Herein, we use lessons learned in exercise physiology and metabolism to propose that augmented lactate production ('lactagenesis'), initiated by gene mutations, is the reason and purpose of the Warburg Effect and that dysregulated lactate metabolism and signaling are the key elements in carcinogenesis. Lactate-producing ('lactagenic') cancer cells are characterized by increased aerobic glycolysis and excessive lactate formation, a phenomenon described by Otto Warburg 93 years ago, which still remains unexplained. After a hiatus of several decades, interest in lactate as a player in cancer has been renewed. In normal physiology, lactate, the obligatory product of glycolysis, is an important metabolic fuel energy source, the most important gluconeogenic precursor, and a signaling molecule (i.e. a 'lactormone') with major regulatory properties. In lactagenic cancers, oncogenes and tumor suppressor mutations behave in a highly orchestrated manner, apparently with the purpose of increasing glucose utilization for lactagenesis purposes and lactate exchange between, within and among cells. Five main steps are identified (i) increased glucose uptake, (ii) increased glycolytic enzyme expression and activity, (iii) decreased mitochondrial function, (iv) increased lactate production, accumulation and release and (v) upregulation of monocarboxylate transporters MTC1 and MCT4 for lactate exchange. Lactate is probably the only metabolic compound involved and necessary in all main sequela for carcinogenesis, specifically: angiogenesis, immune escape, cell migration, metastasis and self-sufficient metabolism. We hypothesize that lactagenesis for carcinogenesis is the explanation and purpose of the Warburg Effect. Accordingly, therapies to limit lactate exchange and signaling within and among cancer cells should be priorities for discovery.

Background

Increased muscle mitochondrial mass is characteristic of elite professional endurance athletes (PAs), whereas increased blood lactate levels (lactatemia) at the same absolute submaximal exercise intensities and decreased mitochondrial oxidative capacity are characteristics of individuals with low aerobic power. In contrast to PAs, patients with metabolic syndrome (MtS) are characterized by a decreased capacity to oxidize lipids and by early transition from fat to carbohydrate oxidation (FATox/CHOox), as well as elevated blood lactate concentration [La^-] as exercise power output (PO) increases, a condition termed 'metabolic inflexibility'.

Objective

The aim of this study was to assess metabolic flexibility across populations with different metabolic characteristics.

Methods

We used indirect calorimetry and [La^-] measurements to study the metabolic responses to exercise in PAs, moderately active individuals (MAs), and MtS individuals.

Results

FATox was significantly higher in PAs than MAs and patients with MtS (p < 0.01), while [La^-] was significantly lower in PAs compared with MAs and patients with MtS. FATox and [La^-] were inversely correlated in all three groups (PA: r = -0.97, p < 0.01; MA: r = -0.98, p < 0.01; MtS: r = -0.92, p < 0.01). The correlation between FATox and [La^-] for all data points corresponding to all populations studied was r = -0.76 (p < 0.01).

Conclusions

Blood lactate accumulation is negatively correlated with FATox and positively correlated with CHOox during exercise across populations with widely ranging metabolic capabilities. Because both lactate and fatty acids are mitochondrial substrates, we believe that measurements of [La^-] and FATox rate during exercise provide an indirect method to assess metabolic flexibility and oxidative capacity across individuals of widely different metabolic capabilities.

Lactate is a ubiquitous molecule in cancer. In this exploratory study, our aim was to test the hypothesis that lactate could function as an oncometabolite by evaluating whether lactate exposure modifies the expression of oncogenes, or genes encoding transcription factors, cell division, and cell proliferation in MCF7 cells, a human breast cancer cell line. Gene transcription was compared between MCF7 cells incubated in (a) glucose/glutamine-free media (control), (b) glucose-containing media to stimulate endogenous lactate production (replicating some of the original Warburg studies), and (c) glucose-containing media supplemented with L-lactate (10 and 20 mM). We found that both endogenous, glucose-derived lactate and exogenous, lactate supplementation significantly affected the transcription of key oncogenes (MYC, RAS, and PI3KCA), transcription factors (HIF1A and E2F1), tumor suppressors (BRCA1, BRCA2) as well as cell cycle and proliferation genes involved in breast cancer (AKT1, ATM, CCND1, CDK4, CDKN1A, CDK2B) (0.001 < p < 0.05 for all genes). Our findings support the hypothesis that lactate acts as an oncometabolite in MCF7 cells. Further research is necessary on other cell lines and biopsy cultures to show generality of the findings and reveal the mechanisms by which dysregulated lactate metabolism could act as an oncometabolite in carcinogenesis.

Lactate is a ubiquitous molecule in cancer. In this exploratory study, our aim was to test the hypothesis that lactate could function as an oncometabolite by evaluating whether lactate exposure modifies the expression of oncogenes, or genes encoding transcription factors, cell division, and cell proliferation in MCF7 cells, a human breast cancer cell line. Gene transcription was compared between MCF7 cells incubated in (a) glucose/glutamine-free media (control), (b) glucose-containing media to stimulate endogenous lactate production (replicating some of the original Warburg studies), and (c) glucose-containing media supplemented with L-lactate (10 and 20 mM). We found that both endogenous, glucose-derived lactate and exogenous, lactate supplementation significantly affected the transcription of key oncogenes (MYC, RAS, and PI3KCA), transcription factors (HIF1A and E2F1), tumor suppressors (BRCA1, BRCA2) as well as cell cycle and proliferation genes involved in breast cancer (AKT1, ATM, CCND1, CDK4, CDKN1A, CDK2B) (0.001 < p < 0.05 for all genes). Our findings support the hypothesis that lactate acts as an oncometabolite in MCF7 cells. Further research is necessary on other cell lines and biopsy cultures to show generality of the findings and reveal the mechanisms by which dysregulated lactate metabolism could act as an oncometabolite in carcinogenesis.

Introduction

Lactate is an important signaling molecule with autocrine, paracrine and endocrine properties involved in multiple biological processes including regulation of gene expression and metabolism. Levels of lactate are increased chronically in diseases associated with cardiometabolic disease such as heart failure, type 2 diabetes, and cancer. Using neonatal ventricular myocytes, we tested the hypothesis that chronic lactate exposure could decrease the activity of cardiac mitochondria that could lead to metabolic inflexibility in the heart and other tissues.

Methods

Neonatal rat ventricular myocytes (NRVMs) were treated for 48 h with 5, 10, or 20 mM lactate and CPT I and II activities were tested using radiolabelled assays. The molecular species profile of the major mitochondrial phospholipid, cardiolipin, was determined using electrospray ionization mass spectrometry along with reactive oxygen species (ROS) levels measured by Amplex Red and mitochondrial oxygen consumption using the Seahorse analyzer.

Results

CPT I activity trended downward (p = 0.07) and CPT II activity significantly decreased with lactate exposure (p < 0.001). Cardiolipin molecular species containing four 18 carbon chains (72 carbons total) increased with lactate exposure, but species of other sizes decreased significantly. Furthermore, ROS production was strongly enhanced with lactate (p < 0.001) and mitochondrial ATP production and maximal respiration were both significantly down regulated with lactate exposure (p < 0.05 and p < 0.01 respectively).

Conclusions

Chronic lactate exposure in cardiomyocytes leads to a decrease in fatty acid transport, alterations of cardiolipin remodeling, increases in ROS production and decreases in mitochondrial oxygen consumption that could have implications for both metabolic health and flexibility. The possibility that both intra-, or extracellular lactate levels play roles in cardiometabolic disease, heart failure, and other forms of metabolic inflexibility needs to be assessed in vivo.