Glycolysis modulates tumor immunology through immune checkpoint regulation, gene expression, and cytokine secretion, particularly in melanoma (4). Enhanced glycolysis leads to the accumulation of lactate, which inhibits approximately 95% of immune cell proliferation, attenuates cytokine secretion and activity, and supports regulatory T cells (Tregs) (5, 6). Monocarboxylate transporter-1 (MCT-1) on cytotoxic T lymphocytes (CTLs) exports lactate; however, excessive lactate impairs the functionality of CTLs despite their increased presence (5). Lactate-induced extracellular acidification via monocarboxylate transporter-4 (MCT-4) inhibits nuclear factor of activated T cells (NFAT) in T and natural killer (NK) cells, reducing interferon-γ (IFN-γ) production and antitumor activity (7). Competition for glycolytic resources with melanoma cells weakens the T cell efficacy and promotes Treg differentiation, facilitating immune evasion (8). Additionally, lactate upregulates vascular endothelial-derived growth factor (VEGF), driving tumor-associated macrophages to the protumorigenic M2 phenotype (9). High-glycolytic tumors express elevated levels of immune inhibitors, such as TGF-B1, CD274, and PDCD1LG2, which suppress immune functions (10). TGF-β inhibits the mammalian target of rapamycin (mTOR) pathway in NK cells, decreasing IFN-γ and antitumor responses (11). Thus, targeting glycolysis may mitigate immune evasion and resistance in melanoma. However, whether the efficacy of immunosuppressants can be improved by directly inhibiting melanoma glycolysis remains to be seen. ( Figure 1 Interactions between tumor metabolism and immune cells in melanoma).

The mitogen-activated protein kinase (MAPK) pathway is intricately linked to glycolysis; the v-raf murine sarcoma viral oncogene homolog B (BRAF)/MAPK pathway is initiated by growth factors, cytokines or hormones binding to a membrane-bound receptor tyrosine kinase (RTK), which induces an interaction between activated RAS and the RAF domain, modulating the shift from oxidative phosphorylation (OXPHOS) to glycolysis in melanoma cells (12, 13). In the MAPK pathway, the BRAFV600E mutation negatively regulates the microphthalmia-associated transcription factor (MITF)-peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC1α) axis, thereby promoting glycolysis (14). BRAF inhibitors (BRAFis) can inhibit the driving effect of hypoxia-induced factor 1α (HIF1α) and Myc on glycolysis, and they can also enhance the tolerance of PGC1α to oxidative stress, which not only maintains the energy balance but also provides a survival environment for tumor cells, thus affecting the therapeutic effect (15). Pyruvate kinase M2 (PKM2) is upregulated in melanoma and resistant cells; its inhibition suppresses growth and resensitizes cells to BRAFis (16). Although combining BRAFis with mitogen-activated protein kinase kinase inhibitors (MEKis) can improve patient prognosis and inhibit the growth of advanced melanoma, this combination cannot achieve an eradication effect. More importantly, upregulation of glycolysis generates resistance to BRAFis and MEKis, allowing previously suppressed melanoma cells to regain their growth advantage, leading to tumor recurrence (17, 18). These mechanisms indicate that glycolysis modulation is pivotal in resistance development.

Amino acids, including glutamate, cysteine, leucine, tryptophan, and arginine, play pivotal roles by supporting melanoma proliferation and facilitating antitumor immunity (4). Although amino acid depletion can activate the inositol-requiring enzyme 1α (IRE1α) and retinoic acid-inducible gene 1 (RIG1) pathways to increase cytokine production and the immune response, amino acid depletion impairs essential immune function, making potential strategies for reducing amino acids to enhance antitumor immunity infeasible (19).

In recent years, the gut microbiota has emerged as a pivotal regulator of tumor progression. Empirical studies have revealed that melanoma patients predominantly harboring the Bacteroides genus within their gut microbiota tend to exhibit elevated levels of circulating CD4⁺ and CD8⁺ T cells. In contrast, individuals with a gut microbiota dominated by the Prevotella genus display increased populations of regulatory Tregs and MDSCs in their peripheral blood (56). Notably, the gut microbiota can negatively regulate MDSCs, thereby diminishing immune evasion and decelerating tumor progression. Additionally, the gut microbiota can activate antigen-presenting cells (APCs), which may inhibit tumor immune escape (57). The gut microbiota plays dual roles in tumor regulation, as antitumor responses within the TME can be attenuated by elevated levels of inflammatory cells and cytokines, thereby facilitating tumor immune evasion. Using a melanoma mouse model, researchers reported that mice subjected to a high-fat diet present increases in the Clostridia and Desulfovibrio populations. This shift in population activates the HMG-B1/NF-κB signaling pathway in macrophages, leading to secretion of the C-C motif chemokine ligand 2 (CCL2) and tumor necrosis factor-α (TNF-α) chemokines, which in turn promotes MDSC infiltration and tumor metastasis (58).

Moreover, the gut microbiota has been identified as a key component of antitumor immunity in the context of PD-1 antibody therapy. Research has indicated that mice colonized with fecal microbiota from melanoma patients who achieved a complete response to PD-1 antibody therapy develop significantly smaller MC38 tumors when treated with anti-PD-L1 monotherapy than mice colonized with fecal microbiota from nonresponding patients (59). Furthermore, the combination of anti-PD-L1 and anti-PD-L2 therapies markedly enhances the antitumor response in mice harboring nonresponder fecal microbiota, resulting in a substantial increase in the overall survival. Additionally, the combined administration of anti-PD-L2 and anti-PD-L1 treatments tends to reduce the growth of ovalbumin-expressing B16 melanoma tumors (B16-L1) (60). These findings underscore the critical role of gut microbiota modulation in melanoma development and the emergence of resistance to immune therapies. However, unlike PD-1 antibody therapy, not all combination therapies have an antitumor effect.

Accumulating evidence supports the potential of fecal microbiota transplantation (FMT) in mitigating immune tolerance in patients with melanoma. FMT alters the gut microbiota composition of the recipient to enrich beneficial bacteria that regulate tumor immunity and potentially enhance therapeutic outcomes. In melanoma mice transplanted with Bacteroides, the expression of chemokines and antigen presentation-related genes increases, which promotes the activation of dendritic cells (DCs) and CD8+ T cells in tumor cells, thereby inhibiting tumor growth (61). In a study conducted at the Hillman Cancer Center at the University of Pittsburgh Medical Center (UPMC) involving 15 participants undergoing FMT, 3 of 5 patients demonstrated positive clinical responses, thereby indicating the efficacy of FMT in melanoma treatment (62). However, the efficacy of FMT is not stable. Antibiotics are commonly used as infection control agents in clinical settings, and their frequent use is extremely detrimental to patient prognosis; the timing of antibiotic administration during PD-1 therapy is strongly associated with patient survival (63, 64). Furthermore, interest in microbiota-modulating pharmacological agents has increased. Studies have demonstrated that treatment of the gut microbiota in melanoma-bearing mice with Ganoderma lucidum polysaccharides inhibits melanoma metastasis (65). Astragalus polysaccharides, through modulation of Lactobacillus spp. and Lactobacillus johnsonii, can enhance immune suppression within the TME, thereby promoting CD8⁺ T cell-mediated cytotoxic functions (57). Similarly, ginseng polysaccharides and inulin have been shown to augment the efficacy of PD-1 antibody therapy by modulating microbial metabolism; however, long-term follow-up and further validation of these effects are necessary.

In summary, the intricate interplay between the gut microbiota and tumor immunity highlights the potential of microbiota-targeted interventions in enhancing the melanoma treatment efficacy and overcoming immune resistance. Most current studies are in the preclinical validation stage, and clinical studies confirming the significant effects of microbiota-targeted interventions are lacking. The inconsistency of the results encourages investigation into more feasible immune mechanisms to explore therapeutic strategies that benefit most patients.