Cardiometabolic diseases-including Type 2 diabetes and obesity-remain leading causes of global mortality. Recent advancements in metabolomics have facilitated the identification of metabolites that are integral to the development of insulin resistance, a characteristic feature of cardiometabolic disease. Key metabolites, such as branched-chain amino acids (BCAAs), ceramides, glycine, and glutamine, have emerged as valuable biomarkers for early diagnosis, risk stratification, and potential therapeutic targets. Elevated BCAAs and ceramides are strongly associated with insulin resistance and Type 2 diabetes, whereas glycine exhibits an inverse relationship with insulin resistance, making it a promising therapeutic target. Metabolites involved in energy stress, including ketone bodies, lactate, and nicotinamide adenine dinucleotide (NAD⁺), regulate insulin sensitivity and metabolic health, with ketogenic diets and NAD⁺ precursor supplementation showing potential benefits. Additionally, the novel biomarker N-lactoyl-phenylalanine further underscores the complexity of metabolic regulation and its therapeutic potential. This review underscores the potential of metabolite-based diagnostics and precision medicine, which could enhance efforts in the prevention, diagnosis, and treatment of cardiometabolic diseases, ultimately improving patient outcomes and quality of life.

H+ facilitates metabolic blood flow regulation while negatively impacting cardiac contractility. Cardiovascular consequences of conjugate bases accumulating alongside H+ remain unclear. Here, we evaluate the cardiovascular effects of nine prominent carboxylates-particularly lactate, 3-hydroxybutyrate, and butyrate-linked to metabolic and microbial activity.

Comparing the actions of pH-adjusted Na-carboxylates to equiosmolar NaCl, we study arteries and veins isolated from healthy rats and humans with ischaemic heart disease, isolated perfused rat hearts, and rat cardiovascular function in vivo.

The tested carboxylates generally relax arteries and veins. L-lactate relaxes human and rat arteries up to 70% (EC50 = 10.1 mM) and rat brachial and mesenteric veins up to 30% of pre-contractions, yet stands out by augmenting contractions of rat femoral, saphenous, and lateral marginal veins and human internal thoracic and great saphenous veins up to 50%. D-lactate shows only minor actions. In isolated perfused hearts, 10 mM L-lactate increases coronary flow (17.1 ± 7.7%) and left ventricular developed pressure (10.1 ± 3.0%) without affecting heart rate. L-lactate infusion in rats-reaching 3.7 ± 0.3 mM in the circulation-increases left ventricular end-diastolic volume (11.3 ± 2.8%), stroke volume (22.6 ± 3.0%), cardiac output (23.4 ± 3.5%), and ejection fraction (10.6 ± 2.0%), and lowers systemic vascular resistance (34.1 ± 3.7%) without influencing blood pressure or heart rate. The ketone body 3-hydroxybutyrate causes lactate accumulation and elevates left ventricular end-diastolic volume in vivo.

Carboxylate metabolites generally relax arteries and veins. L-lactate relaxes arteries, lowering systemic vascular resistance, causes preferential venocontraction with increased ventricular diastolic filling, and elevates cardiac contractility and cardiac output. We propose that L-lactate optimizes cardiovascular function during metabolic disturbances.

L-Lactate, once considered a metabolic waste product of glycolysis, is now recognized as a vitally important metabolite and signaling molecule in multiple biological pathways. However, exploring L-lactate's emerging intra- and extra-cellular roles is hindered by a lack of tools to perturb L-lactate concentration intracellularly and extracellularly. Photocaged compounds are a powerful way to introduce bioactive molecules with spatiotemporal precision using illumination. Here, we report the development of a photocaged derivative of L-lactate, 4-methoxy-7-nitroindolinyl-L-lactate (MNI-L-lac), that releases L-lactate upon illumination. We validated MNI-L-lac in cell culture by demonstrating that the photorelease of L-lactate elicits a response from genetically encoded extra- and intracellular L-lactate biosensors (eLACCO1, eLACCO2.1, R-iLACCO1.2). To demonstrate the utility of MNI-L-lac, we employed the photorelease of L-lactate to activate G protein-coupled receptor 81 (GPR81), as revealed by the inhibition of adenylyl cyclase activity and concomitant decrease of cAMP. These results indicate that MNI-L-lac may be useful for perturbing the concentration of endogenous L-lactate in order to investigate L-lactate's roles in metabolic and signaling pathways.

L-Lactate has emerged as a crucial metabolic intermediate, moving beyond its traditional view as a mere waste product. The recent discovery of L-lactate-driven protein lactylation as a post-translational modification has unveiled a pathway that highlights the role of lactate in cellular signalling. In this Perspective, we explore the enzymatic and metabolic mechanisms underlying protein lactylation and its impacts on both histone and non-histone proteins in the contexts of physiology and diseases. We discuss growing evidence suggesting that this modification regulates a wide range of cellular functions and is involved in various physiological and pathological processes, such as cell-fate determination, development, cardiovascular diseases, cancer and autoimmune disorders. We propose that protein lactylation acts as a pivotal mechanism, integrating metabolic and signalling pathways to enable cellular adaptation, and highlight its potential as a therapeutic target in various diseases.

Hydroxycarboxylic acid receptor 1 (HCAR1), also known as lactate receptor or GPR81, is a class A G-protein-coupled receptor with key roles in regulating lipid metabolism, neuroprotection, angiogenesis, cardiovascular function, and inflammatory response in humans. HCAR1 is highly expressed in numerous types of cancer cells, where it participates in controlling cancer cell metabolism and defense mechanisms, rendering it an appealing target for cancer therapy. However, the molecular basis of HCAR1-mediated signaling remains poorly understood. Here, we report four cryo-EM structures of human HCAR1 and HCAR2 in complex with the Gi1 protein, in which HCAR1 binds to the subtype-specific agonist CHBA (3.16 Å) and apo form (3.36 Å), and HCAR2 binds to the subtype-specific agonists MK-1903 (2.68 Å) and SCH900271 (3.06 Å). Combined with mutagenesis and cellular functional assays, we elucidate the mechanisms underlying ligand recognition, receptor activation, and G protein coupling of HCAR1. More importantly, the key residues that determine ligand selectivity between HCAR1 and HCAR2 are clarified. On this basis, we further summarize the structural features of agonists that match the orthosteric pockets of HCAR1 and HCAR2. These structural insights are anticipated to greatly accelerate the development of novel HCAR1-targeted drugs, offering a promising avenue for the treatment of various diseases.

Given its various roles in cellular functions, lactate is no longer considered a waste product of metabolism and lactate sensing is a pivotal step in the transduction of lactate signals. Lysine lactylation is a recently identified post-translational modification that serves as an intracellular mechanism of lactate sensing and transfer. Although acetyltransferases such as p300 exhibit general acyl transfer activity, no bona fide lactyltransferases have been identified. Recently, the protein synthesis machinery, alanyl-tRNA synthetase 1 (AARS1), AARS2 and their Escherichia coli orthologue AlaRS, have been shown to be able to sense lactate and mediate lactyl transfer and are thus considered pan-lactyltransferases. Here we highlight the mechanisms and functions of these lactyltransferases and discuss potential strategies that could be exploited for the treatment of human diseases.

Lactate, once considered a mere byproduct of anaerobic metabolism, is now recognized as a critical signaling molecule with diverse roles in physiology and pathology. There are two stereoisomers of lactate: L- and D-lactate. Recent studies have shown that disruptions in these two lactate stereoisomers have distinct effects on health and disease. L-lactate is central to glycolysis and energy transfer through the Cori cycle but also acts as the dominant lactylation isomer induced by glycolysis, influencing metabolism and cell survival. Although less studied, D-lactate is linked to metabolic disorders and plays a role in mitochondrial dysfunction and oxidative stress. This review focuses on both L- and D-lactate and examines their biosynthesis, transport, and expanding roles in physiological and pathological processes, particularly their functions in cancer, immune regulation, inflammation, neurodegeneration and other diseases. Finally, we assess the therapeutic prospects of targeting lactate metabolism, highlighting emerging strategies for intervention in clinical settings. Our review synthesizes the current understanding of L- and D-lactate, offering insights into their potential as targets for therapeutic innovation.

Introduction: Polygonatum cyrtonema Hua (PC) is an essential herbal medicine in China, known for improving muscle quality and enhancing physical function; its active ingredients are polysaccharides (PCPs). A previous study revealed the anti-atrophy effects of PCPs in cachectic mice. However, whether the effects of PCPs on anti-atrophy are associated with gut microenvironment remain elusive. This research endeavored to assess the medicinal efficacy of PCPs in alleviating muscle atrophy and fat lipolysis and explore the potential mechanisms. Methods: A cancer cachexia model was induced by male C57BL/6 mice bearing Lewis lung tumor cells and chemotherapy. The pharmacodynamics of PCPs (32 and 64 mg/kg/day) was investigated through measurements of tumor-free body weight, gastrocnemius muscle weight, soleus muscle weight, epididymal fat weight, tissue histology analysis, and pro-inflammatory cytokines. Immunohistochemistry and Western blotting assays were further used to confirm the effects of PCPs. 16S rRNA sequencing, LC-MS and GC-MS-based metabolomics were used to analyze the gut microbiota composition and metabolite alterations. Additionally, the agonist of free fatty acid receptor 2 (FFAR2)-a crucial short-chain fatty acid (SCFA) signaling molecule-was used to investigate the role of gut microbiota metabolites, specifically SCFAs, in the treatment of cancer cachexia, with comparisons to PCPs. Results: This study demonstrated that PCPs significantly mitigated body weight loss, restored muscle fiber atrophy and mitochondrial disorder, alleviated adipose tissue wasting, strengthened the intestinal barrier integrity, and decreased the intestinal inflammation in chemotherapy-induced cachexia. Furthermore, the reversal of specific bacterial taxa including Klebsiella, Akkermansia, norank_f__Desulfovibrionaceae, Enterococcus, NK4A214_group, Eubacterium_fissicatena_group, Eubacterium_nodatum_group, Erysipelatoclostridium, Lactobacillus, Monoglobus, Ruminococcus, Odoribacter, and Enterorhabdus, along with alterations in metabolites such as amino acids (AAs), eicosanoids, lactic acid and (SCFAs), contributed to the therapeutic effects of PCPs. Conclusion: Our findings suggest that PCPs can be used as prebiotic drugs targeting the microbiome-metabolomics axis in cancer patients undergoing chemotherapy.

Lactate is among the highest flux circulating metabolites. It is made by glycolysis and cleared by both tricarboxylic acid (TCA) cycle oxidation and gluconeogenesis. Severe lactate elevations are life-threatening, and modest elevations predict future diabetes. How lactate homeostasis is maintained, however, remains poorly understood. Here, we identify, in mice, homeostatic circuits regulating lactate production and consumption. Insulin induces lactate production by upregulating glycolysis. We find that hyperlactatemia inhibits insulin-induced glycolysis, thereby suppressing excess lactate production. Unexpectedly, insulin also promotes lactate TCA cycle oxidation. The mechanism involves lowering circulating fatty acids, which compete with lactate for mitochondrial oxidation. Similarly, lactate can promote its own consumption by lowering circulating fatty acids via the adipocyte-expressed G-protein-coupled receptor hydroxycarboxylic acid receptor 1 (HCAR1). Quantitative modeling suggests that these mechanisms suffice to produce lactate homeostasis, with robustness to noise and perturbation of individual regulatory mechanisms. Thus, through regulation of glycolysis and lipolysis, lactate homeostasis is maintained.

Acute kidney injury (AKI) remains a major public health challenge, posing serious threats to human health. Increasing evidence indicates that renal cells undergo significant metabolic alterations following AKI, with inflammatory responses persisting throughout both injury and repair phases. Our previous research has demonstrated that heightened aerobic glycolysis after AKI leads to increased secretion of metabolic byproducts such as lactate, which plays a critical role in tissue repair. However, the relationship between metabolic reprogramming and inflammatory responses, as well as the underlying mechanisms, remain poorly understood. This study aims to clarify the regulatory effects of the glycolytic byproduct lactate on macrophage activation and phenotypic differentiation following AKI. We observed increased expression of M1/M2 macrophages and elevated secretion of inflammatory cytokines after folic acid-induced AKI. Immunofluorescence staining showed co-localization of macrophages with α-SMA. Manipulating lactate levels post-injury led to a decrease in macrophage expression and a reduction in fibroblast activation and proliferation, ultimately impairing renal tissue repair. These findings suggest that targeting lactate as a key regulator of macrophage phenotype differentiation may provide a theoretical and clinical foundation for therapeutic strategies in AKI repair.

Neutrophils are pivotal in the immune system and have been recognized as significant contributors to cancer development and progression. These cells undergo metabolic reprogramming in response to various stimulus, including infections, diseases, and the tumor microenvironment (TME). Under normal conditions, neutrophils primarily rely on aerobic glucose metabolism for energy production. However, within the TME featured by hypoxic and nutrient-deprived conditions, they shift to altered anaerobic glycolysis, lipid metabolism, mitochondrial metabolism and amino acid metabolism to perform their immunosuppressive functions and facilitate tumor progression. Targeting neutrophils within the TME is a promising therapeutic approach. Yet, focusing on their metabolic pathways presents a novel strategy to enhance cancer immunotherapy. This review synthesizes the current understanding of neutrophil metabolic reprogramming in the TME, with an emphasis on the underlying molecular mechanisms and signaling pathways. Studying neutrophil metabolism in the TME poses challenges, such as their short lifespan and the metabolic complexity of the environment, necessitating the development of advanced research methodologies. This review also discusses emerging solutions to these challenges. In conclusion, given their integral role in the TME, targeting the metabolic pathways of neutrophils could offer a promising avenue for cancer therapy.

Hydroxy-carboxylic acid receptors HCA1, HCA2, and HCA3 can be activated by important intermediates of energy metabolism. Despite the research focusing on HCA2, its clinical application has been limited by adverse effects. Therefore, the role of HCA1 as a promising target for the treatment of lipolysis warrants further exploration. As HCAs exhibit high similarity when activated with diverse selective agonists, a conserved yet unique activation mechanism for HCAs remains undisclosed. Herein, we unveil the cryo-electron microscopy structures of the 3,5-DHBA-HCA1-Gi signaling complex, the acifran- and MK6892-bound HCA2-Gi signaling complexes, and the acifran-HCA3-Gi signaling complex. Comparative analysis across HCAs reveals key residues in HCA1 contributing to the stabilization of the ligand-binding pocket. Furthermore, chimeric complexes and mutational analyses identify residues that are pivotal for HCA2 and HCA3 selectivity. Our findings elucidate critical structural insights into the mechanisms of ligand recognition and activation within HCA1 and broaden our comprehension of ligand specificity binding across the HCA family.

The effacement of podocyte foot processes, which form slit diaphragms, are common features of proteinuria. Exploring podocyte energy metabolism, especially under diabetic conditions, may offer insights into the pathogenesis of diabetic kidney disease. Lipid accumulation is recognized as a cause of podocyte cytoskeleton remodeling and insulin resistance. Thus, the role of the metabolic sensor G-protein-coupled receptor 81 (GPR81) was examined in the molecular pathway of lipid accumulation in podocytes under hyperglycemic conditions. It was discovered that hyperglycemia downregulated the cyclic adenosine monophosphate/protein kinase A signaling pathway, which downregulated the expression of adipose triglyceride lipase (ATGL). Perilipin 1 was also downregulated; simultaneously, lipid droplet accumulation was enhanced. Glycerol and free fatty acid concentrations were also reduced, providing evidence of lipolysis inhibition. Interestingly, the expression of GPR81 decreased under hyperglycemia conditions despite the evidence of its activation, indicating strict lipolysis regulation. More importantly, cell functions were altered, reflected by an increase in albumin permeability and rearrangement of the actin cytoskeleton. The effect of ATGL activity inhibition on lipolysis, actin cytoskeleton arrangement, and permeability of the podocyte monolayer was investigated. The results were similar to GPR81 downregulation. Altogether, the present data indicate that GPR81 is likely a crucial part of the lipid sensing system, and its alterations during hyperglycemia might contribute to glomerular filtration barrier deterioration in diabetic kidney disease.

This review aims to discuss the process of cardiomyocyte maturation, with a focus on the underlying molecular mechanisms required to form a fully functional heart. We examine both long-standing concepts associated with cardiac maturation and recent developments, and the overall complexity of molecularly integrating all the processes that lead to a mature heart.

Cardiac maturation, defined here as the sequential changes that occurring before the heart reaches full maturity, has been a subject of investigation for decades. Recently, there has been a renewed, highly focused interest in this process, driven by clinically motivated research areas where enhancing maturation may lead to improved therapeutic opportunities. These include using pluripotent stem cell models for cell therapy and disease modeling, as well as recent advancements in adult cardiac regeneration approaches. We highlight key processes underlying maturation of the heart, including cellular and organ growth, and electrophysiological, metabolic, and contractile maturation. We further discuss how these processes integrate and interact to contribute to the overall complexity of the developing heart. Finally, we emphasize the transformative potential for translating relevant maturation concepts to emerging models of heart disease and regeneration.

Heat stroke, a severe heat illness with organ damage, is a major cause of cause irreparable organ damage and higher death rates among military persons and athletes.

To study the changes in blood lactate (Lac) levels and lactate clearance rate (LCR) in athletes with heat illness of varying degrees after high-intensity exercise and to evaluate their prognostic value.

In present study, acute care unit admitted 36 heat sickness patients following high-intensity exercise from December 2019 to July 2024, with comprehensive medical records, for retrospective study. The study population consisted of two groups of high level athletes: the favourable Prognosis Group (< 7 days, 22 cases), comprising 21 males and 1 female with a mean age of 21.8 ± 2.7 years, and the bad Prognosis Group (≥ 7 days, 14cases), consisting of 14 males with a mean age of 22.6 ± 3.2 years. Lac levels were assessed at admission (0 h) and early in therapy (2 h, 6 h), and the LCR was computed. Lac and LCR values at each time point were compared between the two groups to see how they affected patient prognosis.

After 2 and 6 h of therapy, lactate levels decreased significantly in the good prognosis group (1.2 ± 0.5 mmol/L at 2 h and 0.8 ± 0.3 mmol/L at 6 h), but remained elevated in the poor prognosis group (4.2 ± 1.2 mmol/L at 2 h and 3.5 ± 1.5 mmol/L at 6 h). Core body temperature normalized in both groups, but the good prognosis group showed a more rapid decline, with temperatures of 37.4 ± 0.6 °C at 2 h and 36.8 ± 0.4 °C at 6 h in the good prognosis group, and 38.8 ± 0.8 °C at 2 h and 38.2 ± 0.9 °C at 6 h in the poor prognosis group. Notably, a significant positive correlation existed between lactate levels and APACHE II scores at admission (P < 0.01). Furthermore, logistic regression analysis revealed that the 2-hour lactate clearance rate (LCR) (R2 = 0.83) was an independent predictor of outcomes.

The study suggests that athletes with elevated lactate levels after heat illness may be at higher risk of adverse outcomes. The 2-hour lactate clearance rate (LCR) appears to be a valuable prognostic indicator, with potential applications in evaluating the severity of heat illness and guiding treatment decisions. Furthermore, dynamic monitoring of lactate levels in conjunction with LCR may provide valuable insights into the clinical management and prognosis of athletes with heat-related illnesses.

The blastocyst develops a unique metabolism that facilitates the creation of a specialized microenvironment at the site of implantation characterized by high levels of lactate and reduced pH. While historically perceived as a metabolic waste product, lactate serves as a signaling molecule which facilitates the invasion of surrounding tissues by cancers and promotes blood vessel formation during wound healing. However, the role of lactate in reproduction, particularly at the implantation site, is still being considered. Here, we detail the biological significance of the microenvironment created by the blastocyst at implantation, exploring the origin and significance of blastocyst-derived lactate, its functional role at the implantation site and how understanding this mediator of the maternal-fetal dialogue may help to improve implantation in assisted reproduction.

In children with type 1 diabetes (T1D), diabetic ketoacidosis (DKA) triggers a significant inflammatory response; however, the specific effector proteins and signaling pathways involved remain largely unexplored. This pediatric case-control study utilized plasma proteomics to explore protein alterations associated with severe DKA and to identify signaling pathways that associate with clinical variables.

We conducted a proteome analysis of plasma samples from 17 matched pairs of pediatric patients with T1D; one cohort with severe DKA and another with insulin-controlled diabetes. Proximity extension assays were used to quantify 3072 plasma proteins. Data analysis was performed using multivariate statistics, machine learning, and bioinformatics.

This study identified 214 differentially expressed proteins (162 upregulated, 52 downregulated; adj P < 0.05 and a fold change > 2), reflecting cellular dysfunction and metabolic stress in severe DKA. We characterized protein expression across various organ systems and cell types, with notable alterations observed in white blood cells. Elevated inflammatory pathways suggest an enhanced inflammatory response, which may contribute to the complications of severe DKA. Additionally, upregulated pathways related to hormone signaling and nitrogen metabolism were identified, consistent with increased hormone release and associated metabolic processes, such as glycogenolysis and lipolysis. Changes in lipid and fatty acid metabolism were also observed, aligning with the lipolysis and ketosis characteristic of severe DKA. Finally, several signaling pathways were associated with clinical biochemical variables.

Our findings highlight differentially expressed plasma proteins and enriched signaling pathways that were associated with clinical features, offering insights into the pathophysiology of severe DKA.

The lactate shuttle concept has revolutionized our understanding and study of metabolism in physiology, biochemistry, intermediary metabolism, nutrition, and medicine. Seminal findings of the mitochondrial lactate oxidation complex (mLOC) elucidated the architectural structure of its components. Here, we report that the mitochondrial pyruvate carrier (mPC) is an additional member of the mLOC in mouse muscle and C2C12 myoblasts and myotubes. Immunoblots, mass spectrometry, and co-immunoprecipitation experiments of mitochondrial preparations revealed abundant amounts of mitochondrial lactate dehydrogenase (mLDH), monocarboxylate transporter (mMCT), basigin (CD147), cytochrome oxidase (COx), and pyruvate carriers 1 and 2 (mPC1 and 2). In addition, using confocal laser scanning microscopy (CLSM) and in situ proximity ligation, we also demonstrated planar and three-dimensional (3-D) colocalization of pyruvate and lactate transporters with COx in fixed mouse skeletal muscle sections and C2C12 myoblasts and myotubes skeletal muscle sections, mouse muscle and C2C12 myoblasts and myotubes myotubes, and C2C12 myoblasts. This work serves as a landmark for configuring the final pathway of carbohydrate oxidation.NEW & NOTEWORTHY We expand on knowledge of the architectural design of the mitochondrial lactate oxidation complex (mLOC); key members are: mitochondrial lactate dehydrogenase (mLDH), monocarboxylate transporter 1 (mMCT1), cytochrome oxidase (COx), basigin scaffolding protein (CD147), and the mitochondrial pyruvate carrier (mPC). The mLOC is key in creating the lower end of the concentration gradient for disposal of lactate and pyruvate.

Age-dependent changes in adipose tissue are thought to play a role in development of insulin resistance. A major age-dependent change in adipose tissue is the downregulation of key proteins involved in carbohydrate metabolism. In the current study, we investigate the role of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3) a key governor of the rate of glycolysis in adipocytes via the synthesis of fructose-2,6-bisphosphate that was significantly downregulated in aged mice. We employed an adipocyte-specific PFKFB3 mouse line to investigate the role of PFKFB3 on adipocyte function. In both aged mice and PFKFB3-knockout mice, we observed an increase in O-glcNAcylated proteins consistent with a shift in glucose metabolism toward the hexosamine biosynthetic pathway. Under chow-fed conditions, PFKFB3 knockout resulted in significantly smaller adipocyte area, but no difference in total fat mass. While glucose tolerance was unchanged under chow conditions, when mice were challenged with a 4 weeks high-fat feeding, PFKFB3 deletion led to a greater decrease in glucose tolerance as well as a significant increase in macrophage infiltration. These results indicate that perturbation of the glycolytic pathway in adipose tissue has multiple effects of adipocyte biology and may play a significant role in metabolic changes associated with aging. Results of this student support the notion that changes in glucose metabolism in adipose tissue impact whole-body metabolism.

Poly-L-lactic acid (PLLA), a well-established biostimulator that induces collagenases, is widely used among clinical practice to treat skin aging. However, the precise regulatory effect of PLLA on different dermal cell subsets beyond fibroblast has not been fully elucidated. In this study, we constructed in vivo PLLA injection and in vitro PLLA-adipocyte co-culture models to analyze the regulatory effects of PLLA on the volume, differentiation, lipolysis, and thermogenic capacity of dermal adipocyte. We found that PLLA injection significantly reduced the thickness of dermal fat in mice. In co-culture assay, PLLA showed no effect on adipogenesis, but stimulated the lipolysis activity. Interestingly, PLLA also enhanced the differentiation of fat cells into beige fat cells, which possess higher thermogenic capacity. In mechanical study, we blocked adipocyte lactate uptake with a monocarboxylate transporter (MCT1/4) inhibitor and found that the regulatory effect of PLLA on dermal adipocyte relies on its metabolite lactate. In summary, our results suggest that PLLA has complex regulatory effects on the dermal cells, and its ability to improve skin aging is not fully attributed to stimulating collagen synthesis, but also partially involves adipocytes.No Level Assigned This journal requires that authors assign a level of evidence to each submission to which Evidence-Based Medicine rankings are applicable. This excludes Review Articles, Book Reviews, and manuscripts that concern Basic Science, Animal Studies, Cadaver Studies, and Experimental Studies. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

By the time a tumor reaches clinical detectability, it contains around 108-109 cells. However, during tumor formation, significant cell loss occurs due to cell death. In some estimates, it could take up to a thousand cell generations, over a ~ 20-year life-span of a tumor, to reach clinical detectability, which would correspond to a "theoretical" generation of ~1030 cells. These rough calculations indicate that cancers are under negative selection. The fact that they thrive implies that they "evolve", and that their evolutionary trajectories are shaped by the pressure of the environment. Evolvability of a cancer is a function of its heterogeneity, which could be at the genetic, epigenetic, and ecological/microenvironmental levels [1]. These principles were summarized in a proposed classification in which Evo (evolutionary) and Eco (ecological) indexes are used to label cancers [1]. The Evo index addresses cancer cell-autonomous heterogeneity (genetic/epigenetic). The Eco index describes the ecological landscape (non-cell-autonomous) in terms of hazards to cancer survival and resources available. The reciprocal influence of Evo and Eco components is critical, as it can trigger self-sustaining loops that shape cancer evolvability [2]. Among the various hallmarks of cancer [3], metabolic alterations appear unique in that they intersect with both Evo and Eco components. This is partly because altered metabolism leads to the accumulation of oncometabolites. These oncometabolites have traditionally been viewed as mediators of non-cell-autonomous alterations in the cancer microenvironment. However, they are now increasingly recognized as inducers of genetic and epigenetic modifications. Thus, oncometabolites are uniquely positioned at the crossroads of genetic, epigenetic and ecological alterations in cancer. In this review, the mechanisms of action of oncometabolites will be summarized, together with their roles in the Evo and Eco phenotypic components of cancer evolvability. An evolutionary perspective of the impact of oncometabolites on the natural history of cancer will be presented.

The subretina, composed of the choroid and the retinal pigment epithelium (RPE), plays a critical role in proper vision. In addition to phagocytosis of photoreceptor debris, the RPE shuttles oxygen and nutrients to the neuroretina. For their own energy production, RPE cells mainly rely on lactate, a major by-product of glycolysis. Lactate, in turn, conveys most of its biological effects via the hydroxycarboxylic acid receptor 1 (HCAR1). Herein, the lactate-specific receptor, HCAR1, was found to be exclusively expressed in the RPE cells within the subretina, and Hcar1-/- mice exhibited a substantially thinner choroidal vasculature during development. Notably, the angiogenic properties of lactate on the choroid were impacted by the absence of Hcar1. HCAR1-deficient mice exhibited elevated endoplasmic reticulum stress along with eukaryotic translation initiation factor 2α phosphorylation, a significant decrease in the global protein translation rate, and a lower proliferation rate of choroidal vasculature. Strikingly, inhibition of the integrated stress response using an inhibitor that reverses the effect of eukaryotic translation initiation factor 2α phosphorylation restored protein translation and rescued choroidal thinning. These results provide evidence that lactate signalling via HCAR1 is important for choroidal development/angiogenesis and highlight the importance of this receptor in establishing mature vision.

The neurogenic potential of the brain decreases during ageing, whereas the risk of neurodegenerative diseases and stroke rises. This creates a mismatch between the rate of neuron loss and the brain's capacity for replacement. Adult neurogenesis primarily occurs in the subgranular zone (SGZ) and the ventricular-subventricular zone (V-SVZ). Exercise enhances SGZ neurogenesis, and we previously showed that V-SVZ neurogenesis is induced by exercise via activation of the lactate receptor HCA1. Here, we investigated how high-intensity interval training (HIIT) and medium-intensity interval training (MIIT) affect neurogenesis in these niches. Wild-type (WT) and HCA1 knockout (KO) mice were randomized to sedentary, HIIT or MIIT (n = 5-8 per group) for 3 weeks. In the SGZ, HIIT increased the density of doublecortin (DCX)-positive cells in WT mice by 85% (5.77±1.76 vs. 3.12±1.54 cells/100 µm, P = 0.013) and KO mice (67% increase; 7.91±2.92 vs. 4.73±1.63 cells/100 µm, P = 0.004). MIIT did not alter the density of DCX-positive cells in either genotype. HIIT increased the density of Ki-67-positive cells only in KO mice (P = 0.038), whereas no differences in nestin-positive cells were observed. In the V-SVZ, HIIT increased the density of DCX-positive cells in WT mice by 155% (117.79±39.72 vs. 46.25±19.96 cells/100 µm, P < 0.001) and MIIT increased the density of DCX-positive cells by 80% (83.26±39.48 vs. 46.25±19.96 cells/100µm, P = 0.027). No exercise-induced changes were observed in KO mice. Similar patterns were noted for Ki-67 positive and DCX/Ki-67 double-positive cells in the V-SVZ. These findings suggest that HIIT enhances neurogenesis more robustly than MIIT in both niches, with HCA1 playing a crucial role in V-SVZ neurogenesis. KEY POINTS: The neurogenic potential of the brain decreases with age, whereas the risk of neurodegenerative diseases and stroke increases, highlighting a mismatch between neuronal loss and replacement capacity. Exercise enhances neurogenesis in both the subgranular zone and the ventricular-subventricular zone. High-intensity interval exercise is more effective than medium-intensity interval exercise at promoting neurogenesis in both the subgranular zone and the ventricular-subventricular zone of wild-type mice. The enhancement of neurogenesis in the ventricular-subventricular zone is dependent on the activation of the HCA1 receptor, as evidenced by the ability of medium- and high-intensity interval exercise to induce neurogenesis in wild-type mice and the lack of this effect in HCA1 knockout mice. By contrast, neurogenesis in the subgranular zone is independent on the activation of the HCA1 receptor, highlighting that neurogenesis in the two major neurogenic niches are regulated differently.

The aging population is increasing worldwide, and there is also an increase in the aging population living with overweight and obesity, due to changes in lifestyle and in dietary patterns that elderly individuals experience later in life. Both aging and obesity are conditions of accelerated metabolic dysfunction and dysregulated immune responses. In this review, we summarize published findings showing that obesity induces changes in humoral immunity similar to those induced by aging and that the age-associated B cell defects are mainly due to metabolic changes. We discuss the role of the obese adipose tissue in inducing dysfunctional humoral responses and autoimmune Ab secretion.

It is unknown whether changes in lactate concentration produced by different situations (e.g., glycogen depletion or heat) modify fat oxidation. If confirmed, we could determine a dose-response relationship between lactate and fat. The aim of this study was to determine whether changes in lactate concentration (due to glycogen depletion or heat) alter fat oxidation during exercise. 11 males and eight females performed an incremental exercise test under three situations: control, glycogen depletion, and heat. At rest, in the last minute of each step and immediately post-exhaustion, lactate was analyzed and fat oxidation was estimated by indirect calorimetry. Lactate concentration was inversely associated with fat oxidation in the three aforementioned situations (r > 0.88 and p < 0.05). The highest lactate concentration was found in the heat situation, followed by the control situation, and finally the glycogen depletion situation (all p < 0.05). The opposite was found for fat oxidation, with the highest fat oxidation found in the glycogen depletion situation, followed by the control situation, and finally the heat situation (all p < 0.05). There is no association between the changes in lactate concentration between situations at each intensity and the changes in fat oxidation between situations at each intensity in males or females (p > 0.05). In conclusion, lactatemia is strongly and inversely associated with fat oxidation under the three different situations. Furthermore, the lowest lactate concentrations were accompanied by the highest fat oxidations in the glycogen depletion situation, whereas the highest lactate concentrations were accompanied by the lowest fat oxidations in the heat situation.

Plasma glycerol and free fatty acid concentrations decrease following oral glucose consumption, but changes in the rate of lipolysis during an oral glucose tolerance test (OGTT) have not been documented in conjunction with changes in fatty acid (FA) oxidation or reesterification rates in healthy individuals. After a 12-h overnight fast, 15 young (21-35 yr; 7 men and 8 women) and 14 older (60-80 yr; 7 men and 7 women) participants had the forearm vein catheterized for primed continuous infusion of [1,1,2,3,3-2H]glycerol. A contralateral hand vein was catheterized for arterialized blood sampling. Indirect calorimetry was performed simultaneously to determine total FA and carbohydrate (CHO) oxidation rates (Rox). Total FA reesterification rates (Rs) were estimated from tracer-measured lipolytic and FA oxidation rates. After a 90-min equilibration period, participants underwent a 120-min, 75-g OGTT. Glycerol rate of appearance (Ra), an index of lipolysis, decreased significantly from baseline 5 min postchallenge in young participants and 30 min in older participants. At 60 min, FA Rox decreased in both groups, but was significantly higher in older participants. Between 5 and 90 min, CHO Rox was significantly lower in older participants. In addition, FA Rs was significantly lower in older participants at 60 and 90 min. The area under the curve (AUC) for FA Rox was greater than that for FA Rs in older, but not in young participants. Our results indicate that, in aging, the postprandial suppression of lipolysis and FA oxidation are delayed such that FA oxidation is favored over CHO oxidation and FA reesterification.NEW & NOTEWORTHY To our knowledge, our investigation is the first to demonstrate changes in lipolysis during an oral glucose tolerance test (OGTT) in healthy young and older individuals. Plasma glycerol and free fatty acid concentrations changed after glycerol rate of appearance (Ra), indicating that plasma concentrations are incomplete surrogates of the lipolytic rate. Moreover, simultaneous determinations of substrate oxidation rates are interpreted to indicate that metabolic inflexibility in aging is characterized by delayed changes in postprandial substrate utilization related to the lipolytic rate.

The efficacy of immune checkpoint blockade therapy in patients with hepatocellular carcinoma (HCC) remains poor. Although serine- and arginine-rich splicing factor (SRSF) family members play crucial roles in tumors, their impact on tumor immunology remains unclear. This study aimed to elucidate the role of SRSF10 in HCC immunotherapy.

To identify the key genes associated with immunotherapy resistance, we conducted single-nuclear RNA sequencing, multiplex immunofluorescence, and The Cancer Genome Atlas and Gene Expression Omnibus database analyses. We investigated the biological functions of SRSF10 in immune evasion using in vitro co-culture systems, flow cytometry, various tumor-bearing mouse models, and patient-derived organotypic tumor spheroids.

SRSF10 was upregulated in various tumors and associated with poor prognosis. Moreover, SRSF10 positively regulated lactate production, and SRSF10/glycolysis/ histone H3 lysine 18 lactylation (H3K18la) formed a positive feedback loop in tumor cells. Increased lactate levels promoted M2 macrophage polarization, thereby inhibiting CD8+ T cell activity. Mechanistically, SRSF10 interacted with the 3'-untranslated region of MYB, enhancing MYB RNA stability, and subsequently upregulating key glycolysis-related enzymes including glucose transporter 1 (GLUT1), hexokinase 1 (HK1), lactate dehydrogenase A (LDHA), resulting in elevated intracellular and extracellular lactate levels. Lactate accumulation induced histone lactylation, which further upregulated SRSF10 expression. Additionally, lactate produced by tumors induced lactylation of the histone H3K18la site upon transport into macrophages, thereby activating transcription and enhancing pro-tumor macrophage activity. M2 macrophages, in turn, inhibited the enrichment of CD8+ T cells and the proportion of interferon-γ+CD8+ T cells in the tumor microenvironment (TME), thus creating an immunosuppressive TME. Clinically, SRSF10 could serve as a biomarker for assessing immunotherapy resistance in various solid tumors. Pharmacological targeting of SRSF10 with a selective inhibitor 1C8 enhanced the efficacy of programmed cell death 1 (PD-1) monoclonal antibodies (mAbs) in both murine and human preclinical models.

The SRSF10/MYB/glycolysis/lactate axis is critical for triggering immune evasion and anti-PD-1 resistance. Inhibiting SRSF10 by 1C8 may overcome anti-PD-1 tolerance in HCC.

Physical exercise represents an effective strategy for combating obesity via brown adipose tissue (BAT) activation, but the mechanism remains unclear. In this study, we demonstrated that the cooperation between lactate and adrenoceptor signaling regulated BAT activity during exercise. The lactate receptor GPR81 was highly expressed in the BAT of lean mice, whereas its expression was markedly decreased in obese mice. Notably, the level of GPR81 in BAT could be upregulated by exercise. The blockade of lactate production or GPR81 significantly impaired exercise-induced BAT activation. In addition, dietary lactate intake enhanced the efficacy of physical exercise in alleviating BAT whitening in obese mice, as evidenced by the improved mitochondrial ultrastructure, reduced lipid droplets, increased UCP1 expression, and elevated mitochondrial DNA content. Further data indicated that norepinephrine triggered UCP1 activation through both the cAMP/PKA and Ca2+/CaMK pathways during exercise, while lactate mediated this process via the GPR81-Ca2+/CaMK cascade. Our findings unveil a novel mechanism in the regulation of BAT function by physical exercise, providing a promising lifestyle intervention to improve metabolic health.

Idiopathic pulmonary fibrosis (IPF) is a lethal progressive fibrotic lung disease. The development of IPF involves different molecular and cellular processes, and recent studies indicate that lactate plays a significant role in promoting the progression of the disease. Nevertheless, the mechanism by which lactate metabolism is regulated and the downstream effects remain unclear. The molecular chaperone CCT6A performs multiple functions in a variety of biological processes. Our research has identified a potential association between CCT6A and serum lactate levels in IPF patients. Herein, we found that CCT6A was highly expressed in type 2 alveolar epithelial cells (AEC2s) of fibrotic lung tissues and correlated with disease severity. Lactate increases the accumulation of lipid droplets in epithelial cells. CCT6A inhibits lipid synthesis by blocking the production of lactate in AEC2s and alleviates bleomycin-induced pulmonary fibrosis in mice. In addition, our results revealed that CCT6A blocks HIF-1α-mediated lactate production by driving the VHL-dependent ubiquitination and degradation of HIF-1α and further inhibits lipid accumulation in fibrotic lungs. In conclusion, we propose that there is a pivotal regulatory role of CCT6A in lactate metabolism in pulmonary fibrosis, and strategies aimed at targeting these key molecules could represent potential therapeutic approaches for pulmonary fibrosis.

Adipose tissue is distributed in diverse locations throughout the human body. Not much is known about the extent to which anatomically distinct adipose depots are functionally distinct, specialized organs, nor whether depot-specific characteristics result from intrinsic developmental programs, as opposed to reversible physiological responses to differences in tissue microenvironment. We used DNA microarrays to compare mRNA expression patterns of isolated human adipocytes and cultured adipose stem cells, before and after ex vivo adipocyte differentiation, from seven anatomically diverse adipose tissue depots. Adipocytes from different depots display distinct gene expression programs, which are most closely shared with anatomically related depots. mRNAs whose expression differs between anatomically diverse groups of depots (e.g., subcutaneous vs. internal) suggest important functional specializations. These depot-specific differences in gene expression were recapitulated when adipocyte progenitor cells from each site were differentiated ex vivo, suggesting that progenitor cells from specific anatomic sites are deterministically programmed to differentiate into depot-specific adipocytes. Many developmental transcription factors show striking depot-specific patterns of expression, suggesting that adipocytes in each anatomic depot are programmed during early development in concert with anatomically related tissues and organs. Our results support the hypothesis that adipocytes from different depots are functionally distinct and that their depot-specific specialization reflects distinct developmental programs.

As a solid tumor with high glycolytic activity, hepatocellular carcinoma (HCC) produces excess lactic acid and increases extracellular acidity, thus forming a unique immunosuppressive microenvironment. L-lactate dehydrogenase (LDH) and monocarboxylate transporters (MCTs) play a very important role in glycolysis. LDH is the key enzyme for lactic acid (LA) production, and MCT is responsible for the cellular import and export of LA. The synergistic effect of the two promotes the formation of an extracellular acidic microenvironment. In the acidic microenvironment of HCC, LA can not only promote the proliferation, survival, transport and angiogenesis of tumor cells but also have a strong impact on immune cells, ultimately leading to an inhibitory immune microenvironment. This article reviews the role of LA in HCC, especially its effect on immune cells, summarizes the progress of LDH and MCT-related drugs, and highlights the potential of immunotherapy targeting lactate combined with HCC.

Lower oxidative capacity in skeletal muscles (SKMs) is a prevailing cause of metabolic diseases. Exercise not only enhances the fatty acid oxidation (FAO) capacity of SKMs but also increases lactate levels. Given that lactate may contribute to tricarboxylic acid cycle (TCA) flux and impact monocarboxylate transporter 1 in the SKMs, we hypothesize that lactate can influence glucose and fatty acid (FA) metabolism. To test this hypothesis, we investigated the mechanism underlying lactate-driven FAO regulation in the SKM of mice with diet-induced obesity (DIO). Lactate was administered to DIO mice immediately after exercise for over 3 wk. We found that increased lactate levels enhanced energy expenditure mediated by fat metabolism during exercise recovery and decreased triglyceride levels in DIO mice SKMs. To determine the lactate-specific effects without exercise, we administered lactate to mice on a high-fat diet (HFD) for 8 wk. Similar to our exercise conditions, lactate increased FAO, TCA cycle activity, and mitochondrial respiration in the SKMs of HFD-fed mice. In addition, under sufficient FA conditions, lactate increased uncoupling protein-3 abundance via the NADH-NAD+ shuttle. Conversely, ATP synthase abundance decreased in the SKMs of HFD mice. Taken together, our results suggest that lactate amplifies the adaptive increase in FAO capacity mediated by the TCA cycle and mitochondrial respiration in SKMs under sufficient FA abundance.NEW & NOTEWORTHY Lactate administration post-exercise promotes triglyceride content loss in skeletal muscles (SKMs) and reduced body weight. Lactate enhances fatty acid oxidation in the SKMs of high-fat diet (HFD)-fed mice due to enhanced mitochondrial oxygen consumption. In addition, lactate restores the malate-aspartate shuttle, which is reduced by a HFD, and activates the tricarboxylic acid cycle (TCA) cycle in SKMs. Interestingly, supraphysiological lactate facilitates uncoupling protein-3 expression through NADH/NAD+, which is enhanced under high-fat levels in SKMs.

Cardiovascular diseases (CVDs) are often linked to structural and functional impairments, such as heart defects and circulatory dysfunction, leading to compromised peripheral perfusion and heightened morbidity risks. Metabolic remodeling, particularly in the context of cardiac fibrosis and inflammation, is increasingly recognized as a pivotal factor in the pathogenesis of CVDs. Metabolic syndromes further predispose individuals to these conditions, underscoring the need to elucidate the metabolic underpinnings of CVDs. Lactate, a byproduct of glycolysis, is now recognized as a key molecule that connects cellular metabolism with the regulation of cellular activity. The transport of lactate between different cells is essential for metabolic homeostasis and signal transduction. Disruptions to lactate dynamics are implicated in various CVDs. Furthermore, lactylation, a novel post-translational modification, has been identified in cardiac cells, where it influences protein function and gene expression, thereby playing a significant role in CVD pathogenesis. In this review, we summarized recent advancements in understanding the role of lactate and lactylation in CVDs, offering fresh insights that could guide future research directions and therapeutic interventions. The potential of lactate metabolism and lactylation as innovative therapeutic targets for CVD is a promising avenue for exploration.

Aging is characterized by chronic systemic inflammation and metabolic changes. We compare the metabolic status of B cells from young and elderly donors and found that aging is associated with higher oxygen consumption rates, and especially higher extracellular acidification rates, measures of oxidative phosphorylation and of anaerobic glycolysis, respectively. Importantly, this higher metabolic status, which reflects age-associated expansion of pro-inflammatory B cells, is found associated with higher secretion of lactate and autoimmune antibodies after in vitro stimulation. B cells from elderly individuals induce in vitro polarization of CD4+ T cells from young individuals into pro-inflammatory CD4+ T cells through metabolic pathways mediated by lactate, which can be inhibited by targeting lactate enzymes and transporters, as well as signaling pathways supporting anaerobic glycolysis. Lactate also induces immunosenescent B cells that are glycolytic, express transcripts for multiple pro-inflammatory molecules, and are characterized by a higher metabolic status. These results altogether may have relevant clinical implications and suggest alternative targets for therapeutic interventions in the elderly and patients with inflammatory conditions and diseases.

LOX was recently shown to inhibit cancer cell proliferation and tumor growth. The mechanism of this inhibition, however, has been exclusively attributed to LOX depletion of TME lactate, a cancer cell energy source, and production of H2O2, an oxidative stressor. We report that TME lactate triggers the assembly of the lactate receptor hydroxycarboxylic acid receptor 1 (HCAR1)-associated protein complex, which includes GRB2, SOS1, KRAS, GAB1, and PI3K, for the activation of both the RAS and the PI3K oncogenic signaling pathways in breast cancer (BCa) cells. LOX treatment decreased the levels of the proteins in the protein complex via induction of their proteasomal degradation. In addition, LOX inhibited lactate-stimulated expression of the lactate transporters MCT1 and MCT4. Our data suggest that HCAR1 activation by lactate is crucial for the assembly and function of the RAS and PI3K signaling nexus. Shutting down lactate signaling by disrupting this nexus could be detrimental to cancer cells. HCAR1 is therefore a promising target for the control of the RAS and the PI3K signaling required for BCa progression. Thus, our study provides insights into lactate signaling regulation of cancer progression and extends our understanding of LOX's functional mechanisms that are fundamental for exploring its therapeutic potential.

The intricate molecular mechanisms underlying estrogen receptor-positive (ER+) breast carcinogenesis and resistance to endocrine therapy remain elusive. In this study, we elucidate the pivotal role of GPR81, a G protein-coupled receptor, in ER+ breast cancer (BC) by demonstrating low expression of GPR81 in tamoxifen (TAM)-resistant ER+ BC cell lines and tumor samples, along with the underlying molecular mechanisms.

Fatty acid oxidation (FAO) levels and lipid accumulation were explored using MDA and FAβO assay, BODIPY 493/503 staining, and Lipid TOX staining. Autophagy levels were assayed using CYTO-ID detection and Western blotting. The impact of GPR81 on TAM resistance in BC was investigated through CCK8 assay, colony formation assay and a xenograft mice model.

Aberrantly low GPR81 expression in TAM-resistant BC cells disrupts the Rap1 pathway, leading to the upregulation of PPARα and CPT1. This elevation in PPARα/CPT1 enhances FAO, impedes lipid accumulation and lipid droplet (LD) formation, and subsequently inhibits cell autophagy, ultimately promoting TAM-resistant BC cell growth. Moreover, targeting GPR81 and FAO emerges as a promising therapeutic strategy, as the GPR81 agonist and the CPT1 inhibitor etomoxir effectively inhibit ER+ BC cell and tumor growth in vivo, re-sensitizing TAM-resistant ER+ cells to TAM treatment.

Our data highlight the critical and functionally significant role of GPR81 in promoting ER+ breast tumorigenesis and resistance to endocrine therapy. GPR81 and FAO levels show potential as diagnostic biomarkers and therapeutic targets in clinical settings for TAM-resistant ER+ BC.

Elevated lactate levels are a significant biomarker of sepsis and are positively associated with sepsis-related mortality. Sepsis-associated lung injury (ALI) is a leading cause of poor prognosis in clinical patients. However, the underlying mechanisms of lactate's involvement in sepsis-associated ALI remain unclear. In this study, we demonstrate that lactate regulates N6-methyladenosine (m6A) modification levels by facilitating p300-mediated H3K18la binding to the METTL3 promoter site. The METTL3-mediated m6A modification is enriched in ACSL4, and its mRNA stability is regulated through a YTHDC1-dependent pathway. Furthermore, short-term lactate stimulation upregulates ACSL4, which promotes mitochondria-associated ferroptosis. Inhibition of METTL3 through knockdown or targeted inhibition effectively suppresses septic hyper-lactate-induced ferroptosis in alveolar epithelial cells and mitigates lung injury in septic mice. Our findings suggest that lactate induces ferroptosis via the GPR81/H3K18la/METTL3/ACSL4 axis in alveolar epithelial cells during sepsis-associated ALI. These results reveal a histone lactylation-driven mechanism inducing ferroptosis through METTL3-mediated m6A modification. Targeting METTL3 represents a promising therapeutic strategy for patients with sepsis-associated ALI.

We addressed the question of mitochondrial lactate metabolism using genetically-encoded sensors. The organelle was found to contain a dynamic lactate pool that leads to dose- and time-dependent protein lactylation. In neurons, mitochondrial lactate reported blood lactate levels with high fidelity. The exchange of lactate across the inner mitochondrial membrane was found to be mediated by a high affinity H+-coupled transport system involving the mitochondrial pyruvate carrier MPC. Assessment of electron transport chain activity and determination of lactate flux showed that mitochondria are tonic lactate producers, a phenomenon driven by energization and stimulated by hypoxia. We conclude that an overflow mechanism caps the redox level of mitochondria, while saving energy in the form of lactate.