GLUT4 enhancer factor (GEF) interacts with MEF2A and HDAC5 to regulate the GLUT4 promoter in adipocytes

GRK5 promoted renal fibrosis via HDAC5/Smad3 signaling pathway

Renal fibrosis is a common pathological feature of chronic kidney diseases (CKD), poses a significant burden in the aging population, and is a major cause of end-stage renal disease (ESRD). In this study, we investigated the role of G protein-coupled receptor kinases (GRKs) 5 in the pathogenesis of renal fibrosis. GRK5 is a serine/threonine kinase that regulates G protein-coupled receptor (GPCR) signaling. GRK5 has been shown to play a role in various diseases including cardiac disorders and cancer. However, the role of GRK5 in renal fibrosis remains largely unknown. Our finding revealed that GRK5 was significantly overexpressed in renal fibrosis. Specifically, GRK5 was transferred into the nucleus via its nuclear localization sequence to regulate histone deacetylases (HDAC) 5 expression under renal fibrosis. GRK5 acted as an upstream regulator of HDAC5/Smad3 signaling pathway. HDAC5 regulated and prevented the transcriptional activity of myocyte enhancer factor 2A (MEF2A) to repress the transcription of Smad7 which leading to the activation of Smad3. These findings first revealed that GRK5 may be a potential therapeutic target for the treatment of renal fibrosis. Inhibition of GRK5 activity may be a promising strategy to attenuate the progression of renal fibrosis.

Investigating the shared genetic architecture of uterine leiomyoma and breast cancer: A genome-wide cross-trait analysis

Little is known regarding the shared genetic architecture or causality underlying the phenotypic association observed for uterine leiomyoma (UL) and breast cancer (BC). Leveraging summary statistics from the hitherto largest genome-wide association study (GWAS) conducted in each trait, we investigated the genetic overlap and causal associations of UL with BC overall, as well as with its subtypes defined by the status of estrogen receptor (ER). We observed a positive genetic correlation between UL and BC overall (rg = 0.09, p = 6.00 × 10-3), which was consistent in ER+ subtype (rg = 0.06, p = 0.01) but not in ER- subtype (rg = 0.06, p = 0.08). Partitioning the whole genome into 1,703 independent regions, local genetic correlation was identified at 22q13.1 for UL with BC overall and with ER+ subtype. Significant genetic correlation was further discovered in 9 out of 14 functional categories, with the highest estimates observed in coding, H3K9ac, and repressed regions. Cross-trait meta-analysis identified 9 novel loci shared between UL and BC. Mendelian randomization demonstrated a significantly increased risk of BC overall (OR = 1.09, 95% CI = 1.01-1.18) and ER+ subtype (OR = 1.09, 95% CI = 1.01-1.17) for genetic liability to UL. No reverse causality was found. Our comprehensive genome-wide cross-trait analysis demonstrates a shared genetic basis, pleiotropic loci, as well as a putative causal relationship between UL and BC, highlighting an intrinsic link underlying these two complex female diseases.

Myocyte Enhancer Factor 2A Plays a Central Role in the Regulatory Networks of Cellular Physiopathology

Cell regulatory networks are the determinants of cellular homeostasis. Any alteration to these networks results in the disturbance of cellular homeostasis and induces cells towards different fates. Myocyte enhancer factor 2A (MEF2A) is one of four members of the MEF2 family of transcription factors (MEF2A-D). MEF2A is highly expressed in all tissues and is involved in many cell regulatory networks including growth, differentiation, survival and death. It is also necessary for heart development, myogenesis, neuronal development and differentiation. In addition, many other important functions of MEF2A have been reported. Recent studies have shown that MEF2A can regulate different, and sometimes even mutually exclusive cellular events. How MEF2A regulates opposing cellular life processes is an interesting topic and worthy of further exploration. Here, we reviewed almost all MEF2A research papers published in English and summarized them into three main sections: 1) the association of genetic variants in MEF2A with cardiovascular disease, 2) the physiopathological functions of MEF2A, and 3) the regulation of MEF2A activity and its regulatory targets. In summary, multiple regulatory patterns for MEF2A activity and a variety of co-factors cause its transcriptional activity to switch to different target genes, thereby regulating opposing cell life processes. The association of MEF2A with numerous signaling molecules establishes a central role for MEF2A in the regulatory network of cellular physiopathology.

Exercise and GLUT4

The glucose transporter GLUT4 is critical for skeletal muscle glucose uptake in response to insulin and muscle contraction/exercise. Exercise increases GLUT4 translocation to the sarcolemma and t-tubule and, over the longer term, total GLUT4 protein content. Here, we review key aspects of GLUT4 biology in relation to exercise, with a focus on exercise-induced GLUT4 translocation, postexercise metabolism and muscle insulin sensitivity, and exercise effects on GLUT4 expression.

Understanding Dietary Intervention-Mediated Epigenetic Modifications in Metabolic Diseases

The global prevalence of metabolic disorders, such as obesity, diabetes and fatty liver disease, is dramatically increasing. Both genetic and environmental factors are well-known contributors to the development of these diseases and therefore, the study of epigenetics can provide additional mechanistic insight. Dietary interventions, including caloric restriction, intermittent fasting or time-restricted feeding, have shown promising improvements in patients' overall metabolic profiles (i.e., reduced body weight, improved glucose homeostasis), and an increasing number of studies have associated these beneficial effects with epigenetic alterations. In this article, we review epigenetic changes involved in both metabolic diseases and dietary interventions in primary metabolic tissues (i.e., adipose, liver, and pancreas) in hopes of elucidating potential biomarkers and therapeutic targets for disease prevention and treatment.

Genomic landscape of metastatic breast cancer identifies preferentially dysregulated pathways and targets

Nearly all breast cancer deaths result from metastatic disease. Despite this, the genomic events that drive metastatic recurrence are poorly understood. We performed whole-exome and shallow whole-genome sequencing to identify genes and pathways preferentially mutated or copy-number altered in metastases compared with the paired primary tumors from which they arose. Seven genes were preferentially mutated in metastases - MYLK, PEAK1, SLC2A4RG, EVC2, XIRP2, PALB2, and ESR1 - 5 of which are not significantly mutated in any type of human primary cancer. Four regions were preferentially copy-number altered: loss of STK11 and CDKN2A/B, as well as gain of PTK6 and the membrane-bound progesterone receptor, PAQR8. PAQR8 gain was mutually exclusive with mutations in the nuclear estrogen and progesterone receptors, suggesting a role in treatment resistance. Several pathways were preferentially mutated or altered in metastases, including mTOR, CDK/RB, cAMP/PKA, WNT, HKMT, and focal adhesion. Immunohistochemical analyses revealed that metastases preferentially inactivate pRB, upregulate the mTORC1 and WNT signaling pathways, and exhibit nuclear localization of activated PKA. Our findings identify multiple therapeutic targets in metastatic recurrence that are not significantly mutated in primary cancers, implicate membrane progesterone signaling and nuclear PKA in metastatic recurrence, and provide genomic bases for the efficacy of mTORC1, CDK4/6, and PARP inhibitors in metastatic breast cancer.

Adaptations in GLUT4 Expression in Response to Exercise Detraining Linked to Downregulation of Insulin-Dependent Pathways in Cardiac but not in Skeletal Muscle Tissue

Insulin resistance is associated with cardiometabolic risk factors, and exercise training can improve insulin-mediated glucose uptake. However, few studies have demonstrated the reversibility of exercise-induced benefits. Thus, the authors examine the time-response effects of exercise training and detraining on glucose transporter 4 (GLUT4) content, insulin-dependent and insulin-independent pathways in cardiac and gastrocnemius muscle tissues of spontaneously hypertensive rats. Thirty-two male spontaneously hypertensive rats, 4 months old, were assigned to (n = 8/group): T (exercise training: 10-week treadmill exercise, 50-70% maximum effort capacity, 1 hr/day, 5 days/week); D2 (exercise training + 2-day detraining), D4 (exercise training + 4-day detraining); and S (no exercise). The authors evaluated insulin resistance, maximum effort capacity, GLUT4 content, p-IRS-1Tyr1179, p-AS160Ser588, p-AMPKα1Thr172, and p-CaMKIIThr286 in cardiac and gastrocnemius muscle tissues (Western blot). In response to exercise training, there were improvements in insulin resistance (15.4%; p = .010), increased GLUT4 content (microsomal, 29.4%; p = .012; plasma membrane, 27.1%; p < .001), p-IRS-1 (42.2%; p < .001), p-AS160 (60.0%; p < .001) in cardiac tissue, and increased GLUT4 content (microsomal, 29.4%; p = .009; plasma membrane, 55.5%; p < .001), p-IRS-1 (28.1%; p = .018), p-AS160 (76.0%; p < .001), p-AMPK-α1 (37.5%; p = .026), and p-CaMKII (30.0%; p = .040) in the gastrocnemius tissue. In D4 group, the exercise-induced increase in GLUT4 was reversed (plasma membrane, -21.3%; p = .027), p-IRS1 (-37.1%; p = .008), and p-AS160 (-82.6%; p < .001) in the cardiac tissue; p-AS160 expression (-35.7%; p = .034) was reduced in the gastrocnemius. In conclusion, the cardiac tissue is more susceptible to exercise adaptations in the GLUT4 content and signaling pathways than the gastrocnemius muscle. This finding may be explained by particular characteristics of insulin-dependent and insulin-independent pathways in the muscle tissues studied.

TFAM Enhances Fat Oxidation and Attenuates High-Fat Diet-Induced Insulin Resistance in Skeletal Muscle

Diet-induced insulin resistance (IR) adversely affects human health and life span. We show that muscle-specific overexpression of human mitochondrial transcription factor A (TFAM) attenuates high-fat diet (HFD)-induced fat gain and IR in mice in conjunction with increased energy expenditure and reduced oxidative stress. These TFAM effects on muscle are shown to be exerted by molecular changes that are beyond its direct effect on mitochondrial DNA replication and transcription. TFAM augmented the muscle tricarboxylic acid cycle and citrate synthase facilitating energy expenditure. TFAM enhanced muscle glucose uptake despite increased fatty acid (FA) oxidation in concert with higher β-oxidation capacity to reduce the accumulation of IR-related carnitines and ceramides. TFAM also increased pAMPK expression, explaining enhanced PGC1α and PPARβ, and reversing HFD-induced GLUT4 and pAKT reductions. TFAM-induced mild uncoupling is shown to protect mitochondrial membrane potential against FA-induced uncontrolled depolarization. These coordinated changes conferred protection to TFAM mice against HFD-induced obesity and IR while reducing oxidative stress with potential translational opportunities.

Histone deacetylases in cardiovascular and metabolic diseases

Histone deacetylases (HDACs) regulate gene transcription by catalyzing the removal of acetyl groups from key lysine residues in nucleosomal histones and via the recruitment of other epigenetic regulators to DNA promoter/enhancer regions. Over the past two decades, HDACs have been implicated in multiple processes pertinent to cardiovascular and metabolic diseases, including cardiac hypertrophy and remodeling, fibrosis, calcium handling, inflammation and energy metabolism. The development of small molecule HDAC inhibitors and genetically modified loss- and gain-of-function mouse models has allowed interrogation of the roles of specific HDAC isoforms in these processes. Isoform-selective HDAC inhibitors may prove to be powerful therapeutic agents for the treatment of cardiovascular diseases, obesity and diabetes.

The Role of the C-Clamp in Wnt-Related Colorectal Cancers

T-cell Factor/Lymphoid Enhancer Factor (TCF/LEF) transcription factors are major regulators of Wnt targets, and the products of the TCF7 and TCF7L2 genes have both been implicated in the progression of colorectal cancer in animal models and humans. TCFs recognize specific DNA sequences through their high mobility group (HMG) domains, but invertebrate TCFs and some isoforms of vertebrate TCF7 and TCF7L2 contain a second DNA binding domain known as the C-clamp. This review will cover the basic properties of C-clamps and their importance in Wnt signaling, using data from Drosophila, C. elegans, and mammalian cell culture. The connection between C-clamp containing TCFs and colorectal cancer will also be discussed.

Epigenetic changes in diabetes

The incidence of diabetes is increasing worldwide. Diabetes is quickly becoming one of the leading causes of death. Diabetes is a genetic disease; however, the environment plays critical roles in its development and progression. Epigenetic changes often translate environmental stimuli to changes in gene expression. Changes in epigenetic marks and differential regulation of epigenetic modulators have been observed in different models of diabetes and its associated complications. In this minireview, we will focus DNA methylation, Histone acetylation and methylation and their roles in the pathogenesis of diabetes.

Co-activator binding protein PIMT mediates TNF-α induced insulin resistance in skeletal muscle via the transcriptional down-regulation of MEF2A and GLUT4

The mechanisms underlying inflammation induced insulin resistance are poorly understood. Here, we report that the expression of PIMT, a transcriptional co-activator binding protein, was up-regulated in the soleus muscle of high sucrose diet (HSD) induced insulin resistant rats and TNF-α exposed cultured myoblasts. Moreover, TNF-α induced phosphorylation of PIMT at the ERK1/2 target site Ser²⁹⁸. Wild type (WT) PIMT or phospho-mimic Ser298Asp mutant but not phospho-deficient Ser298Ala PIMT mutant abrogated insulin stimulated glucose uptake by L6 myotubes and neonatal rat skeletal myoblasts. Whereas, PIMT knock down relieved TNF-α inhibited insulin signaling. Mechanistic analysis revealed that PIMT differentially regulated the expression of GLUT4, MEF2A, PGC-1α and HDAC5 in cultured cells and skeletal muscle of Wistar rats. Further characterization showed that PIMT was recruited to GLUT4, MEF2A and HDAC5 promoters and overexpression of PIMT abolished the activity of WT but not MEF2A binding defective mutant GLUT4 promoter. Collectively, we conclude that PIMT mediates TNF-α induced insulin resistance at the skeletal muscle via the transcriptional modulation of GLUT4, MEF2A, PGC-1α and HDAC5 genes.

MicroRNA-16 modulates macrophage polarization leading to improved insulin sensitivity in myoblasts

Uncontrolled inflammation leads to several diseases such as insulin resistance, T2D and several types of cancers. The functional role of microRNAs in inflammation induced insulin resistance is poorly studied. MicroRNAs are post-transcriptional regulatory molecules which mediate diverse biological processes. We here show that miR-16 expression levels are down-regulated in different inflammatory conditions such as LPS/IFNγ or palmitate treated macrophages, palmitate exposed myoblasts and insulin responsive tissues of high sucrose diet induced insulin resistant rats. Importantly, forced expression of miR-16 in macrophages impaired the production of TNF-α, IL-6 and IFN-β leading to enhanced insulin stimulated glucose uptake in co-cultured skeletal myoblasts. Further, ectopic expression of miR-16 enhanced insulin stimulated glucose uptake in skeletal myoblasts via the up-regulation of GLUT4 and MEF2A, two key players involved in insulin stimulated glucose uptake. Collectively, our data highlight the important role of miR-16 in ameliorating inflammation induced insulin resistance.

The role of butyrate, a histone deacetylase inhibitor in diabetes mellitus: experimental evidence for therapeutic intervention

The contribution of epigenetic mechanisms in diabetes mellitus (DM), β-cell reprogramming and its complications is an emerging concept. Recent evidence suggests that there is a link between DM and histone deacetylases (HDACs), because HDAC inhibitors promote β-cell differentiation, proliferation, function and improve insulin resistance. Moreover, gut microbes and diet-derived products can alter the host epigenome. Furthermore, butyrate and butyrate-producing microbes are decreased in DM. Butyrate is a short-chain fatty acid produced from the fermentation of dietary fibers by microbiota and has been proven as an HDAC inhibitor. The present review provides a pragmatic interpretation of chromatin-dependent and independent complex signaling/mechanisms of butyrate for the treatment of Type 1 and Type 2 DM, with an emphasis on the promising strategies for its drugability and therapeutic implication.

Selective class IIa HDAC inhibitors: myth or reality

The prospect of intervening, through the use of a specific molecule, with a cellular alteration responsible for a disease, is a fundamental ambition of biomedical science. Epigenetic-based therapies appear as a remarkable opportunity to impact on several disorders, including cancer. Many efforts have been made to develop small molecules acting as inhibitors of histone deacetylases (HDACs). These enzymes are key targets to reset altered genetic programs and thus to restore normal cellular activities, including drug responsiveness. Several classes of HDAC inhibitors (HDACis) have been generated, characterized and, in certain cases, approved for the use in clinic. A new frontier is the generation of subtype-specific inhibitors, to increase selectivity and to manage general toxicity. Here we will discuss about a set of molecules, which can interfere with the activity of a specific subclass of HDACs: the class IIa.

Enhancement of glucose uptake in mouse skeletal muscle cells and adipocytes by P2Y6 receptor agonists

Glucose uptake by peripheral tissues such as skeletal muscles and adipocytes is important in the maintenance of glucose homeostasis. We previously demonstrated that P2Y₆ receptor (P2Y₆R) agonists protect pancreatic islet cells from apoptosis and stimulate glucose-dependent insulin release. Here, we investigated the effects of P2Y₆R activation on glucose uptake in insulin target tissues. An agonist of the P2Y₆R, P¹-(5′-uridine)-P³-(5′-N⁴-methoxycytidine)-triphosphate (MRS2957), significantly increased the uptake of [³H]2-deoxyglucose in mouse C2C12 myotubes and 3T3-L1 adipocytes, and this stimulation was significantly decreased by a selective P2Y₆R antagonist N,N″-1,4-butanediyl-bis[N′-(3-isothiocyanatophenyl)thiourea] (MRS2578). Pre-incubation with Compound C (an inhibitor of 5′-AMP-activated protein kinase, AMPK), or AMPK siRNA abolished the stimulatory effect of MRS2957 on glucose uptake. Also, MRS2957 (60 min incubation) increased recruitment of the facilitated glucose transporter-4 (GLUT4) to the cell membrane, which was blocked by MRS2578. Treatment of C2C12 myotubes with MRS2957 induced significant phosphorylation of AMPK, which increase GLUT4 expression through histone deacetylase (HDAC)5 signaling. Glucose uptake in primary mouse adipocytes from wild-type mice was stimulated upon P2Y₆R activation by either MRS2957 or native agonist UDP, and the P2Y₆R effect was antagonized by MRS2578. However, in adipocytes from P2Y₆R-knockout mice P2Y₆R agonists had no effect on glucose uptake, and there was no change in the glucose uptake by insulin. Our results indicate that the P2Y₆R promotes glucose metabolism in peripheral tissues, which may be mediated through AMPK signaling.

Rab5 activity regulates GLUT4 sorting into insulin-responsive and non-insulin-responsive endosomal compartments: a potential mechanism for development of insulin resistance

Glucose transporter isoform 4 (GLUT4) is the insulin-responsive glucose transporter mediating glucose uptake in adipose and skeletal muscle. Reduced GLUT4 translocation from intracellular storage compartments to the plasma membrane is a cause of peripheral insulin resistance. Using a chronic hyperinsulinemia (CHI)-induced cell model of insulin resistance and Rab5 mutant overexpression, we determined these manipulations altered endosomal sorting of GLUT4, thus contributing to the development of insulin resistance. We found that CHI induced insulin resistance in 3T3-L1 adipocytes by retaining GLUT4 in a Rab5-activity-dependent compartment that is unable to equilibrate with the cell surface in response to insulin. Furthermore, CHI-mediated retention of GLUT4 in this non-insulin-responsive compartment impaired filling of the transferrin receptor (TfR)-positive and TfR-negative insulin-responsive storage compartments. Our data suggest that hyperinsulinemia may inhibit GLUT4 by chronically maintaining GLUT4 in the Rab5 activity-dependent endosomal pathway and impairing formation of the TfR-negative and TfR-positive insulin-responsive GLUT4 pools. This model suggests that an early event in the development of insulin-resistant glucose transport in adipose tissue is to alter the intracellular localization of GLUT4 to a compartment that does not efficiently equilibrate with the cell surface when insulin levels are elevated for prolonged periods of time.

Insulin acutely triggers transcription of Slc2a4 gene: participation of the AT-rich, E-box and NFKB-binding sites

AIMS

The insulin-sensitive glucose transporter protein GLUT4 (solute carrier family 2 member 4 (Slc2a4) gene) plays a key role in glycemic homeostasis. Decreased GLUT4 expression is a current feature in insulin resistant conditions such as diabetes, and the restoration of GLUT4 content improves glycemic control. This study investigated the effect of insulin upon Slc2a4/GLUT4 expression, focusing on the AT-rich element, E-box and nuclear factor NF-kappa-B (NFKB) site.

MAIN METHODS

Rat soleus muscles were incubated during 180 min with insulin, added or not with wortmannin (phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit gamma isoform (PI3K)-inhibitor), ML9 (serine/threonine protein kinase (AKT) inhibitor) and tumor necrosis factor (TNF, GLUT4 repressor), and processed for analysis of GLUT4 protein (Western blotting); Slc2a4, myocyte enhancer factor 2a/d (Mef2a/d), hypoxia inducible factor 1a (Hif1a), myogenic differentiation 1 (Myod1) and nuclear factor of kappa light polypeptide gene enhancer in B-cells 1 (Nfkb1) messenger ribonucleic acids (mRNAs) (polymerase chain reaction (PCR)); and AT-rich- (myocyte-specific enhancer factor 2 (MEF2)-binding site), E-box- (hypoxia inducible factor 1 alpha (HIF1A)- and myoblast determination protein 1 (MYOD1)-binding site), and NFKB-binding activity (electrophoretic mobility assay).

KEY FINDINGS

Insulin increased Slc2a4 mRNA expression (140%) and nuclear proteins binding to AT-rich and E-box elements (~90%), all effects were prevented by wortmannin and ML9. Insulin also increased Mef2a/d and Myod1 mRNA expression, suggesting the participation of these transcriptional factors in the Slc2a4 enhancing effect. Conversely, insulin decreased Nfkb1 mRNA expression and protein binding to the NFKB-site (~50%). Furthermore, TNF-induced inhibition of GLUT4 expression (~40%) was prevented by insulin in an NFKB-binding repressing mechanism. GLUT4 protein paralleled the Slc2a4 mRNA regulations.

SIGNIFICANCE

Insulin enhances the Slc2a4/GLUT4 expression in the skeletal muscle by activating AT-rich and E-box elements, in a PI3K/AKT-dependent mechanism, and repressing NFKB-site activity as well. These results unravel how post-prandial increase of insulin may guarantee GLUT4 expression, and how the insulin signaling impairment can participate in insulin resistance-induced repression of GLUT4.

Computational identification of potential transcriptional regulators of TGF-ß1 in human atherosclerotic arteries

TGF-ß is protective in atherosclerosis but deleterious in metastatic cancers. Our aim was to determine whether TGF-ß transcriptional regulation is tissue-specific in early atherosclerosis. The computational methods included 5 steps: (i) from microarray data of human atherosclerotic carotid tissue, to identify the 10 best co-expressed genes with TGFB1 (TGFB1 gene cluster), (ii) to choose the 11 proximal promoters, (iii) to predict the TFBS shared by the promoters, (iv) to identify the common TFs co-expressed with the TGFB1 gene cluster, and (v) to compare the common TFs in the early lesions to those identified in advanced atherosclerotic lesions and in various cancers. Our results show that EGR1, SP1 and KLF6 could be responsible for TGFB1 basal expression, KLF6 appearing specific to atherosclerotic lesions. Among the TFs co-expressed with the gene cluster, transcriptional activators (SLC2A4RG, MAZ) and repressors (ZBTB7A, PATZ1, ZNF263) could be involved in the fine-tuning of TGFB1 expression in atherosclerosis.

Loss of ACE2 exaggerates high-calorie diet-induced insulin resistance by reduction of GLUT4 in mice

ACE type 2 (ACE2) functions as a negative regulator of the renin-angiotensin system by cleaving angiotensin II (AII) into angiotensin 1-7 (A1-7). This study assessed the role of endogenous ACE2 in maintaining insulin sensitivity. Twelve-week-old male ACE2 knockout (ACE2KO) mice had normal insulin sensitivities when fed a standard diet. AII infusion or a high-fat, high-sucrose (HFHS) diet impaired glucose tolerance and insulin sensitivity more severely in ACE2KO mice than in their wild-type (WT) littermates. The strain difference in glucose tolerance was not eliminated by an AII receptor type 1 (AT1) blocker but was eradicated by A1-7 or an AT1 blocker combined with the A1-7 inhibitor (A779). The expression of GLUT4 and a transcriptional factor, myocyte enhancer factor (MEF) 2A, was dramatically reduced in the skeletal muscles of the standard diet-fed ACE2KO mice. The expression of GLUT4 and MEF2A was increased by A1-7 in ACE2KO mice and decreased by A779 in WT mice. A1-7 enhanced upregulation of MEF2A and GLUT4 during differentiation of myoblast cells. In conclusion, ACE2 protects against high-calorie diet-induced insulin resistance in mice. This mechanism may involve the transcriptional regulation of GLUT4 via an A1-7-dependent pathway.

Regulation of GLUT4 and Insulin-Dependent Glucose Flux

GLUT4 has long been known to be an insulin responsive glucose transporter. Regulation of GLUT4 has been a major focus of research on the cause and prevention of type 2 diabetes. Understanding how insulin signaling alters the intracellular trafficking of GLUT4 as well as understanding the fate of glucose transported into the cell by GLUT4 will be critically important for seeking solutions to the current rise in diabetes and metabolic disease.

Transcriptional and translational regulation of cytokine signaling in inflammatory β-cell dysfunction and apoptosis

Disease is conventionally viewed as the chaotic inappropriate outcome of deranged tissue function resulting from aberrancies in cellular processes. Yet the patho-biology of cellular dysfunction and death encompasses a coordinated network no less sophisticated and regulated than maintenance of homeostatic balance. Cellular demise is far from passive subordination to stress but requires controlled coordination of energy-requiring activities including gene transcription and protein translation that determine the graded transition between defensive mechanisms, cell cycle regulation, dedifferentiation and ultimately to the activation of death programmes. In fact, most stressors stimulate both homeostasis and regeneration on one hand and impairment and destruction on the other, depending on the ambient circumstances. Here we illustrate this bimodal ambiguity in cell response by reviewing recent progress in our understanding of how the pancreatic β cell copes with inflammatory stress by changing gene transcription and protein translation by the differential and interconnected action of reactive oxygen and nitric oxide species, microRNAs and posttranslational protein modifications.

The role of CaMKII in regulating GLUT4 expression in skeletal muscle

Contractile activity during physical exercise induces an increase in GLUT4 expression in skeletal muscle, helping to improve glucose transport capacity and insulin sensitivity. An important mechanism by which exercise upregulates GLUT4 is through the activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in response to elevated levels of cytosolic Ca(2+) during muscle contraction. This review discusses the mechanism by which Ca(2+) activates CaMKII, explains research techniques currently used to alter CaMK activity in cells, and highlights various exercise models and pharmacological agents that have been used to provide evidence that CaMKII plays an important role in regulating GLUT4 expression. With regard to transcriptional mechanisms, the key research studies that identified myocyte enhancer factor 2 (MEF2) and GLUT4 enhancer factor as the major transcription factors regulating glut4 gene expression, together with their binding domains, are underlined. Experimental evidence showing that CaMK activation induces hyperacetylation of histones in the vicinity of the MEF2 domain and increases MEF2 binding to its cis element to influence MEF2-dependent Glut4 gene expression are also given along with data suggesting that p300 might be involved in acetylating histones on the Glut4 gene. Finally, an appraisal of the roles of other calcium- and non-calcium-dependent mechanisms, including the major HDAC kinases in GLUT4 expression, is also given.

Class II histone deacetylases downregulate GLUT4 transcription in response to increased cAMP signaling in cultured adipocytes and fasting mice

Insulin-mediated glucose uptake is highly sensitive to the levels of the facilitative glucose transporter protein, GLUT4. Repression of GLUT4 expression is correlated with insulin resistance in adipose tissue. We have shown that differentiation-dependent GLUT4 transcription was under control of class II histone deacetylases (HDACs). We hypothesized that HDACs may regulate gene expression in adipocytes as a result of adrenergic activation. To test this hypothesis, we activated cAMP signaling in 3T3-L1 adipocytes and in mice after an overnight fast. Chromatin immunoprecipitation experiments showed the association of HDAC4/5 with the GLUT4 promoter in vivo and in vitro in response to elevated cAMP. Knockdown of HDACs by small interfering RNA in cultured adipocytes prevented the cAMP-dependent decrease in GLUT4 transcription. HDAC4/5 recruitment to the GLUT4 promoter was dependent on the GLUT4 liver X receptor (LXR) binding site. Treatment of cells with an LXR agonist prevented the cAMP-dependent decrease in GLUT4 transcription. A loss of function mutation in the LXR response element was required for cAMP-dependent downregulation of GLUT4 expression in vitro, in fasted mice, and in mice subjected to diet-induced obesity. This suggests that activation of LXR signaling can prevent loss of GLUT4 expression in diabetes and obesity.

Epigenetic regulation of glucose transporters in non-small cell lung cancer

Due to their inherently hypoxic environment, cancer cells often resort to glycolysis, or the anaerobic breakdown of glucose to form ATP to provide for their energy needs, known as the Warburg effect. At the same time, overexpression of the insulin receptor in non-small cell lung cancer (NSCLC) is associated with an increased risk of metastasis and decreased survival. The uptake of glucose into cells is carried out via glucose transporters or GLUTs. Of these, GLUT-4 is essential for insulin-stimulated glucose uptake. Following treatment with the epigenetic targeting agents histone deacetylase inhibitors (HDACi), GLUT-3 and GLUT-4 expression were found to be induced in NSCLC cell lines, with minimal responses in transformed normal human bronchial epithelial cells (HBECs). Similar results for GLUT-4 were observed in cells derived from liver, muscle, kidney and pre-adipocytes. Bioinformatic analysis of the promoter for GLUT-4 indicates that it may also be regulated by several chromatin binding factors or complexes including CTCF, SP1 and SMYD3. Chromatin immunoprecipitation studies demonstrate that the promoter for GLUT-4 is dynamically remodeled in response to HDACi. Overall, these results may have value within the clinical setting as (a) it may be possible to use this to enhance fluorodeoxyglucose (18F) positron emission tomography (FDG-PET) imaging sensitivity; (b) it may be possible to target NSCLC through the use of HDACi and insulin mediated uptake of the metabolic targeting drugs such as 2-deoxyglucose (2-DG); or (c) enhance or sensitize NSCLC to chemotherapy.

Histone deacetylase (HDAC) inhibition as a novel treatment for diabetes mellitus

Both common forms of diabetes have an inflammatory pathogenesis in which immune and metabolic factors converge on interleukin-1β as a key mediator of insulin resistance and β-cell failure. In addition to improving insulin resistance and preventing β-cell inflammatory damage, there is evidence of genetic association between diabetes and histone deacetylases (HDACs); and HDAC inhibitors (HDACi) promote β-cell development, proliferation, differentiation and function and positively affect late diabetic microvascular complications. Here we review this evidence and propose that there is a strong rationale for preclinical studies and clinical trials with the aim of testing the utility of HDACi as a novel therapy for diabetes.

Class II histone deacetylases limit GLUT4 gene expression during adipocyte differentiation

Insulin-dependent glucose homeostasis is highly sensitive to the levels of insulin-responsive glucose transporter 4 (GLUT4) expression in adipocytes. The level of GLUT4 protein expression is highly dependent on the rate of GLUT4 gene transcription. GLUT4 gene transcription is decreased in a variety of physiologic states of insulin resistance including type 2 diabetes, obesity, and prolonged fasting. GLUT4 gene expression in adipocytes is differentiation-dependent, with full expression delayed until late in the differentiation program. In this paper, we have tested the hypothesis that differentiation-dependent GLUT4 gene expression in 3T3-L1 adipocytes is dependent on the nuclear concentration of a class II histone deacetylase (HDAC) protein, HDAC5. We have tested this hypothesis by reducing the levels of class II HDACs in the nuclear compartment of 3T3-L1 preadipocytes using two experimental approaches. First, preadipocytes were treated with phenylephrine, an α-adrenergic receptor agonist, to drive HDACS out of the nuclear compartment. Also, the class II HDAC concentrations were reduced using siRNA knockdown. In each case, reduction of nuclear class II HDAC concentration resulted in increased expression of endogenous GLUT4 mRNA in preadipocytes. Together, our data indicate that class II HDAC expression is the major regulatory mechanism for inhibiting GLUT4 expression in the predifferentiated state.

The beneficial effects of exercise in rodents are preserved after detraining: a phenomenon unrelated to GLUT4 expression

BACKGROUND

Although exercise training has well-known cardiorespiratory and metabolic benefits, low compliance with exercise training programs is a fact, and the harmful effects of physical detraining regarding these adaptations usually go unnoticed. We investigated the effects of exercise detraining on blood pressure, insulin sensitivity, and GLUT4 expression in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY).

METHODS

Studied animals were randomized into sedentary, trained (treadmill running/5 days a week, 60 min/day for 10 weeks), 1 week of detraining, and 2 weeks of detraining. Blood pressure (tail-cuff system), insulin sensitivity (kITT), and GLUT4 (Western blot) in heart, gastrocnemius and white fat tissue were measured.

RESULTS

Exercise training reduced blood pressure (19%), improved insulin sensitivity (24%), and increased GLUT4 in the heart (+34%); gastrocnemius (+36%) and fat (+22%) in SHR. In WKY no change in either blood pressure or insulin sensitivity were observed, but there was an increase in GLUT4 in the heart (+25%), gastrocnemius (+45%) and fat (+36%) induced by training. Both periods of detraining did not induce any change in neither blood pressure nor insulin sensitivity in SHR and WKY. One-week detraining reduced GLUT4 in SHR (heart: -28%; fat: -23%) and WKY (heart: -19%; fat: -22%); GLUT4 in the gastrocnemius was reduced after a 2-week detraining (SHR: -35%; WKY: -25%). There was a positive correlation between GLUT4 (gastrocnemius) and the maximal velocity in the exercise test (r = 0.60, p = 0.004).

CONCLUSIONS

The study findings show that in detraining, despite reversion of the enhanced GLUT4 expression, cardiorespiratory and metabolic beneficial effects of exercise are preserved.

Heterogeneous myocyte enhancer factor-2 (Mef2) activation in myocytes predicts focal scarring in hypertrophic cardiomyopathy

Unknown molecular responses to sarcomere protein gene mutations account for pathologic remodeling in hypertrophic cardiomyopathy (HCM), producing myocyte growth and increased cardiac fibrosis. To determine if hypertrophic signals activated myocyte enhancer factor-2 (Mef2), we studied mice carrying the HCM mutation, myosin heavy-chain Arg403Gln, (MHC(403/+)) and an Mef2-dependent β-galactosidase reporter transgene. In young, prehypertrophic MHC(403/+) mice the reporter was not activated. In hypertrophic hearts, activation of the Mef2-dependent reporter was remarkably heterogeneous and was observed consistently in myocytes that bordered fibrotic foci with necrotic cells, MHC(403/+) myocytes with Mef2-dependent reporter activation reexpressed the fetal myosin isoform (βMHC), a molecular marker of hypertrophy, although MHC(403/+) myocytes with or without βMHC expression were comparably enlarged over WT myocytes. To consider Mef2 roles in severe HCM, we studied homozygous MHC(403/403) mice, which have accelerated remodeling, widespread myocyte necrosis, and neonatal lethality. Levels of phosphorylated class II histone deacetylases that activate Mef2 were substantially increased in MHC(403/403) hearts, but Mef2-dependent reporter activation was patchy. Sequential analyses showed myocytes increased Mef2-dependent reporter activity before death. Our data dissociate myocyte hypertrophy, a consistent response in HCM, from heterogeneous Mef2 activation and reexpression of a fetal gene program. The temporal and spatial relationship of Mef2-dependent gene activation with myocyte necrosis and fibrosis in MHC(403/+) and MHC(403/403) hearts defines Mef2 activation as a molecular signature of stressed HCM myocytes that are poised to die.

Acute inhibition of fatty acid import inhibits GLUT4 transcription in adipose tissue, but not skeletal or cardiac muscle tissue, partly through liver X receptor (LXR) signaling

OBJECTIVE

Insulin-mediated glucose uptake is highly sensitive to the levels of the facilitative GLUT protein GLUT4. Transcription of the GLUT4 gene is repressed in states of insulin deficiency and insulin resistance and can be induced by states of enhanced energy output, such as exercise. The cellular signals that regulate GLUT4 transcription are not well understood. We hypothesized that changes in energy substrate flux regulate GLUT4 transcription.

RESEARCH DESIGN AND METHODS

To test this hypothesis, we used transgenic mice in which expression of the chloramphenicol acetyltransferase (CAT) gene is driven by a functional 895-bp fragment of the human GLUT4 promoter, thereby acting as a reporter for transcriptional activity. Mice were treated with a single dose of etomoxir, which inhibits the transport of long-chain fatty acids into mitochondria and increases basal, but not insulin-mediated, glucose flux. GLUT4 and transgenic CAT mRNA were measured.

RESULTS

Etomoxir treatment significantly reduced CAT and GLUT4 mRNA transcription in adipose tissue, but did not change transcription in heart and skeletal muscle. Downregulation of GLUT4 transcription was cell autonomous, since etomoxir treatment of 3T3-L1 adipocytes resulted in a similar downregulation of GLUT4 mRNA. GLUT4 transcriptional downregulation required the putative liver X receptor (LXR) binding site in the human GLUT4 gene promoter in adipose tissue and 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with the LXR agonist, TO901317, partially restored GLUT4 expression in etomoxir-treated cells.

CONCLUSIONS

Our data suggest that long-chain fatty acid import into mitochondria in adipose tissue may produce ligands that regulate expression of metabolic genes.

Multiple signalling pathways redundantly control glucose transporter GLUT4 gene transcription in skeletal muscle

Increased glucose transporter GLUT4 expression in skeletal muscle is an important benefit of regular exercise, resulting in improved insulin sensitivity and glucose tolerance. The Ca(2+)-calmodulin-dependent kinase II (CaMKII), calcineurin and AMPK pathways have been implicated in GLUT4 gene regulation based on pharmacological evidence. Here, we have used a more specific genetic approach to establish the relative role of the three pathways in fast and slow muscles. Plasmids coding for protein inhibitors of CaMKII or calcineurin were co-transfected in vivo with a GLUT4 enhancer-reporter construct either in normal mice or in mice expressing a kinase dead (KD) AMPK mutant. GLUT4 reporter activity was not inhibited in the slow soleus muscle by blocking either CaMKII or calcineurin alone, but was inhibited by blocking both pathways. GLUT4 reporter activity was likewise unchanged in the soleus of KD-AMPK mice, but was significantly reduced by incapacitation of either CaMKII or calcineurin in these mice. On the other hand, in the fast tibialis anterior (TA) muscle, calcineurin appears to exert a prominent role in the control of GLUT4 reporter activity, independent of CaMKII and AMPK. The results point to a muscle type-specific and redundant regulation of GLUT4 enhancer based on the interplay of multiple signalling pathways, all of which are known to affect myocyte enhancing factor 2 (MEF2) transcriptional activity, a point of convergence of different pathways on muscle gene regulation.

AMP-activated protein kinase in contraction regulation of skeletal muscle metabolism: necessary and/or sufficient?

In skeletal muscle, the contraction-activated heterotrimeric 5'-AMP-activated protein kinase (AMPK) protein is proposed to regulate the balance between anabolic and catabolic processes by increasing substrate uptake and turnover in addition to regulating the transcription of proteins involved in mitochondrial biogenesis and other aspects of promoting an oxidative muscle phenotype. Here, the current knowledge on the expression of AMPK subunits in human quadriceps muscle and evidence from rodent studies suggesting distinct AMPK subunit expression pattern in different muscle types is reviewed. Then, the intensity and time dependence of AMPK activation in human quadriceps and rodent muscle are evaluated. Subsequently, a major part of this review critically examines the evidence supporting a necessary and/or sufficient role of AMPK in a broad spectrum of skeletal muscle contraction-relevant processes. These include glucose uptake, glycogen synthesis, post-exercise insulin sensitivity, fatty acid (FA) uptake, intramuscular triacylglyceride hydrolysis, FA oxidation, suppression of protein synthesis, proteolysis, autophagy and transcriptional regulation of genes relevant to promoting an oxidative phenotype.

Insulin regulates fusion of GLUT4 vesicles independent of Exo70-mediated tethering

Insulin regulates cellular glucose uptake by changing the amount of glucose transporter-4 (GLUT4) in the plasma membrane through stimulation of GLUT4 exocytosis. However, how the particular trafficking, tethering, and fusion steps are regulated by insulin is still debated. In a 3T3-L1 adipocyte cell line, the Exocyst complex and its Exo70 subunit were shown to critically affect GLUT4 exocytosis. Here we investigated the effects of Exo70 on tethering and fusion of GLUT4 vesicles in primary isolated rat adipose cells. We found that Exo70 wild type was sequestered away from the plasma membrane in non-stimulated cells, and its overexpression had no effect on GLUT4 trafficking. The addition of insulin increased the amount of Exo70 in the vicinity of the plasma membrane and stimulated the tethering and fusion of GLUT4 vesicles, but the rates of fusion and GLUT4 exposure were not affected by overexpression of Exo70. Surprisingly, the Exo70-N mutant induced insulin-independent tethering of GLUT4 vesicles, which, however, did not lead to fusion and exposure of GLUT4 at the plasma membrane. Upon insulin stimulation, the stationary pretethered GLUT4 vesicles in Exo70-N mutant cells underwent fusion without relocation. Taken together, our data suggest that fusion of GLUT4 vesicles is the rate-limiting step regulated by insulin downstream of Exo70-mediated tethering.

Board-invited review: the biology and regulation of preadipocytes and adipocytes in meat animals

The quality and value of the carcass in domestic meat animals are reflected in its protein and fat content. Preadipocytes and adipocytes are important in establishing the overall fatness of a carcass, as well as being the main contributors to the marbling component needed for consumer preference of meat products. Although some fat accumulation is essential, any excess fat that is deposited into adipose depots other than the marbling fraction is energetically unfavorable and reduces efficiency of production. Hence, this review is focused on current knowledge about the biology and regulation of the important cells of adipose tissue: preadipocytes and adipocytes.