Factors involved in GLUT-1 glucose transporter gene transcription in cardiac muscle

Cadmium contributes to cardiac metabolic disruption by activating endothelial HIF1A-GLUT1 axis

Cadmium (Cd) is an environmental risk factor of cardiovascular diseases. Researchers have found that Cd exposure causes energy metabolic disorders in the heart decades ago. However, the underlying molecular mechanisms are still elusive. In this study, male C57BL/6 J mice were exposed to cadmium chloride (CdCl2) through drinking water for 4 weeks. We found that exposure to CdCl2 increased glucose uptake and utilization, and disrupted normal metabolisms in the heart. In vitro studies showed that CdCl2 specifically increased endothelial glucose uptake without affecting cardiomyocytic glucose uptake and endothelial fatty acid uptake. The glucose transporter 1 (GLUT1) as well as its transcription factor HIF1A was significantly increased after CdCl2 treatment in endothelial cells. Further investigations found that CdCl2 treatment upregulated HIF1A expression by inhibiting its degradation through ubiquitin-proteasome pathway, thereby promoted its transcriptional activation of SLC2A1. Administration of HIF1A small molecule inhibitor echinomycin and A-485 reversed CdCl2-mediated increase of glucose uptake in endothelial cells. In accordance with this, intravenous injection of echinomycin effectively ameliorated CdCl2-mediated metabolic disruptions in the heart. Our study uncovered the molecular mechanisms of Cd in contributing cardiac metabolic disruption by inhibiting HIF1A degradation and increasing GLUT1 transcriptional expression. Inhibition of HIF1A could be a potential strategy to ameliorate Cd-mediated cardiac metabolic disorders and Cd-related cardiovascular diseases.

MicroRNAs-mediated regulation of glucose transporter (GLUT) expression in glioblastoma

Current knowledge about the role of microRNAs (miRNAs) in tumor glucose metabolism is growing, and a number of studies regularly confirm the impact miRNAs can have on glucose metabolism reprogramming in tumors. However, there remains a lack of understanding of the broader perspective on the role of miRNAs in energy reprogramming in glioblastoma. An important role in the metabolism of glucose is played by carrier proteins that ensure its transmembrane movement. Carrier proteins in mammalian cells are glucose transporters (GLUTs). In total, 12 types of GLUTs are distinguished, differing in localization, affinity for glucose and ability to regulate. The fact of increased consumption of glucose in tumors compared to non-proliferating normal tissues is known. Tumor cells need glucose to ensure their survival and growth, so the type of transport proteins like GLUT are critical for them. Previous studies have shown that GLUT-1 and GLUT-3 may play an important role in the development of some types of malignant tumors, including glioblastoma. In addition, there is evidence of how GLUT-1 and GLUT-3 expression is regulated by miRNAs in glioblastoma. Thus, the aim of this study is to highlight the role of specific miRNAs in modulating GLUT levels in order to take into account the use of miRNAs expression modulators as a useful strategy to increase the sensitivity of glioblastoma to current therapies.

Antioxidants Supplementation Reduces Ceramide Synthesis Improving the Cardiac Insulin Transduction Pathway in a Rodent Model of Obesity

Obesity-related disruption in lipid metabolism contributes to cardiovascular dysfunction. Despite numerous studies on lipid metabolism in the left ventricle, there is no data describing the influence of n-acetylcysteine (NAC) and α-lipoic acid (ALA), as glutathione precursors, on sphingolipid metabolism, and insulin resistance (IR) occurrence. The aim of our experiment was to evaluate the influence of chronic antioxidants administration on myocardial sphingolipid state and intracellular insulin signaling as a potential therapeutic strategy for obesity-related cardiovascular IR. The experiment was conducted on male Wistar rats fed a standard rodent chow or a high-fat diet with intragastric administration of NAC or ALA for eight weeks. Cardiac and plasma sphingolipid species were assessed by high-performance liquid chromatography (HPLC). The proteins expressed from sphingolipid and insulin signaling pathways were determined by Western blot. Antioxidant supplementation markedly reduced ceramide accumulation by lowering the expression of selected proteins from the sphingolipid pathway and simultaneously increased the myocardial sphingosine-1-phosphate level. Moreover, NAC and ALA augmented the expression of GLUT4 and the phosphorylation state of Akt (Ser473) and GSK3β (Ser9), which improved the intracellular insulin transduction pathway. Based on our results, we may postulate that NAC and ALA have a beneficial influence on the cardiac ceramidose under IR conditions.

Protein O-GlcNAcylation in the heart

O-GlcNAcylation is a ubiquitous post-translational modification that is extremely labile and plays a significant role in physiology, including the heart. Sustained activation of cardiac O-GlcNAcylation is frequently associated with alterations in cellular metabolism, leading to detrimental effects on cardiovascular function. This is particularly true during conditions such as diabetes, hypertension, cardiac remodelling, heart failure and arrhythmogenesis. Paradoxically, transient elevation of cardiac protein O-GlcNAcylation can also exert beneficial effects in the heart. There is compelling evidence to suggest that a complex interaction between O-GlcNAcylation and phosphorylation also exists in the heart. Beyond direct functional consequences on cardiomyocytes, O-GlcNAcylation also acts indirectly by altering the function of transcription factors that affect downstream signalling. This review focuses on the potential cardioprotective role of protein O-GlcNAcylation during ischaemia-reperfusion injury, the deleterious consequences of chronically elevated O-GlcNAc levels, the interplay between O-GlcNAcylation and phosphorylation in the cardiomyocytes and the effects of O-GlcNAcylation on other major non-myocyte cell types in the heart.

Identification of Keratin 19-Positive Cancer Stem Cells Associating Human Hepatocellular Carcinoma Using 18F-Fluorodeoxyglucose Positron Emission Tomography

Purpose: The current lack of tools for easy assessment of cancer stem cells (CSC) prevents the development of therapeutic strategies for hepatocellular carcinoma (HCC). We previously reported that keratin 19 (K19) is a novel HCC-CSC marker and that PET with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) is an effective method for predicting postoperative outcome in hepatocellular carcinoma. Herein, we examined whether K19⁺ HCC-CSCs can be tracked using ¹⁸F-FDG-PET.Experimental Design: K19 and glucose transporter-1 (GLUT1) expression was evaluated by IHC in 98 hepatocellular carcinoma patients who underwent ¹⁸F-FDG-PET scans before primary tumor resection. Standardized uptake values (SUV) for primary tumors and tumor-to-nontumor SUV ratios (TNR) were calculated using FDG accumulation levels, and values were compared among K19⁺/K19^- patients. Using hepatocellular carcinoma cell lines encoding with a K19 promoter-driven enhanced GFP, ¹⁸F-FDG uptake and GLUT1 expression were examined in FACS-isolated K19⁺/K19^- cells.Results: In hepatocellular carcinoma patients, K19 expression was significantly correlated with GLUT1 expression and FDG accumulation. ROC analyses revealed that among preoperative clinical factors, TNR was the most sensitive indicator of K19 expression in hepatocellular carcinoma tumors. In hepatocellular carcinoma cells, FACS-isolated K19⁺ cells displayed significantly higher ¹⁸F-FDG uptake than K19^- cells. Moreover, gain/loss-of-function experiments confirmed that K19 regulates ¹⁸F-FDG uptake through TGFβ/Smad signaling, including Sp1 and its downstream target GLUT1.Conclusions:¹⁸F-FDG-PET can be used to predict K19 expression in hepatocellular carcinoma and should thereby aid in the development of novel therapeutic strategies targeting K19⁺ HCC-CSCs. Clin Cancer Res; 23(6); 1450-60. ©2016 AACR.

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.

Advanced glycation end products: role in pathology of diabetic cardiomyopathy

Increasing evidence demonstrates that advanced glycation end products (AGEs) play a pivotal role in the development and progression of diabetic heart failure, although there are numerous other factors that mediate the disease response. AGEs are generated intra- and extracellularly as a result of chronic hyperglycemia. Then, following the interaction with receptors for advanced glycation end products (RAGEs), a series of events leading to vascular and myocardial damage are elicited and sustained, which include oxidative stress, increased inflammation, and enhanced extracellular matrix accumulation resulting in diastolic and systolic dysfunction. Whereas targeting glycemic control and treating additional risk factors, such as obesity, dyslipidemia, and hypertension, are mandatory to reduce chronic complications and prolong life expectancy in diabetic patients, drug therapy tailored to reducing the deleterious effects of the AGE-RAGE interactions is being actively investigated and showing signs of promise in treating diabetic cardiomyopathy and associated heart failure. This review shall discuss the formation of AGEs in diabetic heart tissue, potential targets of glycation in the myocardium, and underlying mechanisms that lead to diabetic cardiomyopathy and heart failure along with the use of AGE inhibitors and breakers in mitigating myocardial injury.

Prenatal caloric restriction enhances DNA methylation and MeCP2 recruitment with reduced murine placental glucose transporter isoform 3 expression

Diminished transplacental glucose transport plays an important role in prenatal calorie restriction (CR) induced reduction in fetal growth. Fetal growth restriction (FGR) has an impact in shaping the adult phenotype with transgenerational implications. To understand the mechanisms underlying prenatal CR-induced transplacental glucose transport, we examined the epigenetic regulation of placental glucose transporter (Glut1 and Glut3) expression. We restricted calories by 50% in C57BL6 pregnant mice from gestational days 10 to 19 (CR; n=8) vs. controls (CON; n=8) and observed a 50% diminution in placental Glut3 expression (P<.05) with no effect on Glut1 expression by reverse transcription and quantitative real-time polymerase chain reaction (PCR). CR enhanced DNA methylation of a CpG island situated ~1000 bp upstream from the transcriptional start site of the glut3 gene, with no such effect on the glut1 gene as assessed by methylation-sensitive PCR and bisulfite sequencing. Chromatin immunoprecipitation (ChIP) assays demonstrated enhanced MeCP2 binding to the CpG island of the glut3 gene in response to CR vs. CON (P<.05). Sequential ChIP demonstrated that enhanced MeCP2 binding of the glut3-(m)CpG island enhanced histone deacetylase 2 recruitment (P<.05) but interfered with Sp1 binding (P<.001), although it did not affect Sp3 or Creb/pCreb interaction. We conclude that late-gestation CR enhanced DNA methylation of the placental glut3 gene. This epigenetic change augmented specific nuclear protein-DNA complex formation that was associated with prenatal CR-induced reduction of placental glut3 expression and thereby transplacental glucose transport. This molecular complex provides novel targets for developing therapeutic interventions aimed at reversing FGR.

Egr-1 is a critical regulator of EGF-receptor-mediated expansion of subventricular zone neural stem cells and progenitors during recovery from hypoxia-hypoglycemia

We recently established that the EGF-R (epidermal growth factor receptor) (EGF-R) is an essential regulator of the reactive expansion of SVZ (subventricular zone) NPs (neural precursors) that occurs during recovery from hypoxic-ischemic brain injury. The purpose of the current studies was to identify the conditions and the transcription factor (s) responsible for inducing the EGF-R. Here, we show that the increase in EGF-R expression and the more rapid division of the NPs can be recapitulated in in vitro by exposing SVZ NPs to hypoxia and hypoglycemia simultaneously, but not separately. The EGF-R promoter has binding sites for multiple transcription factors that includes the zinc finger transcription factor, Egr-1. We show that Egr-1 expression increases in NPs, but not astrocytes, following hypoxia and hypoglycemia where it accumulates in the nucleus. To determine whether Egr-1 is necessary for EGF-R expression, we used SiRNAs (small interfering RNA) specific for Egr-1 to decrease Egr-1 expression. Knocking-down Egr-1 decreased basal levels of EGF-R and it abolished the stress-induced increase in EGF-R expression. By contrast, HIF-1 accumulation did not contribute to EGF-R expression and FGF-2 only modestly induced EGF-R. These studies establish a new role for Egr-1 in regulating the expression of the mitogenic EGF-R. They also provide new information into mechanisms that promote NP expansion and provide insights into strategies for amplifying the numbers of stem cells for CNS (central nervous system) regeneration.

Expression of conventional and novel glucose transporters, GLUT1, -9, -10, and -12, in vascular smooth muscle cells

Intimal hyperplasia is characterized by exaggerated proliferation of vascular smooth muscle cells (VSMCs). Enhanced VSMC growth is dependent on increased glucose uptake and metabolism. Facilitative glucose transporters (GLUTs) are comprised of conventional GLUT isoforms (GLUT1-5) and novel GLUT isoforms (GLUT6-14). Previous studies demonstrate that GLUT1 overexpression or GLUT10 downregulation contribute to phenotypic changes in VSMCs. To date, the expression profile of all 14 GLUT isoforms has not been fully examined in VSMCs. Using the proliferative and differentiated phenotypes of human aortic VSMCs, the present study has determined the relative abundance of GLUT1-14 mRNAs by quantitative real-time PCR analysis. Twelve GLUT mRNAs excluding GLUT7 and GLUT14 were detectable in VSMCs. In the proliferative phenotype, the relative abundance of key GLUT mRNAs was GLUT1 (∼43%)>GLUT10 (∼26%)>GLUT9 (∼13%)>GLUT12 (∼4%), whereas in the differentiated phenotype the relative abundance was GLUT10 (∼28%)>GLUT1 (∼25%)>GLUT12 (∼20%)>GLUT9 (∼14%), together constituting 86-87% of total GLUT transcripts. To confirm the expression of key GLUT proteins, immunoblot and immunocytochemical analyses were performed using GLUT isoform-specific primary antibodies. The protein bands characteristic of GLUT1, -9, -10, and -12 were detected in VSMCs in parallel with respective positive controls. In particular, GLUT1 protein expression showed different molecular forms representative of altered glycosylation. While GLUT1 protein displayed a predominant distribution in the plasma membrane, GLUT9, -10, and -12 proteins were mostly distributed in the intracellular compartments. The present study provides the first direct evidence for GLUT9 and GLUT12 expression in VSMCs in conjunction with the previously identified GLUT1 and GLUT10.

Will the original glucose transporter isoform please stand up!

Monosaccharides enter cells by slow translipid bilayer diffusion by rapid, protein-mediated, cation-dependent cotransport and by rapid, protein-mediated equilibrative transport. This review addresses protein-mediated, equilibrative glucose transport catalyzed by GLUT1, the first equilibrative glucose transporter to be identified, purified, and cloned. GLUT1 is a polytopic, membrane-spanning protein that is one of 13 members of the human equilibrative glucose transport protein family. We review GLUT1 catalytic and ligand-binding properties and interpret these behaviors in the context of several putative mechanisms for protein-mediated transport. We conclude that no single model satisfactorily explains GLUT1 behavior. We then review GLUT1 topology, subunit architecture, and oligomeric structure and examine a new model for sugar transport that combines structural and kinetic analyses to satisfactorily reproduce GLUT1 behavior in human erythrocytes. We next review GLUT1 cell biology and the transcriptional and posttranscriptional regulation of GLUT1 expression in the context of development and in response to glucose perturbations and hypoxia in blood-tissue barriers. Emphasis is placed on transgenic GLUT1 overexpression and null mutant model systems, the latter serving as surrogates for the human GLUT1 deficiency syndrome. Finally, we review the role of GLUT1 in the absence or deficiency of a related isoform, GLUT3, toward establishing the physiological significance of coordination between these two isoforms.

The Glut1 and Glut4 glucose transporters are differentially expressed during perinatal and postnatal erythropoiesis

Glucose is a major source of energy for living organisms, and its transport in vertebrates is a universally conserved property. Of all cell lineages, human erythrocytes express the highest level of the Glut1 glucose transporter with more than 200 000 molecules per cell. However, we recently reported that erythrocyte Glut1 expression is a specific trait of vitamin C–deficient mammalian species, comprising only higher primates, guinea pigs, and fruit bats. Here, we show that in all other tested mammalian species, Glut1 was transiently expressed in erythrocytes during the neonatal period. Glut1 was up-regulated during the erythroblast stage of erythroid differentiation and was present on the vast majority of murine red blood cells (RBCs) at birth. Notably though, Glut1 was not induced in adult mice undergoing anemia-induced erythropoiesis, and under these conditions, the up-regulation of a distinct transporter, Glut4, was responsible for an increased glucose transport. Sp3 and Sp1 transcriptions factors have been proposed to regulate Glut1 transcription, and we find that the concomitant repression of Glut1 and induction of Glut4 was associated with a significantly augmented Sp3/Sp1 ratio. Glucose transporter expression patterns in mice and human erythrocytes are therefore distinct. In mice, there is a postnatal switch from Glut1 to Glut4, with Glut4 further up-regulated under anemic conditions.

Metabolic and genetic regulation of cardiac energy substrate preference

Proper heart function relies on high efficiency of energy conversion. Mitochondrial oxygen-dependent processes transfer most of the chemical energy from metabolic substrates into ATP. Healthy myocardium uses mainly fatty acids as its major energy source, with little contribution of glucose. However, lactate, ketone bodies, amino acids or even acetate can be oxidized under certain circumstances. A complex interplay exists between various substrates responding to energy needs and substrate availability. The relative substrate concentration is the prime factor defining preference and utilization rate. Allosteric enzyme regulation and protein phosphorylation cascades, partially controlled by hormones such as insulin, modulate the concentration effect; together they provide short-term adjustments of cardiac energy metabolism. The expression of metabolic machinery genes is also dynamically regulated in response to developmental and (patho)physiological conditions, leading to long-term adjustments. Specific nuclear receptor transcription factors and co-activators regulate the expression of these genes. These include peroxisome proliferator-activated receptors and their nuclear receptor co-activator, estrogen-related receptor and hypoxia-inducible transcription factor 1. Increasing glucose and reducing fatty acid oxidation by metabolic regulation is already a target for effective drugs used in ischemic heart disease and heart failure. Interaction with genetic factors that control energy metabolism could provide even more powerful pharmacological tools.

T3 strongly regulates GLUT1 and GLUT3 mRNA in cerebral cortex of hypothyroid rat neonates

Experimental hypothyroidism alters the expression of the GLUT1 and GLUT4 glucose transporters in brown adipose tissue, skeletal muscle and heart. Congenital hypothyroidism disrupts the development and function of the CNS, and the importance of GLUT1 for proper brain function has been dramatically evidenced in the cases of GLUT1 deficiency syndrome. Because of this, we hypothesised that the expression of GLUT1 and GLUT3, glucose transporters expressed in brain cortex, may be altered in congenital hypothyroidism. GLUT3 mRNA was induced during postnatal development whereas GLUT1 mRNA was initially repressed and further induced; both processes were essentially similar in control and hypothyroid animals. Under these conditions GLUT1 protein expression was reduced in cerebral cortex from 15-day-old hypothyroid neonates, which suggests the existence of post-transcriptional alterations. The most striking differences were observed when hypothyroid animals at different developmental stages were treated acutely with T(3). GLUT1 and GLUT3 mRNA expression behaved in opposite ways in response to treatment with the hormone. Furthermore, the behaviour of each glucose transporter isoform against T(3) was not uniform but changed alongside development. In all, our data show that the regulation of GLUT1 and GLUT3 in cerebral cortex is regulated by T(3) in a complex way and suggest that alterations in the expression of glucose transporters induced by hypothyroidism might have a functional impact on brain glucose uptake.

Regulation of myocardial glucose transporters GLUT1 and GLUT4 in chronically anemic fetal lambs

Little is known about the chronic adaptations that take place in the fetal heart to allow for increased substrate delivery in response to chronic stress. Because glucose is an important fuel for the fetal cardiomyocytes, we hypothesized that myocardial glucose transporters 1 and 4 (GLUT1 and GLUT4, respectively) are up-regulated in the fetal sheep heart that is chronically stressed by anemia. Fetal sheep at 128 d gestation underwent daily isovolumic hemorrhage and determination of myocardial blood flow, oxygen consumption, and substrate utilization. At the end of 3 or 7 d of anemia, myocardial levels of GLUT1 and GLUT4 mRNA and protein were measured and subcellular localization was determined. Despite stable heart rate and blood pressure, anemia caused a nearly 4-fold increase in right and left ventricular (RV and LV) free wall blood flow. No significant change in myocardial glucose uptake was found and serum insulin levels remained stable. Both 3-d RV and LV and 7-d RV mRNA and protein levels of GLUT1 and GLUT4 were unchanged; 7-d LV GLUT1 and GLUT4 mRNA levels were also stable. However, LV GLUT1 protein levels declined significantly, whereas LV GLUT4 protein levels were increased. In the steady state, GLUT4 protein localized to the sarcolemma membrane. These findings suggest that the glucose transporters are post-transcriptionally regulated in myocardium of chronically anemic fetal sheep with changes that mimic normal postnatal development. Unlike the postnatal heart, localization of GLUT4 to the cell membrane suggests the importance of GLUT4 in basal glucose uptake in the stressed fetal heart.

Control of Glut1 promoter activity under basal conditions and in response to hyperosmolarity: role of Sp1

We previously identified (Hwang DY and Ismail-Beigi F. Am J Physiol Cell Physiol 281: C1365-C1372, 2001) a 44-bp GC-rich segment of the rat proximal glucose transporter (Glut)1 promoter, located at -104 to -61, as necessary for basal transcription of the Glut1 gene. Using deletion and mutational analysis and expression of transfected reporter constructs, we report in the present study that mutation of the Sp1 site located within this segment of the promoter leads to a marked ( approximately 4-fold) decrease in basal promoter activity. Double mutations located in the Sp1 site and in a second downstream GC-rich region (-71 to -51) did not cause a further decrease in promoter activity. Gel shift and supershift assays verified the importance of the Sp1 site. Exposure of cells to trichostatin A resulted in increased expression of the endogenous Glut1 as well as the transfected wild-type construct. Finally, the presence of the Sp1 site was found to be essential for the positive response of the promoter to hyperosmolarity. We conclude that the consensus Sp1 site located in the rat proximal Glut1 promoter is necessary and sufficient for basal expression of the Glut1 gene, as well as for its response to hyperosmolarity.

Hepatic expression of the tumor necrosis factor family member lymphotoxin-beta is regulated by interleukin (IL)-6 and IL-1beta: transcriptional control mechanisms in oval cells and hepatoma cell lines

BACKGROUND

Lymphotoxin-beta (LT-beta) plays an important role in inflammation and its promoter contains a functional nuclear factor-kappaB (NF-kappaB) element, rendering it a likely target of pro-inflammatory cytokines. Inflammatory cytokines play a central role in liver regeneration resulting from acute or chronic liver injury, with interleukin (IL)-6 signaling essential for liver regeneration induced by partial hepatectomy. In hepatic oval cells observed following chronic liver injury, LT-beta levels are upregulated, suggesting a link between LT-beta and liver regeneration.

RESULTS

The expression of LT-beta in hepatic oval cell and hepatocellular carcinoma cell lines was further investigated, along with its responsiveness to IL-6 and IL-1beta. Key regulatory cis-acting elements of the LT-beta promoter that mediate IL-6 responsiveness (Sp/BKLF, Ets, NF-kappaB and Egr-1/Sp1) and IL-1beta responsiveness (NF-kappaB and Ets) of hepatic LT-beta expression were identified. The novel binding of basic Kruppel-like factor (BKLF) proteins to an apparent composite Sp/BKLF site of the LT-beta promoter was shown to mediate IL-6 responsiveness. Binding of NF-kappaB p65/p50 heterodimers and Ets-related transcription factors to their respective sites mediates responsiveness to IL-1beta.

CONCLUSION

The identification of IL-6 and IL-1beta as activators of LT-beta supports their involvement in LT-beta signaling in liver regeneration associated with chronic liver damage.

Effects of cortisol on cardiac myocytes and on expression of cardiac genes in fetal sheep

In 17 fetal sheep aged 129 days, the effects of large-dose infusions of cortisol (72.1 mg/day for 2–3 days) on proliferation, binucleation, and hypertrophy of cardiac myocytes, cardiac expression of angiotensinogen, angiotensin receptor subtypes 1 and 2, Glut-1, glucocorticoid and mineralocorticoid receptors, proteins of the MAPK pathways and calcineurin were studied. Cortisol levels were 8.7 ± 2.3 nM (SE) in 8 control and 1,028 ± 189 nM in 9 treated fetuses ( P < 0.001). Cortisol had no effect on myocyte binucleation. Left ventricular free wall (LVFW) uni- and binucleated myocytes were larger in cortisol-treated fetuses ( P < 0.001, P < 0.05). Cortisol-treated fetuses had higher right ventricular free wall (RVFW) and LVFW angiotensinogen (Aogen) mRNA levels (treated: 2.30 ± 0.37, n = 8 and 2.05 ± 0.45, n = 7 vs. control: 0.94 ± 0.12, n = 8 and 0.67 ± 0.09, n = 7, P < 0.02). Levels of the glucose transporter Glut-1 mRNA were lower in the LVFW of treated fetuses (0.83 ± 0.23 vs. 1.47 ± 0.30 in control, P < 0.05, n = 7, 8). The higher the cortisol level, the greater the Aogen mRNA level (RVFW, r = 0.61, P < 0.01, n = 16; LVFW, r = 0.83, P < 0.0003, n = 14). There were no other changes in mRNA levels nor in levels of extracellular kinase, JNK, p38, their phosphorylated forms, and calcineurin. Thus high levels of cortisol such as occur after birth do not affect fetal cardiac myocyte binucleation or number but are associated with higher levels of ventricular Aogen mRNA, lower levels of Glut-1 mRNA, and hypertrophy of LVFW myocytes. These effects could impact on postnatal cardiac development.

Insulin acts via mitogen-activated protein kinase phosphorylation in rabbit blastocysts

The addition of insulin during in vitro culture has beneficial effects on rabbit preimplantation embryos leading to increased cell proliferation and reduced apoptosis. We have previously described the expression of the insulin receptor (IR) and the insulin-responsive glucose transporters (GLUT) 4 and 8 in rabbit preimplantation embryos. However, the effects of insulin on IR signaling and glucose metabolism have not been investigated in rabbit embryos. In the present study, the effects of 170 nM insulin on IR, GLUT4 and GLUT8 mRNA levels, Akt and Erk phosphorylation, GLUT4 translocation and methyl glucose transport were studied in cultured day 3 to day 6 rabbit embryos. Insulin stimulated phosphorylation of the mitogen-activated protein kinase (MAPK) Erk1/2 and levels of IR and GLUT4 mRNA, but not phosphorylation of the phosphatidylinositol 3-kinase-dependent protein kinase, Akt, GLUT8 mRNA levels, glucose uptake or GLUT4 translocation. Activation of the MAPK signaling pathway in the absence of GLUT4 translocation and of a glucose transport response suggest that in the rabbit preimplantation embryo insulin is acting as a growth factor rather than a component of glucose homeostatic control.

Akt mediates the cross-talk between beta-adrenergic and insulin receptors in neonatal cardiomyocytes

Upregulation of the sympathetic nervous system plays a key role in the pathogenesis of insulin resistance. Although the heart is a target organ of insulin, few studies have examined the mechanisms by which beta-adrenergic stimulation affects insulin sensitivity in cardiac muscle. In this study, we explored the molecular mechanisms involved in the regulation of the cross-talk between beta adrenergic and insulin receptors in neonatal rat cardiomyocytes and in transgenic mice with cardiac overexpression of a constitutively active mutant of Akt (E40K Tg). The results of this study show that beta-adrenergic receptor stimulation has a biphasic effect on insulin-stimulated glucose uptake. Short-term stimulation induces an additive effect on insulin-induced glucose uptake, and this effect is mediated by phosphorylation of Akt in threonine 308 through PKA/Ca2+-dependent and PI3K-independent pathway, whereas insulin-evoked threonine phosphorylation of Akt is exclusively PI3K-dependent. On the other hand, long-term stimulation of beta-adrenergic receptors inhibits both insulin-stimulated glucose uptake and insulin-induced autophosphorylation of the insulin receptor, and at the same time promotes threonine phosphorylation of the insulin receptor. This is mediated by serine 473 phosphorylation of Akt through PKA/Ca2+ and PI3K-dependent pathways. Under basal conditions, E40K Tg mice show increased levels of threonine phosphorylation of the beta subunit of the insulin receptor and blunted tyrosine autophosphorylation of the beta-subunit of the insulin receptor after insulin stimulation. These results indicate that, in cardiomyocytes, beta-adrenergic receptor stimulation impairs insulin signaling transduction machinery through an Akt-dependent pathway, suggesting that Akt is critically involved in the regulation of insulin sensitivity.

Exposure to maternal diabetes is associated with altered fetal growth patterns: A hypothesis regarding metabolic allocation to growth under hyperglycemic-hypoxemic conditions

The prevalence of diabetes is rising worldwide, including women who grew poorly in early life, presenting intergenerational health problems for their offspring. It is well documented that fetuses exposed to maternal diabetes during pregnancy experience both macrosomia and poor growth outcomes in birth size. Less is known about the in utero growth patterns that precede these risk factor expressions. Fetal growth patterns and the effects of clinical class and glycemic control were investigated in 37 diabetic pregnant women and their fetuses and compared to 29 nondiabetic, nonsmoking maternal/fetal pairs who were participants in a biweekly longitudinal ultrasound study with measurements of the head, limb, and trunk dimensions. White clinical class of the diabetic women was recorded (A2-FR) and glycosylated hemoglobin levels taken at the time of measurement assessed glycemic control (median 6.9%, interquartile range 5.6-9.2%). No significant difference in fetal weight was found by exposure. The exposed sample had greater abdominal circumferences from 21 weeks (P < or = 0.05) and shorter legs, but greater upper arm and thigh circumferences accompanied increasing glycemia in the second trimester. In the third trimester, exposed fetuses had a smaller slope for the occipital frontal diameter (P = 0.00) and were brachycephalic. They experienced a proximal/distal growth gradient in limb proportionality with higher humerus / femur ratios (P = 0.04) and arms relatively long by comparison with legs (P = 0.02). HbA1c levels above 7.5% accompanied shorter femur length for thigh circumference after 30 gestational weeks of age. Significant effects of diabetic clinical class and glycemic control were identified in growth rate timing. These growth patterns suggest that hypoxemic and hyperglycemic signals cross-talk with their target receptors in a developmentally regulated, hierarchical sequence. The increase in fetal fat often documented with diabetic pregnancy may reflect altered growth at the level of cell differentiation and proximate mechanisms controlling body composition. These data suggest that the maternal-fetal interchange circuit, designed to share and capture resources on the fetal side, may not have had a long evolutionary history of overabundance as a selective force, and modern health problems drive postnatal sequelae that become exacerbated by increasing longevity.

GLUT4 but not GLUT1 expression decreases early in the development of feline obesity

The increase in obesity in people and pets has been phenomenal. As in man, obesity in pets is a risk factor for many diseases including diabetes mellitus. Recently, tissue-specific regulation of glucose metabolism in fat and muscle tissue has been identified as an important factor for insulin sensitivity and it has been hypothesized that glucose uptake into tissues is altered in obesity causing insulin resistance. The purpose of this study was to determine the expression of the glucose transporter proteins GLUT4 and GLUT1 in muscle and fat from lean and obese cats. Seventeen domestic felines were tested in the lean state and again after a 6-month period of ad libitum food intake which led to a significant increase in weight (P < 0.0001). Obese cats showed a significantly higher area under the curve (AUC) for glucose, AUC for insulin and a significant decrease in glucose percentage disappearance per min (K-value) (P = 0.013, 0.018 and 0.017, respectively) during an intravenous glucose tolerance test, but no change in baseline glucose or glycosylated hemoglobin concentrations. GLUT4 expression was decreased in biopsies of both muscle (P = 0.002) and fat (P = 0.001) in the obese animals. GLUT4 in muscle and fat significantly and negatively correlated with the insulin AUC (r2 = 0.36, P = 0.004 and r2 = 0.18, P = 0.040, respectively). GLUT1 expression showed no significant change in the obese cats in either tissue. It is concluded that the changes in GLUT4 are early derangements in obesity and occur before glucose intolerance is clinically evident.

Regulation of the SM-20 prolyl hydroxylase gene in smooth muscle cells

SM-20 encodes an intracellular prolyl hydroxylase that acts on hypoxia inducible factor (HIF)-1alpha, targeting it for proteasomal degradation. By decreasing HIF-alpha, SM-20 is thought to modulate the expression of hypoxia-regulated genes. SM-20 expression in the arterial wall is restricted to smooth muscle cells, which play a critical role in atherosclerosis and arterial injury. To further elucidate the regulation of SM-20 in smooth muscle, we cloned and analyzed the rat SM-20 promoter. In transient transfections, the SM-20 promoter displayed approximately 6-fold greater activity in smooth muscle cells vs. fibroblasts. Deletion analysis and electrophoretic mobility shift assays demonstrated that SM-20 transcription was regulated by two Sp1/Sp3 sites. A shift in binding to the Sp1/Sp3 sites, a decrease in Sp1 and Sp3 protein levels, and the emergence of a lower molecular weight form of Sp1 were seen in serum-deprived or post-confluent SMC, suggesting that SM-20 is regulated during smooth muscle cell differentiation.

Trans-activators regulating neuronal glucose transporter isoform-3 gene expression in mammalian neurons

The murine facilitative glucose transporter isoform 3 is developmentally regulated and is predominantly expressed in neurons. By employing the primer extension assay, the transcription start site of the murine Glut 3 gene in the brain was localized to -305 bp 5' to the ATG translation start codon. Transient transfection assays in N2A neuroblasts using murine GLUT3-luciferase reporter constructs mapped enhancer activities to two regions located at -203 to -177 and -104 to -29 bp flanking a previously described repressor element (-137 to -130 bp). Dephosphorylated Sp1 and Sp3 proteins from the 1- and 21-day-old mouse brain nuclear extracts bound the repressor elements, whereas both dephosphorylated and phosphorylated cAMP-response element-binding protein (CREB) in N2A, 1- and 21-day-old mouse brain nuclear extracts bound the 5'-enhancer cis-elements (-187 to -180 bp) of the Glut 3 gene, and the Y box protein MSY-1 bound the sense strand of the -83- to -69-bp region. Sp3, CREB, and MSY-1 binding to the GLUT 3 DNA was confirmed by the chromatin immunoprecipitation assay, whereas CREB and MSY-1 interaction was detected by the co-immunoprecipitation assay. Furthermore, small interference RNA targeted at CREB in N2A cells decreased endogenous CREB concentrations, and CREB mediated GLUT 3 transcription. Thus, in the murine brain similar to the N2A cells, phosphorylated CREB and MSY-1 bound the Glut 3 gene trans-activating the expression in neurons, whereas Sp1/Sp3 bound the repressor elements. We speculate that phosphorylated CREB and Sp3 also interacted to bring about GLUT 3 expression in response to development/cell differentiation and neurotransmission.

Mitofusin-2 determines mitochondrial network architecture and mitochondrial metabolism. A novel regulatory mechanism altered in obesity

In many cells and specially in muscle, mitochondria form elongated filaments or a branched reticulum. We show that Mfn2 (mitofusin 2), a mitochondrial membrane protein that participates in mitochondrial fusion in mammalian cells, is induced during myogenesis and contributes to the maintenance and operation of the mitochondrial network. Repression of Mfn2 caused morphological and functional fragmentation of the mitochondrial network into independent clusters. Concomitantly, repression of Mfn2 reduced glucose oxidation, mitochondrial membrane potential, cell respiration, and mitochondrial proton leak. We also show that the Mfn2-dependent mechanism of mitochondrial control is disturbed in obesity by reduced Mfn2 expression. In all, our data indicate that Mfn2 expression is crucial in mitochondrial metabolism through the maintenance of the mitochondrial network architecture, and reduced Mfn2 expression may explain some of the metabolic alterations associated with obesity.

The Krüppel-like factor KLF15 regulates the insulin-sensitive glucose transporter GLUT4

Resistance to the stimulatory effects of insulin on glucose utilization is a key feature of type 2 diabetes, obesity, and the metabolic syndrome. Recent studies suggest that insulin resistance is primarily caused by a defect in glucose transport. GLUT4 is the main insulin-responsive glucose transporter and is expressed predominantly in muscle and adipose tissues. Whereas GLUT4 has been shown to play a critical role in maintaining systemic glucose homeostasis, the mechanisms regulating its expression are incompletely understood. We have cloned the murine homologue of KLF15, a member of the Krüppel-like family of transcription factors. KLF15 is highly expressed in adipocytes and myocytes in vivo and is induced when 3T3-L1 preadipocytes are differentiated into adipocytes. Overexpression of KLF15 in adipose and muscle cell lines potently induces GLUT4 expression. This effect is specific to KLF15 as overexpression of two other Krüppel-like factors, KLF2/LKLF and KLF4/GKLF, did not induce GLUT4 expression. Both basal (3.3-fold, p < 0.001) and insulin-stimulated (2.4-fold, p < 0.00001) glucose uptake are increased in KLF15-overexpressing adipocytes. In co-transfection assays, KLF15 and MEF2A, a known activator of GLUT4, synergistically activates the GLUT4 promoter. Promoter deletion and mutational analyses provide evidence that this activity requires an intact KLF15-binding site proximal to the MEF2A site. Finally, co-immunoprecipitation assays show that KLF15 specifically interacts with MEF2A. These studies indicate that KLF15 is an important regulator of GLUT4 in both adipose and muscle tissues.

Enhanced placental GLUT1 and GLUT3 expression in dexamethasone-induced fetal growth retardation

Intrauterine growth retardation (IUGR) increases the risk of developing glucose intolerance and cardiovascular disease in adulthood. Fetal exposure to excess glucocorticoids may contribute to IUGR. Despite the importance of glucose supply for fetal growth, studies on glucose transporter expression in IUGR are few. Two glucose transporters, GLUT1 and GLUT3, are expressed in placenta. In rodent placenta, GLUT1 is replaced by GLUT3 during late gestation. We examined placental GLUT protein expression in 21-day pregnant rats administered dexamethasone (DEX) from day 15 of gestation via osmotic minipump (at doses of 100 or 200 microg/kg body wt. per day). A dose-dependent decline in placental and fetal weight occurred in the DEX groups at day 21. Placental GLUT3 protein expression increased dose-dependently in the DEX groups (by 1.3-fold (n.s) and 2.3-fold (P<0.01), respectively). GLUT1 protein expression also increased dose-dependently in the DEX groups (by 1.6-fold (P<0.05) and 1.9-fold (P<0.01), respectively). In the DEX-treated groups, altered GLUT protein expression occurred in the absence of altered peroxisome proliferator-activated receptor-gamma (PPAR-gamma) protein expression in day 21 placenta; however, PPAR-gamma protein expression in day 21 fetal hearts was greatly suppressed. We conclude that increased placental GLUT1 protein expression may reflect an attempt to increase placental or fetal glucose supply to attenuate the effect of excessive exposure to glucocorticoids to diminish fetal growth, whereas suppression of cardiac PPAR-gamma expression during cardiac development may contribute to the increased risk of developing heart disease found in people of below average birthweight.

A novel functional co-operation between MyoD, MEF2 and TRalpha1 is sufficient for the induction of GLUT4 gene transcription

We report tripartite co-operation between MyoD, myocyte enhancer factor-2 (MEF2) and the thyroid hormone receptor (TRalpha1) that takes place in the context of an 82-bp muscle-specific enhancer in the rat insulin-responsive glucose transporter (GLUT4) gene that is active in both cardiac and skeletal muscle. In the L6E9 skeletal muscle cell line and in 10T1/2 fibroblasts, a powerful synergistic activation of the GLUT4 enhancer relied on the over-expression of MyoD, MEF2 and TRalpha1 and the integrity of their respective binding sites, and occurred when linked to either a heterologous promoter or in the context of the native GLUT4 promoter. In cardiac myocytes, enhancer activity was dependent on the binding sites for MEF2 and TRalpha1. Furthermore, we show that in 10T1/2 fibroblasts, the forced expression of MyoD, MEF2 and TRalpha1 induced the expression of the endogenous, otherwise silent, GLUT4 gene. In all, our results indicate a novel functional co-operation between these three factors which is required for full activation of GLUT4 transcription.

Stimulation of GLUT-1 glucose transporter expression in response to hyperosmolarity

Glucose transporter isoform-1 (GLUT-1) expression is stimulated in response to stressful conditions. Here we examined the mechanisms mediating the enhanced expression of GLUT-1 by hyperosmolarity. GLUT-1 mRNA, GLUT-1 protein, and glucose transport increased after exposure of Clone 9 cells to 600 mosmol/l (produced by addition of mannitol). The stimulation of glucose transport was biphasic: in the early phase (0-6 h) a approximately 2.5-fold stimulation of glucose uptake was associated with no change in the content of GLUT-1 mRNA, GLUT-1 protein, or GLUT-1 in the plasma membrane, whereas the approximately 17-fold stimulation of glucose transport during the late phase (12-24 h) was associated with increases in both GLUT-1 mRNA (approximately 7.5-fold) and GLUT-1 protein content. Cell sorbitol increased after 3 h of exposure to hyperosmolarity. The increase in GLUT-1 mRNA content was associated with an increase in the half-life of the mRNA from 2 to 8 h. A 44-bp region in the proximal GLUT-1 promoter was necessary for basal activity and for the two- to threefold increases in expression by hyperosmolarity. It is concluded that the increase in GLUT-1 mRNA content is mediated by both enhanced transcription and stabilization of GLUT-1 mRNA and is associated with increases in GLUT-1 content and glucose transport activity.

GLUT1 glucose transporter gene transcription is repressed by Sp3. Evidence for a regulatory role of Sp3 during myogenesis

GLUT1 glucose transporters are highly expressed in proliferating and transformed cells as well as in tissues during fetal life. However, the mechanisms that regulate GLUT1 gene expression remain largely unknown. Here, we demonstrate that Sp3 proteins bind to the GLUT1 proximal promoter gene and inhibit transcriptional activity in muscle and non-muscle cells. Two different Sp3 translational products (110 and 74 kDa) derived from differential translational initiation were detected in nuclear extracts from myoblast cells, and both Sp3 protein species inhibited GLUT1 gene transcriptional activity. The inhibitory effect of Sp3 was dominant over the stimulatory effect of Sp1 on transcriptional activity of GLUT1 gene. Furthermore, abolition of Sp3 binding to the proximal promoter of GLUT1 gene completely blocked the response to Sp3. We provide evidence that the expression of Sp3 protein is subject to regulation in muscle cells and that this is likely to control GLUT1. Thus, Sp3 protein was up-regulated in the absence of changes in Sp1 early after the induction of IGF-II-dependent myogenesis. Furthermore, forced over-expression of MyoD caused an enhancement in the cellular Sp3/Sp1 ratio which was concomitant to a reduced GLUT1 expression. Later during myogenesis, Sp3 expression was substantial whereas Sp1 was markedly down-regulated. In summary, we provide direct evidence that the transcription factor Sp3 represses gene expression in non-muscle and muscle cells and this is likely to operate in fetal heart by binding to the GLUT1 gene promoter. This is the first description of a repressor of GLUT1 gene transcription. Furthermore, we propose that variations in the ratio of Sp3 versus Sp1 regulate GLUT1 promoter activity and this is crucial in the down-regulation of GLUT1 associated to myogenesis.