Hydrogen sulfide: an emerging gasotransmitter involved in the survival of pancreatic islets during diabetes therapy

Diabetes is a complex multisystem metabolic disease characterized by persistent hyperglycemia that lowers quality of life and increases mortality. In regard to type 1 diabetes, the phenotypic spectrum includes nearly total insulin insufficiency, whereas type 2 diabetes is characterized by relative insulin deficiency in the context of insulin resistance. In both types of diabetes, β-cell loss typically culminates in apoptosis, which is caused by many factors that initiate signaling cascades to mediate this destructive process. In recent decades, strategies to prevent “functional β-cell loss” have been considered potential treatments for both type 1 diabetes and type 2 diabetes, as β-cell function is critical for preserving normal glucose tolerance.1,2 Mitochondrial aldehyde dehydrogenase 2 (ALDH2), which is recognized primarily for its function in the metabolism of ethanol, has been reported to improve not only ethanol disorders but also a variety of clinical conditions, including cardiovascular and pulmonary diseases as well as metabolic and neurological disorders.3-5 Nevertheless, no research has investigated ALDH2 expression or function in pancreatic β-cells in relation to either form of diabetes. Research is currently being conducted to determine whether structural and functional abnormalities in ALDH2 play a role in the onset of diabetes. Notably, by upregulating antioxidant mechanisms that prevent oxidative damage, hydrogen sulfide (H2S), in addition to ALDH2, has been linked to the health-promoting effects of pancreatic β-cells.6,7 It is still uncertain what kind of H2S cellular signaling influences the survival of pancreatic β-cells. From this perspective, we investigated the intricate mechanism by which H2S signaling affects β-cell survival under hyperglycemic conditions. Recently, our findings revealed that pancreatic β-cells are directly protected against apoptotic cell death resulting from hyperglycemia via endogenous H2S generated by ALDH2.8 Importantly, H2S concentrations are controlled at the levels of generation and consumption. Cystathionine β-synthase and cystathionine-γ-lyase, two pyridoxal-5′phosphate-dependent enzymes, are primarily responsible for the natural production of H2S from L-cysteine in the cell cytosol. Moreover, L-cysteine aminotransferase and 3-mercapto-pyruvate sulfur transferase in the cytosol and mitochondria also produce H2S in the cellular system. Interestingly, the pancreatic islets contain cystathionine β-synthase, cystathionine-γ-lyase and 3-mercapto-pyruvate sulfur transferase enzymes, and the metabolism of L-cysteine produces H2S, which prevents the release of insulin by activating KATP channels, preventing the depolarization of the plasma membrane and the entry of Ca2+ into pancreatic β‐cells.9 However, H2S can accumulate as bound or acid-labile sulfur in the cellular system, which is subsequently released in response to an external stimulus. Most remarkably, bound sulfur can liberate H2S when the cellular environment becomes alkaline due to an increase in reducing substance activity in an alkaline environment. In contrast to the abovementioned studies, intracellular alkalization of islets induces insulin secretion in parallel with an increase in Ca2+ uptake.10 We have no readily available explanation for this disparity, and the exact mechanism underlying glucose-induced alkalinization remains unclear. Notably, insulin secretion is temporarily suppressed by H2S generation, which also protects against glucotoxicity following prolonged exposure to elevated glucose levels by inhibiting reactive oxygen species (ROS) production and increasing the glutathione content.11 Importantly, the reducing capacity of the two main reducing substances, glutathione and cysteine, is greater under alkaline conditions; this could account for the H2S release from the cells.12-14 It has been proposed that intracellular alkaline conditions trigger adenosine monophosphate-activated protein kinase (AMPK), which decreases growth factor withdrawal-induced apoptosis.15 Additionally, increasing the intracellular pH causes important metabolic gatekeeper enzymes to preferentially engage in catalysis, which increases the carbon flux through the pentose phosphate and glycolytic pathways. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the pentose phosphate pathway, is required for the supply of NADPH; however, a redox imbalance during oxidative stress results in a deficiency of NADPH, which is required to maintain reduced glutathione regeneration and neutralize ROS, which causes metabolic abnormalities and β-cell death.16 Most remarkably, we discovered that G6PD degradation caused by diabetes is associated with the inhibition of the capacity of G6PD to produce NADPH via cyclin-dependent kinase 5. Notably, cyclin-dependent kinase 5 activation by high glucose potently enhances the acetylation of G6PD and its degradation via the inhibition of nicotinamide adenine dinucleotide (NAD)-dependent protein deacetylase Sirtuin (SIRT) 2 expression.8 Nevertheless, we provide the first evidence that H2S-induced cyclin-dependent kinase 5 inhibition prevents G6PD acetylation and degradation by inducing SIRT2 activity, which in turn increases G6PD activity and controls ROS detoxification by increasing the glutathione/GSSG ratio. Furthermore, it has been suggested that SIRT2-mediated deacetylation of G6PD at K403 enhances G6PD activity to generate NADPH and reduce oxidative damage.17 While H2S may promote SIRT1 activity through acceleration of SIRT1 sulfhydration and protect against pathological conditions,18 its effect on the sulfhydration of SIRT2 is not known and warrants future investigation. Interestingly, we discovered that H2S increases the stability and activity of the SIRT2 protein by activating AMPK. We previously revealed that AMPK activation potently prevents high-glucose-induced mitochondrial dysfunction and death by enhancing tumor necrosis factor receptor-associated protein 1-glutaminase 1-mediated glutathione antioxidant defense in β-cells.19 Specifically, H2S-induced S-sulfhydration of Cys302 in the AMPK α2 catalytic subunit has been shown to cause AMPK activation and decrease α-synuclein accumulation in dopaminergic cells.20 In addition, L-cysteine and H2S increase phosphatidylinositol (3,4,5)-trisphosphate and AMPK/peroxisome proliferator-activated receptor γ expression and decrease ROS and vascular inflammatory marker levels in high glucose–treated U937 monocytes.21 AMPK activation increases nicotinamide phosphoribosyltransferase and NAD+ levels in mitochondrial compartments via a protein kinase Cε (PKCε)-dependent process that is connected to neuroprotection during ischemia preconditioning.22 Specifically, PKCε activation by H2S has cardioprotective benefits against ischemia–reperfusion injury.23 Furthermore, a previous study demonstrated that ethanol preconditioning or PKCε activation causes PKCε to translocate into the mitochondria, phosphorylating and activating mitochondrial ALDH2, which is correlated with cardioprotection.24,25 Importantly, inhibition of ALDH2 via CVT10216, a selective and potent inhibitor of ALDH2, blocks NaHS-induced AMPK activity and causes mitochondrial dysfunction-mediated caspase-3 induction and β-cell death, suggesting that ALDH2 is directly involved in H2S signaling. Our observation of elevated PKC-ε-ALDH2 activity in NaHS-treated cells supports these findings and suggests that H2S promotes β-cell survival in high-glucose environments (Figure 1).Figure 1: H 2 S-mitochondrial aldehyde dehydrogenase-2 activation promotes the AMPK-G6PD-mediated glutathione antioxidant system to decrease high-glucose–induced pancreatic β-cell dysfunction.3-MST: 3-Mercapto-pyruvate sulfur transferase; 4HNE: 4-hydroxynonenal; ALDH2: aldehyde dehydrogenase 2; AMPK: adenosine monophosphate-activated protein kinase; AP37: a H2S donor; CAT: cysteine aminotransferase; CBS: cystathionine β-synthase; CSE: cystathionine-γ-lyase; G6PD: glucose-6-phosphate dehydrogenase; GR: glutathione reductase; GSH: reduced glutathione; GSSG: oxidized glutathione; GYY4137: a H2S donor; H2S: hydrogen sulfide; NaHS: sodium hydrosulfide; NAMPT: nicotinamide phosphoribosyltransferase; NADP+: nicotinamide adenine dinucleotide phosphate; NADPH: reduced NADP+; PKCε: protein kinase Cε; ROS: reactive oxygen species; SIRT2: Sirtuin 2.Targeting mitochondria is one way in which H2S has been suggested to prevent stress-related cell damage or death. Mitochondrial H2S donors maintain the integrity of the mitochondria, prevent the synthesis of ROS in the mitochondria, and shield islet grafts from ischemia–reperfusion injury.26,27 H2S, however, has two different effects on mitochondrial bioenergetics: at low concentrations, it donates electrons to the electron transport chain, whereas at higher concentrations, it inhibits cytochrome c oxidase. At low micromolar concentrations, H2S has been demonstrated to be the first inorganic donor for mitochondrial energy production. Notably, the dose–response curve for the biological effects of H2S is biphasic or bell shaped. Sulfide quinone oxidoreductase (SQR) is a member of the disulfide oxidoreductase flavoprotein family that is attached to the inner mitochondrial membrane and closely related to the respiratory chain “supercomplex.” SQR uses H2S as a substrate to donate electrons to coenzyme Q at the level of the enzyme. Several mammalian tissues, such as the liver, kidney, and heart, exhibit SQR-mediated sulfide oxidation; however, SQR activity in pancreatic β-cells was not detected in either study. Moreover, cytochrome c increases protein persulfidation when H2S is present, indicating a potential role for the electron transport chain in H2S-dependent signaling in general and in controlling apoptosis specifically through caspase 9 persulfidation.28 Moreover, the thioredoxin/thioredoxin reductase (Trx/TrxR) system also naturally regulates the level of sulfhydration. Numerous investigations have confirmed the function of this system in signaling cascades that span from endoplasmic reticulum stress to apoptosis.29,30 Thus, through maintaining mitochondrial homeostasis, the mitochondrial thioredoxin system (TrxR2/Trx2) may also be involved in the regulation of sulfhydration, which may be important for cellular function. In recent years, it has been revealed that S-persulfidation of cysteine may be employed as a regulatory mechanism of metabolic enzyme enzymatic activity. In contrast, metabolic enzyme activity is inhibited by redox-sensitive cysteine glutathionylation. For example, the cysteine persulfidation of insulin is catalyzed by treating recombinant insulin with NaHS in the presence of the cystathionine-γ-lyase enzyme. Indeed, in vitro treatment with H2S can restore the enzymatic activity of glyceraldehyde-3-phosphate dehydrogenase, which has been S-glutathionylated at the catalytic cysteine site, suggesting that the redox thiol switch from S-glutathionylation to S-persulfidation regulates energy metabolism.31-33 Although cysteines are saved from hyperoxidation by persulfidation, which plays a protective role, an understanding of the dynamics of these H2S signals with relative specificity may have translation implications. Importantly, the protective effects of H2S therapy have mostly been observed at relatively low H2S concentrations or dosages, and relatively high H2S concentrations are unquestionably linked to considerable toxicity. Notably, H2S has a very short half-life in the bloodstream because of its rapid metabolism into inactive metabolites, which presents one of the main obstacles to the development of H2S therapy. The window for H2S therapy is limited, and it is challenging to keep the concentration of H2S at a reasonably constant level. Moreover, a variety of challenges exist in accurately measuring the amounts of H2S in both tissue and circulation. Owing to the difficulties in administering H2S gas directly in a variety of biological contexts, alternative sulfide sources, such as sodium sulfide and sodium salts, are frequently utilized. These salts are referred to as fast-releasing donors because they dissolve quickly in water and release a significant amount of H2S. The susceptibility of these salts to oxidation, their volatility and/or shorter effective residence time, and the quick release of H2S upon dissolution are some potential issues that might be problematic under particular conditions. More recently, peptide-based releasers that can release H2S in a more regulated way as well as slow-releasing donors that release tiny amounts of H2S progressively have been developed. Notably, H2S-related pharmacological research is a quickly emerging field that is expected to provide a number of therapeutic possibilities in the near future. Recent developments in H2S medications suggest that different H2S drugs may improve metabolic balance and significantly lower the likelihood of β-cell dysfunction under stress.