Arginine, a semi-essential amino acid, plays multiple roles in cellular metabolism. As a precursor, arginine contributes to the synthesis of proteins, urea, creatine, homoarginine, putrescine, γ-aminobutyric acid (GABA), glutamate, and proline, which are essential for various physiological functions in the human body. The activity of key enzymes involved in arginine synthesis and metabolism, such as argininosuccinate synthase (ASS), arginase (ARG), and nitric oxide synthase (NOS), significantly affects human health and is associated with diseases such as hyperargininemia and creatine deficiency syndrome. Additionally, arginine is highly expressed in various cancers and serves as a key amino acid in regulating innate and adaptive immunity. Its metabolic derivatives, including nitric oxide (NO) and polyamines, also contribute to tumorigenesis. Despite extensive research on arginine metabolism, fully elucidating its metabolic pathways and the physiological or pathological roles of its metabolites remains crucial. This article discusses the metabolic pathways of arginine, key enzymes involved, and its relationship with cancer, aiming to provide a foundation for treating diseases, including cancer.
Arginine Biosynthesis
In adults, the small intestinal epithelial cells express carbamoyl phosphate synthetase I (CPS1) and ornithine transcarbamylase (OTC), thereby converting dietary absorbed glutamine and proline into citrulline. Once produced, citrulline is absorbed by the renal proximal tubules and converted into arginine under the action of argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL). Although renal metabolism constitutes the major part of the conversion of citrulline to arginine in the body, up to 40% of citrulline is converted to arginine in extrarenal tissues. Secondly, in the liver, the synthesis of arginine is closely linked to the urea cycle, where ornithine reacts with carbamoyl phosphate to form citrulline, which is then further converted into arginine. This process involves the catalysis of enzymes such as CPSI, OTC, and ASS. Meanwhile, not all cell types can synthesize arginine de novo, and the rate of synthesis specifically depends on the exchange rate of arginase between different cellular compartments.
Arginine Catabolism
Arginine metabolism generates various bioactive molecules, such as NO, polyamines, proline, and creatine, which are crucial for cell survival and proliferation. Below, we describe the synthesis of these molecules and their key enzymes. Arginine serves as a precursor for protein synthesis, NO, creatine, polyamines, guanidinoacetate, and urea, and is involved in multiple metabolic pathways.
Figure 1. Arginine metabolism pathways.
NO Synthesis
Dietary arginine is converted into NO and L-citrulline by NO synthase (NOS). The generated L-citrulline can be recycled back into the arginine biosynthetic pathway. As a signaling molecule, NO plays a crucial role in various biological processes and disease mechanisms. In the cardiovascular system, endothelial-derived NO acts as a potent vasodilator, regulating vascular tone and blood pressure. In the nervous system, neuron-derived NO modulates neurodevelopment and affects cognitive functions and stress responses. In the peripheral nervous system, NO regulates smooth muscle tone and motility in the gastrointestinal tract. NO is also involved in tumorigenesis, acting as either a tumor promoter or suppressor.
There are three isoforms of NOS: neuronal (nNOS), inducible (iNOS), and endothelial (eNOS). All NOS isoforms use arginine as a substrate, along with oxygen, NADPH, and tetrahydrobiopterin (BH4), to generate NO and citrulline. Since NO is vital for physiological functions, the competitive enzyme ARG, which converts arginine into ornithine and urea, plays a regulatory role. For instance, during lipopolysaccharide (LPS)-induced inflammation, activated M1 macrophages express iNOS, whereas during inflammation resolution, macrophages switch to expressing ARG1, leading to increased ornithine and polyamine production while reducing NO synthesis. Thus, regulating arginine availability for NOS is critical.
Urea Cycle
Maintaining low ammonium ion concentrations is crucial for mammalian cells. In human plasma, ammonium levels must remain below 70 µM to prevent toxicity. When plasma ammonium levels rise, the body detoxifies excess nitrogen via the urea cycle, excreting urea through urine or sweat. In hepatocytes, free ammonia is converted into carbamoyl phosphate using ATP, which then reacts with ornithine to form citrulline. Citrulline subsequently donates its amine group to aspartate, forming argininosuccinate, which is converted into arginine. Arginase then hydrolyzes arginine into urea and ornithine, completing the cycle.
ARGs are key enzymes in the urea cycle, converting toxic ammonia into urea. There are two isoforms: ARG1, predominantly expressed in the liver, facilitates urea formation and is linked to diseases such as uremia; ARG2, more widely distributed, participates in dopamine synthesis, protein metabolism, glutamate biosynthesis, and inflammatory processes. ARG deficiency results in hyperargininemia, characterized by elevated serum levels of arginine and guanidine compounds. Moreover, tumor cells overexpress ARG1 to deplete arginine, suppressing T-cell activation and promoting immune evasion.
Creatine Synthesis
Creatine is an organic acid that aids in providing energy to muscle and nerve cells. It can rapidly enhance muscle strength and alleviate muscle fatigue. Creatine is primarily synthesized in the kidneys and liver. Initially, arginine and glycine are converted into ornithine and guanidinoacetate under the action of arginine:glycine amidinotransferase (AGAT). Subsequently, guanidinoacetate methyltransferase (GAMT) facilitates the transfer of a methyl group from S-adenosylmethionine to guanidinoacetate, resulting in the production of creatine and S-adenosylhomocysteine. The rate of creatine synthesis is linked to enzyme activity, with AGAT activity being upregulated by growth hormone and downregulated by dietary creatine intake. Moreover, the amount of creatine synthesized is closely related to the levels of its substrate, arginine. Deficiencies in key enzymes such as GAMT and/or AGAT, as well as in the creatine transporter (CrT), can lead to symptoms such as mental and language developmental delays and epilepsy, a condition known as creatine deficiency syndrome. For athletes or fitness enthusiasts, direct supplementation with arginine is less effective due to the body's limited ability to absorb arginine and the rapid decline in blood arginine levels. Therefore, supplementing with intermediates in arginine metabolism, such as citrulline or ornithine, is a more effective alternative to direct arginine supplementation.
Homoarginine
In the first step of creatine synthesis, homoarginine is produced instead of guanidinoacetate when lysine replaces glycine in the reaction catalyzed by AGAT. Homoarginine is a structural analog of arginine and is present at low concentrations in healthy adults. Studies suggest that plasma homoarginine is weakly correlated with the arginine-to-ornithine ratio, indicating its potential role as an arginase inhibitor. Additionally, homoarginine may exert cardiovascular protective effects.
Guanidinoacetate, Agmatine, and Putrescine
Arginine can be converted into guanidinoacetate and ornithine via AGAT. Ornithine is further decarboxylated by ornithine decarboxylase (ODC) to form putrescine, which is subsequently methylated by putrescine-N-methyltransferase using S-adenosylmethionine to generate agmatine. Guanidinoacetate serves as a precursor for creatine synthesis and exhibits anti-inflammatory, antioxidant, and anti-apoptotic properties, making it a potential therapeutic target for cardiovascular and metabolic diseases. Agmatine plays neuroprotective roles by modulating imidazoline receptors, norepinephrine release, and nitric oxide production, contributing to blood pressure regulation and ischemic heart disease protection. Putrescine is involved in cell growth, ion transport, gut development, microbiota regulation, and reproduction.
Glutamate, Proline, and γ-Aminobutyric Acid (GABA)
Although these three metabolites are not directly derived from arginine, they are linked through the urea cycle. Arginine is converted into ornithine and urea by arginase. Ornithine can be further transformed into glutamate and glutamate-γ-semialdehyde via ornithine aminotransferase (OAT). The latter is then reduced to proline by pyrroline-5-carboxylate reductase (P5CS). Meanwhile, GABA is synthesized from putrescine through the action of diamine oxidase (DAO) and aldehyde dehydrogenase (ALDH). While glutamate and proline are important non-essential amino acids in human physiology, their roles will not be detailed here. GABA, an essential inhibitory neurotransmitter, plays a crucial role in the central nervous system of mammals.
Arginine Metabolism and Cancer
During the transformation of normal cells into cancer cells, tumor cells undergo metabolic reprogramming to meet the demands of rapid cell division and adapt to the nutrient-deficient microenvironment. In cells, arginine activates the downstream mTOR signaling pathway via SLC38A9 (a solute carrier family protein). As a key nutrient sensor, mTOR regulates protein synthesis, lipid biosynthesis, and nucleotide production, making arginine indispensable for tumor cells. To satisfy their arginine demand, tumor cells express specific types of solute carriers (SLCs).
Meanwhile, during T cell activation, elevated arginine levels induce a metabolic shift from glycolysis to oxidative phosphorylation, promoting T cell survival, the generation of memory T cells, and enhanced anti-tumor activity. To facilitate this process, immune T cells upregulate different types of SLCs, increasing arginine uptake to activate T cells and enhance their anti-tumor function.
Arginine Metabolic Enzymes and Tumors
ARGs are key enzymes in arginine metabolism, and elevated ARG activity has been detected in both serum and saliva samples from breast cancer patients, significantly correlating with disease progression and tumor metastasis. Among them, ARG1 plays a crucial role in cancer immune evasion by activating immunosuppressive cells such as myeloid-derived suppressor cells (MDSCs). ARGs are highly expressed in tumor-associated macrophages (TAMs) and MDSCs within the tumor microenvironment (TME), leading to arginine depletion, which impairs T cell activation and proliferation.Ren et al. reported that cancer patients exhibit a high abundance of circulating and tumor-infiltrating MDSCs. In the early stages of cancer, MDSCs accumulate in the peripheral blood of patients and express high levels of ARG1. In head and neck squamous cell carcinoma (HNSCC), induced ARG1 promotes polyamine biosynthesis, thereby facilitating tumor growth. After depleting arginine to create an immunosuppressive microenvironment, tumor cells release factors such as granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and chemokine ligand 2 (CCL2). These factors drive myeloid cells toward immunosuppressive phenotypes, including MDSCs and M2 macrophages. These immunosuppressive cells exhibit high ARG expression, depleting extracellular arginine and further inhibiting T cell activation and proliferation.
In many types of cancer cells, the expression of argininosuccinate synthetase 1 (ASS1) is frequently downregulated or lost, reducing endogenous arginine synthesis. As a result, tumor cells rely on exogenous arginine uptake to sustain their growth and proliferation. Based on this, researchers have explored arginine deprivation as an anti-tumor strategy. However, since arginine is also crucial for T cell activation, its depletion negatively impacts immune cells.To overcome this limitation, research teams have proposed an alternative therapeutic strategy that targets cancer-specific arginine-binding factors instead of depleting arginine itself. One study discovered that arginine regulates the expression of metabolism-related genes by binding to RNA-binding motif protein 39 (RBM39). Leveraging this mechanism, they developed inhibitors that specifically degrade RBM39, blocking cancer cell metabolic reprogramming without reducing overall arginine levels, thereby avoiding immune system suppression.
Figure 2.Arginine reprograms metabolism in liver cancer via RBM39.(Mossmann, D., et.al. 2023)
Arginine Metabolic Derivatives and Tumors
In various cancers, such as colorectal and breast cancer, increased polyamine synthesis has been observed, which may be linked to the upregulation of arginase (ARGs) in tumor cells. Growing evidence suggests that arginase and its downstream metabolic products, such as polyamines, are closely associated with tumor development and progression.
Nitric oxide (NO), a nitrogen oxide compound produced during arginine metabolism, plays a crucial role in tumorigenesis. NO is known to promote angiogenesis, tumor metastasis, resistance to apoptosis, and immune evasion. Elevated levels of NO and nitric oxide synthase (NOS) have been detected in cancer patients, which may be strongly associated with increased vascular endothelial growth factor (VEGF) expression, angiogenesis, and metastasis.
Conclusion
As a semi-essential amino acid, arginine plays a crucial role in human metabolism. Its synthesis and catabolism involve multiple enzymes and biological processes that are closely related to health, disease onset, and progression. In cancer, elevated arginine metabolism supports tumor cell proliferation. However, since different tumor cells exhibit distinct arginine metabolic pathways under various physiological conditions, developing personalized therapeutic strategies targeting the metabolic heterogeneity of different cancer types is essential.
Designing inhibitors or activators that regulate arginine synthesis, degradation, or transport could help modulate intracellular arginine levels in tumor cells. As an adjunctive cancer treatment, dietary arginine restriction may also slow the growth of ASS1-deficient tumors. While studies have shown that arginine deprivation can reduce tumor arginine levels and inhibit tumor proliferation, the essential role of arginine in immune cells suggests an alternative therapeutic approach. Instead of depleting arginine, targeting cancer-specific arginine-binding factors may offer a more effective strategy. In the future, identifying more arginine metabolism targets with tumor-specific lethality could provide new opportunities for cancer treatment.
References
- Ren, W., Zhang, X., Li, W., et.al. (2020). Circulating and tumor-infiltrating arginase 1-expressing cells in gastric adenocarcinoma patients were mainly immature and monocytic Myeloid-derived suppressor cells. Scientific reports, 10(1), 8056. https://doi.org/10.1038/s41598-020-64841-4
- Mossmann, D., Müller, C., Park, S.,et.al. (2023). Arginine reprograms metabolism in liver cancer via RBM39. Cell, 186(23), 5068–5083.e23. https://doi.org/10.1016/j.cell.2023.09.011
