Hydrogen Sulfide Metabolism and Pulmonary Hypertension

Abstract

Pulmonary hypertension (PH) is a severe and multifactorial disease characterized by a progressive elevation of pulmonary arterial resistance and pressure due to remodeling, inflammation, oxidative stress, and vasoreactive alterations of pulmonary arteries (PAs). Currently, the etiology of these pathological features is not clearly understood and, therefore, no curative treatment is available. Since the 1990s, hydrogen sulfide (H₂S) has been described as the third gasotransmitter with plethoric regulatory functions in cardiovascular tissues, especially in pulmonary circulation. Alteration in H₂S biogenesis has been associated with the hallmarks of PH. H₂S is also involved in pulmonary vascular cell homeostasis via the regulation of hypoxia response and mitochondrial bioenergetics, which are critical phenomena affected during the development of PH. In addition, H₂S modulates ATP-sensitive K⁺ channel (K_ATP) activity, and is associated with PA relaxation. In vitro or in vivo H₂S supplementation exerts antioxidative and anti-inflammatory properties, and reduces PA remodeling. Altogether, current findings suggest that H₂S promotes protective effects against PH, and could be a relevant target for a new therapeutic strategy, using attractive H₂S-releasing molecules. Thus, the present review discusses the involvement and dysregulation of H₂S metabolism in pulmonary circulation pathophysiology.

Keywords: hydrogen sulfide; inflammation; oxidative stress; pulmonary hypertension; vascular reactivity; vascular remodeling.

Figure 1

Rising number of publications in H₂S research fields. A search for original and review articles published from 1996 to 2020 was performed in the PubMed database (www.ncbi.nlm.nih.gov/pubmed, accessed on 10 June 2021) using the key words “hydrogen sulfide”.

Figure 2

Main H₂S anabolic and catabolic pathways in mammalian cells. (a) Endogenous hydrogen sulfide (H₂S) production is mainly due to enzymatic pathways through the activity of cystathionine γ-lyase (CSE), cystathionine β-synthase (CBS), and 3-mercaptopyruvate sulfur transferase/cysteine aminotransferase (3-MST/CAT) coupling. Cytosolic CBS and CSE are involved in the interconversion of homocysteine to L-cysteine (transsulfuration pathway). In the context of cysteine catabolism, CBS and CSE desulfhydrate L-cysteine (and homocysteine) to produce H₂S. L-cysteine can also act as a substrate with α-ketoglutarate to produce 3-mercaptopyruvate through a transamination reaction via activity of the CAT1 and CAT2 enzymes (cytosolic and mitochondrial, respectively). CSE activity is selectively inhibited by DL-propargylglycine (PAG) and β-cyanoalanine (BCA). CBS and CSE activities—especially the latter—are both inhibited by AOA. In a reducing environment (presence of thioredoxin (Trx) and dihydrolipoic acid (DHLA)—two endogenous reducing molecules), 3-mercaptopyruvate is then used by 3-MST to release pyruvate and H₂S, mainly in the mitochondria. To a lesser extent, non-enzymatic H₂S production can occur in physiological conditions, caused by reactive sulfur groups of thiosulfates (S₂O₃²⁻) or polysulfides (RS_nS) in the presence of glutathione (GSH) or reducing equivalents (nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate (NADPH)). (b) Oxidation, the major catabolic pathway to maintaining H₂S homeostasis, occurs in the mitochondria. H₂S is rapidly oxidized by sulfide quinone oxidoreductase (SQR) to form persulfides (R-SSH), which undergo another oxidation step by persulfide dioxygenase (ETHE1) to produce sulfites (SO₃²⁻). In this process, two electrons (e⁻) are released to ubiquinone (Q) and transferred to complex III of the mitochondrial electron transfer chain (ETC) [33,83]. Sulfites are either converted to thiosulfates or directly to sulfates (SO₄²⁻), thanks to rhodanese (Rhod) and sulfite oxidase (SO), respectively. The final catabolic products—sulfates—are finally excreted via urine. In an additional pathway, persulfides can be degraded to thiosulfates by sulfur transferase (SR). Thiosulfates can also be converted to sulfites and H₂S by thiosulfate reductase (TR) in the presence of GSH, which, ultimately, leads to sulfate production by SO. H₂S methylation is the other, although minor, clearance pathway. H₂S is first converted to methanethiol (CH₄S) and dimethyl sulfide ((CH₃)₂S) via S-methyltransferase (TMT) activity. Then, Rhod breaks down dimethyl sulfide into sulfates, which are then excreted through urine. L-cysteine is degraded to cysteine sulfinate by cysteine dioxygenase (CDO) activity. Cysteine sulfinate is then converted to sulfites by CAT1/2 and, ultimately, to sulfate by SO, or to taurine by cysteine sulfinate decarboxylase (CSAD). 3-MST: 3-mercaptopyruvate sulfur transferase; AOA: aminooxyacetate; BCA: β-cyanoalanine; CAT1/2: cysteine aminotransferase 1/2; CBS: cystathionine β-synthase; CDO: cysteine dioxygenase; CSE: cystathionine γ-lyase; CSAD: cysteine sulfinate decarboxylase; DHLA: dihydrolipoic acid; e⁻: electron; ETC: electron transfer chain; I, II, III, and IV: mitochondria complexes I–IV of ETC; ETHE1: persulfide dioxygenase; GSH: glutathione; H₂S: hydrogen sulfide; IMS: intermembrane space; NADH: nicotinamide adenine dinucleotide; NADPH: nicotinamide adenine dinucleotide phosphate; PAG: DL-propargylglycine; Q: ubiquinone; Rhod: rhodanese; SO: sulfite oxidase; SQR: sulfide quinone oxidoreductase; SR: sulfur transferase; TMT: S-methyltransferase; TR: thiosulfate reductase; Trx: thioredoxin.

Figure 3

Endogenous production of H₂S and its roles in pulmonary circulation. H₂S-producing enzymes have been found to be expressed in the endothelium and the media of pulmonary arteries. CSE and 3-MST are detected in both PAECs and PASMCs, whereas CBS exhibits an endothelial predominance. H₂S is involved in lung and PA development by (1) promoting vascular growth and associated PAEC migration and (2) decreasing media muscularization. Variation in the partial pressure of oxygen is linked to regulation of H₂S metabolism. Hypoxia exposure triggers a decrease in H₂S clearance and an increase in H₂S production, leading to a global elevation of intracellular H₂S levels. Modulation of the H₂S clearance/production balance may play a pivotal role in oxygen sensing in pulmonary circulation. PA treatment with H₂S donors is associated with paradoxical influence on vascular tone. On the one hand, H₂S induces biphasic contraction of PAs that could be explained by the balance of the promotion (oxidation by SQR) versus poisoning (blocking of complex IV) effects of H₂S on mitochondrial ETC function. In other hand, H₂S also dose-dependently relaxes PAs through the activation of K_ATP channels, leading to vascular cells’ hyperpolarization and, thus, to relaxation. Crosstalk between H₂S and endothelial NO pathways was also observed, suggesting a potential role of the endothelium in the relaxing effects of H₂S. 3-MST: 3-mercaptopyruvate sulfur transferase; CSE: cystathionine γ-lyase; GYY4137: morpholin-4-ium 4-methoxyphenyl (morpholino) phosphinodithioate; H₂S: hydrogen sulfide; K_ATP: ATP-sensitive K⁺ channel; Na₂S: sodium sulfide; NaHS: sodium hydrosulfide; NO: nitric oxide; PA: pulmonary artery; PAECs: pulmonary artery endothelial cells; PASMCs: pulmonary artery smooth muscle cells.

Figure 4

Alteration of H₂S metabolism and protective influence of H₂S donors on PH hallmarks. Clinical and experimental data suggest a pivotal dysregulation of endogenous H₂S production through the inhibition of activity and/or expression of H₂S-generating enzymes (namely, CSE, CBS, and 3-MST/CAT). These alterations promote the development of PH-associated hallmarks in PA, such as endothelial dysfunction, inflammation, and oxidative and ER stress, as well as increased media thickness of PAs. Altogether these pathological phenomena lead to increased PA resistance, mPAP, subsequent RV hypertrophy and, finally, PH, H₂S donors such as NaHS and GYY4137 demonstrate multifaceted protective effects on PA alterations, counteracting PH development. 3-MST: 3-mercaptopyruvate sulfur transferase; BPD: bronchopulmonary dysplasia; CAT: cysteine aminotransferase; CBS: cystathionine β-synthase; CHDs: congenital heart diseases; COPD: chronic obstructive pulmonary disease; CSE: cystathionine γ-lyase; ER: endoplasmic reticulum; H₂S: hydrogen sulfide; mPAP: mean pulmonary artery pressure; PA: pulmonary artery; PASMCs: pulmonary artery smooth muscle cells; PH: pulmonary hypertension; RV: right ventricle. Flat arrows represent inhibition interactions.