Effects of 3-mercaptopicolinic acid and a derivative of chlorogenic acid (S-3483) on hepatic and islet glucose-6-phosphatase activity

Chlorogenic acid ameliorates obesity by preventing energy balance shift in high-fat diet induced obese mice

BACKGROUND

Chlorogenic acid is a type of phenolic acid found in many plants. Chlorogenic acid has an anti-obesity effect with an unclear mechanism. The present study aimed to investigate the regulatory effect of chlorogenic acid on energy balance in high-fat diet (HFD) induced obese C57BL/6J mice administrated 100 mg kg^-1 chlorogenic acid for 13 weeks.

RESULTS

The consumption of chlorogenic acid ameliorated HFD induced obesity. Chlorogenic acid did not change the physical activity but significantly decreased food intake and increased body temperature, thermal dissipation and brown adipose tissue activity. Moreover, chlorogenic acid improved glucose tolerance but had a moderate impact on other blood indices. Additionally, chlorogenic acid failed to restore the microbiota change associated with HFD induced obesity, but modified the gut bacterial composition in a unique way.

CONCLUSION

Supplementation with chlorogenic acid can improve HFD induced obesity and associated glucose intolerance mainly via regulating food intake and energy expenditure. © 2020 Society of Chemical Industry.

Characterization of 3-[(Carboxymethyl)thio]picolinic Acid: A Novel Inhibitor of Phosphoenolpyruvate Carboxykinase

Phosphoenolpyruvate carboxykinase (PEPCK) has traditionally been characterized for its role in the first committed step of gluconeogenesis. The current understanding of PEPCK's metabolic role has recently expanded to include it serving as a general mediator of tricarboxylic acid cycle flux. Selective inhibition of PEPCK in vivo and in vitro has been achieved with 3-mercaptopicolinic acid (MPA) (K_i ∼ 8 μM), whose mechanism of inhibition has been elucidated only recently. On the basis of crystallographic and mechanistic data of various inhibitors of PEPCK, MPA was used as the initial chemical scaffold to create a potentially more selective inhibitor, 3-[(carboxymethyl)thio]picolinic acid (CMP), which has been characterized both structurally and kinetically here. These data demonstrate that CMP acts as a competitive inhibitor at the OAA/PEP binding site, with its picolinic acid moiety coordinating directly with the M1 metal in the active site (K_i ∼ 29-55 μM). The extended carboxy tail occupies a secondary binding cleft that was previously shown could be occupied by sulfoacetate (K_i ∼ 82 μM) and for the first time demonstrates the simultaneous occupation of both OAA/PEP subsites by a single molecular structure. By occupying both the OAA/PEP binding subsites simultaneously, CMP and similar molecules can potentially be used as a starting point for the creation of additional selective inhibitors of PEPCK.

Maintenance of luminal NADPH in the endoplasmic reticulum promotes the survival of human neutrophil granulocytes

The present study demonstrates the expression of hexose-6-phosphate dehydrogenase and 11 beta-hydroxysteroid dehydrogenase type 1 in human neutrophils, and the presence and activity of these enzymes in the microsomal fraction of the cells. Their concerted action together with the previously described glucose-6-phosphate transporter is responsible for cortisone-cortisol interconversion detected in human neutrophils. Furthermore, the results suggest that luminal NADPH generation by the cortisol dehydrogenase activity of 11 beta-hydroxysteroid dehydrogenase type 1 prevents neutrophil apoptosis provoked by the inhibition of the glucose-6-phosphate transporter. In conclusion, the maintenance of the luminal NADPH pool is an important antiapoptotic factor in neutrophil granulocytes.

Pathways in beta-cell stimulus-secretion coupling as targets for therapeutic insulin secretagogues

Physiologically, insulin secretion is subject to a dual, hierarchal control by triggering and amplifying pathways. By closing ATP-sensitive K+ channels (KATP channels) in the plasma membrane, glucose and other metabolized nutrients depolarize beta-cells, stimulate Ca2+ influx, and increase the cytosolic concentration of free Ca2+ ([Ca2+]i), which constitutes the indispensable triggering signal to induce exocytosis of insulin granules. The increase in beta-cell metabolism also generates amplifying signals that augment the efficacy of Ca2+ on the exocytotic machinery. Stimulatory hormones and neurotransmitters modestly increase the triggering signal and strongly activate amplifying pathways biochemically distinct from that set into operation by nutrients. Many drugs can increase insulin secretion in vitro, but only few have a therapeutic potential. This review identifies six major pathways or sites of stimulus-secretion coupling that could be aimed by potential insulin-secreting drugs and describes several strategies to reach these targets. It also discusses whether these perspectives are realistic or theoretical only. These six possible beta-cell targets are 1) stimulation of metabolism, 2) increase of [Ca2+]i by closure of K+ ATP channels, 3) increase of [Ca2+]i by other means, 4) stimulation of amplifying pathways, 5) action on membrane receptors, and 6) action on nuclear receptors. The theoretical risk of inappropriate insulin secretion and, hence, of hypoglycemia linked to these different approaches is also envisaged.

Coffee consumption, type 2 diabetes and impaired glucose tolerance in Swedish men and women

OBJECTIVES

The association between coffee consumption, type 2 diabetes and impaired glucose tolerance was examined. In addition, indicators of insulin sensitivity and beta-cell function according to homeostasis model assessment were studied in relation to coffee consumption.

DESIGN

Population-based cross-sectional study.

SETTING AND SUBJECTS

The study comprised 7949 healthy Swedish subjects aged 35-56 years residing within five municipalities of Stockholm. An oral glucose tolerance test identified 55 men and 52 women with previously undiagnosed type 2 diabetes and 172 men and 167 women with impaired glucose tolerance. Information about coffee consumption and other factors was obtained by questionnaire.

RESULTS

The relative risks (adjusted for potential confounders) of type 2 diabetes and impaired glucose tolerance when drinking >/=5 cups of coffee per day compared with </=2 cups per day in men were 0.45 [95% confidence intervals (CI) 0.22-0.92] and 0.63 (CI: 0.41-0.97), respectively, and in women 0.27 (CI: 0.11-0.66) and 0.47 (CI: 0.29-0.76) respectively. In subjects with type 2 diabetes and impaired glucose tolerance, high coffee consumption (>/=5 cups day(-1)) was inversely associated with insulin resistance. In addition, in those with type 2 diabetes and in women (not in men) with impaired glucose tolerance high coffee consumption was inversely associated with low beta-cell function. In women, but not obviously in men, with normal glucose tolerance, coffee consumption was associated with a reduced risk of insulin resistance.

CONCLUSIONS

The results of this study indicated that high consumers of coffee have a reduced risk of type 2 diabetes and impaired glucose tolerance. The beneficial effects may involve both improved insulin sensitivity and enhanced insulin response.

Enzymatic characterization of the pancreatic islet-specific glucose-6-phosphatase-related protein (IGRP)

Glucose is the main physiological stimulus for insulin biosynthesis and secretion by pancreatic beta-cells. Glucose-6-phosphatase (G-6-Pase) catalyzes the dephosphorylation of glucose-6-phosphate to glucose, an opposite process to glucose utilization. G-6-Pase activity in pancreatic islets could therefore be an important factor in the control of glucose metabolism and, consequently, of glucose-dependent insulin secretion. While G-6-Pase activity has been shown to be present in pancreatic islets, the gene responsible for this activity has not been conclusively identified. A homolog of liver glucose-6-phosphatase (LG-6-Pase) specifically expressed in islets was described earlier; however, the authors could not demonstrate enzymatic activity for this protein. Here we present evidence that the previously identified islet-specific glucose-6-phosphatase-related protein (IGRP) is indeed the major islet glucose-6-phosphatase. IGRP overexpressed in insect cells possesses enzymatic activity comparable to the previously described G-6-Pase activity in islets. The K(m) and V(max) values determined using glucose-6-phosphate as the substrate were 0.45 mm and 32 nmol/mg/min by malachite green assay, and 0.29 mm and 77 nmol/mg/min by glucose oxidase/peroxidase coupling assay, respectively. High-throughput screening of a small molecule library led to the identification of an active compound that specifically inhibits IGRP enzymatic activity. Interestingly, this inhibitor did not affect LG-6-Pase activity, while conversely LG-6-Pase inhibitors did not affect IGRP activity. These data demonstrate that IGRP is likely the authentic islet-specific glucose-6-phosphatase catalytic subunit, and selective inhibitors to this molecule can be obtained. IGRP inhibitors may be an attractive new approach for the treatment of insulin secretion defects in type 2 diabetes.

Evidence for the expression of both the hydrolase and translocase components of hepatic glucose-6-phosphatase in murine pancreatic islets

Glucose-6-phosphatase (G6Pase) is a multicomponent enzyme system which regulates the catalysis of glucose-6-phosphate (G6P) to glucose and inorganic phosphate. G6Pase can antagonize glucose phosphorylation, a step prerequisite in the regulation of insulin secretion from pancreatic beta cells, and G6Pase activity is increased in islets isolated from animal models of type II diabetes. Using RT-PCR with hepatic G6Pase catalytic subunit primers, we demonstrate that the sizes of amplified products from ob/ob mouse islets are identical to those from liver cDNA. This was confirmed by PCR-based cloning and sequencing of the hepatic G6Pase catalytic subunit open reading frame from islet cDNA. The expression in islets of the G6P transporter, G6PT1, was also demonstrated, suggesting that all of the identified hepatic G6Pase system genes are expressed in pancreatic islets. Finally, the expression of islet-specific G6Pase-related protein (IGRP) in pancreatic islets was confirmed and its expression in liver was also observed.

Inhibition of microsomal glucose-6-phosphate transport in human neutrophils results in apoptosis: a potential explanation for neutrophil dysfunction in glycogen storage disease type 1b

Mutations in the gene of the hepatic glucose-6-phosphate transporter cause glycogen storage disease type 1b. In this disease, the altered glucose homeostasis and liver functions are accompanied by an impairment of neutrophils/monocytes. However, neither the existence of a microsomal glucose-6-phosphate transport, nor the connection between its defect and cell dysfunction has been demonstrated in neutrophils/monocytes. In this study we have characterized the microsomal glucose-6-phosphate transport of human neutrophils and differentiated HL-60 cells. The transport of glucose-6-phosphate was sensitive to the chlorogenic acid derivative S3483, N-ethylmaleimide, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, known inhibitors of the hepatic microsomal glucose-6-phosphate transporter. A glucose-6-phosphate uptake was also present in microsomes from undifferentiated HL-60 and Jurkat cells, but it was insensitive to S3483. The treatment with S3484 of intact human neutrophils and differentiated HL-60 cells mimicked some leukocyte defects of glycogen storage disease type 1b patients (ie, the drug inhibited phorbol myristate acetate-induced superoxide anion production and reduced the size of endoplasmic reticulum Ca(2+) stores). Importantly, the treatment with S3484 also resulted in apoptosis of human neutrophils and differentiated HL-60 cells, while undifferentiated HL-60 and Jurkat cells were unaffected by the drug. The proapoptotic effect of S3483 was prevented by the inhibition of nicotinamide adenine dinucleotide phosphate oxidase or by antioxidant treatment. These results suggest that microsomal glucose-6-phosphate transport has a role in the antioxidant protection of neutrophils, and that the genetic defect of the transporter leads to the impairment of cellular functions and apoptosis.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

Hepatic glucose uptake, gluconeogenesis and the regulation of glycogen synthesis

Hepatic glycogen is replenished during the absorptive period postprandially. This repletion is prompted partly by an increased hepatic uptake of glucose by the liver, partly by metabolite and hormonal signals in the portal vein, and partly by an increased gluconeogenic flux to glycogen (glyconeogenesis). There is some evidence that the direct formation of glycogen from glucose and that formed by gluconeogenic pathways is linked. This includes: (i) the inhibition of all glycogen synthesis, in vivo, when gluconeogenic flux is blocked by inhibitors; (ii) a dual relationship between glucose concentrations, lactate uptake by the liver and glycogen synthesis (by both pathways) which indicates that glucose sets the maximal rates of glycogen synthesis while lactate uptake determines the actual flux rate to glycogen; (iii) the decrease of both gluconeogenesis and glycogen synthesis by the biguanide, metformin; and (iv) correlations between increased gluconeogenesis and liver glycogen in obese patients and animal models. The degree to which the liver extracts portal glucose is not entirely agreed upon although a preponderance of evidence points to about a 5% extraction rate, following meals, which is dependent on a stimulation of glucokinase. This enzyme may be linked to the expression of other enzymes in the gluconeogenic pathway. Perivenous cells in the liver may induce additional gluconeogenesis in the periportal cells by increasing glycolytically produced lactate. A number of potential mechanisms therefore exist which could link glycogen synthesis from glucose and gluconeogenic substrate.

Alterations of carbohydrate and lipid intermediary metabolism during inhibition of glucose-6-phosphatase in rats

S 4048 (1-[2-(4-Chloro-phenyl)-cyclopropylmethoxy]-3, 4-dihydroxy-5-(3-imidazo[4, 5-b]pyridin-1-yl-3-phenyl-acryloyloxy)-cyclohexanecarboxylic acid), a derivative of chlorogenic acid, specifically inhibits the glucose-6-phosphate translocating component T1 of the glucose-6-phosphatase system. Its pharmacological effect was studied on carbohydrate and lipid parameters in rats. In starved and fed rats, S 4048 caused a dose-dependent reduction of blood glucose levels with a corresponding increase in hepatic and renal glycogen and glucose-6-phosphate. The major quantitative route of carbon flux in the liver during S 4048-induced inhibition of the glucose-6-phosphatase activity seemed to be glycogenesis. Plasma free fatty acids were increased secondarily due to the S 4048-induced hypoglycemia. Hepatic triglycerides were increased possibly due to increased re-esterification of the readily available free fatty acids. Glucose-6-phosphate translocase inhibitors may be useful for experimentally studying aspects of type 1 glycogen storage disease in laboratory animals as well as for the therapeutic modulation of inappropriately high rates of hepatic glucose production in type 2 diabetes.