LDH and PDH – maintaining homeostasis of lactate production and metabolism

LDH is among key rate-limiting enzymes that catalyse the bidirectional conversion of pyruvate and lactate in glycolysis. It is a tetramer composed of different subunits LDHA and LDHB (Ref. Reference Frank, Raue, Fuhrmann, Sirait-Fischer, Reuse, Weigert, Lütjohann, Hiller, Syed and Brüne22). LDHA is mainly responsible for converting pyruvate into lactate and NAD. In contrast, LDHB converts lactate into pyruvate to promote oxidative metabolism and the proportion of the two in the tetramer determines the overall direction of the reaction (Refs. Reference Certo, Tsai, Pucino, Ho and Mauro9, Reference Yang, Luo, Zhao, Li, Wang, Zeng, Yang, Zheng, Jie, Kang, Li, Liu, Zhou and Liu23). Previous studies have shown that LDHA is mainly expressed mainly in the proximal nephron segment, whereas LDHB is mainly expressed mainly in the distal nephron under normal physiological conditions, suggesting that the proximal tubule is more inclined to produce lactate, whereas the distal tubule is more inclined to utilize lactate physiologically (Ref. Reference Osis, Traylor, Black, Spangler, George, Zarjou, Verlander and Agarwal24). Similarly, single-cell RNA-sequencing (scRNA-seq) datasets derived from Wu and Malone et al. as well as Wu and Uchimura et al. revealed that Ldha mRNA expression was greater in the proximal tubules (PTs) than in the distal tubules, whereas Ldhb mRNA expression was higher in the distal nephrons of the loop of Henle and distal convoluted tubules than in the PTs (Refs. Reference Wu, Malone, Donnelly, Kirita, Uchimura, Ramakrishnan, Gaut and Humphreys25, Reference Wu, Uchimura, Donnelly, Kirita, Morris and Humphreys26). Moreover, exposure to injurious stimuli induces corresponding alterations in LDH expression. Analysis of the single-nucleus RNA-sequencing (scRNA-seq) datasets in this study revealed upregulation of the expression of both Ldha and Ldhb gene transcripts following ischaemia–reperfusion injury (IRI), highlighting the initial metabolic shift towards glycolysis postinjury (Ref. Reference Osis, Traylor, Black, Spangler, George, Zarjou, Verlander and Agarwal24). Interestingly, LDHA and LDHB expression in different nephron segments displays spatial and time-dependent responses to ischaemic injury. Both LDHA and LDHB decrease after ischaemic acute kidney injury and recover later; LDHA decreases early (Day 1), followed by a decrease in LDHB on Day 3 postinjury. in vitro experiments conducted by Osis et al. revealed a significant upregulation of LDHA expression in metanephric tubular epithelial cells (TECs) under hypoxic conditions, whereas LDHB levels remained unchanged. Notably, distal nephron expression of LDHB increased between 3 and 7 days after injury but normalised by 14 days. These data suggest that proximal tubules are lactate producers, whereas distal nephron segments are lactate consumers regardless of whether they are in a physiological state or when confronted with stress dependency on LDH subunits and differential expression among distinct renal units. However, several studies have reported a reduction in LDH protein expression in the kidney following the establishment of mouse models simulating unilateral or bilateral renal ischaemia-reperfusion, whereas concurrent elevations in plasma and urine LDH levels have been noted (Refs. Reference Osis, Traylor, Black, Spangler, George, Zarjou, Verlander and Agarwal24, Reference Zager, Johnson and Becker27, Reference Zager, Johnson and Becker28). This phenomenon could be attributed to the release of LDH from damaged cells into the bloodstream or urine (Ref. Reference Zager, Johnson and Becker28).

PDH facilitates the irreversible decarboxylation of pyruvate to acetyl-CoA, thereby regulating carbon entry into the TCA cycle and indirectly facilitating lactate clearance (Refs. Reference Jha and Suk29, Reference Jha, Lee and Suk30). It functions as a pivotal dehydrogenase enzyme that regulates mitochondrial metabolic pathways, connects glycolysis and the TCA cycle and plays a critical role in cellular metabolism. In the case of hypoxia or mitochondrial damage, PDH inhibition leads to reduced lactate clearance, accompanied by a compensatory increase in lactate production, thereby resulting in abnormally elevated lactate levels in the microenvironment and circulation (Refs. Reference Luengo, Li, Gui, Sullivan, Zagorulya, do, Ferreira, Naamati, Ali, Lewis, Thomas, Spranger, Matheson and Vander Heiden31, Reference Kamel, Oh and Halperin32). When cisplatin-induced kidney injury occurs, there is an increase in PDH phosphorylation increases, leading to PDH deactivation (Ref. Reference Galgamuwa, Hardy, Dahlstrom, Blackburn, Wium, Rooke, Cappello, Tummala, Patel, Chuah, Tian, McMorrow, Board and Theodoratos33). Similarly, Lan et al. reported that PDH phosphorylation was upregulated in the kidneys of IRI mice, concomitant with the accumulation of lactate (Ref. Reference Lan, Geng, Singha, Saikumar, Bottinger, Weinberg and Venkatachalam34). The reduced levels of PDH during kidney injury may impede the conversion of pyruvate to acetyl-CoA, thereby compromising the clearance of lactate (Ref. Reference Zager, Johnson and Becker27).

Monocarboxylate transporters – maintaining equilibrium of lactate transport

The transport of lactate across the plasma membrane occurs through two selective transporters, namely proton-coupled monocarboxylate transporters (MCT1–4) and sodium-coupled monocarboxylate transporters (SMCT1–2) (Refs. Reference Yanase, Takebe, Nio-Kobayashi, Takahashi-Iwanaga and Iwanaga35–Reference Certo, Llibre, Lee and Mauro37). The homeostatic balance of lactate in different tissue microenvironments can be maintained through the expression and activity of these monocarboxylate transporters (Refs. Reference Rabinowitz and Enerback16, Reference Becker, Mohebbi, Perna, Ganapathy, Capasso and Wagner38). MCTs need to bind to glycosylated helper proteins to ensure correct expression on the plasma membrane for normal transport protein activity (Ref. Reference Ippolito, Morandi, Giannoni and Chiarugi39). In the transport process, free protons first bind to MCT, and then bind to lactate and then conformational changes occur in the transporter to a ‘closed’, state, where protons and lactate are exposed on the other side of the membrane and the release of protons is followed by the release of lactate. When MCT loses protons, it undergoes a conformational change that restores its initial structure before the next transport (Refs. Reference Li, Yang, Zhang, Lin, Fu, An, Zou, Wang, Wang and Yu17, Reference Halestrap36). In general, MCTs with high substrate affinity, such as MCT2 (Km ≈ 0.7 mM), is predominantly expressed in lactate-consuming tissues (e.g., the brain and heart), where they facilitate efficient cellular lactate uptake. In contrast, MCTs with lower affinity, such as MCT4 (Km ≈ 35 mM), are primarily found in highly glycolytic cells (e.g., astrocytes), enabling rapid lactate export to maintain glycolytic flux (Ref. Reference Sheikh-Hamad40). Owing to their different affinities for lactate, MCTs play disparate roles in both the uptake and efflux of lactate from cells, contingent upon the concentration gradient of this metabolite (Ref. Reference Certo, Llibre, Lee and Mauro37). However, research on SMCTs has long been relatively scarce. Lactate transport mediated by SMCT1–2 primarily facilitates the reabsorption of lactate by tissues, so as to maintain the normal physiological functions of the body (Ref. Reference Iwanaga and Kishimoto41). Thus, MCTs and SMCTs are jointly responsible for lactate homeostasis and the impairment or inactivation of transporters leads to compromised lactate transportation and functionality.

In the kidney, MCT1 (Km ≈ 3.5–10 mM) has been detected in proximal renal tubular cells, whereas MCT2 is predominantly localised in the thick ascending limb of the loop of Henle and the distal convoluted tubule under unperturbed physiological conditions, as reported (Refs. Reference Becker, Mohebbi, Perna, Ganapathy, Capasso and Wagner38, Reference Sheikh-Hamad40). MCT4, a potent lactate output channel, that is expressed in glycolytic cells and induced by hypoxia and redox signalling, has been poorly studied in normal kidneys (Ref. Reference Ippolito, Morandi, Giannoni and Chiarugi39). The tissue distribution of MCT3 is limited, with no detectable expression observed in the kidney. Using the renal transcriptomics database (Nephroseq), we analysed the expression levels of SLC16A1 (encoding MCT1), SLC16A7 (encoding MCT2) and SLC16A3 (encoding MCT4) in a microarray dataset consisting of kidney biopsy specimens from patients with CKD, as constructed by Shunsaku Nakagawa et al. (Ref. Reference Nakagawa, Nishihara, Miyata, Shinke, Tomita, Kajiwara, Matsubara, Iehara, Igarashi, Yamada, Fukatsu, Yanagita, Matsubara and Masuda42). The results showed greater SLC16A1, SLC16A7 and SLC16A3 expression in the kidneys of CKD patients (n = 53) than in those of normal patients (n = 8; Figure 2). Despite not being a specific lactate transport receptor, these findings still have implications for the enhancement of lactate-mediated communication in the kidneys of patients with CKD. In addition, both SMCT1 and SMCT2 are found to be expressed in the proximal tubule (Ref. Reference Gopal, Umapathy, Martin, Ananth, Gnana-Prakasam, Becker, Wagner, Ganapathy and Prasad43). Researchers utilize c/ebpδ^−/− mice, a model for the kidney-specific double knockout of SLC5A8 (encoding SMCT1) and SLC5A12 (encoding SMCT2) and observe a significant upregulation of urine lactate, whereas blood lactate is downregulated compared with that in wild-type mice (Ref. Reference Thangaraju, Ananth, Martin, Roon, Smith, Sterneck, Prasad and Ganapathy44). These findings suggest that the majority of lactate reabsorption in the kidney is attributed to SMCTs. MCT-mediated lactate transport establishes a communication pathway between cells and provides a proper condition for lactate homeostasis and the balance between glycolysis and gluconeogenesis in the kidney.

Figure 2. The expression of MCT1, MCT2 and MCT4 exhibit significant upregulation in the kidneys of patients with CKD. The mRNA levels of SLC16A1, SLC16A7 and SLC16A3 in kidney specimens of CKD (n = 53 samples) and control (n = 8 samples) in GSE66494 dataset are showed. Unpaired t-test, p < 0.05.

In summary, the kidneys serve as both producers and consumers of lactate. Considering the anatomical proximity of these segments, there is a conceivably efficient ‘lactate production-clearance axis’ through the spatiotemporal-specific expression of LDH/PDH and MCTs in proximal tubules and distal tubules to preserve intrarenal lactate equilibrium (Figure 3), similar to the lactate shuttle existing in the brain or the transfer from skeletal muscle to the heart during physical exercise (Ref. Reference Brooks20). Specifically, lactate is primarily generated in the proximal nephron and is subsequently transported to the distal nephron, where it is taken up by cells to meet their energy requirements or for signalling. However, on the basis of the conclusions of other previous studies that the lactate concentration in the medulla is significantly greater than that in the cortex in terms of the distribution of lactate in the kidney and that the gluconeogenesis of lactate in the kidney is performed mainly by the proximal tubule, another hypothesis has been proposed (Refs. Reference Legouis, Faivre, Cippà and de Seigneux45, Reference Bankir46). According to this hypothesis, the medulla mainly generates lactate through glycolysis, whereas the cortex is responsible for absorbing and oxidising lactate to generate energy or synthesising glucose through the gluconeogenesis pathway and then rereleasing this glucose back to the medulla to participate in glycolysis and energy metabolism. To more thoroughly clarify the metabolic fate of lactate in the kidneys, an important physiological process, it is necessary to conduct more systematic and extensive research. These findings provide a new theoretical basis for the study of renal physiology and may offer scientific evidence for the diagnosis and treatment strategies of related diseases.

Figure 3. Schematic Diagram of the "lactate production-clearance axis" between the proximal and distal renal tubules. Under physiological conditions of the kidney, lactate is primarily generated in the proximal nephron under the catalysis of LDHA and is subsequently transported to the distal nephron via MCTs, where it is taken up by cells to meet their energy requirements or for signalling. (Figure was created with Biorender.com.)

Genetic susceptibility

Genome-wide association studies (GWAS) can be used to identify potential associations between genetic regions in the genome and diseases. Large-scale GWAS datasets have shown that loci of lactate metabolism-related genes, namely GCKR (rs1260326) and PFKFB2 (rs2808454), are associated with eGFRcrea, suggesting a possible link to the risk of renal function impairment (Refs. Reference Wuttke47–Reference Liu, Doke, Guo, Sheng, Ma, Park, Vy, Nadkarni, Abedini, Miao, Palmer, Voight, Li, Brown, Ritchie, Shu and Susztak49). In addition, existing studies indicate that specific genetic backgrounds may enhance the susceptibility to lactate metabolism disorders in individuals with kidney diseases. When renal pathological damage is present, genetic factors synergistically exacerbate the risk of local or systemic lactate accumulation. Guo et al. developed a prognostic model for clear cell renal cell carcinoma based on lactate metabolism- and transport-related genes (PNKD, SLC16A8 and SLC5A8), demonstrating that genetic susceptibility contributes to disease progression through lactate metabolic dysregulation, remodelling of the immunosuppressive tumour microenvironment and activation of cell cycle signalling pathways (Ref. Reference Guo, Zhang, Wang, Yuan, Tang, Zhang, Chen and Wang50). Subsequent studies further constructed analogous prognostic models for renal cancer by identifying lactate-associated gene signatures, revealing that genetic heterogeneity mediates immunosuppressive microenvironment formation and therapeutic resistance via lactate metabolism reprogramming (Refs. Reference Sun, Tao, Guo, Jing, Zhang, Wang, Kong, Suo, Jiang and Wang51, Reference Wu, Wu, Sun, You, Zhang and Zhao52). However, current investigations into susceptibility to lactate metabolism disorders in kidney diseases predominantly focus on a limited number of candidate genes, with a notable absence of systematic research approaches, making it difficult to fully unravel the underlying genetic association mechanisms. The development of a genetic risk model for renal lactate metabolism disorder holds significant potential in improving risk stratification and enabling personalised management strategies for high-risk patients.

Hyperlactatemia and kidney

Hyperlactatemia refers to the accumulation of lactate in the blood due to excessive production or insufficient clearance of lactate and lactateosis refers to increased lactate accompanied by metabolic acidosis (Refs. Reference Rabinowitz and Enerback16, Reference Bellomo19). Lactateosis is the most common cause of metabolic acidosis in severe patients and elevated blood lactate concentrations have important effects on patient prognosis and even mortality (Refs. Reference Kamel, Oh and Halperin32, Reference Sun, Li, Chen and Qian53). Acidosis increases lactate metabolism in the kidneys. In the condition of hyperlactatemia, the normal function of the kidneys contributes to lactate clearance, mostly through lactate metabolism rather than excretion (Ref. Reference Madias54). This also means that in the case of lactate overproduction in the body, the kidney will bear a greater burden to maintain acid–base balance and the kidney will be more prone to lactateosis when it is dysfunctional. Lactateosis is often associated with AKI. When AKI occurs in patients, the consumption and clearance of lactate from the kidneys is reduced and impaired, which may lead to the accumulation of lactate in the blood (Refs. Reference Wen, Li, Li, Hu, Wei and Dong55, Reference Legouis, Ricksten, Faivre, Verissimo, Gariani, Verney, Galichon, Berchtold, Feraille, Fernandez, Placier, Koppitch, Hertig, Martin, Naesens, Pugin, McMahon, Cippà and de Seigneux56). From another standpoint, when the production of lactate increases because of lesions in other tissues in the body, the associated inflammatory mechanisms may lead to reduced lactate clearance by the kidney as well (Ref. Reference Kamel, Oh and Halperin32). Numerous clinical studies have consistently demonstrated a positive association between serum lactate levels and the occurrence of AKI during surgical procedures and severe trauma (Refs. Reference García, Manzano-Nunez, Bayona, Naranjo, Villa, Moreno, Ossa, Martinez, Martinez and Puyana57–Reference Choi60). In other words, the blood lactate concentration can serve as an independent risk factor for AKI, and plays a pivotal warning role in early disease detection. More importantly, using a two-sample Mendelian randomisation analysis, we recently reported a causal relationship between hyperlactatemia and a broad range of CKD outcomes indicating that serum lactate levels may also be risk factors for CKD.

Kidney injury and fibrosis

When kidney injury occurs, there is a shift in energy metabolism within the kidney, leading to an increase in the activity of the glycolytic pathway, which contributes to the provision of energy and subsequently mitigates TEC injury (Refs. Reference Zhu, Hu, Chen, Feng, Yang, Liang and Ding14, Reference Lan, Geng, Singha, Saikumar, Bottinger, Weinberg and Venkatachalam34). Enhancement of glycolysis has been demonstrated in various models of AKI, including those induced by ischaemia-reperfusion, sepsis, folic acid, cisplatin and so on (Refs. Reference Shen, Jiang, Wen, Ye, Zhang, Ding, Luo, Xu, Zen, Zhou and Yang15, Reference Lan, Geng, Singha, Saikumar, Bottinger, Weinberg and Venkatachalam34, Reference Gómez, Kellum and Ronco61, Reference Xie, He, Zhang, Xu, Wen, Cao, Zhou, Luo, Yang and Jiang62). This is accompanied with by the accumulation of lactate, the end product of glycolysis, accumulated within damaged renal tubular cells and the tubulointerstitial microenvironment (Ref. Reference Shen, Jiang, Wen, Ye, Zhang, Ding, Luo, Xu, Zen, Zhou and Yang15). Considering the short-term replenishment of energy supply by lactate during injury, it can be postulated that lactate may have a protective effect on kidney injury at this stage, as evidenced by our results that lactate improves tubule injury and subsequent fibrosis after preoperative peritoneal injection of sodium lactate (Figure 4). Interestingly, Zeng et al. reported that both PDHA1 hyperacetylation-mediated lactate overproduction and intraperitoneal injection of exogenous sodium lactate before caecal ligation and puncture procedure promote sepsis-induced acute kidney injury (SAKI), indicating lactate plays a negative role in the AKI process (Ref. Reference An, Yao, Hu, Wu, Li, Li, Wu, Sun, Deng, Zhang, Gong, Huang, Chen and Zeng63). These studies suggest that the effects of lactate may differ in various types of AKI, and more research is necessary to elucidate the role of lactate in acute kidney injury.

Figure 4. The histopathological alterations in renal injury and fibrosis following lactate injection in the UIRI mouse model. LA: lactate intraperitoneal injection. Scale bar, 100 μm.

Although glycolysis initially enhances survival in TECs, sustained activation of glycolysis may contribute to progressive renal injury. Previous findings indicate that lactate generated through tubular glycolysis during the reparative phase of AKI has possess potential cytotoxic effects, thereby directly contributing to fibrogenesis (Refs. Reference Shen, Jiang, Wen, Ye, Zhang, Ding, Luo, Xu, Zen, Zhou and Yang15, Reference Faivre, Verissimo, Auwerx, Legouis and de Seigneux64–Reference Yang, Huo, Cai, Zhang, Dong, Asara and Wei66). We examined the histopathological features of the kidneys of UIRI mice treated with continuous sodium intraperitoneal injection of sodium lactate and discovered that compared with those in the UIRI-saline group, the mice in the UIRI-lactate group had more obvious tubulointerstitial injury and more severe fibrosis (Figure 4). Notably, Shen et al. Injected the glycolysis inhibitors, 2-DG and oxamate, into mice with folate-induced AKI and reported that the lactate level in the renal tubules of these mice decreased significantly, whereas the fibrous deposition decreased. However, more morphological damage occurred in renal tubules (Ref. Reference Shen, Jiang, Wen, Ye, Zhang, Ding, Luo, Xu, Zen, Zhou and Yang15). These findings suggest that inhibiting the production of lactate by tubules could provide a certain degree of protection against fibrosis but at the same time may aggravate the mean renal injury in AKI (Refs. Reference Shen, Jiang, Wen, Ye, Zhang, Ding, Luo, Xu, Zen, Zhou and Yang15, Reference Yang, Huo, Cai, Zhang, Dong, Asara and Wei66, Reference Tan, Gu, Li, Chen, Liu, Liu, Zhang and Xiao67). Consistent with our previous study, we treated UIRI mice with 2-DG and found that 2-DG protected them from renal tubule interstitial fibrosis. It can be inferred that the dysregulation of lactate production and clearance in the kidney is intricately associated with the accelerated progression of renal fibrosis during the transition from the acute to chronic phase. The moderate inhibition of aerobic glycolysis is an effective approach for suppressing renal fibrosis, offering a promising therapeutic strategy for the treatment of AKI and CKD (Refs. Reference Ding, Jiang, Xu, Bai, Zhou, Yuan, Luo, Zen and Yang68, Reference Verissimo, Faivre, Rinaldi, Lindenmeyer, Delitsikou, Veyrat-Durebex, Heckenmeyer, Fernandez, Berchtold, Dalga, Cohen, Naesens, Ricksten, Martin, Pugin, Merlier, Haupt, Rutkowski, Moll, Cippà, Legouis and de Seigneux69). At present, the role of lactate in renal fibrosis is not completely clear, and it is urgent to elucidate the exact new mechanism of its activity to identify new therapeutic targets.

Diabetic kidney disease

As one of the leading causes of renal failure worldwide, diabetic kidney disease (DKD) is generally considered to be associated with altered cellular metabolism (Ref. Reference Sas, Kayampilly, Byun, Nair, Hinder, Hur, Zhang, Lin, Qi, Michailidis, Groop, Nelson, Darshi, Sharma, Schelling, Sedor, Pop-Busui, Weinberg, Soleimanpour, Abcouwer, Gardner, Burant, Feldman, Kretzler, Brosius and Pennathur70). Pathologically induced aberrant glycolysis also occurs in DKD and may play a key role in kidney injury (Refs. Reference Wu, Dong, Atefi, Liu, Elshimali and Vadgama71–Reference Srivastava, Li, Kitada, Fujita, Yamada, Goodwin, Kanasaki and Koya74). Specifically, increased upregulation of IGFBP5 expression induces glycolysis, leading to increased renal inflammation and fibrosis, ultimately contributing to the progression of DKD progression (Ref. Reference Song, Wang, Fu, Chi, Geng, Liu, Cai, Chen, Wu and Hong75). Additionally, the formation and activity of the pyruvate kinase (PK) M2 (PKM2) tetramer are decreased in diabetic nephropathy, whereas the assembly of PKM2 plays a pivotal role in facilitating the accumulation of hypoxia-inducible factor 1α, mediating aberrant glycolysis and inducing fibrosis in DKD (Ref. Reference Qi, Keenan, Li, Ishikado, Kannt, Sadowski, Yorek, Wu, Lockhart, Coppey, Pfenninger, Liew, Qiang, Burkart, Hastings, Pober, Cahill, Niewczas, Israelsen, Tinsley, Stillman, Amenta, Feener, Vander Heiden, Stanton and King76). The findings from these studies suggest that the presence of aberrant glycolysis is linked to an unfavourable prognosis in patients with DKD. We previously reported that the production of lactate, the main metabolite of this process, is closely related to the expression of lactate dehydrogenase and Azushima et al. reported that the expression of Ldha and Ldhb isoforms is significantly increased in the kidneys of diabetic nephropathy (DN) mice. In particular, Ldha, the main enzyme that catalyses the conversion of pyruvate to lactate, changed most significantly in the proximal renal tubules (Refs. Reference Azushima72, Reference Schlosser77). Consistent with these findings, all the relevant clinical analyses have shown that the blood lactate level and urinary lactate levels significantly increase in DKD patients (Refs. Reference Wu, Dong, Atefi, Liu, Elshimali and Vadgama71, Reference Blantz78). Moreover, in recent studies, the significant increase in urinary lactate levels in diabetic nephropathy patients has been consistently associated with albuminuria, a remarkable feature of early kidney injury, suggesting that lactate metabolism may be a new biomarker for DKD (Refs. Reference Sas, Kayampilly, Byun, Nair, Hinder, Hur, Zhang, Lin, Qi, Michailidis, Groop, Nelson, Darshi, Sharma, Schelling, Sedor, Pop-Busui, Weinberg, Soleimanpour, Abcouwer, Gardner, Burant, Feldman, Kretzler, Brosius and Pennathur70, Reference Azushima72, Reference Chou, Lee, Chen, Hsieh, Huang, Li and Lee79). Lactate is known to accompany DKD, but whether lactate is also an indicator of DKD has not yet been determined. In addition to revealing the correlation between high lactate levels in the kidney and kidney damage, this undoubtedly provides a new therapeutic idea for diabetic nephropathy, that is, lactate metabolism may be a new intervention point.

Renal cell carcinoma

Proliferating tumour cells usually exhibit enhanced metabolic activity and obtain more energy rapidly through aerobic glycolysis (Warburg effect). In this case, the production of a large amount of lactate not only leads to the acidification of the tumour microenvironment, but also plays a role in tumour migration, invasion and immunosuppression (Ref. Reference Ippolito, Morandi, Giannoni and Chiarugi39). RCC is a typical malignant tumour that exhibits the Warburg effect (Ref. Reference Miranda-Gonçalves, Lameirinhas, Henrique, Baltazar and Jerónimo80). Moreover, lactate produced by glycolysis may increase the aggressiveness of RCC by reducing SIRT1 activity, and affecting and regulating the phenotype of tumour cell and adjacent normal cells, and thus accelerating tumour progression (Ref. Reference Miranda-Goncalves81). Beyond this, research has indicated that kidney cancer cells can promote tumour progression by breaking down fat around the kidney to produce lactate (Ref. Reference Wei82). Recent studies have focused on the role of lactate-related metabolism in the occurrence and development of renal cell carcinoma. The upregulated expression of LDHA is once again regarded as a potential indicator of poor prognosis in RCC patients (Refs. Reference Li, du, Li and Peng83, Reference Girgis, Masui, White, Scorilas, Rotondo, Seivwright, Gabril, Filter, Girgis, Bjarnason, Jewett, Evans, al-Haddad, Siu and Yousef84). Understanding the role of lactate itself in RCC and the underlying mechanism is necessary.

Crosstalk between cells

Renal cells are capable of intercellular communication as well as communication with other cell types resident in the renal tissue (Ref. Reference Ebefors and Nyström85). The crosstalk plays a pivotal role in the exchange of signals in regulation renal physiological and pathological processes and is generally mediated by an array of bioactive substances, such as cytokine, exosome and metabolites, of which lactate is one. Studies using a folic acid mouse model of AKI have demonstrated that augmented tubular lactate excretion contributes to the activation of renal fibroblasts, which have been considered to have a pro-regenerative role in AKI (Refs. Reference Shen, Jiang, Wen, Ye, Zhang, Ding, Luo, Xu, Zen, Zhou and Yang15, Reference Comstock and Udenfriend65, Reference Guo, Cui, Jiao, Yao, Zhao, Tian, Dong and Liao86). Macrophages are the primary immune cells involved in kidney tissue, which also play a critical role in the cell–cell crosstalk in the kidney and development and progression of renal diseases (Ref. Reference Ebefors and Nyström85). Previous studies have demonstrated that tumour cell-derived lactate plays an important role in signal transduction by inducing tumour-associated macrophage polarisation and subsequently promoting tumour growth, which is also the case in renal cell carcinoma through the activation of specific receptors, such as GPR81 and GPR132, or through transport into cells by monocarboxylic acid transporters (Ref. Reference Colegio, Chu, Szabo, Chu, Rhebergen, Jairam, Cyrus, Brokowski, Eisenbarth, Phillips, Cline, Phillips and Medzhitov87). These findings also suggest that exploring the potential role of lactate in mediating the interactions among various cells in the local kidney throughout disease progression still holds considerable promise. In addition, the interaction between tubular epithelial cells and neighbouring endothelial cells is crucial for maintaining normal kidney function and plays a pivotal role in the pathogenesis and recovery of kidney diseases (Ref. Reference Tasnim and Zink88). However, the precise contribution of lactate in to this process remains unclear and warrants additional empirical investigation.

Signalling pathway

As a signalling molecule, lactate can activate various downstream signalling pathways by binding to specific receptors, including extracellular regulatory protein kinase (ERK1/2), nuclear factor κB (NF-κB) and Wnt signalling pathways, among others, thereby modulating the metabolic processes of tissues and cells (Refs. Reference Yang, Xu, Fan, Tu, Wang, Ha, Williams and Li89–Reference Li, Wang, Wang, Chen and Liu91). This process is typically mediated through the cell surface receptor GPR81, which is independent of MCT, protons and cellular glucose metabolism and plays a crucial role in the progression of various systemic diseases, including tumours, cardiovascular diseases, brain diseases and more (Refs. Reference Li, Yang, Zhang, Lin, Fu, An, Zou, Wang, Wang and Yu17, Reference Meng92, Reference Ouyang, Wang and Huang93). GPR81 expression in the kidney has been investigated in previous studies using in situ hybridisation, revealing its predominant localisation at the vascular poles of the glomerulus and association with the renal microvascular system (Ref. Reference Wallenius, Thalén, Björkman, Johannesson, Wiseman, Böttcher, Fjellström and Oakes94). Through the utilisation of scRNA-seq techniques, Azushima’s team has additionally discovered that the renal expression of GPR81 is mainly localised mainly in mesangial cells (Ref. Reference Azushima72). When ischaemia–reperfusion occurs, extracellular lactate in the kidney is elevated, which can lead to GPR81 activation in mesangial cells that regulate the perfusion of large and vessels and microvessels in the kidney and reduce renal arterial blood flow, a process that relies on ET-1 signal transduction and ultimately exacerbates injury (Ref. Reference Jones, Stewart, Czopek, Menzies, Thomson, Moran, Cairns, Conway, Denby, Livingstone, Wiseman, Hadoke, Webb, Dhaun, Dear, Mullins and Bailey95). Furthermore, inflammatory studies have proposed that GPR81 acts as a mediator of the activation of macrophages and fibroblasts induced by lactate, contributing to renal interstitial inflammatory damage and fibrosis (Ref. Reference Manoharan, Prasad, Thangaraju and Manicassamy96). In addition, lactate can participate in signal transduction pathways in a nonreceptor-mediated manner. Lactate impedes renal protection by inhibiting autophagy initiation via the SIRT3/AMPK pathway in LPS-induced AKI, whereas inhibition of lactate production by 2-DG impedes apoptosis and enhances renal function (Ref. Reference Tan, Gu, Li, Chen, Liu, Liu, Zhang and Xiao67). Lactate-induced activation of the PD-1/PD-L1 pathway could suppress the immune response in SAKI models in mice (Ref. Reference Xu, Ma, Yu, Wang, Wang, Liu, Liu, Gao, Yu and Wang97). Additionally, lactate regulates the TRPV4/ TGFβ1/ SMAD2/3/CTGF signalling pathway to promote partial EMT and accelerate renal fibrosis in TECs (Ref. Reference Zhao, Xu, Chen, Cai, Gong, Li, Kuang, Liu, Zhou, Liu and Yin98).

However, to date, research on the signalling pathways associated with lactate activity in the kidney is still lacking, necessitating further exploration. Further research on signalling pathways involving lactate will facilitate a deeper understanding of its regulatory effects on various physiological and pathological processes.

Protein translational modification

Lactate can be converted to lactoyl-CoA, which is involved in the acylation of both histone and nonhistone proteins. By investigating core histones in human MCF7 cells, Zhang et al. has reported that the mass shift in the lysine residues of three proteolytic peptides is consistent with the addition of a lactyl group to lysine. This ground-breaking study provides the first evidence for the presence of lysine lactylation (Kla) and highlights Kla as a novel epigenetic modification in protein posttranslational processes (Ref. Reference Zhang, Tang, Huang, Zhou, Cui, Weng, Liu, Kim, Lee, Perez-Neut, Ding, Czyz, Hu, Ye, He, Zheng, Shuman, Dai, Ren, Roeder, Becker and Zhao99). The findings from this study suggest that lactate accumulated during glycolysis can serve as a precursor for lactate histone lysine, thereby directly influencing gene expression. Furthermore, Wan et al. then identified a cyclic ammonium ion, which serves as a reliable tool for the identification of novel Kla and modification sites, thereby revealing extensive modifications in the human proteome beyond histones (Ref. Reference Wan, Wang, Yu, Zhang, Tang, Wang, Lu, Li, Delafield, Kong, Wang, Shao, Lv, Wang, Tan, Wang, Hao and Ye100). Their findings demonstrated the presence of Kla proteins in the cytoplasm likewise and their predominant involvement in glycolysis. The presence of glycolytic inhibitors is directly correlated with lactate production and Kla, whereas mitochondrial inhibitors and cellular hypoxia can potentially increase lactate production and Kla activity (Refs. Reference Xiang, Zhao, Wu, Li, Fu and Ma101, Reference Shang, Liu and Hua102).

Protein translational modification (PTM) is a type of epigenetic modification. Through covalent modification, different acyl groups can attach to amino acid residues on histones or nonhistones and proteins undergo posttranslational changes in both the C-terminal region and the prominent N-terminal tail, thereby dynamically regulating protein localisation, activity and molecular interactions (Refs. Reference Xie, Hu, Liu, Zhou, Cheng, Huang and Cao103, Reference Millán-Zambrano, Burton, Bannister and Schneider104). Like other protein modifications, lactylation involves enzymes that catalyse the formation of specific types of PTMs (writers), as well as enzymes that remove PTMs (erasures) (Ref. Reference Millán-Zambrano, Burton, Bannister and Schneider104). P300, a well-known acetyltransferase that catalyses the acetylation of transcription factors, histones and other nucleoproteins and has been shown to be responsible for the induction of protein lactylation by lactate (Ref. Reference Zhang, Tang, Huang, Zhou, Cui, Weng, Liu, Kim, Lee, Perez-Neut, Ding, Czyz, Hu, Ye, He, Zheng, Shuman, Dai, Ren, Roeder, Becker and Zhao99). in vitro studies have shown that two deacetylase families, HDAC1–3 and SIRT1–3, reduce the expression of histones (including H3K18 and H4K5) Kla and that HDAC3 is the most effective erasing agent for Kla (Ref. Reference Moreno-Yruela, Zhang, Wei, Bæk, Liu, Gao, Danková, Nielsen, Bolding, Yang, Jameson, Wong, Olsen and Zhao105). In light of this, lactylation and acetylation are frequently coregulated. For instance, in a study conducted by Yang et al., lactate stimulated the lactylation and acetylation processes of high mobility group protein B1 in macrophages through the activation of p300 acetylase activity and inhibition of SIRT1 deacetylase activity (Ref. Reference Yang, Fan, Wang, Xu, Wang, Tu, Gill, Ha, Liu, Williams and Li106). These extensively investigated acetylation-related molecules in the context of kidney disease may also influence lactcylation and participate in the pathogenesis of this condition.

Since it was proposed, lactylation has been shown to be important for the functions of lactate, and it is involved in important life activities, such as glycolytic cell function, macrophage polarisation, vascular function, mitochondrial fission and fusion and nervous system regulation, including some recent findings in kidney disease. In exploring the pathogenesis of histone lactylation and clear cell renal cell carcinoma, Jiefeng Yang’s team reported a positive feedback loop between histone lactylation (H3K18la) and platelet-derived growth factor receptor β signalling, promoting tumour proliferation and migration (Ref. Reference Yang, Luo, Zhao, Li, Wang, Zeng, Yang, Zheng, Jie, Kang, Li, Liu, Zhou and Liu23). Previous studies conducted by our research team have revealed an increase in the level of glycosis in TECs following kidney injury, leading to the gradual accumulation of lactate, which subsequently increased the expression of the transcription factor Twist1 through lactylation at the histone H3K18 site, thereby promoting the progression of renal fibrosis. In addition, a new study revealed that lactate derived from the glycolytic reprogramming of renal tubules significantly enhances histone lactylation, specifically H4K12la, which is enriched on the promoters of NF-κB signal transduction genes and drives kidney fibrosis (Ref. Reference Wang107). Besides, the role of lactylation modification of nonhistone proteins in the occurrence and development of related kidney diseases has been gradually discovered. A study identified numerous increased lactylation sites on mitochondrial proteins that play key roles in mitochondrial metabolism in the kidneys of db/db mice via integrative lactylome analysis, and these results suggest that the lactylation of mitochondrial proteins may play a vital role in the progression of DKD, which warrants further investigation (Ref. Reference Chen, Feng, Qiao, Pan, Liang, Liu, Zhang, Liu, Liu and Liu108). Additionally, in patients with sepsis-induced acute kidney injury, lactate overproduction mediates mitochondrial fission protein 1 lysine 20 (Fis1 K20la) lactylation to promote excessive mitochondrial fission, thereby exacerbating kidney lesions and dysfunction (Ref. Reference An, Yao, Hu, Wu, Li, Li, Wu, Sun, Deng, Zhang, Gong, Huang, Chen and Zeng63). Lactylation of aldehyde dehydrogenase 2 at lysine 52 (ALDH2 K52la) promotes the ubiquitination-proteasomal degradation of prohibitin 2 (PHB2) – a crucial mitophagy receptor – thereby impairing mitophagy and exacerbating tubular injury and mitochondrial dysfunction in AKI (Ref. Reference Li, Shi, Xu, Wang, Hou, Luan and Chen109). Although few studies have investigated the association between lactylation and diseases in renal-related fields, we have shown that the increased production of lactate in the metabolic reprogramming of diseases such as AKI and DKD plays an important role in the progression of those diseases, and the potential impact of lactylation is also worthy of further exploration. Systematic analysis of the regulatory network between lactic acid metabolism and lactylation modification will help reveal their potential functional roles in the process of kidney injury and repair, thus opening up more possibilities for the treatment of diseases.