Mechanism and Therapeutic Progress of One-Carbon Metabolic Key Enzyme: Serine Hydroxymethyltransferase 2 in Cancer

Abstract

Serine hydroxymethyltransferase 2 (SHMT2) is a crucial mitochondrial enzyme in 1-carbon (1C) metabolism. It catalyzes the conversion of serine to glycine, generating 1C units essential for purine and pyrimidine synthesis, thereby supporting DNA replication and repair. Abnormally high expression is associated with malignant progression and treatment tolerance in various cancers. This review systematically summarizes the functions of SHMT2 in different types of cancer, underscoring on its roles in metabolism, immune microenvironment, and key signaling pathways (PI3K/AKT/mTOR, JAK-STAT, etc.) and outlines its epigenetic regulation and posttranslational modification mechanisms. Compared with the existing research, we focused on the latest regulatory mechanisms of SHMT2 and its potential in cancer treatment, such as the development and application of small-molecule inhibitors (SHIN2 and AGF347).

Introduction

One-carbon (1C) metabolism is a fundamental biological process that regulates the transfer of 1C units, playing a pivotal role in nucleotide biosynthesis, methylation reactions, and redox homeostasis. Cancer cells undergo extensive metabolic reprogramming to meet the demands of rapid proliferation, and alterations in 1C metabolism are a hallmark of this adaptation. Among the key enzymes in this network, Serine hydroxymethyltransferase 2 (SHMT2) has emerged as a central regulator of tumor metabolism. Serine hydroxymethyltransferase 2, a mitochondrial enzyme, catalyzes the conversion of serine to glycine, generating 1C units that fuel nucleotide synthesis, produce NAD(P)H, and supply methyl donors for S-adenosylmethionine (SAM)-dependent methylation reactions. ¹ These metabolic functions are essential for sustaining the high proliferation rate and genomic stability of tumor cells. Notably, cancer cells often exhibit metabolic rewiring characterized by the Warburg effect, prioritizing glycolysis over oxidative phosphorylation even in the presence of sufficient oxygen. ² High SHMT2 expression is frequently associated with enhanced glycolysis, providing cancer cells with biosynthetic precursors and ATP to support rapid growth.

Recent studies have established a strong correlation between elevated SHMT2 levels and poor prognosis in various cancers. Meta-analyses reveal that high SHMT2 expression is linked to larger tumor size, increased lymph node invasion, greater metastatic potential, and worse clinical outcomes. ³ While SHMT2 is best known for its role in metabolic reprogramming, accumulating evidence suggests that it also contributes to tumor progression through epigenetic regulation and redox balance. Mechanistically, SHMT2 exerts oncogenic effects via multiple pathways. It enhances PI3K/AKT signaling by facilitating SAM-mediated methylation of the PTEN promoter, leading to PTEN silencing and sustained AKT activation. In addition, SHMT2 directly interacts with AKT, influencing downstream transcription factors such as N-Myc, further driving tumor progression.^4,5 SHMT2 activity is also fine-tuned by posttranslational modifications (PTM), including deacetylation by SIRT3 and desuccinylation by SIRT5, which regulate its enzymatic function and contribute to the dynamic control of tumor metabolism.^6,7

Given its multifaceted roles in tumorigenesis, SHMT2 represents both a promising biomarker and a therapeutic target. This review explores the involvement of SHMT2 in cancer, with a particular focus on metabolic reprogramming, epigenetics, and mitochondrial dynamics, and discusses its potential as a target for cancer therapy.

SHMT and 1C metabolism process

Serine hydroxymethyltransferase exists in two isoforms—SHMT1, located in the cytoplasm, and SHMT2, localized in the mitochondria—sharing approximately 66% amino acid sequence similarity. ⁸ Both isoforms catalyze the conversion of serine to glycine, transferring the hydroxymethyl group (Cβ) to tetrahydrofolate (THF), thereby generating 1C units essential for nucleotide biosynthesis. However, their functional contributions differ significantly across cellular contexts. Mitochondrial SHMT2 is the primary driver of 1C metabolism in rapidly proliferating cancer cells. It supplies 1C units predominantly from serine, fueling purine and thymidylate synthesis, which are essential for DNA replication and repair. In contrast, SHMT1, the cytoplasmic isoform, is more active under conditions where mitochondrial 1C metabolism is restricted.

Folic acid (vitamin B9) serves as the principal carrier of 1C units, entering cells primarily via the reduced folate carrier (RFC, encoded by SLC19A1). ⁹ Once inside the cell, THF undergoes interconversions into various 1C-carrying forms, including 5-methyl-THF (CH3-THF), 5,10-methylene-THF (CH2-THF), 5,10-methenyl-THF (CH+-THF), and 5-formyl-THF (CHO-THF). Another key contributor to the folate cycle is serine, which provides a direct source of 1C units via SHMT-mediated conversion to glycine. Notably, serine availability is more critical for nucleotide synthesis and cancer cell proliferation than glycine uptake, as supported by metabolic flux studies ¹⁰ (Figure 1).

Figure 1.

1C metabolism pathway.

The folate cycle operates in both the cytoplasm and mitochondria, with intermediate metabolites such as serine, glycine, and formate facilitating crosstalk between these compartments. ¹¹ Although both SHMT isoforms catalyze the same reaction, their roles in tumor metabolism vary. In cancer cells with high folate availability, mitochondrial SHMT2 dominates 1C metabolism, providing a continuous supply of formate and NAD(P)H to sustain biosynthetic and antioxidant functions. Conversely, in cells with low folate levels or SLC19A1 deficiency, SHMT1 becomes the primary driver of 1C metabolism, as mitochondrial flux is downregulated. Importantly, studies have demonstrated that SHMT1 inhibition significantly impairs purine synthesis and tumor growth in SLC19A1-low cancer cells, highlighting SLC19A1 as a potential biomarker of SHMT1 dependence. ¹²

In summary, SHMT catalyzes the conversion of serine to glycine, generating 1C units that are incorporated into THF and ultimately converted into formate, which shuttles between the cytoplasm and mitochondria. Exogenous formate can rescue defects in 1C metabolism and cell growth caused by SHMT2 deficiency by directly replenishing 1C units in the THF cycle. ¹³ Mitochondrial 1C metabolism is a major source of NAD(P)H, which supports large-molecule synthesis and helps cells cope with oxidative stress. ¹⁴ Notably, methyl groups derived from 1C metabolism are essential for the methylation of histones, DNA, and RNA, which are crucial for DNA repair and genomic stability. ¹⁵

The Role of SHMT2 in metabolic reprogramming

Nucleotide metabolism reprogramming

Metabolic reprogramming is a hallmark of cancer cells and refers to the adaptation of metabolic pathways to support rapid proliferation and growth under specific environmental conditions. ² Thus, SHMT2 plays a pivotal role in this process. Serine hydroxymethyltransferase 2 catalyzes the conversion of serine to glycine while generating 1C units in the form of 5,10-methylene tetrahydrofolate, which is a crucial step in pyrimidine and purine nucleotide biosynthesis. In various malignancies, including renal cell carcinoma and bladder cancer, inhibition of SHMT2 effectively disrupts nucleotide synthesis and DNA replication, leading to cell cycle arrest and tumor growth.^{16 -18}

Moreover, in diffuse large B-cell lymphoma (DLBCL), SHMT2 activity has been demonstrated to increase the intracellular ratios of NAD(P)H/NAD(P)+ and GSH/GSSG, thereby reducing the accumulation of reactive oxygen species (ROS). ¹⁹ Notably, NAD(P)H serves not only as a critical component of the antioxidant defense system, but also as a key cofactor in lipid and nucleotide biosynthesis. ²⁰ Through the regulation of these metabolic processes, SHMT2 enhances DNA synthesis and repair, promotes cell proliferation, and confers resistance to apoptosis, ultimately facilitating tumor progression.

In addition to its role in tumor metabolism, SHMT2 has been implicated in cancer drug resistance. 5-Fluorouracil (5-FU) is a chemotherapeutic agent widely employed to treat multiple solid malignancies, including colorectal cancer (CRC). ²¹ Its primary mechanism of action involves the inhibition of thymidylate synthase (TS), thereby disrupting nucleotide biosynthesis and exerting cytotoxic effects on cancer cells. ²² However, in 5-FU-resistant CRC cells, SHMT2 promotes purine biosynthesis and enhances DNA damage repair, allowing tumor cells to sustain resistance to 5-FU. ²³ Therefore, targeting SHMT2 in drug-resistant CRC may represent a promising therapeutic strategy for overcoming chemoresistance and improving treatment efficacy (Figures 2 and 3).

Figure 2.

The mechanisms of tumorigenic effect of SHMT2.

Figure 3.

The roles of SHMT2 in cancer.

Amino acid metabolic reprogramming

It is well established that glycine serves as a fundamental precursor for numerous essential biomolecules, including proteins, nucleotides, and GSH, and is in high demand, often surpassing the cellular requirement for 1C units. ²⁴ Studies have revealed that B-cell lymphoma is highly sensitive to SHMT inhibition, primarily because of the low capacity of these cells to uptake glycine. ²⁵ Consequently, these cells rely on SHMT-catalyzed serine-to-glycine conversion to sustain growth. When SHMT inhibitors are combined with formate, a downstream product of 1C metabolism, they suppress tumor cell proliferation and survival more effectively. However, the anti-tumor effect of this combination is primarily attributed to the restriction of de novo glycine synthesis rather than to the disruption of 1C metabolism, which is the classical mechanism of antifolate drugs. ¹⁹

Furthermore, glutamine metabolism is equally crucial for tumor cell growth, serving not only as a key substrate for maintaining mitochondrial function but also plays a pivotal role in regulating ROS balance and facilitating the synthesis of the antioxidant enzyme GSH. ²⁶ Studies have shown that the inhibition of glutamine metabolism in anaplastic thyroid carcinoma cells triggers the activation of 1C metabolism as a compensatory mechanism to maintain redox homeostasis and sustain tumor cell survival. Further investigations indicated that the dual inhibition of glutamine metabolism and 1C metabolism was significantly more effective in suppressing tumor cell proliferation than either strategy alone. This combination therapy markedly enhanced the therapeutic efficacy of lenvatinib and sorafenib in patients. ²⁷ These findings provide novel insights into the targeting of amino acid metabolism as a promising therapeutic strategy, further expanding the potential applications of metabolic interventions in cancer treatment.

Glycometabolism reprogramming

Among metabolic reprogramming, glycolysis and its byproducts, particularly lactic acid, play a crucial role in tumor progression. Enhanced glycolysis promotes the progression of various cancer types. ²⁸ In renal cell carcinoma (RCC), knockdown of SHMT2 leads to a decreased ability of tumor cells to take up glucose, leading to a reduced glycolysis and subsequent inhibition of tumor proliferation and migration. However, the specific underlying mechanisms remain to be elucidated. ²⁹ Numerous reports have shown that lactic acid contributes to the progression of malignant tumors by regulating angiogenesis, nutrient metabolism, immune evasion, and cell cycle activity. ³⁰ In triple-negative breast cancer (TNBC), increased expression of SHMT2 enhances glucose uptake, increases lactic acid production, and promotes aerobic glycolysis in breast cancer cells, thereby facilitating tumor progression. ³¹

Hypoxia is an inevitable process in the development of solid tumors, and tumor cells adapt to hypoxic microenvironments through metabolic reprogramming. ³² In hypoxic environments, tumor cells mainly consume a large amount of glucose through glycolysis to maintain the energy required for growth and produce a large amount of lactate. Lactic acid, a product of glycolysis, is considered as a metabolic waste product. However, studies have revealed that lactic acids can significantly increase the lactylation level of SHMT2, enhance protein stability, and upregulate protein levels. By regulating the 1C metabolic process, they can increase the ratios of NADH/NAD+, NADPH/NADP+, and GSH/GSSG; reduce the accumulation of ROS in cells; supply large molecules for cell synthesis; and cope with oxidative stress, accelerating malignant progression in esophageal cancer. ³³

Overall, the interplay between glycolysis, lactic acid, and SHMT2 forms a key regulatory network in tumor metabolism. Lactylation modulates 1C metabolism and stabilizes SHMT2, regulating redox balance, biosynthesis, and tumor progression.

Oxidative Phosphorylation (OXPHOS) reprogramming

Mitochondria, the powerhouses of cells, are responsible for ATP production and play essential roles in signal transduction and apoptosis. Serine hydroxymethyltransferase 2 influences mitochondrial structure and function, particularly OXPHOS, thus affecting energy metabolism in cancer cells.^34,35

SHMT2 supports the synthesis of the mitochondrial respiratory complexes by providing critical methyl donors that generate taurine and methyluridine. This is vital for maintaining the tRNA pool required for the translation of OXPHOS complexes. ³⁶ In tumors such as bladder cancer, SHMT2 knockdown reduces the expression of proteins associated with OXPHOS complexes I-V through translational regulation.^{37 -39} Furthermore, SHMT2 contributes to the maintenance of normal mitochondrial structure. In RCC, SHMT2 knockout disrupts mitochondrial integrity, leading to defects in OXPHOS. This dysfunction results in insufficient ATP synthesis, which hinders cellular component synthesis, reduces NAD(P)H/NAD(P)+ ratios, and increases ROS levels, ultimately triggering apoptosis. ²⁹

When misfolded SHMT2 accumulates in the mitochondria, mitochondrial proteases such as Lon protease (LONP1) and caseinolytic protease P (ClpP) maintain mitochondrial function by clearing damaged proteins. ⁴⁰ However, inhibiting LONP1 and ClpP leads to the accumulation of unfolded SHMT2, disrupting the mitochondrial structure and suppressing tumor cell growth. ⁴¹

Mitochondria serve as the primary sites of ROS production and are crucial regulators of apoptosis. In DLBCL, SHMT2 deficiency disrupts redox homeostasis, leading to decreased NADH/NAD+, NADPH/NADP+, and GSH/GSSG ratios. This imbalance promotes excessive ROS accumulation, which in turn triggers mitochondrial membrane potential loss, increased membrane permeability, cytochrome C (Cyt C) release, Bcl-2 family protein translocation, and caspase-3 activation, ultimately inducing ROS-dependent, mitochondria-mediated apoptosis.^19,42 Similarly, in breast cancer, SHMT2 knockdown activates a cascade of caspase proteins and promotes the release of Cyt C from the mitochondria into the cytoplasm, further driving mitochondria-mediated apoptosis.^19,42,43

In summary, SHMT2 plays central roles in promoting NADPH production, maintaining redox homeostasis, preserving mitochondrial function, ensuring metabolic stability, and suppressing ROS-induced apoptosis. Targeting SHMT2 and its regulation of NADPH metabolism are promising therapeutic strategies for cancer treatment with the potential to disrupt tumor metabolic adaptation and enhance cancer cell sensitivity to therapy.

Epigenetic roles of SHMT2 in tumors

In 1942, Waddington coined the term “epigenetics” to describe changes in phenotypes that do not involve genotype alterations. Traditional epigenetic changes involve different gene expression patterns that lead to various phenotypic characteristics, including changes in DNA methylation, histone modifications, and chromatin structure remodeling. ⁴⁴ SAM, a key product of SHMT2 in 1C metabolism, plays a critical role in the epigenetic regulation of tumor cells. As a methyl donor, SAM participates directly in the enzymatic reactions of DNA methyltransferases, facilitating the methylation of CpG islands and other genomic regions, thereby regulating gene expression. Tumor cells maintain elevated SAM levels to sustain the methylation status of their genome, promoting cell proliferation and preventing tumor suppressor gene activation. ⁴⁵ Research has shown that in PTC, SHMT2 mediates 1C metabolism to produce SAM, which is used to methylate the CpG islands in the promoter region of the PTEN gene. This methylation reduces PTEN levels and activates the AKT signaling pathway, leading to tumor cell migration. ⁵ In DLBCL, SAM produced through 1C metabolism methylates the promoters of tumor suppressor genes, such as SASH1 and protein tyrosine phosphatase receptor type M, silencing their functions and promoting DLBCL progression. ⁴⁶

Histone modifications play crucial roles in regulating all biological processes involving DNA transcription and serve as dynamic templates for gene expression. ⁴⁷ Among the various histone modifications, acetylation is particularly significant as it leads to the relaxation of chromatin structure and the formation of an open chromatin conformation that facilitates the binding of transcription factors, thereby significantly enhancing gene expression. ⁴⁸ Recent studies have elucidated that in glioblastoma multiforme (GBM), the nuclear transcription factor Y subunit beta (NFYB) inhibits the transcription of SHMT2 through the activation of the transcriptional expression of histone deacetylase 5 (HDAC5). This suppression results in reduced glucose uptake, lactate production, and cell proliferation, migration, and invasion, while simultaneously increasing apoptosis and sensitivity to the chemotherapeutic drug temozolomide, which is commonly used in GBM treatment. Further mechanistic studies demonstrated that the transcription factor NFYB binds to the promoter region of HDAC5. In turn, HDAC5 inhibits the transcription of SHMT2 by reducing the acetylation sites on histone H3 within the SHMT2 promoter region. ⁴⁹ This reduction in acetylation contributes to chromatin compaction, thereby diminishing SHMT2 expression and the associated oncogenic activity in GBM.

In recent years, post-transcriptional modifications have emerged as significant forms of the epigenetic regulation of gene expression. ⁵⁰ Among these modifications, m6A methylation is the most abundant and widespread in eukaryotic mRNA. It occurs at the N6 position of the adenine base and uses SAM as a methyl donor. m6A modification affects nearly all aspects of mRNA metabolism and has been extensively reported to be involved in tumor progression. ⁵¹ Studies have confirmed that SHMT2 regulates the progression of esophageal cancer and immune evasion by modifying c-Myc with m6A. The MYC family of transcription factors, which includes c-Myc, n-Myc, and l-Myc, is crucial for cell growth, metabolism, and development. ⁵² SHMT2-mediated m6A modification of MYC is facilitated by the m6A reader IGF2BP2, writer METTL3, and erasers FTO and ALKBH5, which together maintain the stability of c-Myc mRNA and promote the progression of esophageal cancer. ⁵³ Phosphoribosyl pyrophosphate amido transferase (PPAT) catalyzes the first step of de novo purine biosynthesis and serves as a key regulatory point in purine nucleotide biosynthesis. ⁵⁴ SHMT2 enhances SAM accumulation through 1C metabolism and participates in m6A modification by employing IGF2BP2 as an m6A reader. This process stabilizes PPAT mRNA in a manner dependent on IGF2BP2, promoting tumor progression by upregulating PPAT mRNA and protein levels in RCC.^16,55 The m6A reader IGF2BP1, an RNA-binding protein (RBP), binds and stabilizes the target mRNA. ⁵⁶ IGF2BP1 directly binds to and stabilizes the mRNA of SHMT2 in neuroblastoma cells, enhancing its protein expression. This binding regulates protein loading in neuroblastoma-derived extracellular vesicles (EVs) and influences the formation of the pre-metastatic microenvironment and niche (PMN) in target organs, thereby further promoting tumor metastasis. ⁵⁷ Estrogen-related receptor α (ERRα, NR3B1), a member of the nuclear receptor superfamily and an orphan receptor, acts as a major regulator of energy metabolism in both normal and tumor cells. ⁵⁸ ERRα activates SHMT2 transcription by targeting its promoter region, thus promoting tumor progression and increasing breast cancer cell resistance to the chemotherapeutic agent lapatinib. ⁵⁹

Proteins are regulated by PTM, including phosphorylation, glycosylation, ubiquitination, methylation, and acetylation. Dysregulated PTMs can disrupt protein function and drive cancer progression. ⁶⁰ SHMT2 regulates tumor progression through modifications, such as dehydroxybutyrylation, deacetylation, and lactylation. The protein tetramer of SHMT2 is crucial for maintaining normal enzymatic activity. Deacetylation and succinylation are key mechanisms for stabilizing SHMT2 intracellular expression and maintaining its high activity. SIRT5 enhances the tetramer formation of SHMT2 and activates its enzymatic activity by desuccinylating SHMT2 and eliminating the succinyl group at the K280 position. ⁶ As the main mitochondrial deacetylase, SIRT3 stabilizes its endogenous expression and maintains high activity through deacetylation at the SHMT2 K95 site. Highly active SHMT2 helps cells resist oxidative stress caused by ROS in the mitochondria and maintains rapid proliferation of cancer cells. Conversely, K95 acetylation of SHMT2 disrupts its functional tetramer structure, inhibits enzyme activity, and accelerates its degradation through the glucose-dependent K63 ubiquitin-lysosome pathway, further decreasing CRC cell proliferation and tumor growth. ⁷ Overall, SIRT5 and SIRT3 regulate SHMT2 function, reduce serine consumption, lower NADPH levels, and decrease CRC cell proliferation and tumor growth through their respective desuccinylation and deacetylation processes.

In summary, as a core product of SHMT2-mediated 1C metabolism, SAM plays a pivotal role in the epigenetic modifications of tumors. As a methyl donor, SAM regulates tumor suppressor genes through DNA methylation, for example, by promoting PTEN methylation, thereby facilitating tumor cell migration. Moreover, SAM participates in m6A methylation, stabilizing key oncogenic mRNAs, such as c-Myc, which further drives cancer cell proliferation and metastasis. In addition, SHMT2 undergoes various PTMs, such as acetylation and succinylation, which modulate its enzymatic activity and contribute to cancer progression. By maintaining elevated SAM levels and undergoing precise PTM regulation, tumor cells enhance proliferation, genomic stability, immune evasion, and drug resistance. These intricate regulatory mechanisms enable tumor cells to sustain continuous growth and evade therapeutic interventions within the complex tumor microenvironment.

SHMT2 in tumor immune microenvironment (TIME)

The TIME is a critical component of TME. Immune cells within the TIME have a dual effect on tumors: when immune cells adopt an anti-tumor phenotype, they can play a role in immune surveillance, preventing tumor progression, whereas under the influence of the TIME, immune cells adopt a pro-tumor phenotype, allowing tumors to evade immune surveillance and promote tumor progression. ⁶¹ However, the relationship between SHMT2 and TIME remains poorly understood.

Studies have shown that SHMT2 promotes the infiltration of immunosuppressive cells, including regulatory T-cells (Treg), and mediates immune escape, thereby enhancing RCC proliferation. ⁶² In lung cancer, SHMT2 plays a crucial role in immune cell infiltration, and high levels of SHMT2 expression are associated with increased immune infiltration. Specifically, SHMT2 expression was significantly and negatively correlated with the infiltration levels of CD4⁺ T-cells, macrophages, and dendritic cells. It is also significantly correlated with tumor-associated macrophages, neutrophils, Th1, Th2, Tfh, and T-cell exhaustion. ⁶³ PD-L1 is a classic target for tumor immunotherapy. Research has shown that dipeptidyl peptidase DPP9 dynamically regulates the stability of the BRISC complex through SHMT2, thereby upregulating PD-L1 expression in RCC. ⁶⁴ In esophageal cancer, overexpression of SHMT2 promotes PD-L1 expression, supporting tumor progression and immune escape. ⁵²

In epithelial ovarian carcinoma (EOC), tumor-associated macrophages (TAM) express the FRβ, which is a key component for folate entry into cells. This receptor further influences the function of SHMT2, providing new opportunities for targeting FR to inhibit cancer progression by suppressing TAM activity. ⁶⁵ Research has demonstrated that in RCC and lung cancer, SHMT2 expression level is significantly positively correlated with the infiltration of CD8⁺ T immune cells. Conversely, in liver cancer, SHMT2 expression is significantly negatively correlated with the infiltration of CD8⁺ T immune cells. ⁶⁶ In summary, our findings highlight the complex immunocyte-related functions of SHMT2 in numerous cancer types.

SHMT2 and noncoding RNA (ncRNA)

Noncoding RNAs play a crucial role in cancer regulation by modulating gene expression and cellular processes. Although protein-coding sequences comprise only about 1.5% of the genome, most of the transcribed sequences produce ncRNAs, which contribute to tumor progression and therapeutic resistance.^67,68 In the context of SHMT2, accumulating evidence suggests that ncRNAs regulate its expression and function through different mechanisms, providing insights into cancer metabolism and potential therapeutic strategies (Figure 4).

Figure 4.

The roles of noncoding RNA in tumorigenesis mechanisms of SHMT2.

MicroRNAs (miRNAs) are small RNA molecules (17-24 nucleotides) that regulate gene expression by binding to the 3′-UTR of target mRNAs, leading to translational repression or degradation. ⁶⁹ Several miRNAs negatively regulate SHMT2 in different cancers. In hepatocellular carcinoma, miR-615-5p suppresses tumor proliferation and migration by directly targeting SHMT2. ⁷⁰ In breast cancer, miR-193b inhibits SHMT2 expression, leading to reduced proliferation and G1-phase cell cycle arrest. ⁷¹ Similarly, in lung adenocarcinoma, miR-383-5p downregulates SHMT2, thereby inhibiting tumor cell proliferation and migration. ⁷² These findings highlight miRNAs as tumor suppressors that suppress SHMT2 activity, suggesting their therapeutic potential.

Long noncoding RNAs (lncRNAs) influence SHMT2 expression through multiple regulatory mechanisms, including acting as competing endogenous RNAs (ceRNAs) and interacting with RNA-binding proteins.^73,74 In NSCLC, lncRNA Gm15290 functions as a ceRNA by sponging miR-615-5p, thereby increasing SHMT2 protein levels and promoting tumor proliferation and invasion. ⁷⁵ In GBM, lncRNA HOTAIRM1 stabilizes SHMT2 mRNA by interacting with PTBP1 and IGF2BP2, facilitating m6A site recognition and enhancing mitochondrial activity, ultimately driving glioma malignancy. ⁷⁶ These studies indicate that diverse lncRNAs indirectly upregulate SHMT2, contributing to its oncogenic role in various cancers.

Circular RNAs (circRNAs) serve as important regulators of SHMT2 by sponging miRNAs and modulating gene expression. ⁷⁷ In breast cancer, circ_0072955 binds to miR-149-5p, preventing it from silencing SHMT2, which promotes tumor progression by enhancing the Warburg effect. ³¹ In gastric cancer, exosomal circ_0063526 regulates the miR-449a/SHMT2 axis, leading to cisplatin resistance. ⁷⁸ Similarly, in CRC, MLK7-AS1 reduces miR-149-5p levels, leading to increased SHMT2 expression and altered cell cycle dynamics.^79,80 These findings suggest that circRNAs contribute to metabolic adaptation and drug resistance in cancer by enhancing SHMT2 expression.

Taken together, ncRNAs play a crucial role in modulating SHMT2 expression, impacting tumor metabolism, proliferation, and therapy resistance. Although miRNAs predominantly suppress SHMT2, lncRNAs and circRNAs facilitate its expression by inhibiting miRNA activity or stabilizing SHMT2 mRNA. Understanding these interactions provides deeper insights into tumor biology and offers potential therapeutic strategies targeting the SHMT2-ncRNA axis.

Mitochondrial isoenzyme of SHMT2 and SHMT1

As a key enzyme in the 1C metabolic pathway within the cytoplasm, the role of SHMT1 in tumors remains unclear, with conflicting evidence regarding its potential to promote or inhibit tumor growth. We summarized the research progress on SHMT1 in various cancers and aimed to elucidate its correlation with SHMT2.

SHMT1 can form a complex with the 5′UTR region of SHMT2 mRNA to regulate serine metabolism. The efficiency of this binding is influenced by the cellular metabolic state. When the complex is formed, it inhibits SHMT2 translation and reduces protein formation. Concurrently, the complex inhibited the serine cleavage activity of SHMT1 but did not affect the reverse reaction. Through the binding of SHMT1 and SHMT2 RNA, tumor cells can dynamically regulate serine and glycine concentrations, especially during metabolic reprogramming to adapt to different metabolic demands.^81,82

Research on TNBC has revealed that SHMT1 overexpression is associated with poor clinical outcomes. ⁸³ Similarly, studies on lung cancer have indicated that SHMT1 exerts oncogenic functions by regulating apoptosis. ⁸⁴ These data indicate that SHMT1 acts as a carcinogen in these cancers. In EOC, the transcription factor WT1 promotes SHMT1 transcription, which in turn stimulates the sialylation gene Neu5Ac. This activation enhances the expression of cancer-associated inflammatory cytokines IL-6 and IL-8, thereby promoting the growth and migration of cancer cells. ⁸⁵ In a study on tumor drug resistance mechanisms, miR-6778-5p was identified as an miRNA that functions independently of Drosha. It targets the transcriptional inhibitor of c-Myc, YWHAE, leading to c-Myc silencing. Blocking the miR-6778-5p/YWHAE/SHMT1 signaling axis can enhance the sensitivity of gastric cancer to 5-FU chemotherapy. ⁸⁶

In contrast to the above conclusions, SHMT1 can also function as a tumor suppressor. In HCC, SHMT1 is expressed at low levels and inhibits cell progression by suppressing ROS generation mediated by NADPH oxidase 1 (NOX1). ⁸⁷ Similarly, in RCC, SHMT1 is expressed at low levels and acts as a tumor suppressor. Mechanistically, the transcription factor HOXD8 promotes SHMT1 expression, leading to the blockade of multiple G2/M phase checkpoints, leading to cell cycle arrest and proliferative defects, ultimately inhibiting RCC proliferation and migration. ⁸⁸

Role of SHMT2 in classical signaling pathways

The inherent complexity of the signaling pathways regulated by SHMT2, combined with its interactions and crosstalk with various other pathways, renders its regulatory role challenging to elucidate. This discussion underscores on the complex interactions between SHMT2 and several major signaling pathways during cellular transformation and highlights their significance in targeted cancer therapies.

The PI3K/AKT/mTOR (PAM) pathway is one of the most frequently activated signaling pathways in human cancers and promotes cell survival, growth, and progression throughout the cell cycle. The dysfunction of this pathway is a well-known driver of resistance to cancer treatment and disease progression. ⁸⁹ The MYC family of transcription factors, including c-Myc, n-Myc, and l-Myc, is a crucial downstream component of the PAM pathway and plays an essential role in tumor growth, metabolism, and development. MYC family members are among the most important transcription factors and nuclear oncogenes. ⁵² In high-risk neuroblastoma, silencing SHMT2 reduces AKT-2 expression in the PAM pathway and decreases pAkt-2 activity, thereby inhibiting the expression of n-Myc protein. SHMT2 regulates N-Myc through AKT-2 phosphorylation and plays an important role in NB tumorigenesis by promoting growth, migration, and metastasis of NB cells in vitro. ⁴ In thyroid papillary carcinoma, PTEN serves as a typical negative regulator of AKT, inhibiting the activation of the PAM signaling pathway. ⁹⁰

In cancer biology, the MAPK signaling pathway is one of the most well-defined signaling pathways and its overactivation is responsible for over 40% of human cancers. ⁹¹ Research has shown that in breast cancer, SHMT2 regulates the levels of phosphorylated p38 and ERK, and SHMT2 can promote the proliferation of breast cancer cells by activating the MAPK signaling pathway. ⁴³ Conversely, in prostate cancer, SHMT2 loss significantly promotes the phosphorylation of ERK1/2, thereby activating the ERK signaling pathway, inhibiting EMT, and regulating the invasive behavior of prostate cancer. Knockdown of SHMT2 increased the expression of the mesenchymal factors ZEB1, vimentin, and CDH2 (N-cadherin) in prostate cancer cells and decreased the expression of the epithelial factor CDH1 (E-cadherin). This activation leads to the expression of downstream genes activated by ERK1/2 such as SRF, FOS, EGR1, and JUN. ⁹²

The JAK-STAT pathway is an evolutionarily conserved signaling cascade activated by cytokine stimulation. It transmits signals from the extracellular environment to the nucleus through the cell membrane, resulting in changes in DNA transcription. The JAK-STAT signaling pathway regulates various cellular functions, including proliferation, migration, differentiation, and apoptosis. ⁹³ SHMT2 mediates bladder cancer cell growth by regulating the JAK-STAT signaling pathway. STAT3 is a well-known oncogene and pivotal regulator of many signaling pathways. Silencing SHMT2 reduces the expression of phosphorylated STAT3, whereas overexpression of SHMT2 increases the expression of phosphorylated STAT3.

The Wnt/β-catenin signaling pathway, also referred to as the canonical Wnt pathway, is a highly conserved and critical axis that regulates a series of intricate cell signaling cascades. This pathway is essential for a wide array of physiological processes, including proliferation, differentiation, apoptosis, migration, invasion, and the maintenance of tissue homeostasis. ⁹⁴

As one of the 4 major cancer treatment modalities, along with surgery, radiation therapy, chemotherapy, and immunotherapy, radiation therapy can be employed either as a curative treatment or as adjuvant therapy after surgery for various cancers. Historically, neither single-agent nor combination radiation has been deemed suitable for treating gastric cancer. However, recent research has revealed that SHMT2 downregulation via the regulation of the Wnt/β-catenin pathway enhances the sensitivity of gastric cancer cells to interventional radiation therapy. SHMT2 has also been implicated in radiation-induced apoptosis in GC cells. ⁹⁵ It interacts with β-catenin and inhibits the ubiquitin-proteasome pathway responsible for its degradation, thereby stabilizing it. Moreover, SHMT2 is a target gene of β-catenin, and through interaction with the transcription factor TCF4, β-catenin can increase SHMT2 expression, forming a positive feedback loop between SHMT2 and β-catenin. This loop enhances proliferation and metastasis of CRC cells. ⁹⁶

The PLCε is an isoenzyme of the PLC family. As a significant member of the phospholipase family, it functions not only as an effector for Ras, Rho, and Rap but also mediates signaling through G protein-coupled receptors. ⁹⁷ Research indicates that the PLC family isoenzyme PLCε can regulate genes involved in 1C metabolism, such as SHMT2, and positively influence prostate cancer proliferation. PLCε regulates the expression of proliferative genes (CyclinD1, PCNA) and serine metabolic enzymes (PSAT1, PSPH, SHMT2) by modulating the downstream effector molecule YAP within the Hippo signaling pathway. ⁹⁸

SHMT2 inhibitors for cancer treatment

Targeting folate metabolism and nucleotide biosynthesis has been a cornerstone of cancer therapy for over 70 years. ⁹⁹ Antifolate drugs, including DHFR inhibitors such as methotrexate (MTX) and TS inhibitors such as capecitabine, are widely employed in clinical practice. ¹¹ MTX inhibits DHFR and is the standard treatment for acute T-cell leukemia. The product of DHFR is THF, while TS uses CH3-THF as the substrate. SHMT catalyzes the conversion of THF to CH3-THF using serine as the 1C donor, producing glycine as a byproduct. Given that SHMT’s central role of SHMT in 1C metabolism is positioned between that of DHFR and TS and that SHMT2 is consistently upregulated in numerous cancers, SHMT2 inhibition presents a compelling therapeutic target (Table 1).

Table 1.

Inhibitors of SHMT2.

Primary target enzymes	Inhibitor	CAS number	Structure	Refs
SHMT2	SHIN2	2204289-53-0		Hayashi et al 100
SHMT1/2, GART	AGF347	2294820-23-6		Dekhne et al 101
SHMT2	Glycyrrhetinic acid	471-53-4		Jin et al 102
SHMT2	Metformin	657-24-9		Yao et al 103
SHMT1/2	Sertraline	79617-96-2		Morais et al 104

In 2017, Ducker et al ¹⁹ identified the first small-molecule inhibitor targeting SHMT1/2, which demonstrated efficacy both as a monotherapy and in combination with MTX in treating T-cell acute lymphoblastic leukemia (T-ALL). A selective SHMT inhibitor, SHIN2, effectively arrested the cell cycle in the S phase, thereby inhibiting T-ALL tumor progression. Notably, SHIN2 was also effective against MTX-resistant T-ALL, with combination therapy of MTX and SHIN2 showing superior efficacy in suppressing tumor cell proliferation compared with either agent alone.^105,106

AGF347 acts as a broad-spectrum inhibitor targeting SHMT2, SHMT1, and GART, leading to increased levels of ROS and glutathione consumption, thereby demonstrating potent anti-tumor activity against lung, CRC, and pancreatic cancer cells.^101,107 Further mechanistic studies have demonstrated that in EOC, AGF347 is selectively transported into tumors via FRα and PCFT. Once internalized, AGF347 inhibits SHMT2 and subsequently disrupts purine and deoxyribonucleotide biosynthesis. Given that elevated glutathione levels can confer resistance to cisplatin in EOC, ¹⁰⁸ the combination of AGF347 with cisplatin in cisplatin-resistant EOC leads to glutathione depletion, resulting in synergistic inhibition and enhanced anti-tumor efficacy. However, combination therapy is complicated by dose-limiting gastrointestinal toxicity, which presents significant challenges for its clinical application. ¹⁰⁹

Glycyrrhetinic acid, a bioactive metabolite derived from glycyrrhizin, a key constituent of licorice that is widely used in traditional Chinese medicines, has been shown to exert anti-tumor effects by targeting and competitively inhibiting SHMT2. This inhibition leads to OXPHOS and FAO downregulation, thereby impairing mitochondrial energy metabolism. ¹⁰² Metformin, a drug widely used in diabetes treatment since 1957, inhibits tryptophan uptake by CRC cells by downregulating MYC, resulting in decreased expression of the transporter SLC7A5. ¹¹⁰ Because MYC is a downstream target of SHMT2, ⁴ this suggests a potential link between metformin, mitochondria, and SHMT2. Recent research has uncovered a novel function of metformin as a noncatalytic pyridoxal-5′-phosphate (PLP)-competitive inhibitor of mitochondrial SHMT2. ¹¹¹ Moreover, the antidepressant sertraline, commonly prescribed at therapeutic doses, has emerged as a novel SHMT inhibitor. Sertraline inhibits SHMT1/2 activity and glycine uptake, reducing serine uptake and the de novo synthesis of serine, thus inhibiting breast tumor growth. Moreover, the ability of sertraline to cross the blood-brain barrier suggests its potential in treating brain tumors which is dependent on 1C metabolism. ¹¹²

Limitations

The main limitation of this study is that we did not perform a systemic review of the literature in this rapidly changing field.

Conclusion

SHMT2 serves as a pivotal enzyme in 1C metabolism, influencing tumor progression through its regulatory roles in metabolism, epigenetic modifications, mitochondrial function, and the TIME. ¹¹³ By generating SAM and modulating DNA methylation, SHMT2 contributes to oncogenic signaling activation, as exemplified in PTC, where it methylates CpG islands in the PTEN promoter to drive AKT pathway activation. ⁵ Furthermore, its involvement in NADPH production underscores its role in oxidative stress regulation, with high SHMT2 expression correlating with poor prognosis in multiple cancers, including bladder cancer, renal cell carcinoma, and DLBCL. Inhibition of SHMT2 disrupts NADPH homeostasis, induces ROS accumulation, and triggers apoptosis,^19,42,43 highlighting its therapeutic potential. In addition, SHMT2 modulates the TIME by promoting PD-L1 expression, facilitating immune evasion. ⁵²

Given its central role in tumor metabolism and adaptation, SHMT2 represents a promising therapeutic target. Small-molecule inhibitors, such as SHIN2 and AGF347, have demonstrated potential in overcoming chemotherapy resistance in vitro.^105,106,109 However, their clinical translation faces significant challenges. Cancer cells may compensate for SHMT2 inhibition through alternative metabolic pathways, such as SHMT1 upregulation or exogenous glycine uptake. Therefore, combination therapy strategies are crucial to enhance treatment efficacy. Notably, combining SHMT2 inhibition with glutamine metabolism inhibitors has been shown to further deplete NADPH and potentiate the effects of targeted therapies like lenvatinib and sorafenib in undifferentiated thyroid cancer. ²⁷

Future research should focus on elucidating SHMT2’s tumor-specific roles, optimizing inhibition strategies, and exploring metabolic co-targeting approaches to improve therapeutic outcomes while minimizing side effects. Given its fundamental role in tumor metabolic adaptation—particularly in NADPH homeostasis and oxidative stress regulation—targeting SHMT2 holds promise for advancing precision cancer therapy.