Moataz M. Gadalla

H2S Signals Through Protein S-Sulfhydration

The gaseous messenger hydrogen sulfide regulates target proteins through S-sulfhydration of cysteine residues. Battling for the Same Cysteine? Recent evidence suggests that hydrogen sulfide (H2S)—a gas perhaps best known for its scent of rotten eggs—has joined nitric oxide (NO) and carbon monoxide in the select ranks of gases that act as physiologic messenger molecules. Although H2S is enzymatically generated in vivo and mediates various physiologic functions, including acting as a vasorelaxant and eliciting hibernation states, the mechanisms through which it affects its targets have been unclear. Here, Mustafa et al. show that endogenous H2S modifies cysteine residues in many proteins, including glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and actin, converting cysteine -SH groups to -SSH groups in a process the authors call S-sulfhydration. Intriguingly, H2S enhanced GAPDH activity through sulfhydration of a cysteine residue that is also a target of nitrosylation by NO, which inhibits GAPDH activity, suggesting that some targets might be subject to regulation through competitive nitrosylation and sulfhydration of the same cysteine residues. Hydrogen sulfide (H2S), a messenger molecule generated by cystathionine γ-lyase, acts as a physiologic vasorelaxant. Mechanisms whereby H2S signals have been elusive. We now show that H2S physiologically modifies cysteines in a large number of proteins by S-sulfhydration. About 10 to 25% of many liver proteins, including actin, tubulin, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), are sulfhydrated under physiological conditions. Sulfhydration augments GAPDH activity and enhances actin polymerization. Sulfhydration thus appears to be a physiologic posttranslational modification for proteins.

Hydrogen sulfide-linked sulfhydration of NF-κB mediates its antiapoptotic actions.

Nuclear factor κB (NF-κB) is an antiapoptotic transcription factor. We show that the antiapoptotic actions of NF-κB are mediated by hydrogen sulfide (H(2)S) synthesized by cystathionine gamma-lyase (CSE). TNF-α treatment triples H(2)S generation by stimulating binding of SP1 to the CSE promoter. H(2)S generated by CSE stimulates DNA binding and gene activation of NF-κB, processes that are abolished in CSE-deleted mice. As CSE deletion leads to decreased glutathione levels, resultant oxidative stress may contribute to alterations in CSE mutant mice. H(2)S acts by sulfhydrating the p65 subunit of NF-κB at cysteine-38, which promotes its binding to the coactivator ribosomal protein S3 (RPS3). Sulfhydration of p65 predominates early after TNF-α treatment, then declines and is succeeded by a reciprocal enhancement of p65 nitrosylation. In CSE mutant mice, antiapoptotic influences of NF-κB are markedly diminished. Thus, sulfhydration of NF-κB appears to be a physiologic determinant of its antiapoptotic transcriptional activity.

Hydrogen sulfide as a gasotransmitter

J. Neurochem. (2010) 10.1111/j.1471‐4159.2010.06580.x

H2S mediates O2 sensing in the carotid body

Gaseous messengers, nitric oxide and carbon monoxide, have been implicated in O2 sensing by the carotid body, a sensory organ that monitors arterial blood O2 levels and stimulates breathing in response to hypoxia. We now show that hydrogen sulfide (H2S) is a physiologic gasotransmitter of the carotid body, enhancing its sensory response to hypoxia. Glomus cells, the site of O2 sensing in the carotid body, express cystathionine γ-lyase (CSE), an H2S-generating enzyme, with hypoxia increasing H2S generation in a stimulus-dependent manner. Mice with genetic deletion of CSE display severely impaired carotid body response and ventilatory stimulation to hypoxia, as well as a loss of hypoxia-evoked H2S generation. Pharmacologic inhibition of CSE elicits a similar phenotype in mice and rats. Hypoxia-evoked H2S generation in the carotid body seems to require interaction of CSE with hemeoxygenase-2, which generates carbon monoxide. CSE is also expressed in neonatal adrenal medullary chromaffin cells of rats and mice whose hypoxia-evoked catecholamine secretion is greatly attenuated by CSE inhibitors and in CSE knockout mice.

Signaling by Gasotransmitters

Nitric oxide, carbon monoxide, and hydrogen sulfide act as messengers in the cardiovascular, immune, and nervous systems. Hormones, neurotransmitters, growth factors, and other signaling molecules come in different chemical classes. In recent years, a number of gases have been recognized as important messenger molecules. They include nitric oxide, carbon monoxide, and hydrogen sulfide, well known as noxious environmental pollutants, so that their existence in mammals was a surprise. All three normally regulate blood-vessel function and appear to act as neurotransmitters in addition to several other roles. Ways in which they signal to their targets, which is the subject of this Review, differ from the actions of other messenger molecules. Instead of binding to conventional receptors on the external surface of adjacent cells, the gases diffuse into the cells where nitric oxide and carbon monoxide may bind to iron in the enzyme that generates the second messenger molecular cyclic guanosine monophosphate. Nitric oxide and hydrogen sulfide can affect a wide range of proteins both on the cell surface and inside cells by chemically modifying the sulfur atom in the amino acid cysteine. Nitric oxide is well established as a major signaling molecule. Evidence is accumulating that carbon monoxide and hydrogen sulfide also are physiologic mediators in the cardiovascular, immune, and nervous systems. This Review focuses on mechanisms whereby they signal by binding to metal centers in metalloproteins, such as in guanylyl cyclase, or modifying sulfhydryl groups in protein targets.

Hypoxic regulation of the cerebral microcirculation is mediated by a carbon monoxide-sensitive hydrogen sulfide pathway

Enhancement of cerebral blood flow by hypoxia is critical for brain function, but signaling systems underlying its regulation have been unclear. We report a pathway mediating hypoxia-induced cerebral vasodilation in studies monitoring vascular disposition in cerebellar slices and in intact mouse brains using two-photon intravital laser scanning microscopy. In this cascade, hypoxia elicits cerebral vasodilation via the coordinate actions of H2S formed by cystathionine β-synthase (CBS) and CO generated by heme oxygenase (HO)-2. Hypoxia diminishes CO generation by HO-2, an oxygen sensor. The constitutive CO physiologically inhibits CBS, and hypoxia leads to increased levels of H2S that mediate the vasodilation of precapillary arterioles. Mice with targeted deletion of HO-2 or CBS display impaired vascular responses to hypoxia. Thus, in intact adult brain cerebral cortex of HO-2–null mice, imaging mass spectrometry reveals an impaired ability to maintain ATP levels on hypoxia.

Protein kinase G–regulated production of H2S governs oxygen sensing

Complex interplay between three gases—oxygen, carbon monoxide, and hydrogen sulfide—is necessary to control breathing. Signaling when to breathe When oxygen concentrations in the blood are low, the carotid body triggers breathing reflexes, a response that requires the gasotransmitter hydrogen sulfide, which is generated by the enzyme CSE. When blood is adequately oxygenated, the enzyme HO-2 generates carbon monoxide, which inhibits CSE and decreases neural activity in the carotid body. Yuan et al. identified two cysteine residues that enabled HO-2 to generate carbon monoxide in an oxygen-sensitive manner. They found that carbon monoxide triggered an increase of the second messenger cGMP, which stimulated protein kinase G to phosphorylate CSE, thereby inhibiting this enzyme and suppressing carotid body activity. Reflexes initiated by the carotid body, the principal O2-sensing organ, are critical for maintaining cardiorespiratory homeostasis during hypoxia. O2 sensing by the carotid body requires carbon monoxide (CO) generation by heme oxygenase-2 (HO-2) and hydrogen sulfide (H2S) synthesis by cystathionine-γ-lyase (CSE). We report that O2 stimulated the generation of CO, but not that of H2S, and required two cysteine residues in the heme regulatory motif (Cys265 and Cys282) of HO-2. CO stimulated protein kinase G (PKG)–dependent phosphorylation of Ser377 of CSE, inhibiting the production of H2S. Hypoxia decreased the inhibition of CSE by reducing CO generation resulting in increased H2S, which stimulated carotid body neural activity. In carotid bodies from mice lacking HO-2, compensatory increased abundance of nNOS (neuronal nitric oxide synthase) mediated O2 sensing through PKG-dependent regulation of H2S by nitric oxide. These results provide a mechanism for how three gases work in concert in the carotid body to regulate breathing.

Endogenous H2S is required for Hypoxic Sensing by Carotid Body Glomus Cells

H(2)S generated by the enzyme cystathionine-γ-lyase (CSE) has been implicated in O(2) sensing by the carotid body. The objectives of the present study were to determine whether glomus cells, the primary site of hypoxic sensing in the carotid body, generate H(2)S in an O(2)-sensitive manner and whether endogenous H(2)S is required for O(2) sensing by glomus cells. Experiments were performed on glomus cells harvested from anesthetized adult rats as well as age and sex-matched CSE(+/+) and CSE(-/-) mice. Physiological levels of hypoxia (Po(2) ∼30 mmHg) increased H(2)S levels in glomus cells, and dl-propargylglycine (PAG), a CSE inhibitor, prevented this response in a dose-dependent manner. Catecholamine (CA) secretion from glomus cells was monitored by carbon-fiber amperometry. Hypoxia increased CA secretion from rat and mouse glomus cells, and this response was markedly attenuated by PAG and in cells from CSE(-/-) mice. CA secretion evoked by 40 mM KCl, however, was unaffected by PAG or CSE deletion. Exogenous application of a H(2)S donor (50 μM NaHS) increased cytosolic Ca(2+) concentration ([Ca(2+)](i)) in glomus cells, with a time course and magnitude that are similar to that produced by hypoxia. [Ca(2+)](i) responses to NaHS and hypoxia were markedly attenuated in the presence of Ca(2+)-free medium or cadmium chloride, a pan voltage-gated Ca(2+) channel blocker, or nifedipine, an L-type Ca(2+) channel inhibitor, suggesting that both hypoxia and H(2)S share common Ca(2+)-activating mechanisms. These results demonstrate that H(2)S generated by CSE is a physiologic mediator of the glomus cells response to hypoxia.

Inherent variations in CO-H2S-mediated carotid body O2 sensing mediate hypertension and pulmonary edema

Significance The carotid body chemosensory reflex is a principal regulator of breathing and blood pressure. Humans and experimental animals display marked interindividual variation in the carotid body chemosensory reflex; however, the underlying mechanisms are not known. Here, we demonstrate differences in carotid body O2 sensing to be mediated by inherent variations in carbon monoxide-sensitive hydrogen sulfide signaling in three distinct rat strains. Hyposensitivity of the carotid body to hypoxia was associated with higher CO and lower H2S levels, poor ventilatory adaptation to hypobaric hypoxia, and pulmonary edema. Hypersensitivity of the carotid body to low O2 was accompanied by reduced CO, greater H2S generation, and hypertension. Oxygen (O2) sensing by the carotid body and its chemosensory reflex is critical for homeostatic regulation of breathing and blood pressure. Humans and animals exhibit substantial interindividual variation in this chemosensory reflex response, with profound effects on cardiorespiratory functions. However, the underlying mechanisms are not known. Here, we report that inherent variations in carotid body O2 sensing by carbon monoxide (CO)-sensitive hydrogen sulfide (H2S) signaling contribute to reflex variation in three genetically distinct rat strains. Compared with Sprague-Dawley (SD) rats, Brown-Norway (BN) rats exhibit impaired carotid body O2 sensing and develop pulmonary edema as a consequence of poor ventilatory adaptation to hypobaric hypoxia. Spontaneous Hypertensive (SH) rat carotid bodies display inherent hypersensitivity to hypoxia and develop hypertension. BN rat carotid bodies have naturally higher CO and lower H2S levels than SD rat, whereas SH carotid bodies have reduced CO and greater H2S generation. Higher CO levels in BN rats were associated with higher substrate affinity of the enzyme heme oxygenase 2, whereas SH rats present lower substrate affinity and, thus, reduced CO generation. Reducing CO levels in BN rat carotid bodies increased H2S generation, restoring O2 sensing and preventing hypoxia-induced pulmonary edema. Increasing CO levels in SH carotid bodies reduced H2S generation, preventing hypersensitivity to hypoxia and controlling hypertension in SH rats.

Inositol pyrophosphates promote tumor growth and metastasis by antagonizing liver kinase B1

Significance Inositol pyrophosphates are messenger molecules incorporating the energetic pyrophosphate bond. Although they have been implicated in diverse biologic processes, their physiologic functions remain enigmatic. We show that the catalytic activity of inositol hexakisphosphate kinase 2 (IP6K2), one of the principal enzymes generating the inositol pyrophosphate IP7 (5-diphosphoinositolpentakisphosphate), mediates cancer cell migration and tumor metastasis both in cell culture and intact mice. In this process, IP6K2 diminishes cell–cell adhesion, enabling cells to invade the intercellular matrix. Drugs that inhibit IP6K2 may be beneficial in cancer therapy. The inositol pyrophosphates, molecular messengers containing an energetic pyrophosphate bond, impact a wide range of biologic processes. They are generated primarily by a family of three inositol hexakisphosphate kinases (IP6Ks), the principal product of which is diphosphoinositol pentakisphosphate (IP7). We report that IP6K2, via IP7 synthesis, is a major mediator of cancer cell migration and tumor metastasis in cell culture and in intact mice. IP6K2 acts by enhancing cell-matrix adhesion and decreasing cell–cell adhesion. This action is mediated by IP7-elicited nuclear sequestration and inactivation of the tumor suppressor liver kinase B1 (LKB1). Accordingly, inhibitors of IP6K2 offer promise in cancer therapy.