Regulation by hydrogen sulfide on thyroid cancer

Xin Lu, Hai-Yan Yu, Lei Peng, Pei-Ying Su

1Department of Oncology, Taian Tumor Hospital, Taian, Shandong, China; 2Department of Reproductive Medicine, Taian Central Hospital, Taian, Shandong, China.

Abstract To explore the role in of hydrogen sulfide (H2S) in the occurrence and development of thyroid cancer, through the literature, this paper comprehensively analyzes the physical and chemical characteristics of H2S and the composition of endogenous hydrogen sulfide enzymes, as well as the thyroid cancer pathogenesis of cancer genes and the related signal pathways, discusses its mechanisms in the development of hydrogen sulfide in thyroid carcinoma, and exogenous hydrogen sulfide donor regulation function of thyroid cancer cell growth.In order to further reveal the pathogenesis of thyroid cancer, for the development of new anti-tumor drugs to bring new clinical ideas and value.

Keywords: Hydrogen sulfide,Thyroid Cancer, Regulation

Introduction

Hydrogen sulfide (H2S) is a colorless gas characterized with a strong rotten egg smell under standard conditions of temperature and pressure.It was known only as an environmental pollutant for several centuries but is now widely considered an important biological and pharmacological mediator.As the third gaseous transmitter alongside nitric oxide (NO) and carbon monoxide (CO), H2S plays a variety of physiological regulatory roles in the health and disease of mammalian cells [1], such as dilating blood vessels [2], induces angiogenesis [3], regulates neural activity [4], improves glucose metabolism [5], protects myocardial ischemiareperfusion injury [6], etc.In the past decade, numerous studies have demonstrated the active roles of H2S in cancer biological processes [7-10].In recent years,the new concept has emerged that various cancer cells promote the production of endogenous H2S, which in turn stimulates cell proliferation, bioenergetics, and tumor angiogenesis in the autocrine and paracrine manner [11].In this review, we explore the role of H2S in thyroid cancer and its potential mechanisms, as well as its progress and potential in thyroid cancer treatment.

1.The source of endogenous H2S

In mammalian cells, there are multiple pathways for the generation of endogenous H2S.In the cytoplasm, L-cysteine (L-Cys) is used as a substrate, and cystathionine-β-synthase (CBS)and cystathionine-γ-lyase (CSE) are two kinds of synthetases that produce H2S [12].In mitochondria,3-mercaptopyruvate sulfurtransferase (3-MST) can catalyse β-mercaptopyruvic acid, as a substrate, to produce H2S.Among them, CSE and CBS catalyze the generation of H2S from L-Cys as its main route.CBS is mostly in the nervous system and liver system [13],3-MST mainly distributes in brain tissue and vascular system, and CSE primarily distributes in organs such as liver, kidney, and lung [9].

One-third of H2S in the body exists as gas, and twothirds exists in the form of sodium sulfide sodium(NaHS).NaHS can be dissociated into sodium ions and sulfhydryl ions in the body.The combination generates H2S.There is a dynamic equilibrium between H2S and NaHS [14].Endogenous H2S can be produced not only through enzymatic pathways but also provided exogenously.Exogenous H2S donors include organic sulfur compounds (OSC), inorganic sulfide salts, and synthetic compounds, such as dipropenyl sulfide (DAS),diallyl disulfide (DADS), NaHS, Na2S, GYY4137, etc.[15]

2.Thyroid cancer

Thyroid cancer is the most common endocrine malignancy, accounting for about 3% of global cancer incidence [16].Among them, the incidence of women is about 5.1%, which is significantly higher than that of men [17].According to the data from the China National Cancer Registry (NCCR), the overall incidence of thyroid cancer is 7.56 per 100,000, ranking seventh among all cancers [18].Statistics show that the incidence of thyroid cancer has increased significantly over the past few decades and will continue to grow year by year [17-19].Thyroid carcinomas are heterogeneous groups of neoplasm with typical histopathological features like other tumors.According to the source of thyroid cells, most thyroid cancers can be divided into two categories: most (＞ 95%) originate from follicular cells, and the remaining 3% to 5% are medullary thyroid carcinomas (MTC) caused by C cells.Follicular cell-derived carcinoma can be further divided into papillary thyroid cancer (PTC), follicular thyroid cancer (FTC), anaplastic thyroid cancer (ATC),and so on [19].

The pathogenesis of thyroid cancer is not completely clear, mainly due to the interaction between environmental factors and the body itself.Insufficient or excessive iodine intake, large exposure to radioactive iodine, neck radiation can cause abnormal division and canceration of thyroid cells, promote the abnormal secretion of thyroid hormones, and eventually cause thyroid nodule to become thyroid cancer [20].At the molecular level, the abnormal expression of oncogenes,the inactivation of tumor suppressor genes, and the cascade dysregulation of signaling pathways are important factors in promoting the development of thyroid cancer.

The genetic changes in different types of thyroid cancer are slightly different.For example, BRAF, RET,RAS, and TRK mutations are mainly manifested in PTC; RAS, PTEN, PAX8/PPAR γ mutations in FTC;BRAF, RAS, TERT, and TP53 mutations in ATC.The abnormal mutations of RET and RAS genes are mainly in MTC [21].In 2014, the Cancer Genome Atlas (TCGA) reported the comprehensive genomic characteristics of PTC; 97% of PTCs have unique molecular changes, including 74% of BRAF or RAS mutations [22].In addition to changes in thyroid cancer driver genes, genetic abnormalities in specific signaling pathways are also a key factor in the development of thyroid tumors.Several important cellular signaling pathways are MAPK, P13K-AKT and RAS/RAF/MEK/ERK pathway.

The MAPK signaling pathway employs a series of protein kinases to transmit signals from the cell membrane to the nucleus to control several cellular processes such as proliferation, differentiation,migration, invasion, and apoptosis.According to cBioPortal data from the TCGA PTC cohort, a change in the MAPK pathway is detected in 83% of all tested PTCs [25], and its activation is mainly related to BRAF genetic mutation [23].The P13K-AKT pathway is a critical molecular signaling pathway involved in the key cellular processes; the oncogenic variants of RAS are likely more dependent on P13K activation to initiate tumorigenesis.It is found that 93% of FTC and 96% of ATC underlie the protein-coding gene mutation of the P13K/AKT signaling pathway [24].The RAS/RAF/MEK/ERK pathway includes three bispecific protein kinases RAF, MEK, ERK, and G-protein RAS[25], all of which play vital roles in the occurrence and the development of thyroid cancer.Studies have shown that the RAS/RAF/MEK/ERK pathway is one of the most active oncogenic signaling pathways in thyroid cancer [26].The changes in thyroid cell genes, signaling pathways, and related dysfunction of molecules promote the occurrence, the development,the invasion, and the metastasis of thyroid cancer.Targeting gene mutations and signal pathways to find effective targets can be used to explore more effective treatments for refractory thyroid cancer program.

3.Regulation of H2S on the growth of thyroid cancer cells

Pieces of evidence show that endogenous H2S or relatively low level of exogenous H2S may exhibit a procancer effect.At the same time, exposure to H2S at a higher amount or for a long period may lead to cancer cell death.Owing to the more recent researches in cancer, H2S exhibits a bell-shaped (often also termed'biphasic', 'bimodal' or 'Janus-faced') pharmacological character [27].At low concentrations, H2S shows cytoprotective effects, including anti-apoptosis,anti-necrosis, and cell proliferation effects.At high concentrations, H2S is characterized as presenting cytotoxicity effects, such as stimulating apoptosis and cell death.This indicates that inhibition of H2S biosynthesis and H2S supplementation serve as two distinct ways for cancer treatment.H2S can regulate tumor cell proliferation and apoptosis through various complex pathways [12], participate in the development of several human malignancies, including colon,ovarian, bladder, renal cell, breast, liver, lung, and oral squamous cell carcinomas [28-34].

3.1.Expression of H2S synthase in thyroid cancer

As mentioned above, the synthesis of H2S depends on three critical synthesizing enzymes, including CBS, CSE, and 3-MST.Higher CBS level correlates with increased clinical aggressiveness in several different malignancies [25, 26, 29-31].Researches on the regulation by H2S on the growth and mechanism of human thyroid cancer cells are increasing.The expression of CBS is examined in benign thyroid tissue,thyroid oncocytomas, thyroid follicular adenomas, and various types of thyroid cancer including follicular,papillary, anaplastic, and medullary thyroid carcinomas[35].Compared with benign thyroid tissue, CBS expression increases in all types of thyroid carcinomas,but not in thyroid follicular adenoma or oncocytomas.It may be useful in differentiating follicular adenomas from follicular carcinomas [35].

The expression of H2S synthetase in PTC tissue has been detected by IHC [36].It is found that CSE is over-expressed in PTC tissues, while CBS and 3-MST are slightly expressed.Hedgehog (Hh) signaling pathway components are also widely expressed.It is suggested that high levels of CSE expression and activation of Hh signaling pathway are involved in the occurrence and the progression of PTC.The CBS expression level is different in each study, which is probably due to the inconsistency in the determination of immunohistochemical results.Detection of mRNA levels by combining PCR can get better, and accurate results.

The expression of 3-MST in thyroid cancer is less studied.In the study of colon cancer [37], lung cancer [38], and urothelial cancer [39-41], the 3-MST expression is up-regulated compared with surrounding healthy tissues.According to in vitro studies, the upregulation of 3-MST is particularly significant in cancer cells that recover from oxidative damage and/or develop a multidrug-resistant phenotype [42].Emerging data on newly discovered 3-MST pharmacological inhibitors, as well as data on the 3-MST silencing method, suggest that the 3-MST/H2S system plays a role in maintaining cancer cell proliferation; and may also regulate biological energy and cellular signaling functions [7].

The regulatory effect of CBS on tumors is mainly achieved through the forms of autocrine and paracrine.By regulating a series of autocrine mechanisms such as bioenergetics, antioxidant capacity, and apoptosisrelated pathways, oncogenes are induced.Tumor microenvironment can be regulated through paracrine mechanisms, such as stimulating angiogenesis and expansion, and regulating macrophage activation[11].CSE also plays a vital role in tumorigenesis and metastasis.In breast cancer studies, CSE actively regulates VEGF expression and increases the levels of specific critical proteins in the VEGF pathway,including the P13K-AKT pathway [43].

In addition to the increased expression of CBS in thyroid cancer cells, thyroid hormones (TH) also plays a role in the regulation of H2S biosynthesis.TH directly regulates cystathionine g-lyase (CGL) and CBS gene expression via TH receptor b (TRb1) as a transcriptional repressor, and H2S negatively regulates circulating TH levels [44].In the hyperthyroid rats, H2S levels increase in the liver and decrease in serum and muscles [45].It is confirmed that the expression of H2S in thyroid cancer and hyperthyroidism is inconsistent.

3.2.Effects of exogenous H2S donors on thyroid cancer cells

The effect of H2S on cancer cells depends on the dose of H2S, which is called a bell-shaped.H2S can promote the proliferation of colon cancer cell lines in the range of 10-50 μmol, and the effect reaches a plateau at a concentration of 200 μmol.In the high concentration range close to 1000 μmol, H2S can significantly inhibit the proliferation of colon cancer cells [46].Similar changes in concentration are also found in thyroid cancer cells [47-49].The proliferation, viability,migration, and invasion of human thyroid carcinoma cells are enhanced by 25-50 μmol NaHS (H2S donor) and inhibited by 200 μmol NaHS.However,H2S showes no obvious effects on the proliferation,viability, and migration of human normal thyroid cells.The mechanism of NaHS on thyroid cancer cells is mainly through a series of signaling pathways.It is found that 200 μmol NaHS can reduce the expression levels of ROS, PI3K, AKT and mTOR proteins, and regulate the growth of human thyroid cancer cells through the ROS/PI3K/AKT/mTOR signaling pathway;and also regulates the proliferation, survival, migration,and invasion of human thyroid cancer cells through the RAS/RAF/MEK/ERK signaling pathway [47].

In addition to NaSH, other H2S donors are also being studied.Diallyl trisulfide (DATS) is an H2S donor extracted from garlic.DATS and two other H2S donors,NaHS and GYY4137, significantly inhibit the growth of papillary thyroid carcinoma KTC-1 cells.The main mechanism is that DATS can increase the expression of cystathionine-γ-lyase (CSE, also known as CTH),and then catalyze the production of higher levels of endogenous H2S.DATS releases H2S and activates the NF-κB signaling pathway, while NF-κB acts as a transcription factor, binds to the CSE promoter and increases CSE transcription.In turn, CSE further promotes the generation of H2S.This positive feedback loop exists between H2S and CSE and helps DATS induce PTC cell death [48].

DATS exerted an apoptosis-inducing effect on thyroid papillary cancer cells, mainly by activating the MAPK signaling pathway [49].DAS can also inhibit cancer cell proliferation of both papillary thyroid cancer and anaplastic thyroid cancer, and induce apoptosis through mitochondrial signaling pathways [50].With a deeper understanding of the molecular mechanisms regulating H2S system in thyroid cancer, as well as the availability of novel H2S-based antitumor agents with high specificity targeting H2S or H2S producing enzymes,the possibility of applying H2S for clinical therapy becomes one step closer.These findings may shed light on the development of novel H2S-based antitumor agents.

4.Conclusion

As a gas signal molecule with a wide range of functions, H2S has set off a wave of research in various disease fields.In the area of thyroid cancer, current research has identified a biphasic model of H2S that promotes tumor progression at low concentrations and inhibits tumor progression at high concentrations.In view of this action characteristic, H2S release agents,H2S synthetase inhibitors, have been widely studied as new anti-cancer drugs.Further research based on the relationship between H2S and thyroid cancer will bring new ideas for further revealing the mechanism of thyroid cancer and the development of new anti-tumor drugs, which will generate substantial potential clinical value.