Metformin suppresses gluconeogenesis by inhibiting mitochondrial glycerophosphate dehydrogenase.

Metformin is considered to be one of the most effective therapeutics for treating type 2 diabetes because it specifically reduces hepatic gluconeogenesis without increasing insulin secretion, inducing weight gain or posing a risk of hypoglycaemia. For over half a century, this agent has been prescribed to patients with type 2 diabetes worldwide, yet the underlying mechanism by which metformin inhibits hepatic gluconeogenesis remains unknown. Here we show that metformin non-competitively inhibits the redox shuttle enzyme mitochondrial glycerophosphate dehydrogenase, resulting in an altered hepatocellular redox state, reduced conversion of lactate and glycerol to glucose, and decreased hepatic gluconeogenesis. Acute and chronic low-dose metformin treatment effectively reduced endogenous glucose production, while increasing cytosolic redox and decreasing mitochondrial redox states. Antisense oligonucleotide knockdown of hepatic mitochondrial glycerophosphate dehydrogenase in rats resulted in a phenotype akin to chronic metformin treatment, and abrogated metformin-mediated increases in cytosolic redox state, decreases in plasma glucose concentrations, and inhibition of endogenous glucose production. These findings were replicated in whole-body mitochondrial glycerophosphate dehydrogenase knockout mice. These results have significant implications for understanding the mechanism of metformin's blood glucose lowering effects and provide a new therapeutic target for type 2 diabetes.

Insulin-independent regulation of hepatic triglyceride synthesis by fatty acids.

A central paradox in type 2 diabetes is the apparent selective nature of hepatic insulin resistance--wherein insulin fails to suppress hepatic glucose production yet continues to stimulate lipogenesis, resulting in hyperglycemia, hyperlipidemia, and hepatic steatosis. Although efforts to explain this have focused on finding a branch point in insulin signaling where hepatic glucose and lipid metabolism diverge, we hypothesized that hepatic triglyceride synthesis could be driven by substrate, independent of changes in hepatic insulin signaling. We tested this hypothesis in rats by infusing [U-(13)C] palmitate to measure rates of fatty acid esterification into hepatic triglyceride while varying plasma fatty acid and insulin concentrations independently. These experiments were performed in normal rats, high fat-fed insulin-resistant rats, and insulin receptor 2'-O-methoxyethyl chimeric antisense oligonucleotide-treated rats. Rates of fatty acid esterification into hepatic triglyceride were found to be dependent on plasma fatty acid infusion rates, independent of changes in plasma insulin concentrations and independent of hepatocellular insulin signaling. Taken together, these results obviate a paradox of selective insulin resistance, because the major source of hepatic lipid synthesis, esterification of preformed fatty acids, is primarily dependent on substrate delivery and largely independent of hepatic insulin action.

Hypophosphatemia promotes lower rates of muscle ATP synthesis.

Hypophosphatemia can lead to muscle weakness and respiratory and heart failure, but the mechanism is unknown. To address this question, we noninvasively assessed rates of muscle ATP synthesis in hypophosphatemic mice by using in vivo saturation transfer [31P]-magnetic resonance spectroscopy. By using this approach, we found that basal and insulin-stimulated rates of muscle ATP synthetic flux (VATP) and plasma inorganic phosphate (Pi) were reduced by 50% in mice with diet-induced hypophosphatemia as well as in sodium-dependent Pi transporter solute carrier family 34, member 1 (NaPi2a)-knockout (NaPi2a-/-) mice compared with their wild-type littermate controls. Rates of VATP normalized in both hypophosphatemic groups after restoring plasma Pi concentrations. Furthermore, VATP was directly related to cellular and mitochondrial Pi uptake in L6 and RC13 rodent myocytes and isolated muscle mitochondria. Similar findings were observed in a patient with chronic hypophosphatemia as a result of a mutation in SLC34A3 who had a 50% reduction in both serum Pi content and muscle VATP After oral Pi repletion and normalization of serum Pi levels, muscle VATP completely normalized in the patient. Taken together, these data support the hypothesis that decreased muscle ATP synthesis, in part, may be caused by low blood Pi concentrations, which may explain some aspects of muscle weakness observed in patients with hypophosphatemia.-Pesta, D. H., Tsirigotis, D. N., Befroy, D. E., Caballero, D., Jurczak, M. J., Rahimi, Y., Cline, G. W., Dufour, S., Birkenfeld, A. L., Rothman, D. L., Carpenter, T. O., Insogna, K., Petersen, K. F., Bergwitz, C., Shulman, G. I. Hypophosphatemia promotes lower rates of muscle ATP synthesis.

Genetic activation of pyruvate dehydrogenase alters oxidative substrate selection to induce skeletal muscle insulin resistance.

The pyruvate dehydrogenase complex (PDH) has been hypothesized to link lipid exposure to skeletal muscle insulin resistance through a glucose-fatty acid cycle in which increased fatty acid oxidation increases acetyl-CoA concentrations, thereby inactivating PDH and decreasing glucose oxidation. However, whether fatty acids induce insulin resistance by decreasing PDH flux remains unknown. To genetically examine this hypothesis we assessed relative rates of pyruvate dehydrogenase flux/mitochondrial oxidative flux and insulin-stimulated rates of muscle glucose metabolism in awake mice lacking pyruvate dehydrogenase kinase 2 and 4 [double knockout (DKO)], which results in constitutively activated PDH. Surprisingly, increased glucose oxidation in DKO muscle was accompanied by reduced insulin-stimulated muscle glucose uptake. Preferential myocellular glucose utilization in DKO mice decreased fatty acid oxidation, resulting in increased reesterification of acyl-CoAs into diacylglycerol and triacylglycerol, with subsequent activation of PKC-θ and inhibition of insulin signaling in muscle. In contrast, other putative mediators of muscle insulin resistance, including muscle acylcarnitines, ceramides, reactive oxygen species production, and oxidative stress markers, were not increased. These findings demonstrate that modulation of oxidative substrate selection to increase muscle glucose utilization surprisingly results in muscle insulin resistance, offering genetic evidence against the glucose-fatty acid cycle hypothesis of muscle insulin resistance.

Laforin and malin knockout mice have normal glucose disposal and insulin sensitivity.

Lafora disease is a fatal, progressive myoclonus epilepsy caused in ~90% of cases by mutations in the EPM2A or EPM2B genes. Characteristic of the disease is the formation of Lafora bodies, insoluble deposits containing abnormal glycogen-like material in many tissues, including neurons, muscle, heart and liver. Because glycogen is important for glucose homeostasis, the aberrant glycogen metabolism in Lafora disease might disturb whole-body glucose handling. Indeed, Vernia et al. [Vernia, S., Heredia, M., Criado, O., Rodriguez de Cordoba, S., Garcia-Roves, P.M., Cansell, C., Denis, R., Luquet, S., Foufelle, F., Ferre, P. et al. (2011) Laforin, a dual-specificity phosphatase involved in Lafora disease, regulates insulin response and whole-body energy balance in mice. Hum. Mol. Genet., 20, 2571-2584] reported that Epm2a-/- mice had enhanced glucose disposal and insulin sensitivity, leading them to suggest that laforin, the Epm2a gene product, is involved in insulin signaling. We analyzed 3-month- and 6-7-month-old Epm2a-/- mice and observed no differences in glucose tolerance tests (GTTs) or insulin tolerance tests (ITTs) compared with wild-type mice of matched genetic background. At 3 months, Epm2b-/- mice also showed no differences in GTTs and ITTs. In the 6-7-month-old Epm2a-/- mice, there was no evidence for increased insulin stimulation of the phosphorylation of Akt, GSK-3 or S6 in skeletal muscle, liver and heart. From metabolic analyses, these animals were normal with regard to food intake, oxygen consumption, energy expenditure and respiratory exchange ratio. By dual-energy X-ray absorptiometry scan, body composition was unaltered at 3 or 6-7 months of age. Echocardiography showed no defects of cardiac function in Epm2a-/- or Epm2b-/- mice. We conclude that laforin and malin have no effect on whole-body glucose metabolism and insulin sensitivity, and that laforin is not involved in insulin signaling.

Fasting induces ketoacidosis and hypothermia in PDHK2/PDHK4-double-knockout mice.

The importance of PDHK (pyruvate dehydrogenase kinase) 2 and 4 in regulation of the PDH complex (pyruvate dehydrogenase complex) was assessed in single- and double-knockout mice. PDHK2 deficiency caused higher PDH complex activity and lower blood glucose levels in the fed, but not the fasted, state. PDHK4 deficiency caused similar effects, but only after fasting. Double deficiency intensified these effects in both the fed and fasted states. PDHK2 deficiency had no effect on glucose tolerance, PDHK4 deficiency produced only a modest effect, but double deficiency caused a marked improvement and also induced lower insulin levels and increased insulin sensitivity. In spite of these beneficial effects, the double-knockout mice were more sensitive than wild-type and single-knockout mice to long-term fasting, succumbing to hypoglycaemia, ketoacidosis and hypothermia. Stable isotope flux analysis indicated that hypoglycaemia was due to a reduced rate of gluconeogenesis and that slightly more glucose was converted into ketone bodies in the double-knockout mice. The findings establish that PDHK2 is more important in the fed state, PDHK4 is more important in the fasted state, and survival during long-term fasting depends upon regulation of the PDH complex by both PDHK2 and PDHK4.

Reassembly of a bioluminescent protein Renilla luciferase directed through DNA hybridization.

Reassembly of split reporter proteins, also referred to as protein complementation, is utilized in the detection of protein-protein or protein-nucleic acid interactions. In this strategy, a reporter protein is fragmented into two inactive polypeptides to which interacting/binding partners are fused. The interaction between fused partners leads to the formation of a reassembled, active reporter. In this Communication, we have presented a proof-of-concept for the detection of a target nucleic acid sequence based on the reassembly of the bioluminescent reporter Renilla luciferase (Rluc), which is driven by DNA hybridization. Although, reassembly of Rluc though protein interactions has been demonstrated by others, the Rluc reassembly through DNA hybridization has not been shown yet, which is the novelty of this work. It is well established that bioluminescence detection offers significant advantages due to the absence of any background signal. In our study, two rationally designed fragments of Rluc were conjugated to complementary oligonucleotide probes. Hybridization of the two probes with fused Rluc fragments resulted in the reassembly of the fragments, generating active Rluc, measurable by the intensity of light given off upon addition of coelenterazine. Our study also shows that the reassembly of Rluc can be inhibited by an oligonucleotide probe that competes to bind to the hybridized probe-Rluc fragment complex, indicating a potential strategy for the quantitative detection of target nucleic acid. We were able to achieve the reassembly of Rluc fused to oligonucleotide probes using femtomole amounts of the probe-fragment protein conjugate. This concentration is approximately 4 orders of magnitude less than that reported using green fluorescent protein (GFP) as the reporter. A DNA-driven Rluc reassembly study performed in a cellular matrix did not show any interference from the matrix.

Author Correction: Metformin inhibits gluconeogenesis via a redox-dependent mechanism in vivo.

In the version of this article originally published, the VPC and VCS flux data shown in Fig. 6e,f were inadvertently duplicated from Fig. 5j,k. The correct data are now shown in Fig. 6e,f. In these corrected data, VPC flux in response to chronic oral metformin treatment was still significantly decreased (Fig. 6e), and there was still no impact of metformin on VCS flux (Fig. 6f). Therefore, the text describing these data remains the same and this correction does not change the conclusion of this study.

Mechanism of copper induced fluorescence quenching of red fluorescent protein, DsRed.

The red fluorescent protein, DsRed, and a few of its mutants have been shown to bind copper ions resulting in quenching of its fluorescence. The response to Cu(2+) is rapid, selective, and reversible upon addition of a copper chelator. DsRed has been employed as an in vitro probe for Cu(2+) determination by us and other groups. It is also envisioned that DsRed can serve as an intracellular genetically encoded indicator of Cu(2+) concentration, and can be targeted to desired subcellular locations for Cu(2+) determination. However, no information has been reported yet regarding the mechanism of the fluorescence quenching of DsRed in the presence of Cu(2+). In this work, we have performed spectroscopic investigations to determine the mechanism of quenching of DsRed fluorescence in the presence of Cu(2+). We have studied the effect of Cu(2+) addition on two representative mutants of DsRed, specifically, DsRed-Monomer and DsRed-Express. Both proteins bind Cu(2+) with micromolar affinities. Stern-Volmer plots generated at different temperatures indicate a static quenching process in the case of both proteins in the presence of Cu(2+). This mechanism was further studied using absorption spectroscopy. Stern-Volmer constants and quenching rate constants support the observation of static quenching in DsRed in the presence of Cu(2+). Circular dichroism (CD)-spectroscopic studies revealed no effect of Cu(2+)-binding on the secondary structure or conformation of the protein. The effect of pH changes on the quenching of DsRed fluorescence in the presence of copper resulted in pK(a) values indicative of histidine and cysteine residue involvement in Cu(2+)-binding.

Metformin inhibits gluconeogenesis via a redox-dependent mechanism in vivo.

Metformin, the universal first-line treatment for type 2 diabetes, exerts its therapeutic glucose-lowering effects by inhibiting hepatic gluconeogenesis. However, the primary molecular mechanism of this biguanide remains unclear, though it has been suggested to act, at least partially, by mitochondrial complex I inhibition. Here we show that clinically relevant concentrations of plasma metformin achieved by acute intravenous, acute intraportal or chronic oral administration in awake normal and diabetic rats inhibit gluconeogenesis from lactate and glycerol but not from pyruvate and alanine, implicating an increased cytosolic redox state in mediating metformin's antihyperglycemic effect. All of these effects occurred independently of complex I inhibition, evidenced by unaltered hepatic energy charge and citrate synthase flux. Normalizing the cytosolic redox state by infusion of methylene blue or substrates that contribute to gluconeogenesis independently of the cytosolic redox state abrogated metformin-mediated inhibition of gluconeogenesis in vivo. Additionally, in mice expressing constitutively active acetyl-CoA carboxylase, metformin acutely decreased hepatic glucose production and increased the hepatic cytosolic redox state without altering hepatic triglyceride content or gluconeogenic enzyme expression. These studies demonstrate that metformin, at clinically relevant plasma concentrations, inhibits hepatic gluconeogenesis in a redox-dependent manner independently of reductions in citrate synthase flux, hepatic nucleotide concentrations, acetyl-CoA carboxylase activity, or gluconeogenic enzyme protein expression.

3,5 Diiodo-L-Thyronine (T2) Does Not Prevent Hepatic Steatosis or Insulin Resistance in Fat-Fed Sprague Dawley Rats.

Thyroid hormone mimetics are alluring potential therapies for diseases like dyslipidemia, nonalcoholic fatty liver disease (NAFLD), and insulin resistance. Though diiodothyronines are thought inactive, pharmacologic treatment with 3,5- Diiodo-L-Thyronine (T2) reportedly reduces hepatic lipid content and improves glucose tolerance in fat-fed male rats. To test this, male Sprague Dawley rats fed a safflower-oil based high-fat diet were treated with T2 (0.25 mg/kg-d) or vehicle. Neither 10 nor 30 days of T2 treatment had an effect on weight, adiposity, plasma fatty acids, or hepatic steatosis. Insulin action was quantified in vivo by a hyperinsulinemic-euglycemic clamp. T2 did not alter fasting plasma glucose or insulin concentration. Basal endogenous glucose production (EGP) rate was unchanged. During the clamp, there was no difference in insulin stimulated whole body glucose disposal. Insulin suppressed EGP by 60% ± 10 in T2-treated rats as compared with 47% ± 4 suppression in the vehicle group (p = 0.32). This was associated with an improvement in hepatic insulin signaling; insulin stimulated Akt phosphorylation was ~2.5 fold greater in the T2-treated group as compared with the vehicle-treated group (p = 0.003). There was no change in expression of genes thought to mediate the effect of T2 on hepatic metabolism, including genes that regulate hepatic lipid oxidation (ppara, carnitine palmitoyltransferase 1a), genes that regulate hepatic fatty acid synthesis (srebp1c, acetyl coa carboxylase, fatty acid synthase), and genes involved in glycolysis and gluconeogenesis (L-pyruvate kinase, glucose 6 phosphatase). Therefore, in contrast with previous reports, in Sprague Dawley rats fed an unsaturated fat diet, T2 administration failed to improve NAFLD or whole body insulin sensitivity. Though there was a modest improvement in hepatic insulin signaling, this was not associated with significant differences in hepatic insulin action. Further study will be necessary before diiodothyronines can be considered an effective treatment for NAFLD and dyslipidemia.

Regulation of glycolysis by Pdk functions as a metabolic checkpoint for cell cycle quiescence in hematopoietic stem cells.

Defining the metabolic programs that underlie stem cell maintenance will be essential for developing strategies to manipulate stem cell capacity. Mammalian hematopoietic stem cells (HSCs) maintain cell cycle quiescence in a hypoxic microenvironment. It has been proposed that HSCs exhibit a distinct metabolic phenotype under these conditions. Here we directly investigated this idea using metabolomic analysis and found that HSCs generate adenosine-5'-triphosphate by anaerobic glycolysis through a pyruvate dehydrogenase kinase (Pdk)-dependent mechanism. Elevated Pdk expression leads to active suppression of the influx of glycolytic metabolites into mitochondria. Pdk overexpression in glycolysis-defective HSCs restored glycolysis, cell cycle quiescence, and stem cell capacity, while loss of both Pdk2 and Pdk4 attenuated HSC quiescence, glycolysis, and transplantation capacity. Moreover, treatment of HSCs with a Pdk mimetic promoted their survival and transplantation capacity. Thus, glycolytic metabolic status governed by Pdk acts as a cell cycle checkpoint that modulates HSC quiescence and function.

Reversal of hypertriglyceridemia, fatty liver disease, and insulin resistance by a liver-targeted mitochondrial uncoupler.

Nonalcoholic fatty liver disease (NAFLD) affects one in three Americans and is a major predisposing condition for the metabolic syndrome and type 2 diabetes (T2D). We examined whether a functionally liver-targeted derivative of 2,4-dinitrophenol (DNP), DNP-methyl ether (DNPME), could safely decrease hypertriglyceridemia, NAFLD, and insulin resistance without systemic toxicities. Treatment with DNPME reversed hypertriglyceridemia, fatty liver, and whole-body insulin resistance in high-fat-fed rats and decreased hyperglycemia in a rat model of T2D with a wide therapeutic index. The reversal of liver and muscle insulin resistance was associated with reductions in tissue diacylglycerol content and reductions in protein kinase C epsilon (PKCε) and PKCθ activity in liver and muscle, respectively. These results demonstrate that the beneficial effects of DNP on hypertriglyceridemia, fatty liver, and insulin resistance can be dissociated from systemic toxicities and suggest the potential utility of liver-targeted mitochondrial uncoupling agents for the treatment of hypertriglyceridemia, NAFLD, metabolic syndrome, and T2D.

Mutations in BCKD-kinase lead to a potentially treatable form of autism with epilepsy.

Autism spectrum disorders are a genetically heterogeneous constellation of syndromes characterized by impairments in reciprocal social interaction. Available somatic treatments have limited efficacy. We have identified inactivating mutations in the gene BCKDK (Branched Chain Ketoacid Dehydrogenase Kinase) in consanguineous families with autism, epilepsy, and intellectual disability. The encoded protein is responsible for phosphorylation-mediated inactivation of the E1α subunit of branched-chain ketoacid dehydrogenase (BCKDH). Patients with homozygous BCKDK mutations display reductions in BCKDK messenger RNA and protein, E1α phosphorylation, and plasma branched-chain amino acids. Bckdk knockout mice show abnormal brain amino acid profiles and neurobehavioral deficits that respond to dietary supplementation. Thus, autism presenting with intellectual disability and epilepsy caused by BCKDK mutations represents a potentially treatable syndrome.

Bioluminescence-based detection of microRNA, miR21 in breast cancer cells.

A hybridization assay for the detection of microRNA, miR21 in cancer cells using the bioluminescent enzyme Renilla luciferase (Rluc) as a label, has been developed. MicroRNAs are small RNAs found in plants, animals, and humans that perform key functions in gene silencing and affect early-stage cell development, cell differentiation, and cell death. miRNAs are considered useful early diagnostic and prognostic markers of cancer, candidates for therapeutic intervention, and targets for basic biomedical research. However, methods for highly sensitive and rapid detection of miRNA directly from samples such as cells that can serve as a suitable diagnostics platform are lacking. In that regard, the utilization of the bioluminescent label, Rluc, that offers the advantage of high signal-to-noise ratio, allows for the development of highly sensitive assays for the determination of miRNA in a variety of matrixes. In this paper, we have described the development of a competitive oligonucleotide hybridization assay for the detection of miR21 using the free miR21 and Rluc-labeled miR21 that competes to bind to an immobilized miR21 complementary probe. The miR21 microRNA chosen for this study is of biomedical significance because its levels are elevated in a variety of cancers. Using the optimized assay, a detection limit of 1 fmol was obtained. The assay was employed for the detection of miR21 in human breast adenocarcinoma MCF-7 cells and nontumorigenic epithelial MCF-10A cells. The comparison of miR21 expression level in two cell lines demonstrated higher expression of miR21 in breast cancer cell line MCF-7 compared to the nontumorigenic MCF-10A cells. Further, using the assay developed, the miR21 quantification could be performed directly in cell extracts. The hybridization assay was developed in a microplate format with a total assay time of 1.5 h and without the need for sample PCR amplification. The need for early molecular markers and their detection methods in cancer diagnosis is tremendous. The characteristics of the assay developed in this work show its suitability for early cancer diagnosis based on miRNA as a biomarker.

Copper sensing based on the far-red fluorescent protein, HcRed, from Heteractis crispa.

In this article, we report for the first time on the copper (Cu(2+)) binding characteristics of the far-red fluorescent protein, HcRed, and its application in the development of a reagentless sensing system for copper. The far-red emission of HcRed (lambda(max) = 645 nm) where background cellular fluorescence is low should prove to be advantageous in the development of the sensing system. In the studies performed in our laboratory, we found that the fluorescence of HcRed is quenched in the presence of copper ions (Cu(2+)). The results obtained through UV-visible and circular dichroism spectra generated in the presence and absence of copper, as well as Stern-Volmer plots at different temperatures, indicate static quenching of HcRed fluorescence in the presence of copper, possibly through the formation of a copper-protein complex. On the basis of this observation, we developed a reagentless sensing system for the detection of copper(II) based on HcRed as the biosensing element. A detection limit for Cu(2+) in the nanomolar range was obtained. HcRed was found to bind copper ions selectively when compared with other divalent ions. A dissociation constant of 3.6muM was observed for copper binding. Histidine and cysteine residues are commonly involved in copper binding within proteins; therefore, to investigate the role of these amino acids present in HcRed, we chemically modified Cys and His residues using iodoacetamide and diethyl pyrocarbonate, respectively. The effect of copper addition on the fluorescence of the chemically modified HcRed was investigated. The His modification of HcRed substantially affected copper ion binding, pointing to histidine as the possible amino acid residue involved in the binding of copper ions in HcRed. A purification strategy for HcRed was also developed based on a copper immobilized affinity column without the addition of any affinity tag on the protein. The HcRed-based copper sensing system can potentially be employed to perform intracellular copper detection by genetically encoding the biosensing element or can be employed in environmental sensing.