Developmental Expression of Monocarboxylate Transporter 1 and 4 in Rat Liver

PURPOSE: Monocarboxylate transporters (MCT) are proton-coupled integral membrane proteins that control the influx and efflux of endogenous monocarboxylates such as lactate, acetate and pyruvate. They also transport and mediate the clearance of drugs such as valproate and gamma-hydroxybutyrate. CD147 functions as ancillary protein that chaperones MCT1 and MCT4 to the cell membrane. There is limited data on the maturation of MCT and CD147 expression in tissues related to drug distribution and clearance. The objective of the present study was to quantify hepatic MCT1, MCT4, and CD147 mRNA, whole cell and membrane protein expression from birth to sexual maturity. METHODS: Liver tissues were collected from male and female Sprague Dawley rats at postnatal days (PND) 1, 3, 5, 7, 10, 14, 18, 21, 28, 35, and 42 (n = 3 - 5). Hepatic mRNA, total and membrane protein expression of MCT1, MCT4, and CD147 was evaluated via qPCR and western blot. RESULTS: MCT1 mRNA and protein demonstrated nonlinear maturation patterns. MCT1 and CD147 membrane protein exhibited low expression at birth, with expression increasing three-fold by PND14, followed by a decline in expression at sexual maturity. MCT4 mRNA had highest expression at PND 1, with decreasing expression towards sexual maturity. In contrast, MCT4 membrane protein exhibited minimal expression from birth through weaning before a 10-fold surge at PND35, whereupon there was a sharp decline in expression at PND42. There was a significant positive correlation between MCT1 and CD147 whole cell and membrane expression, while MCT4 membrane expression demonstrated a weak negative correlation with CD147. CONCLUSION: Our study elucidates the transcriptional and translational maturation patterns of MCT1, MCT4 and CD147 expression, with isoform- dependent differences in the liver. Changes in transporter expression during development may greatly influence drug distribution and clearance in pediatric populations

Peripherally restricted transthyretin-based delivery system for probes and therapeutics avoiding opioid-related side effects

Several investigations into the sites of action of opioid analgesics have utilized peripherally acting mu-opioid receptor antagonists (PAMORAs), which have been incorrectly assumed to possess limited permeability across the blood-brain barrier. Unfortunately, the poor pharmacokinetic properties of current PAMORAs have resulted in misunderstandings of the role of central nervous system and gastrointestinal tract in precipitating side effects such as opioid-induced constipation. Here, we develop a drug delivery approach for restricting the passage of small molecules across the blood-brain barrier. This allows us to develop naloxone- and oxycodone-based conjugates that display superior potency, peripheral selectivity, pharmacokinetics, and efficacy in rats compared to other clinically used PAMORAs. These probes allow us to demonstrate that the mu-opioid receptors in the central nervous system have a fundamental role in precipitating opioid-induced constipation. Therefore, our conjugates have immediate use as pharmacological probes and potential therapeutic agents for treating constipation and other opioid-related side effects

Overview of the Proton-coupled MCT (SLC16A) Family of Transporters: Characterization, Function and Role in the Transport of the Drug of Abuse γ-Hydroxybutyric Acid

The transport of monocarboxylates, such as lactate and pyruvate, is mediated by the SLC16A family of proton-linked membrane transport proteins known as monocarboxylate transporters (MCTs). Fourteen MCT-related genes have been identified in mammals and of these seven MCTs have been functionally characterized. Despite their sequence homology, only MCT1–4 have been demonstrated to be proton-dependent transporters of monocarboxylic acids. MCT6, MCT8 and MCT10 have been demonstrated to transport diuretics, thyroid hormones and aromatic amino acids, respectively. MCT1–4 vary in their regulation, tissue distribution and substrate/inhibitor specificity with MCT1 being the most extensively characterized isoform. Emerging evidence suggests that in addition to endogenous substrates, MCTs are involved in the transport of pharmaceutical agents, including γ-hydroxybuytrate (GHB), 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase inhibitors (statins), salicylic acid, and bumetanide. MCTs are expressed in a wide range of tissues including the liver, intestine, kidney and brain, and as such they have the potential to impact a number of processes contributing to the disposition of xenobiotic substrates. GHB has been extensively studied as a pharmaceutical substrate of MCTs; the renal clearance of GHB is dose-dependent with saturation of MCT-mediated reabsorption at high doses. Concomitant administration of GHB and l-lactate to rats results in an approximately two-fold increase in GHB renal clearance suggesting that inhibition of MCT1-mediated reabsorption of GHB may be an effective strategy for increasing renal and total GHB elimination in overdose situations. Further studies are required to more clearly define the role of MCTs on drug disposition and the potential for MCT-mediated detoxification strategies in GHB overdose

Developmental Expression of Monocarboxylate Transporter 1 and 4 in Rat Liver

PURPOSE: Monocarboxylate transporters (MCT) are proton-coupled integral membrane proteins that control the influx and efflux of endogenous monocarboxylates such as lactate, acetate and pyruvate. They also transport and mediate the clearance of drugs such as valproate and gamma-hydroxybutyrate. CD147 functions as ancillary protein that chaperones MCT1 and MCT4 to the cell membrane. There is limited data on the maturation of MCT and CD147 expression in tissues related to drug distribution and clearance. The objective of the present study was to quantify hepatic MCT1, MCT4, and CD147 mRNA, whole cell and membrane protein expression from birth to sexual maturity. METHODS: Liver tissues were collected from male and female Sprague Dawley rats at postnatal days (PND) 1, 3, 5, 7, 10, 14, 18, 21, 28, 35, and 42 (n = 3 - 5). Hepatic mRNA, total and membrane protein expression of MCT1, MCT4, and CD147 was evaluated via qPCR and western blot. RESULTS: MCT1 mRNA and protein demonstrated nonlinear maturation patterns. MCT1 and CD147 membrane protein exhibited low expression at birth, with expression increasing three-fold by PND14, followed by a decline in expression at sexual maturity. MCT4 mRNA had highest expression at PND 1, with decreasing expression towards sexual maturity. In contrast, MCT4 membrane protein exhibited minimal expression from birth through weaning before a 10-fold surge at PND35, whereupon there was a sharp decline in expression at PND42. There was a significant positive correlation between MCT1 and CD147 whole cell and membrane expression, while MCT4 membrane expression demonstrated a weak negative correlation with CD147. CONCLUSION: Our study elucidates the transcriptional and translational maturation patterns of MCT1, MCT4 and CD147 expression, with isoform- dependent differences in the liver. Changes in transporter expression during development may greatly influence drug distribution and clearance in pediatric populations

Concentration-Effect Relationships for the Drug of Abuse ␥- Hydroxybutyric Acid

ABSTRACT ␥-Hydroxybutyric acid (GHB) is an endogenous neurotransmitter that is abused because of its sedative/hypnotic and euphoric effects. The objectives of this study were to evaluate the concentration-effect relationships of GHB in plasma, cerebrospinal fluid (CSF), brain (whole and discrete brain regions), and brain frontal cortex extracellular fluid. This information is crucial for future studies to evaluate effects of therapeutic interventions on the toxicodynamics of GHB. GHB (200 -1000 mg/kg) was administered intravenously to rats, and plasma and frontal cortex microdialysate samples were collected for up to 6 h after the dose, or plasma, CSF, and brain (whole, frontal cortex, striatum, and hippocampus) concentrations were determined at the offset of its sedative/hypnotic effect [return to righting reflex (RRR)]. GHB-induced changes in the brain neurotransmitters ␥-aminobutyric acid (GABA) and glutamate were also determined. GHB, GABA, and glutamate concentrations were measured by liquid chromatography/tandem mass spectrometry. GHB-induced sleep time significantly increased in a dosedependent manner (20-fold increase from 200 to 1000 mg/kg). GHB concentrations in plasma (300 -400 g/ml), whole brain (70 g/g), discrete brain regions (80 -100 g/g), and brain microdialysate (29 -39 g/ml) correlated with RRR. In contrast, CSF GHB and GABA and glutamate concentrations in discrete brain regions exhibited no relationship with RRR. Our results suggest that GHB-induced sedative/hypnotic effects are mediated directly by GHB and that at high GHB doses, GABA formation from GHB may not contribute to the observed sedative/hypnotic effect. These results support the use of a clinical GHB detoxification strategy aimed at decreasing plasma and brain GHB concentrations after GHB overdoses

Monocarboxylate Transporters (SLC16): Function, Regulation, and Role in Health and Disease

The solute carrier family 16 (SLC16) is comprised of 14 members of the monocarboxylate transporter (MCT) family that play an essential role in the transport of important cell nutrients and for cellular metabolism and pH regulation. MCTs 1-4 have been extensively studied and are involved in the proton-dependent transport of L-lactate, pyruvate, short-chain fatty acids, and monocarboxylate drugs in a wide variety of tissues. MCTs 1 and 4 are overexpressed in a number of cancers, and current investigations have focused on transporter inhibition as a novel therapeutic strategy in cancers. MCT1 has also been used in strategies aimed at enhancing drug absorption due to its high expression in the intestine. Other MCT isoforms are less well characterized, but ongoing studies indicate that MCT6 transports xenobiotics such as bumetanide, nateglinide, and probenecid, whereas MCT7 has been characterized as a transporter of ketone bodies. MCT8 and MCT10 transport thyroid hormones, and recently, MCT9 has been characterized as a carnitine efflux transporter and MCT12 as a creatine transporter. Expressed at the blood brain barrier, MCT8 mutations have been associated with an X-linked intellectual disability, known as Allan-Herndon-Dudley syndrome. Many MCT isoforms are associated with hormone, lipid, and glucose homeostasis, and recent research has focused on their potential roles in disease, with MCTs representing promising novel therapeutic targets. This review will provide a summary of the current literature focusing on the characterization, function, and regulation of the MCT family isoforms and on their roles in drug disposition and in health and disease. SIGNIFICANCE STATEMENT: The 14-member solute carrier family 16 of monocarboxylate transporters (MCTs) plays a fundamental role in maintaining intracellular concentrations of a broad range of important endogenous molecules in health and disease. MCTs 1, 2, and 4 (L-lactate transporters) are overexpressed in cancers and represent a novel therapeutic target in cancer. Recent studies have highlighted the importance of MCTs in glucose, lipid, and hormone homeostasis, including MCT8 in thyroid hormone brain uptake, MCT12 in carnitine transport, and MCT11 in type 2 diabetes

Monocarboxylate Transporter-Mediated Transport of γ-Hydroxybutyric Acid in Human Intestinal Caco-2 Cells

The objectives of this study were to determine mRNA expression of monocarboxylate transporters (MCT) and to evaluate intestinal transport of the MCT substrates γ-hydroxybutyrate (GHB) and d-lactate in human intestinal Caco-2 cells. The presence of mRNA for MCT1, 2, 3, and 4 was observed in Caco-2 cells. The uptake of both GHB and d-lactate in Caco-2 cells was demonstrated to be pH- and concentration-dependent and sodium-independent. The uptake of GHB and d-lactate was best described by a Michaelis-Menten equation with passive diffusion (GHB: Km = 17.6 ± 10.5 mM, Vmax = 17.3 ± 11.7 nmol/min/mg, and P = 0.38 ± 0.15 μl/min/mg; and d-lactate: Km = 6.0 ± 2.9 mM, Vmax = 35.0 ± 18.4 nmol/min/mg, and P = 1.3 ± 0.6 μl/min/mg). The uptake of GHB and d-lactate was significantly decreased by the known MCT inhibitor α-cyano-4-hydroxycinnamate and the MCT substrates GHB and d-lactate but not by the organic cation tetraethylammonium chloride. Directional flux studies with both GHB and d-lactate suggested the involvement of carrier-mediated transport with the permeability in the apical to basolateral direction higher than that in the basolateral to apical direction. These findings confirm the presence of MCT1–4 in Caco-2 cells and demonstrate GHB and d-lactate transport characteristics consistent with proton-dependent MCT-mediated transport