TGFβ2 and TGFβ3 mediate appropriate context-dependent phenotype of rat valvular interstitial cells

This study focused on characterizing the potential mechanism of valvular toxicity caused by TGFβ receptor inhibitors (TGFβRis) using rat valvular interstitial cells (VICs) to evaluate early biological responses to TGFβR inhibition. Three TGFβRis that achieved similar exposures in the rat were assessed. Two dual TGFβRI/-RII inhibitors caused valvulopathy, whereas a selective TGFβRI inhibitor did not, leading to a hypothesis that TGFβ receptor selectivity may influence the potency of valvular toxicity. The dual valvular toxic inhibitors had the most profound effect on altering VIC phenotype including altered morphology, migration, and extracellular matrix production. Reduction of TGFβ expression demonstrated that combined TGFβ2/β3 inhibition by small interfering RNA or neutralizing antibodies caused similar alterations as TGFβRis. Inhibition of TGFβ3 transcription was only associated with the dual TGFβRis, suggesting that TGFβRII inhibition impacts TGFβ3 transcriptional regulation, and that the potency of valvular toxicity may relate to alteration of TGFβ2/β3-mediated processes involved in maintaining proper balance of VIC phenotypes in the heart valve.

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TGFβ signaling blockade causes valvulopathy; VICs may be the cellular target

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VICs express TGFβ receptors, ligands, and pSMAD2/3, indicating autocrine regulation

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TGFβ2 and TGFβ3 maintain VIC phenotype; TGFβRis altered shape, migration, and ECM

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Maintaining TGFβ3 transcription may reduce the potency of toxicity

A systematic strategy for estimating hERG block potency and its implications in a new cardiac safety paradigm

Introduction

hERG block potency is widely used to calculate a drug's safety margin against its torsadogenic potential. Previous studies are confounded by use of different patch clamp electrophysiology protocols and a lack of statistical quantification of experimental variability. Since the new cardiac safety paradigm being discussed by the International Council for Harmonisation promotes a tighter integration of nonclinical and clinical data for torsadogenic risk assessment, a more systematic approach to estimate the hERG block potency and safety margin is needed.

Methods

A cross-industry study was performed to collect hERG data on 28 drugs with known torsadogenic risk using a standardized experimental protocol. A Bayesian hierarchical modeling (BHM) approach was used to assess the hERG block potency of these drugs by quantifying both the inter-site and intra-site variability. A modeling and simulation study was also done to evaluate protocol-dependent changes in hERG potency estimates.

Results

A systematic approach to estimate hERG block potency is established. The impact of choosing a safety margin threshold on torsadogenic risk evaluation is explored based on the posterior distributions of hERG potency estimated by this method. The modeling and simulation results suggest any potency estimate is specific to the protocol used.

Discussion

This methodology can estimate hERG block potency specific to a given voltage protocol. The relationship between safety margin thresholds and torsadogenic risk predictivity suggests the threshold should be tailored to each specific context of use, and safety margin evaluation may need to be integrated with other information to form a more comprehensive risk assessment.

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hERG potency/safety margin is a widely used nonclinical cardiac safety strategy.

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A new regulatory paradigm promotes the integration of nonclinical and clinical data.

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Lack of uncertainty quantification hindered using hERG potency in the new paradigm.

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A systematic method was established to address this limitation.

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Analysis supports using different safety margin thresholds in different context.

In Vitro Metabolite Formation in Human Hepatocytes and Cardiomyocytes and Metabolism and Tissue Distribution in Monkeys of the 2'-C-Methylguanosine Prodrug BMS-986094

BMS-986094, a 2'-C-methylguanosine prodrug for the treatment of chronic hepatitis C virus infection, was withdrawn from phase 2 clinical trials because of unexpected cardiac and renal toxicities. To better understand these toxicities, the in vitro metabolism of BMS-986094 in human hepatocytes (HHs) and human cardiomyocytes (HCMs) and the measurement of BMS-986094 and selected metabolites in monkey plasma and tissues were assessed. BMS-986094 was extensively metabolized by HHs and HCMs, resulting in more efficient formation and accumulation of the active triphosphorylated metabolite, INX-09114, and less efficient efflux of metabolites in HCMs. The predominant metabolism pathway (hydrolysis) in HHs and HCMs was not associated with the formation of reactive metabolites or oxidative stress. In cynomolgus monkeys dosed with BMS-986094 of 15 or 30 mg/kg/d for 3 weeks, the nucleoside metabolite M2 was the major plasma analyte (66%-68% of the combined area under the curve). INX-09114 was the highest drug-related species in the heart and kidney (2,610-4,280 ng/mL [males]; ∼2-420× the concentration of other analytes). Other analytes increased dose dependently, with BMS-986094 highest in diaphragm (≤4,400 ng/mL) followed by M2 in liver and kidney (≤1,360 ng/mL), and M7 and M8 in other tissues (≤124 ng/mL). Three weeks after the last dose, INX-09114 remained high in the heart and kidney (≤1,870 ng/mL), with low M2 (≤37 ng/mL) in plasma and tissues. Persistent high concentrations of INX-09114 in the heart and kidney appeared to correlate with toxicities in these tissues in monkeys.

Effects of BMS-986094, a Guanosine Nucleotide Analogue, on Mitochondrial DNA Synthesis and Function

BMS-986094, the prodrug of a guanosine nucleotide analogue (2'-C-methylguanosine), was withdrawn from clinical trials due to serious safety issues. Nonclinical investigative studies were conducted as a follow up to evaluate the potential for BMS-986094-related mitochondrial-toxicity. In vitro, BMS-986094 was applied to human hepatoma cells (HepG2 and Huh-7) or cardiomyocytes (hiPSCM) up to 19 days to assess mitochondrial DNA content and specific gene expression. There were no mitochondrial DNA changes at concentrations ≤10 µM. Transcriptional effects, such as reductions in Huh-7 MT-ND1 and MT-ND5 mRNA content and hiPSCM MT-ND1, MT-COXII, and POLRMT protein expression levels, occurred only at cytotoxic concentrations (≥10 µM) suggesting these transcriptional effects were a consequence of the observed toxicity. Additionally, BMS-986094 has a selective weak affinity for inhibition of RNA polymerases as opposed to DNA polymerases. In vivo, BMS-986094 was given orally to cynomolgus monkeys for 3 weeks or 1 month at doses of 15 or 30 mg/kg/day. Samples of heart and kidney were collected for assessment of mitochondrial respiration, mitochondrial DNA content, and levels of high energy substrates. Although pronounced cardiac and renal toxicities were observed in some monkeys at 30 mg/kg/day treated for 3-4 weeks, there were no changes in mitochondrial DNA content or ATP/GTP levels. Collectively, these data suggest that BMS-986094 is not a direct mitochondrial toxicant.

A novel method for the determination of 1,5-anhydroglucitol, a glycemic marker, in human urine utilizing hydrophilic interaction liquid chromatography/MS(3)

Plasma levels of 1,5-anhydroglucitol (1-deoxyglucose), a short-term marker of glycemic control, have been measured and used clinically in Japan since the early 1990s. Plasma levels of 1,5-anhydroglucitol are typically measured using either a commercially available enzymatic kit or GC/MS. A more sensitive method is needed for the analysis of 1,5-anhydroglucitol in urine, where levels are significantly lower than in plasma. We have developed a sensitive and selective LC/MS(3) assay utilizing hydrophilic interaction liquid chromatography and ion trap mass spectrometry for the quantitative determination of 1,5-anhydroglucitol in human urine. Diluted human urine samples were analyzed by LC/MS(3) using an APCI source operated in the negative ionization mode. Use of an ion trap allowed monitoring of MS(3) transitions for both 1,5-anhydroglucitol and the internal standard which provided sufficient selectivity and sensitivity for analysis from 50 microL of human urine. Quantitation of 1,5-anhydroglucitol levels in urine was accomplished using a calibration curve generated in water (calibration range 50 ng/mL to 10 microg/mL). Method ruggedness and reproducibility were evaluated by determining the intra- and inter-day accuracies and precision of the assay, as well as the bench-top and freeze-thaw stability. For both inter- and intra-assay evaluations, the accuracy of the assay was found to be acceptable, with the concentrations of all QCs tested not deviating more than 8% from theoretical. Four-hour bench-top and freeze-thaw stabilities were also evaluated; 1,5-anhydroglucitol was found to be stable at room temperature (<18% deviation from theoretical) and during 3 freeze-thaw cycles (<1% deviation from theoretical, except at the lowest QC level). The LC/MS(3) assay was then used to successfully determine the concentration of 1,5-AG in more than 200 urine samples from diabetic patients enrolled in a clinical study.

From clinicogenetic studies of maturity-onset diabetes of the young to unraveling complex mechanisms of glucokinase regulation

Glucokinase functions as a glucose sensor in pancreatic beta-cells and regulates hepatic glucose metabolism. A total of 83 probands were referred for a diagnostic screening of mutations in the glucokinase (GCK) gene. We found 11 different mutations (V62A, G72R, L146R, A208T, M210K, Y215X, S263P, E339G, R377C, S453L, and IVS5 + 1G>C) in 14 probands. Functional characterization of recombinant glutathionyl S-transferase-G72R glucokinase showed slightly increased activity, whereas S263P and G264S had near-normal activity. The other point mutations were inactivating. S263P showed marked thermal instability, whereas the stability of G72R and G264S differed only slightly from that of wild type. G72R and M210K did not respond to an allosteric glucokinase activator (GKA) or the hepatic glucokinase regulatory protein (GKRP). Mutation analysis of the role of glycine at position 72 by substituting E, F, K, M, S, or Q showed that G is unique since all these mutants had very low or no activity and were refractory to GKRP and GKA. Structural analysis provided plausible explanations for the drug resistance of G72R and M210K. Our study provides further evidence that protein instability in combination with loss of control by a putative endogenous activator and GKRP could be involved in the development of hyperglycemia in maturity-onset diabetes of the young, type 2. Furthermore, based on data obtained on G264S, we propose that other and still unknown mechanisms participate in the regulation of glucokinase.

Effects of a GTP-insensitive mutation of glutamate dehydrogenase on insulin secretion in transgenic mice

Glutamate dehydrogenase (GDH) plays an important role in insulin secretion as evidenced in children by gain of function mutations of this enzyme that cause a hyperinsulinism-hyperammonemia syndrome (GDH-HI) and sensitize beta-cells to leucine stimulation. GDH transgenic mice were generated to express the human GDH-HI H454Y mutation and human wild-type GDH in islets driven by the rat insulin promoter. H454Y transgene expression was confirmed by increased GDH enzyme activity in islets and decreased sensitivity to GTP inhibition. The H454Y GDH transgenic mice had hypoglycemia with normal growth rates. H454Y GDH transgenic islets were more sensitive to leucine- and glutamine-stimulated insulin secretion but had decreased response to glucose stimulation. The fluxes via GDH and glutaminase were measured by tracing 15N flux from [2-15N]glutamine. The H454Y transgene in islets had higher insulin secretion in response to glutamine alone and had 2-fold greater GDH flux. High glucose inhibited both glutaminase and GDH flux, and leucine could not override this inhibition. 15NH4Cl tracing studies showed 15N was not incorporated into glutamate in either H454Y transgenic or normal islets. In conclusion, we generated a GDH-HI disease mouse model that has a hypoglycemia phenotype and confirmed that the mutation of H454Y is disease causing. Stimulation of insulin release by the H454Y GDH mutation or by leucine activation is associated with increased oxidative deamination of glutamate via GDH. This study suggests that GDH functions predominantly in the direction of glutamate oxidation rather than glutamate synthesis in mouse islets and that this flux is tightly controlled by glucose.

Green tea polyphenols modulate insulin secretion by inhibiting glutamate dehydrogenase

Insulin secretion by pancreatic beta-cells is stimulated by glucose, amino acids, and other metabolic fuels. Glutamate dehydrogenase (GDH) has been shown to play a regulatory role in this process. The importance of GDH was underscored by features of hyperinsulinemia/hyperammonemia syndrome, where a dominant mutation causes the loss of inhibition by GTP and ATP. Here we report the effects of green tea polyphenols on GDH and insulin secretion. Of the four compounds tested, epigallocatechin gallate (EGCG) and epicatechin gallate were found to inhibit GDH with nanomolar ED(50) values and were therefore found to be as potent as the physiologically important inhibitor GTP. Furthermore, we have demonstrated that EGCG inhibits BCH-stimulated insulin secretion, a process that is mediated by GDH, under conditions where GDH is no longer inhibited by high energy metabolites. EGCG does not affect glucose-stimulated insulin secretion under high energy conditions where GDH is probably fully inhibited. We have further shown that these compounds act in an allosteric manner independent of their antioxidant activity and that the beta-cell stimulatory effects are directly correlated with glutamine oxidation. These results demonstrate that EGCG, much like the activator of GDH (BCH), can facilitate dissecting the complex regulation of insulin secretion by pharmacologically modulating the effects of GDH.

Insights into the structure and regulation of glucokinase from a novel mutation (V62M), which causes maturity-onset diabetes of the young

Glucokinase (GCK) serves as the pancreatic glucose sensor. Heterozygous inactivating GCK mutations cause hyperglycemia, whereas activating mutations cause hypoglycemia. We studied the GCK V62M mutation identified in two families and co-segregating with hyperglycemia to understand how this mutation resulted in reduced function. Structural modeling locates the mutation close to five naturally occurring activating mutations in the allosteric activator site of the enzyme. Recombinant glutathionyl S-transferase-V62M GCK is paradoxically activated rather than inactivated due to a decreased S0.5 for glucose compared with wild type (4.88 versus 7.55 mM). The recently described pharmacological activator (RO0281675) interacts with GCK at this site. V62M GCK does not respond to RO0281675, nor does it respond to the hepatic glucokinase regulatory protein (GKRP). The enzyme is also thermally unstable, but this lability is apparently less pronounced than in the proven instability mutant E300K. Functional and structural analysis of seven amino acid substitutions at residue Val62 has identified a non-linear relationship between activation by the pharmacological activator and the van der Waals interactions energies. Smaller energies allow a hydrophobic interaction between the activator and glucokinase, whereas larger energies prohibit the ligand from fitting into the binding pocket. We conclude that V62M may cause hyperglycemia by a complex defect of GCK regulation involving instability in combination with loss of control by a putative endogenous activator and/or GKRP. This study illustrates that mutations that cause hyperglycemia are not necessarily kinetically inactivating but may exert their effects by other complex mechanisms. Elucidating such mechanisms leads to a deeper understanding of the GCK glucose sensor and the biochemistry of beta-cells and hepatocytes.

Evolution of glutamate dehydrogenase regulation of insulin homeostasis is an example of molecular exaptation

Glutamate dehydrogenase (GDH) is found in all organisms and catalyzes the oxidative deamination of glutamate to 2-oxoglutarate. While this enzyme does not exhibit allosteric regulation in plants, bacteria, or fungi, its activity is tightly controlled by a number of compounds in mammals. We have previously shown that this regulation plays an important role in insulin homeostasis in humans and evolved concomitantly with a 48-residue "antenna" structure. As shown here, the antenna and some of the allosteric regulation first appears in the Ciliates. This primitive regulation is mediated by fatty acids and likely reflects the gradual movement of fatty acid oxidation from the peroxisomes to the mitochondria as the Ciliates evolved away from plants, fungi, and other protists. Mutagenesis studies where the antenna is deleted support this contention by demonstrating that the antenna is essential for fatty acid regulation. When the antenna from the Ciliates is spliced onto human GDH, it was found to fully communicate all aspects of mammalian regulation. Therefore, we propose that glutamate dehydrogenase regulation of insulin secretion is a example of exaptation at the molecular level where the antenna and associated fatty acid regulation was created to accommodate the changes in organelle function in the Ciliates and then later used to link amino acid catabolism and/or regulation of intracellular glutamate/glutamine levels in the pancreatic beta cells with insulin homeostasis in mammals.

A signaling role of glutamine in insulin secretion

Children with hypoglycemia due to recessive loss of function mutations of the beta-cell ATP-sensitive potassium (K(ATP)) channel can develop hypoglycemia in response to protein feeding. We hypothesized that amino acids might stimulate insulin secretion by unknown mechanisms, because the K(ATP) channel-dependent pathway of insulin secretion is defective. We therefore investigated the effects of amino acids on insulin secretion and intracellular calcium in islets from normal and sulfonylurea receptor 1 knockout (SUR1-/-) mice. Even though SUR1-/- mice are euglycemic, their islets are considered a suitable model for studies of the human genetic defect. SUR1-/- islets, but not normal islets, released insulin in response to an amino acid mixture ramp. This response to amino acids was decreased by 60% when glutamine was omitted. Insulin release by SUR1-/- islets was also stimulated by a ramp of glutamine alone. Glutamine was more potent than leucine or dimethyl glutamate. Basal intracellular calcium was elevated in SUR1-/- islets and was increased further by glutamine. In normal islets, methionine sulfoximine, a glutamine synthetase inhibitor, suppressed insulin release in response to a glucose ramp. This inhibition was reversed by glutamine or by 6-diazo-5-oxo-l-norleucine, a non-metabolizable glutamine analogue. High glucose doubled glutamine levels of islets. Methionine sulfoximine inhibition of glucose stimulated insulin secretion was associated with accumulation of glutamate and aspartate. We hypothesize that glutamine plays a critical role as a signaling molecule in amino acid- and glucose-stimulated insulin secretion, and that beta-cell depolarization and subsequent intracellular calcium elevation are required for this glutamine effect to occur.