Muscle-specific knockout of PKC-lambda impairs glucose transport and induces metabolic and diabetic syndromes

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.

AMPK and Beyond: The Signaling Network Controlling RabGAPs and Contraction-Mediated Glucose Uptake in Skeletal Muscle

Impaired skeletal muscle glucose uptake is a key feature in the development of insulin resistance and type 2 diabetes. Skeletal muscle glucose uptake can be enhanced by a variety of different stimuli, including insulin and contraction as the most prominent. In contrast to the clearance of glucose from the bloodstream in response to insulin stimulation, exercise-induced glucose uptake into skeletal muscle is unaffected during the progression of insulin resistance, placing physical activity at the center of prevention and treatment of metabolic diseases. The two Rab GTPase-activating proteins (RabGAPs), TBC1D1 and TBC1D4, represent critical nodes at the convergence of insulin- and exercise-stimulated signaling pathways, as phosphorylation of the two closely related signaling factors leads to enhanced translocation of glucose transporter 4 (GLUT4) to the plasma membrane, resulting in increased cellular glucose uptake. However, the full network of intracellular signaling pathways that control exercise-induced glucose uptake and that overlap with the insulin-stimulated pathway upstream of the RabGAPs is not fully understood. In this review, we discuss the current state of knowledge on exercise- and insulin-regulated kinases as well as hypoxia as stimulus that may be involved in the regulation of skeletal muscle glucose uptake.

Chicken GLUT4 undergoes complex alternative splicing events and its expression in striated muscle changes dramatically during development

Glucose transporter protein 4 (GLUT4) plays an important role in regulating insulin-mediated glucose homeostasis in mammals. Until now, studies on GLUT4 have focused on mammals mostly, while chicken GLUT4 has been rarely investigated. In this study, chicken GLUT4 mRNA sequences were obtained by combining conventional amplification, 5'- and 3'- rapid amplification of cDNA ends technique (RACE), then bioinformatics analysis on its genomic structure, splicing pattern, subcellular localization prediction and homologous comparisons were carried out. In addition, the distribution of GLUT4 was detected by RT-qPCR in bird's liver and striated muscles (cardiac muscle, pectoralis and leg muscle) at different ages, including embryonic day 14 (E14), E19, 7-day-old (D7), D21 and D49 (n = 3-4). Results showed that chicken GLUT4 gene produced at least 14 transcripts (GenBank accession No: OP491293-OP491306) through alternative splicing and polyadenylation, which predicted encoding 12 types of amino acid (AA) sequences (with length ranged from 65 AA to 519 AA). These proteins contain typical major facilitator superfamily domain of glucose transporters with length variations, sharing a common sequence of 59 AA, and were predicted to have distinct subcellular localization. The dominant transcript (named as T1) consists of 11 exons with an open reading frame being predicted encoding 519 AA. In addition, analyzing on the spatio-temporal expression of chicken GLUT4 showed it dominantly expressed in pectoralis, leg muscles and cardiac muscle, and the mRNA level of chicken GLUT4 dramatically fluctuated with birds' development in cardiac muscle, pectoralis and leg muscles, with the level at D21 significantly higher than that at E14, E19, and D49 (P < 0.05). These data indicated that chicken GLUT4 undergoes complex alternative splicing events, and GLUT4 expression level in striated muscle was subjected to dynamic regulation with birds' development. Results indicate these isoforms may play overlapping and distinct roles in chicken.

Prkci Regulates Autophagy and Pancreatic Tumorigenesis in Mice

Protein kinase C iota (PKCι) functions as a bonafide human oncogene in lung and ovarian cancer and is required for KrasG12D-mediated lung cancer initiation and progression. PKCι expression is required for pancreatic cancer cell growth and maintenance of the transformed phenotype; however, nothing is known about the role of PKCι in pancreas development or pancreatic tumorigenesis. In this study, we investigated the effect of pancreas-specific ablation of PKCι expression on pancreatic cellular homeostasis, susceptibility to pancreatitis, and KrasG12D-mediated pancreatic cancer development. Knockout of pancreatic Prkci significantly increased pancreatic immune cell infiltration, acinar cell DNA damage, and apoptosis, but reduced sensitivity to caerulein-induced pancreatitis. Prkci-ablated pancreatic acinar cells exhibited P62 aggregation and a loss of autophagic vesicles. Loss of pancreatic Prkci promoted KrasG12D-mediated pancreatic intraepithelial neoplasia formation but blocked progression to adenocarcinoma, consistent with disruption of autophagy. Our results reveal a novel promotive role for PKCι in pancreatic epithelial cell autophagy and pancreatic cancer progression.

Genes controlling skeletal muscle glucose uptake and their regulation by endurance and resistance exercise

Exercise improves the insulin sensitivity of glucose uptake in skeletal muscle. Due to that, exercise has become a cornerstone treatment for type 2 diabetes mellitus (T2DM). The mechanisms by which exercise improves skeletal muscle insulin sensitivity are, however, incompletely understood. We conducted a systematic review to identify all genes whose gain or loss of function alters skeletal muscle glucose uptake. We subsequently cross-referenced these genes with recently generated data sets on exercise-induced gene expression and signaling. Our search revealed 176 muscle glucose-uptake genes, meaning that their genetic manipulation altered glucose uptake in skeletal muscle. Notably, exercise regulates the expression or phosphorylation of more than 50% of the glucose-uptake genes or their protein products. This included many genes that previously have not been associated with exercise-induced insulin sensitivity. Interestingly, endurance and resistance exercise triggered some common but mostly unique changes in expression and phosphorylation of glucose-uptake genes or their protein products. Collectively, our work provides a resource of potentially new molecular effectors that play a role in the incompletely understood regulation of muscle insulin sensitivity by exercise.

Heparin impairs skeletal muscle glucose uptake by inhibiting insulin binding to insulin receptor

Aim

Heparin, a widely used antithrombotic drug has many other anticoagulant-independent physiological functions. Here, we elucidate a novel role of heparin in glucose homeostasis, suggesting an approach for developing heparin-targeted therapies for diabetes.

Methods

For serum heparin levels and correlation analysis, 122 volunteer's plasma, DIO (4 weeks HFD) and db/db mice serums were collected and used for spectrophotometric determination. OGTT, ITT, 2-NBDG uptake and muscle GLUT4 immunofluorescence were detected in chronic intraperitoneal injection of heparin or heparinase (16 days) and muscle-specific loss-of-function mice. In 293T cells, the binding of insulin to its receptor was detected by fluorescence resonance energy transfer (FRET), Myc-GLUT4-mCherry plasmid was used in GLUT4 translocation. In vitro, C2C12 cells as mouse myoblast cells were further verified the effects of heparin on glucose homeostasis through 2-NBDG uptake, Western blot and co-immunoprecipitation.

Results

Serum concentrations of heparin are positively associated with blood glucose levels in humans and are significantly increased in diet-induced and db/db obesity mouse models. Consistently, a chronic intraperitoneal injection of heparin results in hyperglycaemia, glucose intolerance and insulin resistance. These effects are independent of heparin's anticoagulant function and associated with decreases in glucose uptake and translocation of glucose transporter type 4 (GLUT4) in skeletal muscle. By using a muscle-specific loss-of-function mouse model, we further demonstrated that muscle GLUT4 is required for the detrimental effects of heparin on glucose homeostasis.

Conclusions

Heparin reduced insulin binding to its receptor by interacting with insulin and inhibited insulin-mediated activation of the PI3K/Akt signalling pathway in skeletal muscle, which leads to impaired glucose uptake and hyperglycaemia.

Roles of hepatic atypical protein kinase C hyperactivity and hyperinsulinemia in insulin-resistant forms of obesity and type 2 diabetes mellitus

Diet-induced obesity, the metabolic syndrome, type 2 diabetes (DIO/MetS/T2DM), and their adverse sequelae have reached pandemic levels. In mice, DIO/MetS/T2DM initiation involves diet-dependent increases in lipids that activate hepatic atypical PKC (aPKC) and thereby increase lipogenic enzymes and proinflammatory cytokines. These or other hepatic aberrations, via adverse liver-to-muscle cross talk, rapidly impair postreceptor insulin signaling to glucose transport in muscle. The ensuing hyperinsulinemia further activates hepatic aPKC, which first blocks the ability of Akt to suppress gluconeogenic enzyme expression, and later impairs Akt activation, further increasing hepatic glucose production. Recent findings suggest that hepatic aPKC also increases a proteolytic enzyme that degrades insulin receptors. Fortunately, all hepatic aberrations and muscle impairments are prevented/reversed by inhibition or deficiency of hepatic aPKC. But, in the absence of treatment, hyperinsulinemia induces adverse events, some by using "spare receptors" to bypass receptor defects. Thus, in brain, hyperinsulinemia increases Aβ-plaque precursors and Alzheimer risk; in kidney, hyperinsulinemia activates the renin-angiotensin-adrenal axis, thus increasing vasoconstriction, sodium retention, and cardiovascular risk; and in liver, hyperinsulinemia increases lipogenesis, obesity, hepatosteatosis, hyperlipidemia, and cardiovascular risk. In summary, increases in hepatic aPKC are critically required for development of DIO/MetS/T2DM and its adverse sequelae, and therapeutic approaches that limit hepatic aPKC may be particularly effective.

Cardiometabolic Syndrome: An Update on Available Mouse Models

Cardiometabolic syndrome (CMS), a disease entity characterized by abdominal obesity, insulin resistance (IR), hypertension, and hyperlipidemia, is a global epidemic with approximately 25% prevalence in adults globally. CMS is associated with increased risk for cardiovascular disease (CVD) and development of diabetes. Due to its multifactorial etiology, the development of several animal models to simulate CMS has contributed significantly to the elucidation of the disease pathophysiology and the design of therapies. In this review we aimed to present the most common mouse models used in the research of CMS. We found that CMS can be induced either by genetic manipulation, leading to dyslipidemia, lipodystrophy, obesity and IR, or obesity and hypertension, or by administration of specific diets and drugs. In the last decade, the ob/ob and db/db mice were the most common obesity and IR models, whereas Ldlr-/- and Apoe-/- were widely used to induce hyperlipidemia. These mice have been used either as a single transgenic or combined with a different background with or without diet treatment. High-fat diet with modifications is the preferred protocol, generally leading to increased body weight, hyperlipidemia, and IR. A plethora of genetically engineered mouse models, diets, drugs, or synthetic compounds that are available have advanced the understanding of CMS. However, each researcher should carefully select the most appropriate model and validate its consistency. It is important to consider the differences between strains of the same animal species, different animals, and most importantly differences to human when translating results.

Investigations of mouse models during tumorigenesis revealed essential but distinct in vivo functions among the PKC family

PKC isozymes have been put in place as oncoproteins since the discovery that they can function as receptors for potent tumor-promoting phorbol esters in the 1980s. Despite nearly two decades of research, a clear in vivo proof of that concept was missing. The availability of so-called knock out mouse lines of individual PKC genes provided a tool to investigate isozyme specific in vivo functions in the context of tumor initiation, development and progression. This review aims to provide a limited overview of how the application of these mouse lines in combination with a cancer mouse model helped to understand PKC's in vivo function during tumorigenesis. The focus of this review will be on skin, colon and lung cancer.

Undercarboxylated Osteocalcin Improves Insulin-Stimulated Glucose Uptake in Muscles of Corticosterone-Treated Mice

Short-term administration of glucocorticoids (GCs) impairs muscle insulin sensitivity at least in part via the reduction of undercarboxylated osteocalcin (ucOC). However, whether ucOC treatment reverses the GC-induced muscle insulin resistance remains unclear. To test the hypothesis that ucOC directly ameliorates impaired insulin-stimulated glucose uptake (ISGU) induced by short-term GC administration in mice muscle and to identify the molecular mechanisms, mice were implanted with placebo or corticosterone (CS) slow-release pellets. Two days post-surgery, insulin-tolerance tests (ITTs) were performed. On day 3, serum was collected and extensor digitorum longus (EDL) and soleus muscles were isolated and treated ex vivo with vehicle, ucOC (30 ng/mL), insulin (60 µU/mL), or both. Circulating hormone levels, muscle glucose uptake, and muscle signaling proteins were assessed. CS administration reduced both serum osteocalcin and ucOC levels, whole-body insulin sensitivity, and muscle ISGU in EDL. Ex vivo ucOC treatment restored ISGU in CS-affected muscle, without increasing non-insulin-stimulated glucose uptake. In CS-affected EDL muscle, ucOC enhanced insulin action on phosphorylated (p-)protein kinase B (Akt)^Ser473 and the p-extracellular signal-regulated kinase isoform 2 (ERK2)^{Thr202/Tyr204} /total (t)ERK2 ratio, which correlated with ISGU. In CS-affected soleus muscle, ucOC enhanced insulin action on p-mammalian target of rapamycin (mTOR)^Ser2481 , the p-mTOR^Ser2481 /tmTOR ratio, p-Akt substrate of 160kD (AS160)^Thr642 , and p-protein kinase C (PKC) (pan)^Thr410 , which correlated with ISGU. Furthermore, p-PKC (pan)^Thr410 correlated with p-Akt^Ser473 and p-AS160^Thr642 . ucOC exerts direct insulin-sensitizing effects on CS-affected mouse muscle, likely through an enhancement in activity of key proteins involved in both insulin and ucOC signaling pathways. Furthermore, these effects are muscle type-dependent. © 2019 American Society for Bone and Mineral Research.

Protein kinase C iota facilitates insulin-induced glucose transport by phosphorylation of soluble nSF attachment protein receptor regulator (SNARE) double C2 domain protein b

AIMS/INTRODUCTION

Double C2 domain protein b (DOC2b), one of the synaptotagmins, has been shown to translocate to the plasma membrane, and to initiate membrane-fusion processes of vesicles containing glucose transporter 4 proteins on insulin stimulation. However, the mechanism by which DOC2b is regulated remains unclear. Herein, we identified the upstream regulatory factors of DOC2b in insulin signal transduction. We also examined the role of DOC2b on systemic homeostasis using DOC2b knockout (KO) mice.

MATERIALS AND METHODS

We first identified DOC2b binding proteins by immunoprecipitation and mutagenesis experiments. Then, DOC2b KO mice were generated by disrupting the first exon of the DOC2b gene. In addition to the histological examination, glucose metabolism was assessed by measuring parameters on glucose/insulin tolerance tests. Insulin-stimulated glucose uptake was also measured using isolated soleus muscle and epididymal adipose tissue.

RESULTS

We identified an isoform of atypical protein kinase C (protein kinase C iota) that can bind to DOC2b and phosphorylates one of the serine residues of DOC2b (S34). This phosphorylation is essential for DOC2b translocation. DOC2b KO mice showed insulin resistance and impaired oral glucose tolerance on insulin and glucose tolerance tests, respectively. Insulin-stimulated glucose uptake was impaired in isolated soleus muscle and epididymal adipose tissues from DOC2b KO mice.

CONCLUSIONS

We propose a novel insulin signaling mechanism by which protein kinase C iota phosphorylates DOC2b, leading to glucose transporter 4 vesicle translocation, fusion and facilitation of glucose uptake in response to insulin. The present results also showed DOC2b to play important roles in systemic glucose homeostasis.

Increased Serine and One-Carbon Pathway Metabolism by PKCλ/ι Deficiency Promotes Neuroendocrine Prostate Cancer

Increasingly effective therapies targeting the androgen receptor have paradoxically promoted the incidence of neuroendocrine prostate cancer (NEPC), the most lethal subtype of castration-resistant prostate cancer (PCa), for which there is no effective therapy. Here we report that protein kinase C (PKC)λ/ι is downregulated in de novo and during therapy-induced NEPC, which results in the upregulation of serine biosynthesis through an mTORC1/ATF4-driven pathway. This metabolic reprogramming supports cell proliferation and increases intracellular S-adenosyl methionine (SAM) levels to feed epigenetic changes that favor the development of NEPC characteristics. Altogether, we have uncovered a metabolic vulnerability triggered by PKCλ/ι deficiency in NEPC, which offers potentially actionable targets to prevent therapy resistance in PCa.

PKC and PKN in heart disease

The protein kinase C (PKC) and closely related protein kinase N (PKN) families of serine/threonine protein kinases play crucial cellular roles. Both kinases belong to the AGC subfamily of protein kinases that also include the cAMP dependent protein kinase (PKA), protein kinase B (PKB/AKT), protein kinase G (PKG) and the ribosomal protein S6 kinase (S6K). Involvement of PKC family members in heart disease has been well documented over the years, as their activity and levels are mis-regulated in several pathological heart conditions, such as ischemia, diabetic cardiomyopathy, as well as hypertrophic or dilated cardiomyopathy. This review focuses on the regulation of PKCs and PKNs in different pathological heart conditions and on the influences that PKC/PKN activation has on several physiological processes. In addition, we discuss mechanisms by which PKCs and the closely related PKNs are activated and turned-off in hearts, how they regulate cardiac specific downstream targets and pathways, and how their inhibition by small molecules is explored as new therapeutic target to treat cardiomyopathies and heart failure.

Protein kinase C: perfectly balanced

Protein kinase C (PKC) isozymes belong to a family of Ser/Thr kinases whose activity is governed by reversible release of an autoinhibitory pseudosubstrate. For conventional and novel isozymes, this is effected by binding the lipid second messenger, diacylglycerol, but for atypical PKC isozymes, this is effected by binding protein scaffolds. PKC shot into the limelight following the discovery in the 1980s that the diacylglycerol-sensitive isozymes are "receptors" for the potent tumor-promoting phorbol esters. This set in place a concept that PKC isozymes are oncoproteins. Yet three decades of cancer clinical trials targeting PKC with inhibitors failed and, in some cases, worsened patient outcome. Emerging evidence from cancer-associated mutations and protein expression levels provide a reason: PKC isozymes generally function as tumor suppressors and their activity should be restored, not inhibited, in cancer therapies. And whereas not enough activity is associated with cancer, variants with enhanced activity are associated with degenerative diseases such as Alzheimer's disease. This review describes the tightly controlled mechanisms that ensure PKC activity is perfectly balanced and what happens when these controls are deregulated. PKC isozymes serve as a paradigm for the wisdom of Confucius: "to go beyond is as wrong as to fall short."

Combined Hyperglycemia- and Hyperinsulinemia-Induced Insulin Resistance in Adipocytes Is Associated With Dual Signaling Defects Mediated by PKC-ζ

A hyperglycemic and hyperinsulinemic environment characteristic of type 2 diabetes causes insulin resistance. In adipocytes, defects in both insulin sensitivity and maximum response of glucose transport have been demonstrated. To investigate the molecular mechanisms, freshly isolated rat adipocytes were incubated in control (5.6 mM glucose, no insulin) and high glucose (20 mM)/high insulin (100 nM) (HG/HI) for 18 hours to induce insulin resistance. Insulin-resistant adipocytes manifested decreased sensitivity of glucose uptake associated with defects in insulin receptor substrate (IRS)-1 Tyr phosphorylation, association of p85 subunit of phosphatidylinositol-3-kinase, Akt Ser473 and Thr308 phosphorylation, accompanied by impaired glucose transporter 4 translocation. In contrast, protein kinase C (PKC)-ζ activity was augmented by chronic HG/HI. Inhibition of PKC-ζ with a specific cell-permeable peptide reversed the signaling defects and insulin sensitivity of glucose uptake. Transfection of dominant-negative, kinase-inactive PKC-ζ blocked insulin resistance, whereas constitutively active PKC-ζ recapitulated the defects. The HG/HI incubation was associated with stimulation of IRS-1 Ser318 and Akt Thr34 phosphorylation, targets of PKC-ζ. Transfection of IRS-1 S318A and Akt T34A each partially corrected insulin signaling, whereas combined transfection of both completely normalized insulin signaling. In vivo hyperglycemia/hyperinsulinemia in rats for 48 hours similarly resulted in activation of PKC-ζ and increased phosphorylation of IRS-1 Ser318 and Akt Thr34. These data indicate that impairment of insulin signaling by chronic HG/HI is mediated by dual defects at IRS-1 and Akt mediated by PKC-ζ.

Insulin signalling and glucose transport in the ovary and ovarian function during the ovarian cycle

Data derived principally from peripheral tissues (fat, muscle and liver) show that insulin signals via diverse interconnecting intracellular pathways and that some of the major intersecting points (known as critical nodes) are the IRSs (insulin receptor substrates), PI3K (phosphoinositide kinase)/Akt and MAPK (mitogen-activated protein kinase). Most of these insulin pathways are probably also active in the ovary and their ability to interact with each other and also with follicle-stimulating hormone (FSH) and luteinizing hormone (LH) signalling pathways enables insulin to exert direct modulating influences on ovarian function. The present paper reviews the intracellular actions of insulin and the uptake of glucose by ovarian tissues (granulosa, theca and oocyte) during the oestrous/menstrual cycle of some rodent, primate and ruminant species. Insulin signals through diverse pathways and these are discussed with specific reference to follicular cell types (granulosa, theca and oocyte). The signalling pathways for FSH in granulosa cells and LH in granulosa and theca cells are summarized. The roles of glucose and of insulin-mediated uptake of glucose in folliculogenesis are discussed. It is suggested that glucose in addition to its well-established role of providing energy for cellular function may also have insulin-mediated signalling functions in ovarian cells, involving AMPK (AMP-dependent protein kinase) and/or hexosamine. Potential interactions of insulin signalling with FSH or LH signalling at critical nodes are identified and the available evidence for such interactions in ovarian cells is discussed. Finally the action of the insulin-sensitizing drugs metformin and the thiazolidinedione rosiglitazone on follicular cells is reviewed.

Lipid Use and Misuse by the Heart

The heart utilizes large amounts of fatty acids as energy providing substrates. The physiological balance of lipid uptake and oxidation prevents accumulation of excess lipids. Several processes that affect cardiac function, including ischemia, obesity, diabetes mellitus, sepsis, and most forms of heart failure lead to altered fatty acid oxidation and often also to the accumulation of lipids. There is now mounting evidence associating certain species of these lipids with cardiac lipotoxicity and subsequent myocardial dysfunction. Experimental and clinical data are discussed and paths to reduction of toxic lipids as a means to improve cardiac function are suggested.

Deletion of Protein Kinase C λ in POMC Neurons Predisposes to Diet-Induced Obesity

Effectors of the phosphoinositide 3-kinase (PI3K) signal transduction pathway contribute to the hypothalamic regulation of energy and glucose homeostasis in divergent ways. Here we show that central nervous system (CNS) action of the PI3K signaling intermediate atypical protein kinase C (aPKC) constrains food intake, weight gain, and glucose intolerance in both rats and mice. Pharmacological inhibition of CNS aPKC activity acutely increases food intake and worsens glucose tolerance in chow-fed rodents and causes excess weight gain during high-fat diet (HFD) feeding. Similarly, selective deletion of the aPKC isoform Pkc-λ in proopiomelanocortin (POMC) neurons disrupts leptin action, reduces melanocortin content in the paraventricular nucleus, and markedly increases susceptibility to obesity, glucose intolerance, and insulin resistance specifically in HFD-fed male mice. These data implicate aPKC as a novel regulator of energy and glucose homeostasis downstream of the leptin-PI3K pathway in POMC neurons.

Spatiotemporal Regulators for Insulin-Stimulated GLUT4 Vesicle Exocytosis

Insulin increases glucose uptake and storage in muscle and adipose cells, which is accomplished through the mobilization of intracellular GLUT4 storage vesicles (GSVs) to the cell surface upon stimulation. Importantly, the dysfunction of insulin-regulated GLUT4 trafficking is strongly linked with peripheral insulin resistance and type 2 diabetes in human. The insulin signaling pathway, key signaling molecules involved, and precise trafficking itinerary of GSVs are largely identified. Understanding the interaction between insulin signaling molecules and key regulatory proteins that are involved in spatiotemporal regulation of GLUT4 vesicle exocytosis is of great importance to explain the pathogenesis of diabetes and may provide new potential therapeutic targets.

Brain Insulin Signaling Is Increased in Insulin-Resistant States and Decreases in FOXOs and PGC-1α and Increases in Aβ1-40/42 and Phospho-Tau May Abet Alzheimer Development

Increased coexistence of Alzheimer disease (AD) and type 2 diabetes mellitus (T2DM) suggests that insulin resistance abets neurodegenerative processes, but linkage mechanisms are obscure. Here, we examined insulin signaling factors in brains of insulin-resistant high-fat-fed mice, ob/ob mice, mice with genetically impaired muscle glucose transport, and monkeys with diet-dependent long-standing obesity/T2DM. In each model, the resting/basal activities of insulin-regulated brain protein kinases, Akt and atypical protein kinase C (aPKC), were maximally increased. Moreover, Akt hyperactivation was accompanied by hyperphosphorylation of substrates glycogen synthase kinase-3β and mammalian target of rapamycin and FOXO proteins FOXO1, FOXO3A, and FOXO4 and decreased peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) expression. Akt hyperactivation was confirmed in individual neurons of anterocortical and hippocampal regions that house cognition/memory centers. Remarkably, β-amyloid (Aβ1-40/42) peptide levels were as follows: increased in the short term by insulin in normal mice, increased basally in insulin-resistant mice and monkeys, and accompanied by diminished amyloid precursor protein in monkeys. Phosphorylated tau levels were increased in ob/ob mice and T2DM monkeys. Importantly, with correction of hyperinsulinemia by inhibition of hepatic aPKC and improvement in systemic insulin resistance, brain insulin signaling normalized. As FOXOs and PGC-1α are essential for memory and long-term neuronal function and regeneration and as Aβ1-40/42 and phospho-tau may increase interneuronal plaques and intraneuronal tangles, presently observed aberrations in hyperinsulinemic states may participate in linking insulin resistance to AD.

The phosphatidylethanolamine derivative diDCP-LA-PE mimics intracellular insulin signaling

Insulin facilitates glucose uptake into cells by translocating the glucose transporter GLUT4 towards the cell surface through a pathway along an insulin receptor (IR)/IR substrate 1 (IRS-1)/phosphatidylinositol 3 kinase (PI3K)/3-phosphoinositide-dependent protein kinase-1 (PDK1)/Akt axis. The newly synthesized phosphatidylethanolamine derivative 1,2-O-bis-[8-{2-(2-pentyl-cyclopropylmethyl)-cyclopropyl}-octanoyl]-sn-glycero-3-phosphatidylethanolamine (diDCP-LA-PE) has the potential to inhibit protein tyrosine phosphatase 1B (PTP1B) and to directly activate PKCζ, an atypical isozyme, and PKCε, a novel isozyme. PTP1B inhibition enhanced insulin signaling cascades downstream IR/IRS-1 by preventing tyrosine dephosphorylation. PKCζ and PKCε directly activated Akt2 by phosphorylating at Thr309 and Ser474, respectively. diDCP-LA-PE increased cell surface localization of GLUT4 and stimulated glucose uptake into differentiated 3T3-L1 adipocytes, still with knocking-down IR or in the absence of insulin. Moreover, diDCP-LA-PE effectively reduced serum glucose levels in type 1 diabetes (DM) model mice. diDCP-LA-PE, thus, may enable type 1 DM therapy without insulin injection.

Protein Scaffolds Control Localized Protein Kinase Cζ Activity

Atypical protein kinase C (aPKC) isozymes modulate insulin signaling and cell polarity, but how their activity is controlled in cells is not well understood. These enzymes are constitutively phosphorylated, insensitive to second messengers, and have relatively low activity. Here we show that protein scaffolds not only localize but also differentially control the catalytic activity of the aPKC PKCζ, thus promoting activity toward localized substrates and restricting activity toward global substrates. Using cellular substrate readouts and scaffolded activity reporters in live cell imaging, we show that PKCζ has highly localized and differentially controlled activity on the scaffolds p62 and Par6. Both scaffolds tether aPKC in an active conformation as assessed through pharmacological inhibition of basal activity, monitored using a genetically encoded reporter for PKC activity. However, binding to Par6 is of higher affinity and is more effective in locking PKCζ in an active conformation. FRET-based translocation assays reveal that insulin promotes the association of both p62 and aPKC with the insulin-regulated scaffold IRS-1. Using the aPKC substrate MARK2 as another readout for activity, we show that overexpression of IRS-1 reduces the phosphorylation of MARK2 and enhances its plasma membrane localization, indicating sequestration of aPKC by IRS-1 away from MARK2. These results are consistent with scaffolds serving as allosteric activators of aPKCs, tethering them in an active conformation near specific substrates. Thus, signaling of these intrinsically low activity kinases is kept at a minimum in the absence of scaffolding interactions, which position the enzymes for stoichiometric phosphorylation of substrates co-localized on the same protein scaffold.

Optogenetic activation reveals distinct roles of PIP3 and Akt in adipocyte insulin action

Glucose transporter 4 (GLUT4; also known as SLC2A4) resides on intracellular vesicles in muscle and adipose cells, and translocates to the plasma membrane in response to insulin. The phosphoinositide 3-kinase (PI3K)-Akt signaling pathway plays a major role in GLUT4 translocation; however, a challenge has been to unravel the potentially distinct contributions of PI3K and Akt (of which there are three isoforms, Akt1-Akt3) to overall insulin action. Here, we describe new optogenetic tools based on CRY2 and the N-terminus of CIB1 (CIBN). We used these 'Opto' modules to activate PI3K and Akt selectively in time and space in 3T3-L1 adipocytes. We validated these tools using biochemical assays and performed live-cell kinetic analyses of IRAP-pHluorin translocation (IRAP is also known as LNPEP and acts as a surrogate marker for GLUT4 here). Strikingly, Opto-PIP3 largely mimicked the maximal effects of insulin stimulation, whereas Opto-Akt only partially triggered translocation. Conversely, drug-mediated inhibition of Akt only partially dampened the translocation response of Opto-PIP3 In spatial optogenetic studies, focal targeting of Akt to a region of the cell marked the sites where IRAP-pHluorin vesicles fused, supporting the idea that local Akt-mediated signaling regulates exocytosis. Taken together, these results indicate that PI3K and Akt play distinct roles, and that PI3K stimulates Akt-independent pathways that are important for GLUT4 translocation.

Glucose Transporters in Cardiac Metabolism and Hypertrophy

The heart is adapted to utilize all classes of substrates to meet the high-energy demand, and it tightly regulates its substrate utilization in response to environmental changes. Although fatty acids are known as the predominant fuel for the adult heart at resting stage, the heart switches its substrate preference toward glucose during stress conditions such as ischemia and pathological hypertrophy. Notably, increasing evidence suggests that the loss of metabolic flexibility associated with increased reliance on glucose utilization contribute to the development of cardiac dysfunction. The changes in glucose metabolism in hypertrophied hearts include altered glucose transport and increased glycolysis. Despite the role of glucose as an energy source, changes in other nonenergy producing pathways related to glucose metabolism, such as hexosamine biosynthetic pathway and pentose phosphate pathway, are also observed in the diseased hearts. This article summarizes the current knowledge regarding the regulation of glucose transporter expression and translocation in the heart during physiological and pathological conditions. It also discusses the signaling mechanisms governing glucose uptake in cardiomyocytes, as well as the changes of cardiac glucose metabolism under disease conditions.

Mechanisms for greater insulin-stimulated glucose uptake in normal and insulin-resistant skeletal muscle after acute exercise

Enhanced skeletal muscle and whole body insulin sensitivity can persist for up to 24-48 h after one exercise session. This review focuses on potential mechanisms for greater postexercise and insulin-stimulated glucose uptake (ISGU) by muscle in individuals with normal or reduced insulin sensitivity. A model is proposed for the processes underlying this improvement; i.e., triggers initiate events that activate subsequent memory elements, which store information that is relayed to mediators, which translate memory into action by controlling an end effector that directly executes increased insulin-stimulated glucose transport. Several candidates are potential triggers or memory elements, but none have been conclusively verified. Regarding potential mediators in both normal and insulin-resistant individuals, elevated postexercise ISGU with a physiological insulin dose coincides with greater Akt substrate of 160 kDa (AS160) phosphorylation without improved proximal insulin signaling at steps from insulin receptor binding to Akt activity. Causality remains to be established between greater AS160 phosphorylation and improved ISGU. The end effector for normal individuals is increased GLUT4 translocation, but this remains untested for insulin-resistant individuals postexercise. Following exercise, insulin-resistant individuals can attain ISGU values similar to nonexercising healthy controls, but after a comparable exercise protocol performed by both groups, ISGU for the insulin-resistant group has been consistently reported to be below postexercise values for the healthy group. Further research is required to fully understand the mechanisms underlying the improved postexercise ISGU in individuals with normal or subnormal insulin sensitivity and to explain the disparity between these groups after similar exercise.

Protein kinase Cζ exhibits constitutive phosphorylation and phosphatidylinositol-3,4,5-triphosphate-independent regulation

Atypical protein kinase C (aPKC) isoenzymes are key modulators of insulin signalling, and their dysfunction correlates with insulin-resistant states in both mice and humans. Despite the engaged interest in the importance of aPKCs to type 2 diabetes, much less is known about the molecular mechanisms that govern their cellular functions than for the conventional and novel PKC isoenzymes and the functionally-related protein kinase B (Akt) family of kinases. Here we show that aPKC is constitutively phosphorylated and, using a genetically-encoded reporter for PKC activity, basally active in cells. Specifically, we show that phosphorylation at two key regulatory sites, the activation loop and turn motif, of the aPKC PKCζ in multiple cultured cell types is constitutive and independently regulated by separate kinases: ribosome-associated mammalian target of rapamycin complex 2 (mTORC2) mediates co-translational phosphorylation of the turn motif, followed by phosphorylation at the activation loop by phosphoinositide-dependent kinase-1 (PDK1). Live cell imaging reveals that global aPKC activity is constitutive and insulin unresponsive, in marked contrast to the insulin-dependent activation of Akt monitored by an Akt-specific reporter. Nor does forced recruitment to phosphoinositides by fusing the pleckstrin homology (PH) domain of Akt to the kinase domain of PKCζ alter either the phosphorylation or activity of PKCζ. Thus, insulin stimulation does not activate PKCζ through the canonical phosphatidylinositol-3,4,5-triphosphate-mediated pathway that activates Akt, contrasting with previous literature on PKCζ activation. These studies support a model wherein an alternative mechanism regulates PKCζ-mediated insulin signalling that does not utilize conventional activation via agonist-evoked phosphorylation at the activation loop. Rather, we propose that scaffolding near substrates drives the function of PKCζ.

Contraction stimulates muscle glucose uptake independent of atypical PKC

Exercise increases skeletal muscle glucose uptake, but the underlying mechanisms are only partially understood. The atypical protein kinase C (PKC) isoforms λ and ζ (PKC‐λ/ζ) have been shown to be necessary for insulin‐, AICAR‐, and metformin‐stimulated glucose uptake in skeletal muscle, but not for treadmill exercise‐stimulated muscle glucose uptake. To investigate if PKC‐λ/ζ activity is required for contraction‐stimulated muscle glucose uptake, we used mice with tibialis anterior muscle‐specific overexpression of an empty vector (WT), wild‐type PKC‐ζ (PKC‐ζ^WT), or an enzymatically inactive T410A‐PKC‐ζ mutant (PKC‐ζ^T410A). We also studied skeletal muscle‐specific PKC‐λ knockout (MλKO) mice. Basal glucose uptake was similar between WT, PKC‐ζ^WT, and PKC‐ζ^T410A tibialis anterior muscles. In contrast, in situ contraction‐stimulated glucose uptake was increased in PKC‐ζ^T410A tibialis anterior muscles compared to WT or PKC‐ζ^WT tibialis anterior muscles. Furthermore, in vitro contraction‐stimulated glucose uptake was greater in soleus muscles of MλKO mice than WT controls. Thus, loss of PKC‐λ/ζ activity increases contraction‐stimulated muscle glucose uptake. These data clearly demonstrate that PKC‐λ/ζ activity is not necessary for contraction‐stimulated glucose uptake.

BMI-related progression of atypical PKC-dependent aberrations in insulin signaling through IRS-1, Akt, FoxO1 and PGC-1α in livers of obese and type 2 diabetic humans

Information on insulin resistance in human liver is limited. In mouse diet-induced obesity (DIO), hepatic insulin resistance initially involves: lipid+insulin-induced activation of atypical protein kinase C (aPKC); elevated Akt activity/activation but selective impairment of compartmentalized Akt-dependent FoxO1 phosphorylation; and increases in gluconeogenic and lipogenic enzymes. In advanced stages, e.g., in hepatocytes of type 2 diabetes (T2D) humans, insulin activation of insulin receptor substrate-1(IRS-1) and Akt fails, further increasing FoxO1-dependent gluconeogenic/lipogenic enzyme expression. Increases in hepatic PGC-1α also figure prominently, but uncertainly, in this scheme. Here, we examined signaling factors in liver samples harvested from human transplant donors with increasing BMI, 20→25→30→35→40→45. We found, relative to lean (BMI=20-25) humans, obese (BMI>30) humans had all abnormalities seen in early mouse DIO, but, surprisingly, at all elevated BMI levels, had decreased insulin receptor-1 (IRS-1) levels, decreased Akt activity, and increased expression/abundance of aPKC-ι and PGC-1α. Moreover, with increasing BMI, there were: progressive increases in aPKC activity and PKC-ι expression/abundance; progressive decreases in IRS-1 levels, Akt activity and FoxO1 phosphorylation; progressive increases in expression/abundance of PGC-1α; and progressive increases in gluconeogenic and lipogenic enzymes. Remarkably, all abnormalities reached T2D levels at higher BMI levels. Most importantly, both "early" and advanced abnormalities were largely reversed by 24-hour treatment of T2D hepatocytes with aPKC inhibitor. We conclude: hepatic insulin resistance in human obesity is: advanced; BMI-correlated; and sequentially involves increased aPKC-activating ceramide; increased aPKC levels and activity; decreases in IRS-1 levels, Akt activity, and FoxO1 phosphorylation; and increases in expression/abundance of PGC-1α and gluconeogenic and lipogenic genes.

Protein kinase C in cellular transformation: a valid target for therapy?

The protein kinase C (PKC) family of serine/threonine protein kinases share structural homology, while exhibiting substantial functional diversity. PKC isoforms are ubiquitously expressed in tissues which makes it difficult to define roles for individual isoforms, with complexity compounded by the finding that PKC isoforms can co-operate with or antagonize other PKC family members. A number of studies suggest the involvement of PKC family members in regulating leukaemic cell survival and proliferation. Chronic lymphocytic leukaemia (CLL), the most common leukaemia in the Western world, exhibits dysregulated expression of PKC isoforms, with recent reports indicating that PKCβ and δ play a critical role in B-cell development, due to their ability to link the B-cell receptor (BCR) with downstream signalling pathways. Given the prognostic significance of the BCR in CLL, inhibition of these BCR/PKC-mediated signalling pathways is of therapeutic relevance. The present review discusses the emerging role of PKC isoforms in the pathophysiology of CLL and assesses approaches that have been undertaken to modulate PKC activity.

Rac1 and ROCK are implicated in the cell surface delivery of GLUT4 under the control of the insulin signal mimetic diDCP-LA-PE

The phosphatidylethanolamine derivative 1,2-O-bis-[8-{2-(2-pentyl-cyclopropylmethyl)-cyclopropyl}-octanoyl]-sn-glycero-3-phosphatidylethanolamine (diDCP-LA-PE) promoted GLUT4 translocation to the cell surface in differentiated 3T3-L1-GLUT4myc adipocytes through a pathway along a phosphatidylinositol 3-kinase (PI3K)/3-phosphoinositide-dependent protein kinase-1 (PDK1)/Akt axis, that mimics insulin signaling. Moreover, diDCP-LA-PE-induced GLUT4 translocation was suppressed by inhibitors of the Rho GTPase Rac1 and Rho-associated coiled-coil-containing protein kinase (ROCK) or knocking-down Rac1 and ROCK1. The results of the present study show that Rac1 and ROCK are critical for regulation of GLUT4 trafficking by diDCP-LA-PE as well as insulin.

Diabetes mellitus: The epidemic of the century

The epidemic nature of diabetes mellitus in different regions is reviewed. The Middle East and North Africa region has the highest prevalence of diabetes in adults (10.9%) whereas, the Western Pacific region has the highest number of adults diagnosed with diabetes and has countries with the highest prevalence of diabetes (37.5%). Different classes of diabetes mellitus, type 1, type 2, gestational diabetes and other types of diabetes mellitus are compared in terms of diagnostic criteria, etiology and genetics. The molecular genetics of diabetes received extensive attention in recent years by many prominent investigators and research groups in the biomedical field. A large array of mutations and single nucleotide polymorphisms in genes that play a role in the various steps and pathways involved in glucose metabolism and the development, control and function of pancreatic cells at various levels are reviewed. The major advances in the molecular understanding of diabetes in relation to the different types of diabetes in comparison to the previous understanding in this field are briefly reviewed here. Despite the accumulation of extensive data at the molecular and cellular levels, the mechanism of diabetes development and complications are still not fully understood. Definitely, more extensive research is needed in this field that will eventually reflect on the ultimate objective to improve diagnoses, therapy and minimize the chance of chronic complications development.

Fucosylated chondroitin sulfate from sea cucumber improves insulin sensitivity via activation of PI3K/PKB pathway

This study was to investigate the effects of fucosylated chondroitin sulfate (CHS) from sea cucumber on insulin sensitivity in skeletal muscle of type 2 diabetic mice induced by a high-fat high-sucrose diet (HFSD). CHS supplementation for 19 wk significantly improved insulin sensitivity by 20%, and reduced blood glucose and insulin levels. Western blotting assay showed that CHS significantly increased insulin-stimulated glucose transporter 4 (GLUT4) translocation to 1.7-fold, phosphorylation of phosphoinositide 3-kinase (PI3K) at p85 to 5.0-fold, protein kinase B (PKB) at Ser473 to 1.5-fold, and Thr308 to 1.6-fold in skeletal muscle. However, PI3K, PKB, and GLUT4 total proteins expression were unchangeable. In addition, qRT-PCR analysis proved that the insulin signaling was activated by CHS treatment, showing the increased mRNA expressions of glucose uptake-related key genes. It indicated that CHS improved insulin sensitivity by activation of PI3K/PKB signaling in skeletal muscle of type 2 diabetic mice. Identification of potential mechanism by which CHS increased insulin sensitivity might provide a new functional food or pharmaceutical application of sea cucumber.

Nicotine pre-exposure reduces stroke-induced glucose transporter-1 activity at the blood-brain barrier in mice

Background

With growing electronic cigarette usage in both the smoking and nonsmoking population, rigorous studies are needed to investigate the effects of nicotine on biological systems to determine long-term health consequences. We have previously shown that nicotine exerts specific neurovascular effects that influence blood brain barrier (BBB) function in response to stroke. In this study, we investigated the effects of nicotine on carrier-mediated glucose transport into ischemic brain. Specifically, the present study investigates glucose transporter-1 (GLUT1) function and expression at the BBB in a focal brain ischemia model of mice pre-exposed to nicotine.

Methods

Nicotine was administrated subcutaneously by osmotic pump at the dose of 4.5 mg/kg/day for 1, 7, or 14 days to reflect the plasma levels seen in smokers. Ischemic-reperfusion (IR) injury was induced by 1 h transient middle cerebral artery occlusion (tMCAO) and 24 h reperfusion. Glucose transport was estimated using an in situ brain perfusion technique with radiolabeled glucose and brain vascular GLUT1 expression was detected with immunofluorescence.

Results

The nicotine pre-exposure (1, 7 & 14 day) resulted in significant reduction in D-glucose influx rate (K_in) across the BBB, with a 49% reduction in 14 day nicotine-infused animals. We observed a 41% increase in carrier-mediated glucose transport across the BBB in saline-infused tMCAO animals compared to saline-infused sham animals. Interestingly, in the tMCAO group of animals pre-exposed to nicotine for 14 days had significantly attenuated increased glucose transport by 80% and 38% compared to saline-infused tMCAO and sham animals respectively. Furthermore, immunofluorescence studies of GLUT1 protein expression in the brain microvascular endothelium confirmed that GLUT1 was also induced in saline-infused tMCAO animals and this protein expression induction was reduced significantly (P < 0.01) with 14 day nicotine pre-exposure in tMCAO animals.

Conclusions

Nicotine pre-exposure reduced the IR-enhanced GLUT1 transporter function and expression at the BBB in a focal brain ischemia mouse model. These studies suggest that nicotine exposure prior to stroke could create an enhanced glucose deprived state at the neurovascular unit (NVU) and could provide an additional vulnerability to enhanced stroke injury.

Atypical PKC: a target for treating insulin-resistant disorders of obesity, the metabolic syndrome and type 2 diabetes mellitus

INTRODUCTION

The prevalence of obesity, the metabolic syndrome and type 2 diabetes mellitus have reached pandemic levels. Present therapies do not directly target the key factor responsible for the insulin resistance that underlies the development of these syndromes.

AREAS COVERED

This review focuses on hepatic atypical PKC (aPKC) as a key target for treating these disorders. It reviews data obtained from multiple experimental mouse models of obesity and type 2 diabetes, and hepatocytes of type 2 diabetic humans.

EXPERT OPINION

The review shows that hepatic aPKC is excessively activated by diet-derived lipids and by insulin itself in hyperinsulinemic states. It also shows how excessively activated hepatic aPKC increases expression of gluconeogenic, lipogenic and proinflammatory factors that underlie the development of glucose intolerance, insulin resistance, obesity, hepatosteatosis and hyperlipidemia. Most importantly, the review shows how the selective inhibition of hepatic aPKC by a variety of means, including expression of inhibitory forms of aPKC, genetic deletion of aPKC and use of several newly developed small-molecular-weight chemical agents result in correction of hepatic abnormalities, such as excessive expression of gluconeogenic, lipogenic and proinflammatory factors, and correction or improvement in clinical abnormalities (glucose intolerance, obesity, hepatosteatosis and hyperlipidemia).

Palmitate induces ER stress and autophagy in H9c2 cells: implications for apoptosis and adiponectin resistance

The association between obesity and heart failure is well documented and recent studies have indicated that understanding the physiological role of autophagy will be of great significance. Cardiomyocyte apoptosis is one component of cardiac remodeling which leads to heart failure and in this study we used palmitate-treated H9c2 cells as an in vitro model of lipotoxicity to investigate the role of autophagy in cell death. Temporal analysis revealed that palmitate (100 μM) treatment induced a gradual increase of intracellular lipid accumulation as well as apoptotic cell death. Palmitate induced autophagic flux, determined via increased LC3-II formation and p62 degradation as well as by detecting reduced colocalization of GFP with RFP in cells overexpressing tandem fluorescent GFP/RFP-LC3. The increased level of autophagy indicated by these measures were confirmed using transmission electron microscopy (TEM). Upon inhibiting autophagy using bafilomycin we observed an increased level of palmitate-induced cell death assessed by Annexin V/PI staining, detection of active caspase-3 and MTT cell viability assay. Interestingly, using TEM and p-PERK or p-eIF2α detection we observed increased endoplasmic reticulum (ER) stress in response to palmitate. Autophagy was induced as an adaptive response against ER stress since it was sensitive to ER stress inhibition. Palmitate-induced ER stress also induced adiponectin resistance, assessed via AMPK phosphorylation, via reducing APPL1 expression. This effect was independent of palmitate-induced autophagy. In summary, our data indicate that palmitate induces autophagy subsequent to ER stress and that this confers a prosurvival effect against lipotoxicity-induced cell death. Palmitate-induced ER stress also led to adiponecin resistance.

Epidermal polarity genes in health and disease

The epidermis of the skin is a highly polarized, metabolic tissue with important innate immune functions. The polarity of the epidermis is, for example, reflected in controlled changes in cell shape that accompany differentiation, oriented cell division, and the planar orientation of hair follicles and cilia. The establishment and maintenance of polarity is organized by a diverse set of polarity proteins that include transmembrane adhesion proteins, cytoskeletal scaffold proteins, and kinases. Although polarity proteins have been extensively studied in cell culture and in vivo in simple epithelia of lower organisms, their role in mammalian tissue biology is only slowly evolving. This article will address the importance of polarizing processes and their molecular regulators in epidermal morphogenesis and homeostasis and discuss how alterations in polarity may contribute to skin disease.

Hepatic insulin resistance in ob/ob mice involves increases in ceramide, aPKC activity, and selective impairment of Akt-dependent FoxO1 phosphorylation

Pathogenesis of insulin resistance in leptin-deficient ob/ob mice is obscure. In another form of diet-dependent obesity, high-fat-fed mice, hepatic insulin resistance involves ceramide-induced activation of atypical protein kinase C (aPKC), which selectively impairs protein kinase B (Akt)-dependent forkhead box O1 protein (FoxO1) phosphorylation on scaffolding protein, 40 kDa WD(tryp-x-x-asp)-repeat propeller/FYVE protein (WD40/ProF), thereby increasing gluconeogenesis. Resultant hyperinsulinemia activates hepatic Akt and mammalian target of rapamycin C1, and further activates aPKC; consequently, lipogenic enzyme expression increases, and insulin signaling in muscle is secondarily impaired. Here, in obese minimally-diabetic ob/ob mice, hepatic ceramide and aPKC activity and its association with WD40/ProF were increased. Hepatic Akt activity was also increased, but Akt associated with WD40/ProF was diminished and accounted for reduced FoxO1 phosphorylation and increased gluconeogenic enzyme expression. Most importantly, liver-selective inhibition of aPKC decreased aPKC and increased Akt association with WD40/ProF, thereby restoring FoxO1 phosphorylation and reducing gluconeogenic enzyme expression. Additionally, lipogenic enzyme expression diminished, and insulin signaling in muscle, glucose tolerance, obesity, hepatosteatosis, and hyperlipidemia improved. In conclusion, hepatic ceramide accumulates in response to CNS-dependent dietary excess irrespective of fat content; hepatic insulin resistance is prominent in ob/ob mice and involves aPKC-dependent displacement of Akt fromWD40/ProF and subsequent impairment of FoxO1 phosphorylation and increased expression of hepatic gluconeogenic and lipogenic enzymes; and hepatic alterations diminish insulin signaling in muscle.

Akt-dependent phosphorylation of hepatic FoxO1 is compartmentalized on a WD40/ProF scaffold and is selectively inhibited by aPKC in early phases of diet-induced obesity

Initiating mechanisms that impair gluconeogenic enzymes and spare lipogenic enzymes in diet-induced obesity (DIO) are obscure. Here, we examined insulin signaling to Akt and atypical protein kinase C (aPKC) in liver and muscle and hepatic enzyme expression in mice consuming a moderate high-fat (HF) diet. In HF diet-fed mice, resting/basal and insulin-stimulated Akt and aPKC activities were diminished in muscle, but in liver, these activities were elevated basally and were increased by insulin to normal levels. Despite elevated hepatic Akt activity, FoxO1 phosphorylation, which diminishes gluconeogenesis, was impaired; in contrast, Akt-dependent phosphorylation of glycogenic GSK3β and lipogenic mTOR was elevated. Diminished Akt-dependent FoxO1 phosphorylation was associated with reduced Akt activity associated with scaffold protein WD40/Propeller/FYVE (WD40/ProF), which reportedly facilitates FoxO1 phosphorylation. In contrast, aPKC activity associated with WD40/ProF was increased. Moreover, inhibition of hepatic aPKC reduced its association with WD40/ProF, restored WD40/ProF-associated Akt activity, restored FoxO1 phosphorylation, and corrected excessive expression of hepatic gluconeogenic and lipogenic enzymes. Additionally, Akt and aPKC activities in muscle improved, as did glucose intolerance, weight gain, hepatosteatosis, and hyperlipidemia. We conclude that Akt-dependent FoxO1 phosphorylation occurs on the WD/Propeller/FYVE scaffold in liver and is selectively inhibited in early DIO by diet-induced increases in activity of cocompartmentalized aPKC.

Hepatic Atypical Protein Kinase C: An Inherited Survival-Longevity Gene that Now Fuels Insulin-Resistant Syndromes of Obesity, the Metabolic Syndrome and Type 2 Diabetes Mellitus

This review focuses on how insulin signals to metabolic processes in health, why this signaling is frequently deranged in Western/Westernized societies, how these derangements lead to, or abet development of, insulin-resistant states of obesity, the metabolic syndrome and type 2 diabetes mellitus, and what our options are for restoring insulin signaling, and glucose/lipid homeostasis. A central theme in this review is that excessive hepatic activity of an archetypal protein kinase enzyme, “atypical” protein kinase C (aPKC), plays a critically important role in the development of impaired glucose metabolism, systemic insulin resistance, and excessive hepatic production of glucose, lipids and proinflammatory factors that underlie clinical problems of glucose intolerance, obesity, hepatosteatosis, hyperlipidemia, and, ultimately, type 2 diabetes. The review suggests that normally inherited genes, in particular, the aPKC isoforms, that were important for survival and longevity in times of food scarcity are now liabilities in times of over-nutrition. Fortunately, new knowledge of insulin signaling mechanisms and how an aberration of excessive hepatic aPKC activation is induced by over-nutrition puts us in a position to target this aberration by diet and/or by specific inhibitors of hepatic aPKC.

PKCλ haploinsufficiency prevents diabetes by a mechanism involving alterations in hepatic enzymes

Tissue-specific knockout (KO) of atypical protein kinase C (aPKC), PKC-λ, yields contrasting phenotypes, depending on the tissue. Thus, whereas muscle KO of PKC-λ impairs glucose transport and causes glucose intolerance, insulin resistance, and liver-dependent lipid abnormalities, liver KO and adipocyte KO of PKC-λ increase insulin sensitivity through salutary alterations in hepatic enzymes. Also note that, although total-body (TB) homozygous KO of PKC-λ is embryonic lethal, TB heterozygous (Het) KO (TBHetλKO) is well-tolerated. However, beneath their seemingly normal growth, appetite, and overall appearance, we found in TBHetλKO mice that insulin receptor phosphorylation and signaling through insulin receptor substrates to phosphatidylinositol 3-kinase, Akt and residual aPKC were markedly diminished in liver, muscle, and adipose tissues, and glucose transport was impaired in muscle and adipose tissues. Furthermore, despite these global impairments in insulin signaling, other than mild hyperinsulinemia, glucose tolerance, serum lipids, and glucose disposal and hepatic glucose output in hyperinsulinemic clamp studies were normal. Moreover, TBHetλKO mice were protected from developing glucose intolerance during high-fat feeding. This metabolic protection (in the face of impaired insulin signaling) in HetλKO mice seemed to reflect a deficiency of PKC-λ in liver with resultant 1) increases in FoxO1 phosphorylation and decreases in expression of hepatic gluconeogenic enzymes and 2) diminished expression of hepatic lipogenic enzymes and proinflammatory cytokines. In keeping with this postulate, adenoviral-mediated supplementation of hepatic PKC-λ induced a diabetic state in HetλKO mice. Our findings underscore the importance of hepatic PKC-λ in provoking abnormalities in glucose and lipid metabolism.

Insulin, osteoblasts, and energy metabolism: why bone counts calories

Recent studies have demonstrated that insulin stimulates bone cells to produce and activate osteocalcin, an endocrine hormone that increases the efficiency of glucose metabolism through its actions on the pancreas and other peripheral tissues. In this issue of the JCI, Wei and colleagues directly explore the contribution of insulin signaling in osteoblasts to the disturbances in whole-body glucose metabolism associated with a high-fat diet. In mice fed a high-fat diet, increased uptake of saturated fatty acids by the osteoblast accelerates the ubiquitination and degradation of the insulin receptor. In this setting, impairments in osteoblast insulin signaling reduce serum levels of undercarboxylated osteocalcin, which in turn exacerbate insulin resistance in muscle and white adipose tissue. These findings underscore the importance of insulin-responsive skeletal cells as components of a newly appreciated endocrine network critical for regulating global energy homeostasis.

A proteolytic pathway that controls glucose uptake in fat and muscle

Insulin regulates glucose uptake by controlling the subcellular location of GLUT4 glucose transporters. GLUT4 is sequestered within fat and muscle cells during low-insulin states, and is translocated to the cell surface upon insulin stimulation. The TUG protein is a functional tether that sequesters GLUT4 at the Golgi matrix. To stimulate glucose uptake, insulin triggers TUG endoproteolytic cleavage. Cleavage accounts for a large proportion of the acute effect of insulin to mobilize GLUT4 to the cell surface. During ongoing insulin exposure, endocytosed GLUT4 recycles to the plasma membrane directly from endosomes, and bypasses a TUG-regulated trafficking step. Insulin acts through the TC10α GTPase and its effector protein, PIST, to stimulate TUG cleavage. This action is coordinated with insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases, and with other signals to direct overall GLUT4 targeting. Data support the idea that the N-terminal TUG cleavage product, TUGUL, functions as a novel ubiquitin-like protein modifier to facilitate GLUT4 movement to the cell surface. The C-terminal TUG cleavage product is extracted from the Golgi matrix, which vacates an "anchoring" site to permit subsequent cycles of GLUT4 retention and release. Together, GLUT4 vesicle translocation and TUG cleavage may coordinate glucose uptake with physiologic effects of other proteins present in the GLUT4-containing vesicles, and with potential additional effects of the TUG C-terminal product. Understanding this TUG pathway for GLUT4 retention and release will shed light on the regulation of glucose uptake and the pathogenesis of type 2 diabetes.

Meta-analysis identifies gene-by-environment interactions as demonstrated in a study of 4,965 mice

Identifying environmentally-specific genetic effects is a key challenge in understanding the structure of complex traits. Model organisms play a crucial role in the identification of such gene-by-environment interactions, as a result of the unique ability to observe genetically similar individuals across multiple distinct environments. Many model organism studies examine the same traits but under varying environmental conditions. For example, knock-out or diet-controlled studies are often used to examine cholesterol in mice. These studies, when examined in aggregate, provide an opportunity to identify genomic loci exhibiting environmentally-dependent effects. However, the straightforward application of traditional methodologies to aggregate separate studies suffers from several problems. First, environmental conditions are often variable and do not fit the standard univariate model for interactions. Additionally, applying a multivariate model results in increased degrees of freedom and low statistical power. In this paper, we jointly analyze multiple studies with varying environmental conditions using a meta-analytic approach based on a random effects model to identify loci involved in gene-by-environment interactions. Our approach is motivated by the observation that methods for discovering gene-by-environment interactions are closely related to random effects models for meta-analysis. We show that interactions can be interpreted as heterogeneity and can be detected without utilizing the traditional uni- or multi-variate approaches for discovery of gene-by-environment interactions. We apply our new method to combine 17 mouse studies containing in aggregate 4,965 distinct animals. We identify 26 significant loci involved in High-density lipoprotein (HDL) cholesterol, many of which are consistent with previous findings. Several of these loci show significant evidence of involvement in gene-by-environment interactions. An additional advantage of our meta-analysis approach is that our combined study has significantly higher power and improved resolution compared to any single study thus explaining the large number of loci discovered in the combined study.

Identifying gene-by-environment interactions is important for understand the architecture of a complex trait. Discovering gene-by-environment interaction requires the observation of the same phenotype in individuals under different environments. Model organism studies are often conducted under different environments. These studies provide an unprecedented opportunity for researchers to identify the gene-by-environment interactions. A difference in the effect size of a genetic variant between two studies conducted in different environments may suggest the presence of a gene-by-environment interaction. In this paper, we propose to employ a random-effect-based meta-analysis approach to identify gene-by-environment interaction, which assumes different or heterogeneous effect sizes between studies. Our approach is motivated by the observation that methods for discovering gene-by-environment interactions are closely related to random effects models for meta-analysis. We show that interactions can be interpreted as heterogeneity and can be detected without utilizing the traditional approaches for discovery of gene-by-environment interactions, which treats the gene-by-environment interactions as covariates in the analysis. We provide a intuitive way to visualize the results of the meta-analysis at a locus which allows us to obtain the biological insights of gene-by-environment interactions. We demonstrate our method by searching for gene-by-environment interactions by combining 17 mouse genetic studies totaling 4,965 distinct animals.

Insulin receptor signaling in normal and insulin-resistant states

In the wake of the worldwide increase in type-2 diabetes, a major focus of research is understanding the signaling pathways impacting this disease. Insulin signaling regulates glucose, lipid, and energy homeostasis, predominantly via action on liver, skeletal muscle, and adipose tissue. Precise modulation of this pathway is vital for adaption as the individual moves from the fed to the fasted state. The positive and negative modulators acting on different steps of the signaling pathway, as well as the diversity of protein isoform interaction, ensure a proper and coordinated biological response to insulin in different tissues. Whereas genetic mutations are causes of rare and severe insulin resistance, obesity can lead to insulin resistance through a variety of mechanisms. Understanding these pathways is essential for development of new drugs to treat diabetes, metabolic syndrome, and their complications.

Impairment of insulin-stimulated glucose transport and ERK activation by adipocyte-specific knockout of PKC-λ produces a phenotype characterized by diminished adiposity and enhanced insulin suppression of hepatic gluconeogenesis

Tissue-specific knockout (KO) of atypical protein kinase C-λ (PKC-λ) impairs insulin-stimulated glucose transport in muscle (M) and lipid synthesis in liver (L), thereby producing insulin resistance in MλKO mice and insulin-hypersensitivity in LλKO mice. Here, we generated mice with KO of PKC-λ in adipocytes, i.e., AλKO mice. In isolated adipocytes of AλKO mice, insulin-stimulated aPKC activity and glucose transport were diminished, as were ERK levels and activity. Insulin-stimulated glucose transport and insulin activation of ERK in adipocytes of wild-type mice were similarly inhibited by acute inhibition of PKC-λ with a highly-specific chemical inhibitor. With impairments in glucose transport and ERK activation, AλKO mice had diminished adiposity and serum leptin levels. In addition, AλKO mice had normal glucose tolerance and insulin hypersensitivity owing to enhanced suppression of hepatic glucose output, which apparently reflected increases in Akt activity and FoxO1 phosphorylation, and subsequent decreases in expression of gluconeogenic phosphoenolpyruvate carboxykinase. We conclude that: PKC-λ is required for insulin-stimulated glucose transport and ERK signaling in mouse adipocytes; and diminution of these processes is attended by leanness and therefore hypoleptinemia. How these and perhaps other PKC-λ-dependent processes communicate to liver and improve insulin suppression of hepatic gluconeogenesis remains unclear.

Metabolic syndrome and insulin resistance: underlying causes and modification by exercise training

Metabolic syndrome (MS) is a collection of cardiometabolic risk factors that includes obesity, insulin resistance, hypertension, and dyslipidemia. Although there has been significant debate regarding the criteria and concept of the syndrome, this clustering of risk factors is unequivocally linked to an increased risk of developing type 2 diabetes and cardiovascular disease. Regardless of the true definition, based on current population estimates, nearly 100 million have MS. It is often characterized by insulin resistance, which some have suggested is a major underpinning link between physical inactivity and MS. The purpose of this review is to: (i) provide an overview of the history, causes and clinical aspects of MS, (ii) review the molecular mechanisms of insulin action and the causes of insulin resistance, and (iii) discuss the epidemiological and intervention data on the effects of exercise on MS and insulin sensitivity.

Cardiac lipotoxicity: molecular pathways and therapeutic implications

Diabetes and obesity are both associated with lipotoxic cardiomyopathy exclusive of coronary artery disease and hypertension. Lipotoxicities have become a public health concern and are responsible for a significant portion of clinical cardiac disease. These abnormalities may be the result of a toxic metabolic shift to more fatty acid and less glucose oxidation with concomitant accumulation of toxic lipids. Lipids can directly alter cellular structures and activate downstream pathways leading to toxicity. Recent data have implicated fatty acids and fatty acyl coenzyme A, diacylglycerol, and ceramide in cellular lipotoxicity, which may be caused by apoptosis, defective insulin signaling, endoplasmic reticulum stress, activation of protein kinase C, MAPK activation, or modulation of PPARs.

Skeletal myoblast transplantation on gene expression profiles of insulin signaling pathway and mitochondrial biogenesis and function in skeletal muscle

AIM

The study aims to investigate the gene expression profiling of insulin signaling pathway and mitochondrial biogenesis and function in the skeletal muscle of KK mice.

METHODS

KK mice were divided into the following groups: KK control group, basal medium (M199) only; KK fibroblast group, with human fibroblast transplantation; KK myoblast group, with human skeletal myoblast transplantation. C57BL mice received hSkM transplantation as a normal control. Cells were transplanted into mice hind limb skeletal muscle. All animals were treated with cyclosporine for 6 weeks only. The mice were sacrificed in a fasting state at 12 weeks after treatment. Hind limb skeletal muscle was harvested and used for study of gene expression profiling.

RESULTS

hSkMs survived extensively in mice skeletal muscle at 12 weeks after cell transplantation. Glucose tolerance test showed a significant decrease of blood glucose in the mice of KK myoblast group compared to the KK control and fibroblast groups. Transcriptional patterns of insulin signaling pathway showed alterations in KK myoblast as compared with KK control group (23 genes), KK fibroblast group (7 genes), and C57BL group (8 genes). Transcriptional patterns of mitochondrial biogenesis and function also had alterations in KK myoblast as compared with KK control group (27 genes), KK fibroblast group (9 genes), and C57BL group (6 genes).

CONCLUSIONS

These data demonstrated for the first time that hSKM transplantation resulted in a change of gene transcript in multiple genes involved in insulin signaling pathway and mitochondrial biogenesis and function.