Metformin mitigates impaired testicular lactate transport/utilisation and improves sexual behaviour in streptozotocin-induced diabetic rats

CONTEXT

Lactate is the preferred energy substrate for developing testicular germ cells. Diabetes is associated with impaired testicular lactate transport/utilisation, and poor sexual behaviour.

OBJECTIVE

To examine the effects of metformin on parameters involved in testicular lactate production, transport/utilisation, and sexual behaviour in diabetic state.

METHODS

Male Sprague-Dawley rats were assigned into normal control (NC), diabetic control (DC), and metformin-treated diabetic group (n = 6/group). Metformin (300 mg/kg b.w./day) was administrated orally for 4 weeks.

RESULTS

Intra-testicular glucose and lactate levels, and lactate dehydrogenase (LDH) activity increased, while the mRNA transcript levels of genes responsible for testicular glucose and lactate transport/utilisation (glucose transporter 3, monocarboxylate transporter 4 (MCT4), MCT2, and LDH type C) decreased in DC group. Furthermore, penile nitric oxide increased, while cyclic guanosine monophosphate decreased, with impaired sexual behaviour in DC group. Treatment with metformin improved these parameters.

CONCLUSIONS

Metformin increases testicular lactate transport/utilisation and improves sexual behaviour in diabetic state.

A Key Role of DNA Damage-Inducible Transcript 4 (DDIT4) Connects Autophagy and GLUT3-Mediated Stemness To Desensitize Temozolomide Efficacy in Glioblastomas

DNA damage-inducible transcript 4 (DDIT4) is known to participate in various cancers, including glioblastoma multiforme (GBM). However, contradictory roles of DDIT4 exist in inducing cell death and possessing anti-apoptotic functions against cancer progression. Herein, we investigated DDIT4 signaling in GBM and temozolomide (TMZ) drug resistance. We identified that TMZ induced DDIT4 upregulation, leading to desensitization against TMZ cytotoxicity in GBM cells. Higher DDIT4 levels were found in glioma cells and mesenchymal-type GBM patients, and these higher levels were positively correlated with mesenchymal markers. Furthermore, patients with lower DDIT4 levels, especially O-6-methylguanine-DNA methyltransferase (MGMT)-methylated patients, exhibited better TMZ therapeutic efficacy. We determined that higher levels of 5 DDIT4-associated downstream genes, including SLC2A3 (also known as glucose transporter 3 (GLUT3)), can be used to predict a poor prognosis. Among these 5 genes, only GLUT3 was upregulated in both TMZ-treated and DDIT4-overexpressing cells. DDIT4-mediated GLUT3 expression was also identified, and its expression decreased TMZ's cytotoxicity. A significant correlation existed between DDIT4 and GLUT3. DDIT4 signaling was found to be involved in both glycolytic and autophagic pathways. However, GLUT3 only participated in the exhibition of DDIT4-mediated stemness, resulting from glycolytic regulation, but not in DDIT4-mediated autophagic signaling. Finally, we identified TMZ-upregulated activating transcription factor 4 (ATF4) as an upstream regulator of DDIT4-mediated GLUT3/stemness signaling and autophagy. Consequently, ATF4/DDIT4 signaling was connected to both autophagy and GLUT3-regulated stemness, which are involved in TMZ drug resistance and the poor prognoses of GBM patients. Targeting DDIT4/GLUT3 signaling might be a new direction for glioma therapy.

Functional significance of post-myocardial infarction inflammation evaluated by 18F-fluorodeoxyglucose imaging in swine model

BACKGROUND

The aim of the study was to investigate the relationship between post-myocardial infarction (MI) inflammation and left ventricular (LV) remodeling in a swine model by 18F-fluorodeoxyglucose (FDG) imaging.

METHODS

MI was induced in swine by balloon occlusion of the left anterior descending coronary artery. A series of FDG positron emission tomography (PET) images were taken within 2 weeks post-MI, employing a comprehensive strategy to suppress the physiological uptake of cardiomyocytes. Echocardiography was applied to evaluate LV volume, global and regional function. CD68+ macrophage and glucose transporters (GLUT-1, -3 and -4) were investigated by immunostaining.

RESULTS

The physiological uptake of myocardium was adequately suppressed in 92.3% of PET scans verified by visual analysis, which was further confirmed by the minimal expression of myocardial GLUT-4. Higher FDG uptake was observed in the infarct than in the remote area and persisted within the observational period of 2 weeks. The FDG uptake of infarcted myocardium on day 1 post-MI was correlated with LV global remodeling, and the FDG uptake of infarcted myocardium on days 1 and 8 post-MI had a trend of correlating with regional remodeling of the infarct area.

CONCLUSIONS

We here report a feasible swine model for investigating post-MI inflammation. FDG signal in the infarct area of swine persisted for a longer duration than has been reported in small animals. FDG activity in the infarct area could predict LV remodeling.

Synaptic activity-induced glycolysis facilitates membrane lipid provision and neurite outgrowth

The formation of neurites is an important process affecting the cognitive abilities of an organism. Neurite growth requires the addition of new membranes, but the metabolic remodeling necessary to supply lipids for membrane expansion is poorly understood. Here, we show that synaptic activity, one of the most important inducers of neurite growth, transcriptionally regulates the expression of neuronal glucose transporter Glut3 and rate-limiting enzymes of glycolysis, resulting in enhanced glucose uptake and metabolism that is partly used for lipid synthesis. Mechanistically, CREB regulates the expression of Glut3 and Siah2, the latter and LDH activity promoting the normoxic stabilization of HIF-1α that regulates the expression of rate-limiting genes of glycolysis. The expression of dominant-negative HIF-1α or Glut3 knockdown blocks activity-dependent neurite growth in vitro while pharmacological inhibition of the glycolysis and specific ablation of HIF-1α in early postnatal mice impairs the neurite architecture. These results suggest that the manipulation of neuronal glucose metabolism could be used to treat some brain developmental disorders.

Metabolic signature genes associated with susceptibility to pyruvate kinase, muscle type 2 gene ablation in cancer cells

Pyruvate kinase, muscle type 2 (PKM2), is a key factor in the aerobic glycolysis of cancer cells. In our experiments, liver cancer cell lines exhibited a range of sensitivity to PKM2 knockdown-mediated growth inhibition. We speculated that this differential sensitivity is attributable to the variable dependency on glycolysis for the growth of different cell lines. Transcriptome data revealed overexpression of a glucose transporter (GLUT3) and a lactate transporter (MCT4) genes in PKM2 knockdown-sensitive cells. PKM2 knockdown-resistant cells expressed high levels of the lactate dehydrogenase B (LDHB) and glycine decarboxylase (GLDC) genes. Concordant with the gene expression results, PKM2 knockdown-sensitive cells generated high levels of lactate. In addition, ATP production was significantly reduced in the PKM2 knockdown-sensitive cells treated with a glucose analog, indicative of dependency of their cellular energetics on lactate-producing glycolysis. The PKM2 knockdown-resistant cells were further subdivided into less glycolytic and more (glycolysis branch pathway-dependent) glycolytic groups. Our findings collectively support the utility of PKM2 as a therapeutic target for high lactate-producing glycolytic hepatocellular carcinoma (HCC).

Hypoxic adaptation engages the CBP/CREST-induced coactivator complex of Creb-HIF-1α in transactivating murine neuroblastic glucose transporter

We have shown in vitro a hypoxia-induced time-dependent increase in facilitative glucose transporter isoform 3 (GLUT3) expression in N2A murine neuroblasts. This increase in GLUT3 expression is partially reliant on a transcriptional increase noted in actinomycin D and cycloheximide pretreatment experiments. Transient transfection assays in N2A neuroblasts using murine glut3-luciferase reporter constructs mapped the hypoxia-induced enhancer activities to -857- to -573-bp and -203- to -177-bp regions. Hypoxia-exposed N2A nuclear extracts demonstrated an increase in HIF-1α and p-Creb binding to HRE (-828 to -824 bp) and AP-1 (-187 to -180 bp) cis-elements, respectively, in electromobility shift and supershift assays, which was confirmed by chromatin immunoprecipitation assays. In addition, the interaction of CBP with Creb and HIF-1α and CREST with CBP in hypoxia was detected by coimmunoprecipitation. Furthermore, small interference (si)RNA targeting Creb in these cells decreased endogenous Creb concentrations that reduced by twofold hypoxia-induced glut3 gene transcription. Thus, in N2A neuroblasts, phosphorylated HIF-1α and Creb mediated the hypoxia-induced increase in glut3 transcription. Coactivation by the Ca⁺⁺-dependent CREST and CBP proteins may enhance cross-talk between p-Creb-AP-1 and HIF-1α/HRE of the glut3 gene. Collectively, these processes can facilitate an adaptive response to hypoxic energy depletion targeted at enhancing glucose transport and minimizing injury while fueling the proliferative potential of neuroblasts.

IGF-1 induces hypoxia-inducible factor 1α-mediated GLUT3 expression through PI3K/Akt/mTOR dependent pathways in PC12 cells

Glucose metabolism is essential for most mammalian neurons, and the passage of glucose across cell membranes is mainly facilitated by glucose transporter 3 (GLUT3). In ischemia/reperfusion injured brains, increase of IGF-1 secretion and GLUT3 up-regulation, are regarded as protective processes. Recent works have shown that various growth factors and cytokines including IGF-1 can stimulate HIF-1α expression, thereby triggering transcription of numerous hypoxia-inducible genes by oxygen-independent mechanisms. So, we hypothesized that HIF-1α might play important role in the process of IGF-1 induced GLUT3. Using echinomycin, a HIF-1 inhibitor, and HIF-1α siRNA, we demonstrated IGF-1 induced GLUT3 expression through HIF-1α in neuronal PC12 cells. Moreover, IGF-1 stimulated HIF-1α and GLUT3 protein expression through phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR dependent pathways. Analysis of GLUT3 promoter deletion sequences indicated that a putative hypoxia-response element (HRE) was critical in GLUT3 promoter activity when PC12 cells were treatment with CoCl(2) and IGF-1. In conclusion, we showed that the expression of GLUT3 in response to IGF-1 was dependent on PI-3-kinase and mTOR activity, and required the transcription factor HIF-1α.

Altered placental development in undernourished rats: role of maternal glucocorticoids

Maternal undernutrition (MUN) during pregnancy may lead to fetal intrauterine growth restriction (IUGR), which itself predisposes to adult risk of obesity, hypertension, and diabetes. IUGR may stem from insufficient maternal nutrient supply or reduced placental nutrient transfer. In addition, a critical role for maternal stress-induced glucocorticoids (GCs) has been suggested to contribute to both IUGR and the ensuing risk of adult metabolic syndrome. While GC-induced fetal organ defects have been examined, there have been few studies on placental responses to MUN-induced maternal stress. Therefore, we hypothesize that 50% MUN associates with increased maternal GC levels and decreased placental HSD11B. This in turn leads to decreased placental and fetal growth, hence the need to investigate nutrient transporters. We measured maternal serum levels of corticosterone, and the placental basal and labyrinth zone expression of glucocorticoid receptor (NR3C1), 11-hydroxysteroid dehydrogenase B 1 (HSD11B-1) predominantly activates cortisone to cortisol and 11-dehydrocorticosterone (11-DHC) to corticosterone, although can sometimes drive the opposing (inactivating reaction), and HSD11B-2 (only inactivates and converts corticosterone to 11-DHC in rodents) in control and MUN rats at embryonic day 20 (E20). Moreover, we evaluated the expression of nutrient transporters for glucose (SLC2A1, SLC2A3) and amino acids (SLC38A1, 2, and 4). Our results show that MUN dams displayed significantly increased plasma corticosterone levels compared to control dams. Further, a reduction in fetal and placental weights was observed in both the mid-horn and proximal-horn positions. Notably, the placental labyrinth zone, the site of feto-maternal exchange, showed decreased expression of HSD11B1-2 in both horns, and increased HSD11B-1 in proximal-horn placentas, but no change in NR3C1. The reduced placental GCs catabolic capacity was accompanied by downregulation of SLC2A3, SLC38A1, and SLC38A2 expression, and by increased SLC38A4 expression, in labyrinth zones from the mid- and proximal-horns. In marked contrast to the labyrinth zone, the basal zone, which is the site of hormone production, did not show significant changes in any of these enzymes or transporters. These results suggest that dysregulation of the labyrinth zone GC "barrier", and more importantly decreased nutrient supply resulting from downregulation of some of the amino acid system A transporters, may contribute to suboptimal fetal growth under MUN.

Cerebral metabolism after forced or voluntary physical exercise

The pathophysiology of stroke, a leading cause of morbidity and mortality, is still in the process of being understood. Pre-ischemic exercise has been known to be beneficial in reducing the severity of stroke-induced brain injury in animal models. Forced exercise with a stressful component, rather than voluntary exercise, was better able to induce neuroprotection. This study further determined the changes in cerebral metabolism resulting from the two methods of exercise (forced versus voluntary). Adult male Sprague-Dawley rats were randomly assigned to 3 groups: the control group (no exercise), the forced treadmill exercise group, and the voluntary running wheel exercise group. In order to measure the extent of cerebral metabolism in animals with different exercise regimens, mRNA levels and protein expression of glucose transporter 1 and glucose transporter 3 (GLUT-1 and GLUT-3), phosphofructokinase (PFK), lactate dehydrogenase (LDH), and adenosine monophosphate kinase (AMPK) were measured utilizing real-time reverse transcription polymerase chain reaction (PCR) analysis as well as Western blot analysis. Phosphorylated AMPK activity was also measured using an ELISA activity kit, and hypoxic inducible factor (HIF)-1α was measured at transcription and translation levels. The data show that the forced exercise group had a significant (p < 0.05) increase in cerebral glycolysis, including expressions of GLUT-1, GLUT-3, PFK, LDH, phosphorylated AMPK activity and HIF-1α, when compared to the voluntary exercise and the control groups. Our results suggest that the effects of different exercise on HIF-1α expression and cerebral glycolysis may provide a possible reason for the discrepancy in neuroprotection, with forced exercise faring better than voluntary exercise through increased cerebral metabolism.

Molecular mechanisms involved in Sertoli cell adaptation to glucose deprivation

Sertoli cells provide the physical support and the necessary environment for germ cell development. Among the products secreted by Sertoli cells, lactate, the preferred energy substrate for spermatocytes and spermatids, is present. Considering the essential role of lactate on germ cell metabolism, it is supposed that Sertoli cells must ensure its production even in adverse conditions, such as those that would result from a decrease in glucose levels in the extracellular milieu. The aim of the present study was to investigate 1) a possible effect of glucose deprivation on glucose uptake and on the expression of glucose transporters in rat Sertoli cells and 2) the participation of different signal transduction pathways in the above-mentioned regulation. Results obtained show that decreasing glucose levels in Sertoli cell culture medium provokes 1) an increase in glucose uptake accompanied by only a slight decrease in lactate production, 2) an increase in GLUT1 and a decrease in GLUT3 expression, and 3) an activation of AMP-activated protein kinase (AMPK)-, phosphatidylinositol 3-kinase (PI3K)/PKB-, and p38 MAPK-dependent pathways. Additionally, by using specific inhibitors of these pathways, a possible participation of AMPK- and p38MAPK-dependent pathways in the regulation of glucose uptake and GLUT1 expression is shown. These results suggest that Sertoli cells adapt to conditions of glucose deprivation to ensure an adequate lactate concentration in the microenvironment where germ cell development occurs.

Chronic Exposure of Human Glomerular Epithelial Cells to High Glucose Concentration Results in Modulation of High-Affinity Glucose Transporters Expression

INTRODUCTION

GLUTs are specific membrane proteins that transport glucose down a concentration gradient. There have been few studies on their expression in the kidney. The aim of this study was to identify the expression of GLUTs 1, 3, and 4 in HGEC and their regulation under diabetic milieu.

MATERIAL AND METHODS

An immortalized cell line of HGEC was used. Cells were cultured in medium containing 5 or 25 mM D-glucose. Western blotting and flow cytometry were used to examine the presence of GLUTs (1, 3, 4) and alterations in expression.

RESULTS

Western blotting analysis revealed that GLUT-1 levels were increased by 53% in HGEC cultured under experimental diabetes compared to cells grown in 5mM glucose. GLUT-3 levels were also increased by 15% under diabetic conditions. GLUT-4 levels were decreased by 20% in diabetes. Fluorescence Activated Cell Sorting (FACS) analysis demonstrated that cell surface expression of GLUT-1 was increased by 28% in cells grown in 25mM glucose. High glucose concentration did not affect cell surface expression of GLUT-3 and GLUT-4.

DISCUSSION

These findings suggest that depressed GLUT4 expression in glomerulus and overexpression of GLUT-1 and in a lesser extent of GLUT-3 may alter the glucose uptake in these cells. It has been suggested that the overexpression of GLUT-1 in glomerulus, being the major isoform, may lead to the initial pathologic hallmarks of diabetic nephropathy.

Expression of glucose transporters in epithelial ovarian carcinoma: correlation with clinical characteristics and tumor angiogenesis

We immunohistochemically examined the expression of the glucose transporters (GLUT)1, GLUT3 and GLUT4, in 154 tumor samples of epithelial ovarian carcinoma. In addition, we investigated the correlations between the expression of GLUTs and the vascular endothelial growth factor (VEGF), and microvessel count and clinical parameters. The rates of expression of GLUT1, GLUT3 and GLUT4 were 98.7%, 92.8% and 84.4%, respectively. GLUT1 and GLUT4 were both strongly expressed in serous adenocarcinoma, but weakly expressed in clear cell adenocarcinoma. The expressions of GLUT1 and GLUT4 correlated with the clinical disease stage. The expressions of GLUT1, GLUT3 and GLUT4 correlated positively with VEGF expression. The expression status for GLUT1, GLUT3, GLUT4 and VEGF did not represent a prognostic factor. These findings suggest that characteristic differences in the patterns of glucose uptake can exist according to the histological type and that GLUT1, GLUT3 and GLUT4 could be related to tumor angiogenesis in epithelial ovarian carcinoma.

Tungstate administration improves the sexual and reproductive function in female rats with streptozotocin-induced diabetes

BACKGROUND

Diabetes induces great alterations in female reproductive function. We analyzed the effects of tungstate, an anti-diabetic agent, on the reproductive function of healthy and diabetic female rats.

METHODS

Healthy and streptozotocin-induced diabetic rats were treated with sodium tungstate (2 mg/ml in their drinking water) for 12 weeks. Markers of reproductive function and diabetes were measured in serum, and in uterus and ovaries by Western blot or RT-PCR. Reproductive function was also assessed by mating.

RESULTS

Diabetic rats showed great impairment of libido, which was accompanied by a total loss of fertility (P < 0.05) and a decrease in the serum levels of FSH (P < 0.05) and LH (P < 0.05) compared with healthy rats. Tungstate treatment of diabetic rats partially recovered libido while fertility rate increased to 66.6%. This improvement was accompanied by a recovery of serum FSH (to a level higher than healthy rats) and LH. Moreover, tungstate treatment normalized ovarian expression of GLUT 3 hexose transporter, and estrogen, progesterone and FSH receptors, whereas only GLUT 3 and FSH receptors were normalized in the uterus.

CONCLUSIONS

Our results indicate that the alterations in female reproduction in diabetes were partially reversed after tungstate treatment by a mechanism(s) involving the normalization of serum FSH and LH levels, and ovarian and uterine expression of FSH receptors and GLUT3.

[Value of 18F-FDG metabolic imaging in diagnosis and treatment of head and neck tumors and its mechanism study]

OBJECTIVE

To study the value of (18)F-FDG dual-head tomography with coincidence (DHTC) and single photon emission computerized tomography (SPECT) coincidence imaging in diagnosis and treatment of head and neck tumors and mechanism thereof and analyze the value of glucose transporter proteins in the mechanism of increased uptake glucose of head and neck malignant tumor.

METHODS

Twenty-five patients with head and neck tumors were examined by CT or MRI and underwent (18)F-FDG DHTC and coincidence imaging. The results of these 2 different methods were compared. Fresh tissues of 38 patients with malignant tumors of the head-and-neck underwent RT-PCR and immunohistochemical examination.

RESULTS

The sensitivity, specificity, and accuracy of (18)F-FDG coincidence imaging and registration with integrated CT (SPECT/CT) in diagnosis of the head and neck tumors was 100.0%, 87.5%, and 96.0% respectively, all significantly higher than those of the anatomical imaging (64.7%, 50.0%, and 60.0% respectively, all P < 0.05). For the lesions on the same site, SPECT/CT could diagnose exactly the primary tumor site of neck metastasis in four cases and diagnose the malignancy or benignancy of other four cases that anatomical imaging (CT/MRI) could not diagnose exactly. (18)F-FDG coincidence imaging and registration with integrated CT could find extra lesions of tumors. The results of RT-PCR and immunohistochemistry showed that the mRNA expression and protein expression of Glut-1 and Glut-3 were higher in the head and neck cancer than that in the normal tissue of head and neck or in the adjacent tissue (all P < 0.05).

CONCLUSION

(18)F-FDG coincidence imaging and registration with integrated CT can be as a prospective tool that can judge the malignancy or benignancy of head and neck tumor, and stage and classify the tumor, and distinguish recurrence or necrosis of tumor after treatment by surgery or radiotherapy, and detect unknown primary tumor. The abnormal expressions of Glut-1 and Glut-3 may be correlated with the increased uptake of glucose of head and head cancer.

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.

Insulin neuroprotection against oxidative stress in cortical neurons--involvement of uric acid and glutathione antioxidant defenses

In this study we investigated the effect of insulin on neuronal viability and antioxidant defense mechanisms upon ascorbate/Fe2+-induced oxidative stress, using cultured cortical neurons. Insulin (0.1 and 10 microM) prevented the decrease in neuronal viability mediated by oxidative stress, decreasing both necrotic and apoptotic cell death. Moreover, insulin inhibited ascorbate/Fe2+-mediated lipid and protein oxidation, thus decreasing neuronal oxidative stress. Increased 4-hydroxynonenal (4-HNE) adducts on GLUT3 glucose transporters upon exposure to ascorbate/Fe2+ were also prevented by insulin, suggesting that this peptide can interfere with glucose metabolism. We further analyzed the influence of insulin on antioxidant defense mechanisms in the cortical neurons. Oxidative stress-induced decreases in intracellular uric acid and GSH/GSSG levels were largely prevented upon treatment with insulin. Inhibition of phosphatidylinositol-3-kinase (PI-3K) or mitogen-induced extracellular kinase (MEK) reversed the effect of insulin on uric acid and GSH/GSSG, suggesting the activation of insulin-mediated signaling pathways. Moreover, insulin stimulated glutathione reductase (GRed) and inhibited glutathione peroxidase (GPx) activities under oxidative stress conditions, further supporting that insulin neuroprotection was related to the modulation of the glutathione redox cycle. Thus, insulin may be useful in preventing oxidative stress-mediated injury that occurs in several neurodegenerative disorders.