Functional characteristics of H+ -dependent nicotinate transport in primary cultures of astrocytes from rat cerebral cortex

Mitochondrial targets in hyperammonemia: Addressing urea cycle function to improve drug therapies

The urea cycle (UC) is a critically important metabolic process for the disposal of nitrogen (ammonia) produced by amino acids catabolism. The impairment of this liver-specific pathway induced either by primary genetic defects or by secondary causes, namely those associated with hepatic disease or drug administration, may result in serious clinical consequences. Urea cycle disorders (UCD) and certain organic acidurias are the major groups of inherited rare diseases manifested with hyperammonemia (HA) with UC dysregulation. Importantly, several commonly prescribed drugs, including antiepileptics in monotherapy or polytherapy from carbamazepine to valproic acid or specific antineoplastic agents such as asparaginase or 5-fluorouracil may be associated with HA by mechanisms not fully elucidated. HA, disclosing an imbalance between ammoniagenesis and ammonia disposal via the UC, can evolve to encephalopathy which may lead to significant morbidity and central nervous system damage. This review will focus on biochemical mechanisms related with HA emphasizing some poorly understood perspectives behind the disruption of the UC and mitochondrial energy metabolism, namely: i) changes in acetyl-CoA or NAD+ levels in subcellular compartments; ii) post-translational modifications of key UC-related enzymes, namely acetylation, potentially affecting their catalytic activity; iii) the mitochondrial sirtuins-mediated role in ureagenesis. Moreover, the main UCD associated with HA will be summarized to highlight the relevance of investigating possible genetic mutations to account for unexpected HA during certain pharmacological therapies. The ammonia-induced effects should be avoided or overcome as part of safer therapeutic strategies to protect patients under treatment with drugs that may be potentially associated with HA.

Metabolic engineering of Escherichia coli for biosynthesis of β-nicotinamide mononucleotide from nicotinamide

The β-nicotinamide mononucleotide (NMN) is a key intermediate of an essential coenzyme for cellular redox reactions, NAD. Administration of NMN is reported to improve various symptoms, such as diabetes and age-related physiological decline. Thus, NMN is attracting much attention as a promising nutraceutical. Here, we engineered an Escherichia coli strain to produce NMN from cheap substrate nicotinamide (NAM) and glucose. The supply of in vivo precursor phosphoribosyl pyrophosphate (PRPP) and ATP was enhanced by strengthening the metabolic flux from glucose. A nicotinamide phosphoribosyltransferase with high activity was newly screened, which is the key enzyme for converting NAM to NMN with PRPP as cofactor. Notably, the E. coli endogenous protein YgcS, which function is primarily in the uptake of sugars, was firstly proven to be beneficial for NMN production in this study. Fine-tuning regulation of ygcS gene expression in the engineered E. coli strain increased NMN production. Combined with process optimization of whole-cell biocatalysts reaction, a final NMN titre of 496.2 mg l-1 was obtained.

Characteristics of Hemichannel-Mediated Substrate Transport in Human Retinal Pigment Epithelial Cells under Deprivation of Extracellular Ca2

Retinal pigment epithelial (RPE) cells form the outer blood-retinal barrier (BRB) and regulate drug/compound exchange between the neural retina and blood in the fenestrated blood vessels of retinal choroid via membrane transporters. Recent studies have elucidated that RPE cells express hemichannels, which are opened by extracellular Ca²⁺ depletion and accept several drugs/compounds as a transporting substrate. The objective of this study was to elucidate the hemichannel-mediated compound transport properties of the outer BRB. In human RPE cells, namely ARPE-19 cells, time-dependent uptake of fluorescent hemichannel substrates, such as Lucifer Yellow, sulforhodamine-101 (SR-101), and propidium iodide (PI) was promoted under Ca²⁺-depleted conditions. The uptake of these substrates under Ca²⁺-depleted conditions exhibited saturable kinetics with a Michaelis-Menten constant (K_m) of 87-109 µM. In addition, SR-101 and PI uptake by ARPE-19 cells was dependent of extracellular Ca²⁺ concentration, and that under Ca²⁺-depleted conditions was significantly decreased by typical substrates and/or inhibitors for hemichannels. Moreover, Ca²⁺-depleted conditions promoted the efflux transport of calcein from ARPE-19 cells, and the promoted calcein efflux transport was significantly inhibited by a typical hemichannel inhibitor. These results suggested that hemichannels at the outer BRB were involved in the influx and efflux transport of drugs/compounds.

Effect of diclofenac on SLC16A3/MCT4 by the Caco-2 cell line

In the present study, we demonstrated that monocarboxylate transporter 4 (MCT4) is functionally expressed in Caco-2 cells. We studied the effects of 4 nonsteroidal anti-inflammatory drugs on the uptake of l-lactate as a good substrate of MCT4 by the cells. The monocarboxylate drugs inhibited the uptake of l-lactate into the cells. Diclofenac, as a member of the aryl-acetic acid group of nonsteroidal anti-inflammatory drugs, was the most potent inhibitor, with an inhibition constant of 20 μM. In the next study, we determined the type of inhibition for diclofenac. An l-lactate carrier is non-competitively inhibitable by the drug. We also demonstrated, in Xenopus oocyte expression system, potential of diclofenac for MCT4 inhibitor. The present results could provide a useful tool to discover MCT4-specific inhibitors.

The hypophagic response to heat stress is not mediated by GPR109A or peripheral β-OH butyrate

Rising temperatures resulting from climate change will increase the incidence of heat stress, negatively impacting the labor force and food animal production. Heat stress elevates circulating β-OH butyrate, which induces vasodilation through GPR109a. Interestingly, both heat stress and intraperitoneal β-OH butyrate administration induce hypophagia. Thus, we aimed to investigate the role of β-OH butyrate in heat stress hypophagia in mice. We found that niacin, a β-OH butyrate mimetic that cannot be oxidized to generate ATP, also reduces food intake. Interestingly, the depression in food intake as a result of 8-h intraperitoneal niacin or 48-h heat exposure did not result from changes in hypothalamic expression of orexigenic or anorexigenic signals (AgRP, NPY, or POMC). Genetically eliminating GPR109a expression did not prevent the hypophagic response to heat exposure, intraperitoneal β-OH butyrate (5.7 mmol/kg), or niacin (0.8 mmol/kg). Hepatic vagotomy eliminated the hypophagic response to β-OH butyrate and niacin but did not affect the hypophagic response to heat exposure. We subsequently hypothesized that the hypophagic response to heat stress may depend on direct effects of β-OH butyrate at the central nervous system: β-OH butyrate induced hormonal changes (hyperinsulinemia, hypercorticosteronemia, and hyperleptinemia), or gene expression changes. To test these possibilities, we blocked expression of hepatic hydroxyl methyl glutaryl CoA synthase II (HMGCS2) to prevent hepatic β-OH butyrate synthesis. Mice that lack HMGCS2 maintain a hypophagic response to heat stress. Herein, we establish that the hypophagia of heat stress is independent of GPR109a, the hepatic vagus afferent nerve, and hepatic ketone body synthesis.

Involvement of Histidine Residue His382 in pH Regulation of MCT4 Activity

Monocarboxylate transporter 4 (MCT4) is a pH-dependent bi-directional lactate transporter. Transport of lactate via MCT4 is increased by extracellular acidification. We investigated the critical histidine residue involved in pH regulation of MCT4 function. Transport of lactate via MCT4 was measured by using a Xenopus laevis oocyte expression system. MCT4-mediated lactate transport was inhibited by Zn2+ in a pH physiological condition but not in an acidic condition. The histidine modifier DEPC (diethyl pyrocarbonate) reduced MCT4 activity but did not completely inactivate MCT4. After treatment with DEPC, pH regulation of MCT4 function was completely knocked out. Inhibitory effects of DEPC were reversed by hydroxylamine and suppressed in the presence of excess lactate and Zn2+. Therefore, we performed an experiment in which the extracellular histidine residue was replaced with alanine. Consequently, the pH regulation of MCT4-H382A function was also knocked out. Our findings demonstrate that the histidine residue His382 in the extracellular loop of the transporter is essential for pH regulation of MCT4-mediated substrate transport activity.

Decreased astroglial monocarboxylate transporter 4 expression in temporal lobe epilepsy

Efflux of monocaroxylates like lactate, pyruvate, and ketone bodies from astrocytes through monocarboxylate transporter 4 (MCT4) supplies the local neuron population with metabolic intermediates to meet energy requirements under conditions of increased demand. Disruption of this astroglial-neuron metabolic coupling pathway may contribute to epileptogenesis. We measured MCT4 expression in temporal lobe epileptic foci excised from patients with intractable epilepsy and in rats injected with pilocarpine, an animal model of temporal lobe epilepsy (TLE). Cortical MCT4 expression levels were significantly lower in TLE patients compared with controls, due at least partially to MCT4 promoter methylation. Expression of MCT4 also decreased progressively in pilocarpine-treated rats from 12 h to 14 days post-administration. Underexpression of MCT4 in cultured astrocytes induced by a short hairpin RNA promoted apoptosis. Knockdown of astrocyte MCT4 also suppressed excitatory amino acid transporter 1 (EAAT1) expression. Reduced MCT4 and EAAT1 expression by astrocytes may lead to neuronal hyperexcitability and epileptogenesis in the temporal lobe by reducing the supply of metabolic intermediates and by allowing accumulation of extracellular glutamate.

Role of monocarboxylate transporters in drug delivery to the brain

Monocarboxylate transporters (MCTs) are known to mediate the transport of short chain monocarboxylates such as lactate, pyruvate and butyrate. Currently, fourteen members of this transporter family have been identified by sequence homology, of which only the first four members (MCT1- MCT4) have been shown to mediate the proton-linked transport of monocarboxylates. Another transporter family involved in the transport of endogenous monocarboxylates is the sodium coupled MCTs (SMCTs). These act as a symporter and are dependent on a sodium gradient for their functional activity. MCT1 is the predominant transporter among the MCT isoforms and is present in almost all tissues including kidney, intestine, liver, heart, skeletal muscle and brain. The various isoforms differ in terms of their substrate specificity and tissue localization. Due to the expression of these transporters in the kidney, intestine, and brain, they may play an important role in influencing drug disposition. Apart from endogenous short chain monocarboxylates, they also mediate the transport of exogenous drugs such as salicylic acid, valproic acid, and simvastatin acid. The influence of MCTs on drug pharmacokinetics has been extensively studied for γ-hydroxybutyrate (GHB) including distribution of this drug of abuse into the brain and the results will be summarized in this review. The physiological role of these transporters in the brain and their specific cellular localization within the brain will also be discussed. This review will also focus on utilization of MCTs as potential targets for drug delivery into the brain including their role in the treatment of malignant brain tumors.

Nitric oxide induces the expression of the monocarboxylate transporter MCT4 in cultured astrocytes by a cGMP-independent transcriptional activation

The monocarboxylate transporter MCT4 is a proton-linked carrier particularly important for lactate release from highly glycolytic cells. In the central nervous system, MCT4 is exclusively expressed by astrocytes. Surprisingly, MCT4 expression in primary cultures of mouse cortical astrocytes is conspicuously low, suggesting that an external, nonastrocytic signal is necessary to obtain the observed pattern of expression in vivo. Here, we demonstrate that nitric oxide (NO), delivered by various NO donors, time- and dose-dependently induces MCT4 expression in cultured cortical astrocytes both at the mRNA and protein levels. In contrast, NO does not enhance the expression of MCT1, the other astrocytic monocarboxylate transporter. The transcriptional effect of NO is not mediated by a cGMP-dependent mechanism as shown by the absence of effect of a cGMP analog or of a selective guanylate cyclase inhibitor. NO causes an increase in astrocytic lactate transport capacity which requires the enhancement of MCT4 expression as both are prevented by the use of a specific siRNA against MCT4. In addition, cumulated lactate release by astrocytes over a period of 24 h was also enhanced by NO treatment. Our data suggest that NO represents a putative intercellular signal to control MCT4 expression in astrocytes and in doing so, to facilitate lactate transfer to other surrounding cell types in the central nervous system. © 2011 Wiley-Liss, Inc.

Retinal transfer of nicotinate by H+ -monocarboxylate transporter at the inner blood-retinal barrier

Nicotinic acid is a constituent of the coenzymes NAD and NADP. It also serves as an agonist for the G-protein-coupled receptor GPR109A. Nicotinic acid is widely used at high doses as a lipid-lowering drug, which is associated with an ocular side effect known as niacin maculopathy. Here we investigated the mechanism by which nicotinate is transferred into retina across the inner blood-retinal barrier (BRB). In vivo the blood-to-retina transport of [(3)H]-nicotinate was studied using the carotid artery injection technique. The characteristics of nicotinate transport at the inner BRB were examined in a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2), an in vitro model of inner BRB. The expression of transporters in TR-iBRB2 cells was determined by reverse transcription-polymerase chain reaction. In vivo [(3)H]-nicotinate uptake by the retina was 5.4-fold greater than that of [(14)C]-sucrose, a BRB impermeable vascular space marker. Excess amounts of unlabeled nicotinate and salicylate significantly decreased the in vivo retinal uptake of [(3)H]-nicotinate. [(3)H]-Nicotinate was taken up by TR-iBRB2 cells via an H(+)-dependent saturable process with a Michaelis constant of ~7 mM. Na(+) had minimal effect on the uptake. The H(+)-dependent uptake was significantly inhibited by endogenous monocarboxylates such as lactate and pyruvate, and monocarboxylic drugs such as valproate, salicylate, and ibuprofen. These characteristics are consistent with those of H(+)-coupled monocarboxylate transporters (MCTs). MCT1, MCT2, and MCT4 mRNAs were expressed in TR-iBRB2 cells. The Na(+)-dependent monocarboxylate transporters SMCT1 and SMCT2 were not expressed in these cells. In conclusion, transfer of nicotinate from blood to retina across the inner BRB occurs primarily via H(+)-coupled monocarboxylate transporters.

Basal and stimulated lactate fluxes in primary cultures of astrocytes are differentially controlled by distinct proteins

Lactate release by astrocytes is postulated to be of importance for neuroenergetics but its regulation is poorly understood. Basigin, a chaperone protein for specific monocarboxylate transporters (MCTs), represents a putatively important regulatory element for lactate fluxes. Indeed, basigin knockdown by RNA interference in primary cultures of astrocytes partially reduced both proton-driven lactate influx and efflux. But more strikingly, enhancement of lactate efflux induced by glutamate was prevented while the effect of sodium azide was significantly reduced by treatment of cultured astrocytes with anti-basigin small interfering RNA. Enhancement of glucose utilization was unaffected under the same conditions. Basal lactate uptake and release were significantly reduced by MCT1 knockdown, even more so than with basigin knockdown, whereas glutamate-driven or sodium azide-induced enhancement of lactate release was not inhibited by either MCT1, 2, or 4 small interfering RNAs. In conclusion, MCT1 plays a pivotal role in the control of basal proton-driven lactate flux in astrocytes while basigin is only partly involved, most likely via its interaction with MCT1. In contrast, basigin appears to critically regulate the enhancement of lactate release caused by glutamate (or sodium azide) but via an effect on another unidentified transporter at least present in astrocytes in vitro.

The SLC16 monocaboxylate transporter family

1. The monocarboxylate transporter (MCT, SLC16) family comprises 14 members, of which to date only MCT1-4 have been shown to carry monocarboxylates, transporting important metabolic compounds such as lactate, pyruvate and ketone bodies in a proton-coupled manner. The transport of such compounds is fundamental for metabolism, and the tissue locations, properties and regulation of these isoforms is discussed. 2. Of the other members of the MCT family, MCT8 (a thyroid hormone transporter) and TAT1 (an aromatic amino acid transporter) have been characterized more recently, and their physiological roles are reviewed herein. The endogenous substrates and functions of the remaining members of the MCT family await elucidation. 3. The MCT proteins have the typical twelve transmembrane-spanning domain (TMD) topology of membrane transporter proteins, and their structure-function relationship is discussed, especially in relation to the future impact of the single nucleotide polymorphism (SNP) databases and, given their ability to transport pharmacologically relevant compounds, the potential impact for pharmacogenomics.

Regulation of pH in the mammalian central nervous system under normal and pathological conditions: facts and hypotheses

The maintenance of pH homeostasis in the CNS is of key importance for proper execution and regulation of neurotransmission, and deviations from this homeostasis are a crucial factor in the mechanism underlying a spectrum of pathological conditions. The first few sections of the review are devoted to the brain operating under normal conditions. The article commences with an overview of how extrinsic factors modelling the brain at work: neurotransmitters, depolarising stimuli (potassium and voltage changes) and cyclic nucleotides as major signal transducing vehicles affect pH in the CNS. Further, consequences of pH alterations on the major aspects of CNS function and metabolism are outlined. Next, the major cellular events involved in the transport, sequestration, metabolic production and buffering of protons that are common to all the mammalian cells, including the CNS cells. Since CNS function reflects tight interaction between astrocytes and neurons, the pH regulatory events pertinent to either cell type are discussed: overwhelming evidence implicates astrocytes as a key player in pH homeostasis in the brain. The different classes of membrane proteins involved in proton shuttling are listed and their mechanisms of action are given. These include: the Na+/H+ exchanger, different classes of bicarbonate transporters acting in a sodium-dependent- or -independent mode, monocarboxylic acid transporters and the vacuolar-type proton ATPase. A separate section is devoted to carbonic anhydrase, which is represented by multiple isoenzymes capable of pH buffering both in the cell interior and in the extracellular space. Next, impairment of pH regulation and compensatory responses occurring in brain affected by different pathologies: hypoxia/ischemia, epilepsy, hyperammonemic encephalopathies, cerebral tumours and HIV will be described. The review is limited to facts and plausible hypotheses pertaining to phenomena directly involved in pH regulation: changes in pH that accompany metabolic stress but have no distinct implications for the pH regulatory mechanisms are not dealt with. In most cases, the vast body of knowledge derived from in vitro studies remains to be verified in in vivo settings.

Mechanism of nicotinic acid transport in human liver cells: experiments with HepG2 cells and primary hepatocytes

This study reports on the functional expression of a specific, high-affinity carrier-mediated mechanism for the transport of niacin (nicotinic acid) in human liver cells. Both human-derived liver HepG2 cells and human primary hepatocytes were used as models in these investigations. The initial rate of transport of nicotinic acid into HepG2 cells was found to be acidic pH, temperature, and energy dependent; it was, however, Na(+) independent in nature. Evidence for the existence of a carrier-mediated system that is specific for [(3)H]nicotinic acid transport was found and included the following: 1) saturability as a function of concentration with an apparent K(m) of 0.73 +/- 0.16 microM and V(max) of 25.02 +/- 1.45 pmol.mg protein(-1).3 min(-1), 2) cis-inhibition by unlabeled nicotinic acid and nicotinamide but not by unrelated organic anions (lactate, acetate, butyrate, succinate, citrate, and valproate), and 3) trans-stimulation of [(3)H]nicotinic acid efflux by unlabeled nicotinic acid. Transport of the vitamin into human primary hepatocytes occurs similarly via an acidic pH-dependent and specific carrier-mediated process. Inhibitors of the Ca(2+)-calmodulin-mediated pathway (but not modulators of the PKC-, PKA-, and protein tyrosine kinase-mediated pathways) inhibited nicotinic acid transport into both HepG2 cells and human primary hepatocytes. Maintenance of HepG2 cells (for 48 h) in growth medium oversupplemented with nicotinic acid (or nicotinamide) did not affect the subsequent transport of [(3)H]nicotinic acid into HepG2 cells. These results show, for the first time, the existence of a specific and regulated membrane carrier-mediated system for nicotinic acid transport in human liver cells.