One-Carbon Metabolism in Cancer

Overview of One-Carbon Metabolism Pathways

One-carbon metabolism encompasses several interconnected pathways involving various substrates, enzymes, and co-factors. The principal components of OCM include:

• Methionine Cycle: Involves the conversion of methionine to S-adenosylmethionine (SAM), a universal methyl donor, which is subsequently transformed into S-adenosylhomocysteine (SAH) and eventually into homocysteine.
• Folate Pathway: Central to the production of nucleotides, this pathway is responsible for the synthesis of tetrahydrofolate (THF), which carries one-carbon units in various oxidation states.
• Serine-Glycine Interconversion: This involves the conversion of serine to glycine, providing additional one-carbon units for metabolic processes.

Key Enzymes and Regulatory Factors

Several enzymes play pivotal roles in one-carbon metabolism, including:

• Methionine Synthase (MTR): Catalyzes the remethylation of homocysteine to methionine using 5-methyltetrahydrofolate (5-methylTHF) as a methyl donor.
• Thymidylate Synthase (TS): Facilitates the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), a critical step in DNA synthesis.
• Dihydrofolate Reductase (DHFR): Reduces dihydrofolate to tetrahydrofolate, essential for nucleotide biosynthesis.

The Association Between One-Carbon Metabolism and Cancer

One-carbon metabolism (OCM) is closely tied to cancer biology, with its essential roles in nucleotide synthesis, methylation, and redox balance contributing to tumor growth and adaptation. Cancer cells, unlike their normal counterparts, often undergo a metabolic shift to increase OCM activity, which supports their high demands for DNA synthesis and epigenetic alterations necessary for rapid proliferation. The interconnected folate and methionine cycles within OCM generate one-carbon units that supply key intermediates for purine and thymidine synthesis, maintaining genomic stability during cell division and reducing susceptibility to DNA damage. Elevated expression of enzymes in these cycles, such as serine hydroxymethyltransferase (SHMT) and methylenetetrahydrofolate dehydrogenase (MTHFD), is commonly observed in cancers, underscoring OCM's role in supporting the high metabolic needs of tumors.

In addition to fueling cell growth, the one-carbon units from these pathways are pivotal in driving epigenetic modifications through the production of SAM, a primary methyl donor. SAM availability allows for DNA and histone methylation, processes that regulate gene expression and can lead to the silencing of tumor suppressor genes or activation of oncogenes. This epigenetic reprogramming is crucial for cancer cells, allowing them to adapt their gene expression profiles to promote survival and evade immune detection. Furthermore, the redox balance maintained by the one-carbon pathway contributes to antioxidant defenses within cancer cells, protecting them from oxidative stress—a common byproduct of the high metabolic activity and environmental pressures within tumors.

The Role of Methyl Donors in Tumorigenesis

Methyl donors, predominantly through methionine and folate metabolism, serve as central drivers of tumorigenesis by regulating methylation reactions that influence gene expression, DNA synthesis, and cellular interaction with the tumor microenvironment. The metabolic dependencies of cancer cells on exogenous methyl donors, particularly methionine, underscore the potential for targeting these pathways in cancer therapy.

Folate Metabolism and Its Tumorigenic Contributions

Folate is central to nucleotide synthesis and the generation of methyl groups in cells, functioning as a co-factor in crucial reactions that supply one-carbon units for DNA synthesis and methylation. THF, a reduced form of folate, is sequentially modified to form 5,10-methylenetetrahydrofolate (5,10-methylTHF) and 5-methyltetrahydrofolate (5-methylTHF), intermediates required in DNA synthesis and methylation cycles.

DNA Synthesis and Repair

5,10-methylTHF is critical in synthesizing thymidylate for DNA synthesis. Cancer cells, which exhibit rapid proliferation, show increased demand for folate to support the synthesis of DNA and RNA precursors. Thymidylate synthase (TS), an enzyme that converts deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), requires 5,10-methylTHF as a co-factor. This reaction is crucial for maintaining DNA integrity, as it supplies thymidine, a nucleotide specific to DNA, preventing uracil misincorporation that can cause strand breaks and mutations. Elevated TS expression is frequently observed in cancer cells, allowing them to meet increased thymidine demands, maintain high proliferation rates, and avoid apoptosis.

Genomic Stability and Folate Deficiency

Folate insufficiency leads to imbalanced one-carbon units in DNA synthesis pathways, resulting in an increased incorporation of uracil in place of thymidine, generating single and double-strand DNA breaks. These breaks create a genome instability that predisposes cells to tumorigenic transformation. Folate deficiency has also been linked to heightened chromosomal damage, impaired DNA repair, and increased mutation rates, contributing to carcinogenesis in various tissues. Epidemiological studies support this connection, as low folate levels are associated with elevated risks for colorectal, breast, and pancreatic cancers. However, high folate levels in established tumors may have dual effects, potentially enhancing proliferation by supplying a surplus of one-carbon units essential for continued nucleotide and SAM production.

Methionine, SAM, and Epigenetic Reprogramming in Cancer

Methionine, obtained through dietary sources or recycled via the remethylation of homocysteine, is essential for SAM synthesis, the principal methyl donor in cellular methylation reactions. Cancer cells exhibit unique dependencies on methionine and SAM due to their requirement for sustained methylation capacity and DNA replication.

Methionine Dependence and SAM Production

Many cancers display "methionine addiction," wherein they rely heavily on exogenous methionine to maintain elevated SAM levels for methylation processes. This methionine dependency limits the ability of cancer cells to utilize homocysteine remethylation as a compensatory source, thereby restricting their methylation potential and affecting cell viability under methionine-deprived conditions. Methionine adenosyltransferase (MAT) enzymes catalyze the conversion of methionine to SAM, and their upregulation in certain cancers sustains the methylation machinery, thus maintaining epigenetic reprogramming conducive to oncogenesis.

Epigenetic Modifications Driven by SAM Availability

High SAM levels in cancer cells facilitate DNA and histone methylation, critical for regulating gene expression and chromatin structure. SAM donates methyl groups to DNA methyltransferases (DNMTs) and histone methyltransferases (HMTs), resulting in the hypermethylation of CpG islands in the promoters of tumor suppressor genes. This epigenetic silencing of genes like p16INK4a (CDKN2A), MLH1, and RB1 disrupts cell cycle regulation, apoptosis, and DNA repair, creating an environment conducive to tumor growth and survival. Simultaneously, global hypomethylation, often associated with oncogene activation and genomic instability, is a consequence of SAM metabolic reprogramming in cancer cells, permitting the activation of transposable elements, oncogenes, and repetitive sequences that promote malignant behavior.

Differential Methyl Donor Availability Across Cancer Types

The differential expression of enzymes involved in methyl donor pathways leads to distinct metabolic adaptations among various cancers, highlighting methyl donor utilization as a potential biomarker for cancer prognosis and treatment selection. Tumors that heavily rely on folate and methionine pathways may exhibit heightened sensitivity to antifolate agents (e.g., methotrexate, pemetrexed) and methionine-restricted diets, while cancers with intrinsic folate metabolism dysregulation or methionine-independent SAM synthesis may be resistant to such treatments. The variability in methyl donor utilization suggests that precise metabolic profiling could aid in the stratification of patients for therapies targeting one-carbon metabolism.

Methyl Donor Influence on Tumor Microenvironment and Metabolic Crosstalk

In addition to supporting cancer cell growth, methyl donors also influence the surrounding tumor microenvironment (TME) by modulating the metabolism of immune cells and fibroblasts that interact with cancer cells. The availability of methyl donors in the TME can affect the functional capacity of immune cells, such as T cells and macrophages, which require SAM for cytokine production and immune response regulation. By modulating methylation in these cells, tumors may suppress immune responses or encourage pro-tumorigenic inflammation, supporting immune evasion.

Metabolic Crosstalk in the TME

Cancer cells release metabolites like homocysteine and methylated derivatives, which can modulate the activity of neighboring stromal and immune cells. Homocysteine, in particular, can alter immune cell methylation profiles and pro-inflammatory cytokine expression, creating a microenvironment supportive of tumor progression. Studies have shown that targeting methyl donor pathways in immune cells within the TME could enhance immune infiltration and anti-tumor responses, thus presenting an indirect strategy for anti-cancer therapies.

An overview of one-carbon metabolism and the established/future therapeutics that target this pathway (Newman et al., 2017).

Metabolic Reprogramming and the Tumor Microenvironment

Metabolic reprogramming is a hallmark of cancer, enabling tumor cells to meet the high demands for energy, biosynthesis, and redox balance required for continuous growth. This metabolic shift is not limited to the tumor cells alone but significantly influences the surrounding tumor microenvironment (TME), which includes immune cells, fibroblasts, endothelial cells, and other stromal components. The metabolic interactions between cancer cells and their microenvironment create a unique, often hostile, niche that supports tumor progression, immune evasion, and therapy resistance.

Cancer Cell Metabolism and Its Impact on the TME

Cancer cells adapt their metabolism to thrive in low-oxygen, nutrient-poor conditions typically present in the TME. These adaptations often involve increased glucose uptake and its conversion to lactate via aerobic glycolysis (the Warburg effect), even in the presence of oxygen. This metabolic pathway not only meets the energy and biosynthetic needs of cancer cells but also acidifies the microenvironment due to lactate production. Acidification impairs the function of immune cells, such as T cells and natural killer (NK) cells, which are sensitive to low pH, effectively dampening the immune response against the tumor. Moreover, lactate acts as a signaling molecule, influencing the activity of tumor-associated macrophages (TAMs) and promoting a pro-tumorigenic M2-like macrophage phenotype that supports tissue remodeling, angiogenesis, and immune suppression.

Glutamine Metabolism and Redox Balance

Cancer cells also display a high dependence on glutamine metabolism, which supports various biosynthetic and antioxidant pathways. Glutamine serves as a carbon and nitrogen source, fueling the tricarboxylic acid (TCA) cycle and supporting the synthesis of non-essential amino acids, nucleotides, and lipids. Importantly, glutamine contributes to the generation of glutathione, a major cellular antioxidant that helps cancer cells counteract reactive oxygen species (ROS) and maintain redox homeostasis. This ability to maintain a favorable redox state is essential for cancer cells, as the high levels of ROS generated by rapid metabolism and hypoxia in the TME would otherwise induce apoptosis or cell death. Cancer-associated fibroblasts (CAFs) within the TME are also influenced by tumor glutamine metabolism, as they can be driven to secrete metabolites, such as lactate and pyruvate, that further fuel the tumor's metabolic needs, thus establishing a bidirectional metabolic interaction that enhances tumor resilience.

One-Carbon Metabolism in the Tumor Microenvironment

One-carbon metabolism, through its integration of the folate and methionine cycles, also plays a pivotal role in the TME by supporting nucleotide synthesis, methylation, and redox balance. Cancer cells upregulate enzymes in one-carbon metabolism to produce nucleotides essential for DNA synthesis, thereby maintaining proliferative capacity. Additionally, the production of SAM, a key methyl donor generated in the methionine cycle, allows cancer cells to sustain DNA and histone methylation, leading to epigenetic changes that favor tumor growth. This hyperactive one-carbon metabolism often results in the depletion of available methionine and folate in the TME, depriving immune and stromal cells of these essential nutrients and limiting their ability to mount an effective anti-tumor response. For example, T cells in the TME, already suppressed by acidic and hypoxic conditions, are further impaired by limited access to methionine, which is necessary for their activation and proliferation.

Hypoxia and Angiogenesis in the Tumor Microenvironment

Hypoxia is a common feature in rapidly growing tumors due to insufficient blood supply, and it acts as a potent modulator of both tumor metabolism and the TME. Under hypoxic conditions, the transcription factor hypoxia-inducible factor 1-alpha (HIF-1α) is stabilized, driving the expression of genes involved in glycolysis, angiogenesis, and survival. HIF-1α promotes the Warburg effect by upregulating glucose transporters (e.g., GLUT1) and glycolytic enzymes, which increase glucose uptake and lactate production. Lactate accumulation further supports angiogenesis by activating pro-angiogenic factors, such as vascular endothelial growth factor (VEGF), which stimulates the formation of new blood vessels to supply the tumor with oxygen and nutrients. This pro-angiogenic environment attracts endothelial cells and pericytes, creating abnormal, leaky blood vessels that paradoxically exacerbate hypoxia and lead to further metabolic adaptations in tumor cells and the surrounding stroma.

Immune Modulation through Metabolic Competition and Signaling

Metabolic reprogramming in cancer cells also creates a competitive metabolic environment that deprives immune cells of essential nutrients such as glucose, amino acids, and oxygen, hampering their anti-tumor activity. T cells, for example, rely on glucose and amino acids like glutamine for activation, proliferation, and effector functions. The increased consumption of these nutrients by cancer cells results in their scarcity within the TME, limiting T cell function and survival. Additionally, immunosuppressive cells within the TME, such as regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), often exploit these metabolic conditions by adapting to low glucose and high lactate, thus sustaining their immunosuppressive functions even in nutrient-deprived environments.

Tumor cells also release metabolic byproducts, such as adenosine, which accumulate in the hypoxic TME and further suppress immune responses. Adenosine interacts with A2A receptors on T cells, inhibiting their cytotoxic function and promoting a more tolerogenic immune environment. The metabolic reprogramming of both cancer cells and immune cells leads to a self-perpetuating cycle of immunosuppression, where tumor growth is facilitated by an immune system that is functionally compromised due to metabolic deprivation and suppression.

Implications for Therapeutic Targeting

The complex metabolic landscape within the TME, characterized by cancer-driven metabolic reprogramming and altered nutrient availability, offers potential avenues for targeted therapies. Approaches that modulate tumor metabolism—such as inhibitors of glycolysis, glutaminolysis, and one-carbon metabolism—have shown promise in preclinical models. Furthermore, therapies aimed at reprogramming the metabolism of immune cells to withstand the nutrient-deprived TME, such as metabolic checkpoint inhibitors, may help restore effective anti-tumor immunity. Targeting metabolic pathways involved in lactate production or glutamine dependency could disrupt the metabolic interplay within the TME, shifting it toward a less favorable environment for tumor growth and a more supportive one for immune cell function.

References

1. Newman, Alice C., and Oliver DK Maddocks. "One-carbon metabolism in cancer." British journal of cancer 116.12 (2017): 1499-1504.
2. Yang, Ming, and Karen H. Vousden. "Serine and one-carbon metabolism in cancer." Nature Reviews Cancer 16.10 (2016): 650-662.