The competition of amino acid among tumors, tumor-associated macrophages and T cells based on metabolic reprogramming

Summary

Amino acids are important nutrients in the process of tumor proliferation. Dysregulated amino acid metabolism profoundly influences tumor growth and immune cell function. Within the tumor microenvironment (TME), metabolic reprogramming of amino acids modulates the polarization of tumor-associated macrophages (TAMs) and the differentiation of T cells, processes intimately linked to tumor immune evasion. Meanwhile, metabolic reprogramming leads to amino acid competition between tumor cells and immune cells, particularly TAMs and T cells. To meet their own amino acid needs, tumors carry out a series of optimized metabolic strategies by expressing specific enzymes, cytokines, and amino acid transporters, and so forth promoting the formation of an immunosuppressive microenvironment and hindering anti-tumor immunity. Notably, this metabolic competition may exhibit spatial heterogeneity and temporal dynamics. Given the central role of amino acid metabolism in tumor progression and immune evasion, targeting key metabolic pathways represents a promising therapeutic strategy for cancer treatment.

Graphical abstract

Human metabolism; Immune response; Cancer systems biology; Cancer

Introduction

In the 20th century, the German scientist Warburg first discovered the reprogramming phenomenon of glucose metabolism in tumor cells, namely the well-known Warburg Effect, the phenomenon that tumor cells still prioritize glycolysis to metabolize glucose even under conditions of sufficient oxygen.¹ With advances in the study of nutrient metabolism in the tumor microenvironment (TME), accumulating evidence has revealed that amino acid metabolism also plays a critical role in tumor development. Tumor cells and various immune cells in the TME require amino acids to varying degrees to maintain their activity.² To meet the demands of energy and nutrients, tumor cells and immune cells compete for amino acids in the TME. Therefore, understanding the metabolism of important amino acids in the TME and the metabolic competition between tumor cells and immune cells, especially tumor-associated macrophages (TAMs) and T cells, helps to further understand the relationship between tumors and immunity.

Amino acid metabolism in the TME

Amino acids in the TME are indispensable nutrients for tumor cells to sustain rapid proliferation and growth. Amino acids can support the survival of cancer cells by sustaining redox homeostasis, maintaining energy production, and regulating epigenetic modifications.³ Cancer cells can take up large amounts of amino acids to provide carbon and nitrogen sources for the synthesis of biological macromolecules such as nucleotides. Their metabolic products enter the tricarboxylic acid cycle (TCA cycle) to produce adenosine triphosphate (ATP). However, under steady-state conditions, normal cells have a relatively low demand for amino acids.⁴ Amino acids provide approximately 30–50% of the carbon source for tumors, while glucose only provides 10–15%, indicating that tumors have a huge demand for amino acids.⁵ In addition, many intermediate products of amino acid metabolism, such as polyamines (mainly including putrescine, spermidine and spermine), also affect the life activities of tumors. Polyamines produced by arginine metabolism could promote the proliferation and invasion of tumors.⁶ Another product, nitric oxide (NO), has dual roles: It can promote tumor angiogenesis while also inhibiting tumors via p53 upregulation.⁷

Key enzymes and intermediate metabolites of amino acid metabolism not only regulate immune cell functions but also serve as indispensable nutrients for their activation and differentiation. For example, amino acids can further activate the T cell receptor (TCR)-CD3 complex by directly activating sensor proteins such as mammalian target of rapamycin (mTOR) in the cytoplasm, or affect the activation and function of T cells through metabolism.² In addition, amino acid transporters, most of which belong to the solute carrier (SLC) superfamily, also play a critical role in mediating nutrient uptake. In B cells, amino acids also promote differentiation via specific transporters and mTOR, enhancing the secretion of antibodies such as IgG and IgM, as well as cytokines.⁸ Meanwhile, glutamine supports the anti-tumor function of NK cells. When glutamine is removed, or the core functional subunit SLC7A5 of L-type amino acid transporter-1 (LAT1) is blocked, the level of cMyc protein rapidly decreases, thereby leading to impaired NK cell function.⁹ The metabolic pathways of distinct amino acids differentially affect macrophage differentiation and function. In M1-macrophages, arginine is metabolized to NO, which inhibits tumor development, while in M2-macrophages, arginine is catabolized by arginase-1 (Arg-1), suppressing anti-tumor immunity.¹⁰ In addition, arginine can also be metabolized by glycine amidinotransferase (AGAT) and guanidinoacetate methyltransferase (GAMT) to produce creatine, thereby promoting M2 polarization in macrophages.¹¹ The glutamine-derived metabolite α-ketoglutarate (α-KG) also promotes M2-macrophage activation, whereas a reduced α-KG/succinate ratio enhances the pro-inflammatory function of M1-macrophages.¹² Amino acids can also influence the infiltration of immunosuppressive myeloid-derived suppressor cells (MDSCs) in the TME. On the one hand, studies have shown that glutamine deficiency upregulates G-CSF and GM-CSF expression, promoting MDSC enrichment in breast cancer.¹³ On the other hand, MDSCs can preferentially express Arg-1 to consume arginine in the TME, thereby suppressing the function of T cells.¹⁴

The development of immunosuppressive TME is closely related to abnormal amino acid metabolism.¹⁵ Excessive amino acid catabolism by tumor cells increases immunosuppressive metabolites in the TME. Concurrently, tumor cells deplete amounts of amino acids, leading to local shortages that impair immune cell differentiation and function. Single-cell sequencing and transcriptome analysis revealed that the high-amino acid group in colon cancer epithelial cells exhibited a relatively weak immune capacity, and the activation of immune cells and anti-tumor functions in the TME were also significantly weakened.¹⁶ With the decline of immune function and the formation of an immunosuppressive microenvironment, tumor immune evasion subsequently occurs. For instance, in head and neck squamous cell carcinoma, kynurenine (KYN), a tryptophan metabolite, upregulates Siglec-15 in cancer cells, which in turn impairs T cell infiltration and activation, facilitating tumor immune escape.¹⁷

In summary, amino acid metabolism regulates tumor progression and immune responses through multiple mechanisms. Notably, TAMs are the most abundant immune cells in the TME and are closely associated with the formation of an immunosuppressive microenvironment.¹⁸ T cells, particularly CD8⁺ cytotoxic T lymphocytes, serve as the core effectors of anti-tumor immunity by directly killing cancer cells.¹⁹ Both cell types are highly sensitive to amino acid availability, and amino acid metabolic reprogramming profoundly influences their activation, polarization, and effector functions.^20,21 Emerging evidence suggests that competition for amino acids between tumor cells and these immune cells has gradually become a key mechanism of immune evasion. Therefore, this review focuses on amino acid metabolism in TAMs and T cells and their metabolic competition with tumor cells.

The influence of amino acid metabolic reprogramming on TAMs and T cells

Amino acid metabolic reprogramming in TAMs

The traditional dichotomy classifies TAMs into M1 and M2 types, with M1-TAMs usually associated with pro-inflammatory and anti-tumor effects, while M2-TAMs are often related to anti-inflammatory and pro-tumor activities.²² However, some TAMs co-express both M1 and M2 markers.²³ The simple M1/M2 classification has limitations in reflecting the dynamic, complex, and plastic state of TAMs.^24,25 Nevertheless, many studies on the metabolism of TAMs still rely on the traditional dichotomy. Therefore, this article continues to use the M1/M2 dichotomy to discuss the impact of amino acid metabolism on TAMs (Table 1).

Table 1.

Amino acid metabolic reprogramming on TAMs

Function	Amino acids and metabolite	Mechanism
Regulation of M2 polarization	spermidine	spermidine promotes the hypusination of eIF5A.26
	spermine	spermine promotes PPARG transcription via p53/TDG-mediated DNA demethylation.27
	glutamine	GS expression maintains glutamine supply.28
	α-KG	α-KG promotes M2 polarization.12
	methionine	high expression of MAT2A upregulates SAM levels and promotes H3K4me3 histone methylation modification.29
	tryptophan	tryptophan activates AHR to promote the expression of Arg-1.30
	ILA	ILA inhibits pro-inflammatory polarization in vitro.31
	BCAAs	BCAAs increase the OXPHOS of M2-macrophages.32
Mediated immunosuppression	arginine	TAMs consume arginine and inhibit NO synthesis and protein translation in T cells.33
	tryptophan	IDO1 consumes tryptophan, activates AHR, induces the expression of IL-10, TGF-β and Arg-1 and the differentiation of Tregs.34,35
	glutamine	under hypoxia, TAMs produce IL-23, which activates Tregs.36
Metabolic plasticity	arginine	SEP restores BH4 levels, reprogramming M2-type cells back to M1-type.37
	serine	outside the TME, M1 polarization is promoted through the serine-SAM-IGF1-P38-STAT1 axis or PHGDH38; within the TME, IL-4 upregulates PHGDH, and α-KG activates mTORC1 to promote M2 polarization.39

eIF5A: eukaryotic translation initiation factor 5A, GS: glutamine synthetase, α-KG: α-ketoglutarate, MAT2A: methionine adenosyltransferase II alpha, SAM: S-adenosylmethionine, ILA: indole-3-lactic acid, BCAAs: branched-chain amino acids, OXPHOS: oxidative phosphorylation, TAMs: tumor-associated macrophages, NO: nitric oxide, IDO1: indoleamine 2,3-dioxygenase 1, Tregs: regulatory T cells, SEP: sepiapterin, BH4: tetrahydrobiopterin, TME: tumor microenvironment.

Various amino acids can promote the pro-tumor polarization of TAMs through different metabolic mechanisms. Studies have found that breast cancer cells can secrete arginine to drive the pro-tumor polarization of TAMs.²¹ Furthermore, the arginine metabolite spermidine can support M2 polarization by promoting the hypusination of eukaryotic translation initiation factor 5A (eIF5A) to regulate mitochondrial respiration.²⁶ Spermine can also promote the transcription of the nuclear receptor PPARG in TAMs through p53/TDG-mediated DNA demethylation, and PPARG is a key regulator of M2 polarization in TAMs.²⁷

Other amino acids similarly regulate the M2 polarization of TAMs. Glutamine synthetase (GS), the key enzyme for glutamine synthesis, is often highly expressed in most cancers and M2-TAMs.²⁸ Glutamine and its metabolite α-KG also promote the expression of a pro-tumor phenotype in TAMs.¹² It is well known that methionine, catalyzed by methionine adenosyltransferase II alpha (MAT2A), consumes ATP to become activated S-adenosylmethionine (SAM). In gastric cancer, the TME upregulates MAT2A in macrophages, leading to elevated SAM levels. High levels of SAM promote H3K4me3 histone methylation modification on the RIP1 gene promoter, further activating RIP1 expression and ultimately driving pro-tumor macrophage polarization.²⁹

Tryptophan regulates TAM polarization in association with an important sensor, the aryl hydrocarbon receptor (AHR).⁴⁰ Tryptophan metabolism activates AHR in TAMs, thereby promoting the upregulation of Arg-1 expression and the formation of an immunosuppressive phenotype.³⁰ Additionally, indole-3-lactic acid (ILA) released by Bifidobacterium breve during tryptophan breakdown can inhibit the pro-inflammatory polarization of TAMs in vitro.³¹ Current knowledge regarding the impact of branched-chain amino acid (BCAA) metabolism on macrophages is incomplete, but BCAA supplementation may be associated with the polarization of M2 macrophages in vitro and in vivo, and increase the oxidative phosphorylation (OXPHOS) process in M2 macrophages.³²

Thus, metabolic reprogramming of most amino acids can often drive the dynamic pro-tumor polarization of TAMs, and M2-TAMs could acquire immunosuppressive functions through this specific amino acid metabolic pathway. Polarized TAMs often exhibit a significant capacity for tryptophan and arginine uptake and metabolism, primarily manifesting the phenotypic and functional characteristics of immunosuppressive macrophages.⁴¹ For example, M2-TAMs consume arginine, which is associated with the inhibition of NO and protein synthesis in T cells, thereby affecting TCR function.³³ M2-TAMs can highly express indoleamine 2,3-dioxygenase 1 (IDO1), directly depleting tryptophan and subsequently activating AHR,³⁴ inducing the expression of the immunosuppressive cytokine IL-10, and promoting the expression of TGF-α, TGF-β and Arg-1, thereby suppressing immune function and facilitating tumor immune evasion.³⁵ Furthermore, in glutamine-addicted clear cell renal cell carcinoma, TAMs undergo conversion to the M2 phenotype and, under hypoxic conditions, produce IL-23 to activate regulatory T cells (Tregs), participating in the formation of an immunosuppressive microenvironment.³⁶ Given the dependence of TAM proliferation and differentiation on amino acid metabolism, amino acid depletion in the TME can repolarize TAMs toward an anti-tumor phenotype, conversely controlling tumor progression. Short-term methionine deprivation inhibits the TCA cycle, regulates M1 polarization through uncharged tRNA and PDHA1 phosphorylation, while simultaneously suppressing M2 polarization.⁴² In vitro experimental studies have also shown that the traditional Chinese medicine Jianpi Jiedu Decoction inhibits AHR expression and M2 polarization of TAMs, thereby suppressing the growth of colon cancer cells.⁴³

Although during tumor proliferation and development, amino acid metabolic reprogramming triggers changes in cytokines, metabolites, and signaling molecules, subsequently reshaping the TAM phenotype and promoting an immunosuppressive microenvironment, this does not mean that amino acids solely drive pro-tumor polarization. TAM metabolism is plastic. When the signals received by TAMs, the expression of key enzymes, or the concentrations of metabolites change, their metabolic activity and functional status can also change. For instance, arginine is transported into activated macrophages by SLC7A2.² M1-macrophages primarily metabolize arginine via iNOS to produce NO, which exerts anti-tumor effects, whereas M2-macrophages utilize Arg-1. Notably, Fernando et al. discovered that this differential metabolic pathway is associated with differences in the availability of the NO synthase cofactor tetrahydrobiopterin (BH4). The BH4 precursor sepiapterin (SEP) can restore BH4 levels in M2 macrophages, redirecting arginine metabolism toward NO synthesis, thereby converting M2 macrophages to an M1 phenotype.³⁷

Similarly, the role of serine metabolism in macrophage polarization also exhibits significant plasticity. In inflammatory models outside the TME, serine metabolism promotes IFN-γ-induced M1 polarization via the serine-SAM-IGF1-p38-STAT1 axis.³⁸ Additionally, PHGDH, the key enzyme in the serine synthesis pathway, consumes NAD+ to inhibit the activity of deacetylase SIRT1/3, thereby mediating M1 macrophage polarization and IL-1β expression.⁴⁴ However, in TME, Cai et al. found that IL-4 and tumor-conditioned medium can upregulate PHGDH expression in TAMs, generating α-KG to activate the mTORC1 signaling pathway, thereby driving TAM polarization toward an immunosuppressive M2 phenotype.³⁹ This differential effect may stem from differences in the expression of signals such as IL-4 and tumor-derived factors in TME. Differential functional differentiation of metabolites may also contribute to the differences in polarization. For example, SAM is biased toward epigenetic regulation, while α-KG directly participates in metabolic reprogramming and mTORC1 signaling activation. Furthermore, factors such as the use of primary macrophages versus cell lines and conditioned media from different tumor sources may also influence research outcomes.

This metabolic plasticity often leads to the diversity and heterogeneity of the TAM population. With the continuous development of new technologies such as single-cell RNA sequencing, the traditional M1/M2 dichotomy of macrophages has proven inadequate to describe the heterogeneity of TAMs in the TME. Recent single-cell RNA sequencing data from various solid tumors indicate that TAMs can be classified into four subgroups, including two major subgroups, C1Q + TAMs and SPP1+ TAMs, and two minor subgroups, FCN1+ TAMs and CCL18+ TAMs.⁴⁵ Rakina et al. revealed more TAM subpopulations from the perspectives of single-cell and spatial background. Among them, IDO1 is listed as the signature gene of C1QC + TAMs and CTSB+ TAMs, while ARG1 is the signature gene of GPNMB+ TAMs.⁴⁶ Amino acid metabolism is also closely related to the functions of these subgroups. For instance, C1Q + TAMs can highly express IDO, metabolizing tryptophan into KYN. KYN promotes Treg differentiation by activating AHR on infiltrating immune cells, leading to immunosuppression.⁴⁷ And the upregulated Arg-1 in SPP1+ TAMs and GPNMB+ TAMs inhibits the effector function of T cells by consuming arginine.⁴⁶ Thus, single-cell sequencing technology based on transcriptome expression profiles can more objectively identify subpopulations of TAMs with unique molecular characteristics and functional properties. This also suggests that molecular classification can more precisely link amino acid metabolism to specific TAM subsets, aiding the identification of key targets for metabolic reprogramming. In the future, the association between different TAM subgroups and amino acid metabolism should be further analyzed based on omics technology, providing new therapeutic strategies for targeting amino acid pathways.

Amino acid metabolic reprogramming in T cells

T cells are the core effector cells in anti-tumor immunity. Based on different membrane markers, T cells are classified into two main categories: CD4⁺ T cells and CD8⁺ T cells. CD4⁺ T cells include follicular helper T cells (Tfh), helper T cells (Th), and Tregs, while CD8⁺ T cells include cytotoxic T cells (CTLs) and memory T cells (Tm).⁴⁸ Amino acid metabolism can influence T cell activation, differentiation, effector function, and exhaustion status (Table 2).

Table 2.

Amino acid metabolic reprogramming on T cells

Function	Amino acids and metabolite	Mechanism
Regulation of activation and proliferation	glutamine	glutamine metabolism fuels the TCA cycle, supports nucleotide synthesis, and maintains redox balance.49
	leucine, methionine, tryptophan	these amino acids are ingested through SLC7A5, activating mTORC1 and driving protein synthesis and glycolysis.50
Bidirectional regulation of differentiation and functional effects	high-level arginine	high-level arginine promotes the activation of mTORC1 and supports the formation of effector T cells and Tm.51
	low-level serine	in a low-arginine environment, Tregs activate the ATF4-SLC7A11-GSH axis to maintain inhibitory function.52
	high-level serine	high-level serine recruits c-Fos and promotes PD-1-dependent CD4+ Tregs differentiation.53
	serine deficiency	a serine-free diet can slow the growth of colorectal cancer and promote CD8+ T cell infiltration.54
T cell exhaustion and immunosuppression	spermidine	spermidine inhibits TCR clustering required for CD8+ T cell activation.55
	KYN	KYN activates AHR to mediate Treg differentiation and functional inhibition of effector T cells.56
	5-HTP	5-HTP activates AHR and induces T cell exhaustion.57
	high-level SAM	high-level SAM reprograms CD8+ T cell chromatin accessibility, leading to T cell exhaustion.58
	BCAAs	BCAAs deficiency leads to abnormal histone acetylation, which impairs the expression of effector genes in CD8+ T cells.59
	methionine deficiency	in CD4+ T cells, methionine deficiency decreases AMPK and upregulates PD-1, inducing Th cell exhaustion.60
		in CD8+ T cells, methionine deficiency reduces H3K79me2 methylation, impairs STAT5 signaling, and induces apoptosis.61

TCA cycle: tricarboxylic acid cycle, mTORC1: mechanistic target of rapamycin complex 1, Tm: memory T cells, TCR: T cell receptor, KYN: kynurenine, AHR: aryl hydrocarbon receptor, 5-http: 5-hydroxytryptophan, BCAAs: branched-chain amino acids, PD-1: death protein 1, Th: helper T cells.

During T cell activation, amino acid metabolism is critical for supporting clonal expansion and functional acquisition. Resting T cells primarily rely on OXPHOS to maintain energy homeostasis, but upon activation, they rapidly shift to aerobic glycolysis and glutaminolysis to meet biosynthetic demands.⁶² This metabolic shift is often regulated by the upregulation of amino acid transporters and key metabolic enzymes. For example, glutamine is the most abundant amino acid taken up by activated T cells and supports T cell activation and clonal expansion through multiple mechanisms. SLC1A5-mediated glutamine uptake activates mTORC1. Subsequently, intracellular glutamine is metabolized to glutamate by glutaminase (GLS) and then to α-KG, which enters the TCA cycle to replenish energy. This metabolic pathway also contributes to glutathione synthesis, redox balance, and the hexosamine biosynthetic pathway.⁴⁹ Notably, among the numerous amino acid transporters, SLC7A5 is one of the most significantly upregulated transporters.⁶³ SLC7A5 is involved in the transport of various amino acids such as leucine, methionine, and tryptophan, and is crucial for maintaining mTORC1 activity.⁵⁰ Deficiency in SLC7A5 impedes glycolysis and OXPHOS.⁶⁴ Furthermore, transporters such as SLC7A1, SLC1A5, and SLC7A5 can synergistically maintain mTORC1 activity. The mTORC1/c-Myc signaling pathway further upregulates these same transporters, driving glycolysis and protein synthesis to provide the necessary bioenergetic and macromolecular precursors for T cells to progress from the G1 phase to the S phase.⁶⁵

Amino acid metabolism also plays a significant role in regulating T cell differentiation and effector function. L-arginine, an essential amino acid, promotes mTORC1 activation, anti-tumor responses and Tm formation.⁵¹ Conversely, CD4⁺ Tregs activated in a low-arginine environment undergo metabolic and transcriptional reprogramming via the ATF4-SLC7A11-GSH axis, thereby exerting immunosuppressive effects.⁵² Similarly, serine enrichment in melanoma promotes programmed cell death-1 (PD-1)-dependent CD4⁺ Treg differentiation via sphinganine-mediated c-Fos recruitment.⁵³ Conversely, a serine-free diet can slow the growth of colorectal cancer and promote CD8⁺ T cell infiltration, enhancing anti-tumor immune responses.⁵⁴ Thus, it is evident that the impact of amino acids on T cell differentiation and function is not unidirectional. Based on differences in cytokines, metabolite concentrations, metabolic pathways, and metabolic sources within the microenvironment, amino acid metabolism possesses the potential for the bidirectional regulation of T cell function.

However, within the nutrient-poor TME, amino acid metabolic reprogramming often drives T cell exhaustion and anti-tumor immune suppression. This process is typically associated with competitive nutrient uptake, accumulation of inhibitory metabolites, and epigenetic remodeling.

Cancer cells share similar metabolic demands with anti-tumor CD8⁺ T cells, leading to competition for the limited amino acids within the TME. Tumor cells frequently enhance the uptake of critical amino acids such as glutamine, leucine, and arginine by upregulating transporters such as SLC25A22, SLC7A5, and SLC1A5, or metabolic enzymes such as IDO1 and Arg-1. This excessive consumption of amino acids can impact T cell biosynthesis and energy supply, thereby impairing their anti-tumor function.^66,67,68 This depletion of T cell nutrient supply caused by amino acid metabolic competition will be elaborated on later.

Various inhibitory metabolites accumulating in the TME can also regulate T cell activity and mediate immune suppression. Spermidine, a product of tumor arginine metabolism, inhibits TCR clustering required for CD8⁺ T cell activation within the TME.⁵⁵ Among these, the tryptophan-KYN-AHR pathway is a classic paradigm of metabolite-mediated immunosuppression. Tryptophan is often catabolized by IDO or tryptophan 2,3-dioxygenase (TDO2) into KYN. KYN can directly mediate the differentiation of CD4⁺ T cells into Tregs by activating AHR, thereby suppressing effector T cell function.⁵⁶ In addition to KYN, high IL-2 levels in the TME induce sustained STAT5 activation in CD8⁺ T cells, promoting the conversion of tryptophan to 5-hydroxytryptophan (5-HTP), which also activates AHR and impairs CD8⁺ T cell function.⁵⁷ Additionally, in hepatocellular carcinoma, the active methionine metabolism in tumor cells elevates levels of its products SAM and 5-methylthioadenosine (MTA), which can reprogram the chromatin accessibility of CD8⁺ T cells, ultimately leading to T cell exhaustion and dysfunction.⁵⁸

Finally, amino acid metabolism can alter T cell chromatin states and gene expression by influencing modifications such as methylation and acetylation, ultimately resulting in T cell exhaustion or irreversible loss of function. For instance, BCAA deficiency in T cells reduces acetyl-CoA production, leading to abnormal histone acetylation and impaired effector gene expression in CD8⁺ T cells.⁵⁹ Furthermore, methionine deficiency drives T cell dysfunction through multiple synergistic signaling pathways. In CD8⁺ T cells, reduced SAM leads to decreased histone H3K79me2 methylation levels and impaired STAT5 signaling, subsequently inducing T cell apoptosis and functional loss.⁶¹ In CD4⁺ T cells, the decrease in H3K79me2 caused by methionine deficiency leads to reduced AMPK activity and increased PD-1 expression, resulting in the exhaustion of CD4⁺ Th cells and compromised anti-tumor immune function.⁶⁰ These two pathways reflect the multi-dimensional regulatory capacity of the methionine metabolic network across different T cell subsets. From a metabolic network perspective, methionine deficiency simultaneously affects methyl donor supply, energy-sensing pathways, and immune checkpoint expression, forming a synergistic immunosuppressive network.

In summary, the inhibition of T cell immune function resulting from amino acid metabolic reprogramming is not regulated by a single independent mechanism. Tumor-driven amino acid competition not only induces metabolic defects in T cells but also generates immunosuppressive metabolites via active metabolism of tumor cells. Ultimately, persistent epigenetic alterations lock T cells into an exhausted state, impairing anti-tumor function. This interconnected network, involving nutrient competition, metabolite accumulation, and epigenetic remodeling, synergistically drives immunosuppression. Nevertheless, T cell exhaustion and functional inhibition are not irreversible. By supplementing key amino acids or blocking inhibitory pathways, T cell activity and function can be restored once again.^69,70,71

The metabolic competition of key amino acids between tumor cells and immune cells, especially TAMs and T cells

The TME exhibits characteristics such as hypoxia, low pH, nutrient deficiency, accumulation of immunosuppressive metabolic products, and extensive infiltration of immunosuppressive cells. These features combine with the high demand for amino acids by tumor cells and immune cells such as TAMs and T cells, creating an environment where amino acid resources are scarce but highly competitive. Just as the reprogramming of amino acid metabolism in TME often prompts TAMs to polarize toward the tumor-promoting M2 type and promotes the exhaustion and dysfunction of T cells, the metabolic competition for amino acids between tumor cells and immune cells also contributes to the establishment of an immunosuppressive microenvironment (Figure 1 created with BioGDP.com).⁷²

Figure 1.

The key amino acid metabolic competition between tumor cells and immune cells

As important nutrients, amino acids are subject to competitive uptake by tumor cells and immune cells.

(A) Tumor cells compete with immune cells for amino acid uptake via transporters and key enzymes, leading to immune cell dysfunction and the generation of immunosuppressive cells. A deep green arrow indicates a higher intake of amino acids, while a lighter green arrow indicates a lower intake.

(B) Differentiation of Tregs and M2-TAMs mediated by amino acid metabolic competition further exacerbates arginine competition and secretes cytokines such as IL-10 and TGF-β, enhancing T cell exhaustion and dysfunction.

(C) Amino acid metabolic competition exhibits spatial heterogeneity in the TME. In malignant regions and immune cell-enriched border zones, metabolism of amino acids such as glutamine is highly active, while metabolic activity remains normal in other areas.

(D) Temporal dynamics of amino acid competition during tumor progression. During the early stage of immune surveillance, immune cells such as T cells remain active and compete with tumor cells for amino acids. In the advanced stage of tumor development, chronic amino acid deprivation leads to T cell exhaustion and promotes immunosuppression. PD-L1: programmed death-ligand 1, SLC: solute carrier, GFPT2: glutamine-fructose-6-phosphate transaminase 2, PD-1: programmed cell death-1, ER: endoplasmic reticulum, mTORC1: mechanistic target of rapamycin complex 1, GCN2: general control nonderepressible 2, MDSCs: myeloid-derived suppressor cells, TAMs: tumor-associated macrophages, Arg-2: arginase-2, Arg arginine, Gln: glutamine.

The mediators of amino acid competition: amino acid transporters and key enzymes

Amino acid metabolic competition is often mediated by amino acid transporters and key intracellular metabolic enzymes. Tumor cells and inhibitory immune cells can gain an advantage by upregulating these transporters and enzymes, and this competitive pattern has been verified in various amino acids.

The upregulation of transporters leads to the depletion of amino acids required by immune cells. In hepatocellular carcinoma, tumor cells upregulate SLC1A5 to compete with myeloid cells for glutamine, resulting in glutamine deprivation in TME. This deficiency induces endoplasmic reticulum stress, upregulates GPR109A in myeloid cells, and subsequently activates the ERK signaling pathway and drives their immunosuppressive polarization, promoting G-MDSC infiltration and M2-TAMs differentiation, ultimately inhibiting CTL-mediated anti-tumor immunity.⁷³ Exosomes derived from myofibroblastic cancer-associated fibroblasts (myCAFs) promote tumor cell uptake of glutamine from the TME by upregulating SLC38A2, thereby reducing glutamine levels in immune cells such as T cells, macrophages, and NK cells, affecting their biosynthesis and energy supply, and subsequently leading to a decline in anti-tumor immunity and tumor-killing ability.⁷⁴ Additionally, tumor cells highly express SLC7A5 and SLC43A2, competing for more methionine and thereby affecting the SAM level and methylation modification in T cells, causing T cell dysfunction.⁷⁵ Melanoma cells compete with CD8⁺ T cells for cysteine, which is associated with impaired cysteine synthesis, accumulation of reactive oxygen species (ROS), and ferroptosis, manifested as impaired T cell memory formation, upregulated PD-1/TIM-3 expression, and reduced cytokine secretion.⁷⁶

The metabolic imbalance of key enzymes also promotes the competition of amino acid metabolism. Macrophages often require sufficient glutamine to regulate mitochondrial division and thereby exert the function of engulfing tumor cells. Cancer cells overexpress glutamine-fructose-6-phosphate transaminase 2 (GFPT2) to consume glutamine in the TME, which prevents TAMs from utilizing glutamine, disrupts mitochondrial fission, causes fragmentation, and consequently allows tumor cells to escape immune clearance.⁷⁷ In metastatic melanoma, Tregs highly express arginase-2 (Arg-2), which consumes intracellular and extracellular arginine, inhibits the proliferation of CD4⁺ effector T cells, and reduces the activity of mTOR signaling.⁷⁸

In summary, amino acid transporters and key enzymes are the central nodes of the immune-metabolic competition. They determine the outcome of metabolic competition by regulating the nutrient flux and metabolite availability mediated by tumors and immune cells, and drive the shaping of the immunosuppressive TME.

Spatial heterogeneity and temporal dynamics of amino acid metabolic competition

The intensity of amino acid metabolism competition is not uniform throughout the entire tumor. By integrating spatial transcriptomics and metabolomics, it has been discovered that there is significant spatial heterogeneity in amino acid metabolism within tumor tissues. In pancreatic cancer tissues, abnormal cell communication in the malignant regions shows significant depletion of glutamine, synchronous activation of polyamine metabolism, and an increase in related metabolic enzymes. In contrast, glycine, alanine, proline, and arginine are enriched in normal regions, while histidine is enriched in immune regions and stromal regions. This spatial distribution reflects the differences in the amino acid requirements of cancer cells and immune cells in different pathological regions. Tumor cells in the malignant regions may compete for the uptake of glutamine to support proliferation, while immune regions and stromal regions retain specific amino acids to maintain immune regulatory functions, resulting in the heterogeneous spatial distribution of amino acid metabolism competition.⁷⁹

Similar phenomena were also observed in gastric cancer. Notably, a tumor boundary area containing multiple immune cells was first discovered in gastric cancer tissues. This area exhibits unique amino acid metabolic competition characteristics.⁸⁰ Compared to normal tissues, the glutamine metabolism of immune cells in this area was significantly upregulated. This finding may suggest that metabolic competition for glutamine occurs at the junction between tumors and normal tissues. This particular spatial metabolic heterogeneity may be closely related to the formation of the tumor immune barrier and immune escape, providing a new spatial perspective for understanding amino acid metabolic competition in the TME.

Beyond spatial heterogeneity, amino acid competition also exhibits temporal dynamics. Tumor progression, metastasis, and treatment resistance are continuous and dynamic processes, and metabolic reprogramming of nutrients such as amino acids is a key feature associated with these changes.⁸¹ In the early stage of tumor development, immune cells such as T cells can compete with tumor cells for amino acids and other nutrients while maintaining a certain level of activity. However, as the tumor progresses, tumor cells consume key amino acids by highly expressing related amino acid transporters and metabolic enzymes, while toxic metabolites such as lactic acid and KYN gradually accumulate in the TME, ultimately jointly contributing to the functional exhaustion of T cells and immune resistance.⁸²

However, this trend is not completely irreversible. Developing corresponding treatment strategies can reverse this situation. For instance, the glutamine transporter inhibitor V-9302 selectively blocks the uptake of glutamine by TNBC cells, but does not block the uptake by CD8⁺ T cells, thereby promoting the synthesis of glutathione and improving the function of CD8⁺ T cells.⁸³ Metformin, by down-regulating SLC7A5, inhibits the competition of colorectal cancer cells for tryptophan, which helps to restore the function of CD8⁺ T cells.⁸⁴

Thus, the competitive balance between tumor cells and immune cells is not fixed but dynamically evolves during tumor progression and treatment.

Metabolic network crosstalk mediated by amino acid metabolic competition

In the TME, the competition among various amino acid metabolic pathways that affect downstream functions and mediate immunosuppression is not an independent process. Instead, these pathways are interconnected and collectively contribute to immune suppression.

When tumor cells compete for amino acid uptake through the overexpression of amino acid transporters or key enzymes, leading to amino acid deficiency in T cells, the activation of the mTORC1 pathway is inhibited, thereby affecting the proliferation and activation of T cells. At the same time, the general control nonderepressible 2 (GCN2) pathway is activated, inhibiting the translation process and promoting the tumor’s adaptation to the amino acid-deficient environment.⁸⁵ This dynamic balance and interaction system between mTORC1 and GCN2 coordinates with each other, jointly promoting the functional exhaustion of T cells and forming the basis of the entire immune suppression network.

Based on this, the metabolic products of amino acids act as signaling factors to further amplify the inhibitory effect and promote changes in the function of immune cells. Arginine-derived polyamines in TAMs, via the P53-TDG axis, induce DNA demethylation and drive M2 polarization.²¹ The metabolite KYN of tryptophan promotes the differentiation of immunosuppressive Tregs and the M2 polarization of TAMs by activating AHR.⁵⁶ Methionine-derived SAM accumulates in tumor cells but is deficient in T cells. This deficiency impairs H3K79me2 methylation and contributes to the dysfunction and exhaustion of CD8⁺ and CD4⁺ T cells through distinct pathways.⁷⁵ In addition, the reprogrammed M2-TAMs continuously secrete inhibitory cytokines and express immune checkpoint molecules,⁸⁶ which in turn exacerbates amino acid deficiency, reinforce immunosuppression, and continuously worsen the imbalance state of mTORC1/GCN2.

The amino acid sensors mTORC1 and GCN2 integrate signals of amino acid availability, such as arginine, glutamine, and tryptophan. Then, this integration triggers cross-talk among downstream signaling pathways, including P53, SAM, AHR, and H3K79me2. Moreover, the inhibitory immune cells, including Tregs and M2-TAMs, further reinforce immune evasion and T cell exhaustion. This series of metabolic pathways is interrelated and jointly forms a complex immune regulatory network. This also reveals the reasons why single-target treatment strategies are ineffective, as they lack an understanding of the systematic operation of the amino acid metabolism. Therefore, in the future, efforts should be focused on developing more comprehensive and effective new drugs based on the entire amino acid metabolism network.

Therapeutic strategies targeting amino acid metabolism

Given the significant role that amino acid metabolism plays in tumor growth and proliferation, as well as in the functioning of immune cells, further investigation into the complex amino acid metabolism between tumor cells and immune cells and the development of new drugs targeting amino acid metabolism for tumor treatment have emerged as potential therapeutic strategies. The following will introduce targeted therapies focusing on transporters, metabolic enzymes, and sensors, which are related to amino acid metabolic competition (Table 3).

Table 3.

Drugs/intervention targeting amino acid metabolism of tumors

Drug (Intervention)	Target	Mechanism
Targeted amino acid transporters
JPH203 (KYT0353)87	SLC7A5	inhibit SLC7A5 and down-regulate the mTORC1 signaling pathway
Genetic engineering of CAR-NK/T cells88	SLC1A5, SLC3A2, SLC7A5	enhance the expression of amino acid transporters in immune cells
Target amino acid metabolic enzymes
CB-115889	Arg-1	inhibit Arg-1
OATD-0290	Arg-1, Arg-2	inhibit Arg-1, Arg-2
Targeted amino acid sensor
BAY 241696491	AHR	inhibit AHR
Azole [4,5-day] pyrimidine compounds92	GCN2	inhibit GCN2
Prodrugs
Sirpiglenastat (DRP-104)93	DON	Release DON in tumor tissues

mTORC1: mechanistic target of rapamycin complex 1, CAR: chimeric antigen receptors, AHR: aryl hydrocarbon receptor, GCN2: general control nonderepressible 2, DON: 6-diazo-5-oxo-L-norleucine.

Amino acid transport across the plasma membrane is mediated by various transporters, which play a key role in tumor proliferation. Therefore, they are considered pharmacological targets to block the growth and survival of cancer cells. Most previous studies have focused on inhibiting the synthesis and utilization of amino acids by tumors. For instance, a specific inhibitor JPH203 (KYT0353) with high affinity for SLC7A5 can inhibit tumor protein synthesis by suppressing SLC7A5’s function to down-regulate the mTORC1 signaling pathway and activate the general amino acid control pathway. Meanwhile, JPH203 has a relatively minor impact on other transporters and shows no obvious toxicity.⁸⁷ In addition, SLC7A5 inhibitors have achieved encouraging results in Phase I clinical trials for the treatment of advanced solid tumors (UMIN000016546), and currently, Phase II randomized controlled trials have also demonstrated relatively significant efficacy in biliary tract cancers (UMIN000034080).⁹⁴ Phase III trials are underway. Recent studies have shown that upregulating the expression of amino acid transporters can enhance the ability of immune cells to obtain amino acids from the TME. NK cells and T cells expressing chimeric antigen receptors (CARs) can better recognize and eliminate tumors. However, they need to compete with tumor cells for nutrients to meet their requirements. SLC1A5, SLC7A5, and the auxiliary subunit SLC3A2 are essential transporters for immune cell uptake of glutamine and leucine. c-Myc, HIF-2a, YAP1/TAZ, YBX3, and some cytokines can enhance their expression, thereby strengthening the metabolic and effector functions of CAR-NK and T cells against tumors.⁸⁸ This finding may inform new strategies for targeting amino acid transporters by modulating immune cell metabolism. However, the relevant research is still in the proof-of-concept and preclinical stages and has not yet entered clinical trials.

Amino acid metabolism is also regulated by biosynthetic and catabolic enzymes. Many inhibitor drugs targeting key enzymes in amino acid metabolism have been widely studied. For example, oral inhibitors of arginine metabolism, CB-1158 and OATD-02, have been developed. CB-1158 is a small molecule inhibitor of arginase, which can inhibit Arg-1 to shift the immune microenvironment to a pro-inflammatory state, weaken the immune regulation mediated by myeloid cells, and inhibit tumor growth.⁸⁹ The novel boric acid derivative OATD-02 is the only dual inhibitor that can simultaneously inhibit both Arg-1 and Arg-2.⁹⁰ Their inhibitory effect on arginase may be effective across multiple advanced cancer types, either as monotherapy or in combination with immune checkpoint inhibitors (ICIs) or chemotherapy. At present, these two inhibitors have entered clinical trials. The Phase I trial of CB-1158 (NCT02903914) confirmed its inhibitory effect on the target. However, grade 3 adverse events, including fatigue, nausea, and diarrhea, occurred in approximately half of the patients, and the response rate when combined with ICIs was limited.⁹⁵ Currently, the clinical development has been halted. In contrast, the phase I trial (NCT05759923) of OATD-02 is evaluating its safety and preliminary efficacy in advanced colorectal, ovarian, pancreatic, and renal cell carcinoma.⁹⁶ At present, this inhibitor is undergoing further clinical trials. Inhibitors of other amino acids, such as GLS, have also shown certain effects in clinical trials of patients with cancer at different stages. Furthermore, a recent study has suggested that the key enzyme ALDH18A1, which is part of the glutamine-arginine-proline metabolic axis in tumors, is highly expressed in various types of tumors. The research on its targeted therapy may provide new targets for cancer treatment.⁹⁷ However, at present, there is still a lack of specific ALDH18A1 inhibitors entering clinical development, and the translational potential of this target remains to be further verified.

Advances in amino acid metabolism research have revealed that sensors such as AHR and GCN2 detect amino acid levels and regulate the immune microenvironment, positioning them as novel therapeutic targets. AHR promotes tumor growth in various tumors through the TRP-KYN-AHR signaling pathway. Hezaveh et al. found that knocking out AHR in myeloid cells or inhibiting AHR with drugs could reduce the growth of pancreatic ductal adenocarcinoma and enhance the efficacy of ICIs.³⁰ An AHR inhibitor, BAY 2416964, has demonstrated enhanced immune cell function and potent anti-tumor effects both in vivo and in vitro.⁹¹ Its first-in-human phase I clinical trial (NCT04069026) also demonstrated promising efficacy and a favorable tolerability profile. The most common adverse symptoms, fatigue and nausea, were mostly grade 1–2. These findings further indicate that inhibiting AHR is a new approach for restoring the function of immune cells and overcoming various resistance to cancer immunotherapy. At present, GCN2 inhibitors have also garnered considerable interest. Among them, triazolo [4,5-day] pyrimidine compounds can specifically target GCN2 within cancer cells, thereby exerting powerful anti-tumor effects.⁹² At present, the research on such compounds as GCN2 inhibitors is still in the early exploration stage and has not yet entered clinical trials. These studies have demonstrated the potential of targeting amino acid sensors in treating tumors. However, more and deeper exploration of related mechanisms and drug development research is still needed in the future.

Notably, drugs targeting amino acid metabolism are often combined with ICIs. This is because metabolic intervention reshapes the tumor immune microenvironment at multiple levels, creating favorable conditions for immunotherapy. First, amino acid deprivation promotes PD-L1 upregulation in tumor cells and also leads to the accumulation of immunosuppressive metabolites, directly inhibiting T cell function.⁹⁸ Secondly, after ICIs activate T cells, these cells also require sufficient amino acids to support their activation and proliferation. ICIs can target upregulated immune checkpoints, while amino acid-targeted drugs alleviate immunosuppression and provide energy for ICI-activated T cells.⁹⁹ Therefore, the combined use of the two often demonstrates a synergistic effect, significantly enhancing the efficacy of immunotherapy. A phase II trial of the IDO pathway inhibitor indoximod in combination with pembrolizumab for the treatment of patients with advanced melanoma shows an encouraging effect, demonstrating the synergistic effect of this combination therapy.¹⁰⁰

In addition, amino acid transporters such as SLC7A5 and some key enzymes are often co-expressed in both tumor cells and immune cells. Therefore, therapeutic strategies targeting these transporters and enzymes not only inhibit tumor growth but also weaken the function of immune cells. To avoid this dual inhibition, it is necessary to improve the precision of the therapeutic window. A novel broad-spectrum glutamine antagonist prodrug, sirpiglenastat (DRP-104), takes advantage of the enriched protease activity in tumor tissues to preferentially bioactivate it into the active form 6-diazo-5-oxo-L-norleucine (DON), while being biologically inactivated in gastrointestinal tissues.⁹³ This specific activation mechanism ensures that the active drug is mainly concentrated in tumor sites, reducing its impact on normal tissues. The research on such prodrugs offers a promising approach to overcome the dual inhibition caused by targeting amino acid metabolism drugs. Future efforts should focus on developing highly selective agents that target tumor cells while minimizing adverse effects on immune cells.

Although significant progress has been made in the development of drugs targeting amino acid metabolism, most of the candidate drugs are still in preclinical or early clinical trial stages. The main bottlenecks in their clinical translation lie in the dual effects of the drugs on the immune system and the drug resistance caused by tumor metabolic remodeling. Therefore, the focus of future research and development should be on improving the target selectivity of the drugs and developing better combination treatment strategies based on a deeper understanding of the metabolic and immune regulatory network.

Conclusion

Amino acids, as a key nutrient in TME, undergo metabolic reprogramming that not only drives the polarization of TAMs toward the M2 phenotype, but also mediates the dysfunction and exhaustion of T cells. This process of reshaping the immune cell function and constructing an immunosuppressive microenvironment is closely related to the amino acid metabolic competition between tumor cells and immune cells. From amino acid sensors to downstream signaling molecules and metabolic products, various pathways interweave with each other, integrating the metabolic competition of various amino acids into a strict metabolic network, which collaboratively regulates the dysfunction of TAMs and T cells, and ultimately promotes the occurrence of immune escape. A comprehensive understanding of this network may pave the way for novel therapeutic strategies targeting amino acid metabolism. However, overcoming these challenges, including limited monotherapy efficacy, dual effects on tumor and immune cells, and therapeutic resistance, will be critical to advancing this approach. In the future, based on the key nodes of the amino acid metabolic network, biomarkers for dynamic monitoring of efficacy and resistance should be developed. By integrating spatial metabolomics and other technologies to analyze metabolic competition in different regions of tumors, and design highly selective and controllable combined treatment strategies based on the tumor evolution stage and treatment response status. To achieve a leap from broad intervention to precise targeting, and ultimately promote the development of individualized and precise treatment.

Acknowledgments

All the figures were prepared with the help of the free BioGDP online tool (https://biogdp.com). This research was funded by the National Natural Science Foundation of China (grant no. 82272928).

Author contributions

Conceptualization, B.Z. and Y.S.; Literature research, C.X., Y.S., C.Z., and Q.M.; writing – original draft, C.X.; writing – review and editing, C.X., Y.S., M.Z., and Z.W.; Foundation resource, B.Z.; supervision, B.Z.

Declaration of interests

The authors declare no competing interests.