S‐adenosylmethionine: A metabolite critical to the regulation of autophagy

Abstract Autophagy is a mechanism that enables cells to maintain cellular homeostasis by removing damaged materials and mobilizing energy reserves in conditions of starvation. Although nutrient availability strongly impacts the process of autophagy, the specific metabolites that regulate autophagic responses have not yet been determined. Recent results indicate that S‐adenosylmethionine (SAM) represents a critical inhibitor of methionine starvation–induced autophagy. SAM is primarily involved in four key metabolic pathways: transmethylation, transsulphuration, polyamine synthesis and 5′‐deoxyadenosyl 5′‐radical–mediated biochemical transformations. SAM is the sole methyl group donor involved in the methylation of DNA, RNA and histones, modulating the autophagic process by mediating epigenetic effects. Moreover, the metabolites of SAM, such as homocysteine, glutathione, decarboxylated SAM and spermidine, also exert important influences on the regulation of autophagy. From our perspective, nuclear‐cytosolic SAM is a conserved metabolic inhibitor that connects cellular metabolic status and the regulation of autophagy. In the future, SAM might be a new target of autophagy regulators and be widely used in the treatment of various diseases.

modifications, including acetylation 14 and methylation, 15 of proteins may also impact autophagy by targeting these pathways, at least in part. These modifications are different from kinase-mediated modulation. The methylation status of multiple regulatory factors and core components of the autophagy mechanism undergo changes, and the methylation of promoter CpG islands on DNA molecules or histone lysine residues is related to different aspects of autophagy. 16 S-adenosylmethionine (SAM) is a significant biological sulphonium compound that participates in a variety of biochemical processes. SAM is biosynthesized through the reaction of methionine with ATP that is catalysed by SAM synthetase or methionine adenosyltransferase (MAT). The reaction mainly occurs in cytosolic pools via the one-carbon metabolism pathway that encompasses both the folate and methionine cycles. 17 Several amino acids, namely serine, glycine and threonine, can initiate the folate cycle to facilitate the generation of SAM in the methionine cycle. SAM is considered to be the main methyl donor reagent for significant methylation reactions that occur in all living organisms, which are essential in cell differentiation and survival by regulating key metabolic pathways, including methylation and polyamine synthesis. Recent studies have shown that transcriptional regulation and epigenetic regulation are critical for the autophagic process. 18 Mutations in core enzymes associated with one-carbon metabolism and SAM are observed in cancer and are accompanied by aberrant methylation states. 19 SAM is an important metabolite and can act as a nutrition, energy and stress sensor in vivo and in vitro, thus regulating autophagy. Moreover, SAM is involved in transmethylation, transsulphuration and polyamine synthesis and participates in the regulation of autophagy through different mechanisms. Thus, given that the autophagy response is influenced by the metabolic state of a cell, we concluded that the SAM level is a key regulator of autophagy and that regulating the SAM level can be a new therapeutic strategy for disease treatment.

| Biosynthesis of SAM
S-adenosylmethionine can be synthesized in the cytoplasm of every cell, especially hepatocytes. 20 In liver, SAM synthesis is mainly derived from methionine, which accounts for half of the daily intake of amino acids. 21,22 Methionine is converted into SAM by MAT in an ATP-dependent process ( Figure 1). 21 During this process, methionine is transformed to carry a sulphonium ion as a high-energy reagent by combining with the adenosyl moiety of ATP; then, the activator transfers its methyl group to various substrates, including proteins, DNA, RNA and lipids. 23 Methylation is an important protein post-translational modification that not only regulates target gene transcription and expression but also controls the activity of various signalling pathways, including autophagy initiation pathways. 15,24 S-adenosylhomocysteine (SAH) is the by-product of the methyltransferase reaction and is transformed into homocysteine. Then, homocysteine is a precursor involved in the synthesis of cysteine and glutathione by the transsulphuration pathway. 5,10-Methylenetetrahydrofolate (5,10-methylene-THF), which is derived from the folate cycle, can be irreversibly converted into 5-methyl-THF (5-MTHF) by methylenetetrahydrofolate reductase (MTHFR), which donates its methyl group to homocysteine in the methionine synthase (MS) reaction to produce methionine and THF, requiring vitamin B12 as a cofactor. 25 Sustaining the SAM cycle is dependent on intermediary metabolism and can be negatively affected by a diet lacking several essential nutrients, for example methionine, folate, vitamin B12 and vitamin B6. Impaired dietary supply, absorption, distribution, metabolism or enzymatic processing of these nutrients negatively impacts the cellular level of SAM. [26][27][28][29] Methionine restriction, for instance, results in a decrease in SAM level and extends the lifespan of different species by modulating autophagy. 30

| Degradation of SAM
S-adenosylmethionine acts as a link between four key metabolic pathways, transmethylation, transsulphuration, polyamine synthesis and 5′-deoxyadenosine 5′-radical-mediated biochemical transformation ( Figure 1), influencing the regulation of autophagy. In the following sections, the role of SAM in each of these four metabolic pathways will be described.

| Transmethylation
The most prominent metabolic function of SAM is its role as a methyl group donor in a variety of reactions catalysed by enzymes from the methyltransferase family. 31 Indeed, more than 90% of the formed SAM molecules are consumed to sustain methylation reactions.
In humans, most SAM is generated in liver, where more than half of the methionine from daily intake is metabolized and more than 85% of all methylation reactions occur. In transmethylation, SAM donates its methyl group to a large variety of receptor molecules.
The reactions are catalysed by methyltransferases. In the human genome, more than 200 SAM-dependent methyltransferases have been identified. 32 Moreover, accumulating evidence has shown that several methyltransferases and demethylases are involved in regulating autophagy, such as H3K27 trimethylation by enhancer of zeste homolog 2 (EZH2). 33 H3K9 dimethylation by G9a 34 and non-histone protein methylation are also associated with autophagy regulation. 15 After delivery of the methyl group to the recipient compound, SAM is converted to SAH. In addition, it was confirmed that elevated levels of SAH and decreased levels of SAM inhibit transmethylation. 20,21 The ratio of SAM to SAH is generally considered to be a metabolic indicator that controls methylation in vivo, and a decrease in this ratio indicates a reduction in methylation ability. SAH is further converted to homocysteine (HCY), which is catalysed by SAH hydrolase (AHCY). 21 It has been proven that HCY modulates autophagy in different cell types. However, the regulatory effect of HCY on autophagy has opposite effects (upregulation and downregulation) in different cell types and biological contexts. 35 Then, HCY enters two metabolic pathways, remethylation to regenerate methionine or the transsulphuration pathway to generate cysteine and α-ketobutyrate.
Two enzymes are needed for the remethylation of HCY to form methionine, MS, which requires folate and vitamin B12, and betaine Hcy methyltransferase (BHMT), which requires betaine as a cofactor, which is a metabolite of choline.

| Transsulphuration
The transsulphuration pathway is another crucial catabolism process that links SAM to cysteine biosynthesis. Cysteine is generated from HCY via the transsulphuration reaction, and this process principally occurs in liver and lens. 36 First, HCY is transformed into cystathionine by condensation with homoserine, and it is then converted to cysteine and α-ketobutyrate. The above reactions are catalysed by cystathionine β-synthase (CBS) and cystathionase (CSE), respectively, and both require vitamin B6 as a cofactor. Cysteine is converted to various sulphur-containing molecules, including glutathione (GSH), taurine, sulphate (SO 4 ) and hydrogen sulphide (H 2 S), whereas α-ketobutyrate is converted to succinyl-CoA, and this reaction mainly occurs in the mitochondria. SAM is a significant precursor of GSH, and the transsulphuration pathway is especially active in liver. 37 GSH protects against oxidative damage in many tissues, and GSH depletion may induce autophagy. 38 GSH redox homeostasis may be central to the proteostasis maintenance realized through autophagic regulation. 39 In liver, SAM inhibits the activity of MTHFR and activates CBS. 40,41 Therefore, an increase in SAM reduces 5-MTHF, the substrate for MS. Thus, the SAM level is the main control of the flux of sulphurcontaining molecules; when SAM is decreased, Hcy is directed to F I G U R E 1 S-adenosylmethionine (SAM) biosynthesis and metabolism. Methionine (Met) is converted into SAM by MAT in an ATP-dependent process. SAM is linked to four key metabolic pathways: transmethylation, transsulphuration, polyamine synthesis and 5′-deoxyadenosyl 5′-radical-mediated biochemical transformations. In transmethylation, SAM donates its methyl group to many substrates, including DNA, RNA and proteins, which are catalysed by specific methyltransferases (MTs). After transmethylation reactions, SAM is converted to S-adenosylhomocysteine (SAH), which is hydrolysed by a reversible enzyme called SAH hydrolase (AHCY) to form homocysteine (Hcy) and adenosine. Hcy has two fates: to be remethylated to regenerate Met or to enter the transsulphuration pathway. In transsulphuration, Hcy is first converted to cystathionine and then to cysteine (Cys) and α-ketobutyrate, catalysed by the enzyme cystathionine β-synthase (CBS). Then, Cys is converted to various sulphur-containing molecules, including glutathione (GSH), taurine, hydrogen sulphide (H 2 S) and sulphate (SO 4 ), which is catalysed by the enzyme cystathionase (CSE). Both CBS and CSE require vitamin B6 as a cofactor. α-Ketobutyrate is converted to succinyl-CoA, which is metabolized in the mitochondria. Hcy is remethylated to regenerate Met via two routes: the MS pathway and the betaine homocysteine S-methyltransferase (BHMT) pathway. In the MS pathway, 5-methyltetrahydrofolate (MTHF) donates a methyl group to Hcy, which requires folate and vitamin B12. In the BHMT route, Hcy uses betaine as a methyl donor. In polyamine synthesis, SAM is first decarboxylated to form decarboxylated SAM (dcSAM). Then, putrescine uses dcSAM as a propylamine donor, which is transformed to spermidine (SPD) and spermine (SPM), yielding 5′-methylthioadenosine (MTA) as a by-product. MTA is used to regenerate methionine through the methionine salvation pathway. In 5′-deoxyadenosyl 5′-radicalmediated biochemical transformations, SAM initiates various radical chemical reactions, which are catalysed by a large family of SAM radical enzymes. These enzymes share a CX 3 CX 2 C motif forming a characteristic [4Fe-4S] cluster. SAM is converted to [4Fe-4S]-methionine and a 5′-deoxyadenosyl 5′-radical through binding to the [4Fe-4S] cluster regenerate SAM through the remethylation pathway, and when the SAM level is increased, Hcy is directed to the transsulphuration pathway.

| Polyamine synthesis
S-adenosylmethionine also participates in the synthesis of polyamines. Spermidine (SPD) and spermine (SPM) are the primary polyamines in mammalian cells. SPD and SPM have crucial effects on senescence, immunity and cancer. [42][43][44] There is accumulating evidence showing that polyamines, including putrescine, SPD and SPM, are novel autophagy and longevity inducers. 42,45 In this pathway, SAM is decarboxylated and generates decarboxylated SAM (dcSAM), which is catalysed by the enzyme SAM decarboxylase; subsequently, the first aminopropyl group from dcSAM is added to putrescine to generate SPD, which is catalysed by SPD synthase, SPD is converted to SPM via the addition of the second aminopropyl group, which is catalysed by SPM synthase, and both the reactions yield 5′-methylthioadenosine (MTA) as a byproduct. 46,47 Then, MTA is used to regenerate methionine through a methionine salvation pathway. 48 In this pathway, MTA is first phosphorylated by MTA phosphorylase (MTAP), and then MTA is converted to adenine and 5-methylthioribose-1-phosphate, which is further metabolized to methionine. When MTAP is deficient, endogenous MTA is unable to rescue methionine or adenine. This deficiency results in impaired polyamine biosynthesis and accumulation of dcSAM and MTA, and both inhibit the methylation reactions. 49-51

| SAM AND AUTOPHAGY
Autophagy involves the lysosomal degradation pathway, which removes damaged and potentially harmful substances and replenishes energy reserves under starvation conditions to maintain cellular homeostasis. Although the regulation of autophagy is strongly dependent on the availability of nutrients, there are few descriptions of the specific metabolites that regulate autophagy. Recently, SAMTOR was found to link methionine and one-carbon metabolism to mTORC1 signalling, thereby acting as a SAM sensor. 58

| Non-histone methylation
Previous studies have shown that SAM production inhibits non-ni-

| Histone methylation
S-adenosylmethionine can regulate the histone methylation state as a metabolic coenzyme, which is linked to autophagy control. G9A is a methyltransferase catalysing monomethylation and dimethylation of H3K9 (H3K9me1 and H3K9me2) in chromatin. 74,75 SAM is also necessary for the stable interaction of G9a with H3K9M histones. 59 Recent studies have demonstrated that G9A inhibition or silencing represses serine-glycine biosynthesis by reducing H3K9me1 levels, which inhibits mTORC1 kinase activity and induces cell death in association with autophagy. 76,77 Under nutrient-deficient conditions, G9A dissociates from the promoters in autophagosome genes such as LC3B, WIPI1 and DOR, with a subsequent H3K9me2 reduction at these sites, consequently resulting in transcriptional activation and autophagy stimulation. 34 The conversion of serine to glycine leads to the formation of 5,10-MTHF, a major contributor to SAM biosynthesis. Serine supplementation, but not glycine supplementation, can rescue the cell death phenotype inducted by G9A inhibition or silencing. 76 This finding indicates that G9A inhibition or silencing represses the conversion of serine to glycine, which decreases the synthesis of 5,10-MTHF and SAM. Therefore, a low level of SAM is sufficient to stimulate autophagy via G9A-mediated epigenetic regulation.

F I G U R E 2
Working model for SAM-mediated modulation of autophagy. SAM is an essential metabolite that acts as a high-energy methyl donor for most methylation modifications of non-histone, histone, DNA and RNA, which epigenetically affect autophagic flux. Moreover, SAM is the biosynthetic precursor for HCY and cysteine and other sulphur-containing metabolites such as GSH. All of these roles significantly contribute to the modulation of autophagy. In addition, the SAM metabolite SPD has also been shown to be a physiological inducer of autophagy. Collectively, the metabolites of SAM and the key enzymes in SAM biosynthesis and metabolism influence the core autophagy machinery. S-adenosylmethionine: SAM, homocysteine: HCY, glutathione: GSH, spermidine: SPD, AMP-dependent protein kinase: AMPK, mechanistic target of rapamycin: mTOR, migration inhibitory factor: MIF, cystic fibrosis transmembrane conductance regulator: CFTR, mTOR complex 1: mTORC1, protein phosphatase 2A: PP2A, nitrogen permease regulating protein: NPR2P, ten-eleven translocation methylcytosine dioxygenases: TET, N 6 -methyladenosine: RNA methylation, m 6 A RNA methylation

| DNA methylation
Among epigenetic modifications, DNA methylation is perhaps the best studied and the most common. 78 DNA methylation of cytosine and adenine residues is carried out by SAM-dependent methyltransferases. 67 Recently, a study has shown that methionine deprivation results in a rapid decrease in intracellular SAM, inducing H3K4me3 and DNA demethylation and triggering p53 signalling in human embryonic stem cells. 60 p53 is a critical component of stress signalling and adaptation, which can regulate autophagy. 79   Furthermore, HCY also transforms into cysteine and α-ketobutyrate by the transsulphuration pathway. Cysteine can be converted to various sulphur-containing molecules, such as GSH, an important cellular antioxidant and free radical scavenger. SAM treatment in patients with cirrhosis leads to recovery of liver GSH levels. 103 The response of the liver to endotoxaemia preserves hepatic SAM storage by increasing MAT gene expression and decreasing the utilization of SAM, leading to the inhibition of GSH synthesis. 104 As a result, SAM is a key mediator and precursor of GSH. Mounting evidence suggests that GSH may play a role in the control of autophagy. 63,105 GSH deficiency triggers AMPK-independent induction of autophagy. 106 GSH is an intracellular antioxidant that helps to alleviate oxidative stress in proliferating cells. Nutrient deficiencies result in a significant decrease in intracellular GSH levels in cancer cells. This phenomenon relies on ABCC1-mediated GSH degradation, as well as glutamate-cysteine ligase inhibition and to a lesser extent on the formation of GSH-protein mixed disulphides. All of these factors synergistically promote the regulation of autophagy by transforming the intracellular redox conditions to a more oxidizing state. 63 In addition, α-ketobutyrate inhibits ATP synthase in a manner that is dependent on downstream target of rapamycin (TOR) to prolong life of C elegans, which results in a decrease in ATP content and an increase in autophagy. 64 Consist of this, α-ketobutyrate reduction activates TORC1 by a Gtr1/RRAG-independent mechanism in yeast. 107,108 Interestingly, in contrast with those observations, α-ketobutyrate enhances the GTP load of RRAG proteins, thereby activating MTORC1 and inhibiting autophagy in mammalian cells. 109,110 It is worth noting that the difference in α-ketobutyrate in the modulation of autophagy and the exact molecular mechanism remain to be explored in future studies.

| RNA methylation
In conclusion, SAM, as a mediator of sulphur-containing molecules, regulates the activity of TOR, which is a key modulator of autophagy.  65 When added to culture medium, SPD also directly induces autophagy in a manner that is independent of transcription. The mechanism has not yet been well elucidated but may be a consequence of enhanced deacetylation of essential autophagy-related proteins such as ATG5 and ATG7. 42 Furthermore, consumption of intracellular polyamines by inhibition of the biosynthetic enzyme ODC with DFMO inhibits autophagy induction in response to starvation or rapamycin treatment, with a decrease in the levels of LC3 and ATG5. 66 Thus, deficiency in the biosynthesis of polyamines and cellular accumulation of dcSAM inhibits autophagy, which may regulate cell proliferation.

| THER APEUTI C S TR ATEG IE S
The connection between SAM and autophagy was described above.
As an important metabolite in the body, SAM plays an essential role in cell growth, apoptosis, death and differentiation. 116  chronic pain, 125 osteoarthritis 126 and liver support. 120 Regulating the levels of SAM in vivo could improve the prognosis and the effects of treatment. 117,120,127 Given the relationship between SAM and autophagy, new therapeutic strategies for SAM will be introduced in the following sections to provide new ideas for the treatment of diseases.

| SAM as a treatment for inflammationinduced diseases
Many experimental models and clinical trials have demonstrated the therapeutic effects of SAM in acute liver injury and liver fibrosis. 40,104,128,129 The hepatoprotective effects of SAM may be attributed to reduced hepatocytes oxidative stress and inflammatory responses through inhibiting nuclear factor-kappa B (NF-κB) nuclear translocation and activating nuclear factor erythroid 2-related factor (Nrf2). 130 Lipopolysaccharide (LPS), an inducer of endotoxaemia, inactivates hepatic MAT. 131 SAM treatment has been demonstrated to prevent the LPS-induced serum tumour necrosis factor-α (TNFα) increase in rats. 131 In addition, SAM also plays a role in preserving the GSH store, 103 which moderates LPS-induced hepatotoxicity, thus preventing liver injury. 104 GSH modulates autophagy by transforming the intracellular redox conditions to a more oxidizing state when the intracellular GSH levels in cancer cells decrease. 63 We proposed that SAM treatment promotes the production of GSH and blocks the core autophagy machinery, which may play a role in the regulation of oxidative stress in cancer cells. In addition, SAM could have anti-inflammatory and anti-fibrotic effects on allergic airway inflammation and remodelling in a murine model of chronic asthma, most likely by reducing oxidative stress and inhibiting activation of the TGF-β1 signalling pathway. 132 SAM and MTA are also effective in preventing inflammation-induced colon cancer in mice by reducing the expression of NF-κB and STAT3, both of which are essential for promoting the survival and growth of cells during colon tumorigenesis. 133 Previous studies have also demonstrated that SAM inhibits inflammation through the suppression of cytokines, such as IL-6, IL-8, IL-10, TNF-α and IFN-γ. 130,133,134 A randomized controlled trial of patients with Alzheimer's disease also demonstrated that higher plasma SAM levels are connected with a decrease in presenilin, IL-6 and TNF-α. 135 Furthermore, SAM administration has been used in patients with hepatitis-related cirrhosis 136 and osteoarthritis 137 by reducing inflammation. In summary, SAM treatment might inhibit autophagy by restoring intracellular GSH levels to reduce oxidative stress and exert an anti-inflammatory effect by suppressing the secretion of inflammatory cytokines. Because SAM is a metabolite in the body and appears to be safe for regular use, it may be an ideal candidate for chemoprevention of inflammation-induced diseases.

| Metformin enhances anti-tumour effects by regulating SAM levels
Metformin is an oral anti-hyperglycaemic agent for type 2 diabetes (T2D) that promotes health through the modulation of epigenomic metabolism. Metformin has recently been shown to promote global DNA methylation in non-cancerous, cancer-prone and metastatic cancer cells through positively modulating the activity of AHCY (the SAH hydrolase) in an AMPK-dependent manner and promoting the accumulation of SAM. 148,149 These results indicate that metformininduced DNA methylation plays a role in delaying or reversing the hallmarks of ageing or age-related diseases, such as cancer. However, in contrast to those observations in mammalian cells, metformin disrupts the folate cycle, leading to a reduction in SAM levels in microbes and slowing ageing in C elegans. 150 Moreover, metformin is also an inhibitor of the genes that participate in methionine biosynthesis transcription. It is interesting to note that limiting methionine in the diet can extend the lifespan of fruit flies and rodents. Bacteria in the human gut have a significant effect on nutrition and host biology. Metformin could also control the response of tumour therapy by modulating the tumour microenvironment, 151 particularly in immunotherapy. 152 Metformin might have potential therapeutic efficacy through attenuating microbial methionine biosynthesis and decreasing SAM levels 150,153 or affecting the composition of the intestinal microflora. 154 Furthermore, it has been reported that metformin treatment modulates the autophagy mediators becn1, atg7, and LC3 II/I, activates AMPK and represses both the mTORC1 and mTORC2 signalling pathways, thus inducing autophagy 155,156 and playing a role in T2D treatment and anti-tumour therapies. [157][158][159] Therefore, metformin might regulate the metabolism of the epigenome by changing the level of SAM, altering mammalian physiology through its effects on gut microbiota, and playing an anti-tumour role by activating autophagy. Hence, combined metformin and chemotherapy or immunotherapy can enhance anti-tumour effects.

| DFMO as a tumour chemopreventive agent
Difluoromethylornithine (DFMO), an inhibitor of ODC, antagonizes polyamine synthesis, resulting in a huge increase in dcSAM, leading to dcSAM levels that are 3-fold 4-fold greater than SAM levels. 114,115 DFMO has long been used for the treatment of sleeping sickness, since polyamines are essential for the proliferation of these protozoa. 160 However, polyamines are also essential for mammalian cell proliferation. Recently, DFMO has been shown to strongly inhibit the autophagic response by decreasing the cellular levels of polyamine and increasing dcSAM. Moreover, dcSAM inhibits methylation reactions, such as DNA methylation, 112 which may regulate cell proliferation. DFMO treatment inhibits the promotion and proliferation/progression stages of cancer both in vitro and vivo. 161 Considering that DFMO is already approved by the US. Food and Drug Administration, DFMO could be a candidate agent in cancer treatment to control host autophagy. In addition, DFMO depletes tumours of polyamines and inhibits the growth of MYC-deregulated tumours in animals. 162,163 Pre-emptive blockade of polyamine synthesis effectively prevents neuroblastoma formation in the transgenic TH-MYCN model. 162 DFMO as a cancer chemopreventive agent that has attracted much interest in recent years since ODC is transactivated by the c-myc oncogene in malignant tumours. 164 Identical findings in multiple cancer initiation models have led to a focus on DFMO as a chemopreventive agent and a treatment in combination with other therapies for use in treating diseases, including neuroblastoma, colorectal neoplasia, pancreatic cancer, and skin cancer, in a large number of clinical settings. 161,[165][166][167][168][169] In addition, polyamine depletion shapes the tumour microenvironment to change it from a tumour-permissive microenvironment by suppressing immune system effectors to a microenvironment that promotes anti-tumour immunity. 170  The most important function of SAM is to be the primary methyl donor involved in epigenetic regulation. In addition, SAM is also a precursor of (a) GSH, the main endogenous antioxidant; (b) polyamines, which participate in the modulation of transcription, translation, cell growth and apoptosis; and (c) 5′-deoxyadenosyl 5′-radical, which initiates a radical reaction. The metabolites of SAM and the key enzymes in SAM biosynthesis and metabolism influence the core autophagy machinery. The levels of SAM modulate autophagy accompanied by targeting the AMPK/mTOR pathway and regulating methyltransferase activity. SAM metabolism has received increasing attention from autophagy researchers and has provided new insights into the importance of SAM levels for autophagy regulation, thereby opening new avenues for autophagic control. Moreover, SAM treatment has been indicated that to inhibit autophagy and suppress the secretion of inflammatory cytokines, playing a role in ameliorating inflammatory cancers. There is evidence that some therapeutic strategies work against tumour cells by modulating the levels of SAM, GSH or dcSAM and then stimulating autophagy.
These therapeutic methods could also be combined with other treatments, such as chemotherapy and immunotherapy, and may provide a new effective therapeutic method to improve prognosis. However, new clinical trials are necessary to unambiguously establish the benefit of these methods in tumour therapy. Future research may investigate how the formation of the transcriptional complex and epigenetic alterations mediate the relationships among SAM accumulation, hypermethylation of DNA and histones, and autophagy repression. The therapeutic potential of SAM in tumours and other diseases should also be investigated.

ACK N OWLED G EM ENTS
We thank a professional English editor (American Journal Experts) for assistance in improving the quality of language.

CO N FLI C T O F I NTE R E S T
No potential conflicts of interest were disclosed.

DATA AVA I L A B I L I T Y S TAT E M E N T
Research data are not shared.