Experimental diabetes increases the formation of sulfane by transsulfuration and inactivation of tyrosine aminotransferase in cytosols from rat liver

The Role of H2S in the Metabolism of Glucose and Lipids

Glucose and lipids are essential elements for maintaining the body's homeostasis, and their dysfunction may participate in the pathologies of various diseases, particularly diabetes, obesity, metabolic syndrome, cardiovascular ailments, and cancers. Among numerous endogenous mediators, the gasotransmitter hydrogen sulfide (H₂S) plays a central role in the maintenance of glucose and lipid homeostasis. Current evidence from both pharmacological studies and transgenic animal models suggest a complex relationship between H₂S and metabolic dysregulation, especially in diabetes and obesity. This notion is achieved through tissue-specific expressions and actions of H₂S on target metabolic and hormone organs including the pancreas, skeletal muscle, livers, and adipose. In this chapter, we will summarize the roles and mechanisms of H₂S in several metabolic organs/tissues that are necessary for glucose and lipid metabolic homeostasis. In addition, future research directions and valuable therapeutic avenues around the pharmacological regulation of H₂S in glycolipid metabolism disorder will be also discussed.

Hydrogen Sulfide and Glucose Homeostasis: A Tale of Sweet and the Stink

SIGNIFICANCE

Among many endogenous mediators, the gasotransmitter hydrogen sulfide (H₂S) plays an important role in the regulation of glucose homeostasis. In this article we discuss different functional roles of H₂S in several metabolic organs/tissues required in the maintenance of glucose homeostasis. Recent Advances: New evidence has emerged revealing the insulin sensitizing role of H₂S in adipose tissue and skeletal muscle biology. In addition, H₂S was demonstrated to be a potent stimulator of gluconeogenesis via the induction and stimulation of various glucose-producing pathways in the liver.

CRITICAL ISSUES

Similar to its other physiological effects, H₂S exhibits paradoxical characteristics in the regulation of glucose homeostasis: (1) H₂S stimulates glucose production via activation of gluconeogenesis and glycogenolysis in hepatocytes, yet inhibits lipolysis in adipocytes; (2) H₂S stimulates glucose uptake into adipocytes and skeletal muscle but inhibits glucose uptake into hepatocytes; (3) H₂S inhibits insulin secretion from pancreatic β cells, yet sensitizes insulin signaling and insulin-triggered response in adipose tissues and skeletal muscle. It is also unclear the impact H₂S may have on glucose metabolism and utilization by other vital organs, such as the brain.

FUTURE DIRECTIONS

Recent reports and ongoing studies lay the foundation for a general, although highly incomplete, understanding of the effect of H₂S on regulating glucose homeostasis. In this review, we describe the molecular mechanisms and physiological outcomes of the gasotransmitter H₂S on organs and tissues required for homeostatic maintenance of blood glucose. Future directions highlighting the H₂S-mediated homeostatic control of glucose metabolism under physiological and insulin-resistant conditions are also discussed. Antioxid. Redox Signal. 28, 1463-1482.

Hydrogen Sulfide in Renal Physiology and Disease

SIGNIFICANCE

Hydrogen sulfide (H2S) has only recently gained recognition for its physiological effects. It is synthesized widely in the mammalian tissues and regulates several biologic processes ranging from development, angiogenesis, neurotransmission to protein synthesis. Recent Advances: The aim of this review is to critically evaluate the evidence for a role for H2S in kidney function and disease.

CRITICAL ISSUES

H2S regulates fundamental kidney physiologic processes such as glomerular filtration and sodium reabsorption. In kidney disease states H2S appears to play a complex role in a context-dependent manner. In some disease states such as ischemia-reperfusion and diabetic kidney disease it can serve as an agent that ameliorates kidney injury. In other diseases such as cis-platinum-induced kidney disease it may mediate kidney injury although more investigation is needed. Recent studies have revealed that the actions of nitric oxide and H2S may be integrated in kidney cells.

FUTURE DIRECTIONS

Further studies are needed to understand the full impact of H2S on kidney physiology. As it is endowed with the properties of regulating blood flow, oxidative stress, and inflammation, H2S should be investigated for its role in inflammatory and toxic diseases of the kidney. Such in-depth exploration may identify specific kidney diseases in which H2S may constitute a unique target for therapeutic intervention. Antioxid. Redox Signal. 25, 720-731.

Effect of Insulin Resistance on Monounsaturated Fatty Acid Levels: A Multi-cohort Non-targeted Metabolomics and Mendelian Randomization Study

Insulin resistance (IR) and impaired insulin secretion contribute to type 2 diabetes and cardiovascular disease. Both are associated with changes in the circulating metabolome, but causal directions have been difficult to disentangle. We combined untargeted plasma metabolomics by liquid chromatography/mass spectrometry in three non-diabetic cohorts with Mendelian Randomization (MR) analysis to obtain new insights into early metabolic alterations in IR and impaired insulin secretion. In up to 910 elderly men we found associations of 52 metabolites with hyperinsulinemic-euglycemic clamp-measured IR and/or β-cell responsiveness (disposition index) during an oral glucose tolerance test. These implicated bile acid, glycerophospholipid and caffeine metabolism for IR and fatty acid biosynthesis for impaired insulin secretion. In MR analysis in two separate cohorts (n = 2,613) followed by replication in three independent studies profiled on different metabolomics platforms (n = 7,824 / 8,961 / 8,330), we discovered and replicated causal effects of IR on lower levels of palmitoleic acid and oleic acid. A trend for a causal effect of IR on higher levels of tyrosine reached significance only in meta-analysis. In one of the largest studies combining "gold standard" measures for insulin responsiveness with non-targeted metabolomics, we found distinct metabolic profiles related to IR or impaired insulin secretion. We speculate that the causal effects on monounsaturated fatty acid levels could explain parts of the raised cardiovascular disease risk in IR that is independent of diabetes development.

Decreased Gluconeogenesis in the Absence of Cystathionine Gamma-Lyase and the Underlying Mechanisms

AIMS

To investigate the regulation of hepatic glucose production by cystathionine γ-lyase (CSE)-generated hydrogen sulfide (H2S) in hepatic glucose production under physiological conditions.

RESULTS

We found that CSE knockout (KO) mice had a reduced rate of gluconeogenesis, which was reversed by administration of NaHS (an H2S donor) (i.p.). Interestingly, isolated CSE KO hepatocytes exhibited a reduced glycemic response to chemical-induced activation of the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) and glucocorticoid pathways compared with wild-type (WT) hepatocytes. Treatment with the inhibitors for PKA (KT5720) or glucocorticoid receptor (GR) (RU-486) significantly reduced H2S-stimulated glucose production from both WT and CSE KO mouse hepatocytes. NaHS treatment upregulated the protein levels of key gluconeogenic transcription factors, such as peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and CCAAT-enhancer-binding protein-β (C/EBP-β). Moreover, exogenous H2S augmented the S-sulfhydration of the rate-limiting gluconeogenic enzymes and PGC-1α and increased their activities, which were lower in untreated CSE KO hepatocytes. Finally, knockdown of PGC-1α, but not C/EBP-β, significantly decreased NaHS-induced glucose production from the primary hepatocytes.

INNOVATION

This study demonstrates the stimulatory effect of endogenous H2S on liver glucose production and reveals three underlying mechanisms; that is, H2S upregulates the expression levels of PGC-1α and phosphoenolpyruvate carboxykinase via the GR pathway; H2S upregulates the expression level of PGC-1α through the activation of the cAMP/PKA pathway as well as PGC-1α activity via S-sulfhydration; and H2S upregulates the expression and the activities (by S-sulfhydration) of glucose-6-phosphatase and fructose-1,6-bisphosphatase.

CONCLUSION

This study may offer clues for the homeostatic regulation of glucose metabolism under physiological conditions and its dysregulation in metabolic syndrome.

Tyrosine Is Associated with Insulin Resistance in Longitudinal Metabolomic Profiling of Obese Children

In obese children, hyperinsulinaemia induces adverse metabolic consequences related to the risk of cardiovascular and other disorders. Branched-chain amino acids (BCAA) and acylcarnitines (Carn), involved in amino acid (AA) degradation, were linked to obesity-associated insulin resistance, but these associations yet have not been studied longitudinally in obese children. We studied 80 obese children before and after a one-year lifestyle intervention programme inducing substantial weight loss >0.5 BMI standard deviation scores in 40 children and no weight loss in another 40 children. At baseline and after the 1-year intervention, we assessed insulin resistance (HOMA index), fasting glucose, HbA1c, 2 h glucose in an oral glucose tolerance test, AA, and Carn. BMI adjusted metabolite levels were associated with clinical markers at baseline and after intervention, and changes with the intervention period were evaluated. Only tyrosine was significantly associated with HOMA (p < 0.05) at baseline and end and with change during the intervention (p < 0.05). In contrast, ratios depicting BCAA metabolism were negatively associated with HOMA at baseline (p < 0.05), but not in the longitudinal profiling. Stratified analysis revealed that the children with substantial weight loss drove this association. We conclude that tyrosine alterations in association with insulin resistance precede alteration in BCAA metabolism. This trial is registered with ClinicalTrials.gov Identifier NCT00435734.

Enzymology of H2S biogenesis, decay and signaling

SIGNIFICANCE

Hydrogen sulfide (H2S), produced by the desulfuration of cysteine or homocysteine, functions as a signaling molecule in an array of physiological processes including regulation of vascular tone, the cellular stress response, apoptosis, and inflammation.

RECENT ADVANCES

The low steady-state levels of H2S in mammalian cells have been recently shown to reflect a balance between its synthesis and its clearance. The subversion of enzymes in the cytoplasmic trans-sulfuration pathway for producing H2S from cysteine and/or homocysteine versus producing cysteine from homocysteine, presents an interesting regulatory problem.

CRITICAL ISSUES

It is not known under what conditions the enzymes operate in the canonical trans-sulfuration pathway and how their specificity is switched to catalyze the alternative H2S-producing reactions. Similarly, it is not known if and whether the mitochondrial enzymes, which oxidize sulfide and persulfide (or sulfane sulfur), are regulated to increase or decrease H2S or sulfane-sulfur pools.

FUTURE DIRECTIONS

In this review, we focus on the enzymology of H2S homeostasis and discuss H2S-based signaling via persulfidation and thionitrous acid.

Hydrogen sulfide impairs glucose utilization and increases gluconeogenesis in hepatocytes

Mounting evidence has established hydrogen sulfide (H(2)S) as an important gasotransmitter with multifaceted physiological functions. The aim of the present study was to investigate the role of H(2)S on glucose utilization, glycogen synthesis, as well as gluconeogenesis in both HepG(2) cells and primary mouse hepatocytes. Incubation with NaHS (a H(2)S donor) impaired glucose uptake and glycogen storage in HepG(2) cells via decreasing glucokinase activity. Adenovirus-mediated cystathionine γ-lyase (CSE) overexpression increased endogenous H(2)S production and lowered glycogen content in HepG(2) cells. Glycogen content was significantly higher in liver tissues from CSE knockout (KO) mice compared to that from wild type (WT) mice in fed condition. Glucose consumption was less in primarily cultured hepatocytes isolated from WT mice than those from CSE KO mice, but more glucose was produced by hepatocytes via gluconeogenesis and glycogenolysis pathways in WT mice than in CSE KO mice. NaHS treatment reduced the phosphorylation of AMP-activated protein kinase, whereas stimulation of AMP-activated protein kinase by 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside reversed H(2)S-impaired glucose uptake. H(2)S-increased glucose production was likely through increased phosphoenolpyruvate carboxykinase activity. In addition, insulin at the physiological range inhibited CSE expression, and H(2)S decreased insulin-stimulated phosphorylation of Akt in HepG(2) cells. CSE expression was increased, however, in insulin-resistant state induced by exposing cells to high levels of insulin (500 nm) and glucose (33 mm) for 24 h. Taken together, these data suggest that the interaction of H(2)S and insulin in liver plays a pivotal role in regulating insulin sensitivity and glucose metabolism.

Increased renal methylglyoxal formation with down-regulation of PGC-1α-FBPase pathway in cystathionine γ-lyase knockout mice

We have previously reported that hydrogen sulfide (H(2)S), a gasotransmitter and vasodilator has cytoprotective properties against methylglyoxal (MG), a reactive glucose metabolite associated with diabetes and hypertension. Recently, H(2)S was shown to up-regulate peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α, a key gluconeogenic regulator that enhances the gene expression of the rate-limiting gluconeogenic enzyme, fructose-1,6-bisphosphatase (FBPase). Thus, we sought to determine whether MG levels and gluconeogenic enzymes are altered in kidneys of 6-22 week-old cystathionine γ-lyase knockout (CSE(-/-); H(2)S-producing enzyme) male mice. MG levels were determined by HPLC. Plasma glucose levels were measured by an assay kit. Q-PCR was used to measure mRNA levels of PGC-1α and FBPase-1 and -2. Coupled-enzymatic assays were used to determine FBPase activity, or triosephosphate levels. Experimental controls were either age-matched wild type mice or untreated rat A-10 cells. Interestingly, we observed a significant decrease in plasma glucose levels along with a significant increase in plasma MG levels in all three age groups (6-8, 14-16, and 20-22 week-old) of the CSE(-/-) mice. Indeed, renal MG and triosephosphates were increased, whereas renal FBPase activity, along with its mRNA levels, were decreased in the CSE(-/-) mice. The decreased FBPase activity was accompanied by lower levels of its product, fructose-6-phosphate, and higher levels of its substrate, fructose-1,6-bisphosphate in renal extracts from the CSE(-/-) mice. In agreement, PGC-1α mRNA levels were also significantly down-regulated in 6-22 week-old CSE(-/-) mice. Furthermore, FBPase-1 and -2 mRNA levels were reduced in aorta tissues from CSE(-/-) mice. Administration of NaHS, a H(2)S donor, increased the gene expression of PGC-1α and FBPase-1 and -2 in cultured rat A-10 cells. In conclusion, overproduction of MG in CSE(-/-) mice is due to a H(2)S-mediated down-regulation of the PGC-1α-FBPase pathway, further suggesting the important role of H(2)S in the regulation of glucose metabolism and MG generation.

Emerging perspectives on essential amino acid metabolism in obesity and the insulin-resistant state

Dysregulation of insulin action is most often considered in the context of impaired glucose homeostasis, with the defining feature of diabetes mellitus being elevated blood glucose concentration. Complications arising from the hyperglycemia accompanying frank diabetes are well known and epidemiological studies point to higher risk toward development of metabolic disease in persons with impaired glucose tolerance. Although the central role of proper blood sugar control in maintaining metabolic health is well established, recent developments have begun to shed light on associations between compromised insulin action [obesity, prediabetes, and type 2 diabetes mellitus (T2DM)] and altered intermediary metabolism of fats and amino acids. For amino acids, changes in blood concentrations of select essential amino acids and their derivatives, in particular BCAA, sulfur amino acids, tyrosine, and phenylalanine, are apparent with obesity and insulin resistance, often before the onset of clinically diagnosed T2DM. This review provides an overview of these changes and places recent observations from metabolomics research into the context of historical reports in the areas of biochemistry and nutritional biology. Based on this synthesis, a model is proposed that links the FFA-rich environment of obesity/insulin resistance and T2DM with diminution of BCAA catabolic enzyme activity, changes in methionine oxidation and cysteine/cystine generation, and tissue redox balance (NADH/NAD+).

The pathogenic role of cystathionine γ-lyase/hydrogen sulfide in streptozotocin-induced diabetes in mice

Reduced β-cell mass and increased activities of ATP-sensitive K(+) channels in pancreatic β cells are associated with the pathogenesis of diabetes. Cystathionine γ-lyase (CSE) is a major hydrogen sulfide (H(2)S)-producing enzyme in pancreatic β cells. Herein, we examine the effects of genetic and pharmacologic ablation of CSE on β-cell functions and their correlation with streptozotocin (STZ)-induced diabetes. Compared with wild-type mice, CSE knockout (CSE KO) mice that received STZ injections exhibited a delayed onset of diabetic status. The application of dl-propargylglycine (PPG) to inhibit CSE activity protected wild-type mice from STZ-induced hyperglycemia and hypoinsulinemia. STZ significantly increased pancreatic H(2)S production in wild-type mice but not in CSE KO mice. STZ induced more apoptotic β-cell death in wild-type mice than in CSE KO mice. STZ exposure decreased the viability of cultured INS-1E cells, which was partly reversed by PPG co-treatment. STZ also significantly stimulated H(2)S production in cultured INS-1E cells. In addition, STZ stimulated ATP-sensitive K(+) currents in pancreatic β cells from wild-type mice but not in the presence of PPG or in β cells from CSE KO mice. Sodium hydrosulfide injection instantly increased blood glucose, decreased plasma insulin, and deteriorated glucose tolerance in mice. Take together, these results provide evidence that the CSE/H(2)S system plays a critical role in regulating β-cell functions.

Streptozotocin-induced diabetes in the rat is associated with enhanced tissue hydrogen sulfide biosynthesis

This investigation is aimed to determine whether the biosynthesis of H(2)S, an endogenous vasodilator gas, is altered in the streptozotocin-diabetic rat. Plasma H(2)S concentration as well as the activity, and expression, of H(2)S synthesizing enzymes (namely cystathionine-gamma-lyase (CSE) and cystathionine-beta-synthetase (CBS)) were measured in various tissues of non-diabetic, streptozotocin-diabetic and insulin-treated diabetic rats. H(2)S formation in pancreas and liver was increased in diabetic rats. Both CSE and CBS mRNAs were increased in liver of diabetic animals. Similarly, CBS mRNA was increased in pancreas. Insulin treatment restored the changes in H(2)S metabolism seen. The findings of this study suggest that the metabolism of H(2)S in pancreas and liver is altered in the streptozotocin-diabetic rat. This is the first study in which a derangement in H(2)S biosynthesis in diabetes has been demonstrated. H(2)S may play a part in the aetiology or development of diabetes in this animal model.

Calpain is a mediator of preservation-reperfusion injury in rat liver transplantation

Proteases as well as alterations in intracellular calcium have important roles in hepatic preservation-reperfusion injury, and increased calpain activity recently has been demonstrated in liver allografts. Experiments were designed to evaluate (i) hepatic cytosolic calpain activity during different periods of cold ischemia (CI), rewarming, or reperfusion, and (ii) effects of inhibition of calpain on liver graft function using the isolated perfused rat liver and arterialized orthotopic liver transplantation models. Calpain activity was assayed using the fluorogenic substrate Suc-Leu-Leu-Val-Tyr-7-amino-4-methyl coumarin (AMC) and expressed as mean +/- SD pmol AMC released/min per mg of cytosolic protein. Calpain activity rose significantly after 24 hr of CI in University of Wisconsin solution and further increased with longer preservation. Activity also increased within 30 min of rewarming, peaking at 120 min. Increased durations of CI preceding rewarming resulted in significantly higher activity (P < 0.01). Calpain activity increased rapidly upon reperfusion and was significantly enhanced by previous CI (P < 0.01). Calpain inhibition with Cbz-Val-Phe methyl ester significantly decreased aspartate aminotransferase released in the isolated perfused rat liver perfusate (P < 0.05). Duration of survival after orthotopic liver transplantation using livers cold-preserved for 40 hr was also significantly increased (P < 0.05) with calpain inhibitor. In conclusion, calpain proteases are activated during each phase of transplantation and are likely to play an important role in the mechanisms of preservation-reperfusion injury.