Effect of in vitro exposure to hydrogen sulfide on rabbit alveolar macrophages cultured on gas-permeable membranes

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

Hydrogen sulfide: An endogenous regulator of the immune system

Hydrogen sulfide (H₂S) is now recognized as an endogenous signaling gasotransmitter in mammals. It is produced by mammalian cells and tissues by various enzymes - predominantly cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MST) - but part of the H₂S is produced by the intestinal microbiota (colonic H₂S-producing bacteria). Here we summarize the available information on the production and functional role of H₂S in the various cell types typically associated with innate immunity (neutrophils, macrophages, dendritic cells, natural killer cells, mast cells, basophils, eosinophils) and adaptive immunity (T and B lymphocytes) under normal conditions and as it relates to the development of various inflammatory and immune diseases. Special attention is paid to the physiological and the pathophysiological aspects of the oral cavity and the colon, where the immune cells and the parenchymal cells are exposed to a special "H₂S environment" due to bacterial H₂S production. H₂S has many cellular and molecular targets. Immune cells are "surrounded" by a "cloud" of H₂S, as a result of endogenous H₂S production and exogenous production from the surrounding parenchymal cells, which, in turn, importantly regulates their viability and function. Downregulation of endogenous H₂S producing enzymes in various diseases, or genetic defects in H₂S biosynthetic enzyme systems either lead to the development of spontaneous autoimmune disease or accelerate the onset and worsen the severity of various immune-mediated diseases (e.g. autoimmune rheumatoid arthritis or asthma). Low, regulated amounts of H₂S, when therapeutically delivered by small molecule donors, improve the function of various immune cells, and protect them against dysfunction induced by various noxious stimuli (e.g. reactive oxygen species or oxidized LDL). These effects of H₂S contribute to the maintenance of immune functions, can stimulate antimicrobial defenses and can exert anti-inflammatory therapeutic effects in various diseases.

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 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.

Occupational exposure to hydrogen sulfide in the sour gas industry: some unresolved issues

Occupational exposure to hydrogen sulfide (H2S) and the medical management of H2S-associated toxicity remains a problem in the sour gas industry and some other industrial settings. The acute effects of exposure to H2S are well recognized, but accurate exposure-response data are limited to acutely lethal effects, even in animal studies. Odor followed by olfactory paralysis and keratoconjunctivitis are the characteristics effects of H2S at lower concentrations. H2S-induced acute central toxicity leading to reversible unconsciousness is a "knockdown"; it is controversial whether repeated or prolonged knockdowns are associated with chronic neurologic sequelae but the evidence is suggestive. Knockdowns can be acutely fatal as a consequence of respiratory paralysis and cellular anoxia. Pulmonary edema is also a well-recognized acute effect of H2S toxicity. Human studies of sublethal exposure with satisfactory exposure assessment are almost nonexistent. There are indications, poorly documented at present, of other chronic health problems associated with H2S exposure, including neurotoxicity, cardiac arrhythmia, and chronic eye irritation but apparently not cancer. Rigorous and comprehensive studies in the sour gas industry are difficult, in part because of confounding exposures and uncertain end points.

Cytotoxic effects of hydrogen sulfide on pulmonary alveolar macrophages in rats

Respiratory rates (basal and zymosan-stimulated) and cell viability were monitored in pulmonary alveolar macrophages (PAM) from rats exposed to 0, 70, 280, and 560 mg/m3 (0, 50, 200, and 400 ppm) hydrogen sulfide (H2S) gas for 4 h. Zymosan-stimulated respiratory rates were markedly reduced in PAM collected from rats exposed to 280 and 560 mg/m3 H2S; however, their basal respiratory rates were not affected. Significant decrease in cell viability was also observed in samples from 560 mg/m3 H2S-treated rats, but it remained high and unchanged in other treatments. In vitro incubation of PAM from control rats with sulfide (a precursor of H2S) and its two oxidation products, sulfite and sulfate, showed that sulfide was markedly more inhibitory to both respiratory rates than sulfite or sulfate. These treatments did not affect cell viability.

A critical review of the literature on hydrogen sulfide toxicity

The information available on the biological activity of hydrogen sulfide has been examined for present status of critical results pertaining to the toxicity of hydrogen sulfide. This review of the literature is intended as an evaluative report rather than an annotated bibliography of all the source material examined on hydrogen sulfide. The information was selected as it might relate to potential toxic effects of hydrogen sulfide to man and summarized, noting information gaps that may require further investigation. Several recommendations are listed for possible consideration for either toxicological research or additional short- and long-term tests. Two bibliographies have been provided to assist in locating references considered in this report: (1) literature examined but not cited and (2) reference citations. The majority of the references in the first bibliography were considered peripheral information and less appropriate for inclusion in this report.

Improved method for transmission electron microscopy of ciliated cell monolayers maintained on gas-permeable membranes

A new method is described for preparing thin sections for transmission electron microscopy of ciliated respiratory epithelial cells cultivated in vitro on Teflon membranes. Hamster tracheal explant cultures were treated with collagenase to promote epithelial migration. Patches of monolayer growth occurred in the vicinity of the explants when the latter were incubated on FEP-etched Teflon membranes in Chamber/Dishes. Cells in the monolayer maintained a high degree of differentiation as evidenced by active ciliary motion. The monolayer was fixed and processed in situ. During the embedding process the chamber was supported on a solid Teflon cylinder to keep the membrane taut as it was heated to polymerize the resin. The membrane was removed and the monolayered cells were re-embedded and sectioned. Electron microscopic examination of cultures revealed clear cell profiles on a flat, even basal plane. This method provides for a normal spatial configuration of cells in a monolayer format, and should prove useful for electron microscopy studies of any cells cultured on flexible films.

Cytotoxicity and mutagenicity of vapor-phase pollutants in rat lung epithelial cells and Chinese hamster ovary cells grown on collagen gels

Lung epithelial cell (cell line designated LEC) and Chinese hamster ovary (CHO) cells were grown on hydrated collagen gels and exposed directly to toxic vapor-phase pollutants. The cells were exposed to graded concentrations of phenol, formaldehyde, a volatile fraction of process stream material from an experimental coal gasifier and the nonparticulate, vapor phase of diesel engine exhaust. During exposures, the cells were maintained at an air/collagen interface by removing the medium overlying the hydrated collagen gel. Morphological changes indicative of cell retraction were found in LEC cell cultures exposed to phenol, formaldehyde, or diesel exhaust. Damage following exposure to the toxicants was quantitated in LEC and CHO cells by Trypan blue dye exclusion, a measure of plasma membrane integrity. Clone-forming ability was also used to measure cell survival in CHO cells. When measured by Trypan blue dye exclusion, phenol (EC50 = 2.1 mg/l) caused membrane damage to LEC cells but not CHO cells, while formaldehyde (EC50 = 31 and 42 micrograms/l for LEC and CHO, respectively) and diesel exhaust (EC50 = 11 and 29% of tailpipe exhaust in LEC and CHO cells, respectively) caused damage to both cell types. No cytotoxicity was observed in LEC or CHO cells exposed to the fraction from the coal gasifier. Essentially no mutagenic activity was associated with the exposure of CHO cells to formaldehyde or the vapor phase of diesel exhaust. Mutagenic activity was found in CHO cells exposed to ethylene oxide, the positive control. The results of this study indicate that mammalian cells grown on collagen gels can readily be exposed to vapors of chemicals and chemical mixtures. The cell exposure system may be generally useful in the analysis of toxic damage to mammalian cells resulting from gaseous or vapor-phase pollutants.

Evaluation of an exposure system using cells grown on collagen gels for detecting highly volatile mutagens in the CHO/HGPRT mutation assay

Chinese hamster ovary (CHO) cells were grown on hydrated collagen gels, the overlaying medium removed leaving the cells at an air/collagen interface, and the cells exposed to a dynamic flow of ethylene oxide. Increases in CHO cell mutant frequency and decreases in cell viability were observed. To establish if the exposure system could be simplified, cells were exposed in sealed bottles (static system) to ethylene oxide. No substantial changes in cytotoxicity, mutant frequency, or effective concentration were noted when comparing static versus dynamic exposure systems. The general usefulness of the exposure system using cells grown on collagen gels was evaluated in a static system using propylene oxide and 1,2-dichloroethane, both of which were found to be mutagenic and cytotoxic. Comparatively, the exposure of cells by the collagen gel method was as effective in detecting genotoxic damage as were conventional methods (cells covered with medium) using cells grown on glass substrates. The exposure of CHO cells on collagen gels to highly volatile mutagens was simple and inexpensive, and may be generally useful in the detection of gaseous or volatile mutagens.