1. Introduction

Cardiovascular disease (CVD) is the leading cause of death worldwide, accounting for almost one third of all global deaths ^[1]. CVD is a cluster of disorders of the heart and blood vessels and is comprised of coronary heart disease, peripheral vascular disease, cerebrovascular disease and other conditions. Although CVD is most common in older adults, atherosclerosis can begin in childhood and progress slowly across the life span ^[2]. Therefore, reducing the global burden of CVD by identifying children at risk and providing preventive interventions early are extremely important. Noteworthy, CVD can originate from the early stages of life, not only childhood but tracing back into the fetal life. This theory is now termed the developmental origins of health and disease (DOHaD) by observing how a suboptimal environment in utero has an adverse influence on offspring outcomes in later life ^[3].

The fetal cardiovascular system is vulnerable to adverse early-life environmental insults ^[4]. Developmental plasticity accommodates morphological and functional changes during organogenesis, leading to endothelial dysfunction, stiffer vascular tree, small coronary arteries, low nephron endowment, and fewer cardiomyocytes, through a process known as cardiovascular programming ^[4][5][6]. So far, several mechanisms underlying cardiovascular programming have been proposed, like oxidative stress, nitric oxide (NO) deficiency, activation of the renin–angiotensin system (RAS), dysregulated nutrient-sensing signals, and dysbiosis of gut microbiota ^[4][5][6].

Hydrogen sulfide (H₂S), the third gasotransmitter, has emerged as a crucial regulator of cardiovascular homeostasis ^[7][8][9]. H₂S exerts multifaceted biological functions, including vasodilatation, angiogenesis, antioxidant, anti-inflammation, mitochondria bioenergetics, and antiapoptosis ^[10][11]. In this regard, H₂S-releasing drugs have been considered as potential therapeutics for CVD ^[7][8]. It is noteworthy that the DOHaD concept provides a strategy termed reprogramming to reverse or postpone the programming processes in early life, accordingly protecting offspring against many adult diseases of developmental origins ^[12]. Emerging evidence suggests that H₂S can be used as a reprogramming strategy in hypertension of developmental origins ^[13]. Although H₂S has been shown to have beneficial effects on CVD ^[7][8], whether it could serve as a reprogramming intervention for developmental origins of CVD remains largely unclear.

2. Hydrogen Sulfide in the Cardiovascular System

2.1. H₂S Signaling Pathway

H₂S, a colorless gas with a characteristic foul odor of rotten eggs, was first identified as an environmental toxin in the 1700s and opened three centuries of research into its biological roles ^[14]. In the late 1990s, H₂S was reclassified as the third gaseous signaling molecule, alongside nitric oxide (NO) and carbon monoxide (CO) ^[10]. Currently, H₂S is known as a ubiquitous second messenger molecule with important functions in cardiovascular physiology ^[10][15]. Much of the previous work investigating the actions of H₂S has been directly focused on incident CVD; however, there is a growing need to better understand the mechanisms and pathways of H₂S signaling in CVD of developmental origins.

Figure 1 illustrates three major pathways of H₂S synthesis, including enzymatic pathway, nonenzymatic pathway, and bacteria origins. Three enzymes have been identified to enzymatically generate H₂S, cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulphurtransferase (3MST) ^[10]. CBS and CSE are cytosolic enzymes, but 3-MST is mainly existing in the mitochondria. l-cysteine is the principal substrate for both CBS and CSE to generate H₂S. CBS and CSE can also produce H₂S using other substrates. Homocysteine can be catalyzed by CBS to generate cystathionine, followed by CSE to produce l-cysteine. All of the above-mentioned H₂S-generating enzymes are expressed in the heart and blood vessels ^[10][16]. In an alternative pathway, 3-mercaptopyruvate, the substrate for 3-MST to produce H₂S, is provided by cysteine aminotransferase (CAT) and D-amino acid oxidase (DAO). In the peroxisome, d-cysteine can be catabolized by DAO to generate H₂S ^[17]. Besides the enzymatic pathway, H₂S can be nonenzymatically produced through thiosulfate, glucose, polysulfides, glutathione, and elemental sulfur.

Another source of H₂S is coming from the gut microbiota. Approximately fifty percent of fecal H₂S is derived from bacteria. In the gut, sulfate-reducing bacteria (SRB) obtain energy from the oxidation of organic compounds, reducing sulfate to H₂S. Desulfovibrio account for 66% of all SRB in the human colon ^[18]. Other gut bacteria may also produce H₂S by sulfite reduction, including species E. coli, Enterobacter, Salmonella, Klebsiella, Bacillus, Corynebacterium, Staphylococcus, and Rhodococcus ^[19]. Conversely, sulfur-oxidizing bacteria (SOB) reduces H₂S via sulfur oxidation. The SOB members include genera Acidithiobacillus, Bacillus, Paracoccus, Pseudomonas, and Xanthobacter. In the gut, a huge quantity of H₂S is oxidized by colonocytes to thiosulfate. The existence of thiosulfate in cecal venous blood not only reflects the detoxification of H₂S but also the recycling of H₂S.

In the circulation and tissues, free H₂S can be scavenged and stored in the bound-sulfate and sulfane sulfur pools. Methylation and oxidation are two major mechanisms of H₂S metabolism. H₂S can be excreted in urine and flatus as free sulfate, free sulfide or thiosulfate.

Figure 1. Schematic representation of three major sources of H₂S: enzymatic pathway, nonenzymatic pathway, and bacterial origins. Cystathionine β-synthase (CBS) catalyzes homocysteine to produce Cystathionine. Cystathionine γ-lyase (CSE) catalyzes cystathionine to form l-cysteine or l-cysteine to generate H₂S. 3-Mercaptopyruvate sulfurtransferase (3MST) produces H₂S from 3-mercaptopyruvate, which is generated by cysteine aminotransferase (CAT) and d-amino acid oxidase (DAO) from l-cysteine and d-cysteine, respectively. Another source of endogenous H₂S is coming from nonenzymatic processes. The other source of H₂S is derived from gut microbes, mainly by the sulfate-reducing bacteria (SRB).

2.2. The Role of H₂S in the Pathophysiology of CVD

Multiple lines of evidence indicate that H₂S plays a crucial role in the pathogenesis of CVD. The first are reports on knockout mice lacking genes encoding for CSE, CBS, and 3-MST. CSE is the most relevant H₂S-producing enzyme in the cardiovascular system. Mutant mice lacking CSE had decreased H₂S levels in the serum, heart, vessels, and other tissues ^[20]. CSE knockout mice displayed hypertension, endothelial dysfunction, and accelerated atherosclerosis ^[20][21]. CBS-deficient mice developed endothelial dysfunction ^[22] and cerebral vascular dysfunction ^[23]. 3-MST knockout mice developed hypertension and cardiac hypertrophy ^[17]. Second, are observations that impaired H₂S-generating pathways were found in CVDs, including atherosclerosis ^[24], coronary artery disease ^[25], stroke ^[26], and peripheral vascular disease ^[15].

Third, are studies of protein S-sulfhydration, a vital post-translational modification induced by H₂S ^[9]. S-sulfhydration usually increases the reactivity of target proteins via formation of a cysteine persulfide to target proteins ^[9]. H₂S is able to S-sulfhydrate Kelch-like ECH associated protein 1 (Keap1), specificity protein-1 (SP-1), nuclear factor kappa-B (NF-κB) and interferon regulatory factor-1 (IRF-1) to regulate target gene transcription, which is crucial for the regulation of endothelial phenotypes, myocardial hypertrophy, mitochondrial biogenesis, oxidative stress, apoptosis and inflammation ^[9].

Fourth, several H₂S-releasing drugs have demonstrated considerable promise for beneficial effects against CVDs in various animal models ^[7][8]. As reviewed elsewhere ^[7], several cytoprotective actions of H₂S have been reported in the heart and vasculature. In the heart, the protective effects of H₂S signaling was related to anti-inflammation, antiapoptosis, reduction of oxidative stress, and antifibrosis that leads to cardiac remodeling and functional improvements. In the vessels, H₂S signaling can preserve endothelial NO synthase (eNOS)-derived NO production, while reducing oxidative stress, inflammation, fibrosis, and smooth muscle cell proliferation.

2.3. H₂S Signaling in Various CVDs

Endothelial dysfunctions are associated with various CVDs, including hypertension, atherosclerosis, myocardial infarction, and the cardiovascular complications of diabetes. H₂S can prime endothelial cells toward angiogenesis and contribute to relax vascular smooth muscle cells, and thereby reducing BP ^[27]. A deficit in H₂S homeostasis is involved in the pathogenesis of endothelial dysfunction, while the application of H₂S-releasing drugs to increase endogenous H₂S level can restore endothelial function and antagonize the progression of CVDs.

Hypertension is a key risk factor for multiple CVDs. Like NO, H₂S is a vasodilator. H₂S has been reported to relax various blood vessels, such as the rat thoracic aorta, portal vein, and peripheral resistance vessels ^[28][29][30]. The involvement of H₂S deficiency in hypertension has been examined in various animal models of hypertension, including the spontaneously hypertensive rat (SHR) ^[31], the renovascular hypertensive model ^[32], Dahl salt-sensitive rats ^[33], and NO-deficient rats ^[34]. Conversely, several prior studies have shown the beneficial effects of exogenous and endogenous H₂S on hypertension, as reviewed elsewhere ^[35]. However, little is known about whether these H₂S-based therapies could be used as reprogramming interventions perinatally to reduce the vulnerability to developing cardiovascular programming in offspring.

ApoE knockout mice developed advanced atherosclerosis related to a decreased plasma H₂S level and vascular CSE expression/activity, suggesting disturbance of the vascular CSE/H2S pathway plays a role in the pathogenesis of atherosclerosis ^[36]. Additionally, a reduction in circulating H₂S has also been noted in diabetic animal models and diabetic patients ^[37]. Conversely, H2S therapy proved beneficial in diabetes-accelerated atherosclerosis in diabetic mice ^[38]. In a rat model of myocardial ischemia–reperfusion (I/R), pharmacologic inhibition of CSE resulted in an increase in infarct size, whereas H₂S replacement displayed myocardial protection ^[39]. Likewise, cardiac-specific overexpression of CSE in mice protects against myocardial I/R injury ^[40]. Summarizing, in clinical and preclinical studies of various CVDs, endogenous H₂S production is diminished in these pathological conditions and H₂S deficiency contributes to the progression of disease ^[7].