements such as vitamins and minerals are widely used in the hope of improving health but may have unidentified risks and side effects. In particular, a pathogenic link between dietary supplements and specific oncogenes remains unknown. Here we report that chondroitin-4-sulfate (CHSA), a natural glycosaminoglycan approved as a dietary supplement used for osteoarthritis, selectively promotes the tumor growth potential of BRAF V600E-expressing human melanoma cells in patient- and cell line-derived xenograft mice and confers resistance to BRAF inhibitors. Mechanistically, chondroitin sulfate glucuronyltransferase (CSGlcA-T) signals through its product CHSA to enhance casein kinase 2 (CK2)-PTEN binding and consequent phosphorylation and inhibition of PTEN, which requires CHSA chains and is essential to sustain AKT activation in BRAF V600E-expressing melanoma cells. However, this CHSA-dependent PTEN inhibition is dispensable in cancer cells expressing mutant NRAS or PI3KCA, which directly activate the PI3K-AKT pathway. These results suggest that dietary supplements may exhibit oncogene-dependent pro-tumor effects.

Effect of sub-toxic exposure to Malathion on glucose uptake and insulin signaling in L6 myoblast derived myotubes

Biochemical basis of Malathion exposure-induced diabetes mellitus is not known. Hence, effects of its sub-toxic exposure on redox sensitive kinases (RSKs), insulin signaling and insulin-induced glucose uptake were assessed in rat muscle cell line. In this in vitro study, rat myoblast (L6) cells were differentiated to myotubes and were exposed to sub-toxic concentrations (10 mg/l and 20 mg/l) of Malathion for 18 hours. Total antioxidant level and insulin-stimulated glucose uptake by myotubes were assayed. Activation of JNK, NFκB, p38MAPK and insulin signaling from tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and serine phosphorylation of Akt were assessed in myotubes after Malathion exposure by western blot and was compared with those in controls. Paraoxonase (PON) activity was measured in cell lysate using p-nitrophenyl acetate as substrate. PON1 and PON2 expression in myotubes were assessed by PCR. The glucose uptake and total antioxidant level in L6-derived myotubes after sub-toxic exposure to Malathion were decreased in a dose-dependent manner. Phosphorylation levels of RSKs (JNK, p38MAPK and IκBα component of NFκB) were increased and that of IRS-1 and Akt on insulin stimulation was decreased following Malathion exposure as compared to those in controls. PON1 and PON2 genes were expressed in myotubes with and without Malathion exposure. Significant PON activity was present in cell lysate. We conclude that sub-toxic Malathion exposure induces oxidative stress in muscle cells activating RSKs that impairs insulin signaling and thereby insulin-stimulated glucose uptake in muscle cells. This probably explains the biochemical basis of Malathion-induced insulin resistance state and diabetes mellitus.

Crosstalk in transition: the translocation of Akt

Akt/PKB is an important crosstalk node at the junction between a number of major signalling pathways in the mammalian cell. As a significant nutrient sensor, Akt plays a central role in many cellular processes, including cell growth, cell survival and glucose metabolism. The dysregulation of Akt signalling is implicated in the development of many diseases, from diabetes to cancer. The translocation of Akt from cytosol to plasma membrane is a crucial step in Akt activation. Akt is initially synthesized on the endoplasmic reticulum, but translocates to the plasma membrane (PM) in response to insulin stimulation, where it may be activated. The Akt is then recycled to the cytoplasm. The activated Akt may propagate signals to downstream substrates both at the PM and in the cytosol, hence understanding the translocation dynamics is an important step in dissecting the signalling system. At the present time, however, knowledge concerning the translocation of either activated and unactivated Akt is scant. Here we present a simple, deterministic, three-compartment ordinary differential equation model of Akt translocation in vitro. This model can reproduce the salient features of Akt translocation in a manner consistent with the experimental data. Furthermore, we demonstrate that this system is equivalent to a damped harmonic oscillator, and analyse the steady state and transient behaviour of the model over the entire parameter space.

The cell biology of systemic insulin function

Insulin is the paramount anabolic hormone, promoting carbon energy deposition in the body. Its synthesis, quality control, delivery, and action are exquisitely regulated by highly orchestrated intracellular mechanisms in different organs or "stations" of its bodily journey. In this Beyond the Cell review, we focus on these five stages of the journey of insulin through the body and the captivating cell biology that underlies the interaction of insulin with each organ. We first analyze insulin's biosynthesis in and export from the β-cells of the pancreas. Next, we focus on its first pass and partial clearance in the liver with its temporality and periodicity linked to secretion. Continuing the journey, we briefly describe insulin's action on the blood vasculature and its still-debated mechanisms of exit from the capillary beds. Once in the parenchymal interstitium of muscle and adipose tissue, insulin promotes glucose uptake into myofibers and adipocytes, and we elaborate on the intricate signaling and vesicle traffic mechanisms that underlie this fundamental function. Finally, we touch upon the renal degradation of insulin to end its action. Cellular discernment of insulin's availability and action should prove critical to understanding its pivotal physiological functions and how their failure leads to diabetes.

Mitochondrial genome modulates myocardial Akt/Glut/HK salvage pathway in spontaneously hypertensive rats adapted to chronic hypoxia

Recently we have shown that adaptation to continuous normobaric hypoxia (CNH) decreases myocardial ischemia/reperfusion injury in spontaneously hypertensive rats (SHR) and in a conplastic strain (SHR-mt^BN). The protective effect was stronger in the latter group characterized by a selective replacement of the SHR mitochondrial genome with that of a more ischemia-resistant Brown Norway strain. The aim of the present study was to examine the possible involvement of the hypoxia inducible factor (HIF)-dependent pathway of the protein kinase B/glucose transporters/hexokinase (Akt/GLUT/HK) in this mitochondrial genome-related difference of the cardioprotective phenotype. Adult male rats were exposed for 3 wk to CNH ([Formula: see text] 0.1). The expression of dominant isoforms of Akt, GLUT, and HK in left ventricular myocardium was determined by real-time RT-PCR and Western blotting. Subcellular localization of GLUTs was assessed by quantitative immunofluorescence. Whereas adaptation to hypoxia markedly upregulated protein expression of HK2, GLUT1, and GLUT4 in both rat strains, Akt2 protein level was significantly increased in SHR-mt^BN only. Interestingly, a higher content of HK2 was revealed in the sarcoplasmic reticulum-enriched fraction in SHR-mt^BN after CNH. The increased activity of HK determined in the mitochondrial fraction after CNH in both strains suggested an increase of HK association with mitochondria. Interestingly, HIF1a mRNA increased and HIF2a mRNA decreased after CNH, the former effect being more pronounced in SHR-mt^BN than in SHR. Pleiotropic effects of upregulated Akt2 along with HK translocation to mitochondria and mitochondria-associated membranes can potentially contribute to a stronger CNH-afforded cardioprotection in SHR-mt^BN compared with progenitor SHR.

Sphingolipid changes do not underlie fatty acid-evoked GLUT4 insulin resistance nor inflammation signals in muscle cells

Ceramides contribute to obesity-linked insulin resistance and inflammation in vivo, but whether this is a cell-autonomous phenomenon is debated, particularly in muscle, which dictates whole-body glucose uptake. We comprehensively analyzed lipid species produced in response to fatty acids and examined the consequence to insulin resistance and pro-inflammatory pathways. L6 myotubes were incubated with BSA-adsorbed palmitate or palmitoleate in the presence of myriocin, fenretinide, or fumonisin B1. Lipid species were determined by lipidomic analysis. Insulin sensitivity was scored by Akt phosphorylation and glucose transporter 4 (GLUT4) translocation, while pro-inflammatory indices were estimated by IκBα degradation and cytokine expression. Palmitate, but not palmitoleate, had mild effects on Akt phosphorylation but significantly inhibited insulin-stimulated GLUT4 translocation and increased expression of pro-inflammatory cytokines Il6 and Ccl2 Ceramides, hexosylceramides, and sphingosine-1-phosphate significantly heightened by palmitate correlated negatively with insulin sensitivity and positively with pro-inflammatory indices. Inhibition of sphingolipid pathways led to marked changes in cellular lipids, but did not prevent palmitate-induced impairment of insulin-stimulated GLUT4 translocation, suggesting that palmitate-induced accumulation of deleterious lipids and insulin resistance are correlated but independent events in myotubes. We propose that muscle cell-endogenous ceramide production does not evoke insulin resistance and that deleterious effects of ceramides in vivo may arise through ancillary cell communication.