Role of Hydrogen Sulfide in the Endocrine System

Hydrogen sulfide (H2S), as one of the three known gaseous signal transduction molecules in organisms, has attracted a surging amount of attention. H2S is involved in a variety of physiological and pathological processes in the body, such as dilating blood vessels (regulating blood pressure), protecting tissue from ischemia-reperfusion injury, anti-inflammation, carcinogenesis, or inhibition of cancer, as well as acting on the hypothalamus and pancreas to regulate hormonal metabolism. The change of H2S concentration is related to a variety of endocrine disorders, and the change of hormone concentration also affects the synthesis of H2S. Understanding the effect of biosynthesis and the concentration of H2S on the endocrine system is useful to develop drugs for the treatment of hypertension, diabetes, and other diseases.


INTRODUCTION
It was not until the 1990s when endogenous and relatively high concentrations of hydrogen sulfide (H 2 S) were found in the brains of mice, humans, and cattle (1)(2)(3), that people's inherent perception of H 2 S as a toxic gas changed. Exogenous H 2 S is a strong neurotoxin with a sturdy stimulating effect on mucosa. Inhalation of a small amount of high concentration of H 2 S can be fatal in a short time. A low concentration of H 2 S can affect the eyes, respiratory system, and central nervous system. With the progress of the research, more and more functions of endogenous H 2 S have been clarified, including vasodilation and angiogenesis (4,5), cell death (6), intracellular signal transduction (7), inflammatory regulation (8), and mitochondrial energetics (9). Abnormal synthesis and decomposition of H 2 S can lead to a variety of diseases, including but not limited to hypertension, obesity, diabetes, arteriosclerosis, and cancer.
The endocrine syndrome occurs when the secretion and/or structure of the endocrine gland or endocrine tissue itself is abnormal. It also includes the syndromes of abnormal hormone sources, abnormal hormone receptors, and physiological disorders caused by the abnormal metabolism of hormones or substances. In the endocrine system, H 2 S can act on the thyroid (10), adrenal gland (11), and gonad (12) through the hypothalamus-pituitary axis, as well as on the pancreas (13), thereby participating in the regulation of many hormones in the body, and the hypothalamuspituitary axis can, in turn, regulate the production of H 2 S (14).
The purpose of this paper is to discuss the effect of H 2 S on the endocrine system, hoping to have a better understanding of this new gas signal transduction molecule.

SYNTHESIS AND METABOLISM OF H 2 S IN THE ENDOCRINE SYSTEM
At present, three main known enzymes produce H 2 S in organisms: cystathionine b-synthase (CBS), cystathionine g-lyase (CSE), and 3mercaptopyruvate sulfurtransferase (3-MST) (15). The substrate of CBS and CSE is L-cysteine (16), while 3-MST can catalyze 3mercaptopyruvate to produce H 2 S (17,18). Recently, it has been discovered that a kind of human methanethiol oxidase-selenium binding protein 1 (SELENBP1) can convert methanethiol into H 2 0 2 , formaldehyde, and H 2 S. In adipocytes, H 2 S can be produced by SELENBP1 and is related to the expression of CBS, CSE, and 3-MST (19)(20)(21). But the amount of the enzyme that produces H 2 S varies in different tissues and organs. In endocrine glands and endocrine organs, the RNA and protein expression levels of CBS are the highest in the pancreas, especially in acinar cells (22). Moreover, the CBS is rarely distributed in other endocrine glands such as thyroid, parathyroid, adrenal gland, and pituitary gland (23); however, abnormally increased in thyroid carcinoma (10). The expression of CSE is the most abundant in the liver, but low in the thyroid, pancreas, testis, ovary, and other endocrine glands (24)(25)(26).3-MST is highly expressed in endocrine tissues (thyroid, parathyroid, adrenal), pancreas, gonad (testis, ovary) (27). H 2 S can affect the secretion of many hormones and participate in the occurrence and development of endocrine diseases (11), but the effect of H 2 S on the body may be biphasic (15), that is, the effect of too high or too low concentration is the opposite, so to ensure the normal physiological function of the body, it is necessary to maintain the concentration of H 2 S at an appropriate concentration. It is well known that H 2 S can be regulated in two ways: synthesis and consumption. In terms of synthesis, H 2 S is synthesized mainly through enzymatic and nonenzymatic pathways (such as reduction of sulfur-containing compounds) and, in a few cases, released by bound sulfur stored in cells (28). In the endocrine system, H 2 S is mainly produced by three enzymes that catalyze different substrates. In terms of consumption, H 2 S can remove protons from mitochondria and rapidly oxidize to thiosulfate, which is then converted to sulfite and eventually oxidized to sulfate (28). H 2 S can also accept the methyl of thiol-S-methyltransferase to constitute dimethylsulfide and methanethiol. Or metabolized by thiohemoglobin, a complex formed by H 2 S and methemoglobin (29). The half-life of H 2 S in vivo is very short (a few seconds to a few minutes) (30,31). Nowadays, it mostly inhibits the activity of synthase or increases the donor in vitro, which brings great difficulties to the study of H 2 S.

BIOLOGICAL EFFECTS OF H 2 S IN THE ENDOCRINE SYSTEM
H 2 S can protect endocrine organs and regulate hormone secretion through anti-oxidative stress and ion channel regulation.

Antioxidant Stress
H 2 S protects cells from oxidative stress in two ways: (1) directly scavenging reactive oxygen species (ROS) and (2) up-regulating antioxidant defense. Excessive ROS can cause oxidative stress, resulting in DNA damage, protein misfolding, organelle lesion, and neuronal synaptic dysfunction (32). The main substances that produce ROS in cells are peroxide and nitric oxide (NO) (33). In the heart, H 2 S reduces lipid peroxidation by scavenging peroxides; directly scavenges nitrite peroxides in neuroblasts, and binds to NO-free radicals to reduce oxidative stress (34)(35)(36). H 2 S up-regulates antioxidant defense mainly by activating the nuclear-factor-E2-related factor-2 (Nrf2) pathway and increasing the amount of glutathione (GSH). Nrf2 is an antioxidant regulator which regulates the gene expression of a variety of enzymes used to reduce oxidative stress (37). GSH is the main substance of antioxidant stress in cells, and H 2 S can distribute GSH into mitochondria. The ROS produced in mitochondria can be scavenged by GSH (38).

Regulated Ion Channels
K ATP channel is the first ion channel target of H 2 S (4), and many reported physiological effects of H 2 S are mediated by this channel: (1) vasodilation induced by H 2 S depends on its opening of K ATP channel in vascular smooth muscle cells; (2) H 2 S opens K ATP channel in rat atrial and ventricular myocytes; (3) electrophysiological studies have shown that NaHS increases K ATP channel currents in rat aortic and smooth muscle cells. (4) the opening of the K ATP channel also supports the relaxing effect of H 2 S on colonic and ocular smooth muscle (39)(40)(41)(42)(43)(44). The Ca 2+ channel is another ion channel regulated by H 2 S, which is elucidated in many types of cells, including nerve cells, cardiomyocytes, and endothelial cells. It is involved in cardiac contraction, angiogenesis, inflammation, and sensory transmission, and its effect may be related to the source of Ca 2+ (extracellular or endoplasmic reticulum) (45)(46)(47). We will discuss the effects of the K ATP channel and Ca 2+ channel on insulin secretion below.

THE MECHANISM OF H 2 S PARTICIPATING IN THE PHYSIOLOGICAL PROCESS OF THE ENDOCRINE SYSTEM
H 2 S plays a role in the endocrine system to (1) protect pancreatic cells, regulate insulin secretion, (2) maintain the function of the adrenal cortex, promote the release of catecholamine, (3) maintain reproduction, and (4) regulate the secretion of pituitary hormones and are regulated by hormones ( Figure 1).

H 2 S in the Pancreas
H 2 S in the pancreas was first found in mice, and CBS and CSE were subsequently detected in these tissues (48,49). The main roles of H 2 S in the pancreas are protecting pancreatic b cells and regulating insulin secretion.H 2 S may protect pancreatic b cells in the following three ways: (1) reduce the production of ROS; (2) inhibit the expression of thioredoxin binding protein-2-a redox protein associated with diabetes that promotes apoptosis; and (3) increase the content of GSH, all of which reduce the damage of oxidative stress (50)(51)(52)(53). On the contrary, a high concentration of H 2 S induces apoptosis of pancreatic b cells (54). Insulin secretion is affected by many factors. It is known that the concentration and oscillation of Ca 2+ , K ATP channel is related to H 2 S. H 2 S can not only inhibit the entry of Ca 2+ from the plasma membrane into cells, to reduce insulin secretion, but also promote the release of Ca 2+ in mitochondria and increase insulin secretion (55). It is well known that a high concentration of glucose can promote insulin secretion. In rat insulinoma cell line INS-1E, a high concentration of glucose (16mM) not only inhibits the activity of the K ATP channel but also reduces the concentration of H 2 S, meanwhile H 2 S supplementation can activate the K ATP channel (56). Since the activity of the K ATP channel is also negatively correlated with insulin secretion (57). Therefore, it is unequivocal H 2 S can induce its effects on insulin secretion by utilizing this channel.

The Role of H 2 S in the Adrenal Gland
In the adrenal gland, H 2 S can act on the adrenal cortex and chromaffin cells, respectively. For the adrenal cortex, inhibition of CBS/CSE can result in mitochondrial oxidative stress and dysfunction, while corticosterone response to the adrenocorticotropic hormone is weakened, which is reversed by morpholin-4-iummethoxyphenyl-morpholino-phosphinodithioate (GYY4137, an H 2 S donor). Lipopolysaccharide (LPS) can lead to adrenal dysfunction induced by mitochondrial injury, and LPS inhibits the expression of CBS/CSE and the production of H 2 S in the adrenal gland, subsequent addition of GYY4137 inhibits apoptosis induced by LPS (58,59). Calcium signaling plays a crucial role in the release of catecholamines (60). In mammals (mice and rats), the expression of CSE in chromaffin cells increased the oxygen sensitivity of the carotid body (61). Moreover, it has been proven that H 2 S can inhibit the depolarization of the membrane caused by IK + Ca and induce the release of catecholamines in rat chromaffin cells (62). In calve chromaffin cells, the basal Ca 2+ level increased in the presence of NaHS (H 2 S donor), thus promoting exocytosis (63). In fish (rainbow trout), electrical stimulation and acute hypoxia-induced increased secretion of catecholamines and H 2 S, followed by the addition of amino-oxyacetic acid (AOAA, CBS inhibitor), which significantly inhibited the production of H 2 S, while the effect of DL-Propargylglycine (PAG, CSE inhibitor) was not obvious (64).

The Function of H 2 S in Gonads
The gonads mainly cover the testicles of men and the ovaries of women. Testes can secrete the male hormone testosterone, its main function is to promote the development of gonad and its accessory structure and the appearance of parasexual characteristics, as well as to promote protein synthesis (65). The expression of CBS and CSE are found in rat testes, but they are differentially expressed; CSE is mainly expressed in Sertoli cells and immature spermatogonia, while CBS is mainly distributed in Leydig cells, Sertoli cells, and germ cells (66). Sulfides in garlic regulate the secretion of testosterone by affecting the secretion of luteinizing hormone (LH), indicating that H 2 S may play an important role in the secretion of testosterone (67). Similarly, in rat testis, the provision of NaHS protected the testis from oxidative stress and inflammation induced by cisplatin and increased the activity of antioxidant enzymes (68). Subsequently, it has been found that the expression of CBS decreases in the spermatozoa of human asthenospermic patients, and AOAA inhibited the sperm motility, then the addition of GYY4137 weakens the effect of AOAA, suggesting that H 2 S could promote the production and activity of spermatozoa (69).
The decreased fertility of female offspring of CBS knockout mice suggests that H 2 S may have a greater effect on female fertility (70). In different mammals, H 2 S regulates female reproduction in many ways such as; the LH peak before ovulation in mice increases with the expression of H 2 S producing enzymes, the inhibition of CSE by hydroxylamine hydrochloride prevents ovulation, whereas, the subsequent addition of NaHS reverses the effect of the inhibitor, Na 2 S, an H 2 S donor, increases the plasma level of oxytocin, vasopressin, and corticosterone in rats; CBS increased the level of H 2 S in the uterine artery during sheep production, and the change of CBS protein expression was consistent with the estrous cycle and follicular estrogen level of sheep, so as the estrogen level of the human menstrual cycle (12,71,72).

Interaction Between H 2 S and Hypothalamus-Pituitary Axis
Abundant CSE and CBS were found in the hypothalamus (73). In the paraventricular nucleus (PVN), the high expression of CBS upregulates the level of pre-thyrotropin-releasing hormone (pre-TRH) and decreases the content of corticotropin and corticosterone in blood (74). Interestingly, in hypothalamic explants, NaHS did not affect the physiological secretion of corticotropin-releasing hormone (CRH) but could inhibit the release of CRH stimulated by KCl (75). The results show that the hormones produced by the hypothalamus-pituitary axis also affect the synthesis of H 2 S. Thyroid hormone (TH) and growth hormone (GH) regulates the production of H 2 S in the liver through TH receptor b1 and GH receptor, respectively. In mechanism, TH inhibits the expression of CSE, while GH inhibits the production of H 2 S through substrate availability control by autophagy (14). However, another study showed that in hyperthyroidism rats, the level of H 2 S in the liver increased (76), which may be due to the different effects of different concentrations of TH on the H 2 S producing enzyme.

ENDOCRINE DISEASES INVOLVED IN H 2 S
H 2 S can regulate glucose and fat metabolism through the pancreas, liver, adipose tissue, and skeletal muscle. In human thyroid and ovarian cancer, H 2 S appears to have a dual effect, which may be related to its concentration (or metabolic rate) in the cells. High concentrations of H 2 S can aggravate pancreatitis and induced lung injury, but protect osteoblasts, making it a potential drug for the prevention of osteoporosis.

H 2 S and Type 2 Diabetes
Pancreas, liver, adipose tissue, and skeletal muscle are involved in the regulation of plasma glucose, the enzyme-producing H 2 S is expressed in all the above tissues (48,(77)(78)(79). H 2 S can manage plasma glucose levels by protecting pancreatic b cells and regulating insulin secretion (see section 4.1). H 2 S also seems to have the opposite effect on liver glucose uptake, glycogen storage, and gluconeogenesis. One point of view is that H 2 S promotes liver glycogenesis: in hepatocellular carcinoma cell line HepG2, NaHS inhibits the activity of glucokinase, and CSE overexpression reduces glycogen content, while CSE knockout increases glycogen content (80,81). Another point of view is that H 2 S inhibits gluconeogenesis in the liver: the lack of CSE promotes gluconeogenesis in the liver, knockout CSE of HepG2 increases glucose production, while the addition of NaHS weakens the knockout effect of CSE (82). The role of H 2 S in regulating plasma glucose through adipose tissue also seems to be twofold. On the one hand, H 2 S increases glucose uptake and fat production in adipocytes, for example: (1) in 3T3L1 adipocytes, H 2 S improves high glucose-induced insulin resistance by up-regulating phosphatidylinositol 3,4,5trisphosphate level, (2) in diabetic insulin resistance rats, NaHS and H 2 S gas solution improve insulin resistance by activating insulin receptors (IR), (3) CSE/H 2 S converts glucose into triglycerides in adipocytes through peroxisome proliferatoractivated receptor g (PPARg) and stores them in cells, (4) H 2 S promotes adipogenesis in 3T3L1 cells by increasing the expression of fatty acid protein 4 (83)(84)(85)(86). On the other hand, H 2 S plays a vital role in adipocyte insulin resistance mediated by tumor necrosis factor-a (TNF-a) (87). H 2 S improved insulin resistance of skeletal muscle by increasing the sensitivity of the IR-PI3K-Akt signal pathway. In mouse myoblast C 2 C 12 , the addition of NaHS increased glucose uptake (84,88). The above results show that H 2 S reduces the level of plasma glucose by regulating numerous cellular markers.

Effect of H 2 S on Thyroid Cancer
The effect of H 2 S on cancer is related to its concentration, that is, low concentration (or low production rate) and high concentration (or rapid production rate) can show opposite results (15).In thyroid cancer, two different H 2 S donors showed different results. Diallyl trisulfide (DATS), an H 2 S donor from garlic, has an inhibitory effect on both anaplastic and papillary thyroid cancer. In anaplastic cancer cell line 8505C, DATS induces the accumulation of ROS, which inhibits cell survival and increases apoptosis. In addition, it promotes cell DNA damage and arrests the cell cycle in the G2/M phase (89). In papillary carcinoma cell line KTC-1, treatment with diallyl trisulfide (DATS) activates NK-kB signaling pathway and increases the expression of NF-kB-dependent CSE, while H 2 S induced cell growth inhibition (90). While in another thyroid papillary carcinoma cell line BCPAP, DATS blocks the cell cycle in G0/G1 phase and promotes apoptosis of thyroid cancer cells by inducing mitochondrial apoptosis and activating the MAPK signal pathway (91). When NaHS is used as an H 2 S donor, a low concentration (≤50uM) of NaHS promotes the proliferation, migration, and invasion of thyroid cancer cells in vitro, while a high concentration (200uM) has the opposite result. In BALB/C nude mice with axillary tumorigenesis, the tumor mass increases, the doubling time shortens and the inhibition rate decreases in low concentration (≤2.8mg/kg/day) of NaHS, while in high concentration (≥5.6 mg/kg/day) the effect is reversed (92).

H 2 S Promotes Acute Pancreatitis and Its Induced Lung Injury
In rats and mice with acute pancreatitis (AP), it has been found that the concentration of serum H 2 S is increased and the expressions of CSE and TNF-a are also increased in the lungs. The pro-inflammatory effect of H 2 S may be related to the release of substance P (SP) and the activation of trypsinogen, while the release of SP is mediated by neurokinin-1 receptor (NK-1R) in pancreatic acini (48,93,94). In addition, the number of autophagosomes and autophagy vacuoles increases in AP, which is positively correlated with the level of H 2 S, which associates with the activation of the AMPK/mTOR signal pathway (95). The addition of PAG decreases the activity of peroxidase in the lung and pancreas, the expression of TNF-a, CSE, and NK-1R, and the concentration of H 2 S in serum (93,94,96). Diallyl disulfide in garlic can also trigger an antiinflammatory effect in the pancreas and lung by reducing the expression of TNF-a, CSE, NK-1R, and the production of H 2 S (97). To sum up, H 2 S can aggravate AP and its induced lung injury in many ways, and inhibiting the production of H 2 S may be a potential method for the treatment of AP (Figure 2).

H 2 S and Obesity
H 2 S can increase or decrease fat synthesis in many ways. It is described previously by regulating insulin sensitivity in adipocytes. The expression of H 2 S in different tissues can jointly regulate the formation of fat. Overexpression of CBS in PVN reduces obesity and insulin resistance induced by a high-fat diet (HFD). Mechanistically, leptin up-regulates CBS expression through FOXO3a, promotes pre-TRH production by inhibiting the mTOR pathway, and then increases TH production and decreases corticosterone production, thus reducing fat deposition and improving insulin resistance induced by FHD (74). However, in mouse adipocytes, H 2 S promotes adipogenesis by activating PPARg, where when CSE is deficient, mice show resistance to FDH-induced obesity (98). Perivascular adipose tissue provides protection and support for blood vessels. In obese patients, the secretion of pro-inflammatory macrophages increases, resulting in increased consumption of H 2 S in vascular endothelium and smooth muscle, which directly damages endothelium-dependent vasodilation, and results in a loss of pulmonary vein function (99).

Uncertainty of the Effect of H 2 S on Ovarian Cancer
The expression of CBS is increased in ovarian cancer cell line A2780. After down-regulation of CBS in ovarian cancer cell line A2780, the content of GSH decreased, which aggravated the cascade of apoptosis triggered by oxidative stress and enhanced the sensitivity to cisplatin. In addition, mitochondrial function is inhibited and ATP synthesis decreases after down-regulation of CBS (100). Moreover, selenium-containing chrysin, as a compound with anticancer and antioxidant effects, shows an antitumor effect in many ovarian cancer cell lines, and its inhibitory effect is achieved by inhibiting the expression of CBS and promoting the transformation of GSH (101). However, another data shows that GYY4137 promotes the release of Ca 2+ from the endoplasmic reticulum of A2780, induces endoplasmic reticulum stress, and drives apoptosis, while the content of GSH is decreased (102).

H 2 S Inhibits the Development of Osteoporosis
Osteoporosis occurs in postmenopausal women and can also occur in patients with specific hormone secretion disorders or patients treated with glucocorticoid drugs. Endogenous H 2 S protects osteoblasts MC3T3-E1 from cytotoxicity induced by ROS. NaHS safeguards osteoblasts from oxidative stress-induced cell injury and inhibition of proliferation and differentiation through the MAPK signal pathway (103). Therefore, H 2 S may be used as a drug to prevent osteoporosis.

CONCLUSIONS
The hypothalamic-pituitary-target organ axis is a complex biological structure, which plays an important role in endocrine regulation. H 2 S is distributed in multiple sites in the hypothalamus, pituitary-target organ, making it an important molecule in regulating hormone secretion, which is not only dual but also bi-directional, that is, H 2 S not only promotes the secretion of certain hormones but can also inhibit them. The complex effect of H 2 S on the endocrine system may be caused by its action on different organs. Current studies mainly focus on the effect of H 2 S on a single organ, the role of feedback regulation is unclear, which brings a difficult problem to explore the mechanism of action of H 2 S in the endocrine system.
There's a growing body of evidence that H 2 S plays an important role in type 2 diabetes, but its effect on insulin secretion and insulin target organs has not been consensus, which may be related to its concentration and production rate in the target organs. Similarly, H 2 S is also involved in the occurrence and development of endocrine organ tumors, and its role in thyroid cancer is related to its concentration. Since the exact concentration of H 2 S in cells is difficult to detect, the effect of a specific concentration of H 2 S on endocrine organs remains to be further explained.
Although there are still many problems to be solved, existing studies have shown that H 2 S plays a key role in maintaining the function of endocrine organs and hormone secretion. Therefore, the regulation of H 2 S production may be a potential treatment for endocrine diseases.