The Warburg effect, characterized by a preference for aerobic glycolysis over oxidative phosphorylation, significantly enhances the aggressiveness of tumor cells exhibiting this metabolic phenotype (3). According to the metabolic symbiosis hypothesis, tumor cells can be divided into two distinct subtypes: oxygenated tumor cells, which are located near blood vessels, and hypoxic tumor cells, which are situated further away from blood vessels (22, 23). To address ATP deficiency and enhance glucose uptake, hypoxic tumor cells upregulate the expression of GLUT1. Hypoxia-inducible factor-1 (HIF-1) facilitates the conversion of glucose to pyruvate and subsequently to lactate in the presence of lactate dehydrogenase-5 (LDH-5). This process promotes the regeneration of nicotinamide adenine dinucleotide (NAD⁺), which is essential for maintaining a high glycolytic flux. Lactate and hypoxia-stimulated hypoxia-inducible factor-1α (HIF-1α) or WNT/β-catenin signaling prevent pyruvate from entering the TCA cycle and converting to acetyl-CoA by upregulating pyruvate dehydrogenase kinase 1 (PDK1) and then inhibiting pyruvate dehydrogenase complex (PDC). This drives tumor cells to gain energy through aerobic glycolysis. Finally, MCT4 removes lactate produced in hypoxic tumor cells to avoid intracellular acidification. Oxygenated tumor cells transport lactate produced by hypoxic tumor cells through MCT1 and produce ATP by oxidizing lactate (24).

Oxidative lactate metabolism offers several advantages to oxygenated tumor cells compared to glucose-dependent respiration. Specifically, ATP production through lactate oxidation is 7.5 times greater than that produced through aerobic glycolysis. Tumor cells prioritize lactate utilization to conserve energy required for the synthesis and maintenance of glycolytic enzymes, as well as for the phosphorylation of glucose and fructose-6-phosphate (F6P) during glycolysis. Additionally, the oxidation of lactate by LDH-1 generates ample nicotinamide adenine dinucleotide (NADH), which serves as a fuel source for the mitochondrial electron transport chain (ETC). Ultimately, the LDH-1 catalyzed reaction may facilitate lysosomal acidification and the maturation of autophagy vesicles. Autophagy allows oxidized proteins and organelles to be recycled (4).

In addition to its role in metabolic regulation, MCT1 also promotes tumor progression by modulating the immune microenvironment.

Macrophages can differentiate into either M1 or M2 phenotypes (25), M1 macrophages exhibit anti-tumor properties, while M2 macrophages promote tumor growth (26). Tumor cells release lactate via MCT1 or MCT4, which can influence the polarization of M2 macrophages (27). In a study involving breast cancer by Mu et al, it was observed that tumor-derived lactate facilitated the polarization of M2 macrophages through the activation of the extracellular regulated protein kinase (ERK)/signal transducer and activator of transcription 3 (STAT3) signaling pathway (28). In addition, M2 macrophages were found to release immunosuppressive cytokines that can stimulate the development of regulatory T cells (Treg cells) (29).

Treg cells suppress anti-tumor immune responses, while effector T cells play a crucial role in combating tumors (30, 31). The TME facilitates the recruitment and differentiation of Treg cells by upregulating forkhead box proteins P3 (FOXP3) and MCT1. Increased FOXP3 expression enhances the adaptability of Treg cells to the high-lactate TME by suppressing c-Myc and glycolysis, while promoting oxidative phosphorylation (OXPHOS) and increasing NAD⁺ oxidation (32). Treg cells uptake lactate via MCT1, where it is metabolized intracellularly to phosphoenolpyruvate (PEP). This metabolic process leads to an increase in cytoplasmic calcium ion concentration, facilitating the translocation of nuclear factor of activated T-cells 1 (NFAT1) to the nucleus. This enhances the expression of programmed death-1 (PD-1), while PD-1 expression in effector T cells is inhibited (33). PD-1 blockade invigorates PD-1-expressing Treg cells, leading to treatment failure (33). Activated cytotoxic T lymphocytes (CTLs) predominantly utilize glycolysis for energy production, with the resulting lactate being excreted via MCT1 to sustain their cytokine production and cytotoxic functions (34). Elevated lactate levels within the TME impede T cell function by suppressing lactate efflux (35), thereby creating a conducive environment for tumor cell survival.

Neutrophils, similar to macrophages, exhibit two distinct phenotypes: N1 and N2 (36). The N1 phenotype is characterized by cytotoxicity and anti-inflammatory properties, while the N2 phenotype demonstrates immunosuppressive capabilities (36). It has been shown that the transforming growth factor-β (TGF-β)/Smad3 signaling pathway plays a crucial role in the transformation of neutrophils into the N2 phenotype (37). Tumor-derived lactate enters neutrophils via MCT1 and triggers PD-L1 expression through the nuclear factor-kappa B (NF-κB)/cyclooxygenase-2 (COX-2) pathway. Neutrophils expressing PD-L1 have the potential to suppress T cell cytotoxicity (38), thereby facilitating tumor progression.

In oxygenated tumor cells and endothelial cells, lactate is transported into the cell via MCT1 and subsequently oxidized by LDH-1 to produce pyruvate. Pyruvate is characterized as a pseudo-hypoxic signal that influences HIF prolyl hydroxylases (PHDs), particularly PHD2. When oxygen, α-ketoglutarate, and vitamin C are present, this family of enzymes catalyzes the hydroxylation of HIF-1α on two proline residues, targeting this HIF-1 subunit to proteasome-mediated degradation. This hydroxylation leads to the proteasome-mediated degradation of the HIF-1α subunit. Thus, even in the presence of sufficient oxygen, PHDs can be inhibited by oxidants or α-ketoglutarate competitors. The inhibitory effect of pyruvate on PHDs varies by cell type. In oxygenated tumor cells and endothelial cells, pyruvate stably oxidizes HIF-1α, thereby activating HIF-1 and triggering the transcription of vascular endothelial growth factor A (VEGF-A) in tumor cells and VEGF receptor 2 (VEGFR2) as well as basic fibroblast growth factor (bFGF) in endothelial cells (4). Additionally, endothelial cells possess an autocrine pathway for angiogenesis. Lactate uptake by endothelial cells induces the phosphorylation of the inhibitor of κBα (IκBα), leading to the activation of the NF-κB/interleukin-8 (IL-8) signaling pathway, which promotes angiogenesis in tumor cells (22, 39). VEGF-A, VEGFR2, bFGF, and IL-8 are all pro-angiogenic factors that activate their respective receptors to promote neovascularization (24).

The reverse Warburg effect is observed in the interaction between tumor cells and stromal cells. Hydrogen peroxide released by tumor cells induces oxidative stress in stromal cells. This oxidative stress activates hypoxia-inducible factor-1α (HIF-1α) and nuclear factor-kappa B (NF-κB). HIF-1α not only triggers aerobic glycolysis and angiogenesis but also induces autophagy and lysosomal degradation, leading to the loss of caveolin-1 (Cav-1). The loss of Cav-1 amplifies oxidative stress through a positive feed-forward control mechanism and contributes to metabolic changes in stromal cells. As a result, proteins such as MCT1 are highly activated. Lactate produced by stromal cells enters tumor cells via MCT1 for metabolism (40). This metabolic interaction promotes the creation of a nutrient-rich microenvironment that allows tumor cells to meet their metabolic needs (41).

Obstruction of ferroptosis favors tumor progression. Ferroptosis is a novel form of programmed cell death (42). The primary feature of ferroptosis is the buildup of lipid peroxides (43). Cellular lipids and lipid metabolism play a pivotal role in regulating ferroptosis (44). Hydroxy-carboxylic acid receptor 1 (HCAR1), a member of the G protein-coupled receptor family (45), acts as a receptor for lactate and modulates its metabolism (46). HCAR1 is expressed on the membrane of various cells, including tumor cells, and its activation through lactate binding results in increased MCT1 expression, thereby facilitating lactate uptake by tumor cells (45). In a study involving hepatocellular carcinoma cells by Zhao et al, these cells exhibited resistance to ferroptosis induced by common ferroptosis inducers. The lactate taken up by MCT1 potentially led to increased ATP production and a decreased AMP: ATP ratio within cellular compartments (47). Disruption of the AMP: ATP balance by lactate can deactivate adenosine monophosphate-activated protein kinase (AMPK), upregulate the expression of sterol regulatory element-binding protein 1 (SREBP1) and its target stearoyl-CoA desaturase 1 (SCD1), ultimately resulting in increased synthesis of anti-ferroptosis monounsaturated fatty acids (MUFAs) and reduced lipid peroxidation (47). Furthermore, MUFAs have been shown to enhance the fluidity and curvature of the lipid bilayer, promote the formation of autophagosomes on the endoplasmic reticulum, and activate autophagy (48). Autophagy can counteract cellular lipotoxicity to some extent and is particularly important for the survival of tumor cells (48).

Evidence suggests that autophagy is indispensable for the progression of malignant tumors in numerous instances (49). Osimertinib (OSI), an epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) (50), has been found to upregulate MCT1 expression in colorectal cancer (CRC) cells (51). This upregulation activated the liver kinase B1 (LKB1)/AMPK signaling pathway, inducing protective autophagy in CRC cells (51). In a study involving hepatocellular carcinoma by Colozza et al, researchers discovered that starvation induced autolysosome production, which activated the Wnt/β-catenin signaling pathway and subsequently upregulated MCT1 expression. This enhanced glycolysis and facilitated tumor metastasis (52). Mechanistically, β-catenin served as a crucial second messenger in the classical Wnt signaling pathway. Under normal physiological conditions, the destruction complex—comprising Axin, adenomatous polyposis coli (APC), glycogen synthase kinase 3 (GSK3), and casein kinase 1α (CK1α)—actively facilitated β-catenin turnover via a proteasome-dependent mechanism. GSK3β-mediated phosphorylation led to the eventual degradation of β-catenin (53). Activation of Wnt signaling led to the binding of the Wnt ligand to its cognate receptors, frizzled (FZD) and lipoprotein receptor-related protein 5 and 6 (LRP5/6). This binding initiated the assembly of a multiprotein complex known as the signalosome and suppressed the activity of the destruction complex (53). The subsequent internalization of the signalosome into early endosomes (EEs), which then matured into multivesicular bodies (MVBs), was essential for the propagation of Wnt signals (53). Consequently, β-catenin was stabilized and translocated to the nucleus, where it cooperated with T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) to activate the transcription and translation of the MCT1 gene (53).

When used as a substrate for MCT1, acetate can promote tumor progression through various mechanisms. In a study involving clear cell renal cell carcinoma (ccRCC) by Li et al, researchers identified MCT1 as a significant facilitator of ccRCC, with its role in metabolic reprogramming mediated through acetate transport (54). The transportation of acetate into the cell via MCT1, followed by its translocation into the nucleus, resulted in the activation of oncogenes by enhancing the acetylation of histone H3K27 in the promoter region of the FASN and ACACA genes, ultimately promoting tumor progression (54). In a study involving non-small cell lung cancer by Wang et al, the uptake of acetate by tumor cells, facilitated by high expression of MCT1 and catalyzed by ACSS2 to produce acetyl-CoA, increased lipid synthesis in the tumor cells and led to elevated levels of lysine acetylation at position 148 of c-Myc (13). Most importantly, the researchers found that dihydrolipoamide S-acetyltransferase (DLAT) within the pyruvate dehydrogenase complex (PDC) performed a nonclassical metabolic function and mediated the acetylation of c-Myc (13). Acetylated c-Myc increased binding to the deubiquitinating enzyme USP10, promoting c-Myc protein stabilization and subsequent transcriptional activation of PD-L1, LDHA, MCT1, and Cyclin D1 expression (13). Both the mouse in situ tumorigenic lung cancer model and the subcutaneous tumorigenic model demonstrated that the consumption of acetate-containing drinking water inhibits CD8+ T cell infiltration and promotes tumor growth (13).

Given its pivotal role in tumor progression, MCT1 has emerged as a promising therapeutic target. Various research groups have explored strategies to inhibit MCT1, employing both genetic knockdown techniques and pharmacological inhibitors.

Inhibiting MCT1 disrupts the metabolic symbiosis between tumor cells. When MCT1 is inhibited using α-cyano-4-hydroxycinnamate (CHC) or siRNA, lactate transport in oxygenated tumor cells ceases. As a result, these oxygenated tumor cells must increase glucose consumption to compensate for the lack of lactate. This disruption in metabolic symbiosis between oxygenated and hypoxic tumor cells leads to increased glucose uptake by oxygenated tumor cells from nearby blood vessels. Consequently, hypoxic tumor cells experience glucose deprivation, leading to apoptosis. The inhibition of MCT1 in oxygenated tumor cells indirectly causes the death of hypoxic tumor cells (55). This underscores MCT1 as a promising target for tumor therapy, as its inhibition not only eliminates hypoxic tumor cells but also allows for the treatment of more vulnerable oxygenated tumor cells through methods like chemotherapy or radiation (23).

Inhibition of MCT1 can suppress tumor angiogenesis. Since MCT1 is a key transporter for lactate uptake by endothelial cells, a study has employed CHC and siRNA to inhibit MCT1, thereby effectively suppressing lactate-induced tumor angiogenesis. Mechanistically, targeted inhibition of MCT1 in endothelial cells directly impeded angiogenesis by reducing HIF-1 activity (56).

Inhibition of MCT1 holds potential to impede tumor progression by reducing immunosuppression. Lactate, a hallmark metabolite of TME (57), actively promotes immune evasion of tumor cells by inhibiting immune cell toxicity and proliferation (58). Tumor-derived lactate has been identified as an inhibitor of CD8+ T cell toxicity (59). A nanomedicine containing MCT1 inhibitors released the drug at low pH, inhibiting MCT1 to curb lactate release and thereby enhancing the anti-tumor activity of CD8+ T cells (60).

Moreover, inhibiting MCT1 prevents tumor cells from absorbing lactate from stroma cells, thereby diminishing the role of stroma cells in promoting tumor cell proliferation. Additionally, inhibiting MCT1 using siRNA or MCT1 inhibitors promotes ferroptosis, an effect that is primarily mediated through the regulation of intracellular lipid metabolism (24).

The aforementioned CHC is categorized as a non-selective MCT inhibitor, a group that also encompasses compounds such as phloretin, quercetin, p-CMBS, lonidamine, DIDs, and simvastatin (4). These non-selective MCT inhibitors are characterized by their limited specificity (61). Consequently, the development and utilization of selective MCT1 inhibitors assume heightened significance. AZD3965 stands out as one of the selective MCT1 inhibitors. Other selective MCT1 inhibitors include AR-C155858, BAY-8002, and 7ACC2 (24). Moreover, 3-bromopyruvate (3-BrPA) has been shown to induce an epigenetically driven loss of MCT1 (1). Table 2 summarizes the efficacy of MCTs inhibitors.

AZD3965, BAY-8002, and 7ACC2 induce distinct conformational changes in MCT1, resulting in an outward-open conformation in the presence of BAY-8002 and AZD3965, and an inward-open conformation in the presence of 7ACC2 (61). All three inhibitors directly occupy the substrate binding site, achieving inhibition of MCT1 through direct competition with the substrate binding site and inhibition of conformational changes in the transporter protein (61). 3-BrPA treatment induces silencing of MCT1 by hypermethylating the promoter of the SLC16A1 gene (62). AR-C155858 interacts with transmembrane helices 7–10 in the inward-open conformation of MCT1, resulting in its inhibition (4). Additionally, microRNAs (miRNAs) such as miR-342-3p and miR-124 are found to downregulate MCT1 expression by targeting its messenger RNA (mRNA) (63, 64). MiR-146a indirectly inhibits the expression of MCT1 by inhibiting CD147 (65). The mechanism of action of MCT1 inhibitors is summarized in Figure 3 .

AZD3965 is a pyrrole pyrimidine derivative (66). In a study involving B-cell lymphoma by Beloueche-Babari et al, treatment of immunodeficient mice with Raji xenografts using AZD3965 resulted in tumor growth inhibition and a reduction in choline levels (67). Mechanistically, intracellular lactate accumulation induced by AZD3965 led to decreased expression of choline kinase α (ChoKα) and its mRNA, consequently inhibiting de novo synthesis of choline phosphate (67). Simultaneously, there was an increase in the infiltration of dendritic cells (DCs) and natural killer cells (NKs) (67). In a systematic review assessing the anti-tumor properties of AZD3965 in a murine model, investigators determined that AZD3965 effectively augmented tumor responsiveness to radiation and chemotherapeutic agents (3).

AR-C155858 is also a pyrrole pyrimidine derivative (66). The use of AR-C155858 in combination with anti-CD19 chimeric antigen receptor (CAR)-T cell therapy has shown promising results in the treatment of B-cell malignancies (68). In a study focusing on B-cell lymphoma by Lopez et al, the inhibition of MCT1 by AR-C155858, in conjunction with CAR-T cells, led to enhanced in vitro cytotoxicity and improved anti-tumor control in a mouse model. Mechanistically, B-cell lymphoma cells primarily relied on MCT1 for lactate export and were susceptible to inhibition, with minimal to no expression of MCT4. Conversely, CAR-T cells exhibited elevated levels of both MCT1 and MCT4 following activation, rendering them functionally resistant to AR-C155858 inhibition (69). Furthermore, in the context of breast cancer by Guan et al, AR-C155858 has been shown to reduce tumor cell proliferation in vitro (70).

Malignant pleural effusion (MPE) is a common complication in advanced malignant tumors (71). FOXP3, a pivotal transcription factor, plays a crucial role in the regulation of immune responses. Within MPE, FOXP3 natural killer T (NKT)-like cells leveraged their elevated expression of MCT1 and LDHB to uptake and metabolize lactate, thereby maintaining their immunosuppressive capabilities within the effusion (72). In vitro studies have shown that 7ACC2 significantly reduced FOXP3 expression in NKT-like cells, which can inform the development of NKT-like cell-based therapies aimed at controlling MPE progression (72). In a study involving pancreatic cancer by Sandforth et al, tumor cells expressing MCT1 were protected against gemcitabine-induced apoptosis in a MCT1-dependent manner (73). The administration of 7ACC2 can counteract this protective effect and induce apoptosis in the tumor cells (73).

The DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is known for its pleiotropic protein kinase activity and is often linked to poor prognosis in tumor patients (74). Wagner et al. revealed that overlocalization of DNA-PKcs can confer protection against lentiviral transduction, such as human immunodeficiency virus-1 (HIV-1). Meanwhile, BAY-8002, which inhibited lactate flux, enhanced the nuclear localization of DNA-PKcs, thereby reducing the efficacy of lentiviral transduction (75). In the context of prostate cancer by Matheux et al, MCT1 has been identified as a transporter for afatinib. The inhibition of MCT1 by BAY-8002 partially diminished the sensitivity of 22Rv1 cells, which expressed the pregnane X receptor, to afatinib (76).