The MEF2A and MEF2D isoforms are differentially regulated in muscle and adipose tissue during states of insulin deficiency

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.

CEBPβ binding directly to the promoter region drives CEBPɑ transcription and improves FABP4 transcriptional activity in adipose tissue of yak (Bos grunniens)

Fatty acid binding protein 4 (FABP4) was crucial to fatty acid uptake and intracellular transport. However, the mechanisms regulating yak (Bos grunniens) FABP4 transcription were not determined. In the current study, predominant expression levels of yak FABP4 were identified in subcutaneous fat and longissimus dorsi muscles by quantitative real-time polymerase chain reactions (qPCR). The CCAAT/enhancer binding protein alpha (CEBPα) and myocyte enhancer factor 2A (MEF2A), as transcriptional activator or repressor in the promoter region of FABP4, were confirmed by both site-directed mutagenesis experiment and chromatin immunoprecipitation assay. Additionally, molecular mechanisms of CEBPɑ regulation were analyzed to explore the transcriptional regulatory property of FABP4, which indicated that transcriptional activity of CEBPɑ depended on CCAAT/ enhancer binding protein beta (CEBPβ) transcription factor. Our results demonstrated that CEBPβ binding directly to the promoter region drove CEBPɑ transcription, improving yak FABP4 transcriptional activity in adipocytes. This mechanism expanded the information on the transcriptional regulatory network of adipogenesis.

Insulin Signal Transduction Perturbations in Insulin Resistance

Type 2 diabetes mellitus is a widespread medical condition, characterized by high blood glucose and inadequate insulin action, which leads to insulin resistance. Insulin resistance in insulin-responsive tissues precedes the onset of pancreatic β-cell dysfunction. Multiple molecular and pathophysiological mechanisms are involved in insulin resistance. Insulin resistance is a consequence of a complex combination of metabolic disorders, lipotoxicity, glucotoxicity, and inflammation. There is ample evidence linking different mechanistic approaches as the cause of insulin resistance, but no central mechanism is yet described as an underlying reason behind this condition. This review combines and interlinks the defects in the insulin signal transduction pathway of the insulin resistance state with special emphasis on the AGE-RAGE-NF-κB axis. Here, we describe important factors that play a crucial role in the pathogenesis of insulin resistance to provide directionality for the events. The interplay of inflammation and oxidative stress that leads to β-cell decline through the IAPP-RAGE induced β-cell toxicity is also addressed. Overall, by generating a comprehensive overview of the plethora of mechanisms involved in insulin resistance, we focus on the establishment of unifying mechanisms to provide new insights for the future interventions of type 2 diabetes mellitus.

Sucrose exposure during gestation lactation and postweaning periods increases the pubococcygeus muscle reflex activity in adult male rats

Erectile dysfunction is related to metabolic alterations produced by a high carbohydrate diet, which may affect muscle activity during penile reflex in adulthood. We determined whether sucrose water consumption during gestation lactation and postweaning affects pubococcygeus muscle (Pcm) activity during urethrogenital reflex in adult male rat offspring. Twelve female rats were mated and grouped in control mothers consumed tap water and sucrose mothers consumed sucrose water during gestation lactation. Male pups were weaned and assigned into four groups (n = 6 each): those from control mothers who continued drinking tap water (CM-CO group) or sucrose water (CM-SO group) until adult life, and those from sucrose mothers who drank tap water (SM-CO group) or continued drinking sucrose water (SM-SO group) until adult life. Body weight, Pcm activity during penile stimulation by bipolar electrodes and urethrogenital reflex were analyzed. A catheter was placed into the urethra to record variations in urethral pressure after mechanical stimulation. Two-way ANOVA followed by post hoc tests were used considering P ≤ 0.05 as a significant difference. Males from the SM-SO group showed weight gain compared to the control group (P < 0.001). Also, sucrose intake promoted high Pcm activity (P < 0.0001) but reduced urethrogenital reflex duration CM-CO vs CM-SO (P = 0.02); CM-CO vs SM-CO (P = 0.01); CM-SO vs SM-SO (P < 0.003); and SM-CO vs SM-SO (P < 0.002). Our results suggest that a combination of a sucrose-rich diet during gestation lactation and postweaning modifies Pcm activation during penile reflex. The urethrogenital reflex is a spinal ejaculatory-like reflex, these rats could have penile dysfunction equivalent to premature ejaculation in men.

Genome-wide scan for common variants associated with intramuscular fat and moisture content in rainbow trout

BACKGROUND

Genetic improvement of fillet quality attributes is a priority of the aquaculture industry. Muscle composition impacts quality attributes such as flavor, appearance, texture, and juiciness. Fat and moisture make up about ~ 80% of the tissue weight. The genetic architecture underlying the fat and moisture content of the muscle is still to be fully explored in fish. A 50 K gene transcribed SNP chip was used for genotyping 789 fish with available phenotypic data for fat and moisture content. Genotyped fish were obtained from two consecutive generations produced in the National Center for Cool and Cold Water Aquaculture (NCCCWA) growth-selective breeding program. Estimates of SNP effects from weighted single-step GBLUP (WssGBLUP) were used to perform genome-wide association (GWA) analysis to identify quantitative trait loci (QTL) associated with the studied traits.

RESULTS

Using genomic sliding windows of 50 adjacent SNPs, 137 and 178 SNPs were identified as associated with fat and moisture content, respectively. Chromosomes 19 and 29 harbored the highest number of SNPs explaining at least 2% of the genetic variation in fat and moisture content. A total of 61 common SNPs on chromosomes 19 and 29 affected the aforementioned traits; this association suggests common mechanisms underlying intramuscular fat and moisture content. Additionally, based on single-marker GWA analyses, 8 and 24 SNPs were identified in association with fat and moisture content, respectively.

CONCLUSION

SNP-harboring genes were primarily involved in lipid metabolism, cytoskeleton remodeling, and protein turnover. This work provides putative SNP markers that could be prioritized and used for genomic selection in breeding programs.

Programming Skeletal Muscle Metabolic Flexibility in Offspring of Male Rats in Response to Maternal Consumption of Slow Digesting Carbohydrates during Pregnancy

Skeletal muscle plays a relevant role in metabolic flexibility and fuel usage and the associated muscle metabolic inflexibility due to high-fat diets contributing to obesity and type 2 diabetes. Previous research from our group indicates that a high-fat and rapid-digesting carbohydrate diet during pregnancy promotes an excessive adipogenesis and also increases the risk of non-alcoholic fatty liver disease in the offspring. This effect can be counteracted by diets containing carbohydrates with similar glycemic load but lower digestion rates. To address the role of the skeletal muscle in these experimental settings, pregnant rats were fed high-fat diets containing carbohydrates with similar glycemic load but different digestion rates, a high fat containing rapid-digesting carbohydrates diet (HF/RD diet) or a high fat containing slow-digesting carbohydrates diet (HF/SD diet). After weaning, male offspring were fed a standard diet for 3 weeks (weaning) or 10 weeks (adolescence) and the impact of the maternal HF/RD and HF/SD diets on the metabolism, signaling pathways and muscle transcriptome was analyzed. The HF/SD offspring displayed better muscle features compared with the HF/RD group, showing a higher muscle mass, myosin content and differentiation markers that translated into a greater grip strength. In the HF/SD group, metabolic changes such as a higher expression of fatty acids (FAT/CD36) and glucose (GLUT4) transporters, an enhanced glycogen content, as well as changes in regulatory enzymes such as muscle pyruvate kinase and pyruvate dehydrogenase kinase 4 were found, supporting an increased muscle metabolic flexibility and improved muscle performance. The analysis of signaling pathways was consistent with a better insulin sensitivity in the muscle of the HF/SD group. Furthermore, increased expression of genes involved in pathways leading to muscle differentiation, muscle mass regulation, extracellular matrix content and insulin sensitivity were detected in the HF/SD group when compared with HF/RD animals. In the HF/SD group, the upregulation of the ElaV1/HuR gene could be one of the main regulators in the positive effects of the diet in early programming on the offspring. The long-lasting programming effects of the HF/SD diet during pregnancy may depend on a coordinated gene regulation, modulation of signaling pathways and metabolic flexibility that lead to an improved muscle functionality. The dietary early programming associated to HF/SD diet has synergic and positive crosstalk effects in several tissues, mainly muscle, liver and adipose tissue, contributing to maintain the whole body homeostasis in the offspring.

Dysfunctional EAT thickness may promote maladaptive heart remodeling in CVD patients through the ST2-IL33 system, directly related to EPAC protein expression

Dysfunctional epicardial adipose tissue (EAT) secretome can influence the heart’s stretch response. However, the molecular mechanisms are still poorly understood. The aim of this study was to clarify how dysfunctional EAT promotes maladaptive heart remodeling in cardiovascular disease (CVD) through ST2 production associated with exchange protein directly activated by cAMP (EPAC) proteins. A series of 55 CVD males were enrolled and their EAT thickness, LV mass and volumes were measured by echocardiography. Blood, plasma and EAT biopsies were collected for molecular and proteomic assays. Taking EAT thickness as a continuous variable there was a direct correlation between the ST2 cardiac stretch mediator and EAT thickness (r = 0.54, p < 0.01) and an inverse relation between the ST2 gene and IL-33 expression (r −0.50, p < 0.01). In the CVD population EPAC2 expression directly correlated with the ST2 gene (r = 0.74, p < 0.0001) causing an ST2/IL-33 system local (p < 0.001) and systemic (sST2 = 57.33 ± 3.22 and IL-33 = 0.53 ± 017 pg/mL; p < 0.0001) protein imbalance associated with maladaptive remodeling. This indicated that dysfunctional EAT is a source of both EPAC and ST2 protein and an EPAC2 isoform seems involved in ST2 production in adipose tissue. Both EPAC2 and ST2 expression were directly related to maladaptive heart remodeling indices, suggesting EAT measurements could be useful in the early assessment of CVD complications.

Cbl downregulation increases RBP4 expression in adipocytes of female mice

Obesity leads to adipose tissue dysfunction, insulin resistance and diabetes. Adipose tissue produces adipokines that contribute to regulate insulin sensitivity. In turn, insulin stimulates the production and release of some adipokines. Casitas-b-lymphoma proteins (c-Cbl, Cbl-b and Cbl3) are intracellular adaptor signalling proteins that are rapidly phosphorylated by activation of tyrosine kinase receptors. c-Cbl is rapidly phosphorylated by insulin in adipocytes. Here, we tested the hypothesis that Cbl signalling regulates adipokine expression in adipose tissue. We determined the adipokine profile of WAT of Cbl-/- and Cbl+/+ mice in the C57BL6 background. Female Cbl-/- mice exhibited altered expression of adiponectin, leptin and RBP4 in visceral adipose tissue, while no significant changes were seen in male mice. TNFα and IL6 levels were unaffected by Cbl depletion. RBP4 expression was unchanged in liver. Adipose tissue of Cbl-/- animals showed increased basal activation of extracellular regulated kinases (ERK1/2) compared to Cbl+/+. c-Cbl knockdown in 3T3L1 adipocytes also increased basal ERK phosphorylation and RBP4 expression. Inhibition of ERK1/2 phosphorylation in Cbl-depleted 3T3L1 adipocytes or in adipose tissue explants of Cbl-/- mice reduced RBP4 mRNA. 17β-Estradiol increased RBP4 mRNA in adipocytes. Cbl depletion did not change ER expression but increased phosphorylation of ERα at S118, a target site for ERK1/2. ERK1/2 inhibition reduced phosphoER and RBP4 levels. These findings suggest that Cbl contributes to regulate RBP4 expression in adipose of female mice through ERK1/2-mediated activation of ERα. Since Cbl signalling is compromised in diabetes, these data highlight a novel mechanism that upregulates RBP4 locally.

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.

The molecular characterization and temporal-spatial expression of myocyte enhancer factor 2 genes in the goat and their association with myofiber traits

The myocyte enhancer factor-2 (MEF2) gene family in vertebrates includes MEF2A, MEF2B, MEF2C, and MEF2D, which have important functions in the regulation of muscular growth and development. To investigate their temporal-spatial expression and functions in the goat, these genes were cloned (accession nos. JN967621-24) and their expression patterns characterized at five postnatal stages (3, 30, 60, 90, and 120days). Association analysis was then applied regarding MEF2 expression levels and myofiber diameter and density. MEF2B was shown to be weakly homologous with other species, the distant branches with other members and the lowest expression levels, suggesting that it is distinct from other family members. Expression of the other three MEF2 genes was widely distributed, but this was largely accumulated in the skeletal muscle and myocardium compared with the viscera at all developmental stages. MEF2A and MEF2D expression levels were higher overall than MEF2B and MEF2C in six tissues, and were significantly positively correlated with the myofiber diameter of the longissimus dorsi. These findings suggest that goat MEF2 genes mainly function in the skeletal muscle and myocardium, and that MEF2A and MEF2D are likely to effectively promote muscular growth and development during postnatal stages. MEF2A expression was highest in the myocardium, where MEF2C expression increased with age, implying that both gene products are related to the growth and development of postnatal myocardium.

The possible role of epigenetics in gestational diabetes: cause, consequence, or both

Gestational diabetes mellitus (GDM) is defined as the glucose intolerance that is not present or recognized prior to pregnancy. Several risk factors of GDM depend on environmental factors that are thought to regulate the genome through epigenetic mechanisms. Thus, epigenetic regulation could be involved in the development of GDM. In addition, the adverse intrauterine environment in patients with GDM could also have a negative impact on the establishment of the epigenomes of the offspring.

Chronic ethanol feeding impairs AMPK and MEF2 expression and is associated with GLUT4 decrease in rat myocardium

Chronic and heavy alcohol consumption is one of the causes of heart diseases. However, the effects of ethanol on insulin sensitivity in myocardium has been unclear. To investigate the effects of ethanol on the expression of AMP-activated protein kinase (AMPK), myocyte enhancer factor 2 (MEF2) and glucose transporter 4 (GLUT4), all of which are involved in the regulation of insulin sensitivity, in the myocardium, we performed three parts of experiments in vivo and in vitro. I: Rats were injected with 5-amino-4-imidazolecarboxamide ribonucleotide (AICAR, 0.8 mg.kg(-l)) for 2 h. II: Rats received different dose (0.5, 2.5 or 5 g.kg(-l).d(-l)) of ethanol for 22-week. III: Primary neonatal rat cardiomyocytes were isolated and treated with or without 100 mM ethanol or 1 mM AICAR for 4 h. The cardiac protein and mRNA expression of AMPKalpha subunits, MEF2 and GLUT4 were observed by western-blotting and RT-PCR, respectively. Serum TNFalpha levels were assessed by ELISA. The results showed chronic ethanol exposure induced insulin resistance. Ethanol decreased the mRNA levels of AMPKalpha1 and alpha2, the protein levels of total- and phospho-AMPKalpha in cardiomyocytes. Similarly, ethanol showed inhibitory effects on both the mRNA and protein levels of MEF2A and 2D, and GLUT4 in a dose-response-like fashion. Correlation analysis implied an association between phospho-AMPKalpha and MEF2A or MEF2D, and between the levels of MEF2 protein and GLUT4 transcription. In addition, ethanol elevated serum TNFalpha level. Taken together, chronic ethanol exposure decreases the expression of AMPKalpha and MEF2, and is associated with GLUT4 decline in rat myocardium.

Long-term ethanol exposure inhibits glucose transporter 4 expression via an AMPK-dependent pathway in adipocytes

AIM

The roles of AMP-activated protein kinase (AMPK) and myocyte enhancer factor 2 isoforms (MEF2A, D) as mediators of the effects of ethanol on glucose transporter 4 (GLUT4) expression are unclear. We studied the effects of ethanol in adipocytes in vivo and in vitro.

METHODS

Thirty-six male Wistar rats were divided into three groups and given ethanol in a single daily dose of 0, 0.5, or 5 g/kg for 22 weeks. The expression of AMPK, MEF2 isoforms A and D, and GLUT4 was measured and compared in the three groups. The existence of the AMPK/MEF2/GLUT4 pathway in adipocytes and the effects of ethanol on this pathway were studied in (a) epididymal adipose tissue from six male Wistar rats subcutaneously injected with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR, an AMPK activator) or with 0.9% NaCl (control); and (b) isolated rat and human adipocytes treated with or without ethanol, AICAR, and compound C (a selective AMPK inhibitor). Expression of AMPK, MEF2, and GLUT4 was measured by RT-PCR and Western blotting.

RESULTS

(1) Long-term ethanol exposure decreased activated AMPK, MEF2A, MEF2D, and GLUT4 expression in rat adipose tissue. (2) In rat and human adipocytes, AICAR-induced AMPK activation, with subsequent elevation of MEF2 and GLUT4 expression, was inhibited by compound C. (3) In vitro ethanol-treatment suppressed the AMPK/MEF2/GLUT4 pathway.

CONCLUSION

The AMPK/MEF2/GLUT4 pathway exists in both rat and human adipocytes, and activated AMPK may positively regulate MEF2 and GLUT4 expression. Ethanol inhibition of this pathway leads to decreased GLUT4 expression, thus reducing insulin sensitivity and glucose tolerance.

The glucose-lowering agent sodium tungstate increases the levels and translocation of GLUT4 in L6 myotubes through a mechanism associated with ERK1/2 and MEF2D

AIMS/HYPOTHESIS

The aim of this study was to investigate the action of the glucose-lowering compound sodium tungstate on glucose transport in muscle myotubes and to unravel the molecular events underlying the effects observed.

METHODS

We studied the effects of tungstate on 2-deoxy-D: -glucose uptake, levels and translocation of the glucose transporters GLUT4 and GLUT1, and Glut4 (also known as Slc2a4) promoter activity. We also measured the modifications of individual components of the signalling pathways involved in the effects observed.

RESULTS

Tungstate increased 2-deoxy-D: -glucose uptake in differentiated L6 myotubes through an increase in the total amount and translocation of GLUT4 transporter. The effects on glucose uptake were additive to those of insulin. Tungstate activated transcription of the Glut4 promoter, as shown by an increase in Glut4 mRNA, and by a promoter reporter assay. The assay of deletions of the Glut4 promoter indicated that the effect of tungstate is mediated by the myocyte enhancer factor 2 (MEF2)-binding domain. Accordingly, MEF2 levels and DNA binding activities were increased in response to the treatment. Tungstate-induced glucose uptake and GLUT4 transcriptional activation were dependent on the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), while no changes were observed in the phosphorylation state of the beta subunit of the insulin receptor, in the phosphatidylinositol 3-kinase pathway or in the activation of 5'AMP-activated protein kinase.

CONCLUSIONS/INTERPRETATION

Tungstate activates glucose uptake in myotubes through a novel ERK1/2-dependent mechanism. This effect is exerted by an increase in the content and translocation of the GLUT4 transporter. This is the first report of a glucose-lowering compound activating Glut4 transcription through an ERK1/2-dependent increase in MEF2 levels.

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

The insulin-responsive glucose transporter, GLUT4, is regulated in various physiologic states at the transcriptional level. When expressed in transgenic mice, the human GLUT4 promoter is governed by two cis-acting sequences: an MEF2 binding domain and Domain I, that function both as positive and negative regulators depending on the physiologic state. MEF2 proteins and GLUT4 enhancer factor (GEF) are known ligands for these cis-acting elements, but their mechanism of action is unclear. To begin to understand this important process, we have characterized GEF structural domains and its interactions with the MEF2A isoform. We find that the C terminus of GEF comprises its DNA-binding domain, but does not contribute to GEF homo-oligomerization. We also have found that GEF dimerizes with increased affinity to a hypophosphorylated form of MEF2A. Furthermore, we demonstrated that MEF2A binding to its cognate binding site can increase the DNA binding activity of GEF to Domain I, suggesting a novel mechanism for MEF2A transcriptional activation. Finally, we have demonstrated that the transcriptional co-repressor HDAC5 can interact with GEF in the absence of MEF2 proteins and specifically inhibit GLUT4 promoter activity. These findings lead to the hypothesis that GEF and the MEF2 proteins form a complex on the GLUT4 promoter that allows for recruitment of transcriptional co-regulators (repressors and/or activators) to control GLUT4 promoter activity.

Cardiomyocyte expression of PPARgamma leads to cardiac dysfunction in mice

Three forms of PPARs are expressed in the heart. In animal models, PPARgamma agonist treatment improves lipotoxic cardiomyopathy; however, PPARgamma agonist treatment of humans is associated with peripheral edema and increased heart failure. To directly assess effects of increased PPARgamma on heart function, we created transgenic mice expressing PPARgamma1 in the heart via the cardiac alpha-myosin heavy chain (alpha-MHC) promoter. PPARgamma1-transgenic mice had increased cardiac expression of fatty acid oxidation genes and increased lipoprotein triglyceride (TG) uptake. Unlike in cardiac PPARalpha-transgenic mice, heart glucose transporter 4 (GLUT4) mRNA expression and glucose uptake were not decreased. PPARgamma1-transgenic mice developed a dilated cardiomyopathy associated with increased lipid and glycogen stores, distorted architecture of the mitochondrial inner matrix, and disrupted cristae. Thus, while PPARgamma agonists appear to have multiple beneficial effects, their direct actions on the myocardium have the potential to lead to deterioration in heart function.

Hyperphosphorylation of MEF2A in primary adipocytes correlates with downregulation of human GLUT4 gene promoter activity

GLUT4 promoter activity is regulated by hormonal, metabolic, and tissue-specific controls. This complicates the study of GLUT4 gene transcription, as no cell culture model adequately recapitulates these extracellular regulators. While investigating cultured primary adipocytes as a model system for GLUT4 transcription, we observed that GLUT4 mRNA was specifically and rapidly downregulated upon tissue dispersal. Downregulation of GLUT4 mRNA was mediated in part by loss of regulatory control by the trans-acting factors that control GLUT4 transcriptional activity [the myocyte enhancer factor 2 (MEF2) transcription factor family and the GLUT4 enhancer factor] and their cognate DNA binding sites in transgenic mice. The differences in GLUT4 transcription when whole adipose tissue and cell culture model systems are compared can be correlated to a posttranslational phosphorylation of the transcription factor MEF2A. The difference in the MEF2A phosphorylation state in whole tissue vs. isolated cells may provide a further basis for the development of an in vitro system that could recapitulate fully regulated GLUT4 promoter activity. Development of an in vitro system to reconstitute GLUT4 transcriptional regulation will further efforts to discern the molecular mechanisms that underlie GLUT4 expression.

Activation of the Cbl insulin signaling pathway in cardiac muscle; Dysregulation in obesity and diabetes

In adipocytes, the Cbl/CAP dependent signaling pathway has been involved in regulating insulin-stimulated glucose uptake. We investigated activation of Cbl and its downstream effector TC10 in cardiac and skeletal muscle of Balb/C mice. Insulin administration resulted in Cbl phosphorylation in cardiac, skeletal muscle, and adipose tissue. Subsequent TC10 activation was detected only in heart and adipose tissue. c-Cbl and CAP gene expression was significantly reduced in the heart tissue of streptozotocin-induced diabetic animals, whereas no change was observed for other components of the pathway. No changes in Cbl expression were detected in hindlimb muscle. In leptin-/- obese mice Cbl expression in heart and adipose tissue was maintained, although insulin-mediated Cbl phosphorylation and subsequent TC10 activation were significantly reduced. In conclusion, our data demonstrate that Cbl/CAP/TC10 insulin signaling pathway is active in cardiac muscle and impaired during obesity and insulin deficiency.

Role of histone and transcription factor acetylation in diabetes pathogenesis

Globally, diabetes (and, in particular, type 2 diabetes) represents a major challenge to world health. Currently in the United States, the costs of treating diabetes and its associated complications exceed 100 billion US dollars annually, and this figure is expected to soar in the near future. Despite decades of intense research efforts, the genetic basis of the events involved in the pathogenesis of diabetes is still poorly understood. Diabetes is a complex multigenic syndrome primarily due to beta-cell dysfunction associated with a variable degree of insulin resistance. Recent advances have led to exciting new developments with regard to our understanding of the mechanisms that regulate insulin transcription. These include data that implicate chromatin as a critical regulator of this event. The 'Histone Code' is a widely accepted hypothesis, whereby sequential modifications to the histones in chromatin lead to regulated transcription of genes. One of the modifications used in the histone code is acetylation. This is probably the best characterized modification of histones, which is carried out under the control of histone acetyltransferases (HATs) and histone deacetylases (HDACs). These enzymes also regulate the activity of a number of transcription factors through acetylation. Increasing evidence links possible dysregulation of these mechanisms in the pathogenesis of diabetes, with important therapeutic implications.

Enhanced insulin-stimulated glycogen synthesis in response to insulin, metformin or rosiglitazone is associated with increased mRNA expression of GLUT4 and peroxisomal proliferator activator receptor gamma co-activator 1

AIMS/HYPOTHESIS

The aim of this study was to determine the effect of several antidiabetic agents on insulin-stimulated glycogen synthesis, as well as on mRNA expression.

METHODS

Cultured primary human skeletal myotubes obtained from six healthy subjects were treated for 4 or 8 days without or with glucose (25 mmol/l), insulin (400 pmol/l), rosiglitazone (10 micromol/l), metformin (20 micromol/l) or the AMP-activated kinase activator 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) (200 micromol/l). After this, insulin-stimulated glycogen synthesis was determined. mRNA levels of the glucose transporters GLUT1 and GLUT4, the peroxisomal proliferator activator receptor gamma (PPAR gamma) co-activator 1 (PGC1) and the myocyte-specific enhancer factors (MEF2), MEF2A, MEF2C and MEF2D were determined using real-time PCR analysis after 8 days exposure to the various antidiabetic agents.

RESULTS

Insulin-stimulated glycogen synthesis was significantly increased in cultured human myotubes treated with insulin, rosiglitazone or metformin for 8 days, compared with non-treated cells. Furthermore, an 8-day exposure of myotubes to 25 mmol/l glucose impaired insulin-stimulated glycogen synthesis. In contrast, treatment with AICAR was without effect on insulin-mediated glycogen synthesis. Exposure to insulin, rosiglitazone or metformin increased mRNA expression of PGC1 and GLUT4, while AICAR or 25 mmol/l glucose treatment increased GLUT1 mRNA expression. Metformin also increased mRNA expression of the MEF2 isoforms.

CONCLUSIONS/INTERPRETATION

Enhanced insulin-stimulated glycogen synthesis in human skeletal muscle cell culture coincides with increased GLUT4 and PGC1 mRNA expression following treatment with various antidiabetic agents. These data show that chronic treatment of human myotubes with insulin, metformin or rosiglitazone has a direct positive effect on insulin action and mRNA expression.

A potential link between muscle peroxisome proliferator- activated receptor-alpha signaling and obesity-related diabetes

The role of the peroxisome proliferator-activated receptor-alpha (PPARalpha) in the development of insulin-resistant diabetes was evaluated using gain- and loss-of-function approaches. Transgenic mice overexpressing PPARalpha in muscle (MCK-PPARalpha mice) developed glucose intolerance despite being protected from diet-induced obesity. Conversely, PPARalpha null mice were protected from diet-induced insulin resistance in the context of obesity. In skeletal muscle, MCK-PPARalpha mice exhibited increased fatty acid oxidation rates, diminished AMP-activated protein kinase activity, and reduced insulin-stimulated glucose uptake without alterations in the phosphorylation status of key insulin-signaling proteins. These effects on muscle glucose uptake involved transcriptional repression of the GLUT4 gene. Pharmacologic inhibition of fatty acid oxidation or mitochondrial respiratory coupling prevented the effects of PPARalpha on GLUT4 expression and glucose homeostasis. These results identify PPARalpha-driven alterations in muscle fatty acid oxidation and energetics as a potential link between obesity and the development of glucose intolerance and insulin resistance.

11beta-Hydroxysteroid dehydrogenase Type 1: genetic polymorphisms are associated with Type 2 diabetes in Pima Indians independently of obesity and expression in adipocyte and muscle

AIMS/HYPOTHESIS

The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) modulates tissue-specific glucocorticoid concentrations by generating active cortisol. We have shown that adipose tissue 11beta-HSD1 mRNA levels were associated with adiposity and insulinaemia. Here we conducted further expression and genetic association studies in Pima Indians.

METHODS

The 11beta-HSD1 mRNA concentrations were measured in abdominal subcutaneous adipocytes (n=61) and skeletal muscle tissues (n=64). Single nucleotide polymorphisms in the HSD11B1 gene were genotyped in a larger group of full-blooded Pima Indians.

RESULTS

Two representative SNPs (SNP1, n=706; SNP5, n=839) were associated with Type 2 diabetes mellitus (p=0.01), although neither SNP was associated with obesity. Among subjects with normal glucose tolerance, SNP1 (n=127) and SNP5 (n=159) were associated with insulin-mediated glucose uptake rates (p=0.03 and p=0.04), and SNP1 was further associated with fasting, 30-min, and 2-h plasma insulin concentrations (p=0.002, p=0.002 and p=0.03). Adipocyte 11beta-HSD1 mRNA concentrations were correlated positively with adiposity and insulinaemia, and were additionally negatively correlated with insulin-mediated glucose uptake rates; nevertheless, the adipocyte 11beta-HSD1 expression did not correlate with genotypes of the donors. The muscle 11beta-HSD1 mRNA concentrations did not correlate with any anthropometric or metabolic variables.

CONCLUSIONS/INTERPRETATION

We confirmed that adipocyte 11beta-HSD1 mRNA concentrations were associated with adiposity, and showed that genetic variations in the HSD11B1 gene were associated with Type 2 diabetes mellitus, plasma insulin concentrations and insulin action, independent of obesity. The variable adipose expression might not be a primary consequence of these HSD11B1 SNPs. Therefore, it is possible that the HSD11B1 gene is under tissue-specific regulation, and has tissue-specific consequences.

Regulation of the human GLUT4 gene promoter: interaction between a transcriptional activator and myocyte enhancer factor 2A

The GLUT4 gene is subject to complex tissue-specific and metabolic regulation, with a profound impact on insulin-mediated glucose disposal. We have shown, by using transgenic mice, that the human GLUT4 promoter is regulated through the cooperative function of two distinct regulatory elements, domain 1 and the myocyte enhancer factor 2 (MEF2) domain. The MEF2 domain binds transcription factors MEF2A and MEF2D in vivo. Domain I binds a transcription factor, GLUT4 enhancer factor (GEF). In this report, we show a restricted pattern of GEF expression in human tissues, which overlaps with MEF2A only in tissues expressing high levels of GLUT4, suggesting the hypothesis that GEF and MEF2A function together to activate GLUT4 transcription. Data obtained from transiently transfected cells support this hypothesis. Neither GEF nor MEF2A alone significantly activated GLUT4 promoter activity, but increased promoter activity 4- to 5-fold when expressed together. Deletion of the GEF-binding domain (domain I) and the MEF2-binding domain prevented activation, strengthening the conclusion that promoter regulation occurs through these elements. GEF and MEF2A, isolated from nuclei of transfected cells, bound domain I and the MEF2 domain, respectively, which is consistent with activation through these regulatory elements. Finally, GEF and MEF2A coimmunoprecipitated in vivo, strongly supporting a mechanism of GLUT4 transcription activation that depends on this protein-protein interaction.