Proteomics analysis of lung reveals inflammation and cell death induced by atmospheric H2S exposure in pig

Hydrogen sulfide (H₂S) is a popular toxic environmental gas and industrial pollutant, which can be harmful to multiple organ systems of both human and livestock, especially to the respiratory system. However, the injury mechanism of H₂S exposure to lung remains poorly understood. In this study, pig lung was selected as a H₂S exposure model for the first time. We first examined the histological damage and the mRNA expression of pro-inflammatory genes of lung in pigs exposed to H₂S. Histopathology change and increased mRNA level of pro-inflammatory cytokines demonstrated that H₂S exposure indeed induced inflammatory injury in the porcine lung. We then performed TMT-based quantitative proteomics analysis to probe the injury molecular mechanism. The proteomics results showed that 526 proteins have significant changes in abundance between control and H₂S treated swine. Further validation analysis of some H₂S responsive proteins using both Real-time quantitative PCR and western blotting demonstrated that proteomics data are reliable. KEGG pathway analysis revealed that these proteins were involved in antigen processing and presentation, complement and coagulation cascade, IL-17 signaling pathway, ferroptosis and necroptosis. Our data suggest that H₂S exposure induced immune suppression, inflammatory response and cell death. These findings provide a new insight into the complexity mechanisms of H₂S induced lung injury, and offer therapeutic potential as drug targets with a view towards curing the intoxication caused by H₂S.

Characterization of hydrogen sulfide toxicity to human corneal stromal fibroblasts

Hydrogen sulfide gas (H2 S) is a chemical weapon and a common environmental pollutant. H2 S intoxication is lethal to humans and animals. H2 S contact to the eye can cause vision loss. However, the molecular mechanisms associated with H2 S toxicity to the cornea remain unclear, and no specific therapy exists to mitigate ocular damage from H2 S. Here, we report H2 S-induced cytotoxicity and the parameters contributing to the molecular mechanisms associated with corneal toxicity using primary human corneal stromal fibroblasts (hCSFs) in vitro. Sodium hydrosulfide (NaSH) was used as a source of H2 S, and the cytotoxicity of H2 S was determined by treating hCSF cells with varying concentrations of NaSH (0-10 mM) for 0-72 hours. Changes in cell proliferation, oxidative stress factors, and the expression of inflammatory and fibrotic genes were studied using standard commercial kits and qRT-PCR. NaSH exposure to hCSFs showed dose- and time-dependent cytotoxicity. The IC50 of NaSH was determined to be 5.35 mM. NaSH 5.35 mM exposure led to significantly decreased cytochrome c oxidase activity, increased ROS production, and increased expression of inflammatory and fibrotic genes in hCSF cells. H2 S/NaSH exposure alters normal mitochondrial function, oxidative stress, and inflammatory and fibrotic gene responses in corneal stromal fibroblasts in vitro.

Hydrogen sulphide sensitivity and tolerance in genetically distinct lineages of a selfing mangrove fish (Kryptolebias marmoratus)

Mangroves are critical marine habitats. High hydrogen sulphide (H2S) is a feature of these important ecosystems and its toxicity creates a challenge for mangrove inhabitants. The mangrove rivulus (Kryptolebias marmoratus) is a selfing, hermaphroditic, amphibious fish that can survive exposure to 1116 μM H2S in the wild. These fish rely on cutaneous respiration for gas and ion exchange when emerged. We hypothesized that the skin surface is fundamentally important in H2S tolerance in these mangrove fish by limiting H2S permeability. To test our hypothesis, we first disrupted the skin surface in one isogenic lineage and measured H2S tolerance and sensitivity. We increased water H2S concentration until emersion as a measure of the ability to sense and react to H2S, which we refer to as sensitivity. We then determined H2S tolerance by preventing emersion and increasing H2S until loss of equilibrium (LOE). The H2S concentration at emersion and LOE were significantly affected by disrupting the skin surface, providing support that the skin is involved in limiting H2S permeability. Capitalizing on their unique reproductive strategy, we used three distinct isogenic lineages to test the hypothesis that there would be genetic differences in H2S sensitivity and tolerance. We found significant differences in emersion concentration only among lineages, suggesting a genetic component to H2S sensitivity but not tolerance. Our study also demonstrated that external skin modifications and avoidance behaviours are two distinct strategies used to tolerate ecologically relevant H2S concentrations and likely facilitate survival in challenging mangrove habitats.

Atmospheric H2S exposure does not affect stomatal aperture in maize

Stomatal aperture in maize is not affected by exposure to a subtoxic concentration of atmospheric H₂S. At least in maize, H₂S, thus, is not a gaseous signal molecule that controls stomatal aperture. Sulfur is an indispensable element for the physiological functioning of plants with hydrogen sulfide (H₂S) potentially acting as gasotransmitter in the regulation of stomatal aperture. It is often assumed that H₂S is metabolized into cysteine to stimulate stomatal closure. To study the significance of H₂S for the regulation of stomatal closure, maize was exposed to a subtoxic atmospheric H₂S level in the presence or absence of a sulfate supply to the root. Similar to other plants, maize could use H₂S as a sulfur source for growth. Whereas sulfate-deprived plants had a lower biomass than sulfate-sufficient plants, exposure to H₂S alleviated this growth reduction. Shoot sulfate, glutathione, and cysteine levels were significantly higher in H₂S-fumigated plants compared to non-fumigated plants. Nevertheless, this was not associated with changes in the leaf area, stomatal density, stomatal resistance, and transpiration rate of plants, meaning that H₂S exposure did not affect the transpiration rate per stoma. Hence, it did not affect stomatal aperture, indicating that, at least in maize, H₂S is not a gaseous signal molecule controlling this aperture.

Hydrogen sulfide upregulates miR-16-5p targeting PiK3R1 and RAF1 to inhibit neutrophil extracellular trap formation in chickens

Hydrogen sulfide (H₂S) is a toxic air pollutant that causes immune damage. Recent studies have found that neutrophil extracellular trap (NET) formation is one way in which neutrophils exert immune functions. In addition, the formation of NETs is also related to thrombosis and autoimmune diseases. Recent studies have shown that miRNAs are involved in the regulation of a variety of pathophysiological processes. Here, we investigated the role of H₂S in regulating the formation of NETs by affecting miR-16-5p. Our study established an in vitro H₂S exposure model for neutrophils using phorbol-myristate-acetate (PMA) to induce NET formation. We observed the morphological changes of cells with scanning electron microscopy and fluorescence microscopy. Then, the content of extracellular DNA and the expression of MPO and NE in each group were detected. The results showed that H₂S inhibited the formation of NETs. The expression of miR-16-5p and its target genes PiK3R1 and RAF1 was then measured by qRT-PCR. H₂S upregulated miR-16-5p and inhibited expression of the target genes PiK3R1 and RAF1, and it subsequently inhibited the Pi3K/AKT and ERK pathways and decreased respiratory burst levels. Furthermore, H₂S attenuated inositol 1,4,5-trisphosphate receptor (IP3R)-mediated endoplasmic reticulum calcium outflow as well as autophagy caused by PMA. This study enriches H₂S immunotoxicity research and provides a possible solution for the treatment of NET-related diseases.

Transcriptomic profile analysis of brain inferior colliculus following acute hydrogen sulfide exposure

Hydrogen sulfide (H2S) is a gaseous molecule found naturally in the environment, and as an industrial byproduct, and is known to cause acute death and induces long-term neurological disorders following acute high dose exposures. Currently, there is no drug approved for treatment of acute H2S-induced neurotoxicity and/or neurological sequelae. Lack of a deep understanding of pathogenesis of H2S-induced neurotoxicity has delayed the development of appropriate therapeutic drugs that target H2S-induced neuropathology. RNA sequencing analysis was performed to elucidate the cellular and molecular mechanisms of H2S-induced neurodegeneration, and to identify key molecular elements and pathways that contribute to H2S-induced neurotoxicity. C57BL/6J mice were exposed by whole body inhalation to 700 ppm of H2S for either one day, two consecutive days or 4 consecutive days. Magnetic resonance imaging (MRI) scan analyses showed H2S exposure induced lesions in the inferior colliculus (IC) and thalamus (TH). This mechanistic study focused on the IC. RNA Sequencing analysis revealed that mice exposed once, twice, or 4 times had 283, 193 and 296 differentially expressed genes (DEG), respectively (q-value < 0.05, fold-change> 1.5). Hydrogen sulfide exposure modulated multiple biological pathways including unfolded protein response, neurotransmitters, oxidative stress, hypoxia, calcium signaling, and inflammatory response in the IC. Hydrogen sulfide exposure activated PI3K/Akt and MAPK signaling pathways. Pro-inflammatory cytokines were shown to be potential initiators of the modulated signaling pathways following H2S exposure. Furthermore, microglia were shown to release IL-18 and astrocytes released both IL-1β and IL-18 in response to H2S. This transcriptomic analysis data revealed complex signaling pathways involved in H2S-induced neurotoxicity and may provide important associated mechanistic insights.

Hydrogen sulfide exposure induces apoptosis and necroptosis through lncRNA3037/miR-15a/BCL2-A20 signaling in broiler trachea

Hydrogen sulfide (H₂S) is an air pollutant, has toxic effects on respiratory tract. However, the underlying mechanisms of H₂S induced respiratory toxicity and the roles of long non-coding RNAs (lncRNA) and microRNA (miRNA) in this process remain poorly understood. To clear this, we investigated the change of tracheal tissue ultrastructure and the expression profiles of lncRNAs and miRNAs of chicken trachea exposed to H₂S for 42 days. Results showed that H₂S exposure triggered apoptosis, necroptosis, and differential expression of 16 lncRNAs and 18 miRNAs in broiler tracheas. The results of LMH cells stimulated by NaHS in vitro also showed the occurrence of apoptosis and necroptosis. LncRNA3037 is down-regulated and miR-15a is up-regulated in tracheal tissue and LMH cells under H₂S exposure. Bioinformatics analysis and dual luciferase reporter system showed lncRNA3037 bound directly to miR-15a and negatively regulates each other. A20 and BCL2 are the target genes of miR-15a and negatively regulated by it. Overexpression of miR-15a caused apoptosis and necroptosis and its inhibition partially reversed apoptosis and necroptosis of LMH cells caused by NaHS stimulation and lncRNA3037 knockdown. Taken together, we concluded that H₂S exposure mediates apoptosis and necroptosis through lncRNA3037/miR-15/A20-BCL2. These results provide new insights for unveiling the biological effects of H₂S in vivo and in vitro.

Atmospheric H2S triggers immune damage by activating the TLR-7/MyD88/NF-κB pathway and NLRP3 inflammasome in broiler thymus

Atmospheric hydrogen sulfide (H₂S) is a highly toxic air pollutant that has a negative effect on human health and animal welfare. The immunotoxicity of H₂S has been explored previously, but its mechanism still needs to be clarified, especially in chickens. To further evaluate the immunotoxicity of H₂S, 1-day-old broilers were recruited and exposed to atmospheric H₂S for 42 days of age. Our results showed that H₂S significantly reduced the thymus index and the CD4⁺ and CD8⁺ T-lymphocyte numbers and that it also changed the CD4⁺/CD8⁺ ratio. The morphological analysis showed that H₂S incrassated the medulla and generated inflammatory infiltration. In addition, it caused the mitochondria to swell and the chromatin to condense, and destroyed nuclear structures were observed. We also conducted bioinformation and transcriptomic analyses to delve the mechanism of H₂S toxicity in chicken thymus. We measured 172 differently expression genes (DEGs) after H₂S exposure and further filtrated the DEGs that are related to inflammation and cell death that play a critical role in immune function. We concluded that H₂S significantly increased IL-1β, IL-4 and IL-10 levels, whereas it downregulated IL-12 and IFN-γ. This study confirmed that H₂S triggered the thymus inflammatory response and caused a Th1/Th2 imbalance. Moreover, our results demonstrated that H₂S triggered the TLR-7/MyD88/NF-κB pathway to promote NLRP3 inflammasome activation. In conclusion, atmospheric H₂S actives the TLR-7/MyD88/NF-κB pathway and the NLRP3 inflammasome to promote an inflammatory response, which then causes tissues damage in broiler thymus. These results provide new insights for unveiling the immunotoxic effects of H₂S.

Hydrogen sulfide-induced oxidative stress leads to excessive mitochondrial fission to activate apoptosis in broiler myocardia

Hydrogen sulfide (H₂S), as an environmental gas pollutant, has harmful effects on many tissues and organs, including myocardium. However, the underlying mechanisms of H2S-induced myocardia toxicity remain poorly understood. The present study was designed to investigate the effect of H₂S on myocardia injury in broilers from the perspective of apoptosis. 30 ppm H2S was administered in the broiler chamber for 2, 4 and 6 week, respectively, and the myocardial samples in control groups and H2S groups were collected immediately after euthanized broilers. Transmission electron microscope, test kits, qRT-PCR and western blot were performed. Results showed that H₂S exposure decreased the activities of catalase (CAT) and total antioxidant capability (T-AOC), whereas the content of hydrogen peroxide (H₂O₂) and the activity of inducible nitric oxide synthase (iNOS) enhanced. Besides, we found the excessive expression of mitochondrial fission genes (Drp1 and Mff) by H₂S, the dynamic balance of mitochondrial fission and fusion is destroyed. Furthermore, the levels of pro-apoptotic gene (including CytC, Cas3, Cas8, Cas9, TNF-α and Bax) increased after H₂S exposure, as well as the expression level of anti-apoptotic gene bcl-2 decreased. At the same time, the activities of ATPase (including Na⁺-K⁺-ATPase, Ca²⁺-ATPase, Mg²⁺-ATPase and Ca²⁺-Mg²⁺-ATPase) weakened under H₂S exposure. Therefore, we conclude that H₂S induced oxidative stress and then leaded to excessive mitochondrial fission, which involved in apoptosis and damage broiler myocardia.

A Novel Mechanism To Prevent H2S Toxicity in Caenorhabditis elegans

Hydrogen sulfide (H2S) is an endogenously produced signaling molecule that can be cytoprotective, especially in conditions of ischemia/reperfusion injury. However, H2S is also toxic, and unregulated accumulation or exposure to environmental H2S can be lethal. In Caenorhabditis elegans, the hypoxia inducible factor (hif-1) coordinates the initial transcriptional response to H2S, and is essential to survive exposure to low concentrations of H2S. We performed a forward genetic screen to identify mutations that suppress the lethality of hif-1 mutant animals in H2S. The mutations we recovered are specific for H2S, as they do not suppress embryonic lethality or reproductive arrest of hif-1 mutant animals in hypoxia, nor can they prevent the death of hif-1 mutant animals exposed to hydrogen cyanide. The majority of hif-1 suppressor mutations we recovered activate the skn-1/Nrf2 transcription factor. Activation of SKN-1 by hif-1 suppressor mutations increased the expression of a subset of H2S-responsive genes, consistent with previous findings that skn-1 plays a role in the transcriptional response to H2S. Using transgenic rescue, we show that overexpression of a single gene, rhy-1, is sufficient to protect hif-1 mutant animals in H2S. The rhy-1 gene encodes a predicated O-acyltransferase enzyme that has previously been shown to negatively regulate HIF-1 activity. Our data indicate that RHY-1 has novel, hif-1 independent, function that promotes survival in H2S.

[Sargassum: the atypical irruption of an ancient ecosystem]

Sargassum constitutes an ancient marine ecosystem that circulates clockwise on the Atlantic Ocean. Upon 2011, the pelagic seaweed which is the main component of sargassum started to reach beaches on 19 Caribbean countries, with environmental, health and economic impacts that need to be addressed urgently.

Hydrogen sulfide exposure triggers chicken trachea inflammatory injury through oxidative stress-mediated FOS/IL8 signaling

Hydrogen sulfide (H₂S) is well known to cause irritation and damage to airway following inhalation, but the mechanism by which H₂S contributes to airway toxicity is unclear. In order to assess the respiratory toxicity of H₂S inhalation in chicken trachea, we investigated the change of oxidative stress parameters, tracheal tissue structure and transcriptome profiles of chicken trachea exposed to H₂S for 42 days. The results showed H₂S exposure induced oxidative stress and inflammation in trachea. The ultrastructural analysis revealed loss of cilia and accumulation of mucus in tracheal epithelium. Differentially expressed genes (DEGs) analysis indicated 454 genes were significantly changed, including 136 genes upregulated and 318 genes downregulated. Gene ontology and KEGG analysis showed many genes involved in response to oxidative stress, inflammatory and immune response, which might contribute to H₂S-induced tracheal inflammatory injury. Among those genes, N-acetyl-L-cysteine (NAC) treatment blocked the H₂S-triggered expression of FOS and IL8. Silencing FOS by siRNA inhibited H₂S-induced expression of IL8. Taken together, we concluded that H₂S induced oxidative stress leads to tracheal inflammation through FOS/IL8 signaling, leading to excessive mucus secretion and absence of cilia. These results provide new insights for unveiling the biological effects of H₂S in vivo and in vitro.

Methylene Blue Counteracts H2S-Induced Cardiac Ion Channel Dysfunction and ATP Reduction

We have previously demonstrated that methylene blue (MB) counteracts the effects of hydrogen sulfide (H₂S) cardiotoxicity by improving cardiomyocyte contractility and intracellular Ca²⁺ homeostasis disrupted by H₂S poisoning. In vivo, MB restores cardiac contractility severely depressed by sulfide and protects against arrhythmias, ranging from bundle branch block to ventricular tachycardia or fibrillation. To dissect the cellular mechanisms by which MB reduces arrhythmogenesis and improves bioenergetics in myocytes intoxicated with H₂S, we evaluated the effects of H₂S on resting membrane potential (E_m), action potential (AP), Na⁺/Ca²⁺ exchange current (I_NaCa), depolarization-activated K⁺ currents and ATP levels in adult mouse cardiac myocytes and determined whether MB could counteract the toxic effects of H₂S on myocyte electrophysiology and ATP. Exposure to toxic concentrations of H₂S (100 µM) significantly depolarized E_m, reduced AP amplitude, prolonged AP duration at 90% repolarization (APD₉₀), suppressed I_NaCa and depolarization-activated K⁺ currents, and reduced ATP levels in adult mouse cardiac myocytes. Treating cardiomyocytes with MB (20 µg/ml) 3 min after H₂S exposure restored E_m, APD₉₀, I_NaCa, depolarization-activated K⁺ currents, and ATP levels toward normal. MB improved mitochondrial membrane potential (∆ψ_m) and oxygen consumption rate in myocytes in which Complex I was blocked by rotenone. We conclude that MB ameliorated H₂S-induced cardiomyocyte toxicity at multiple levels: (1) reversing excitation-contraction coupling defects (Ca²⁺ homeostasis and L-type Ca²⁺ channels); (2) reducing risks of arrhythmias (E_m, APD, I_NaCa and depolarization-activated K⁺ currents); and (3) improving cellular bioenergetics (ATP, ∆ψ_m).

A Turn-On Optoacoustic Probe for Imaging Metformin-Induced Upregulation of Hepatic Hydrogen Sulfide and Subsequent Liver Injury

Metformin is currently the most prescribed oral agent for diabetes treatment; however the overdose or long-term use may cause some severe side effects such as liver injury. Researches indicate that metformin-induced liver injury is closely related to upregulation of hepatic H2S. Hence, monitoring hepatic H2S generation induced by metformin could be an effective approach for evaluating hepatoxicity of the drug. Methods: We present a novel turn-on and dual-mode probe for detecting and imaging metformin-induced liver injury by specifically tracking the upregulation of hepatic H2S with fluorescent and optoacoustic methods. After reaction with H2S, the strong electron-withdrawing group dinitrophenyl ether (which acts as both the recognition moiety and the fluorescence quencher) was cleaved and replaced by an electron-donating group hydroxyl. This correspondingly leads to the changes of the probe's electronic state and absorption red-shifting as well as the subsequent turn-on fluorescent and optoacoustic signals. Results: The probe was applied to the colon tumor-bearing mice model and the metformin-induced liver injury mice model to achieve tumor imaging and liver injury assessment. The biosafety of the probe was verified by histological analysis (hematoxylin and eosin staining) and serum biochemical assays. Conclusion: The probe responds quickly to H2S in tumors and the liver, and MSOT imaging with the probe offers cross-secitonal and 3D spatial information of liver injury. This study may provide an effective approach for accessing medication side effects by tracking drug-metabolism-related products.

Hydrogen sulfide inhalation-induced immune damage is involved in oxidative stress, inflammation, apoptosis and the Th1/Th2 imbalance in broiler bursa of Fabricius

Hydrogen sulfide (H₂S) is widely accepted to be a signaling molecule that exhibits some potentially beneficial therapeutic effects at physiological concentrations. At elevated levels, H₂S is highly toxic and has a negative effect on human health and animal welfare. Studies have shown that H₂S exposure induces an immune function in mice, but there are few studies of the effect of continuous H₂S exposure on immune organs in poultry. In this study, one-day-old broilers were selected and exposed to 4 or 20 ppm of H₂S gas for 14, 28 and 42 days of age. After exposure, the bursa of Fabricius (BF) was harvested. The results showed that continuous H₂S exposure reduced the body weight, abdominal fat percentage, and antibody titer in broilers. H₂S exposure also decreased mRNA expression of IgA, IgM and IgG in the broiler BF. A histological study revealed obvious nuclear debris, and a few vacuoles in the BF, and an ultrastructural study revealed mitochondrial and nuclear damage to BF cells after H₂S exposure for 42 d. Terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assay suggested H₂S exposure remarkably increased the number of TUNEL positive nuclei and significantly increased apoptotic index. The expression of apoptotic genes also confirmed that H₂S inhalation damaged the broiler BF. Increased cytokines and reduced antioxidant responses were detected in the BF after exposure to H₂S. Cytokines promoted inflammation and caused a Th1/Th2 imbalance. We suggest that continuous H₂S intoxication triggers oxidative stress, inflammation, apoptosis and a Th1/Th2 imbalance in the BF, leading to immune injury in broilers.

The effects of atmospheric hydrogen sulfide on peripheral blood lymphocytes of chickens: Perspectives on inflammation, oxidative stress and energy metabolism

Excessive hydrogen sulfide (H₂S) affects poultry health. Exposure to air pollution induces inflammation, oxidative stress, energy metabolism dysfunction and adverse health effects. However, few detailed studies have been conducted on the molecular mechanisms of H₂S-induced injury in poultry. To understand how H₂S drives its adverse effects on chickens, twenty-four 14-day-old chickens were randomly divided into two groups. The chickens in the control group were raised in a separate chamber without H₂S, and the chickens in the treatment group were exposed to 30 ppm H₂S. After 14 days of exposure, peripheral blood samples were taken and the lymphocytes were extracted to detect inflammation, oxidative stress and energy metabolism in broilers. Overall, an increase in the inflammatory response was detected in the peripheral blood lymphocytes following H₂S exposure compared to the control group, and the expression levels of the heat shock proteins (HSPs) and the transcription factors nuclear factor κB (NF-κB), cyclooxygenase 2 (COX-2) and inducible nitric oxide synthase (iNOS) were up-regulated in the H₂S group, which further suggested that H₂S induced an inflammatory response via the NF-κB pathway. Because of the activation of NF-κB, which is a major regulator of oxidative stress, we also observed that reactive oxygen species (ROS) production was elevated under H₂S exposure. In addition, we presumed that energy metabolism might be damaged due to the increased ROS production, and we found that H₂S down-regulated the expression levels of energy metabolism-related genes, which indicated the occurrence of energy metabolism dysfunction. Altogether, this study suggests that exposure to excessive atmospheric H₂S induces an inflammatory response, oxidative stress and energy metabolism dysfunction, providing a reference for comparative medicine.

Ontogenetic and temperature-dependent changes in tolerance to hypoxia and hydrogen sulfide during the early life stages of the Manila clam Ruditapes philippinarum

Wind-induced upwelling of hypoxic waters containing hydrogen sulfide (H₂S) sometimes causes mass mortalities of aquatic organisms inhabiting coastal areas, including the hypoxia-tolerant Manila clam Ruditapes philippinarum. We examined the tolerance of Manila clam to H₂S under controlled laboratory conditions. Larvae and juveniles obtained by artificial fertilization or from a wild population were exposed to normoxic or to hypoxic water with or without un-ionized H₂S (concentrations, 0.2-52.2 mg/L). Twenty-four-hour exposure experiments revealed ontogenetic changes in the clam's tolerance to H₂S exposure: tolerance was enhanced from the larval stages to juveniles just after settlement but was attenuated as juveniles grew. Tolerance of larvae and juveniles to H₂S exposure weakened as the water temperature rose from 20 to 28 °C. Prolonged 48-h exposure to H₂S attenuated the tolerance of juveniles to H₂S. Temporary suspension of H₂S exposure by 24-h reoxygenation improved the ability of juveniles to withstand repeated H₂S exposure.

The dichotomous role of H2S in cancer cell biology? Déjà vu all over again

Nitric oxide (NO) a gaseous free radical is one of the ten smallest molecules found in nature, while hydrogen sulfide (H2S) is a gas that bears the pungent smell of rotten eggs. Both are toxic yet they are gasotransmitters of physiological relevance. There appears to be an uncanny resemblance between the general actions of these two gasotransmitters in health and disease. The role of NO and H2S in cancer has been quite perplexing, as both tumor promotion and inflammatory activities as well as anti-tumor and antiinflammatory properties have been described. These paradoxes have been explained for both gasotransmitters in terms of each having a dual or biphasic effect that is dependent on the local flux of each gas. In this review/commentary, I have discussed the major roles of NO and H2S in carcinogenesis, evaluating their dual nature, focusing on the enzymes that contribute to this paradox and evaluate the pros and cons of inhibiting or inducing each of these enzymes.

A timeline of hydrogen sulfide (H2S) research: From environmental toxin to biological mediator

The history of H2S - as an environmental toxin - dates back to 1700, to the observations of the Italian physician Bernardino Ramazzini, whose book "De Morbis Artificum Diatriba" described the painful eye irritation and inflammation of "sewer gas" in sewer workers. The gas has subsequently been identified as hydrogen sulfide (H2S), and opened three centuries of research into the biological roles of H2S. The current article highlights the key discoveries in the field of H2S research, including (a) the toxicological studies, which characterized H2S as an environmental toxin, and identified some of its modes of action, including the inhibition of mitochondrial respiration; (b) work in the field of bacteriology, which, starting in the early 1900s, identified H2S as a bacterial product - with subsequently defined roles in the regulation of periodontal disease (oral bacterial flora), intestinal epithelial cell function (enteral bacterial flora) as well as in the regulation of bacterial resistance to antibiotics; and (c), work in diverse fields of mammalian biology, which, starting in the 1940s, identified H2S as an endogenous mammalian enzymatic product, the functions of which - among others, in the cardiovascular and nervous system - have become subjects of intensive investigation for the last decade. The current review not only enumerates the key discoveries related to H2S made over the last three centuries, but also compiles the most frequently cited papers in the field which have been published over the last decade and highlights some of the current 'hot topics' in the field of H2S biology.

BigR is a sulfide sensor that regulates a sulfur transferase/dioxygenase required for aerobic respiration of plant bacteria under sulfide stress

To cope with toxic levels of H2S, the plant pathogens Xylella fastidiosa and Agrobacterium tumefaciens employ the bigR operon to oxidize H2S into sulfite. The bigR operon is regulated by the transcriptional repressor BigR and it encodes a bifunctional sulfur transferase (ST) and sulfur dioxygenase (SDO) enzyme, Blh, required for H2S oxidation and bacterial growth under hypoxia. However, how Blh operates to enhance bacterial survival under hypoxia and how BigR is deactivated to derepress operon transcription is unknown. Here, we show that the ST and SDO activities of Blh are in vitro coupled and necessary to oxidize sulfide into sulfite, and that Blh is critical to maintain the oxygen flux during A. tumefaciens respiration when oxygen becomes limited to cells. We also show that H2S and polysulfides inactivate BigR leading to operon transcription. Moreover, we show that sulfite, which is produced by Blh in the ST and SDO reactions, is toxic to Citrus sinensis and that X. fastidiosa-infected plants accumulate sulfite and higher transcript levels of sulfite detoxification enzymes, suggesting that they are under sulfite stress. These results indicate that BigR acts as a sulfide sensor in the H2S oxidation mechanism that allows pathogens to colonize plant tissues where oxygen is a limiting factor.

Hydrogen sulfide emissions from a swine building affected by dietary crude protein

Hydrogen sulfide (H₂S) is a toxic air pollutant at animal facilities; but the understanding of its generation and emission processes has been limited. This paper studied H₂S emissions during a complete cycle of wean-finish pigs from a research building, where 12 pig rooms were divided into three groups that were fed with standard feed (control), and 2.1-3.8% (T₁) and 4.4-7.8% (T₂) reduced dietary crude protein (CP) feed. The group cycle mean H₂S emission rates were 4.0 ± 2.9, 4.3 ± 3.2, and 5.4 ± 4.0 g d^-1 AU^-1 (Animal Unit = 500 kg live mass), respectively, for the control, T₁, and T₂ groups. Emissions of H₂S were promoted by 10.0 and 36.7%, respectively, for the T₁ and T₂ groups (p < 0.001), although large variabilities existed in the emissions from different rooms within the same groups. The enhanced H₂S emissions from the T₁ and T₂ groups were related to the reduced manure pH and were possibly affected through a number of pathways, which could involve volatile fatty acids and nitrogen concentrations, and microbial activities in manure.

Cobinamide is effective for treatment of hydrogen sulfide-induced neurological sequelae in a mouse model

Hydrogen sulfide (H2 S) is a highly neurotoxic gas. Acute exposure can lead to neurological sequelae among survivors. A drug for treating neurological sequelae in survivors of acute H2 S intoxication is needed. Using a novel mouse model we evaluated the efficacy of cobinamide (Cob) for increasing survival of, and reducing neurological sequalae in, mice exposed to sublethal doses of H2 S. There were two objectives: (1) to determine the dose-response efficacy of Cob and (2) to determine the effective therapeutic time window of Cob. To explore objective 1, mice were injected intramuscularly with Cob at 0, 50, or 100 mg/kg at 2 min after H2 S exposure. For objective 2, mice were injected intramuscularly with 100 mg/kg Cob at 2, 15, and 30 min after H2 S exposure. For both objectives, mice were exposed to 765 ppm of H2 S gas. Cob significantly reduced H2 S-induced lethality in a dose-dependent manner (P < 0.05). Cob-treated mice exhibited significantly fewer seizures and knockdowns compared with the H2 S-exposed group. Cob also reversed H2 S-induced weight loss, behavioral deficits, neurochemical changes, cytochrome c oxidase enzyme inhibition, and neurodegeneration in a dose- and time-dependent manner (P < 0.01). Overall, these findings show that Cob increases survival and is neuroprotective in a mouse model of H2 S-induced neurological sequelae.

Hydrogen Sulfide Specifically Alters NAD(P)H Quinone Dehydrogenase 1 (NQO1) Olfactory Neurons in the Rat

The regions of the olfactory epithelium affected by hydrogen sulfide (H₂S) toxicity in the rat present a striking similarity with the developmental olfactory zone 1 described in the mouse. This zone which is the only region containing neurons expressing NAD(P)H quinone dehydrogenase 1 (NQO1) is involved in complex behavioral responses in rodents, and other mammals, triggered by specific olfactory stimuli. We therefore sought to determine whether (1) olfactory neurons expressing NQO1 are located in the same regions in the rats and in the mice and (2) there is an overlap between olfactory neurons expressing this protein and those affected by the toxicity of H₂S. Rats were exposed to H₂S - 200 ppm during 3 h, three consecutive days- and displayed symmetric acute segmental necrosis of the neurons and sustentacular cells of the olfactory epithelium in the dorsomedial nasal cavity. We found that expression of NQO1 in Sprague-Dawley rats spatially recapitulated that of the mouse. The degree of agreement or overlap between these two populations of neurons (necrosis vs. NQO1 expression) reached 80.2%. Although the underlying mechanisms accounted for the high sensitivity for NQO1 neurons -or the relative protection of non NQO1 neurons- to sulfide toxicity remain to be established, this observation is offering an intriguing approach that could be used to acutely suppress the pool of neural cells in olfactory zone I and to understand the mechanisms of toxicity and protection of other populations of neurons exposed to sulfide.