Increased expression of monocarboxylate transporter 1 after acute ischemia of isolated, perfused mouse hearts

Monocarboxylate Transporter 1 May Benefit Cerebral Ischemia via Facilitating Lactate Transport From Glial Cells to Neurons

Monocarboxylate transporter 1 (MCT1) is expressed in glial cells and some populations of neurons. MCT1 facilitates astrocytes or oligodendrocytes (OLs) in the energy supplement of neurons, which is crucial for maintaining the neuronal activity and axonal function. It is suggested that MCT1 upregulation in cerebral ischemia is protective to ischemia/reperfusion (I/R) injury. Otherwise, its underlying mechanism has not been clearly discussed. In this review, it provides a novel insight that MCT1 may protect brain from I/R injury via facilitating lactate transport from glial cells (such as, astrocytes and OLs) to neurons. It extensively discusses (1) the structure and localization of MCT1; (2) the regulation of MCT1 in lactate transport among astrocytes, OLs, and neurons; and (3) the regulation of MCT1 in the cellular response of lactate accumulation under ischemic attack. At last, this review concludes that MCT1, in cerebral ischemia, may improve lactate transport from glial cells to neurons, which subsequently alleviates cellular damage induced by lactate accumulation (mostly in glial cells), and meets the energy metabolism of neurons.

Compromised Lactate Efflux Renders Vulnerability of Oligodendrocyte Precursor Cells to Metabolic Stresses

Among the brain cells, oligodendrocyte progenitor cells (OPCs) are the most vulnerable in response to hypoxic and ischemic insults, of which the mechanism remains unknown. Brain cells are known to import or export lactate via differentially expressed monocarboxylate transporters (MCTs) to maintain energy metabolism and pH homeostasis. The present study aims to determine the role of MCT1 in the high vulnerability of OPCs. Here we show that a mild ischemic condition equivalent to ischemic preconditioning caused detectable loss of OPCs. MCT1, which is primarily expressed in oligodendrocyte lineage cells including OPCs, was up-regulated immediately under oxygen-glucose deprivation (OGD) conditions. However, persistent hypoxia, but not hypoglycemia, inhibited the function of MCT1, leading to an intracellular lactate accumulation and acidosis in OPCs. Neurons, which express primarily MCT2, were able to export lactate and maintain an intracellular pH homeostasis under similar conditions. The results support that compromised lactate efflux resulting from hypoxia-induced dysfunction of MCT1 contributes to the high vulnerability of OPCs.

Role of rno-miR-124-3p in regulating MCT1 expression in rat brain after permanent focal cerebral ischemia

This study aimed to assess the role of microRNAs (miRNAs) in regulating monocarboxylate transporter-1 (MCT1) expression in rat brain after permanent focal cerebral ischemia to identify a new target for early treatment of cerebral ischemia. Focal cerebral ischemia was induced by permanent middle cerebral artery occlusion (pMCAO) in rats. Morphology and protein expression levels of MCT1 were assessed by immunofluorescence and Western blotting. Using bioinformatics and double luciferase reporter assays, rno-miR-124-3p was selected as a direct target for rat MCT1. Expression of rno-miR-124-3p after pMCAO was detected. Then, rats were treated with rno-miR-124-3p agomir via lateral ventricle injection, and after 6 h or 24 h ischemia, rno-miR-124-3p expression and gene and protein expression of MCT-1 were detected by qRT-PCR and Western blotting. Brain infarction was identified by 2, 3, 5-triphenyltetrazolium chloride (TTC) staining. Results showed that pMCAO induced brain infarction and increased the expression of MCT1. The levels of rno-miR-124-3p after pMCAO were in contrast to those of MCT1 protein in ischemic region, while declined after 3, 6 and 12 h of pMCAO in ischemic penumbra. After administration of rno-miR-124-3p agomir, MCT1 mRNA and protein levels were increased after 6 h of pMCAO, while decreased after 24 h of pMCAO. Meanwhile, rno-miR-124-3p levels increased after both times. TTC staining showed treatment with rno-miR-124-3p agomir reduced brain infarction. The role of rno-miR-124-3p in regulating MCT1 was as a positive regulator after 6 h of pMCAO, while a negative regulator after 24 h of pMCAO, however, both activities had protective effects against cerebral ischemia.

Reduced cerebral monocarboxylate transporters and lactate levels by ethanol and normobaric oxygen therapy in severe transient and permanent ischemic stroke

OBJECTIVES

Neuroprotective benefits of ethanol (EtOH) and normobaric oxygenation (NBO) were previously demonstrated in transient and permanent ischemic stroke. Here we sought to identify whether the enhanced lactic acidosis and increased expression of monocarboxylate transporters (MCTs) observed after stroke might be attenuated by single and/or combined EtOH and NBO therapies.

METHODS

Sprague-Dawley rats (n=96) were subjected to right middle cerebral artery occlusion (MCAO) for 2 or 4h (transient ischemia), or 28 h (permanent ischemia) followed by 3, 24h, or no reperfusion. Rats received: (1) either an intraperitoneal injection of saline (sham treatment), one dose of EtOH (1.5 g/kg), two doses of EtOH (1.5 g/kg at 2h of MCAO, followed by 1.0 g/kg 2h after 1st dose), or (2) EtOH+95% NBO (at 2h of MCAO for 6h in permanent ischemia). Lactate levels were detected at 3 and 24h of reperfusion. Gene and protein expressions of MCT-1, -2, -4 were assessed by real-time PCR and western blotting.

RESULTS

A dose-dependent EtOH neuroprotection was found in transient ischemia. Following transient ischemia, a single dose of EtOH (in 2h-MCAO) or a double dose (in 4h-MCAO), significantly attenuated lactate levels, as well as the mRNAs and protein expressions of MCT-1, MCT-2, and MCT-4. However, while two doses of EtOH alone was ineffective in permanent stroke, the combined therapy (EtOH+95% NBO) resulted in a more significant attenuation in all the above levels and expressions.

CONCLUSIONS

Our study demonstrates that acute EtOH administration attenuated lactic acidosis in transient or permanent ischemic stroke. This EtOH-induced beneficial effect was potentiated by NBO therapy in permanent ischemia. Because both EtOH and NBO are readily available, inexpensive and easy to administer, their combination could be implemented in the clinics shortly after stroke.

Functional interaction between bicarbonate transporters and carbonic anhydrase modulates lactate uptake into mouse cardiomyocytes

Blood-derived lactate is a precious energy substrate for the heart muscle. Lactate is transported into cardiomyocytes via monocarboxylate transporters (MCTs) together with H(+), which couples lactate uptake to cellular pH regulation. In this study, we have investigated how the interplay between different acid/base transporters and carbonic anhydrases (CA), which catalyze the reversible hydration of CO2, modulates the uptake of lactate into isolated mouse cardiomyocytes. Lactate transport was estimated both as lactate-induced acidification and as changes in intracellular lactate levels measured with a newly developed Förster resonance energy transfer (FRET) nanosensor. Recordings of intracellular pH showed an increase in the rate of lactate-induced acidification when CA was inhibited by 6-ethoxy-2-benzothiazolesulfonamide (EZA), while direct measurements of lactate flux demonstrated a decrease in MCT transport activity, when CA was inhibited. The data indicate that catalytic activity of extracellular CA increases lactate uptake and counteracts intracellular lactate-induced acidification. We propose a hypothetical model, in which HCO3 (-), formed from cell-derived CO2 at the outer surface of the cardiomyocyte plasma membrane by membrane-anchored, extracellular CA, is transported into the cell via Na(+)/HCO3 (-) cotransport to counteract intracellular acidification, while the remaining H(+) stabilizes extracellular pH at the surface of the plasma membrane during MCT activity to enhance lactate influx into cardiomyocytes.

Both butyrate incubation and hypoxia upregulate genes involved in the ruminal transport of SCFA and their metabolites

Butyrate modulates the differentiation, proliferation and gene expression profiles of various cell types. Ruminal epithelium is exposed to a high intraluminal concentration and inflow of n-butyrate. We aimed to investigate the influence of n-butyrate on the mRNA expression of proteins involved in the transmembranal transfer of n-butyrate metabolites and short-chain fatty acids in ruminal epithelium. N-butyrate-induced changes were compared with the effects of hypoxia because metabolite accumulation after O2 depletion is at least partly comparable to the accumulation of metabolites after n-butyrate exposure. Furthermore, in various tissues, O2 depletion modulates the expression of transport proteins that are also involved in the extrusion of metabolites derived from n-butyrate breakdown in ruminal epithelium. Sheep ruminal epithelia mounted in Ussing chambers were exposed to 50 mM n-butyrate or incubated under hypoxic conditions for 6 h. Electrophysiological measurements showed hypoxia-induced damage in the epithelia. The mRNA expression levels of monocarboxylate transporters (MCT) 1 and 4, anion exchanger (AE) 2, downregulated in adenoma (DRA), putative anion transporter (PAT) 1 and glucose transporter (GLUT) 1 were assessed by RT-qPCR. We also examined the mRNA expression of nuclear factor (NF) κB, cyclooxygenase (COX) 2, hypoxia-inducible factor (HIF) 1α and acyl-CoA oxidase (ACO) to elucidate the possible signalling pathways involved in the modulation of gene expression. The mRNA expression levels of MCT 1, MCT 4, GLUT 1, HIF 1α and COX 2 were upregulated after both n-butyrate exposure and hypoxia. ACO and PAT 1 were upregulated only after n-butyrate incubation. Upregulation of both MCT isoforms and NFκB after n-butyrate incubation could be detected on protein level as well. Our study suggests key roles for MCT 1 and 4 in the adaptation to an increased intracellular load of metabolites, whereas an involvement of PAT 1 in the transport of n-butyrate also seems possible.

The NLRP3 inflammasome is up-regulated in cardiac fibroblasts and mediates myocardial ischaemia-reperfusion injury

AIMS

Nucleotide-binding oligomerization domain-Like Receptor with a Pyrin domain 3 (NLRP3) is considered necessary for initiating a profound sterile inflammatory response. NLRP3 forms multi-protein complexes with Apoptosis-associated Speck-like protein containing a Caspase recruitment domain (ASC) and Caspase-1, which activate pro-interleukin-1β (IL-1β) and pro-IL-18. The role of NLRP3 in cardiac cells is not known. Thus, we investigated the expression and function of NLRP3 during myocardial ischaemia.

METHODS AND RESULTS

Myocardial infarction (MI) was induced in adult C57BL/6 mice and Wistar rats by ligation of the coronary artery. A marked increase in NLRP3, IL-1β, and IL-18 mRNA expression was found in the left ventricle after MI, primarily located to myocardial fibroblasts. In vitro studies in cells from adult mice showed that myocardial fibroblasts released IL-1β and IL-18 when primed with lipopolysaccharide and subsequently exposed to the danger signal adenosine triphosphate, a molecule released after tissue damage during MI. When hearts were isolated from NLRP3-deficient mice, perfused and subjected to global ischaemia and reperfusion, a marked improvement of cardiac function and reduction of hypoxic damage was found compared with wild-type hearts. This was not observed in ASC-deficient hearts, potentially reflecting a protective role of other ASC-dependent inflammasomes or inflammasome-independent effects of NLRP3.

CONCLUSION

This study shows that the NLRP3 inflammasome is up-regulated in myocardial fibroblasts post-MI, and may be a significant contributor to infarct size development during ischaemia-reperfusion.

Intracellular lactate signaling cascade in atrial remodeling of mitral valvular patients with atrial fibrillation

Background

Atrial remodeling has emerged as the structural basis for the maintenance and recurrence of atrial fibrillation. Lactate signaling cascade was recently linked to some cardiovascular disorders for its regulatory functions to myocardial structural remodeling. It was hypothesized that lactate signaling cascade was involved in the maintenance and recurrence of atrial fibrillation by regulating atrial structural remodeling.

Methods

Biopsies of right atrial appendage and clinical data were collected from sex- and age-matched 30 persistent atrial fibrillation, 30 paroxysmal atrial fibrillation, 30 sinus rhythm patients undergoing isolated mitral valve surgery and 10 healthy heart donors.

Results

Atrial fibrillation groups had higher atrial lactate expression and this upregulated expression was positively correlated with regulatory indicators of atrial structural remodeling as reflected by severe oxidative stress injury and mitochondrial control of apoptosis.

Conclusions

The present findings suggest a potential role for lactate signaling cascade in the maintenance and recurrence of atrial fibrillation and possibly represent new targets for therapeutic intervention in this disorder.