The physiological functions of hydrogen sulfide (H 2 S) include vasorelaxation, stimulation of cellular bioenergetics, and promotion of angiogenesis. Analysis of human colon cancer biopsies and patient-matched normal margin mucosa revealed the selective upregulation of the H 2 S-producing enzyme cystathionine-β-synthase (CBS) in colon cancer, resulting in an increased rate of H 2 S production. Similarly, colon cancer-derived epithelial cell lines (HCT116, HT-29, LoVo) exhibited selective CBS up-regulation and increased H 2 S production, compared with the nonmalignant colonic mucosa cells, NCM356. CBS localized to the cytosol, as well as the mitochondrial outer membrane. ShRNA-mediated silencing of CBS or its pharmacological inhibition with aminooxyacetic acid reduced HCT116 cell proliferation, migration, and invasion; reduced endothelial cell migration in tumor/endothelial cell cocultures; and suppressed mitochondrial function (oxygen consumption, ATP turnover, and respiratory reserve capacity), as well as glycolysis. Treatment of nude mice with aminooxyacetic acid attenuated the growth of patientderived colon cancer xenografts and reduced tumor blood flow. Similarly, CBS silencing of the tumor cells decreased xenograft growth and suppressed neovessel density, suggesting a role for endogenous H 2 S in tumor angiogenesis. In contrast to CBS, silencing of cystathionine-γ-lyase (the expression of which was unchanged in colon cancer) did not affect tumor growth or bioenergetics. In conclusion, H 2 S produced from CBS serves to (i) maintain colon cancer cellular bioenergetics, thereby supporting tumor growth and proliferation, and (ii) promote angiogenesis and vasorelaxation, consequently providing the tumor with blood and nutritients. The current findings identify CBS-derived H 2 S as a tumor growth factor and anticancer drug target.

Hydrogen sulfide (H 2 S) is a unique gasotransmitter, with regulatory roles in the cardiovascular, nervous, and immune systems. Some of the vascular actions of H 2 S (stimulation of angiogenesis, relaxation of vascular smooth muscle) resemble those of nitric oxide (NO). Although it was generally assumed that H 2 S and NO exert their effects via separate pathways, the results of the current study show that H 2 S and NO are mutually required to elicit angiogenesis and vasodilatation. Exposure of endothelial cells to H 2 S increases intracellular cyclic guanosine 5′-monophosphate (cGMP) in a NO-dependent manner, and activated protein kinase G (PKG) and its downstream effector, the vasodilator-stimulated phosphoprotein (VASP). Inhibition of endothelial isoform of NO synthase (eNOS) or PKG-I abolishes the H 2 S-stimulated angiogenic response, and attenuated H 2 S-stimulated vasorelaxation, demonstrating the requirement of NO in vascular H 2 S signaling. Conversely, silencing of the H 2 S-producing enzyme cystathionine-γ-lyase abolishes NO-stimulated cGMP accumulation and angiogenesis and attenuates the acetylcholine-induced vasorelaxation, indicating a partial requirement of H 2 S in the vascular activity of NO. The actions of H 2 S and NO converge at cGMP; though H 2 S does not directly activate soluble guanylyl cyclase, it maintains a tonic inhibitory effect on PDE5, thereby delaying the degradation of cGMP. H 2 S also activates PI3K/Akt, and increases eNOS phosphorylation at its activating site S1177. The cooperative action of the two gasotransmitters on increasing and maintaining intracellular cGMP is essential for PKG activation and angiogenesis and vasorelaxation. H 2 S-induced wound healing and microvessel growth in matrigel plugs is suppressed by pharmacological inhibition or genetic ablation of eNOS. Thus, NO and H 2 S are mutually required for the physiological control of vascular function. N itric oxide (NO) and hydrogen sulfide (H 2 S) are two endogenous gasotransmitters whose regulatory roles in the cardiovascular system include vasorelaxation and stimulation of angiogenesis (1, 2). In endothelial cells, NO is synthesized by the endothelial isoform of NO synthase (eNOS). The principal pathway of NO signaling involves binding to the heme moiety of the soluble guanylyl cyclase (sGC) and production of the second messenger cyclic guanosine 5′-monophosphate (cGMP), followed by the activation of protein kinase G (PKG) (3, 4). However, vascular H 2 S is generated from L-cysteine by two pyridoxal 5′-phosphate-dependent enzymes, cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE), and by the combined action of cysteine aminotransferase (CAT) and 3-mercaptopyruvate sulfurtransferase (3-MST); activation of the ATP-dependent potassium channel (K ATP channel), modulation of cell metabolism, and posttranslational protein modifications via sulfhydration have been identified as some of its key signaling pathways (5-7).It is generally assumed that the signaling pathways of NO and H 2 S are independent. In the ...

Until recently, hydrogen sulfide (H2S) was exclusively viewed a toxic gas and an environmental hazard, with its toxicity primarily attributed to the inhibition of mitochondrial Complex IV, resulting in a shutdown of mitochondrial electron transport and cellular ATP generation. Work over the last decade established multiple biological regulatory roles of H2S, as an endogenous gaseous transmitter. H2S is produced by cystathionine γ-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). In striking contrast to its inhibitory effect on Complex IV, recent studies showed that at lower concentrations, H2S serves as a stimulator of electron transport in mammalian cells, by acting as a mitochondrial electron donor. Endogenous H2S, produced by mitochondrially localized 3-MST, supports basal, physiological cellular bioenergetic functions; the activity of this metabolic support declines with physiological aging. In specialized conditions (calcium overload in vascular smooth muscle, colon cancer cells), CSE and CBS can also associate with the mitochondria; H2S produced by these enzymes, serves as an endogenous stimulator of cellular bioenergetics. The current article overviews the biochemical mechanisms underlying the stimulatory and inhibitory effects of H2S on mitochondrial function and cellular bioenergetics and discusses the implication of these processes for normal cellular physiology. The relevance of H2S biology is also discussed in the context of colonic epithelial cell physiology: colonocytes are exposed to high levels of sulfide produced by enteric bacteria, and serve as a metabolic barrier to limit their entry into the mammalian host, while, at the same time, utilizing it as a metabolic 'fuel'.

The goal of the present studies was to investigate the role of changes in hydrogen sulfide (H 2 S) homeostasis in the pathogenesis of hyperglycemic endothelial dysfunction. Exposure of bEnd3 microvascular endothelial cells to elevated extracellular glucose (in vitro "hyperglycemia") induced the mitochondrial formation of reactive oxygen species (ROS), which resulted in an increased consumption of endogenous and exogenous H 2 S. Replacement of H 2 S or overexpression of the H 2 S-producing enzyme cystathionine-γ-lyase (CSE) attenuated the hyperglycemia-induced enhancement of ROS formation, attenuated nuclear DNA injury, reduced the activation of the nuclear enzyme poly(ADP-ribose) polymerase, and improved cellular viability. In vitro hyperglycemia resulted in a switch from oxidative phosphorylation to glycolysis, an effect that was partially corrected by H 2 S supplementation. Exposure of isolated vascular rings to high glucose in vitro induced an impairment of endothelium-dependent relaxations, which was prevented by CSE overexpression or H 2 S supplementation. siRNA silencing of CSE exacerbated ROS production in hyperglycemic endothelial cells. Vascular rings from CSE −/− mice exhibited an accelerated impairment of endothelium-dependent relaxations in response to in vitro hyperglycemia, compared with wild-type controls. Streptozotocininduced diabetes in rats resulted in a decrease in the circulating level of H 2 S; replacement of H 2 S protected from the development of endothelial dysfunction ex vivo. In conclusion, endogenously produced H 2 S protects against the development of hyperglycemia-induced endothelial dysfunction. We hypothesize that, in hyperglycemic endothelial cells, mitochondrial ROS production and increased H 2 S catabolism form a positive feed-forward cycle. H 2 S replacement protects against these alterations, resulting in reduced ROS formation, improved endothelial metabolic state, and maintenance of normal endothelial function.

It is well established that exposure of mammalian cells to hydrogen sulfide (H(2)S) suppresses mitochondrial function by inhibiting cytochrome-c oxidase (CcOX; complex IV). However, recent experimental data show that administration of H(2)S to mammalian cells can serve as an electron donor and inorganic source of energy. The aim of our study was to investigate the role of endogenously produced H(2)S in the regulation of mitochondrial electron transport and oxidative phosphorylation in isolated liver mitochondria and in the cultured murine hepatoma cell line Hepa1c1c7. Low concentrations of H(2)S (0.1-1 μM) elicited a significant increase in mitochondrial function, while higher concentrations of H(2)S (3-30 μM) were inhibitory. The positive bioenergetic effect of H(2)S required a basal activity of the Krebs cycle and was most pronounced at intermediate concentrations of succinate. 3-mercaptopyruvate (3-MP), the substrate of the mitochondrial enzyme 3-mercaptopyruvate sulfurtransferase (3-MST) stimulated mitochondrial H(2)S production and enhanced mitochondrial electron transport and cellular bioenergetics at low concentrations (10-100 nM), while at higher concentrations, it inhibited cellular bioenergetics. SiRNA silencing of 3-MST reduced basal bioenergetic parameters and prevented the stimulating effect of 3-MP on mitochondrial bioenergetics. Silencing of sulfide quinone oxidoreductase (SQR) also reduced basal and 3-MP-stimulated bioenergetic parameters. We conclude that an endogenous intramitochondrial H(2)S-producing pathway, governed by 3-MST, complements and balances the bioenergetic role of Krebs cycle-derived electron donors. This pathway may serve a physiological role in the maintenance of mitochondrial electron transport and cellular bioenergetics.

The purpose of the current study was to investigate the effect of the recently synthesized mitochondrially-targeted H2S donor, AP39 [10-oxo-10-(4-(3-thioxo-3H-1,2-dithiol-5yl)phenoxy)decyl) triphenylphosphonium bromide], on bioenergetics, viability, and mitochondrial DNA integrity in bEnd.3 murine microvascular endothelial cells in vitro, under normal conditions, and during oxidative stress. Intracellular H2S was assessed by the fluorescent dye 7-azido-4-methylcoumarin. For the measurement of bioenergetic function, the XF24 Extracellular Flux Analyzer was used. Cell viability was estimated by the combination of the MTT and LDH methods. Oxidative protein modifications were measured by the Oxyblot method. Reactive oxygen species production was monitored by the MitoSOX method. Mitochondrial and nuclear DNA integrity were assayed by the Long Amplicon PCR method. Oxidative stress was induced by addition of glucose oxidase. AP39 (30 – 300 nM) to bEnd.3 cells increased intracellular H2S levels, with a preferential response in the mitochondrial regions. AP39 exerted a concentration-dependent effect on mitochondrial activity, which consisted of a stimulation of mitochondrial electron transport and cellular bioenergetic function at lower concentrations (30–100 nM) and an inhibitory effect at the higher concentration of 300 nM. Under oxidative stress conditions induced by glucose oxidase, an increase in oxidative protein modification and an enhancement in MitoSOX oxidation was noted, coupled with an inhibition of cellular bioenergetic function and a reduction in cell viability. AP39 pretreatment attenuated these responses. Glucose oxidase induced a preferential damage to the mitochondrial DNA; AP39 (100 nM) pretreatment protected against it. In conclusion, the current paper documents antioxidant and cytoprotective effects of AP39 under oxidative stress conditions, including a protection against oxidative mitochondrial DNA damage.

Emerging work demonstrates the dual regulation of mitochondrial function by hydrogen sulfide (H2S), including, at lower concentrations, a stimulatory effect as an electron donor, and, at higher concentrations, an inhibitory effect on cytochrome C oxidase. In the current article, we overview the pathophysiological and therapeutic aspects of these processes. During cellular hypoxia/acidosis, the inhibitory effect of H2S on complex IV is enhanced, which may shift the balance of H2S from protective to deleterious. Several pathophysiological conditions are associated with an overproduction of H2S (e.g. sepsis), while in other disease states H2S levels and H2S bioavailability are reduced and its therapeutic replacement is warranted (e.g. diabetic vascular complications). Moreover, recent studies demonstrate that colorectal cancer cells up-regulate the H2S-producing enzyme cystathionine β-synthase (CBS), and utilize its product, H2S, as a metabolic fuel and tumour-cell survival factor; pharmacological CBS inhibition or genetic CBS silencing suppresses cancer cell bioenergetics and suppresses cell proliferation and cell chemotaxis. In the last chapter of the current article, we overview the field of H2S-induced therapeutic 'suspended animation', a concept in which a temporary pharmacological reduction in cell metabolism is achieved, producing a decreased oxygen demand for the experimental therapy of critical illness and/or organ transplantation. LINKED ARTICLESThis article is part of a themed issue on Mitochondrial Pharmacology: Energy, Injury & Beyond. To view the other articles in this issue visit http://dx.doi. org/10.1111/bph.2014.171.issue-8 Abbreviations 3-MST, 3-mercaptopyruvate sulfurtransferase; AOAA, aminooxyacetic acid; CBS, cystathionine β-synthase; CSE, cystathionine γ-lyase; H2S, hydrogen sulfide; KATP, ATP-sensitive potassium channel; PAG, propargylglycine; SOU, sulfide-oxidizing unit Introduction H2S, a colourless, flammable, water-soluble gas, is gaining increased attention as an endogenous biological mediator. The distribution and regulation of the three H2S-producing enzymes [cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MST)], and the wide range of biological effects of H2S are discussed in separate reviews Szabo, 2007 Snyder, 2010;Kimura, 2010;2013;Predmore and Lefer, 2010;Whiteman and Winyard, 2011;Kimura et al., 2012;Wang, 2012). In a recent article, we have overviewed the dual mitochondrial effects of H2S, which range from stimulatory effects, occurring at lower concentrations, to the suppression of mitochondrial function, which occurs at higher concentrations . The purpose of the current article is to outline the physiological, pathophysiological and therapeutic aspects of this regulation. Similar to our approach in the companion article , in the current article we use the terms 'sulfide' and H2S interchangeably to collectively refer to H2S gas as well as its two ionized forms in solution: HS -and S 2-. H 2 S as a potential inducer of ...

Mammalian cells can utilize hydrogen sulfide (H2S) to support mitochondrial respiration. The aim of our study was to explore the potential role of S-sulfhydration (a H2S-induced posttranslational modification, also known as S-persulfidation) of the mitochondrial inner membrane protein ATP synthase (F1F0 ATP synthase/Complex V) in the regulation of mitochondrial bioenergetics. Using a biotin switch assay, we have detected S-sulfhydration of the α subunit (ATP5A1) of ATP synthase in response to exposure to H2S in vitro. The H2S generator compound NaHS induced S-sulfhydration of ATP5A1 in HepG2 and HEK293 cell lysates in a concentration-dependent manner (50–300 μM). The activity of immunocaptured mitochondrial ATP synthase enzyme isolated from HepG2 and HEK293 cells was stimulated by NaHS at low concentrations (10–100 nM). Site-directed mutagenesis of ATP5A1 in HEK293 cells demonstrated that cysteine residues at positions 244 and 294 are subject to S-sulfhydration. The double mutant ATP synthase protein (C244S/C294S) showed a significantly reduced enzyme activity compared to control and the single-cysteine-mutated recombinant proteins (C244S or C294S). To determine whether endogenous H2S plays a role in the basal S-sulfhydration of ATP synthase in vivo, we compared liver tissues harvested from wild-type mice and mice deficient in cystathionine-gamma-lyase (CSE, one of the three principal mammalian H2S-producing enzymes). Significantly reduced S-sulfhydration of ATP5A1 was observed in liver homogenates of CSE−/− mice, compared to wild-type mice, suggesting a physiological role for CSE-derived endogenous H2S production in the S-sulfhydration of ATP synthase. Various forms of critical illness (including burn injury) upregulate H2S-producing enzymes and stimulate H2S biosynthesis. In liver tissues collected from mice subjected to burn injury, we detected an increased S-sulfhydration of ATP5A1 at the early time points post-burn. At later time points (when systemic H2S levels decrease) S-sulfhydration of ATP5A1 decreased as well. In conclusion, H2S induces S-sulfhydration of ATP5A1 at C244 and C294. This post-translational modification may be a physiological mechanism to maintain ATP synthase in a physiologically activated state, thereby supporting mitochondrial bioenergetics. The sulfhydration of ATP synthase may be a dynamic process, which may be regulated by endogenous H2S levels under various pathophysiological conditions.