Rac1 signaling is required for insulin-stimulated glucose uptake and is dysregulated in insulin-resistant murine and human skeletal muscle

The PI3K/Akt signaling axis and type 2 diabetes mellitus (T2DM): From mechanistic insights into possible therapeutic targets

Type 2 diabetes mellitus (T2DM) is an immensely debilitating chronic disease that progressively undermines the well-being of various bodily organs and, indeed, most patients succumb to the disease due to post-T2DM complications. Although there is evidence supporting the activation of the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway by insulin, which is essential in regulating glucose metabolism and insulin resistance, the significance of this pathway in T2DM has only been explored in a few studies. The current review aims to unravel the mechanisms by which different classes of PI3Ks control the metabolism of glucose; and also to discuss the original data obtained from international research laboratories on this topic. We also summarized the role of the PI3K/Akt signaling axis in target tissues spanning from the skeletal muscle to the adipose tissue and liver. Furthermore, inquiries regarding the impact of disrupting this axis on insulin function and the development of insulin resistance have been addressed. We also provide a general overview of the association of impaired PI3K/Akt signaling pathways in the pathogenesis of the most prevalent diabetes-related complications. The last section provides a special focus on the therapeutic potential of this axis by outlining the latest advances in active compounds that alleviate diabetes via modulation of the PI3K/Akt pathway. Finally, we comment on the future research aspects in which the field of T2DM therapies using PI3K modulators might be developed.

Pharmacological Activation of PDC Flux Reverses Lipid-Induced Inhibition of Insulin Action in Muscle During Recovery From Exercise

Insulin resistance is a risk factor for type 2 diabetes, and exercise can improve insulin sensitivity. However, following exercise, high circulating fatty acid (FA) levels might counteract this. We hypothesized that such inhibition would be reduced by forcibly increasing carbohydrate oxidation through pharmacological activation of the pyruvate dehydrogenase complex (PDC). Insulin-stimulated glucose uptake was examined with a crossover design in healthy young men (n = 8) in a previously exercised and a rested leg during a hyperinsulinemic-euglycemic clamp 5 h after one-legged exercise with 1) infusion of saline, 2) infusion of intralipid imitating circulating FA levels during recovery from whole-body exercise, and 3) infusion of intralipid + oral PDC activator, dichloroacetate (DCA). Intralipid infusion reduced insulin-stimulated glucose uptake by 19% in the previously exercised leg, which was not observed in the contralateral rested leg. Interestingly, this effect of intralipid in the exercised leg was abolished by DCA, which increased muscle PDC activity (130%) and flux (acetylcarnitine 130%) and decreased inhibitory phosphorylation of PDC on Ser293 (∼40%) and Ser300 (∼80%). Novel insight is provided into the regulatory interaction between glucose and lipid metabolism during exercise recovery. Coupling exercise and PDC flux activation upregulated the capacity for both glucose transport (exercise) and oxidation (DCA), which seems necessary to fully stimulate insulin-stimulated glucose uptake during recovery.

ARTICLE HIGHLIGHTS

Positive Effects of Physical Activity on Insulin Signaling

Physical activity is integral to metabolic health, particularly in addressing insulin resistance and related disorders such as type 2 diabetes mellitus (T2DM). Studies consistently demonstrate a strong association between physical activity levels and insulin sensitivity. Regular exercise interventions were shown to significantly improve glycemic control, highlighting exercise as a recommended therapeutic strategy for reducing insulin resistance. Physical inactivity is closely linked to islet cell insufficiency, exacerbating insulin resistance through various pathways including ER stress, mitochondrial dysfunction, oxidative stress, and inflammation. Conversely, physical training and exercise preserve and restore islet function, enhancing peripheral insulin sensitivity. Exercise interventions stimulate β-cell proliferation through increased circulating levels of growth factors, further emphasizing its role in maintaining pancreatic health and glucose metabolism. Furthermore, sedentary lifestyles contribute to elevated oxidative stress levels and ceramide production, impairing insulin signaling and glucose metabolism. Regular exercise induces anti-inflammatory responses, enhances antioxidant defenses, and promotes mitochondrial function, thereby improving insulin sensitivity and metabolic efficiency. Encouraging individuals to adopt active lifestyles and engage in regular exercise is crucial for preventing and managing insulin resistance and related metabolic disorders, ultimately promoting overall health and well-being.

Unresolved questions in the regulation of skeletal muscle insulin action by reactive oxygen species

Reactive oxygen species (ROS) are well-established signaling molecules implicated in a wide range of cellular processes, including both oxidative stress and intracellular redox signaling. In the context of insulin action within its target tissues, ROS have been reported to exert both positive and negative regulatory effects. However, the precise molecular mechanisms underlying this duality remain unclear. This Review examines the complex role of ROS in insulin action, with a particular focus on skeletal muscle. We aim to address three critical aspects: (a) the proposed intracellular pro-oxidative redox shift elicited by insulin, (b) the evidence supporting that redox-sensitive cysteine modifications impact insulin signaling and action, and (c) cellular mechanisms underlying how ROS can paradoxically act as both enhancers and inhibitors of insulin action. This Review underscores the urgent need for more systematic research to identify specific reactive species, redox targets, and the physiological significance of redox signaling in maintaining insulin action and metabolic health, with a particular emphasis on human skeletal muscle.

AMPK as a mediator of tissue preservation: time for a shift in dogma?

Ground-breaking discoveries have established 5'-AMP-activated protein kinase (AMPK) as a central sensor of metabolic stress in cells and tissues. AMPK is activated through cellular starvation, exercise and drugs by either directly or indirectly affecting the intracellular AMP (or ADP) to ATP ratio. In turn, AMPK regulates multiple processes of cell metabolism, such as the maintenance of cellular ATP levels, via the regulation of fatty acid oxidation, glucose uptake, glycolysis, autophagy, mitochondrial biogenesis and degradation, and insulin sensitivity. Moreover, AMPK inhibits anabolic processes, such as lipogenesis and protein synthesis. These findings support the notion that AMPK is a crucial regulator of cell catabolism. However, studies have revealed that AMPK's role in cell homeostasis might not be as unidirectional as originally thought. This Review explores emerging evidence for AMPK as a promoter of cell survival and an enhancer of anabolic capacity in skeletal muscle and adipose tissue during catabolic crises. We discuss AMPK-activating interventions for tissue preservation during tissue wasting in cancer-associated cachexia and explore the clinical potential of AMPK activation in wasting conditions. Overall, we provide arguments that call for a shift in the current dogma of AMPK as a mere regulator of cell catabolism, concluding that AMPK has an unexpected role in tissue preservation.

Insulin-stimulated translocation of the fatty acid transporter CD36 to the plasma membrane is mediated by the small GTPase Rac1 in adipocytes

Cluster of differentiation 36 (CD36) is a scavenger receptor (SR), recognizing diverse extracellular ligands in various types of mammalian cells. Long-chain fatty acids (FAs), which are important constituents of phospholipids and triglycerides, also utilize CD36 as a predominant membrane transporter, being incorporated from the circulation across the plasma membrane in several cell types, including cardiac and skeletal myocytes and adipocytes. CD36 is localized in intracellular vesicles as well as the plasma membrane, and its distribution is modulated by extracellular stimuli. Herein, we aimed to clarify the molecular basis of insulin-stimulated translocation of CD36, which leads to the enhanced uptake of long-chain FAs, in adipocytes. To this end, we developed a novel exofacial epitope-tagged reporter to specifically detect cell surface-localized CD36. By employing this reporter, we demonstrate that the small GTPase Rac1 plays a pivotal role in insulin-stimulated translocation of CD36 to the plasma membrane in 3T3-L1 adipocytes. Additionally, phosphoinositide 3-kinase and the protein kinase Akt2 are shown to be involved in the regulation of Rac1. Downstream of Rac1, another small GTPase RalA directs CD36 translocation. Collectively, these results suggest that CD36 is translocated to the plasma membrane by insulin through mechanisms similar to those for the glucose transporter GLUT4 in adipocytes.

The Effects of Body Fat Reduction through the Metabolic Control of Steam-Processed Ginger Extract in High-Fat-Diet-Fed Mice

The prevalence of metabolic syndrome is increasing globally due to behavioral and environmental changes. There are many therapeutic agents available for the treatment of chronic metabolic diseases, such as obesity and diabetes, but the data on their efficacy and safety are lacking. Through a pilot study by our group, Zingiber officinale rhizomes used as a spice and functional food were selected as an anti-obesity candidate. In this study, steam-processed ginger extract (GGE) was used and we compared its efficacy at alleviating metabolic syndrome-related symptoms with that of conventional ginger extract (GE). Compared with GE, GGE (25-100 μg/mL) had an increased antioxidant capacity and α-glucosidase inhibitory activity in vitro. GGE was better at suppressing the differentiation of 3T3-L1 adipocytes and lipid accumulation in HepG2 cells and promoting glucose utilization in C2C12 cells than GE. In 16-week high-fat-diet (HFD)-fed mice, GGE (100 and 200 mg/kg) improved biochemical profiles, including lipid status and liver function, to a greater extent than GE (200 mg/kg). The supplementation of HFD-fed mice with GGE (200 mg/kg) resulted in the downregulation of SREBP-1c and FAS gene expression in the liver. Collectively, our results indicate that GGE is a promising therapeutic for the treatment of obesity and metabolic syndrome.

P21-activated kinase-1 signaling is required to preserve adipose tissue homeostasis and cardiac function

While P21-activated kinase-1 (PAK1) has been extensively studied in relation to cardiovascular health and glucose metabolism, its roles within adipose tissue and cardiometabolic diseases are less understood. In this study, we explored the effects of PAK1 deletion on energy balance, adipose tissue homeostasis, and cardiac function utilizing a whole-body PAK1 knockout (PAK1-/-) mouse model. Our findings revealed that body weight differences between PAK1-/- and WT mice emerged at 9 weeks of age, with further increases observed at 12 weeks. Furthermore, PAK1-/- mice displayed increased fat mass and decreased lean mass at 12 weeks, indicating a shift towards adiposity. In conjunction with the increased body weight, PAK1-/- mice had increased food intake and reduced energy expenditure. At a mechanistic level, PAK1 deletion boosted the expression of lipogenic markers while diminishing thermogenic markers expression in adipose tissues, contributing to reduced energy expenditure and the overall obesogenic phenotype. Moreover, our findings highlighted a significant impact on cardiac function following PAK1 deletion, including alterations in calcium kinetics and compromised systolic and lusitropy functions. In summary, our study emphasizes the significant role of PAK1 in weight regulation and cardiac function, enriching our comprehension of heart health and metabolism. These findings could potentially facilitate the identification of novel therapeutic targets in cardiometabolic diseases.

The genetic and dietary landscape of the muscle insulin signalling network

Metabolic disease is caused by a combination of genetic and environmental factors, yet few studies have examined how these factors influence signal transduction, a key mediator of metabolism. Using mass spectrometry-based phosphoproteomics, we quantified 23,126 phosphosites in skeletal muscle of five genetically distinct mouse strains in two dietary environments, with and without acute in vivo insulin stimulation. Almost half of the insulin-regulated phosphoproteome was modified by genetic background on an ordinary diet, and high-fat high-sugar feeding affected insulin signalling in a strain-dependent manner. Our data revealed coregulated subnetworks within the insulin signalling pathway, expanding our understanding of the pathway's organisation. Furthermore, associating diverse signalling responses with insulin-stimulated glucose uptake uncovered regulators of muscle insulin responsiveness, including the regulatory phosphosite S469 on Pfkfb2, a key activator of glycolysis. Finally, we confirmed the role of glycolysis in modulating insulin action in insulin resistance. Our results underscore the significance of genetics in shaping global signalling responses and their adaptability to environmental changes, emphasising the utility of studying biological diversity with phosphoproteomics to discover key regulatory mechanisms of complex traits.

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.

Leveraging genetic diversity to identify small molecules that reverse mouse skeletal muscle insulin resistance

Systems genetics has begun to tackle the complexity of insulin resistance by capitalising on computational advances to study high-diversity populations. 'Diversity Outbred in Australia (DOz)' is a population of genetically unique mice with profound metabolic heterogeneity. We leveraged this variance to explore skeletal muscle's contribution to whole-body insulin action through metabolic phenotyping and skeletal muscle proteomics of 215 DOz mice. Linear modelling identified 553 proteins that associated with whole-body insulin sensitivity (Matsuda Index) including regulators of endocytosis and muscle proteostasis. To enrich for causality, we refined this network by focusing on negatively associated, genetically regulated proteins, resulting in a 76-protein fingerprint of insulin resistance. We sought to perturb this network and restore insulin action with small molecules by integrating the Broad Institute Connectivity Map platform and in vitro assays of insulin action using the Prestwick chemical library. These complementary approaches identified the antibiotic thiostrepton as an insulin resistance reversal agent. Subsequent validation in ex vivo insulin-resistant mouse muscle and palmitate-induced insulin-resistant myotubes demonstrated potent insulin action restoration, potentially via upregulation of glycolysis. This work demonstrates the value of a drug-centric framework to validate systems-level analysis by identifying potential therapeutics for insulin resistance.

Impaired Insulin Signaling Mediated by the Small GTPase Rac1 in Skeletal Muscle of the Leptin-Deficient Obese Mouse

Insulin-stimulated glucose uptake in skeletal muscle is mediated by the glucose transporter GLUT4. The small GTPase Rac1 acts as a switch of signal transduction that regulates GLUT4 translocation to the plasma membrane following insulin stimulation. However, it remains obscure whether signaling cascades upstream and downstream of Rac1 in skeletal muscle are impaired by obesity that causes insulin resistance and type 2 diabetes. In an attempt to clarify this point, we investigated Rac1 signaling in the leptin-deficient (Lepob/ob) mouse model. Here, we show that insulin-stimulated GLUT4 translocation and Rac1 activation are almost completely abolished in Lepob/ob mouse skeletal muscle. Phosphorylation of the protein kinase Akt2 and plasma membrane translocation of the guanine nucleotide exchange factor FLJ00068 following insulin stimulation were also diminished in Lepob/ob mice. On the other hand, the activation of another small GTPase RalA, which acts downstream of Rac1, by the constitutively activated form of Akt2, FLJ00068, or Rac1, was partially abrogated in Lepob/ob mice. Taken together, we conclude that insulin-stimulated glucose uptake is impaired by two mechanisms in Lepob/ob mouse skeletal muscle: one is the complete inhibition of Akt2-mediated activation of Rac1, and the other is the partial inhibition of RalA activation downstream of Rac1.

The Rho guanine dissociation inhibitor α inhibits skeletal muscle Rac1 activity and insulin action

The molecular events governing skeletal muscle glucose uptake have pharmacological potential for managing insulin resistance in conditions such as obesity, diabetes, and cancer. With no current pharmacological treatments to target skeletal muscle insulin sensitivity, there is an unmet need to identify the molecular mechanisms that control insulin sensitivity in skeletal muscle. Here, the Rho guanine dissociation inhibitor α (RhoGDIα) is identified as a point of control in the regulation of insulin sensitivity. In skeletal muscle cells, RhoGDIα interacted with, and thereby inhibited, the Rho GTPase Rac1. In response to insulin, RhoGDIα was phosphorylated at S101 and Rac1 dissociated from RhoGDIα to facilitate skeletal muscle GLUT4 translocation. Accordingly, siRNA-mediated RhoGDIα depletion increased Rac1 activity and elevated GLUT4 translocation. Consistent with RhoGDIα's inhibitory effect, rAAV-mediated RhoGDIα overexpression in mouse muscle decreased insulin-stimulated glucose uptake and was detrimental to whole-body glucose tolerance. Aligning with RhoGDIα's negative role in insulin sensitivity, RhoGDIα protein content was elevated in skeletal muscle from insulin-resistant patients with type 2 diabetes. These data identify RhoGDIα as a clinically relevant controller of skeletal muscle insulin sensitivity and whole-body glucose homeostasis, mechanistically by modulating Rac1 activity.

microRNA-96 targets the INS/AKT/GLUT4 signaling axis: Association with and effect on diabetic retinopathy

Background

miR-96-5p is a highly expressed microRNA in the retina of subjects with diabetes. The INS/AKT/GLUT4 signaling axis is the main cell signaling pathway of glucose uptake in cells. Here, we investigated the role of miR-96-5p in this signaling pathway.

Methods

Expression levels of miR-96-5p and its target genes were measured under high glucose conditions, in the retina of streptozotocin-induced diabetic mice, in the retina of AAV-2-eGFP-miR-96 or GFP intravitreal injected mice and in the retina of human donors with diabetic retinopathy (DR). MTT, wound healing, tube formation, Western blot, TUNEL, angiogenesis assays and hematoxylin-eosin staining of the retinal sections were performed.

Results

miR-96-5p expression was increased under high glucose conditions in mouse retinal pigment epithelial (mRPE) cells, in the retina of mice receiving AAV-2 carrying miR-96 and STZ-treated mice. Expression of the miR-96-5p target genes related to the INS/AKT/GLUT4 signaling pathway was reduced following miR-96-5p overexpression. mmu-miR-96-5p expression decreased cell proliferation and thicknesses of retinal layers. Cell migration, tube formation, vascular length, angiogenesis, and TUNEL-positive cells were increased.

Conclusions

In in vitro and in vivo studies and in human retinal tissues, miR-96-5p regulated the expression of the PIK3R1, PRKCE, AKT1, AKT2, and AKT3 genes in the INS/AKT axis and some genes involved in GLUT4 trafficking, such as Pak1, Snap23, RAB2a, and Ehd1. Because disruption of the INS/AKT/GLUT4 signaling axis causes advanced glycation end product accumulation and inflammatory responses, the inhibition of miR-96-5p expression could ameliorate DR.

Hormonal regulation of metabolism-recent lessons learned from insulin and estrogen

Hormonal signaling plays key roles in tissue and metabolic homeostasis. Accumulated evidence has revealed a great deal of insulin and estrogen signaling pathways and their interplays in the regulation of mitochondrial, cellular remodeling, and macronutrient metabolism. Insulin signaling regulates nutrient and mitochondrial metabolism by targeting the IRS-PI3K-Akt-FoxOs signaling cascade and PGC1α. Estrogen signaling fine-tunes protein turnover and mitochondrial metabolism through its receptors (ERα, ERβ, and GPER). Insulin and estrogen signaling converge on Sirt1, mTOR, and PI3K in the joint regulation of autophagy and mitochondrial metabolism. Dysregulated insulin and estrogen signaling lead to metabolic diseases. This article reviews the up-to-date evidence that depicts the pathways of insulin signaling and estrogen-ER signaling in the regulation of metabolism. In addition, we discuss the cross-talk between estrogen signaling and insulin signaling via Sirt1, mTOR, and PI3K, as well as new therapeutic options such as agonists of GLP1 receptor, GIP receptor, and β3-AR. Mapping the molecular pathways of insulin signaling, estrogen signaling, and their interplays advances our understanding of metabolism and discovery of new therapeutic options for metabolic disorders.

Molecular Genetics of Abnormal Redox Homeostasis in Type 2 Diabetes Mellitus

Numerous studies have shown that oxidative stress resulting from an imbalance between the production of free radicals and their neutralization by antioxidant enzymes is one of the major pathological disorders underlying the development and progression of type 2 diabetes (T2D). The present review summarizes the current state of the art advances in understanding the role of abnormal redox homeostasis in the molecular mechanisms of T2D and provides comprehensive information on the characteristics and biological functions of antioxidant and oxidative enzymes, as well as discusses genetic studies conducted so far in order to investigate the contribution of polymorphisms in genes encoding redox state-regulating enzymes to the disease pathogenesis.

Enhancement of cGMP-dependent pathway activity ameliorates hyperglycemia-induced decrease in SIRT1-AMPK activity in podocytes: Impact on glucose uptake and podocyte function

Hyperglycemia significantly decreases 3',5'-cyclic guanosine monophosphate (cGMP)-dependent pathway activity in the kidney. A well-characterized downstream signaling effector of cGMP is cGMP-dependent protein kinase G (PKG), exerting a wide range of downstream effects, including vasodilation and vascular smooth muscle cells relaxation. In podocytes that are exposed to high glucose concentrations, crosstalk between the protein deacetylase sirtuin 1 (SIRT1) and adenosine monophosphate-dependent protein kinase (AMPK) decreased, attenuating insulin responsiveness and impairing podocyte function. The present study examined the effect of enhancing cGMP-dependent pathway activity on SIRT1-AMPK crosstalk in podocytes under hyperglycemic conditions. We found that enhancing cGMP-dependent pathway activity using a cGMP analog was associated with increases in SIRT1 protein levels and activity, with a concomitant increase in the degree of AMPK phosphorylation. The beneficial effects of enhancing cGMP-dependent pathway activity on SIRT1-AMPK crosstalk also included improvements in podocyte function. Based on our findings, we postulate an important role for SIRT1-AMPK crosstalk in the regulation of albumin permeability in hyperglycemia that is strongly associated with activity of the cGMP-dependent pathway.

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.

Insulin controls cytoskeleton reorganization and filtration barrier permeability via the PKGIα-Rac1-RhoA crosstalk in cultured rat podocytes

Podocyte foot processes are an important cellular layer of the glomerular barrier that regulates glomerular permeability. Insulin via the protein kinase G type Iα (PKGIα) signaling pathway regulates the balance between contractility and relaxation (permeability) of the podocyte barrier by regulation of the actin cytoskeleton. This mechanism was shown to be disrupted in diabetes. Rho family guanosine-5'-triphosphates (GTPases) are dynamic modulators of the actin cytoskeleton and expressed in cells that form the glomerular filtration barrier. Thus, changes in Rho GTPase activity may affect glomerular permeability to albumin. The present study showed that Rho family GTPases control podocyte migration and permeability. Moreover these processes are regulated by insulin in PKGIα-dependent manner. Modulation of the PKGI-dependent activity of Rac1 and RhoA GTPases with inhibitors or small-interfering RNA impair glomerular permeability to albumin. We also demonstrated this mechanism in obese, insulin-resistant Zucker rats. We propose that PKGIα-Rac1-RhoA crosstalk is necessary in proper organization of the podocyte cytoskeleton and consequently the stabilization of glomerular architecture and regulation of filtration barrier permeability.

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.

Comparative Morphological, Metabolic and Transcriptome Analyses in elmo1−/−, elmo2−/−, and elmo3−/− Zebrafish Mutants Identified a Functional Non-Redundancy of the Elmo Proteins

The ELMO protein family consists of the homologues ELMO1, ELMO2 and ELMO3. Several studies have shown that the individual ELMO proteins are involved in a variety of cellular and developmental processes. However, it has poorly been understood whether the Elmo proteins show similar functions and act redundantly. To address this question, elmo1−/−, elmo2−/− and elmo3−/− zebrafish were generated and a comprehensive comparison of the phenotypic changes in organ morphology, transcriptome and metabolome was performed in these mutants. The results showed decreased fasting and increased postprandial blood glucose levels in adult elmo1−/−, as well as a decreased vascular formation in the adult retina in elmo1−/−, but an increased vascular formation in the adult elmo3−/− retina. The phenotypical comparison provided few similarities, as increased Bowman space areas in adult elmo1−/− and elmo2−/− kidneys, an increased hyaloid vessel diameter in elmo1−/− and elmo3−/− and a transcriptional downregulation of the vascular development in elmo1−/−, elmo2−/−, and elmo3−/− zebrafish larvae. Besides this, elmo1−/−, elmo2−/−, and elmo3−/− zebrafish exhibited several distinct changes in the vascular and glomerular structure and in the metabolome and the transcriptome. Especially, elmo3−/− zebrafish showed extensive differences in the larval transcriptome and an impaired survivability. Together, the data demonstrated that the three zebrafish Elmo proteins regulate not only similar but also divergent biological processes and mechanisms and show a low functional redundancy.

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.

Gene deletion of γ-actin impairs insulin-stimulated skeletal muscle glucose uptake in growing mice but not in mature adult mice

The cortical cytoskeleton, consisting of the cytoplasmic actin isoforms β and/or γ-actin, has been implicated in insulin-stimulated GLUT4 translocation and glucose uptake in muscle and adipose cell culture. Furthermore, transgenic inhibition of multiple actin-regulating proteins in muscle inhibits insulin-stimulated muscle glucose uptake. The current study tested if γ-actin was required for insulin-stimulated glucose uptake in mouse skeletal muscle. Based on our previously reported age-dependent phenotype in muscle-specific β-actin gene deletion (^-/- ) mice, we included cohorts of growing 8-14 weeks old and mature 18-32 weeks old muscle-specific γ-actin^-/- mice or wild-type littermates. In growing mice, insulin significantly increased the glucose uptake in slow-twitch oxidative soleus and fast-twitch glycolytic EDL muscles from wild-type mice, but not γ-actin^-/- . In relative values, the maximal insulin-stimulated glucose uptake was reduced by ~50% in soleus and by ~70% in EDL muscles from growing γ-actin^-/- mice compared to growing wild-type mice. In contrast, the insulin-stimulated glucose uptake responses in mature adult γ-actin^-/- soleus and EDL muscles were indistinguishable from the responses in wild-type muscles. Mature adult insulin-stimulated phosphorylations on Akt, p70S6K, and ULK1 were not significantly affected by genotype. Hence, insulin-stimulated muscle glucose uptake shows an age-dependent impairment in young growing but not in fully grown γ-actin^-/- mice, bearing phenotypic resemblance to β-actin^-/- mice. Overall, γ-actin does not appear required for insulin-stimulated muscle glucose uptake in adulthood. Furthermore, our data emphasize the need to consider the rapid growth of young mice as a potential confounder in transgenic mouse phenotyping studies.

Ubiquitin-like processing of TUG proteins as a mechanism to regulate glucose uptake and energy metabolism in fat and muscle

In response to insulin stimulation, fat and muscle cells mobilize GLUT4 glucose transporters to the cell surface to enhance glucose uptake. Ubiquitin-like processing of TUG (Aspscr1, UBXD9) proteins is a central mechanism to regulate this process. Here, recent advances in this area are reviewed. The data support a model in which intact TUG traps insulin-responsive "GLUT4 storage vesicles" at the Golgi matrix by binding vesicle cargoes with its N-terminus and matrix proteins with its C-terminus. Insulin stimulation liberates these vesicles by triggering endoproteolytic cleavage of TUG, mediated by the Usp25m protease. Cleavage occurs in fat and muscle cells, but not in fibroblasts or other cell types. Proteolytic processing of intact TUG generates TUGUL, a ubiquitin-like protein modifier, as the N-terminal cleavage product. In adipocytes, TUGUL modifies a single protein, the KIF5B kinesin motor, which carries GLUT4 and other vesicle cargoes to the cell surface. In muscle, this or another motor may be modified. After cleavage of intact TUG, the TUG C-terminal product is extracted from the Golgi matrix by the p97 (VCP) ATPase. In both muscle and fat, this cleavage product enters the nucleus, binds PPARγ and PGC-1α, and regulates gene expression to promote fatty acid oxidation and thermogenesis. The stability of the TUG C-terminal product is regulated by an Ate1 arginyltransferase-dependent N-degron pathway, which may create a feedback mechanism to control oxidative metabolism. Although it is now clear that TUG processing coordinates glucose uptake with other aspects of physiology and metabolism, many questions remain about how this pathway is regulated and how it is altered in metabolic disease in humans.

Changes in Skeletal Muscle PAK1 Levels Regulate Tissue Crosstalk to Impact Whole Body Glucose Homeostasis

Skeletal muscle accounts for ~80% of insulin-stimulated glucose uptake. The Group I p21-activated kinase 1 (PAK1) is required for the non-canonical insulin-stimulated GLUT4 vesicle translocation in skeletal muscle cells. We found that the abundances of PAK1 protein and its downstream effector in muscle, ARPC1B, are significantly reduced in the skeletal muscle of humans with type 2 diabetes, compared to the non-diabetic controls, making skeletal muscle PAK1 a candidate regulator of glucose homeostasis. Although whole-body PAK1 knockout mice exhibit glucose intolerance and are insulin resistant, the contribution of skeletal muscle PAK1 in particular was unknown. As such, we developed inducible skeletal muscle-specific PAK1 knockout (skmPAK1-iKO) and overexpression (skmPAK1-iOE) mouse models to evaluate the role of PAK1 in skeletal muscle insulin sensitivity and glucose homeostasis. Using intraperitoneal glucose tolerance and insulin tolerance testing, we found that skeletal muscle PAK1 is required for maintaining whole body glucose homeostasis. Moreover, PAK1 enrichment in GLUT4-myc-L6 myoblasts preserves normal insulin-stimulated GLUT4 translocation under insulin resistance conditions. Unexpectedly, skmPAK1-iKO also showed aberrant plasma insulin levels following a glucose challenge. By applying conditioned media from PAK1-enriched myotubes or myoblasts to β-cells in culture, we established that a muscle-derived circulating factor(s) could enhance β-cell function. Taken together, these data suggest that PAK1 levels in the skeletal muscle can regulate not only skeletal muscle insulin sensitivity, but can also engage in tissue crosstalk with pancreatic β-cells, unveiling a new molecular mechanism by which PAK1 regulates whole-body glucose homeostasis.

Rho GTPases in Skeletal Muscle Development and Homeostasis

Rho guanosine triphosphate hydrolases (GTPases) are molecular switches that cycle between an inactive guanosine diphosphate (GDP)-bound and an active guanosine triphosphate (GTP)-bound state during signal transduction. As such, they regulate a wide range of both cellular and physiological processes. In this review, we will summarize recent work on the role of Rho GTPase-regulated pathways in skeletal muscle development, regeneration, tissue mass homeostatic balance, and metabolism. In addition, we will present current evidence that links the dysregulation of these GTPases with diseases caused by skeletal muscle dysfunction. Overall, this information underscores the critical role of a number of members of the Rho GTPase subfamily in muscle development and the overall metabolic balance of mammalian species.

Schisandrin C Affects Glucose-Stimulated Insulin Secretion in Pancreatic β-Cells and Glucose Uptake in Skeletal Muscle Cells

The aim of our study was to investigate the effect of three lignans (schisandrol A, schisandrol B, and schisandrin C) on insulin secretion in rat INS-1 pancreatic β-cells and glucose uptake in mouse C2C12 skeletal muscle cells. Schisandrol A and schisandrin C enhanced insulin secretion in response to high glucose levels with no toxic effects on INS-1 cells. The effect of schisandrin C was superior to that of gliclazide (positive control), a drug commonly used to treat type 2 diabetes (T2D). In addition, western blot analysis showed that the expression of associated proteins, including peroxisome proliferator-activated receptor γ (PPARγ), pancreatic and duodenal homeobox 1 (PDX-1), phosphatidylinositol 3-kinase (PI3K), Akt, and insulin receptor substrate-2 (IRS-2), was increased in INS-1 cells after treatment with schisandrin C. In addition, insulin secretion effect of schisandrin C were enhanced by the Bay K 8644 (L-type Ca²⁺ channel agonist) and glibenclamide (K⁺ channel blocker), were abolished by the nifedipine (L-type Ca²⁺ channel blocker) and diazoxide (K⁺ channel activator). Moreover, schisandrin C enhanced glucose uptake with no toxic effects on C2C12 cells. Western blot analysis showed that the expression of associated proteins, including insulin receptor substrate-1 (IRS-1), AMP-activated protein kinase (AMPK), PI3K, Akt, glucose transporter type 4 (GLUT-4), was increased in C2C12 cells after treatment with schisandrin C. Schisandrin C may improve hyperglycemia by enhancing insulin secretion in pancreatic β-cells and improving glucose uptake into skeletal muscle cells. Our findings may provide evidence that schisandrin C may be beneficial in devising novel anti-T2D strategies.

Atrophy of White Adipose Tissue Accompanied with Decreased Insulin-Stimulated Glucose Uptake in Mice Lacking the Small GTPase Rac1 Specifically in Adipocytes

Insulin stimulates glucose uptake in adipose tissue and skeletal muscle by inducing plasma membrane translocation of the glucose transporter GLUT4. Although the small GTPase Rac1 is a key regulator downstream of phosphoinositide 3-kinase (PI3K) and the protein kinase Akt2 in skeletal muscle, it remains unclear whether Rac1 also regulates glucose uptake in white adipocytes. Herein, we investigated the physiological role of Rac1 in white adipocytes by employing adipocyte-specific rac1 knockout (adipo-rac1-KO) mice. Subcutaneous and epididymal white adipose tissues (WATs) in adipo-rac1-KO mice showed significant reductions in size and weight. Actually, white adipocytes lacking Rac1 were smaller than controls. Insulin-stimulated glucose uptake and GLUT4 translocation were abrogated in rac1-KO white adipocytes. On the other hand, GLUT4 translocation was augmented by constitutively activated PI3K or Akt2 in control, but not in rac1-KO, white adipocytes. Similarly, to skeletal muscle, the involvement of another small GTPase RalA downstream of Rac1 was demonstrated. In addition, mRNA levels of various lipogenic enzymes were down-regulated in rac1-KO white adipocytes. Collectively, these results suggest that Rac1 is implicated in insulin-dependent glucose uptake and lipogenesis in white adipocytes, and reduced insulin responsiveness due to the deficiency of Rac1 may be a likely explanation for atrophy of WATs.

The PKGIα-Rac1 pathway is a novel regulator of insulin-dependent glucose uptake in cultured rat podocytes

Insulin plays a major role in regulating glucose homeostasis in podocytes. Protein kinase G type Iα (PKGIα) plays an important role in regulating glucose uptake in these cells. Rac1 signaling plays an essential role in the reorganization of the actin cytoskeleton and is also essential for insulin-stimulated glucose transport. The experiments were conducted using primary rat podocytes. We performed western blot analysis, evaluated small GTPases activity assays, measured radioactive glucose uptake, and performed immunofluorescence imaging to analyze the role of PKGIα-Rac1 signaling in regulating podocyte function. We also utilized a small-interfering RNA-mediated approach to determine the role of PKGIα and Rac1 in regulating glucose uptake in podocytes. The present study investigated the influence of the PKGI pathway on the insulin-dependent regulation of activity and cellular localization of small guanosine triphosphatases in podocytes. We found that the PKGIα-dependent activation of Rac1 signaling induced activation of the PAK/cofilin pathway and increased insulin-mediated glucose uptake in podocytes. The downregulation of PKGIα or Rac1 expression abolished this effect. Rac1 silencing prevented actin remodeling and GLUT4 translocation close to the cell membrane. These data provide evidence that PKGIα-dependent activation of the Rac1 signaling pathways is a novel regulator of insulin-mediated glucose uptake in cultured rat podocytes.

Skeletal Muscle Gene Expression Profile in Response to Caloric Restriction and Aging: A Role for SirT1

SirT1 plays a crucial role in the regulation of some of the caloric restriction (CR) responsive biological pathways. Aging suppresses SirT1 gene expression in skeletal muscle, suggesting that aging may affect the role of CR in muscle. To determine the role of SirT1 in the regulation of CR regulated pathways in skeletal muscle, we performed high-throughput RNA sequencing using total RNA isolated from the skeletal muscles of young and aged wild-type (WT), SirT1 knockout (SirT1-KO), and SirT1 overexpression (SirT1-OE) mice fed to 20 wk ad libitum (AL) or 40% CR diet. Our data show that aging repressed the global gene expression profile, which was restored by CR via upregulating transcriptional and translational process-related pathways. CR inhibits pathways linked to the extracellular matrix and cytoskeletal proteins regardless of aging. Mitochondrial function and muscle contraction-related pathways are upregulated in aged SirT1 KO mice following CR. SirT1 OE did not affect whole-body energy expenditure or augment skeletal muscle insulin sensitivity associated pathways, regardless of aging or diet. Overall, our RNA-seq data showed that SirT1 and CR have different functions and activation of SirT1 by its activator or exercise may enhance SirT1 activity that, along with CR, likely have a better functional role in aging muscle.

Sirtuin 1 is not required for contraction-stimulated glucose uptake in mouse skeletal muscle

While it has long been known that contraction robustly stimulates skeletal muscle glucose uptake, the molecular steps regulating this increase remain incompletely defined. The mammalian ortholog of Sir2, sirtuin 1 (SIRT1), is an NAD⁺-dependent protein deacetylase that is thought to link perturbations in energy flux associated with exercise to subsequent cellular adaptations. Nevertheless, its role in contraction-stimulated glucose uptake has not been described. The objective of this study was to determine the importance of SIRT1 to contraction-stimulated glucose uptake in mouse skeletal muscle. Using a radioactive 2-deoxyglucose uptake (2DOGU) approach, we measured ex vivo glucose uptake in unstimulated (rested) and electrically stimulated (100 Hz contraction every 15 s for 10 min; contracted) extensor digitorum longus (EDL) and soleus from ∼15-wk-old male and female mice with muscle-specific knockout of SIRT1 deacetylase activity and their wild-type littermates. Skeletal muscle force decreased over the contraction protocol, although there were no differences in the rate of fatigue between genotypes. In EDL and soleus, loss of SIRT1 deacetylase activity did not affect contraction-induced increase in glucose uptake in either sex. Interestingly, the absolute rate of contraction-stimulated 2DOGU was ∼1.4-fold higher in female compared with male mice, regardless of muscle type. Taken together, our findings demonstrate that SIRT1 is not required for contraction-stimulated glucose uptake in mouse skeletal muscle. Moreover, to our knowledge, this is the first demonstration of sex-based differences in contraction-stimulated glucose uptake in mouse skeletal muscle.NEW & NOTEWORTHY Here, we demonstrate that glucose uptake in response to ex vivo contractions is not affected by the loss of sirtuin 1 (SIRT1) deacetylase function in muscle, regardless of sex or muscle type. Interestingly, however, similar to studies on insulin-stimulated glucose uptake, we demonstrate that contraction-stimulated glucose uptake is robustly higher in female compared with the male skeletal muscle. To our knowledge, this is the first demonstration of sex-based differences in contraction-stimulated glucose uptake in skeletal muscle.

Rho Family GTPases and Rho GEFs in Glucose Homeostasis

Dysregulation of glucose homeostasis leading to metabolic syndrome and type 2 diabetes is the cause of an increasing world health crisis. New intriguing roles have emerged for Rho family GTPases and their Rho guanine nucleotide exchange factor (GEF) activators in the regulation of glucose homeostasis. This review summates the current knowledge, focusing in particular on the roles of Rho GEFs in the processes of glucose-stimulated insulin secretion by pancreatic β cells and insulin-stimulated glucose uptake into skeletal muscle and adipose tissues. We discuss the ten Rho GEFs that are known so far to regulate glucose homeostasis, nine of which are in mammals, and one is in yeast. Among the mammalian Rho GEFs, P-Rex1, Vav2, Vav3, Tiam1, Kalirin and Plekhg4 were shown to mediate the insulin-stimulated translocation of the glucose transporter GLUT4 to the plasma membrane and/or insulin-stimulated glucose uptake in skeletal muscle or adipose tissue. The Rho GEFs P-Rex1, Vav2, Tiam1 and β-PIX were found to control the glucose-stimulated release of insulin by pancreatic β cells. In vivo studies demonstrated the involvement of the Rho GEFs P-Rex2, Vav2, Vav3 and PDZ-RhoGEF in glucose tolerance and/or insulin sensitivity, with deletion of these GEFs either contributing to the development of metabolic syndrome or protecting from it. This research is in its infancy. Considering that over 80 Rho GEFs exist, it is likely that future research will identify more roles for Rho GEFs in glucose homeostasis.

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.

Exercise-A Panacea of Metabolic Dysregulation in Cancer: Physiological and Molecular Insights

Metabolic dysfunction is a comorbidity of many types of cancers. Disruption of glucose metabolism is of concern, as it is associated with higher cancer recurrence rates and reduced survival. Current evidence suggests many health benefits from exercise during and after cancer treatment, yet only a limited number of studies have addressed the effect of exercise on cancer-associated disruption of metabolism. In this review, we draw on studies in cells, rodents, and humans to describe the metabolic dysfunctions observed in cancer and the tissues involved. We discuss how the known effects of acute exercise and exercise training observed in healthy subjects could have a positive outcome on mechanisms in people with cancer, namely: insulin resistance, hyperlipidemia, mitochondrial dysfunction, inflammation, and cachexia. Finally, we compile the current limited knowledge of how exercise corrects metabolic control in cancer and identify unanswered questions for future research.

Insulin-Stimulated Muscle Glucose Uptake and Insulin Signaling in Lean and Obese Humans

PURPOSE

Skeletal muscle is the primary site for insulin-stimulated glucose disposal, and muscle insulin resistance is central to abnormal glucose metabolism in obesity. Whether muscle insulin signaling to the level of Akt/AS160 is intact in insulin-resistant obese humans is controversial.

METHODS

We defined a linear range of insulin-stimulated systemic and leg glucose uptake in 14 obese and 14 nonobese volunteers using a 2-step insulin clamp (Protocol 1) and then examined the obesity-related defects in muscle insulin action in 16 nonobese and 25 obese male and female volunteers matched for fitness using a 1-step, hyperinsulinemic, euglycemic clamp coupled with muscle biopsies (Protocol 2).

RESULTS

Insulin-stimulated glucose disposal (Si) was reduced by > 60% (P < 0.0001) in the obese group in Protocol 2; however, the phosphorylation of Akt and its downstream effector AS160 were not different between nonobese and obese groups. The increase in phosphorylation of Akt2 in response to insulin was positively correlated with Si for both the nonobese (r = 0.53, P = 0.03) and the obese (r = 0.55, P = 0.01) groups. Total muscle GLUT4 protein was 17% less (P < 0.05) in obese subjects.

CONCLUSIONS

We suggest that reduced muscle glucose uptake in obesity is not due to defects in the insulin signaling pathway at the level of Akt/AS160, which suggests there remain significant gaps in our knowledge of muscle insulin resistance in obesity. Our data imply that models of acute lipotoxicity do not replicate the pathophysiology of obesity.

Effects of Different Exercise Training Protocols on Gene Expression of Rac1 and PAK1 in Healthy Rat Fast- and Slow-Type Muscles

Purpose

Rac1 and its downstream target PAK1 are novel regulators of insulin and exercise-induced glucose uptake in skeletal muscle. However, it is not yet understood how different training intensities affect the expression of these proteins. Therefore, we studied the effects of high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) on Rac1 and PAK1 expression in fast-type (gastrocnemius, GC) and slow-type (soleus, SOL) muscles in rats after HIIT and MICT swimming exercises.

Methods

The mRNA expression was determined using qPCR and protein expression levels with reverse-phase protein microarray (RPPA).

Results

HIIT significantly decreased Rac1 mRNA expression in GC compared to MICT (p = 0.003) and to the control group (CON) (p = 0.001). At the protein level Rac1 was increased in GC in both training groups, but only the difference between HIIT and CON was significant (p = 0.02). HIIT caused significant decrease of PAK1 mRNA expression in GC compared to MICT (p = 0.007) and to CON (p = 0.001). At the protein level, HIIT increased PAK1 expression in GC compared to MICT and CON (by ∼17%), but the difference was not statistically significant (p = 0.3, p = 0.2, respectively). There were no significant differences in the Rac1 or PAK1 expression in SOL between the groups.

Conclusion

Our results indicate that HIIT, but not MICT, decreases Rac1 and PAK1 mRNA expression and increases the protein expression of especially Rac1 but only in fast-type muscle. These exercise training findings may reveal new therapeutic targets to treat patients with metabolic diseases.

Mechanisms and consequences of dysregulation of the Tiam family of Rac activators in disease

The Tiam family proteins - Tiam1 and Tiam2/STEF - are Rac1-specific Guanine Nucleotide Exchange Factors (GEFs) with important functions in epithelial, neuronal, immune and other cell types. Tiam GEFs regulate cellular migration, proliferation and survival, mainly through activating and directing Rac1 signalling. Dysregulation of the Tiam GEFs is significantly associated with human diseases including cancer, immunological and neurological disorders. Uncovering the mechanisms and consequences of dysregulation is therefore imperative to improving the diagnosis and treatment of diseases. Here we compare and contrast the subcellular localisation and function of Tiam1 and Tiam2/STEF, and review the evidence for their dysregulation in disease.

Rho GTPases in cancer radiotherapy and metastasis

Despite treatment advances, radioresistance and metastasis markedly impair the benefits of radiotherapy to patients with malignancies. Functioning as molecular switches, Rho guanosine triphosphatases (GTPases) have well-recognized roles in regulating various downstream signaling pathways in a wide range of cancers. In recent years, accumulating evidence indicates the involvement of Rho GTPases in cancer radiotherapeutic efficacy and metastasis, as well as radiation-induced metastasis. The functions of Rho GTPases in radiotherapeutic efficacy are divergent and context-dependent; thereby, a comprehensive integration of their roles and correlated mechanisms is urgently needed. This review integrates current evidence supporting the roles of Rho GTPases in mediating radiotherapeutic efficacy and the underlying mechanisms. In addition, their correlations with metastasis and radiation-induced metastasis are discussed. Under the prudent application of Rho GTPase inhibitors based on critical evaluations of biological contexts, targeting Rho GTPases can be a promising strategy in overcoming radioresistance and simultaneously reducing the metastatic potential of tumor cells.

Vav2 catalysis-dependent pathways contribute to skeletal muscle growth and metabolic homeostasis

Skeletal muscle promotes metabolic balance by regulating glucose uptake and the stimulation of multiple interorgan crosstalk. We show here that the catalytic activity of Vav2, a Rho GTPase activator, modulates the signaling output of the IGF1- and insulin-stimulated phosphatidylinositol 3-kinase pathway in that tissue. Consistent with this, mice bearing a Vav2 protein with decreased catalytic activity exhibit reduced muscle mass, lack of proper insulin responsiveness and, at much later times, a metabolic syndrome-like condition. Conversely, mice expressing a catalytically hyperactive Vav2 develop muscle hypertrophy and increased insulin responsiveness. Of note, while hypoactive Vav2 predisposes to, hyperactive Vav2 protects against high fat diet-induced metabolic imbalance. These data unveil a regulatory layer affecting the signaling output of insulin family factors in muscle.

Skeletal muscle plays a key role in regulating systemic glucose and metabolic homeostasis. Here, the authors show that the catalytic activity of Vav2, an activator of Rho GTPases, modulates those processes by favoring the responsiveness of this tissue to insulin and related factors.

A Cell-Autonomous Signature of Dysregulated Protein Phosphorylation Underlies Muscle Insulin Resistance in Type 2 Diabetes

Skeletal muscle insulin resistance is the earliest defect in type 2 diabetes (T2D), preceding and predicting disease development. To what extent this reflects a primary defect or is secondary to tissue cross talk due to changes in hormones or circulating metabolites is unknown. To address this question, we have developed an in vitro disease-in-a-dish model using iPS cells from T2D patients differentiated into myoblasts (iMyos). We find that T2D iMyos in culture exhibit multiple defects mirroring human disease, including an altered insulin signaling, decreased insulin-stimulated glucose uptake, and reduced mitochondrial oxidation. More strikingly, global phosphoproteomic analysis reveals a multidimensional network of signaling defects in T2D iMyos going beyond the canonical insulin-signaling cascade, including proteins involved in regulation of Rho GTPases, mRNA splicing and/or processing, vesicular trafficking, gene transcription, and chromatin remodeling. These cell-autonomous defects and the dysregulated network of protein phosphorylation reveal a new dimension in the cellular mechanisms underlying the fundamental defects in T2D.

Integrin-associated ILK and PINCH1 protein content are reduced in skeletal muscle of maintenance haemodialysis patients

KEY POINTS

Patients with renal failure undergoing maintenance haemodialysis are associated with insulin resistance and protein metabolism dysfunction. Novel research suggests that disruption to the transmembrane protein linkage between the cytoskeleton and the extracellular matrix in skeletal muscle may contribute to reduced amino acid metabolism and insulin resistance in haemodialysis. ILK, PINCH1 and pFAK^Tyr397 were significantly decreased in haemodialysis compared to controls, whereas Rac1 and Akt2 showed no different between groups. Rac1 deletion in the Rac1 knockout model did not alter the expression of integrin-associated proteins. Phenylalanine kinetics were reduced in the haemodialysis group at 30 and 60 min post meal ingestion compared to controls; both groups showed similar levels of insulin sensitivity and β-cell function. Key proteins in the integrin-cytoskeleton linkage are reduced in haemodialysis patients, suggesting for the first time that integrin-associated proteins dysfunction may contribute to reduced phenylalanine flux without affecting insulin resistance in haemodialysis patients.

ABSTRACT

Muscle atrophy, insulin resistance and reduced muscle phosphoinositide 3-kinase-Akt signalling are common characteristics of patients undergoing maintenance haemodialysis (MHD). Disruption to the transmembrane protein linkage between the cytoskeleton and the extracellular matrix in skeletal muscle may contribute to reduced amino acid metabolism and insulin resistance in MHD patients. Eight MHD patients (age: 56 ± 5 years: body mass index: 32 ± 2 kg m^-2 ) and non-diseased controls (age: 50 ± 2 years: body mass index: 31 ± 1 kg m^-2 ) received primed continuous l-[ring-² H₅ ]phenylalanine before consuming a mixed meal. Phenylalanine metabolism was determined using two-compartment modelling. Muscle biopsies were collected prior to the meal and at 300 min postprandially. In a separate experiment, skeletal muscle tissue from muscle-specific Rac1 knockout (Rac1 mKO) was harvested to investigate whether Rac1 depletion disrupted the cytoskeleton-integrin linkage, allowing for cross-model examination of proteins of interest. ILK, PINCH1 and pFAK^Tyr397 were significantly lower in MHD (P < 0.01). Rac1 and Akt showed no difference between groups for the human trial. Rac1 deletion in the Rac1 mKO model did not alter the expression of integrin-associated proteins. Phenylalanine rates of appearance and disappearance, as well as metabolic clearance rates, were lower in the MHD group at 30 and 60 min post meal ingestion compared to controls (P < 0.05). Both groups showed similar levels of insulin sensitivity and β-cell function. Key proteins in the integrin-cytoskeleton linkage are reduced in MHD patients, suggesting for the first time that integrin-associated proteins dysfunction may contribute to reduced phenylalanine flux without affecting insulin resistance in haemodialysis patients.

Cytotoxicity and insulin resistance reversal ability of biofunctional phytosynthesized MgO nanoparticles

The present study investigates the cytotoxicity of hexagonal MgO nanoparticles synthesized via Amaranthus tricolor leaf extract and spherical MgO nanoparticles synthesized via Amaranthus blitum and Andrographis paniculata leaf extracts. In vitro cytotoxicity analysis showed that the hexagonal MgO nanoparticles synthesized from A. tricolor extract demonstrated the least toxicity to both diabetic and non-diabetic cells at 600 μl/ml dosage. The viability of the diabetic cells (3T3-L1) after incubation with varying dosages of MgO nanoparticles was observed to be 55.3%. The viability of normal VERO cells was 86.6% and this stabilized to about 75% even after exposure to MgO nanoparticles dosage of up to 1000 μl/ml. Colorimetric glucose assay revealed that the A. tricolor extract synthesized MgO nanoparticles resulted in ~ 28% insulin resistance reversal. A reduction in the expression of GLUT4 protein at 54 KDa after MgO nanopaSrticles incubation with diabetic cells was observed via western blot analysis to confirm insulin reversal ability. Fluorescence microscopic analysis with propidium iodide and acridine orange dyes showed the release of reactive oxygen species as a possible mechanism of the cytotoxic effect of MgO nanoparticles. It was inferred that the synergistic effect of the phytochemicals and MgO nanoparticles played a significant role in delivering enhanced insulin resistance reversal capability in adipose cells.

Gene Expression Profiling of Type 2 Diabetes Mellitus by Bioinformatics Analysis

Objective

The aim of this study was to identify the candidate genes in type 2 diabetes mellitus (T2DM) and explore their potential mechanisms.

Methods

The gene expression profile GSE26168 was downloaded from the Gene Expression Omnibus (GEO) database. The online tool GEO2R was used to obtain differentially expressed genes (DEGs). Gene Ontology (GO) term enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed by using Metascape for annotation, visualization, and comprehensive discovery. The protein-protein interaction (PPI) network of DEGs was constructed by using Cytoscape software to find the candidate genes and key pathways.

Results

A total of 981 DEGs were found in T2DM, including 301 upregulated genes and 680 downregulated genes. GO analyses from Metascape revealed that DEGs were significantly enriched in cell differentiation, cell adhesion, intracellular signal transduction, and regulation of protein kinase activity. KEGG pathway analysis revealed that DEGs were mainly enriched in the cAMP signaling pathway, Rap1 signaling pathway, regulation of lipolysis in adipocytes, PI3K-Akt signaling pathway, MAPK signaling pathway, and so on. On the basis of the PPI network of the DEGs, the following 6 candidate genes were identified: PIK3R1, RAC1, GNG3, GNAI1, CDC42, and ITGB1.

Conclusion

Our data provide a comprehensive bioinformatics analysis of genes, functions, and pathways, which may be related to the pathogenesis of T2DM.

The guanine nucleotide exchange factor FLJ00068 activates Rac1 in adipocyte insulin signaling

Insulin stimulates glucose uptake via the translocation of the glucose transporter GLUT4 to the plasma membrane in adipocytes. Several lines of evidence suggest that the small GTPase Rac1 plays an important role in insulin-stimulated glucose uptake in skeletal muscle and adipocytes. The purpose of this study is to investigate the mechanisms whereby Rac1 is regulated in adipocyte insulin signaling. Here, we show that knockdown of the guanine nucleotide exchange factor FLJ00068 inhibits Rac1 activation and GLUT4 translocation by insulin and a constitutively activated form of the protein kinase Akt2. Furthermore, constitutively activated FLJ00068 induced Rac1 activation and Rac1-dependent GLUT4 translocation. Collectively, these results suggest the involvement of FLJ00068 downstream of Akt2 in insulin-stimulated glucose uptake signaling in adipocytes.

Development and comparison of RNA-sequencing pipelines for more accurate SNP identification: practical example of functional SNP detection associated with feed efficiency in Nellore beef cattle

Background

Optimization of an RNA-Sequencing (RNA-Seq) pipeline is critical to maximize power and accuracy to identify genetic variants, including SNPs, which may serve as genetic markers to select for feed efficiency, leading to economic benefits for beef production. This study used RNA-Seq data (GEO Accession ID: PRJEB7696 and PRJEB15314) from muscle and liver tissue, respectively, from 12 Nellore beef steers selected from 585 steers with residual feed intake measures (RFI; n = 6 low-RFI, n = 6 high-RFI). Three RNA-Seq pipelines were compared including multi-sample calling from i) non-merged samples; ii) merged samples by RFI group, iii) merged samples by RFI and tissue group. The RNA-Seq reads were aligned against the UMD3.1 bovine reference genome (release 94) assembly using STAR aligner. Variants were called using BCFtools and variant effect prediction (VeP) and functional annotation (ToppGene) analyses were performed.

Results

On average, total reads detected for Approach i) non-merged samples for liver and muscle, were 18,362,086.3 and 35,645,898.7, respectively. For Approach ii), merging samples by RFI group, total reads detected for each merged group was 162,030,705, and for Approach iii), merging samples by RFI group and tissues, was 324,061,410, revealing the highest read depth for Approach iii). Additionally, Approach iii) merging samples by RFI group and tissues, revealed the highest read depth per variant coverage (572.59 ± 3993.11) and encompassed the majority of localized positional genes detected by each approach. This suggests Approach iii) had optimized detection power, read depth, and accuracy of SNP calling, therefore increasing confidence of variant detection and reducing false positive detection. Approach iii) was then used to detect unique SNPs fixed within low- (12,145) and high-RFI (14,663) groups. Functional annotation of SNPs revealed positional candidate genes, for each RFI group (2886 for low-RFI, 3075 for high-RFI), which were significantly (P < 0.05) associated with immune and metabolic pathways.

Conclusion

The most optimized RNA-Seq pipeline allowed for more accurate identification of SNPs, associated positional candidate genes, and significantly associated metabolic pathways in muscle and liver tissues, providing insight on the underlying genetic architecture of feed efficiency in beef cattle.

Mechanisms of muscle insulin resistance and the cross-talk with liver and adipose tissue

Insulin resistance is a metabolic disorder affecting multiple tissues and is a precursor event to type 2 diabetes (T2D). As T2D affects over 425 million people globally, there is an imperative need for research into insulin resistance to better understand the underlying mechanisms. The proposed mechanisms involved in insulin resistance include both whole body aspects, such as inflammation and metabolic inflexibility; as well as cellular phenomena, such as lipotoxicity, ER stress, and mitochondrial dysfunction. Despite numerous studies emphasizing the role of lipotoxicity in the pathogenesis of insulin resistance, an understanding of the interplay between tissues and these proposed mechanisms is still emerging. Furthermore, the tissue-specific and unique responses each of the three major insulin target tissues and how each interconnect to regulate the whole body insulin response has become a new priority in metabolic research. With an emphasis on skeletal muscle, this mini-review highlights key similarities and differences in insulin signaling and resistance between different target-tissues, and presents the latest findings related to how these tissues communicate to control whole body metabolism.

Insulin-stimulated glucose uptake partly relies on p21-activated kinase (PAK)2, but not PAK1, in mouse skeletal muscle

KEY POINTS

Muscle-specific genetic ablation of p21-activated kinase (PAK)2, but not whole-body PAK1 knockout, impairs glucose tolerance in mice. Insulin-stimulated glucose uptake partly relies on PAK2 in glycolytic extensor digitorum longus muscle By contrast to previous reports, PAK1 is dispensable for insulin-stimulated glucose uptake in mouse muscle.

ABSTRACT

The group I p21-activated kinase (PAK) isoforms PAK1 and PAK2 are activated in response to insulin in skeletal muscle and PAK1/2 signalling is impaired in insulin-resistant mouse and human skeletal muscle. Interestingly, PAK1 has been suggested to be required for insulin-stimulated glucose transporter 4 translocation in mouse skeletal muscle. Therefore, the present study aimed to examine the role of PAK1 in insulin-stimulated muscle glucose uptake. The pharmacological inhibitor of group I PAKs, IPA-3 partially reduced (-20%) insulin-stimulated glucose uptake in isolated mouse soleus muscle (P < 0.001). However, because there was no phenotype with genetic ablation of PAK1 alone, consequently, the relative requirement for PAK1 and PAK2 in whole-body glucose homeostasis and insulin-stimulated muscle glucose uptake was investigated. Whole-body respiratory exchange ratio was largely unaffected in whole-body PAK1 knockout (KO), muscle-specific PAK2 KO and in mice with combined whole-body PAK1 KO and muscle-specific PAK2 KO. By contrast, glucose tolerance was mildly impaired in mice lacking PAK2 specifically in muscle, but not PAK1 KO mice. Moreover, while PAK1 KO muscles displayed normal insulin-stimulated glucose uptake in vivo and in isolated muscle, insulin-stimulated glucose uptake was slightly reduced in isolated glycolytic extensor digitorum longus muscle lacking PAK2 alone (-18%) or in combination with PAK1 KO (-12%) (P < 0.05). In conclusion, glucose tolerance and insulin-stimulated glucose uptake partly rely on PAK2 in glycolytic mouse muscle, whereas PAK1 is dispensable for whole-body glucose homeostasis and insulin-stimulated muscle glucose uptake.