Hydrogen sulfide in inflammation: friend or foe?

Hydrogen sulfide (H(2)S), the gaseous mediator produced by various cells in our body, was recently discovered to play a major role in human physiology despite its toxic nature known for centuries. In addition to its pathophysiological relevance in cardiovascular and neuronal disorders, there is considerable interest in the significance of H(2)S in inflammation. A number of preclinical studies in our laboratory as well as by others, using H(2)S donors and inhibitors of its endogenous synthesis, have provided evidence for both pro- and anti-inflammatory character of H(2)S. But so far, there is a significant lack of support from relevant clinical studies. One of the major contentious issues being variable dose and sampling time, controversies exist on the precise friend or foe nature of this gaseous transmitter. However, it is well accepted that once a clearer picture of the whole story of H(2)S in inflammation emerges, potential for therapeutic manipulations in this field are immense. This review focuses on the intriguing effects of H(2)S in some of the inflammatory conditions such as acute pancreatitis, sepsis, burn injuries and local inflammation of the joints. Active research projects have been undertaken to elucidate the mechanisms of action of H(2)S in inflammation, including neurogenic inflammation and interaction with other biological mediators and pathways. The early and fragmentary evidence obtained holds promise for a successful drug intervention for these inflammatory diseases.     

The role of brain gaseous transmitters in the regulation of the circulatory system

A number of neurotransmitters, including biologically active gases namely, nitric oxide (NO), hydrogen sulfide (H2S) and carbon monoxide (CO) have been postulated to play an important role in the control of the cardiovascular system by the brain. The attention of researchers has been focused on NO in particular. It has been shown that pharmacological manipulation of NO concentration in the brain produces significant changes in circulatory parameters. Furthermore, significant alterations in the brain NO system have been found in animal models of human cardiovascular diseases. These findings imply that NO in the brain may become a promising target for new treatment strategies. Although H2S and CO have also been proved to serve as transmitters in the central nervous system, their role in the neurogenic regulation of the cardiovascular system remains more obscure. Interestingly, increased synthesis of NO, H2S and CO is found in inflammation and it appears that the gases mediate some of the circulatory responses to inflammatory stimuli. In this review we discuss the role of brain gaseous transmitters in the control of the circulatory system in health and disease.     

Potential importance of alterations in hydrogen sulphide (H2S) bioavailability in diabetes

Despite its long-standing reputation as a foul smelling and toxic gas that is associated with the decay of biological matter,hydrogen sulphide (H2S) has emerged as an important regulator of cardiovascular homoeostasis. H2S promotes a number of cellular signals that regulate metabolism, cardiac function and cell survival. Endogenous H2S bioavailability is regulated by several enzymes involved in the biosynthesis of cysteine. This study by Brancaleone et al. in the current issue of the British Journal of Pharmacology provides novel insights into the impairment of H2S biosynthesis in the setting of diabetes mellitus. The authors report that enzymic H2S biosynthesis is impaired in a murine model of type 1 diabetes and the attenuation in H2S bioavailability is associated with impaired vascular reactivity. This study has profound implications for the use of pharmacological agents to augment endogenous H2S synthesis or agents that release H2S to augment the levels of this gaseous signalling molecule in cardiovascular disease.     

Generation of DNA-Damaging Reactive Oxygen Species via the Autoxidation of Hydrogen Sulfide under Physiologically Relevant Conditions: Chemistry Relevant to Both the Genotoxic and Cell Signaling Properties of H(2)S

Hydrogen sulfide (H(2)S) has long been known for its toxic properties; however, in recent years, evidence has emerged that this small, gaseous molecule may serve as an endogenous cell-signaling agent. Though perhaps surprising in light of its potential role as an endogenous signaling agent, a number of studies have provided evidence that H(2)S is a DNA-damaging mutagen. In the work reported here, the chemical mechanisms of DNA damage by H(2)S were examined. Using a plasmid-based DNA strand cleavage assay, we found that micromolar concentrations of H(2)S generated single-strand DNA cleavage. Mechanistic studies indicate that this process involved autoxidation of H(2)S to generate superoxide, hydrogen peroxide, and, ultimately, the well-known DNA-damaging agent hydroxyl radical via a trace metal-mediated Fenton-type reaction. Strand cleavage by H(2)S proceeded in the presence of physiological thiol concentrations, and the known byproducts of H(2)S oxidation such as thiosulfate, sulfite, and sulfate do not contribute to the strand cleavage process. However, initially generated oxidation products such as persulfide (S(2)(2-)) likely undergo rapid autoxidation reactions that contribute to the generation of superoxide. The potential relevance of autoxidation processes to the genotoxic and cell signaling properties of H(2)S is discussed.     

Therapeutic potential of new hydrogen sulfide-releasing hybrids

A new class of hydrogen sulfide (H(2)S)-donating hybrids combined with pharmacologically active compounds is presented in this article. The pharmacological profiles of some hybrid lead compounds in the areas of inflammation, H(2)S-donating diclofenac (ACS 15); cardiovascular, H(2)S-donating aspirin (ACS 14); urology, H(2)S-donating sildenafil (ACS 6); and neurodegenerative, H(2)S-donating latanoprost (ACS 67) for glaucoma treatment and H(2)S-donating levodopa (ACS 84) for Parkinson's disease, are described. The new H(2)S-releasing hybrids demonstrate remarkable improvement in activity and tolerability as compared with the related parent compounds, suggesting an active pharmacological role for H(2)S. Finally the mechanism(s) of action of glutathione-dependent and independent, and of gas (H(2)S) release (spontaneous or enzymatic) and its implications for clinical pharmacology perspectives will be also discussed.     

Old and new gasotransmitters in the cardiovascular system: focus on the role of nitric oxide and hydrogen sulfide in endothelial cells and cardiomyocytes

The functional relevance of nitric oxide (NO) in the cardiovascular system is well established since the end of the 80', when it was firstly proposed as a key controller of vasodilation. More recent evidences, still debated and partly conflicting, point to a role of NO in the angiogenic progression. On the other hand hydrogen sulfide is a new entry as a gasotransmitter in the cardiovascular system. The variety of its biological functions seems to grow day after day. The first to be described is surely its reversible and poisoning binding of the cytochrome c oxidase that leads to impairment of the respiratory chain in mitochondria. However, sub-toxic concentrations have been later proved to be essential to maintain fundamental physiological functions in several tissues. The basal production of H2S is determined by the activity of, at least, three constitutively expressed enzymes (CBS, CSE, and 3-MPT) with tissue specificity for CBS and CSE in the central nervous and cardiovascular system, respectively. The assumption of a pivotal role of H2S in regulating physiological function is supported by the demonstration that reduced production of this gaseous molecule by CSE induces hypertension in mice. The increasing number of studies showing the regulatory functions of H2S reveals that maintaining the normal blood pressure levels is only one of its multiple biological actions. In this review, we would like to explore the recent literature on NO and H2S roles on cardiovascular system and to elucidate potential outcomes in the use of pharmacological drugs interfering with their metabolism.     

Hydrogen sulfide induced neuronal death occurs via glutamate receptor and is associated with calpain activation and lysosomal rupture in mouse primary cortical neurons

Hydrogen sulfide (H(2)S) is a cytotoxic gas recently proposed as a novel neuromodulator. Endogenous levels of H(2)S in the brain range between 50 and 160 microM and perturbed H(2)S synthesis has been reported in the brains from stroke, Alzheimer's disease and Down syndrome patients. Recently, in immature non-glutamate receptor expressing mouse cortical neurons H(2)S was shown to inhibit cell death exhibited by high concentrations of glutamate whereas H(2)S was not cytotoxic. Due to the reported role of H(2)S in facilitating LTP through NMDA receptors we examined the effects of H(2)S on glutamate receptor functioning using mature cortical neurons expressing functional glutamate receptor subtypes. Addition of 100 microM glutamate exhibited extensive cell death which was exacerbated by co-incubation with < or = 200 microM of the H(2)S donor sodium hydrosulfide (NaHS). At <200 microM NaHS induced apoptosis whereas >200 microM NaHS induced necrosis. Cell death was inhibited by pharmacological glutamate receptor antagonists MK801 and APV (NMDA receptor antagonists), and CNQX (kainate and AMPA receptor antagonist) but not kynurenate (broad spectrum glutamate receptor antagonist), GYKI52466 (more selective AMPA receptor antagonist) and CYZ (AMPA receptor potentiator). Although markers of apoptosis were observed, we did not detect caspase activation either by Western blotting or fluorescence assays and caspase inhibitors did not prevent cell death. Rather, H(2)S induced calpain activation and lysosomal membrane destabilization; processes inhibited by preferential antagonists of NMDA and kainate receptors. These data suggest that H(2)S induced neuronal death through ionotropic glutamate receptors, which recruits apoptosis to ensure cellular demise and employs calpains and lysosomal rupture. This study provides novel insights into cell death observed in neurodegenerative diseases involving glutamate receptor activation and perturbed H(2)S synthesis.     

Hydrogen sulfide in gastrointestinal and liver physiopathology

Hydrogen sulfide (H(2)S) is a gas that can be formed by the action of two enzymes, cystathionine gamma lyase (CSE) and cystathionine beta synthase (CBS). H(2)S has been known for hundreds of years for its poisoning effect, however the idea that H(2)S is not only a poison, but can exert a physiological role in mammalian organisms, originates from the evidence that this gaseous mediator is produced endogenously. In addition to H(2)S synthesis by gastrointestinal tissue, the intestinal mucosa, particularly in the large intestine, is regularly exposed to high concentrations of H(2)S that are generated by some species of bacteria and through the reduction of unabsorbed intestinal inorganic sulphate. This review reports on the effects of H(2)S in the gastrointestinal tract and liver and provides information on the therapeutic applications of H(2)S-donating drugs.     

TRPV1 receptor: a target for the treatment of pain, cough, airway disease and urinary incontinence

The TRPV1 channel is mainly expressed in sensory nerves. Activation of the channel induces neuropeptide release from central and peripheral sensory nerve terminals, resulting in the sensation of pain, neurogenic inflammation, smooth muscle contraction and cough. The TRPV1 channel can be activated by vanilloids such as capsaicin, as well as endogenous stimulators including H(+), heat, lipoxygenase products and anandamide. TRPV1 channel function is upregulated by several endogenous mediators present in inflammatory conditions, which decreases the threshold for activation of the channel. Under these conditions, TRPV1 can be activated by physiological body temperature, slight acidification or lower concentration of TRPV1 agonists. There is evidence that TRPV1 plays a role in the development of pathophysiological changes and symptoms in several diseases. In this review, we discuss TRPV1 channel activation and regulation in normal and diseased conditions, the role of TRPV1 in pain, cough, asthma and urinary incontinence, and the potential use of TRPV1 antagonists as a novel therapy for these diseases.     

Dual role of hydrogen sulfide in mechanical inflammatory hypernociception

Hydrogen sulfide (H(2)S) is an endogenous gas involved in several biological functions, including modulation of nociception. However, the mechanisms involved in such modulation are not fully elucidated. The present study demonstrated that the pretreatment of mice with PAG, a H(2)S synthesis inhibitor, reduced LPS-induced mechanical paw hypernociception. This inhibition of hypernociception was associated with the prevention of neutrophil recruitment to the plantar tissue. Conversely, PAG had no effect on LPS-induced production of the hypernociceptive cytokines, TNF-alpha, IL-1beta and CXCL1/KC and on hypernociception induced by PGE(2), a directly acting hypernociceptive mediator. In contrast with the pro-nociceptive role of endogenous H(2)S, systemic administration of NaHS, a H(2)S donor, reduced LPS-induced mechanical hypernociception in mice. Moreover, this treatment inhibited mechanical hypernociception induced by PGE(2), suggesting a direct effect of H(2)S on nociceptive neurons. The antinociceptive mechanism of exogenous H(2)S depends on K((ATP))(+) channels since the inhibition of PGE(2) hypernociception by NaHS was prevented by glibenclamide (K((ATP))(+) channel blocker). Finally, NaHS did not alter the thermal nociceptive threshold in the hot-plate test, confirming that its effect is mainly peripheral. Taken together, these results suggest that H(2)S has a dual role in inflammatory hypernociception: 1. an endogenous pro-nociceptive effect due to up-regulation of neutrophil migration, and 2. an antinociceptive effect by direct blockade of nociceptor sensitization modulating K((ATP))(+) channels.     

Hydrogen sulphide and angiogenesis: mechanisms and applications

In vascular tissues, hydrogen sulphide (H(2)S) is mainly produced from L-cysteine by the cystathionine gamma-lyase (CSE) enzyme. Recent studies show that administration of H(2)S to endothelial cells in culture stimulates cell proliferation, migration and tube formation. In addition, administration of H(2)S to chicken chorioallantoic membranes stimulates blood vessel growth and branching. Furthermore, in vivo administration of H(2)S to mice stimulates angiogenesis, as demonstrated in the Matrigel plug assay. Pathways involved in the angiogenic response of H(2)S include the PI-3K/Akt pathway, the mitogen activated protein kinase pathway, as well as ATP-sensitive potassium channels. Indirect evidence also suggests that the recently demonstrated role of H(2)S as an inhibitor of phosphodiesterases may play an additional role in its pro-angiogenic effect. The endogenous role of H(2)S in the angiogenic response has been demonstrated in the chicken chorioallantoic membranes, in endothelial cells in vitro and ex vivo. Importantly, the pro-angiogenic effect of vascular endothelial growth factor (but not of fibroblast growth factor) involves the endogenous production of H(2)S. The pro-angiogenic effects of H(2)S are also apparent in vivo: in a model of hindlimb ischaemia-induced angiogenesis, H(2)S induces a marked pro-angiogenic response; similarly, in a model of coronary ischaemia, H(2)S exerts angiogenic effects. Angiogenesis is crucial in the early stage of wound healing. Accordingly, topical administration of H(2)S promotes wound healing, whereas genetic ablation of CSE attenuates it. Pharmacological modulation of H(2)S-mediated angiogenic pathways may open the door for novel therapeutic approaches.     

Digoxin increases hydrogen sulfide concentrations in brain, heart and kidney tissues in mice

The interest in digoxin has recently increased due to the expanding knowledge regarding endogenous cardiac glycosides and a potential oncological application of this drug. Hydrogen sulfide (H(2)S), a crucial co-modulator of various physiological processes, is involved in the pathophysiology of different disorders and may be useful in the treatment of some diseases. The interaction between cardiac glycosides and H(2)S is unknown. The aim of the study is to assess the influence of digoxin on H(2)S tissue concentrations in mouse brain, heart and kidney. Thirty male BALB/c mice were given intraperitoneal injections of digoxin at 0.5 mg/kg body weight (b.w.) per day (group D1, n = 10) or 1 mg/kg b.w. per day (group D2, n = 10). The control group (n = 10) received physiological saline. Free H(2)S tissue concentrations were measured via the Siegel spectrophotometric modified method. There was a significant, progressive increase in the H(2)S concentrations for both the low and high digoxin doses in the brain (7.7% and 8.5%, respectively), heart (by 6.0% and 22.1%, respectively) and kidney (by 7.6% and 13.0%, respectively). This report shows that digoxin administration is followed by an increase in the free H(2)S concentrations in mouse brain, heart and kidney tissues.     

Garlic-derived natural polysulfanes as hydrogen sulfide donors: Friend or foe?

In vitro and in vivo studies reported the anti-cancer potential of organosulfur compounds (OSCs) as they trigger biological effects leading to cell cycle arrest with accumulation of cells in G2/M, alteration of the microtubular network, modulation of Bcl-2 family protein expression patterns and changes of the redox status. Despite these well-described effects, no OSC derivative is yet undergoing clinical trials even though their chemistry is well understood as OSCs act as hydrogen sulfide (H₂S) donors. H₂S is a biological mediator, synthesized through cysteine degradation and modulates vasodilation, cytoprotection, inflammation and angiogenesis. It is well accepted that H₂S plays a biphasic pharmacological role: the inhibition of endogenous synthesis of H₂S and paradoxically also the use of H₂S donors to increase H₂S concentration, induce both anti-cancer effects leading therefore to controversial discussions. Altogether, the role of H₂S in the anti-cancer action of OSCs remains poorly understood. In this review, we hypothesize that OSCs act through H₂S signaling pathways in cancer cells, and that a clearer understanding of the mechanism of action of H₂S in OSC-mediated anti-cancer activity is required for further application of these compounds in translational medicine.     

The neurophysiology of hydrogen sulfide

Hydrogen sulfide (H(2)S) has emerged as the third endogenous gaseous mediator in the central and peripheral nervous system. H(2)S is generated by three enzymes, cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3MST). In the CNS, H(2)S, generated mainly by CBS in astrocytes and 3MST in neurons, appears to participate in cognition, memory, regulation of the cardiopulmonary functions and neuroprotection. In the peripheral nervous system, evidence suggests that H(2)S may be involved in autonomic control of the cardiopulmonary and gastrointestinal functions as well pain and inflammation.     

Hydrogen sulfide regulates intracellular Ca2+ concentration in endothelial cells from excised rat aorta

Hydrogen sulphide (H2S) is a recently discovered gasotransmitter that may regulate a growing number of endothelial functions, including nitric oxide (NO) release, proliferation, adhesion and migration, which are the key steps of angiogenesis. The mechanism whereby H2S impacts on endothelial physiology is still unclear: however, the aforementioned processes are driven by an increase in intracellular Ca2+ concentration ([Ca2+]i). In the present study, we exploited the excised rat aorta to gain insights into the regulation of [Ca2+]i by H2S within in situ endothelial cells (ECs). Sodium hydrosulphide (NaHS), a H2S donor, caused an elevation in [Ca2+]i, which disappeared in absence of extracellular Ca2+. NaHSinduced Ca2+ inflow was sensitive to high doses of Gd3+, but not BTP-2. Inhibition of the reverse-mode of the Na+-Ca2+ exchanger (NCX), with KB-R7943 or upon removal of extracellular Na+, abrogated the Ca2+ response to NaHS. Moreover, NaHS-elicited Ca2+ entry was significantly reduced by TEA and glybenclamide, which hinted at the involvement of ATP-dependent K+ (KATP) channels. Conversely, NaHS-evoked Ca2+ signal was not affected by the reducing agent, dithiothreitol. Acute addition of NaHS hindered both Ca2+ release and Ca2+ entry induced by ATP, a physiological agonist of ECs. Consistently, inhibition of endogenous H2S synthesis with DL-propargylglycine impaired ATP-induced Ca2+ inflow, whereas it did not affect Ca2+ mobilization. These data provide the first evidence that H2S may stimulate Ca2+ influx into ECs by recruiting the reverse-mode of NCX and KATP channels. In addition, they show that such gasotransmitter may modulate the Ca2+ signals elicited by physiological stimuli in intact endothelium.     

Hydrogen sulphide: biopharmacological roles in the cardiovascular system and pharmaceutical perspectives

Hydrogen sulphide (H(2)S) is now viewed as an important endogenous gasotransmitter, which exhibits many beneficial effects on the cardiovascular system. H(2)S is biosynthesized in mammalian tissues by both non-enzymatic processes and several enzymatic pathways ensured by cystathionine-β-synthase and cystathionine-γ-lyase. H(2)S is endowed with the antioxidant properties of inorganic and organic sulphites, being a scavenger of reactive oxygen species. Furthermore, H(2)S triggers other important effects and the activation of ATP-sensitive potassium channels (KATP) accounts for its vasorelaxing and cardioprotective effects. H(2)S also inhibits smooth muscle proliferation and platelet aggregation. Conversely, the impairment of H(2)S contributes to the pathogenesis of hypertension and is involved in cardiovascular complications associated with diabetes mellitus. There is also evidence of a link between H(2)S and endothelial nitric oxide (NO). Recent observations indicate a possible pathogenic link between deficiencies of H(2)S activity and the progress of endothelial dysfunction. These biological aspects of endogenous H(2)S led to consider this mediator as "the new NO" and to evaluate new attractive opportunities to develop innovative classes of drugs. In this review, the main roles played by H(2)S in the cardiovascular system and the first examples of H(2)S-donor drugs are discussed. Some hybrid drugs are also addressed in this review. In such compounds opportune H(2)S-releasing moieties are conjugated to well-known drugs to improve their pharmacodynamic profile or to reduce the potential for adverse effects.     

Hydrogen sulfide inhibits human platelet aggregation

Gaseous mediators such as nitric oxide (NO) play a major regulatory role in the cardiovascular system homeostasis, including platelet aggregation. Here, we investigated whether hydrogen sulfide (H(2)S), a newly recognized endogenous mediator, can affects aggregation of human platelets, using sodium hydrogen sulfide (NaHS) as H(2)S-donor. NaHS inhibited platelet aggregation induced by ADP, collagen, epinephrine, arachidonic acid, thromboxane mimetic, U46619, and thrombin. H(2)S effect was not dependent by cAMP/cGMP generation, NO production or potassium-channels opening. NaHS concentrations (up to 10 mM) did not exert toxic effects on platelet viability. The possible protective role of endogenous H(2)S in cardiovascular system is discussed.     

Hydrogen sulphide in heart and systemic circulation

In the mammalian cardiovascular system, H(2)S joins carbon monoxide (CO) and endothelial derived relaxing factors, (EDRFs)-nitric oxide (NO), as the third gasotransmitter. In the vasculature, cystathionine-γ-lyase (CSE) is the main enzyme responsible for H(2)S biosynthesis starting from the substrate e.g. L-cysteine. There is a growing body of evidence that supports a role for H(2)S in regulating the vascular homeostasis. H(2)S (NaHS) is known to induce a concentration-dependent relaxation of large conduit arteries. Interestingly, H(2)S also relaxes peripheral resistance vessels such as mesenteric arteries suggesting a role for H(2)S also in the regulation of vascular resistance and systemic blood pressure. This vasodilatory effect is dependent on the activation of K(ATP) channels. However, a cross-talk exists between the L-Argine/NO and L-cysteine/H(2)S pathways. Furthermore, it has been shown that H(2)S acts as an endogenous non-selective inhibitor of phosphodiesterase activity. Compelling evidence links H(2)S to regulation of erectile function while it remains unclear whether the L-cysteine/H(2)S pathway plays a pathogenetic role in erectile dysfunction. Despite the rapid growth of the field, it should be noted that several aspects of H(2)S physiology in the cardiovascular system remain unsolved and the lack of reliable inhibitors and donors remains a major limitation.     

The role of CCK2 receptors in the homeostasis of the opioid system

The overlapping distribution of opioid and cholecystokinin (CCK) peptides and their receptors (mu- and delta-opioid receptors, CCK1 and CCK2 receptors) in the central nervous system have led to a large number of studies aimed at clarifying the functional relationships between these two neuropeptides. The existence of regulatory loops between both systems has been proposed, and the physiological antagonism between CCK, through activation of CCK2 receptors, and endogenous opioid systems has been demonstrated. This is illustrated by the large potentiation of the main pharmacological effects of exogenous (morphine) or endogenous (enkephalins) opioids. Thus, co-administration of CCK2 antagonists with morphine or RB-101, a systemically active inhibitor which fully protects enkephalins from their degradation, led to strongly enhanced analgesic responses or antidepressant-like effects of the opioids. All these findings have been recently confirmed using CCK2 receptor knockout mice, and the role of CCK2 receptor in the physiological control of the opioid system has been conclusively demonstrated. In this article, we review the experimental pharmacology of the association of CCK2 antagonists and opioids (exogenous and endogenous), emphasizing the clinical interest of such an association.