Endogenous Sulfur Dioxide: A New Member of Gasotransmitter Family in the Cardiovascular System.

Sulfur dioxide (SO2) was previously regarded as a toxic gas in atmospheric pollutants. But it has been found to be endogenously generated from metabolism of sulfur-containing amino acids in mammals through transamination by aspartate aminotransferase (AAT). SO2 could be produced in cardiovascular tissues catalyzed by its synthase AAT. In recent years, studies revealed that SO2 had physiological effects on the cardiovascular system, including vasorelaxation and cardiac function regulation. In addition, the pathophysiological effects of SO2 were also determined. For example, SO2 ameliorated systemic hypertension and pulmonary hypertension, prevented the development of atherosclerosis, and protected against myocardial ischemia-reperfusion (I/R) injury and isoproterenol-induced myocardial injury. These findings suggested that endogenous SO2 was a novel gasotransmitter in the cardiovascular system and provided a new therapy target for cardiovascular diseases.

1. Introduction

Sulfur dioxide (SO2) was regarded as a toxic gas and environmental pollutant. It is colorless, transparent, odorous, and water-soluble. The harmful effects of SO2 on human, animals, and plants have been extensively investigated [1, 2]. However, SO2 can be endogenously generated from metabolism of the sulfur-containing amino acid L-cysteine in mammals [3]. It has features of low molecular weight, continuous production, and fast diffusion and plays extensive biological action independent of membrane receptors [4, 5]. In neutral fluid or mammal plasma, SO2 is broken down to its derivatives, bisulfite and sulfite (NaHSO3/Na2SO3, 1 : 3 M/M), maintaining organism homeostasis [6]. The sulfite is the physiological form of SO2 in vivo [7, 8]. The reference range for total serum sulfite in healthy human beings was 0–9.85 μmol/L detected by high-performance liquid chromatography with fluorescence detection [9]. Serum sulfite was obviously increased in patients suffering from acute pneumonia and chronic renal failure, as well as pediatric acute lymphoblastic leukemia with bacterial inflammation [10-12]. Of note, Balazy et al. found that SO2 could be produced in the porcine coronary arterial rings after incubation with calcium ionophore by gas chromatography-mass spectrometry [13]. Du et al. firstly found that endogenous SO2/aspartate aminotransferase (AAT) pathway existed in the cardiovascular system [14]. SO2 not only has important physiological effects on vascular tone and cardiac function but also exerts pathophysiological effects in the cardiovascular system, including regulation of hypertension, pulmonary hypertension, atherosclerosis, and cardiac ischemia-reperfusion (I/R) injury [15-18]. The abovementioned evidence suggests that the endogenous SO2 may be a novel gasotransmitter in mammals, similar to nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S). The physiological significance of SO2, particularly its regulatory role in the cardiovascular system, has attracted a great deal of interest in the field [19-21].

Therefore, the objective of this review was to elaborate on the generation and metabolism of endogenous SO2 and give a summary of the physiological and pathophysiological effects of SO2 on the cardiovascular system.

2. Generation and Distribution of Endogenous SO in the Cardiovascular System

SO2 can be generated from the metabolism of L-cysteine which is converted from methionine via the transmethylation-transsulfuration pathway (Figure 1) [3, 22]. Firstly, L-cysteine is oxidized to form L-cysteine sulfinate by cysteine dioxygenase (CDO), and then the latter is transaminated to form β-sulfinylpyruvate by AAT. The β-sulfinylpyruvate spontaneously decomposes to pyruvate and SO2 (Figure 1) [3]. Additionally, H2S which shares the same substrate L-cysteine with SO2 can be transferred to SO2 in vivo through other pathways. Mitsuhashi et al. reported that H2S could be converted to sulfite or SO2 by NADPH oxidase in activated neutrophils [23]. Besides, H2S can be first oxidized to thiosulfate by sulfide oxidase and then converted to SO2 catalyzed by thiosulfate sulfurtransferase or glutathione-dependent thiosulfate reductase (Figure 1) [6, 24, 25]. SO2 can exist in the gaseous form or be hydrated to sulfite, which is subsequently oxidized to sulfate by sulfite oxidase, and then the sulfate is excreted into the urine by the kidney (Figure 1) [3, 22].

Figure 1

Illustration of production and metabolism of endogenous SO2 in mammals. SO2: sulfur dioxide; H2S: hydrogen sulfide; CBS: cystathionine β-synthase; CSE: cystathionine γ-lyase; CDO: cysteine dioxygenase; AAT: aspartate aminotransferase; CAT: 2-cysteine aminotransferase; 3MST: 3-mercaptopyruvate sulfurtransferase.

Du et al. first measured endogenous SO2/AAT pathway in the cardiovascular system of Wistar rats and found that SO2 concentration in rat plasma was 15.54 ± 1.68 μmol/L [14]. Li and Meng reported a similar sulfite level of 12.59 ± 9.03 μmol/L in rat plasma [26]. The content of SO2 in aortic tissue was highest, up to 5.55 ± 0.35 μmol/g protein, followed by pulmonary arteries (3.27 ± 0.21 μmol/g protein), mesenteric arteries (2.67 ± 0.17 μmol/g protein), tail arteries (2.50 ± 0.20 μmol/g protein), and renal arteries (2.23 ± 0.19 μmol/g protein), respectively [14]. Moreover, plasma AAT activity was 87 ± 18 U/L. Unlike SO2 content, the activity of AAT in the renal arteries was higher than that in other vascular tissues mentioned above [14]. Furthermore, AAT mRNA expression was rich in endothelial cells and in vascular smooth muscle cell (VSMC) beneath the endothelial layer [14].

3. Physiological Effects of SO on the Cardiovascular System

3.1. Vasorelaxant Effect of SO2

SO2 derivatives (mixture of sodium bisulfite and sodium sulfite, 1 : 3 M/M in neutral solution) could induce a concentration-dependent relaxation of isolated rat aortic rings, whereas L-aspartate-β-hydroxamate (HDX), an inhibitor of SO2 synthase AAT, caused greater vasoconstriction than that of the control group [14]. And the vasorelaxing effects of SO2 gas and SO2 gas solution were similar [27, 28]. Therefore, SO2 might act as a vasoactive molecule. It had a vital vasodilating function required for maintaining normal vascular tone.

The mechanisms of this physiological vasorelaxation by SO2 were complex. Nicardipine eliminated the vasorelaxing effect induced by SO2 derivatives, indicating that the L-type calcium (L-Ca2+) channel participated in the role of SO2 [14]. Additionally, at low concentration (<450 μmol/L), the vasorelaxing effect of SO2 was related to the big-conductance Ca2+-activated K+ (BKCa) channel, while at a high concentration (>500 μmol/L) the vasorelaxation induced by SO2 was associated to adenosine triphosphate-sensitive potassium (KATP) channel activation and the L-Ca2+ channel [29]. Mechanistically, SO2 and its derivatives induced the KATP and BKCa channels activation through increasing the expressions of Kir6.1, Kir6.2, SUR2B, and BKCa channel subunits α and β1 in rat aortic rings, while SO2 and its derivatives inhibited the L-type calcium channel through decreasing the expressions of Cav1.2 and Cav1.3 [30]. Besides, SO2 derivatives increased levels of 3′-5′-cyclic adenosine monophosphate (cAMP), prostacyclin (PGI2), adenylyl cyclase (AC) activity, and protein kinase A (PKA) activity in rat aortic rings, indicating that the relaxing effect of SO2 was related to the PGI2-AC-cAMP-PKA signal pathway [31, 32]. Moreover, the endothelial nitric oxide synthase- (eNOS-) nitric oxide- (NO-) 3′-5′-cyclic guanosine monophosphate (cGMP) pathway and BKCa channel partially mediated the vasorelaxing effect of SO2 and sodium bisulfite in an endothelium-dependent manner at low concentration (<450 μM), while at high concentration (≥1000 μM) the vasorelaxation induced by SO2 was endothelium independent and relied on the KATP and L-Ca2+ channels [26, 33, 34]. Hence, ion channels, such as L-Ca2+, KATP, and BKCa channels, as well as cGMP and cAMP pathways play important roles in the effects of SO2 on vasodilation.

3.2. Negative Inotropic Effect of SO2

In isolated perfused rat heart, gaseous SO2 and its derivatives (NaHSO3/Na2SO3, 1 : 3 M/M, 0–2000 μmol/L) elicited a dose-dependent negative inotropic effect, which affected the heart rate, left ventricular developed pressure (LVDP), and the first derivatives of LVDP (±LV dp/dt max) [35, 36]. And the gaseous SO2 induced a server negative effect compared to SO2 derivatives. The mechanisms for this inotropic effect are different between high concentration and low concentrations of SO2. At low concentrations, SO2 produced negative inotropic effects through upregulating the activities of protein kinase C (PKC), cyclooxygenase, and cGMP, while, at high concentrations, the inotropic effects induced by SO2 were associated with the activation of KATP channel by increasing the expressions of Kir6.2 and SUR2A and the inhibition of calcium influx via the L-type calcium channel by decreasing the expressions of Cav1.2 and Cav1.3 in rat hearts [36, 37]. Moreover, SO2 could depress L-type calcium channel current in isolated rat cardiomyocytes [38]. These data indicated that SO2 had a negative inotropic effect on myocardial contractility and hemodynamic parameters, which might help to explain some cardiovascular effects induced by SO2.

4. Pathophysiological Effects of SO on the Cardiovascular System

4.1. SO2 and Hypertension

Hypertension is a major risk factor for many cardiovascular disorders. However, the pathogenesis of hypertension has not been fully elucidated. Exposure to SO2 (50 ppm, 6 hr/d, 5 d/wk for 31 weeks) was reported to cause a slight but consistent decrease in blood pressure in susceptible to salt-induced hypertension rats [39], implying that SO2 might regulate blood pressure. Moreover, spontaneously hypertensive rats (SHRs) exhibited a significant decrease in the plasma SO2 content and AAT activity in both serum and aorta [15]. And SO2 derivatives administration markedly inhibited the upregulated tail artery pressure of SHRs [15, 40], which suggested that SO2 played a role in the progress of hypertension. Arterial remodeling predominates in severe hypertension [41]. As well, SO2 alleviated the pressure to media, decreased the ratio of media to lumen radius, and reduced the proliferative index of smooth muscle cells in the thoracic aorta of SHRs compared to those of sterile water-treated rats [15]. These findings further verified that the inhibited endogenous SO2/AAT pathway might participate in the development of hypertension. Vasorelaxation dysfunction is the main component of the pathogenesis of hypertension. SO2 could increase vasorelaxation in SHR arteries by enhancing the vasodilating response to NO in isolated aortic rings and promoting NO production of aortic tissues [40]. The interaction between SO2 and NO is involved in the mechanisms by which SO2 regulates hypertension.

The abnormally increased proliferation of VSMCs induces vascular remodeling and accelerates the development of hypertension [42]. Both exogenous SO2 derivatives and endogenous-derived SO2 by AAT overexpression significantly inhibited serum-stimulated VSMC proliferation through preventing cell cycle progression from G1 to S phase and inhibiting DNA synthesis [43]. Further study demonstrated that SO2 elevated cellular cyclic adenosine monophosphate (cAMP) production to activate the PKA signaling, subsequently phosphorylated c-Raf on Ser259 site to block its activation, and then inhibited the extracellular regulated protein kinase (Erk)/mitogen-activated protein kinase (MAPK) signaling transduction, which finally prevented cell cycle progression and led to the suppression of VSMC proliferation [43]. The inhibition of VSMC proliferation might also be involved in SO2-mediated antihypertensive mechanisms.

4.2. SO2 and Pulmonary Hypertension

4.2.1. SO2 and Hypoxic Pulmonary Hypertension

Pulmonary hypertension, characterized by high pressure in pulmonary artery, is a common complication of congenital heart disease (CHD), ultimately inducing right ventricular failure and even death. A prospective cohort study showed that the serum SO2 levels of children were, respectively, (10.6 ± 2.4), (8.9 ± 2.3), (7.3 ± 2.9), and (4.3 ± 2.1) μM, in the control group, CHD without pulmonary hypertension group, CHD with mild pulmonary hypertension group, and CHD with moderate or severe pulmonary hypertension group [44], suggesting that a negative correlation existed between SO2 and pulmonary hypertension. Consistent with this, a downregulated SO2 level and AAT expression in lung tissue, accompanied with significant pulmonary hypertension, pulmonary vascular remodeling, and increased vascular inflammation, were found in rats under hypoxic condition [16, 45]. Most importantly, SO2 derivatives could markedly lower mean pulmonary artery pressure (mPAP) of hypoxic pulmonary hypertensive rats, whereas HDX advanced pulmonary hypertension [16, 45], indicating that decreased SO2/AAT pathway was involved in the development on hypoxic pulmonary hypertension. The hallmark pathological feature of hypoxic pulmonary hypertension is the pulmonary vascular structural remodeling including extracellular matrix accumulation, vascular smooth muscle proliferation, and inflammatory cells infiltrates [46]. SO2 derivatives prevented pulmonary vascular remodeling in hypoxic pulmonary hypertension through promoting collagen I and III degradation, suppressing abnormal collagen deposition in pulmonary vascular walls and through inhibiting pulmonary arterial SMC proliferation by downregulating Raf-1, MEK-1, and phosphorylating ERK under hypoxia [16]. Inflammation is important in the pathogenesis of hypoxic pulmonary hypertension. In addition, SO2 could inhibit pulmonary inflammation by suppressing expressions of nuclear factor-kappa B (NF-κB) and intercellular adhesion molecule-1 (ICAM-1) [16], indicating the inhibitory effects of SO2 on inflammation may also be involved in the mechanism by which SO2 protects against hypoxic pulmonary hypertension.

4.2.2. SO2 and Monocrotaline-Induced Pulmonary Hypertension

Monocrotaline (MCT), a pyrrolizidine alkaloid, increased mPAP and the ratio of right ventricle to left ventricle plus septum, coincident with the elevated SO2 content, AAT activity, and expression in rats [47]. SO2 derivatives injection significantly lowered mPAP and alleviated small and median pulmonary artery structural remodeling, whereas HDX which inhibited the activity of AAT and the production of endogenous SO2 further augmented mPAP, promoted right ventricular hypertrophy, and worsened pulmonary arteries structural remodeling [47]. These findings implied that the upregulation of endogenous SO2/AAT pathway might play a protective role in the development of MCT-induced pulmonary hypertension. The enhancement of oxidative stress is one of the main pathogenesis of MCT-induced pulmonary hypertension [48]. SO2 could upregulate the activities of antioxidative enzymes, including superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) in lung tissues and plasma from MCT-induced pulmonary hypertensive rats, whereas HDX decreased the activities of antioxidative enzymes [47]. These data suggested that the promotion of endogenous antioxidative capacities might be responsible for the protective role of SO2 in MCT-induced pulmonary hypertension.

4.2.3. SO2 and High Pulmonary Blood Flow-Induced Pulmonary Hypertension

Severe pulmonary hypertension develops secondary to high pulmonary blood flow in patients with left-to-right shunt congenital heart defects or systemic arteriovenous shunt [49, 50]. However, the underlying mechanisms for flow-induced pulmonary hypertension remain poorly understood. The endogenous SO2/AAT2 pathway in pulmonary tissues was also inhibited in rats with pulmonary hypertension induced by high pulmonary blood flow [51]. SO2 reduced systolic pulmonary arterial pressure and improved pulmonary arterial structural remodeling, exhibiting decreased ratio of muscularized arteries to small pulmonary arteries and increased percentage of nonmuscularized arteries in the development of high pulmonary blood flow-induced pulmonary hypertension [51]. The mechanism was unclear, however. Both SO2 and H2S were derived from the methionine metabolism and they could convert to each other in mammals. Moreover, the endogenous H2S pathway exerted obvious mitigation effect on pulmonary hypertension induced by high pulmonary blood flow and H2S had strong vasodilating effect. Therefore, the researchers investigated the impact of SO2 on the endogenous H2S generating pathway in the pathogenesis of high blood flow-induced pulmonary hypertension. And they found that SO2 derivatives could upregulate the concentration of H2S in lung tissues, as well as the expressions of the key generating enzymes of H2S, including cystathionine γ-lyase (CSE), cystathionine β-synthase (CBS), and 3-mercaptopyruvate sulfurtransferase (3MST) [51]. Furthermore, SO2 increased the protein expression of these H2S producing enzymes probably through upregulating their gene transcription. These data suggested that SO2 alleviated pulmonary hypertension induced by high pulmonary blood flow in association with upregulating the reduced endogenous H2S pathway.

4.3. SO2 and Atherosclerosis

Atherosclerosis, a chronic and progressive pathological process in arteries, is a crucial pathological manifestation of cardiovascular diseases. Vascular inflammation, oxidative stress, VSMC proliferation, endothelial injury, and foam cell accumulation contribute to the formation of atherosclerotic plaque. Environmental toxicological study showed that the chronic exposure to gaseous air pollution such as SO2, NO, and CO might lead to the promotion of atherosclerosis [52, 53]. Growing evidence demonstrated that endogenous NO, CO, and H2S were beneficial in alleviating atherosclerosis [54-56]. They exerted significant anti-inflammation effect in the development of atherosclerosis, especially endothelium-derived NO which played a notably protective role in the early stage of atherosclerosis. However, the role of SO2 at physiological concentration in the development of atherosclerosis was unclear. Li et al. found that plasma and aortic SO2 contents were downregulated with the reduced aortic AAT activity in atherosclerosis rats [17], implying that the inhibition of SO2/AAT pathway might be involved in the pathogenesis of atherosclerosis. SO2 derivatives treatment diminished the size of atherosclerotic plaques in the coronary artery, not only by increasing H2S/CSE pathway and the NO/nitric oxide synthase (NOS) pathway, but also by elevating the antioxidative capacities through increasing plasma GSH-Px and SOD activities and decreasing MDA level [17]. Additionally, suppression of VSMC proliferation via cAMP/PKA signaling-mediated Erk/MAPK pathway might also contribute to the antiatherosclerotic effects of SO2 [43].

4.4. SO2 and Myocardial Ischemia Reperfusion

Myocardial ischemia-reperfusion (I/R) injury is an important cause of tissue and cell injury and often leads to heart failure. The main mechanisms involve inflammation, oxidative damage, and intracellular and mitochondrial calcium overload [57]. In rat myocardial I/R models made by ligating the left coronary artery for 30 min and reperfusion for 120 min, AAT1 protein expression was significantly decreased compared to sham operation group [18]. And SO2 derivatives preconditioning for 10 min before ischemia (with a low concentration of sulfur dioxide of 1–10 μmol/kg) significantly decreased myocardial infract size and lowered levels of myocardial enzymes creatine kinase (CK) and lactate dehydrogenase (LDH) in plasma of rats with I/R injury in vivo [18]. SO2 preconditioning also increased cardiac function and attenuated myocardium apoptosis induced by I/R [18]. Ischemic preconditioning-induced endoplasmic reticulum stress (ERS) plays a protective role in the ischemia injury. Glucose-regulated protein 78 (GRP78), C/EBP homologous protein (CHOP), and phosphorylated eukaryotic initiation of the factor 2α-subunit (p-eIF2α) are the markers of myocardial ERS. SO2 pretreatment induced myocardial GRP78 expression and eIF2α phosphorylation prior to myocardial I/R, while inhibiting expressions of myocardial GRP78, CHOP, and p-eIF2α in rats with myocardial I/R [18]. Dithiothreitol, an ERS activator [58], mimicked the cardioprotective effect of SO2, whereas ERS inhibitor 4-phenylbutyrate abolished the cardioprotection of SO2 preconditioning [18, 59]. The above data suggested that augmentation of ERS by SO2 preconditioning before myocardial I/R contributed to cardioprotection against lethal ischemia. Moreover, SO2 preconditioning significantly elevated the phosphorylation of Akt and PI3K p85 and attenuated the myocardial damage in rats with I/R injury [60]. LY294002, a PI3K inhibitor, prevented the protective function of SO2 preconditioning as well as SO2-induced GRP78 and p-eIF2α expression [18, 60], indicating that PI3K/Akt signaling pathway likely mediated the activation of ERS by SO2 pretreatment in rats subjected to myocardial I/R. In addition, oxidative stress is involved in the pathogenesis of myocardial I/R. SO2 preconditioning with low dose of SO2 (1 and 5 μmol/kg) prior to ischemia could significantly elevate plasma levels of SOD, GSH, and GSH-Px and reduce the MDA level [61], indicating that SO2 preconditioning enhanced the antioxidative capacity in rats with myocardial I/R. MAPK signaling, one of the most important pathways in cell signal transduction, is crucial to myocardial I/R. SO2 preconditioning significantly improved cardiac function and reduced myocardial expression of phosphorylated ERK1/2 protein in isolated rat heart with I/R [62]. Pretreated with PD98059, the ERK1/2 inhibitor abolished the above functions of SO2 [62]. These data indicated that inhibition of ERK1/2 signal pathway activation mediated the cardioprotection of SO2 preconditioning in isolated rat heart subjected to I/R. Taken together, elevation of PI3K/AKT signaling, suppression of ERK-MAPK pathway, augmentation of ERS, enhancement of antioxidative capacity, and attenuation of cardiomyocyte apoptosis might be involved in SO2-mediated cardiac protective mechanisms.

4.5. SO2 and Myocardial Injury

Myocardial injury is a common feature in various cardiac diseases. The underlying mechanisms include hypoxia, overactive oxidative stress, and calcium overload. A previous study found that endogenous SO2/AAT pathway was downregulated in isoproterenol- (ISO-) induced myocardial injury in rats [63]. Administration of SO2 (85 mg/(kg day)) could alleviate cardiac dysfunction and myocardial damage induced by ISO [63]. These data demonstrated that endogenous SO2 might be an important regulator in the pathophysiological process of myocardial injury. The molecular mechanisms underlying the cardioprotective effects of SO2 were still unknown. Oxidative stress was involved in the pathogenesis for ISO-induced myocardial injury. ISO produced oxygen free radicals caused membrane lipid peroxidation, injured the structure of cardiomyocytes, and finally resulted in myocardial damage [64]. But SO2 could increase myocardial antioxidant capacity in rats with myocardial injury by increasing the myocardial activity of SOD and GSH, upregulating the mRNA expression of SOD2 and GSH-Px1, and decreasing products of oxidative stress such as H2O2 and O2 ∙− [63]. Oxidative stress could cause ERS in rat cardiomyocytes [65]. And the overactivated ERS would contribute to the development of myocardial injury. SO2 significantly inhibited the excessive activation of ERS, which might be involved in the mechanism by which SO2 derivatives protected against myocardial injury induced by ISO [66]. In addition, the products of oxidative stress cause the cardiomyocyte membrane damage and morphological mitochondrial injury. SO2 also attenuated ISO-induced mitochondrial swelling and deformation, which was important feature in apoptosis [63]. And cardiomyocyte apoptosis is a key pathological change in myocardial injury. Of note, supplementation of SO2 derivatives alleviated ISO-induced myocardial injury partly through reducing cardiomyocyte apoptosis [67]. The antiapoptotic function of SO2 was mediated by promoting bcl-2 expression, suppressing bax expression, enhancing mitochondrial membrane potential, inhibiting mitochondrion MPTP opening, reducing the release of cytochrome c from mitochondrion into cytoplasm, and decreasing the activation of caspase-9 and caspase-3 [67]. Therefore, the bcl-2/cytc/caspase-9/caspase-3 pathway was involved in the ISO-induced myocardial injury in rats. Intracellular calcium homeostasis exerts a fundamental effect on myocardial physiology and pathology. And calcium overload is an important mechanism involved in myocardial injury. SO2 treatment could inhibit the increased intracellular free Ca2+ concentration induced by ISO in H9C2 cells [68], indicating that the protective effect of SO2 in myocardial injury might be related to the calcium homeostasis regulated by SO2 in cardiomyocytes. Moreover, SO2 derivatives could modulate L-type calcium current and voltage-gated potassium channels in rat cardiomyocytes, indicating that ion channels might also be involved in the effect of SO2 on cardiomyocyte injury [69, 70].

5. Interaction among SO and Other Gasotransmitters

SO2 and H2S share the same endogenous substrate L-cysteine, and they can transform into each other under some biochemical condition [6, 23, 71]. Moreover, they share similar regulatory roles including vasorelaxation, antioxidative action, and inhibition of inflammation and apoptosis. Chen et al. found that SO2 upregulated the concentration and production of H2S in hypoxic rats. And SO2 increased the expression of CSE and 3MST in pulmonary arteries of hypoxic pulmonary hypertensive rats [72]. In addition, SO2 alleviated pulmonary hypertension and improved the pulmonary vascular pathological injury induced by high pulmonary blood flow in association with upregulating the endogenous H2S pathway [51]. Furthermore, SO2 derivatives have a marked antiatherogenic effect with an increased aortic H2S/CSE pathway in atherosclerotic rats [17]. In rats with myocardial I/R injury, SO2 preconditioning markedly upregulated the myocardial H2S level and CSE expression [61]. The above findings provide some evidence that there is a crosstalk between SO2 and H2S. Moreover, NO also shares a variety of the similar biological effects of H2S and SO2, including vasodilation, antioxidation, and anti-inflammatory actions. Li and Meng found that a low concentration (3 or 5 nM) of a NO donor sodium-nitroprusside enhanced the vasodilating effect of SO2 by nearly sixfold [26], suggesting that SO2 and NO have a synergistic effect on vasodilation. In contrast, the NOS inhibitor L-NAME could abolish the vasorelaxing effect of SO2 derivatives (0.5 and 1 mM) in endothelium-intact rings, indicating that endothelium-dependent vasorelaxation induced by SO2 was partially mediated by a NOS pathway [73]. Additionally, both acute and prolonged SO2 exposure upregulated the eNOS-NO-cGMP pathway, which might be involved in the vasodilation induced by SO2 [34]. Moreover, SO2 increased vasorelaxation in SHRs by enhancing the vasorelaxing response to NO and upregulating NO production in aortic tissues [40]. And SO2 also increased NO/NOS pathway in rats with atherosclerosis [17]. By contrast, SO2 pretreatment reduced the myocardial tissue levels of NO and expression of iNOS in rats with I/R [61]. These data suggest that there is also an interaction between SO2 and NO. Hence, endogenous SO2 participates in crosstalk with H2S and NO and an endogenous gaseous messenger molecule network might exist in mammals. However, there are still many questions to be answered about the interactions among these gases. For example, the exact pertinence among these gases in the various pathways of cardiovascular protection has not been fully explored. It is also not known if a combination of these gases will provide synergistic effects in the therapy of cardiovascular diseases. Therefore, additional studies are needed to further investigate interactions among the gasotransmitter pathways.

6. Conclusions

In summary, SO2 can be generated in the cardiovascular system of mammals and the SO2/AAT pathway participates in many biological functions [22, 74]. Endogenously derived SO2 or SO2 derivatives at physiological concentrations play a crucial role in normal physiological process including regulation of vascular tone and cardiac function. In addition, SO2/AAT pathway has important pathophysiological significance in many cardiovascular diseases, such as hypertension, pulmonary hypertension, atherosclerosis, ischemia-reperfusion injury, and myocardial injury. Just as NO, carbon monoxide (CO), and H2S, SO2 is also an endogenous gaseous signaling molecule in the cardiovascular system [71, 75]. However, the biological mechanisms by which endogenous SO2 regulates different cardiovascular diseases and the further cardiovascular effects of SO2 still need to be deeply investigated.

Clarifying the interactions among SO2 and other endogenous gasotransmitters could improve clinical translation. SO2 could upregulate endogenous level of H2S or NO in several cardiovascular diseases such as atherosclerosis, systemic hypertension, or pulmonary hypertension [17, 40, 51]. These lines of evidence imply a crosstalk among SO2 and other gasotransmitters (NO, CO, and H2S) in the cardiovascular system, which requires further exploration.

An understanding of the cardiovascular protective function of SO2 may lead to a new therapeutic strategy based on the modulation of SO2 production. Thus, the function and signaling pathway relating to AAT in the cardiovascular system are worthy of further investigation. Additionally, the design of SO2-controlled releasing agents under physiological condition is extremely urgent, because the stable and reliable SO2 donors are not only the useful research tools, but also potential therapeutic agents to treat cardiovascular diseases. Nowadays, the majority of cardiovascular studies on SO2 have been performed in rats and mice, which lack clinical evidence. Exploring the role of SO2 in large animal models with similar cardiovascular features as human suffering from cardiovascular diseases would help a transition to clinical trials.