The D-lactate enigma: exploring the inflammatory influence of D-lactate in cattle

Abstract

D-lactic acidosis is associated with fermentative disturbances and is often marked by elevated levels of D-lactic acid in the blood, ruminal fluid, and synovial fluid in cattle. D-lactic acidosis is linked to various inflammatory manifestations, and although the causative factors have been extensively explored, the exact pathogenesis of the associated inflammation remains elusive. Notably, less attention has been given to D-lactate, a stereoisomer found in the plasma of affected animals, which may lead to D-lactic acidosis. This review aims to highlight the evidence suggesting that D-lactate participates in the modulation of inflammatory processes and explore its potential effects on synoviocytes, polymorphonuclear neutrophils, macrophages, and T-cells. This comprehensive examination of D-lactate's involvement in the inflammatory response process provides timely insights into the pathophysiological aspects of ruminal acidosis in cattle.

Keywords: D-lactate; bovine; inflammation; lameness; ruminal acidosis.

Figure 1

D-lactate metabolism in mammalian cells. D-lactate enters from the extracellular into the cytosol via MCT. Mammalian cells can also generate D-lactate during methylglyoxal metabolism. Methylglyoxal is mainly produced as a byproduct of glycolysis, although it can also be produced during lipid peroxidation and amino acid breakdown. Detoxification of methylglyoxal takes place in the cytosol by the glyoxalase system, consisting of the enzymes GLO1 and GLO2, which employ reduced GSH to form D-lactate. D-lactate metabolism occurs within the mitochondria, entering through MPC located in the inner mitochondrial membrane. Within the mitochondria, D-lactate is oxidized to pyruvate by LDHD, using FAD as a cofactor. Pyruvate continues its oxidation to enter the mitochondrial tricarboxylic acid cycle. MCT, monocarboxylate transporter; TPI, triose phosphate isomerase; MGS, methylglyoxal synthase; AMO, acetol monooxygenase; SSAO, semicarbazide-sensitive amine oxidase; GSH, glutathione; GLOI, glyoxalase 1; GLOII, glyoxalase II; MPC, mitochondrial pyruvate carrier; LDHD, D-lactate dehydrogenase; FAD/FADH₂, flavin adenine dinucleotide; TCA, tricarboxylic acid. Figure created with BioRender.

Figure 2

Key signaling events underlying D-lactate-induced metabolic reprogramming in bFLS. D-lactate enters bovine fibroblast-like synoviocytes (bFLS) through MCT-1 and induces the activation of the PI3K/Akt, p38 MAPK and ERK1/2 MAPK pathways, and the subsequent activation of the transcription factors NF-κB and HIF-1. Through these signaling pathways and transcription factors, D-lactate induces the synthesis and secretion of IL-6, an inflammatory marker characteristic of bovine polysynovitis associated with D-lactic acidosis. Additionally, the local inflammatory response is sustained thanks to the activation of HIF-1, which favors glycolytic metabolism by increasing the expression of GLUT1 (which increases the incorporation of glucose from the extracellular medium), PDK1 (blocking the mitochondrial use of pyruvate through the TCA cycle) and LDHA (ensuring glycolytic flux by favoring the oxidation of pyruvate to lactate). IL-6, interleukin 6; MCT1, monocarboxylate transporter 1; GLUT1, solute carrier family 2 (facilitated glucose transporter) member 1; PI3K, phosphatidyl inositol 3-kinase; Akt, protein kinase B; p38, p38 mitogen-activated protein kinase (MAPK); ERK1/2, extracellular signal-regulated kinase 1/2 MAPK; HIF-1α and HIF-1β, hypoxia inducible factor 1 alpha and beta subunits, respectively; NF-κB, nuclear factor kappa B; LDHA, lactate dehydrogenase A subunit; PDK1, pyruvate dehydrogenase kinase 1; PDH, pyruvate dehydrogenase; TCA, tricarboxylic acid. Figure created with BioRender.

Figure 3

D-lactate-induced NET formation is sustained through metabolic reprogramming of bPMN. Bovine polymorphonuclear neutrophils (bPMN) incorporate D-lactate through MCT-1, which induces the activation of the PI3K/Akt signaling pathway and the subsequent stabilization of HIF-1α. Additionally, D-lactate triggers the activation of PAD4, which catalyzes the transformation of arginine residues to citrulline in histones, leading to chromatin decondensation. HIF-1 activation is primarily responsible for the metabolic reprogramming necessary to energetically sustain NET release, through the overexpression of enzymes associated with glycolysis, gluconeogenesis and glycogen metabolism. D-lactate also increases the production of mtROS, mainly through mitochondrial complex I, a molecular mechanism necessary to induce the stabilization of HIF-1α and, consequently, the greater transcriptional activity of HIF-1. Furthermore, D-lactate promotes bPMN adhesion to endothelial cells through a mechanism involving the increased expression of CD11b and shedding of L-selectin. MCT1, monocarboxylate transporter 1; PI3K, phosphatidylinositol 3-kinase; Akt, protein kinase B; HIF-1α and HIF-1β, hypoxia inducible factor 1 alpha and beta subunits, respectively; mtROS, mitochondrial reactive oxygen species; PAD4, peptidyl arginine deiminase 4; Cit, citrulline; ICAM, intercellular adhesion molecule; NET, neutrophil extracellular traps. Figure created with BioRender.