Novel insights into hydrogen sulfide--mediated cytoprotection

Abstract

Hydrogen sulfide (H(2)S) is a colorless, water soluble, flammable gas that has the characteristic smell of rotten eggs. Like other members of the gasotransmitter family (nitric oxide and carbon monoxide), H(2)S has traditionally been considered to be a highly toxic gas and environmental hazard. However, much like for nitric oxide and carbon monoxide, the initial negative perception of H(2)S has evolved with the discovery that H(2)S is produced enzymatically in mammals under normal conditions. As a result of this discovery, there has been a great deal of work to elucidate the physiological role of H(2)S. H(2)S is now recognized to be cytoprotective in various models of cellular injury. Specifically, it has been demonstrated that the acute administration of H(2)S, either prior to ischemia or at reperfusion, significantly ameliorates in vitro or in vivo myocardial and hepatic ischemia-reperfusion injury. These studies have also demonstrated a cardioprotective role for endogenous H(2)S. This review article summarizes the current body of evidence demonstrating the cytoprotective effects of H(2)S with an emphasis on the cardioprotective effects. This review also provides a detailed description of the current signaling mechanisms shown to be responsible for these cardioprotective actions.

Figures

FIG. 1.

Endogenously produced gases that modulate cardiovascular physiology. Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are three endogenously produced gases that are members of the gasotransmitter family of cellular signaling molecules. Each is produced enzymatically and all have been reported to possess cytoprotective effects. It should be noted that these gases are very labile and in general highly reactive. NO, CO, and H2S all activate multilple signal pathways and promote chemical modifications. CBS, cystathionine β-synthase, CGL or CSE, cystathionine γ-lyase, cGMP, cyclic guanosine monophosphate, eNOS, endothelial nitric oxide synthase, HO-1, heme oxygnase 1, iNOS, inducible nitric oxide synthase, KATP, ATP-sensitive K+ channel, 3MST, 3-mercaptopyruvate sulfur transferase, nNOS, neuronal nitric oxide synthase, sGC, soluble guanylyl cyclase. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 2.

Enzymatic synthesis of hydrogen sulfide. The physiological production of H2S in mammalian systems has been attributed to three enzymes that are part of the cysteine biosynthesis pathway. Cystathionine β-synthase (CBS) is predominantly found in the brain, nervous system, and liver. It produces H2S through a reaction involving the generation of cystathionine from homocysteine, serine, and cysteine. Cystathionine γ-lyase (CGL or CSE) is found in the vasculature and live. It produces H2S through a reaction involving the generation of L-cysteine, pyruvate, and ammonia from L-cystathionine and cysteine. 3-Mercaptopyruvate sulfur transferase (3MST) has been reported to be localized in the mitochondria and can be found in the brain and vasculature. It produces H2S through a reaction involving the generation of pyruvate from 3-mercaptopyruvate (3MP). The 3MP is provided through the metabolism of cysteine and α-ketoglutarate (α-KG) by cysteine aminotransferase (CAT). These enzymes regulate the physiological levels of H2S observed in the bloodstream and tissues and are critical for H2S homeostasis. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 3.

Cardiovascular actions of hydrogen sulfide. H2S is produced in μM concentrations in the circulation and despite its very short half-life in blood and tissues exerts a number of critical effects on the cardiovascular system. Physiological actions include: vasodilation, inhibition of leukocyte-endothelial cell interactions, antioxidant effects, anti-apoptotic effects, increased angiogenesis, and modulation of mitochondrial respiration. Cystathionine gamma lyase (CGL or CSE) is localized primarily in the smooth muscle cell layer of blood vessels while the recently described enzyme, 3-mercaptopyruvate sulfur transferase (3MST) has been identified in vascular endothelial cells as well as in mitochondria. At present, it is not clear if 3MST is localized in cardiac mitochondria. These cytoprotective actions of H2S are observed at physiological levels of H2S (i.e., μM) while very high concentrations of H2S (mM) induce cellular injury and apoptosis. At present, very little is known regarding H2S levels in animals and humans in the setting of cardiovascular disease of the effects of risk factors (i.e., hypertension, diabetes mellitus, dyslipidemia, and obesity) on blood and tissue levels of H2S. Hydrogen sulfide therapy in cardiovascular disease is focused on the use of physiological levels of H2S. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 4.

Deficiency of CGL exacerbates myocardial ischemia-reperfusion injury. In order to determine the role of endogenously produced H2S on myocardial reperfusion injury, mice deficient in CGL (CGL−/−) and wild-type littermates were subjected to 45 min of left coronary artery occlusion, followed by 24 h of reperfusion. At 24 h of reperfusion, myocardial area-at-risk and infarct size were evaluated with the TTC-Evans blue dual staining method. (A) Representative photomicrographs of midventricular heart sections from wild-type and CGL−/− mice at 24 h after reperfusion. Myocardial infarct size (white tissue) is significantly greater in CGL−/− mice as compared to wild-type control hearts. (B) Bar graph of the myocardial area-at-risk per left ventricle (AAR/LV), infarct size per AAR (Inf/AAR), and Inf/LV in wild-type and CGL−/− groups of mice. CGL−/− mice experienced a 48% increase in Inf/AAR (p = 0.004 vs. wild-type) and a 34% (p = 0.01) increase in Inf/LV when compared to wild-type mice. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article at www.liebertonline.com/ars).

FIG. 5.

Hydrogen sulfide activates myocardial KATP channels. (A) Representative recording of ATP-sensitive potassium (KATP) channels measured in the inside-out configuration at −100 mV membrane potential. Note that inward currents are depicted as downward deflections of the current trace. ATP and H2S were applied as shown in the top bars. (B) Mean patch current. On average, the mean patch current recorded with 30 μM ATP was 41% of that recorded in the absence of ATP (n = 7). H2S increased mean patch current by 66%. Values are means ± S.E.M. **p < 0.01 compared to 0.03 mM ATP.

FIG. 6.

Summary of the key H2S signaling targets and pathways. Although the physiological and cytoprotective effects of H2S have previously been documented, the signaling mechanisms that mediate these effects have just begun to be elucidated. The activation of multiple signaling cascades allows H2S to target pathways that can provide cytoprotection independently of each other, but most importantly allows for the merging of multiple pathways into a single cytoprotective signaling cascade. Some of the known protective signal pathways are illustrated. Given the highly reactive property of hydrogen sulfide and the fact that H2S induces S-sulfhydration, it is likely that additional signaling pathways not shown are activated by H2S. Additionally, signaling pathways involved in pharmacological preconditioning may differ from the cytoprotective effects of H2S administered during the pathogenesis of ischemia-reperfusion injury. GPx, glutathione peroxidase; GR, glutathione reductase; GST, glutathione S-transferase; HO-1, hemeoxygenase-1; HSP, heat shock protein; MPTP, mitochondrial permeability transition pore; Nrf-2, nuclear-factor-E2-related factor; PKC, protein kinase C; ROS, reactive oxygen species; SERCA, sarco-endoplasmic reticulum Ca2+-ATPase; Trx, thioredoxin; TrxR, thioredoxin reductase; STAT-3, signal transducers and activators of transcription-3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article at www.liebertonline.com/ars).