Signal pathways involved in the biological effects of sulfur dioxide

Color-reversible fluorescence tracking for the dynamic interaction of SO2 with Hg2+ in living cells

Mercury ion (Hg2+) is one of the most threatening substances to human health, and the mercury poisoning can damage physiological homeostasis severely in human, even cause death. Intriguingly, Sulfur dioxide (SO2), a gas signal molecule in human, can specifically interact with Hg2+ for relieving mercury poisoning. However, the dynamic interaction of Hg2+ with SO2 at the tempospatial level and the correlation between Hg2+ and SO2 in the pathological process of mercury poisoning are still elusive. Herein, we rationally designed a reversible and dual color fluorescent probe (CCS) for dynamically visualizing Hg2+ and SO2 and deciphering their interrelationship in mercury poisoning. CCS held good sensitivity, selectivity and reversibility to Hg2+ and SO2, that enabled CCS to specifically detect SO2 and Hg2+ via cyan fluorescence channel (centered around 485 nm) and red fluorescence channel (centered around 679 nm), respectively. Notably, the separate fluorescence signal changes of CCS realized the dynamic tracing of Hg2+ and SO2 in living cells, and presented the potential for exploring the correlation between SO2 and Hg2+ in mercury poisoning.

Sulfur dioxide inhibits mast cell degranulation by sulphenylation of galectin-9 at cysteine 74

Objectives

Mast cell (MC) degranulation is a key process in allergic reactions and inflammatory responses. Aspartate aminotransferase 1 (AAT1)-derived endogenous sulfur dioxide (SO2) is an important regulator of MC function. However, the mechanism underlying its role in MC degranulation remains unclear. This study aimed to investigate the mechanism by which endogenous SO2 controlled MC degranulation.

Methods

HMC-1 and Rat basophilic leukemia cell MC line (RBL-2H3) were used in the cell experiments. SO2 content was detected by in situ fluorescent probe. MC degranulation represented by the release rate of MC β-hexosaminidase was determined using a colorimetric assay. Sulfenylation of galectin-9 (Gal-9) in MCs and purified protein was detected using a biotin switch assay. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to determine the exact sulfenylation sites of Gal-9 by SO2. Animal models of passive cutaneous anaphylaxis (PCA) and hypoxia-driven pulmonary vascular remodeling were used to investigate the effect of SO2 on mast cell activation in vivo. Site-directed mutation of Gal-9 was conducted to confirm the exact site of SO2 and support the significance of SO2/Gal-9 signal axis in the regulation of MC degranulation.

Results

Degranulation was increased in AAT1-knockdowned MCs, and SO2 supplementation reversed the increase in MC degranulation. Furthermore, deficiency of endogenous SO2 contributed to IgE-mediated degranulation in vitro. Besides, SO2 inhibited IgE-mediated and hypoxia-driven MC degranulation in vivo. Mechanistically, LC-MS/MS analysis and site-directed mutation results showed that SO2 sulfenylated Gal-9 at cysteine 74. Sulfenylation of the 74th cysteine of Gal-9 protein was required in the SO2-inhibited MC degranulation under both physiological and pathophysiological conditions.

Conclusion

These findings elucidated that SO2 inhibited MC degranulation via sulfenylating Gal-9 under both physiological and pathophysiological conditions, which might provide a novel treatment approach for MC activation-related diseases.

ONOO- Activatable Fluorescent Sulfur Dioxide Donor for a More Accurate Assessment of Cell Ferroptosis

Ferroptosis is critical in the treatment of tumor therapies. Thus, monitoring reactive oxygen species (ROS) is of great significance for accurate assessment in ferroptosis without any interference. However, current probes for monitoring ROS during ferroptosis suffer from a drawback in that the probes consume ROS during detection, which inhibits the ferroptosis process and thus affects the accuracy and effectiveness of monitoring the process of ferroptosis. Herein, a new fluorescent donor probe, TFMU-SO2D, with the combination of the moiety of the SO2 donor is designed and synthesized by introducing the aryl boronate moieties that could give it the ability to effectively recognize ONOO-. The released SO2 could consume excess glutathione and regulate oxidative stress by elevating ROS levels, which would offset the ROS depletion by TFMU-SO2D and ensure accuracy in monitoring the ferroptosis process. The experimental results demonstrated that TFMU-SO2D possessed satisfactory performance for monitoring ONOO- as well as simultaneously releasing SO2 in oxidative stress stimulated by monensin and ferroptosis stimulated by erastin and RSL3. Additionally, the capability of SO2 synergized with ferroptosis to inhibit the viability of cancer cells was demonstrated by the CCK8 assay, which may be due to the fact that SO2 can potentiate ferroptosis cell death by increasing the ROS level. Overall, these combined results indicated that TFMU-SO2D possesses the excellent ability to precisely monitor ONOO- during ferroptosis without interference, which is significant for accurately accessing ferroptosis, cancer treatment, and drug development.

Small molecule probes as versatile energy acceptors: A breakthrough in photoelectrochemical sensing for sulfur dioxide recording in rat brain

Microelectrode-based photoelectrochemical (PEC) sensing is a newly developed and promising analytical technique for in vivo analysis. However, the inadequate specificity in complex environment of living bodies restricted its further in vivo application. Herein, we utilized a small molecule probe as the energy acceptor to quench the photocurrent of CdTe quantum dots through energy transfer. The efficiency of energy transfer was modulated by the concentration of target SO2, resulting in changes in photocurrent. The chemical recognition reaction between small molecule probes and SO2 enhanced the specificity of PEC sensing, thus guaranteeing its in vivo applications. Furthermore, with the use of light addressing strategy, simultaneous detection in the multiple brain regions was implemented. The energy transfer based light addressable PEC microsensor achieved monitoring fluctuations of SO2 levels in multiple brain regions of rats with epilepsy.

A novel fluorescent probe based on triphenylamine for detecting sulfur dioxide derivatives

According to the nucleophilicity of sulfur dioxide derivatives, a reactive fluorescent probe was designed and synthesized by linking triphenylamine with benzoindole.

Compensatory role of endogenous sulfur dioxide in nitric oxide deficiency-induced hypertension

OBJECTIVE

This study aimed to determine the communicational pattern of gaseous signaling molecules sulfur dioxide (SO2) and nitric oxide (NO) between vascular endothelial cells (VECs) and vascular smooth muscle cells (VSMCs), and elucidate the compensatory role and significance of endogenous SO2 in the development of hypertension due to NO deficiency.

APPROACH AND RESULTS

Blood pressure was monitored by the tail-cuff and implantable physiological signal telemetry in L-nitro-arginine methyl ester (l-NAME)-induced hypertensive mice, and structural alterations of mouse aortic vessels were detected by the elastic fiber staining method. l-NAME-treated mice showed decreased plasma NO levels, increased SO2 levels, vascular remodeling, and increased blood pressure, and application of l-aspartate-β-hydroxamate, which inhibits SO2 production, further aggravated vascular structural remodeling and increased blood pressure. Moreover, in a co-culture system of HAECs and HASMCs, NO from HAECs did not influence aspartate aminotransferase (AAT)1 protein expression but decreased AAT1 activity in HASMCs, thereby resulting in the inhibition of endogenous SO2 production. Furthermore, NO promoted S-nitrosylation of AAT1 protein in HASMCs and purified AAT1 protein. Liquid chromatography with tandem mass spectrometry showed that the Cys192 site of AAT1 purified protein was modified by S-nitrosylation. In contrast, dithiothreitol or C192S mutations in HASMCs blocked NO-induced AAT1 S-nitrosylation and restored AAT1 enzyme activity.

CONCLUSION

Endothelium-derived NO inhibits AAT activity by nitrosylating AAT1 at the Cys192 site and reduces SO2 production in HASMCs. Our findings suggest that SO2 acts as a compensatory defense system to antagonize vascular structural remodeling and hypertension when the endogenous NO pathway is disturbed.

Sulfur dioxide promotes seed germination by modulating reactive oxygen species production in maize

Sulfur dioxide (SO₂) is generally considered to be toxic to cells, but recent studies have shown that SO₂ has positive roles in stress defense responses in plants. However, whether SO₂ functions as a signaling molecule in the developmental process, especially in seed germination, is yet to be studied. Here, we present data supporting the role of SO₂ in seed germination and possible molecular mechanisms. SO₂ treatment significantly promoted the seed germination and seed vigor in maize. The germinating seeds treated with SO₂ treatment exhibited higher reactive oxygen species (ROS) levels and NADPH oxidase activities. Furthermore, the specific NADPH oxidase inhibitor diphenyleneiodinium (DPI) strongly inhibited ROS accumulations, and SO₂-promoted seed germination and vigor. Meanwhile, α-Amylase activity and transcripts in germinating seeds treated with SO₂ were significantly elevated. These data have demonstrated that NADPH oxidase-dependent ROS production contributes to the induction of α-Amylase activity, thereby promoting seed germination upon SO₂ exposure. SO₂ might function as a signaling molecule in plant growth and development, especially in seed germination. This study might provide a theoretical foundation for the potential exploitation of hydrated SO₂ in seed germination control in crop management.

Sulfur Dioxide: An Endogenous Protector Against Myocardial Injury

Sulfur dioxide (SO2) was previously known as a harmful gas in air pollution. Recently, it was reported that SO2 can be endogenously generated in cardiovascular tissues. Many studies have revealed that endogenous SO2 has important physiological and pathophysiological significance and pharmacological potential. As a novel gasotransmitter, SO2 has important regulatory effects on the heart. It has a dose-dependent negative inotropic effect on cardiac function, in which L-type calcium channels are involved. SO2 can also attenuate myocardial injury caused by various harmful stimuli and play an important role in myocardial ischemia-reperfusion injury and myocardial hypertrophy. These effects are thought to be linked to its ability to reduce inflammation and as an antioxidant. In addition, SO2 regulates cardiomyocyte apoptosis and autophagy. Therefore, endogenous SO2 plays an important role in maintaining cardiovascular system homeostasis. In the present review, the literature concerning the metabolism of endogenous SO2, its cardiac toxicological effects and physiological regulatory effects, mechanisms for SO2-mediated myocardial protection and its pharmacological applications are summarized and discussed.

FRET Modulated Signaling: A Versatile Strategy to Construct Photoelectrochemical Microsensors for In Vivo Analysis

Microelectrode-based electrochemical (EC) and photoelectrochemical (PEC) sensors are promising candidates for in vivo analysis of biologically important chemicals. However, limited selectivity in complicated biological systems and poor adaptability to electrochemically non-active species restrained their applications. Herein, we propose the concept of modulating the PEC output by a fluorescence resonance energy transfer (FRET) process. The emission of energy donor was dependent on the concentration of target SO₂ , which in turn served as the modulator of the photocurrent signal of the photoactive material. The employment of optical modulation circumvented the problem of selectivity, and the as-fabricated PEC microelectrode showed good stability and reproducibility in vivo. It can monitor fluctuations of SO₂ levels in brains of rat models of cerebral ischemia-reperfusion and febrile seizure. More significantly, such a FRET modulated signaling strategy can be extended to diverse analytes.

Are Reactive Sulfur Species the New Reactive Oxygen Species?

Significance: Oxidative stress in moderation positively affects homeostasis through signaling, while in excess it is associated with adverse health outcomes. Both activities are generally attributed to reactive oxygen species (ROS); hydrogen peroxide as the signal, and cysteines on regulatory proteins as the target. However, using antioxidants to affect signaling or benefit health has not consistently translated into expected outcomes, or when it does, the mechanism is often unclear. Recent Advances: Reactive sulfur species (RSS) were integral in the origin of life and throughout much of evolution. Sophisticated metabolic pathways that evolved to regulate RSS were easily "tweaked" to deal with ROS due to the remarkable similarities between the two. However, unlike ROS, RSS are stored, recycled, and chemically more versatile. Despite these observations, the relevance and regulatory functions of RSS in extant organisms are generally underappreciated. Critical Issues: A number of factors bias observations in favor of ROS over RSS. Research conducted in room air is hyperoxic to cells, and promotes ROS production and RSS oxidation. Metabolic rates of rodent models greatly exceed those of humans; does this favor ROS? Analytical methods designed to detect ROS also respond to RSS. Do these disguise the contributions of RSS? Future Directions: Resolving the ROS/RSS issue is vital to understand biology in general and human health in particular. Improvements in experimental design and analytical methods are crucial. Perhaps the most important is an appreciation of all the attributes of RSS and keeping an open mind.

Sulfur dioxide derivatives produce antidepressant- and anxiolytic-like effects in mice

Sulfur dioxide (SO₂) can be endogenously generated from sulfur-containing amino acids in animals and humans. Increasing evidence shows that endogenous SO₂ may act as a gaseous molecule to participate in many physiological and pathological processes. However, the role of SO₂ and its derivatives in the central nervous system remains poorly understood. The present study explored the protective effects of exogenous SO₂ derivatives (Na₂SO₃:NaHSO₃, 3:1 M/M) on cellular injury in vitro by using the cell proliferation assay (MTS), cell counting kit 8 assay (CCK-8), and cyto-flow assay in the corticosterone (CORT)-induced PC12 cell injury model. We also examined the antidepressant and anxiolytic effects of SO₂ derivatives on the chronic mild stress (CMS)-induced depression mouse model by using the open field test, novelty suppressed feeding test, forced swimming test, tail suspension test, and sucrose preference test. In the MTS and CCK-8 assays, we found that preexposure of SO₂ derivatives significantly blocked CORT-induced decrease of cellular survival without causing any negative effects. Results from the cyto-flow assay indicated that treatment with SO₂ derivatives could reverse CORT-induced early and late apoptosis of PC12 cells. Systemic treatment with SO₂ derivatives produced markedly antidepressant- and anxiolytic-like activities in mice under normal condition and rapidly reversed CMS-induced depressive- and anxiety-like behaviors. In conclusion, these findings indicate that exogenous SO₂ derivatives show protective properties against the detrimental effects of stress and exert antidepressant- and anxiolytic-like actions. The present study suggests that exogenous SO₂ derivatives are potential therapeutic agents for the treatment of depression, anxiety, and other stress-related diseases.

ROS-associated immune response and metabolism: a mechanistic approach with implication of various diseases

The immune system plays a pivotal role in maintaining the defense mechanism against external agents and also internal danger signals. Metabolic programming of immune cells is required for functioning of different subsets of immune cells under different physiological conditions. The field of immunometabolism has gained ground because of its immense importance in coordination and balance of immune responses. Metabolism is very much related with production of energy and certain by-products. Reactive oxygen species (ROS) are generated as one of the by-products of various metabolic pathways. The amount, localization of ROS and redox status determine transcription of genes, and also influences the metabolism of immune cells. This review discusses ROS, metabolism of immune cells at different cellular conditions and sheds some light on how ROS might regulate immunometabolism.

An imidazo[1,5-α]pyridine-derivated fluorescence sensor for rapid and selective detection of sulfite

Sulfur-containing species are essential in the composition and the metabolism of the organisms, thus developing a full set of implements to cover all of them is still a favorable choice. Herein, we chose imidazo [1,5-α]pyridine moiety as the basic fluorophore for the detection of sulfite, and preliminarily completed the toolset since biothiols (GSH, Cys, Hcy), H₂S, and PhSH could be detected by sensors based on the same backbone. The designed sensor, IPD-SFT, with structural novelty and large Stokes shift (130 nm), indicated the most attractive advantages of remarkably rapid response period (within 1 min) and high selectivity for sulfite from all the sulfur-containing species. Other practical properties included high sensitivity (LOD = 50 nM) and wide pH adaptability (5.0-11.0). Furthermore, IPD-SFT could monitor both exogenous and endogenous sulfite. It not only raised a potential tool for sulfite detection, but also preliminarily completed the toolset for all the sulfur-containing species. The development of such toolsets might reveal the sulfur-containing metabolism and corresponding physiology and pathological procedures.

Hydrogen Sulfide Impairs Meiosis Resumption in Xenopus laevis Oocytes

The role of hydrogen sulfide (H₂S) is addressed in Xenopus laevis oocytes. Three enzymes involved in H₂S metabolism, cystathionine β-synthase, cystathionine γ-lyase, and 3-mercaptopyruvate sulfurtransferase, were detected in prophase I and metaphase II-arrested oocytes and drove an acceleration of oocyte meiosis resumption when inhibited. Moreover, meiosis resumption is associated with a significant decrease in endogenous H₂S. On another hand, a dose-dependent inhibition was obtained using the H₂S donor, NaHS (1 and 5 mM). NaHS impaired translation. NaHS did not induce the dissociation of the components of the M-phase promoting factor (MPF), cyclin B and Cdk1, nor directly impacted the MPF activity. However, the M-phase entry induced by microinjection of metaphase II MPF-containing cytoplasm was diminished, suggesting upstream components of the MPF auto-amplification loop were sensitive to H₂S. Superoxide dismutase and catalase hindered the effects of NaHS, and this sensitivity was partially dependent on the production of reactive oxygen species (ROS). In contrast to other species, no apoptosis was promoted. These results suggest a contribution of H₂S signaling in the timing of amphibian oocytes meiosis resumption.

Recent progress in the small-molecule fluorescent probes for the detection of sulfur dioxide derivatives (HSO3-/SO32-)

Sulfur dioxide (SO₂) had been recognized as an environmental pollutant produced from industrial processes. SO₂ is water soluble and forms hydrated SO₂ (SO₂·H₂O), bisulfite ion (HSO₃^-), and sulfite ion (SO₃^2-) upon dissolution in water. SO₂ could be also produced endogenously from sulfur-containing amino acids l-cysteine in mammals. Endogenous SO₂ can maintain the balance of biological sulfur and redox equilibrium in vivo, regulate blood insulin levels and reduce blood pressure. Excess intake of exogenous SO₂ can result in respiratory diseases, cardiovascular diseases and neurological disorders. As a result, fluorescent probes to detect HSO₃^-/SO₃^2- have attracted great attention in recent years. Herein, a general overview was provided with the aim to highlight the typical examples of the HSO₃^-/SO₃^2- fluorescent probes reported since 2010, especially those in the past five years. We have classified HSO₃^-/SO₃^2- fluorescent probes through different chemical reaction mechanisms and wish this review will give some help to the researchers in this field.

Near-Infrared Light-Triggered Sulfur Dioxide Gas Therapy of Cancer

The exploitation of gas therapy platforms holds great promise as a "green" approach for selective cancer therapy, however, it is often associated with some challenges, such as uncontrolled or insufficient gas generation and unclear therapeutic mechanisms. In this work, a gas therapy approach based on near-infrared (NIR) light-triggered sulfur dioxide (SO₂) generation was developed, and the therapeutic mechanism as well as in vivo antitumor therapeutic efficacy was demonstrated. A SO₂ prodrug-loaded rattle-structured upconversion@silica nanoparticles (RUCSNs) was constructed to enable high loading capacity without obvious leakage and to convert NIR light into ultraviolet light so as to activate the prodrug for SO₂ generation. In addition, SO₂ prodrug-loaded RUCSNs showed high cell uptake, good biocompatibility, intracellular tracking ability, and high NIR light-triggered cytotoxicity. Furthermore, the cytotoxic SO₂ was found to induce cell apoptosis accompanied by the increase of intracellular reactive oxygen species levels and the damage of nuclear DNA. Moreover, efficient inhibition of tumor growth was achieved, associated with significantly prolonged survival of mice. Such NIR light-triggered SO₂ therapy may provide an effective strategy to stimulate further development of synergistic cancer therapy platforms.

Suppressing NLRP2 expression accelerates hepatic steatosis: A mechanism involving inflammation and oxidative stress

Nonalcoholic fatty liver disease (NAFLD) is characterized by lipid accumulation and inflammation in the liver, contributing to a broad spectrum of severe pathologies, such as metabolic syndrome and hepatocellular carcinoma. Presently, the pathogenesis that attributes to NAFLD has not been fully understood. NLRP2 has been shown to inhibit the NF-κB signaling, and thus may contribute to regulate the inflammatory response. However, its role in NAFLD is largely unclear. In the study, we found that NLRP2 was markedly decreased in liver tissues of individuals with severe steatosis, or in a genetic deficiency (ob/ob) mice. High fat diet (HFD) feeding also led to a significant reduction of NLRP2 in liver of mice. Then, the wild type (WT) and NLRP2 knockout (KO) mice were used to further explore the role of NLRP2 in the NAFLD progression. NLRP2 knockout mice exhibited severer metabolic syndrome and hepatic steatosis after HFD administration, as evidenced by the increased body weight, liver histological changes and lipid accumulation. Moreover, HFD feeding-induced inflammation was significantly accelerated by the loss of NLRP2, as evidenced by the increased expression of pro-inflammatory cytokines and activation of nuclear factor κB (NF-κB) pathway. In addition, oxidative stress triggered by HFD was further promoted by NLRP2 deletion through repressing NF-E2-related factor 2 (Nrf2) pathway. In vitro, we surprisingly found that promoting Nrf2 activation could attenuate NLRP2 knockout-accelerated inflammation and reactive oxygen species (ROS) generation. Therefore, our study indicated that NLRP2 might be a potential target for developing effective therapeutic strategy to prevent NAFLD progression.

SO2 Donors and Prodrugs, and Their Possible Applications: A Review

SO2 is widely recognized as an air pollutant and is a known cause of acid rain. At a sufficiently high level, it also causes respiratory diseases. A much lesser known side of SO2 is its endogenous nature and possible physiological roles. There is mounting evidence that SO2 is produced during normal cellular metabolism and may possibly function as a signaling molecule in normal physiology. The latter aspect is still at the stage of being carefully examined as to the validity of classifying SO2 as a gasotransmitter with endogenous signaling roles. One difficulty in studying the biological and pharmacological roles of SO2 is the lack of adequate tools for its controllable and precise delivery. Traditional methods of using SO2 gas or mixed sulfite salts do not meet research need for several reasons. Therefore, there has been increasing attention on the need of developing SO2 donors or prodrugs that can be used as tools for the elucidation of SO2's physiological roles, pharmacological effects, and possible mechanism(s) of action. In this review, we aim to review basic sulfur chemistry in the context of sulfur signaling and various chemical strategies used for designing SO2 donors. We will also discuss potential pharmacological applications of SO2 donors, lay out desirable features for such donors and possibly prodrugs, analyze existing problems, and give our thoughts on research needs.

Gas Signaling Molecules and Mitochondrial Potassium Channels

Recently, gaseous signaling molecules, such as carbon monoxide (CO), nitric oxide (NO), and hydrogen sulfide (H₂S), which were previously considered to be highly toxic, have been of increasing interest due to their beneficial effects at low concentrations. These so-called gasotransmitters affect many cellular processes, such as apoptosis, proliferation, cytoprotection, oxygen sensing, ATP synthesis, and cellular respiration. It is thought that mitochondria, specifically their respiratory complexes, constitute an important target for these gases. On the other hand, increasing evidence of a cytoprotective role for mitochondrial potassium channels provides motivation for the analysis of the role of gasotransmitters in the regulation of channel function. A number of potassium channels have been shown to exhibit activity within the inner mitochondrial membrane, including ATP-sensitive potassium channels, Ca²⁺-activated potassium channels, voltage-gated Kv potassium channels, and TWIK-related acid-sensitive K⁺ channel 3 (TASK-3). The effects of these channels include the regulation of mitochondrial respiration and membrane potential. Additionally, they may modulate the synthesis of reactive oxygen species within mitochondria. The opening of mitochondrial potassium channels is believed to induce cytoprotection, while channel inhibition may facilitate cell death. The molecular mechanisms underlying the action of gasotransmitters are complex. In this review, we focus on the molecular mechanisms underlying the action of H₂S, NO, and CO on potassium channels present within mitochondria.

Sulfur dioxide attenuates hypoxia-induced pulmonary arteriolar remodeling via Dkk1/Wnt signaling pathway

OBJECTIVE

This study investigated the impact of SO₂ on rats with hypoxic pulmonary vascular structural remodeling and its possible mechanisms.

MATERIALS AND METHODS

Pulmonary vascular morphological change was examined by HE staining. RNA-based high-throughput sequencing (RNA-seq) was performed to detect differential expression of mRNAs in Normal and Hypoxia-induced Pulmonary hypertension (HPH) rats. The Real-time quantitative RT-PCR (q RT-PCR) was used for validation of wnt7b, sfrp2, cacna1f, DKK1, CaSR and vimentin mRNA expression levels. Protein levels of CaSR, Vimentin, Caspase3, E-cadherin and P-Akt1/2/3 were detected by Western blot and immunohistochemistry.

RESULTS

This study showed that SO₂ significantly attenuated the interstitial thickening and prominent media hypertrophy of pulmonary arteries. SO₂ downregulated p-Akt1/2/3 protein level and upregulated E-cadherin protein level in lung tissues, which inhibited the proliferation and epithelial-to-mesenchymal transition (EMT) in HPH rats. RNA-seq and PCR validation results showed that levels of Wnt7b, Sfrp2 and Cacna1f mRNAs decreased and Dkk1 mRNA level increased obviously in HPH rats. Moreover, SO₂ attenuated the mRNA and protein level of CaSR, which was activated in HPH rats and resulted in the proliferation of PASMCs. Besides, the mRNA and protein expression of vimentin in PASMCs significantly reduced after SO₂ treatment.

CONCLUSION

Together, these findings indicate that SO₂ could attenuate hypoxia-induced pulmonary arteriolar remodeling and may suppress the proliferation and migration of PASMCs at least in part through the Dkk1/Wnt signaling pathway.

The Increased Endogenous Sulfur Dioxide Acts as a Compensatory Mechanism for the Downregulated Endogenous Hydrogen Sulfide Pathway in the Endothelial Cell Inflammation

Endogenous hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) are regarded as important regulators to control endothelial cell function and protect endothelial cell against various injuries. In our present study, we aimed to investigate the effect of endogenous H₂S on the SO₂ generation in the endothelial cells and explore its significance in the endothelial inflammation in vitro and in vivo. The human umbilical vein endothelial cell (HUVEC) line (EA.hy926), primary HUVECs, primary rat pulmonary artery endothelial cells (RPAECs), and purified aspartate aminotransferase (AAT) protein from pig heart were used for in vitro experiments. A rat model of monocrotaline (MCT)-induced pulmonary vascular inflammation was used for in vivo experiments. We found that endogenous H₂S deficiency caused by cystathionine-γ-lyase (CSE) knockdown increased endogenous SO₂ level in endothelial cells and enhanced the enzymatic activity of AAT, a major SO₂ synthesis enzyme, without affecting the expressions of AAT1 and AAT2. While H₂S donor could reverse the CSE knockdown-induced increase in the endogenous SO₂ level and AAT activity. Moreover, H₂S donor directly inhibited the activity of purified AAT protein, which was reversed by a thiol reductant DTT. Mechanistically, H₂S donor sulfhydrated the purified AAT1/2 protein and rescued the decrease in the sulfhydration of AAT1/2 protein in the CSE knockdown endothelial cells. Furthermore, an AAT inhibitor l-aspartate-β-hydroxamate (HDX), which blocked the upregulation of endogenous SO₂/AAT generation induced by CSE knockdown, aggravated CSE knockdown-activated nuclear factor-κB pathway in the endothelial cells and its downstream inflammatory factors including ICAM-1, TNF-α, and IL-6. In in vivo experiment, H₂S donor restored the deficiency of endogenous H₂S production induced by MCT, and reversed the upregulation of endogenous SO₂/AAT pathway via sulfhydrating AAT1 and AAT2. In accordance with the results of the in vitro experiment, HDX exacerbated the pulmonary vascular inflammation induced by the broken endogenous H₂S production in MCT-treated rat. In conclusion, for the first time, the present study showed that H₂S inhibited endogenous SO₂ generation by inactivating AAT via the sulfhydration of AAT1/2; and the increased endogenous SO₂ generation might play a compensatory role when H₂S/CSE pathway was downregulated, thereby exerting protective effects in endothelial inflammatory responses in vitro and in vivo.

H2S and polysulfide metabolism: Conventional and unconventional pathways

It is now well established that hydrogen sulfide (H₂S) is an effector of a wide variety of physiological processes. It is also clear that many of the effects of H₂S are mediated through reactions with cysteine sulfur on regulatory proteins and most of these are not mediated directly by H₂S but require prior oxidation of H₂S and the formation of per- and polysulfides (H₂S_n, n = 2-8). Attendant with understanding the regulatory functions of H₂S and H₂S_n is an appreciation of the mechanisms that control, i.e., both increase and decrease, their production and catabolism. Although a number of standard "conventional" pathways have been described and well characterized, novel "unconventional" pathways are continuously being identified. This review summarizes our current knowledge of both the conventional and unconventional.

Hydrogen Sulfide in the Adipose Tissue-Physiology, Pathology and a Target for Pharmacotherapy

Hydrogen sulfide (H₂S) is synthesized in the adipose tissue mainly by cystathionine γ-lyase (CSE). Several studies have demonstrated that H₂S is involved in adipogenesis, that is the differentiation of preadipocytes to adipocytes, most likely by inhibiting phosphodiesterases and increasing cyclic AMP concentration. The effect of H₂S on adipose tissue insulin sensitivity and glucose uptake is controversial. Some studies suggest that H₂S inhibits insulin-induced glucose uptake and that excess of H₂S contributes to adipose tissue insulin resistance in metabolic syndrome. In contrast, other studies have demonstrated that H₂S stimulates glucose uptake and its deficiency contributes to insulin resistance. Similarly, the effect of H₂S on adipose tissue lipolysis is controversial. H₂S produced by perivascular adipose tissue decreases vascular tone by activating ATP-sensitive and/or voltage-gated potassium channels in smooth muscle cells. Experimental obesity induced by high calorie diet has a time dependent effect on H₂S in perivascular adipose tissue; short and long-term obesity increase and decrease H₂S production, respectively. Hyperglycemia has been consistently demonstrated to suppress CSE-H₂S pathway in various adipose tissue depots. Finally, H₂S deficiency may contribute to adipose tissue inflammation associated with obesity/metabolic syndrome.

Association between ambient air pollution and proliferation of umbilical cord blood cells

It has been established as a common knowledge that ambient air pollution (AAP) has an adverse effect on human health. The pathophysiological mechanism of this impact is likely to be related to the oxidative stress. In the current study we estimate the association between AAP and cell proliferation (CP) of umbilical cord blood cells, representing maternal organism most proximal to the fetal body. Blood samples were tested for proliferation in 292 enrolled Arab-Bedouin women at delivery (July 2012-March 2013). The estimates of AAP were defined by a hybrid satellite based model predicting both PM_2.5 (particles<2.5µm in diameter) and PM₁₀ (particles<10µm in diameter) as well as monitoring stations for gaseous air pollutants. Risk estimates of pollution exposure were adjusted to medical history, household risk factors and meteorological factors on the day of delivery or one week prior. Ambient ozone (O₃) levels on 1, 2, 3and 4 days prior to delivery were associated with lower CP (Prevalence ratio (PR)=0.92, 0.92, 0.93, 0.93, respectively). Increase in inter-quartile range (IOR) of PM_2.5 one day before delivery was associated with 9% increase in CP levels (PR=1.09). The positive direction in association was changed to negative association with CP for PM_2.5 levels measured at more distant time periods (PR=0.90 and 0.93 for lags 5 and 6 days, respectively). Investigation of PM₁₀ levels indicated a similar pattern (PR=1.05 for pollution values recorded one day before delivery and 0.93 and 0.95 for lags of 5 and 6 days, respectively). Carbon monoxide (CO) levels were associated with lower CP on the day of delivery and 1day prior (PR=0.92 and PR=0.94). To conclude, the levels of cell proliferation of umbilical cord blood cells appear to be associated with the AAP. More studies are needed to support our findings.

Sulfur dioxide inhalation lowers sperm quality and alters testicular histology via increasing expression of CREM and ACT proteins in rat testes

Sulfur dioxide (SO₂) is one of the main atmospheric pollutants worldwide, and is reported to be responsible for the formation of severe haze in China. Some studies have demonstrated a potential harmful effect of SO₂ on the male reproductive system; however the underlying mechanism is still unknown. The purpose of this study is to investigate the roles of cytochrome P450 (P450), cAMP-responsive element modulator (CREM), and activator of CREM (ACT) in SO₂-induced toxicity. Forty-eight male Wistar rats were randomly divided into an experimental and control group. The experiment group was exposed to SO₂ in ambient air (10ppm, 4h/day), and the control group was treated with filtered air in the same conditions. After 2 weeks, the results showed a significant decrease in body weight and sperm motility, and an increase in the testis weight-to-body weight ratio as compared to the control group. Histological investigation suggested that SO₂ exposure led to loose arrangement of the spermatogenic cells and local structural damage in the seminiferous tubules. Moreover, the expressions of P450, CREM and ACT proteins increased in the testes by 0.22%, 47.26% and 23.38%, respectively. Taken together, SO₂ inhalation lowered sperm quality, altered testicular histology, and increased expressions of CREM and ACT proteins in the testes of rats. Overall, these results could contribute to a better understanding of SO₂-induced male reproductive toxicity.

Volcanic air pollution over the Island of Hawai'i: Emissions, dispersal, and composition. Association with respiratory symptoms and lung function in Hawai'i Island school children

BACKGROUND

Kilauea Volcano on the Island of Hawai'i has erupted continuously since 1983, releasing approximately 300-12000metrictons per day of sulfur dioxide (SO2). SO2 interacts with water vapor to produce an acidic haze known locally as "vog". The combination of wind speed and direction, inversion layer height, and local terrain lead to heterogeneous and variable distribution of vog over the island, allowing study of respiratory effects associated with chronic vog exposure.

OBJECTIVES

We characterized the distribution and composition of vog over the Island of Hawai'i, and tested the hypotheses that chronic vog exposure (SO2 and acid) is associated with increased asthma prevalence, respiratory symptoms, and reduced pulmonary function in Hawai'i Island schoolchildren.

METHODS

We compiled data of volcanic emissions, wind speed, and wind direction over Hawai'i Island since 1992. Community-based researchers then measured 2- to 4-week integrated concentrations of SO2 and fine particulate mass and acidity in 4 exposure zones, from 2002 to 2005, when volcanic SO2 emissions averaged 1600metrictons per day. Concurrently, community researchers recruited schoolchildren in the 4th and 5th grades of 25 schools in the 4 vog exposure zones, to assess determinants of lung health, respiratory symptoms, and asthma prevalence.

RESULTS

Environmental data suggested 4 different vog exposure zones with SO2, PM2.5, and particulate acid concentrations (mean±s.d.) as follows: 1) Low (0.3±0.2ppb, 2.5±1.2μg/m(3), 0.6±1.1nmolH+/m(3)), 2) Intermittent (1.6±1.8ppb, 2.8±1.5μg/m(3), 4.0±6.6nmolH+/m(3)), 3) Frequent (10.1±5.2ppb, 4.8±1.9μg/m(3), 4.3±6.7nmolH+/m(3)), and 4) Acid (1.2±0.4ppb, 7.2±2.3μg/m(3), 25.3±17.9nmolH+/m(3)). Participants (1957) in the 4 zones differed in race, prematurity, maternal smoking during pregnancy, environmental tobacco smoke exposure, presence of mold in the home, and physician-diagnosed asthma. Multivariable analysis showed an association between Acid vog exposure and cough and strongly suggested an association with FEV1/FVC <0.8, but not with diagnosis of asthma, or chronic persistent wheeze or bronchitis in the last 12months.

CONCLUSIONS

Hawai'i Island's volcanic air pollution can be very acidic, but contains few co-contaminants originating from anthropogenic sources of air pollution. Chronic exposure to acid vog is associated with increased cough and possibly with reduced FEV1/FVC, but not with asthma or bronchitis. Further study is needed to better understand how volcanic air pollution interacts with host and environmental factors to affect respiratory symptoms, lung function, and lung growth, and to determine acute effects of episodes of increased emissions.

Benzothiazole Sulfinate: a Water-Soluble and Slow-Releasing Sulfur Dioxide Donor

Sulfur dioxide (SO2) has long been considered a toxic environmental pollutant and byproduct of industrial processing. Recently it has become evident that SO2 may also have regulatory functions in mammalian pulmonary systems. However, the study of these effects has proven to be challenging due to the difficulty in administering SO2 in a reliable manner. In this work, we report the discovery of a new pH-dependent and water-soluble SO2 donor, benzothiazole sulfinate (BTS). We have found BTS to have slow and sustained SO2 release at physiological pH. Additionally, we have explored its vasorelaxation properties as compared to the authentic SO2 gas solutions. The slow release of BTS should make it a useful tool for the study of endogenously generated SO2.

Sodium fluoride and sulfur dioxide affected male reproduction by disturbing blood-testis barrier in mice

Fluoride and sulfur dioxide (SO2), two well-known environmental toxicants, have been implicated to have adverse effects on male reproductive health in humans and animals. The objective of this study to investigate if the BTB is one of the pathways that lead to reproductive toxicity of sodium fluoride and sulfur dioxide alone or in combination, in view of the key role of blood testis barrier (BTB) in testis. The results showed that a marked decrease in sperm quality, and altered morphology and ultrastructure of BTB in testis of mice exposure to fluoride (100 mg NaF/L in drinking water) or/and sulfur dioxide (28 mg SO2/m(3), 3 h/day). Meanwhile, the mRNA expression levels of some vital BTB-associated proteins, including occluding, claudin-11, ZO-1, Ncadherin, α-catenin, and connexin-43 were all strikingly reduced after NaF exposure, although only the reduction of DSG-2 was statistically significant in all treatment groups. Moreover, the proteins expressions also decreased significantly in claudin-11, N-cadherin, α-catenin, connexin-43 and desmoglein-2 in mice treated with fluoride and/or SO2. These changes in BTB structure and constitutive proteins may therefore be connected with the low sperm quality in these mice. The role of fluoride should deserves more attention in this process.

The vasodilatory effect of sulfur dioxide via SGC/cGMP/PKG pathway in association with sulfhydryl-dependent dimerization

The present study was designed to explore the role of soluble guanylate cyclase (sGC)/cyclic guanosine monophosphate (cGMP)/PKG pathway in sulfur dioxide (SO2)-induced vasodilation. We showed that SO2 induced a concentration-dependent relaxation of phenylephrine (PE)-precontracted rat aortic rings in association with an increase in cGMP concentration, whereas l-aspartic acid β-hydroxamate (HDX), an inhibitor of SO2 synthase, contracted rings in a dose-dependent manner. Pretreatment of aortic rings with the sGC inhibitor ODQ (30 μM) attenuated the vasodilatory effects of SO2, suggesting the involvement of cGMP pathway in SO2-induced vasodilation. Mechanistically, SO2 upregulated the protein levels of sGC and PKG dimers, while HDX inhibited it, indicating SO2 could promote cGMP synthesis through sGC activation. Furthermore, the dimerization of sGC and PKG and vasodilation induced by SO2 in precontracted rings were significantly prevented by thiol reductants dithiothreitol (DTT). In addition, SO2 reduced the activity of phosphodiesterase type 5 (PDE5), a cGMP-specific hydrolytic enzyme, implying that SO2 elevated cGMP concentration by inhibiting its hydrolysis. Hence, SO2 exerted its vasodilatory effects at least partly by promoting disulfide-dependent dimerization of sGC and PKG, resulting in an activated sGC/cGMP/PKG pathway in blood vessels. These findings revealed a new mode of action and mechanisms by which SO2 regulated the vascular tone.