Protective effects of hydrogen sulfide on oxidative stress and fibrosis in hepatic stellate cells

Elucidating the Molecular Pathways and Therapeutic Interventions of Gaseous Mediators in the Context of Fibrosis

Fibrosis, a pathological alteration of the repair response, involves continuous organ damage, scar formation, and eventual functional failure in various chronic inflammatory disorders. Unfortunately, clinical practice offers limited treatment strategies, leading to high mortality rates in chronic diseases. As part of investigations into gaseous mediators, or gasotransmitters, including nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S), numerous studies have confirmed their beneficial roles in attenuating fibrosis. Their therapeutic mechanisms, which involve inhibiting oxidative stress, inflammation, apoptosis, and proliferation, have been increasingly elucidated. Additionally, novel gasotransmitters like hydrogen (H2) and sulfur dioxide (SO2) have emerged as promising options for fibrosis treatment. In this review, we primarily demonstrate and summarize the protective and therapeutic effects of gaseous mediators in the process of fibrosis, with a focus on elucidating the underlying molecular mechanisms involved in combating fibrosis.

Inhibition of endogenous hydrogen sulfide production reduces activation of hepatic stellate cells via the induction of cellular senescence

In chronic liver injury, quiescent hepatic stellate cells (HSCs) transdifferentiate into activated myofibroblast-like cells and produce large amounts of extracellular matrix components, e.g. collagen type 1. Cellular senescence is characterized by irreversible cell-cycle arrest, arrested cell proliferation and the acquisition of the senescence-associated secretory phenotype (SASP) and reversal of HSCs activation. Previous studies reported that H2S prevents induction of senescence via its antioxidant activity. We hypothesized that inhibition of endogenous H2S production induces cellular senescence and reduces activation of HSCs. Rat HSCs were isolated and culture-activated for 7 days. After activation, HSCs treated with H2S slow-releasing donor GYY4137 and/or DL-propargylglycine (DL-PAG), an inhibitor of the H2S-producing enzyme cystathionine γ-lyase (CTH), as well as the PI3K inhibitor LY294002. In our result, CTH expression was significantly increased in fully activated HSCs compared to quiescent HSCs and was also observed in activated stellate cells in a in vivo model of cirrhosis. Inhibition of CTH reduced proliferation and expression of fibrotic markers Col1a1 and Acta2 in HSCs. Concomitantly, DL-PAG increased the cell-cycle arrest markers Cdkn1a (p21), p53 and the SASP marker Il6. Additionally, the number of β-galactosidase positive senescent HSCs was increased. GYY4137 partially restored the proliferation of senescent HSCs and attenuated the DL-PAG-induced senescent phenotype. Inhibition of PI3K partially reversed the senescence phenotype of HSCs induced by DL-PAG. Inhibition of endogenous H2S production reduces HSCs activation via induction of cellular senescence in a PI3K-Akt dependent manner. Our results show that cell-specific inhibition of H2S could be a novel target for anti-fibrotic therapy via induced cell senescence.

Hydrogen Sulfide and Functional Therapy: Novel Mechanisms from Epigenetics

Hydrogen sulfide (H2S) is a gasotransmitter with significant physiological effects, including anti-inflammatory properties, regulation of oxidative stress, and vasodilation, thus regulating body functions. Functional therapy involves using treatments that target the underlying cause of a disease, rather than simply treating symptoms. Epigenetics refers to changes in gene expression that occur through modifications to DNA, to the proteins that package DNA, or to noncoding RNA mechanisms. Recent research advances suggest that H2S may play a role in epigenetic regulation by altering DNA methylation patterns and regulating histone deacetylases, enzymes that modify histone proteins, or modulating microRNA mechanisms. These critical findings suggest that H2S may be a promising molecule for functional therapy in various diseases where epigenetic modifications are dysregulated. We reviewed the relevant research progress in this area, hoping to provide new insights into the epigenetic mechanisms of H2S. Despite the challenges of clinical use of H2S, future research may lead to the progress of new therapeutic approaches. Antioxid. Redox Signal. 40, 110-121.

Novel Hydrogen Sulfide Hybrid Derivatives of Keap1-Nrf2 Protein-Protein Interaction Inhibitor Alleviate Inflammation and Oxidative Stress in Acute Experimental Colitis

Ulcerative colitis (UC) is an idiopathic inflammatory disease of unknown etiology possibly associated with intestinal inflammation and oxidative stress. Molecular hybridization by combining two drug fragments to achieve a common pharmacological goal represents a novel strategy. The Kelch-like ECH-associated protein 1 (Keap1)-nuclear factor erythroid 2-related factor 2 (Nrf2) pathway provides an effective defense mechanism for UC therapy, and hydrogen sulfide (H₂S) shows similar and relevant biological functions as well. In this work, a series of hybrid derivatives were synthesized by connecting an inhibitor of Keap1-Nrf2 protein-protein interaction with two well-established H₂S-donor moieties, respectively, via an ester linker, to find a drug candidate more effective for the UC treatment. Subsequently, the cytoprotective effects of hybrids derivatives were investigated, and DDO-1901 was identified as a candidate showing the best efficacy and used for further investigation on therapeutic effect on dextran sulfate sodium (DSS)-induced colitis in vitro and in vivo. Experimental results indicated that DDO-1901 could effectively alleviate DSS-induced colitis by improving the defense against oxidative stress and reducing inflammation, more potent than parent drugs. Compared with either drug alone, such molecular hybridization may offer an attractive strategy for the treatment of multifactorial inflammatory disease.

PCSK9: A emerging participant in heart failure

Heart failure (HF) is a complex clinical syndrome caused by various cardiovascular diseases. Its main pathogenesis includes cardiomyocyte loss, myocardial energy metabolism disorder, and activation of cardiac inflammation. Due to the clinically unsatisfactory treatment of heart failure, different mechanisms need to be explored to provide new targets for the treatment of this disease. Proprotein convertase subtilisin/kexin type 9 (PCSK9), a gene mainly related to familial hypercholesterolemia, was discovered in 2003. Aside from regulating lipid metabolism, PCSK9 may be involved in other biological processes such as apoptosis, autophagy, pyroptosis, inflammation, and tumor immunity and related to diabetes and neurodegenerative diseases. Recently, clinical data have shown that the circulating PCSK9 level is significantly increased in patients with heart failure, and it is related to the prognosis for heart failure. Furthermore, in animal models and patients with myocardial infarction, PCSK9 in the infarct margin area was also found to be significantly increased, which further suggested that PCSK9 might be closely related to heart failure. However, the specific mechanism of how PCSK9 participates in heart failure remains to be further explored. The purpose of this review is to summarize the potential mechanism of PCSK9's involvement in heart failure, thereby providing a new treatment strategy for heart failure.

Function of gaseous hydrogen sulfide in liver fibrosis

Over the past few years, hydrogen sulfide (H₂S) has been shown to exert several biological functions in mammalian. The endogenous production of H₂S is mainly mediated by cystathione β-synthase, cystathione γ-lyase and 3-mercaptopyruvate sulfur transferase. These enzymes are broadly expressed in liver tissue and regulates liver function by working on a variety of molecular targets. As an important regulator of liver function, H₂S is critically involved in the pathogenesis of various liver diseases, such as non-alcoholic steatohepatitis and liver cancer. Targeting H₂S-generating enzymes may be a therapeutic strategy for controlling liver diseases. This review described the function of H₂S in liver disease and summarized recent characterized role of H₂S in several cellular process of the liver.

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, a gaseous signaling molecule, elongates primary cilia on kidney tubular epithelial cells by activating extracellular signal-regulated kinase

Primary cilia on kidney tubular cells play crucial roles in maintaining structure and physiological function. Emerging evidence indicates that the absence of primary cilia, and their length, are associated with kidney diseases. The length of primary cilia in kidney tubular epithelial cells depends, at least in part, on oxidative stress and extracellular signal-regulated kinase 1/2 (ERK) activation. Hydrogen sulfide (H₂S) is involved in antioxidant systems and the ERK signaling pathway. Therefore, in this study, we investigated the role of H₂S in primary cilia elongation and the downstream pathway. In cultured Madin-Darby Canine Kidney cells, the length of primary cilia gradually increased up to 4 days after the cells were grown to confluent monolayers. In addition, the expression of H₂S-producing enzyme increased concomitantly with primary cilia length. Treatment with NaHS, an exogenous H₂S donor, accelerated the elongation of primary cilia whereas DL-propargylglycine (a cystathionine γ-lyase inhibitor) and hydroxylamine (a cystathionine-β-synthase inhibitor) delayed their elongation. NaHS treatment increased ERK activation and Sec10 and Arl13b protein expression, both of which are involved in cilia formation and elongation. Treatment with U0126, an ERK inhibitor, delayed elongation of primary cilia and blocked the effect of NaHS-mediated primary cilia elongation and Sec10 and Arl13b upregulation. Finally, we also found that H₂S accelerated primary cilia elongation after ischemic kidney injury. These results indicate that H₂S lengthens primary cilia through ERK activation and a consequent increase in Sec10 and Arl13b expression, suggesting that H₂S and its downstream targets could be novel molecular targets for regulating primary cilia.

Role of Hydrogen Sulfide in the Treatment of Fibrosis

Hydrogen sulfide (H₂S) is a biological gas, the abnormal metabolism of which has associations with the pathogenesis of fibrosis. The purpose of this paper was to determine the potential of H₂S in the prevention and treatment of fibrosis. The data is obtained mainly from articles found in the PubMed database using the keywords “fibrosis” and “hydrogen sulfide,” limiting the results to those published within the last 10 years. Some additional resources have also been used, such as books and articles within journals. Evidence of decreased H₂S enzyme levels in animal models with fibrotic diseases has been found. The protective role of H₂S has been validated by the administration of exogenous H₂S donors in animal models with fibrosis. It is also evident that H₂S is involved in complex signaling pathways and ion channels that inhibit fibrosis development. These findings support the role of H₂S in the treatment of a variety of fibrotic diseases. A randomized controlled trial in fibrosis patients comparing the efficacy of exogenous H₂S and placebo in addition to standard of care can be implemented to validate this further.

Effect of microbiota metabolites on the progression of chronic hepatitis B virus infection

Accumulating evidence shows that the intestinal microbiota is closely related to the pathophysiology and the disease progression of chronic hepatitis B virus (HBV) infection. The intestinal microbiota acts on the host through its metabolites. This review aimed to discuss the effects of gut microbiota metabolites on the disease progression of chronic HBV infection. A literature search on PubMed database and Wiley Online Library with pre-specified criteria yielded 96 unique results. After consensus by all authors, the contents from 86 original publications were extracted and included in this review. In liver disease with HBV infection, the intestinal microbiota changed in different stages and affected the production of bacterial metabolites. The abundance of bacteria producing short-chain fatty acids such as butyrate reduced, which was associated with bacterial translocation and the progression of liver disease. The intestinal microbiota-bile acid-host axis was destroyed, affecting the progression of the disease. Under the control of intestinal microbiota, tryptophan affected the gut-liver axis through three main metabolic pathways, among which the kynurenine pathway was closely related to the immune response of hepatitis B. The level of trimethylamine-N-oxide decreased in liver cancer with HBV infection and were used as a potential biomarker of liver cancer. Vitamin deficiencies, including those of vitamin D and vitamin A related to microbiota, were common and associated with survival. Hydrogen sulfide regulated by the intestinal microbiota was also closely related to the gut-liver axis. In liver disease with hepatitis B infection, the intestinal microbiota is imbalanced, and a variety of intestinal microbiota metabolites participate in the occurrence and development of the disease.

Gasotransmitters: Potential Therapeutic Molecules of Fibrotic Diseases

Fibrosis is defined as the pathological progress of excessive extracellular matrix (ECM), such as collagen, fibronectin, and elastin deposition, as the regenerative capacity of cells cannot satisfy the dynamic repair of chronic damage. The well-known features of tissue fibrosis are characterized as the presence of excessive activated and proliferated fibroblasts and the differentiation of fibroblasts into myofibroblasts, and epithelial cells undergo the epithelial-mesenchymal transition (EMT) to expand the number of fibroblasts and myofibroblasts thereby driving fibrogenesis. In terms of mechanism, during the process of fibrosis, the activations of the TGF-β signaling pathway, oxidative stress, cellular senescence, and inflammatory response play crucial roles in the activation and proliferation of fibroblasts to generate ECM. The deaths due to severe fibrosis account for almost half of the total deaths from various diseases, and few treatment strategies are available for the prevention of fibrosis as yet. Recently, numerous studies demonstrated that three well-defined bioactive gasotransmitters, including nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S), generally exhibited anti-inflammatory, antioxidative, antiapoptotic, and antiproliferative properties. Besides these effects, a number of studies have reported that low-dose exogenous and endogenous gasotransmitters can delay and interfere with the occurrence and development of fibrotic diseases, including myocardial fibrosis, idiopathic pulmonary fibrosis, liver fibrosis, renal fibrosis, diabetic diaphragm fibrosis, and peritoneal fibrosis. Furthermore, in animal and clinical experiments, the inhalation of low-dose exogenous gas and intraperitoneal injection of gaseous donors, such as SNAP, CINOD, CORM, SAC, and NaHS, showed a significant therapeutic effect on the inhibition of fibrosis through modulating the TGF-β signaling pathway, attenuating oxidative stress and inflammatory response, and delaying the cellular senescence, while promoting the process of autophagy. In this review, we first demonstrate and summarize the therapeutic effects of gasotransmitters on diverse fibrotic diseases and highlight their molecular mechanisms in the process and development of fibrosis.

Wild Bitter Melon Extract Regulates LPS-Induced Hepatic Stellate Cell Activation, Inflammation, Endoplasmic Reticulum Stress, and Ferroptosis

The activation of hepatic stellate cells (HSCs) is a key component of liver fibrosis. Two antifibrosis pathways have been identified, the reversion to quiescent-type HSCs and the clearance of HSCs through apoptosis. Lipopolysaccharide- (LPS-) induced HSCs activation and proliferation have been associated with the development of liver fibrosis. We determined the pharmacological effects of wild bitter melon (WM) on HSC activation following LPS treatment and investigated whether WM treatment affected cell death pathways under LPS-treated conditions, including ferroptosis. WM treatment caused cell death, both with and without LPS treatment. WM treatment caused reactive oxygen species (ROS) accumulation without LPS treatment and reversed the decrease in lipid ROS production in HSCs after LPS treatment. We examined the effects of WM treatment on fibrosis, endoplasmic reticulum (ER) stress, inflammation, and ferroptosis in LPS-activated HSCs. The western blotting analysis revealed that the WM treatment of LPS-activated HSCs induced the downregulation of the connective tissue growth factor (CTGF), α-smooth muscle actin (α-SMA), integrin-β1, phospho-JNK (p-JNK), glutathione peroxidase 4 (GPX4), and cystine/glutamate transporter (SLC7A11) and the upregulation of CCAAT enhancer-binding protein homologous protein (CHOP). These results support WM as an antifibrotic agent that may represent a potential therapeutic solution for the management of liver fibrosis.

Chrysophanol attenuates hepatitis B virus X protein-induced hepatic stellate cell fibrosis by regulating endoplasmic reticulum stress and ferroptosis

Hepatitis B virus X protein (HBx) and hepatic stellate cells (HSCs) are critical for liver fibrosis development. Anti-fibrosis occurs via reversion to quiescent-type HSCs or clearance of HSCs via apoptosis or ferroptosis. We aimed to elucidate the role of chrysophanol in rat HSC-T6 cells expressing HBx and investigate whether chrysophanol (isolated from Rheum palmatum rhizomes) influences cell death via ferroptosis in vitro. Analysis of lipid reactive oxygen species (ROS), Bip, CHOP, p-IRE1α, GPX4, SLC7A11, α-SMA, and CTGF showed that chrysophanol attenuated HBx-repressed cell death. Chrysophanol can impair HBx-induced activation of HSCs via endoplasmic reticulum stress (ER stress) and ferroptosis-dependent and GPX4-independent pathways.

Implications of hydrogen sulfide in liver pathophysiology: Mechanistic insights and therapeutic potential

Background

Over the last several decades, hydrogen sulfide (H₂S) has been found to exert multiple physiological functions in mammal systems. The endogenous production of H₂S is primarily mediated by cystathione β-synthase (CBS), cystathione γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST). These enzymes are widely expressed in the liver tissues and regulate hepatic functions by acting on various molecular targets.

Aim of Review

In the present review, we will highlight the recent advancements in the cellular events triggered by H₂S under liver diseases. The therapeutic effects of H₂S donors on hepatic diseases will also be discussed.

Key Scientific Concepts of Review

As a critical regulator of liver functions, H₂S is critically involved in the etiology of various liver disorders, such as nonalcoholic steatohepatitis (NASH), hepatic fibrosis, hepatic ischemia/reperfusion (IR) injury, and liver cancer. Targeting H₂S-producing enzymes may be a promising strategy for managing hepatic disorders.

Hydrogen Sulfide Protects against Paraquat-Induced Acute Liver Injury in Rats by Regulating Oxidative Stress, Mitochondrial Function, and Inflammation

In addition to the lung, the liver is considered another major target for paraquat (PQ) poisoning. Hydrogen sulfide (H₂S) has been demonstrated to be effective in the inhibition of oxidative stress and inflammation. The aim of this study was to investigate the protective effect of exogenous H₂S against PQ-induced acute liver injury. The acute liver injury model was established by a single intraperitoneal injection of PQ, evidenced by histological alteration and elevated serum aminotransferase levels. Different doses of NaHS were administered intraperitoneally one hour before exposure to PQ. Analysis of the data shows that exogenous H₂S attenuated the PQ-induced liver injury and oxidative stress in a dose-dependent manner. H₂S significantly suppressed reactive oxygen species (ROS) generation and the elevation of malondialdehyde content while it increased the ratio of GSH/GSSG and levels of antioxidant enzymes including SOD, GSH-Px, HO-1, and NQO-1. When hepatocytes were subjected to PQ-induced oxidative stress, H₂S markedly enhanced nuclear translocation of Nrf2 via S-sulfhydration of Keap1 and resulted in the increase in IDH2 activity by regulating S-sulfhydration of SIRT3. In addition, H₂S significantly suppressed NLRP3 inflammasome activation and subsequent IL-1β excretion in PQ-induced acute liver injury. Moreover, H₂S cannot reverse the decrease in SIRT3 and activation of the NLRP3 inflammasome caused by PQ in Nrf2-knockdown hepatocytes. In summary, H₂S attenuated the PQ-induced acute liver injury by enhancing antioxidative capability, regulating mitochondrial function, and suppressing ROS-induced NLRP3 inflammasome activation. The antioxidative effect of H₂S in PQ-induced liver injury can at least partly be attributed to the promotion of Nrf2-driven antioxidant enzymes via Keap1 S-sulfhydration and regulation of SIRT3/IDH2 signaling via Nrf2-dependent SIRT3 gene transcription as well as SIRT3 S-sulfhydration. Thus, H₂S supplementation can form the basis for a promising novel therapeutic strategy for PQ-induced acute liver injury.

Effects of Lespedeza Bicolor Extract on Regulation of AMPK Associated Hepatic Lipid Metabolism in Type 2 Diabetic Mice

Lespedeza bicolor (LB) is one of the ornamental plants used for the treatment of inflammation caused by oxidative damage. However, its beneficial effects on hyperglycemia-induced hepatic damage and the related molecular mechanisms remain unclear. We hypothesized that Lespedeza bicolor extract (LBE) would attenuate hyperglycemia-induced liver injury in type 2 diabetes mellitus (T2DM). Diabetes was induced by a low dosage of streptozotocin (STZ) injection (30 mg/kg) with a high fat diet in male C57BL/6J mice. LBE was administered orally at 100 mg/kg or 250 mg/kg for 12 weeks. LBE supplementation regardless of dosage ameliorated plasma levels of hemoglobin A1c (HbA1c) in diabetic mice. Moreover, both LBE supplementations upregulated AMP-activation kinase (AMPK), which may activate sirtuin1 (SIRT) associated pathway accompanied by decreased lipid synthesis at low dose of LBE supplementation. These changes were in part explained by reduced protein levels of oxidative stress (nuclear factor erythroid 2-related factor 2 (Nrf2) and catalase), inflammation (nuclear factor kappa B (NF-κB), interleukin-1β (IL-1β), interleukin-6 (IL-6), and nitric oxide synthases (iNOS)), and fibrosis (α-smooth muscle actin (α-SMA) and protein kinase C (PKC)) in diabetic liver. Taken together, LBE might be a potential nutraceutical to ameliorate hepatic damage by regulation of AMPK associated pathway via oxidative stress, inflammation, and fibrosis in T2DM.

Apoptotic effects of hsian-tsao (Mesona procumbens Hemsley) on hepatic stellate cells mediated by reactive oxygen species and ERK, JNK, and caspase-3 pathways

The activation of hepatic stellate cells (HSCs) is an important step in the progress of liver fibrosis. Fibrosis can be impeded by HSC reversion to a quiescent state or HSC clearance through apoptosis. To investigate the apoptotic effects of hsian-tsao (Mesona procumbens Hemsl) on human HSCs, the expression levels of cleaved caspase-3, p38, and c-Jun N-terminal kinase (JNK) were assessed using Western blotting, and the caspase-3 activity was measured using caspase-3/CPP32 colorimetric assay kit. Hsian-tsao extract (HTE) increased the activity of caspase-3 and the level of activated caspase-3, indicating the activation of apoptosis. The intracellular reactive oxygen species (ROS) level increased in a dose-dependent manner. This increase was prevented by an antioxidant, suggesting that HTE induces ROS accumulation. In addition, we found that HTE induced the phosphorylation of the mitogen-activated protein kinases JNK and p38. These collective data indicate that HTE induces apoptosis via ROS production through the p38, JNK, and caspase-3-dependent pathways. HTE may decrease HSC activation in liver fibrosis and may have a therapeutic potential.

Hepatic and Intrahepatic Targeting of Hydrogen Sulfide Prodrug by Bioconjugation

Hydrogen sulfide (H₂S) is an endogenous gaseous transmitter known to play an important role in biological functions. For the hepatic and intrahepatic targeting of H₂S prodrug at the cellular level, we developed two types of sulfo-albumins, in which five sulfide groups (source of H₂S) were covalently bound to succinylated (Suc) or galactosylated (Gal) bovine serum albumin (BSA). Sulfo-BSA-Suc and polyethylene glycol (PEG)-Sulfo-BSA-Gal, both released H₂S in the 5 mM glutathione solution, but not in the plasma. Sulfo-BSA-Suc and PEG-Sulfo-BSA-Gal were taken up by RAW264.7 cells (mouse macrophage-like cells) and Hep G2 cells (human hepatocellular carcinoma cells), respectively, and H₂S was released. These results indicate that Sulfo-BSA-Suc and PEG -Sulfo-BSA-Gal selectively released H₂S intracellularly. In a biodistribution study, up to 80% of ¹¹¹In-labeled Sulfo-BSA-Suc and PEG-Sulfo-BSA-Gal rapidly accumulated in the liver, 30 min after intravenous injection in mice. Furthermore, ¹¹¹In-labeled Sulfo-BSA-Suc and PEG-Sulfo-BSA-Gal predominantly accumulated in liver nonparenchymal (endothelial cells and Kupffer cells) and parenchymal cells (hepatocytes), respectively. These findings suggest that targeted delivery of H₂S prodrug to a specific type of liver cells was successfully achieved by bioconjugation.

Hydrogen Sulfide as a Novel Regulatory Factor in Liver Health and Disease

Hydrogen sulfide (H₂S), a colorless gas smelling of rotten egg, has long been recognized as a toxic gas and environment pollutant. However, increasing evidence suggests that H₂S acts as a novel gasotransmitter and plays important roles in a variety of physiological and pathological processes in mammals. H₂S is involved in many hepatic functions, including the regulation of oxidative stress, glucose and lipid metabolism, vasculature, mitochondrial function, differentiation, and circadian rhythm. In addition, H₂S contributes to the pathogenesis and treatment of a number of liver diseases, such as hepatic fibrosis, liver cirrhosis, liver cancer, hepatic ischemia/reperfusion injury, nonalcoholic fatty liver disease/nonalcoholic steatohepatitis, hepatotoxicity, and acute liver failure. In this review, the biosynthesis and metabolism of H₂S in the liver are summarized and the role and mechanism of H₂S in liver health and disease are further discussed.

Hydrogen sulfide alleviates uranium-induced rat hepatocyte cytotoxicity via inhibiting Nox4/ROS/p38 MAPK pathway

As a gasotransmitter, hydrogen sulfide (H₂ S) plays a crucial role in regulating the signaling pathway mediated by oxidative stress. The purpose of this study was to investigate the protective effects of H ₂ S on uranium-induced rat hepatocyte cytotoxicity. Primary hepatocytes were isolated and cultured from Sprague Dawley rat liver tissues. After pretreating with sodium hydrosulfide (an H ₂ S donor) for 1 hour (or GKT-136901 for 30 minutes), hepatocytes were treated by uranyl acetate for 24 hours. Cell viability, reactive oxygen species (ROS), malondialdehyde (MDA), NADPH oxidase 4 (Nox4), and p38 mitogen-activated protein kinase (p38 MAPK) phosphorylation were respectively determined. The effects of direct inhibition of Nox4 expression by GKT-136901 (a Nox4 inhibitor) on ROS and phospho-p38 MAPK levels were examined in uranium-treated hepatocytes. The results implicate that H ₂ S can afford protection of rat hepatocytes against uranium-induced adverse effects through attenuating oxidative stress via prohibiting Nox4/ROS/p38 MAPK signaling.

The smell of renal protection against chronic kidney disease: Hydrogen sulfide offers a potential stinky remedy

Chronic kidney disease (CKD) is a common global health challenge characterized by irreversible pathological processes that reduce kidney function and culminates in development of end-stage renal disease. It is associated with increased morbidity and mortality in addition to increased caregiver burden and higher financial cost. A central player in CKD pathogenesis and progression is renal hypoxia. Renal hypoxia stimulates induction of oxidative and endoplasmic reticulum stress, inflammation and tubulointerstitial fibrosis, which in turn, promote cellular susceptibility and further aggravate hypoxia, thus forming a pathological vicious cycle in CKD progression. Although the importance of CKD is widely appreciated, including improvements in the quality of existing therapies such as dialysis and transplantation, new therapeutic options are limited, as there is still increased morbidity, mortality and poor quality of life among CKD patients. Growing evidence indicates that hydrogen sulfide (H₂S), a small gaseous signaling molecule with an obnoxious smell, accumulates in the renal medulla under hypoxic conditions, and functions as an oxygen sensor that restores oxygen balance and increases medullary flow. Moreover, plasma H₂S level has been recently reported to be markedly reduced in CKD patients and animal models. Also, H₂S has been established to possess potent antioxidant, anti-inflammatory, and anti-fibrotic properties in several experimental models of kidney diseases, suggesting that its supplementation could protect against CKD and retard its progression. The purpose of this review is to discuss current clinical and experimental developments regarding CKD, its pathophysiology, and potential cellular and molecular mechanisms of protection by H₂S in experimental models of CKD.

Cystathionine γ-Lyase Deficiency Exacerbates CCl4-Induced Acute Hepatitis and Fibrosis in the Mouse Liver

AIMS

The present study examined the role of cystathionine γ-lyase (CSE) in carbon tetrachloride (CCl₄)-induced liver damage.

RESULTS

A CSE gene knock-out and luciferase gene knock-in (KI) mouse model was constructed to study the function of CSE and to trace its expression in living status. CCl₄ or lipopolysaccharide markedly downregulated CSE expression in the liver of mice. CSE-deficient mice showed increased serum alanine aminotransferase and aspartate aminotransferase levels, and liver damage after CCl₄ challenge, whereas albumin and endogenous hydrogen sulfide (H₂S) levels decreased significantly. CSE knockout mice showed increased serum homocysteine levels, upregulation of inflammatory cytokines, and increased autophagy and IκB-α degradation in the liver in response to CCl₄ treatment. The increase in pro-inflammatory cytokines, including tumor necrosis factor-alpha in CSE-deficient mice after CCl₄ challenge, was accompanied by a significant increase in liver tissue hydroxyproline and α-smooth muscle actin and histopathologic changes in the liver. However, H₂S donor pretreatment effectively attenuated most of these imbalances.

INNOVATION

Here, a CSE knock-out and luciferase KI mouse model was established for the first time to study the transcriptional regulation of CSE expression in real time in a non-invasive manner, providing information on the effects and potential mechanisms of CSE on CCl₄-induced liver injury.

CONCLUSION

CSE deficiency increases pro-inflammatory cytokines in the liver and exacerbates acute hepatitis and liver fibrosis by reducing H₂S production from L-cysteine in the liver. The present data suggest the potential of an H₂S donor for the treatment of liver diseases such as toxic hepatitis and fibrosis. Antioxid. Redox Signal. 27, 133-149.

Diallyl Trisulfide Suppresses Oxidative Stress-Induced Activation of Hepatic Stellate Cells through Production of Hydrogen Sulfide

Accumulating data reveal that garlic has beneficial effects against chronic liver disease. We previously reported that diallyl trisulfide (DATS), the primary organosulfur compound in garlic, reduced fibrosis and attenuated oxidative stress in rat fibrotic liver. The present study was aimed at elucidating the underlying mechanisms. The primary rat hepatic stellate cells (HSCs) were cultured and stimulated with hydrogen peroxide (H₂O₂) for inducing HSC activation under oxidative stress. We examined the effects of DATS on the profibrogenic properties and oxidative stress in H₂O₂-treated HSCs. The results showed that DATS suppressed and reduced fibrotic marker expression in HSCs. DATS arrested cell cycle at G2/M checkpoint associated with downregulating cyclin B1 and cyclin-dependent kinase 1, induced caspase-dependent apoptosis, and reduced migration in HSCs. Moreover, intracellular levels of reactive oxygen species and lipid peroxide were decreased by DATS, but intracellular levels of glutathione were increased in HSCs. Furthermore, DATS significantly elevated hydrogen sulfide (H₂S) levels within HSCs, but iodoacetamide (IAM) reduced H₂S levels and significantly abrogated DATS production of H₂S within HSCs. IAM also abolished all the inhibitory effects of DATS on the profibrogenic properties and oxidative stress in HSCs. Altogether, we demonstrated an H₂S-associated mechanism underlying DATS inhibition of profibrogenic properties and alleviation of oxidative stress in HSCs. Modulation of H₂S production may represent a therapeutic remedy for liver fibrosis.

Hydrogen sulfide alleviates uranium-induced acute hepatotoxicity in rats: Role of antioxidant and antiapoptotic signaling

As an endogenous gaseous mediator, H₂ S exerts antioxidative, antiapoptotic, and cytoprotective effects in livers. This study was designed to investigate the protective role of H₂ S against uranium-induced hepatotoxicity in adult SD male rats after in vivo effect of uranium on endogenous H₂ S production was determined in livers. The levels of endogenous H₂ S and H₂ S-producing enzymes (CBS and CSE) were measured in liver homogenates from uranium -intoxicated rats. In rats injected intraperitoneally (i.p.) with uranyl acetate or NaHS (an H₂ S donor) alone or in combination, we examined biochemical parameters to assess liver function, revealed hepatic histopathological alteration, investigated oxidative stress markers, and explored apoptotic signaling in liver homogenates. The results suggest that uranium-intoxication in rats decreased CBS and CSE protein expression, H₂ S synthesis capacity, and endogenous H₂ S generation. NaHS administration in uranium-intoxicated rats produced amelioration in liver biochemical indices and histopathological effects, decreased MDA content, and increased GSH level and antioxidative enzymes activities like SOD, CAT, GPx, and GST. NaHS administration in uranium-intoxicated rats attenuated uranium-activated phosphorylation state of JNK. NaHS treatment in uranium-intoxicated rats increased antiapoptotic Bcl-2 but decreased pro-apoptotic Bax, resulting in the rise of Bcl-2/Bax ratio. NaHS treatment in uranium-intoxicated rats reduced the apoptosis mediator caspase-3 and cytochrome c release and elevated ATP contents. Taken together, these data implicate that H₂ S can afford protection to rat livers against uranium-induced adverse effects mediated by up-regulation of antioxidant and antiapoptotic signaling. The anti-apoptotic property of H₂ S may be involved, at least in part, in inhibiting JNK signaling. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 581-593, 2017.

The CBS/CSE system: a potential therapeutic target in NAFLD?

Non-alcoholic fatty liver disease (NAFLD) is a broad spectrum liver disorder diagnosed in patients without a history of alcohol abuse. NAFLD is growing at alarming rates worldwide. Its pathogenesis is complex and incompletely understood. The cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE) system regulates homocysteine and cysteine metabolism and contributes to endogenous hydrogen sulfide (H2S) biosynthesis. This review summarizes our current understanding of the hepatic CBS/CSE system, and for the first time, positions this system as a potential therapeutic target in NAFLD. As will be discussed, the CBS/CSE system is highly expressed and active in the liver. Its dysregulation, presenting as alterations in circulating homocysteine and (or) H2S levels, has been reported in NAFLD patients and in NAFLD-associated co-morbidities such as obesity and type 2 diabetes. Intricate links between the CBS/CSE system and a number of metabolic and stress related molecular mediators have also emerged. Various dysfunctions in the hepatic CBS/CSE system have been reported in animal models representative of each NAFLD spectrum. It is anticipated that a newfound appreciation for the hepatic CBS/CSE system will emerge that will improve our understanding of NAFLD pathogenesis, and give rise to new prospective targets for management of this disorder.

Endogenous carbon monoxide downregulates hepatic cystathionine-γ-lyase in rats with liver cirrhosis

The aim of the present study was to investigate the effect of endogenous carbon monoxide (CO) on the hydrogen sulfide/cystathionine-γ-lyase (H₂S/CSE) pathway in cirrhotic rat livers. The rats were allocated at random into four groups: Sham, cirrhosis, cobalt protoporphyrin (CoPP) and zinc protoporphyrin IX (ZnPP). The expression of hepatic CSE mRNA was evaluated using a quantitative polymerase chain reaction, while CSE protein expression was determined using immunohistochemical analysis. Hematoxylin and eosin staining was performed for the histological evaluation of liver fibrosis. The levels of H₂S, alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin (TBIL) and carboxyhemoglobin (COHb) in the arterial blood were determined, in addition to the portal vein pressure. The mRNA and protein expression levels of hepatic CSE and the serum levels of H₂S were significantly decreased in the cirrhosis group compared with those in the sham group (P<0.05). Compared with the cirrhosis group, rats in the ZnPP group had significantly lower levels of serum ALT, AST and TBIL, arterial COHb and hepatic fibrosis, while hepatic CSE expression and the production of H₂S were significantly increased (P<0.05). The CoPP group exhibited decreased hepatic CSE expression and H₂S production, but aggravated hepatic function and fibrosis (P<0.05). In conclusion, the H₂S/CSE pathway is involved in the formation of liver cirrhosis and serves a crucial function in protecting liver cells against the progression of liver fibrosis. Endogenous CO downregulates hepatic CSE mRNA and protein expression and the production of H₂S in rats with liver cirrhosis.

Endogenous hydrogen sulfide is associated with angiotensin II type 1 receptor in a rat model of carbon tetrachloride-induced hepatic fibrosis

The present study aimed to investigate the effects of endogenous hydrogen sulfide (H₂S) on the expression levels of angiotensin II type 1 receptor (AGTR1) in a rat model of carbon tetrachloride (CCl₄)-induced hepatic fibrosis. A total of 56 Wistar rats were randomly divided into four groups: Normal control group, model group, sodium hydrosulfide (NaHS) group, and DL-propargylglycine (PAG) group. Hepatic fibrosis was induced by CCl₄. The rats in the PAG group were intraperitoneally injected with PAG, an inhibitor of cystathionine-γ-lyase (CSE). The rats in the NaHS group were intraperitoneally injected with NaHS. An equal volume of saline solution was intraperitoneally injected into both the control and model groups. All rats were sacrificed at week three or four following treatment. The serum levels of hyaluronidase (HA), laminin protein (LN), procollagen III (PcIII), and collagen IV (cIV) were detected using ELISA. The serum levels of alanine transaminase (ALT), aspartate transaminase (AST), and albumin (ALB) were detected using an automatic biochemical analyzer. The liver mRNA expression levels of CSE were detected by reverse transcription-quantitative polymerase chain reaction. The liver expression levels of AGTR1 and the plasma expression levels of H₂S were detected using western blot analyses. The results indicated that the severity of hepatic fibrosis, the serum expression levels of HA, LN, PcIII, cIV, ALT, and AST, the liver expression levels of CSE and AGTR1, and the plasma expression levels of H₂S were significantly higher in the PAG group, as compared with the model group (P<0.05). Conversely, the expression levels of ALB were significantly lower in the PAG group, as compared with the model group. In addition, the severity of hepatic fibrosis, the serum expression levels of HA, LN, PcIII, cIV, ALT, and AST, the liver expression levels of CSE and AGTR1, and the plasma expression levels of H₂S were significantly lower in the NaHS group, as compared with the model group (P<0.05). These results suggest that endogenous H₂S is associated with CCl₄-induced hepatic fibrosis in rats, and may exhibit anti-fibrotic effects. Furthermore, H₂S reduced the liver expression levels of AGTR1, which may be associated with the delayed progression of hepatic fibrosis.

Hydrogen Sulfide as a Potential Therapeutic Target in Fibrosis

Hydrogen sulfide (H₂S), produced endogenously by the activation of two major H₂S-generating enzymes (cystathionine β-synthase and cystathionine γ-lyase), plays important regulatory roles in different physiologic and pathologic conditions. The abnormal metabolism of H₂S is associated with fibrosis pathogenesis, causing damage in structure and function of different organs. A number of in vivo and in vitro studies have shown that both endogenous H₂S level and the expressions of H₂S-generating enzymes in plasma and tissues are significantly downregulated during fibrosis. Supplement with exogenous H₂S mitigates the severity of fibrosis in various experimental animal models. The protective role of H₂S in the development of fibrosis is primarily attributed to its antioxidation, antiapoptosis, anti-inflammation, proangiogenesis, and inhibition of fibroblasts activities. Future studies might focus on the potential to intervene fibrosis by targeting the pathway of endogenous H₂S-producing enzymes and H₂S itself.

Sodium thiosulfate attenuates angiotensin II-induced hypertension, proteinuria and renal damage

Hypertension and proteinuria are important mediators of renal damage. Despite therapeutic interventions, the number of patients with end stage renal disease steadily increases. Hydrogen sulfide (H(2)S) is an endogenously produced gasotransmitter with vasodilatory, anti-inflammatory and antioxidant properties. These beneficial characteristics make H(2)S an attractive candidate for pharmacological use in hypertensive renal disease. We investigated the protective properties of H(2)S in angiotensin II (Ang II)-induced hypertensive renal disease in rats. Treatment with the H(2)S donor NaHS and major H(2)S metabolite sodium thiosulfate (STS) during three weeks of Ang II infusion reduced hypertension, proteinuria, oxidative stress and renal functional and structural deterioration. In an ex vivo isolated perfused kidney setup, NaHS, but not STS, reduced intrarenal pressure. The effect of NaHS could partially be explained by its activation of the ATP-sensitive potassium channels. In conclusion, treatment with H(2)S attenuates Ang II-associated functional and structural renal deterioration, suggesting that intervention in H(2)S production pathways has potential therapeutic benefit and might be a valuable addition to the already existing antihypertensive and renoprotective therapies.

Effect of sulfide on the cytotoxicity of arsenite and arsenate in human hepatocytes (HepG2) and human urothelial cells (UROtsa)

Arsenic, a common poison, is known to react with sulfide in vivo, forming thioarsenates. The acute toxicity of the inorganic thioarsenates is currently unknown. Our experiments showed that a fourfold sulfide excess reduced acute arsenite cytotoxicity in human hepatocytes (HepG2) and urothelial cells (UROtsa) significantly, but had little effect on arsenate toxicity. Speciation analysis showed immediate formation of thioarsenates (up to 73 % of total arsenic) in case of arsenite, but no speciation changes for arsenate. Testing acute toxicity of mono- and trithioarsenate individually, both thioarsenates were found to be more toxic than their structural analogue arsenate, but less toxic than arsenite. Toxicity increased with the number of thio groups. The amount of cellular arsenic uptake after 24 h corresponded to the order of toxicity of the four compounds tested. The dominant to almost exclusive intracellular arsenic species was arsenite. The results imply that thiolation is a detoxification process for arsenite in sulfidic milieus. The mechanism could either be that thioarsenates regulate the amount of free arsenite available for cellular uptake without entering the cells themselves, or, based on their chemical similarity to arsenate, they could be taken up by similar transporters and reduced rapidly intracellularly to arsenite.

Hydrogen sulfide protects against bleomycin-induced pulmonary fibrosis in rats by inhibiting NF-κB expression and regulating Th1/Th2 balance

Hydrogen sulfide (H2S) displays vasodilative, anti-oxidative, anti-inflammatory and cytoprotective activities. The objective of this study was to evaluate the inhibitory effect of H2S on bleomycin (BLM)-induced pulmonary fibrosis in rats and its possible mechanisms. Fifty-four pathogen-free Male Wistar rats were randomly divided into three groups: control, BLM and H2S treated groups with 18 rats in each group. Each group was then divided into three subgroups based on time of study (7, 14 and 28 day). Pulmonary fibrosis model was established by a single intratracheal instillation of BLM A5 (5 mg/kg). While control rats received saline, rats of the treated group simultaneously were administered intraperitoneal injections of NaHS (the H2S donor, 28 μmol/kg) once daily. BLM induced pulmonary inflammation and fibrosis, increased lung hydroxyproline levels, lung index, total cell counts, neutrophils and eosinophils counts and expression of NF-κB p65 in lung tissue, decreased lymphocytes and macrophages counts. In addition, Th1 response is suppressed as shown by diminished IFN-γ in bronchoalveolar lavage fluid (BALF) after BLM exposure, and enhancement of Th2 response is marked by increased IL-4 in BALF. H2S administration significantly attenuated these effects. The findings reveal the therapeutic potential of H2S for BLM-induced pulmonary fibrosis in male rats, which were at least partly due to inhibition NF-κB p65 expression and regulation of Th1/Th2 balance.

Sodium/calcium exchanger is upregulated by sulfide signaling, forms complex with the β1 and β3 but not β2 adrenergic receptors, and induces apoptosis

Hydrogen sulfide (H2S) as a novel gasotransmitter regulates variety of processes, including calcium transport systems. Sodium calcium exchanger (NCX) is one of the key players in a regulation calcium homeostasis. Thus, the aims of our work were to determine effect of sulfide signaling on the NCX type 1 (NCX1) expression and function in HeLa cells, to investigate the relationship of β-adrenergic receptors with the NCX1 in the presence and/or absence of H2S, and to determine physiological importance of this potential communication. As a H2S donor, we used morpholin-4-ium-4-methoxyphenyl(morpholino) phosphinodithioate-GYY4137. We observed increased levels of the NCX1 mRNA, protein, and activity after 24 h of GYY4137 treatment. This increase was accompanied by elevated cAMP due to the GYY4137 treatment, which was completely abolished, when NCX1 was silenced. Increased cAMP levels would point to upregulation of β-adrenergic receptors. Indeed, GYY4137 increased expression of β1 and β3 (but not β2) adrenergic receptors. These receptors co-precipitated, co-localized with the NCX1, and induced apoptosis in the presence of H2S. Our results suggest that sulfide signaling plays a role in regulation of the NCX1, β1 and β3 adrenergic receptors, their co-localization, and stimulation of apoptosis, which might be of a potential importance in cancer treatment.