Immunohistochemical and functional characterization of pH-dependent intestinal absorption of weak organic acids by the monocarboxylic acid transporter MCT1

CNS Viral Infections-What to Consider for Improving Drug Treatment: A Plea for Using Mathematical Modeling Approaches

Neurotropic viruses may cause meningitis, myelitis, encephalitis, or meningoencephalitis. These inflammatory conditions of the central nervous system (CNS) may have serious and devastating consequences if not treated adequately. In this review, we first summarize how neurotropic viruses can enter the CNS by (1) crossing the blood-brain barrier or blood-cerebrospinal fluid barrier; (2) invading the nose via the olfactory route; or (3) invading the peripheral nervous system. Neurotropic viruses may then enter the intracellular space of brain cells via endocytosis and/or membrane fusion. Antiviral drugs are currently used for different viral CNS infections, even though their use and dosing regimens within the CNS, with the exception of acyclovir, are minimally supported by clinical evidence. We therefore provide considerations to optimize drug treatment(s) for these neurotropic viruses. Antiviral drugs should cross the blood-brain barrier/blood cerebrospinal fluid barrier and pass the brain cellular membrane to inhibit these viruses inside the brain cells. Some antiviral drugs may also require intracellular conversion into their active metabolite(s). This illustrates the need to better understand these mechanisms because these processes dictate drug exposure within the CNS that ultimately determine the success of antiviral drugs for CNS infections. Finally, we discuss mathematical model-based approaches for optimizing antiviral treatments. Thereby emphasizing the potential of CNS physiologically based pharmacokinetic models because direct measurement of brain intracellular exposure in living humans faces ethical restrictions. Existing physiologically based pharmacokinetic models combined with in vitro pharmacokinetic/pharmacodynamic information can be used to predict drug exposure and evaluate efficacy of antiviral drugs within the CNS, to ultimately optimize the treatments of CNS viral infections.

Enteric and systemic postprandial lactate shuttle phases and dietary carbohydrate carbon flow in humans

Dietary glucose in excess is stored in the liver in the form of glycogen. As opposed to direct conversion of glucose into glycogen, the hypothesis of the postprandial lactate shuttle (PLS) proposes that dietary glucose uptake is metabolized to lactate in the gut, thereby being transferred to the liver for glycogen storage. In the present study, we provide evidence of a PLS in young healthy men and women. Overnight fasted participants underwent an oral glucose tolerance test, and arterialized lactate concentration and rate of appearance were determined. The concentration of lactate in the blood rose before the concentration of glucose, thus providing evidence of an enteric PLS. Secondary increments in the concentration of lactate in the blood and its rate of appearance coincided with those of glucose, which indicates the presence of a larger, secondary, systemic PLS phase driven by hepatic glucose release. The present study challenges the notion that lactate production is the result of hypoxia in skeletal muscles, because our work indicates that glycolysis proceeds to lactate in fully aerobic tissues and dietary carbohydrate is processed via lactate shuttling. Our study proposes that, in humans, lactate is a major vehicle for carbohydrate carbon distribution and metabolism.

The Daily Expression of ABCC4 at the BCSFB Affects the Transport of Its Substrate Methotrexate

The choroid plexuses (CPs), located in the brain ventricles, form an interface between the blood and the cerebrospinal fluid named the blood-cerebrospinal barrier, which, by the presence of tight junctions, detoxification enzymes, and membrane transporters, limits the traffic of molecules into the central nervous system. It has already been shown that sex hormones regulate several CP functions, including the oscillations of its clock genes. However, it is less explored how the circadian rhythm regulates CP functions. This study aimed to evaluate the impact of sex hormones and circadian rhythms on the function of CP membrane transporters. The 24 h transcription profiles of the membrane transporters rAbca1, rAbcb1, rAbcc1, rAbcc4, rAbcg2, rAbcg4, and rOat3 were characterized in the CPs of intact male, intact female, sham-operated female, and gonadectomized rats. We found that rAbcc1 is expressed in a circadian way in the CPs of intact male rats, rAbcg2 in the CPs of intact female rats, and both rAbcc4 and rOat3 mRNA levels were expressed in a circadian way in the CPs of intact male and female rats. Next, using an in vitro model of the human blood–cerebrospinal fluid barrier, we also found that methotrexate (MTX) is transported in a circadian way across this barrier. The circadian pattern of Abcc4 found in the human CP epithelial papilloma cells might be partially responsible for MTX circadian transport across the basal membrane of CP epithelial cells.

The Role of Gut Microbiota and Metabolites in Obesity-Associated Chronic Gastrointestinal Disorders

The gut microbiota is a complex community of microorganisms that has become a new focus of attention due to its association with numerous human diseases. Research over the last few decades has shown that the gut microbiota plays a considerable role in regulating intestinal homeostasis, and disruption to the microbial community has been linked to chronic disease conditions such as inflammatory bowel disease (IBD), colorectal cancer (CRC), and obesity. Obesity has become a global pandemic, and its prevalence is increasing worldwide mostly in Western countries due to a sedentary lifestyle and consumption of high-fat/high-sugar diets. Obesity-mediated gut microbiota alterations have been associated with the development of IBD and IBD-induced CRC. This review highlights how obesity-associated dysbiosis can lead to the pathogenesis of IBD and CRC with a special focus on mechanisms of altered absorption of short-chain fatty acids (SCFAs).

Critical transporters of methionine and methionine hydroxyl analogue supplements across the intestine: What we know so far and what can be learned to advance animal nutrition

DL-methionine (DL-Met) and its analogue DL-2-hydroxy-4-(methylthio) butanoic acid (DL-methionine hydroxyl analogue or DL-MHA) have been used as nutritional supplements in the diets of farmed raised animals. Knowledge of the intestinal transport mechanisms involved in these products is important for developing dietary strategies. This review provides updated information of the expression, function, and transport kinetics in the intestine of known Met-linked transporters along with putative MHA-linked transporters. As a neutral amino acid (AA), the transport of DL-Met is facilitated by multiple apical sodium-dependent/-independent high-/low-affinity transporters such as ASCT2, B⁰AT1 and rBAT/b^0,+AT. The basolateral transport largely relies on the rate-limiting uniporter LAT4, while the presence of the basolateral antiporter y⁺LAT1 is probably necessary for exchanging intracellular cationic AAs and Met in the blood. In contrast, the intestinal transport kinetics of DL-MHA have been scarcely studied. DL-MHA transport is generally accepted to be mediated simply by the proton-dependent monocarboxylate transporter MCT1. However, in-depth mechanistic studies have indicated that DL-MHA transport is also achieved through apical sodium monocarboxylate transporters (SMCTs). In any case, reliance on either a proton or sodium gradient would thus require energy input for both Met and MHA transport. This expanding knowledge of the specific transporters involved now allows us to assess the effect of dietary ingredients on the expression and function of these transporters. Potentially, the resulting information could be furthered with selective breeding to reduce overall feed costs.

Characteristic and Mechanism of Drug-Herb Interaction Between Acetylsalicylic Acid and Danhong Injection Mediated by Organic Anion Transporters

The mixture of Salvia miltiorrhiza and Carthamus tinctorius (Danhong injection, DHI) is widely prescribed in China for the treatment of cardiovascular and cerebrovascular diseases. In most cases, DHI is used in combination with acetylsalicylic acid (aspirin, ASA). However, the interaction between DHI and ASA remains largely undefined. The purpose of this study is to explore the interaction profile and mechanism between DHI and ASA. The frequency of drug combination of DHI and ASA was analyzed based on 5,183 clinical cases. The interaction characteristics were evaluated by analyzing the pharmacokinetics and disposition profile of salicylic acid (SA, the primary metabolite of ASA) in rats. The interaction mechanisms were explored through evaluating the hydrolysis of ASA regulated by ASA esterase, the tubular secretion of SA mediated by influx and efflux transporters, and the tubular reabsorption of SA regulated by urinary acidity-alkalinity. The inhibitory potential of DHI on organic anion transporters (OATs) was further verified in aristolochic acid I (AAI) induced nephropathy. Clinical cases analysis showed that DHI and ASA were used in combination with high frequency of 70.73%. In drug combination of DHI and ASA, the maximum plasma concentration of SA was significantly increased by 1.37 times, while the renal excretion of SA was significantly decreased by 32.54%. The mechanism study showed that DHI significantly inhibited the transport function, gene transcription and protein expression of OATs. In OATs mediated AAI nephropathy, DHI significantly reduced the renal accumulation of AAI by 55.27%, and alleviated renal damage such as glomerulus swelling, tubular blockage and lymphocyte filtration. In drug combination of DHI and ASA, DHI increased the plasma concentration of SA not through enhancing the hydrolysis of ASA, and the tubular reabsorption of SA was not significantly affected. Inhibition of tubular secretion of SA mediated by OATs might be the reason that contributes to the decrease of SA renal excretion.

Characterization of the segmental transport mechanisms of DL-methionine hydroxy analogue along the intestinal tract of rainbow trout with an additional comparison to DL-methionine

The aim of this study was to identify the unknown transport mechanism of the extensively used monocarboxylate methionine feed supplement DL-methionine hydroxy analogue (DL-MHA) in rainbow trout intestine. Transport across the pyloric caeca (PC), midgut (MG), and hindgut (HG) regions were kinetically studied in Na⁺- and H⁺-dependent manners. Gene expression of monocarboxylate (MCTs) and sodium monocarboxylate transporters (SMCTs) were assessed. Results demonstrated that DL-MHA transport from 0.2-20 mM was Na⁺-dependent and obeyed Michaelis-Menten kinetics with low affinity in PC & MG in apical/basal pH of 7.7/7.7. Changes in apical/basal pH (6.0/6.0, 6.0/7.7, and 7.7/8.7) had insignificant effects on kinetics. In contrast, HG flux kinetics were only obtained in pH 7.7/8.7 or in the presence of lactate with medium affinity. Additionally, DL-MHA transport from 0-150 μM demonstrated the presence of a Na⁺-dependent high-affinity transporter in PC & MG. Conclusively, two distinct carrier-mediated DL-MHA transport mechanisms along the trout gut were found: 1) in PC & MG: apical transport was regulated by Na⁺-requiring systems that possibly contained low- and high-affinity transporters, and basolateral transport was primarily achieved through a H⁺-independent transporter; 2) in HG: uptake was apically mediated by a Na⁺-dependent transporter with medium affinity, and basolateral exit was largely controlled by an H⁺-dependent transporter. Finally, two major methionine feed supplements, DL-MHA and DL-methionine (DL-Met) were compared to understand the differences in their bioefficacy. Flux rates of DL-MHA were only about 42.2-66.0% in PC and MG compared to DL-Met, suggesting intestinal transport of DL-MHA was lower than DL-Met.

Involvement of butyrate in electrogenic K+ secretion in rat rectal colon

Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, are synthesized from dietary carbohydrates by colonic bacterial fermentation. These SCFAs supply energy, suppress cancer, and affect ion transport. However, their roles in ion transport and regulation in the intracellular environment remain unknown. In order to elucidate the roles of SCFAs, we measured short-circuit currents (I_SC) and performed RT-PCR and immunohistochemical analyses of ion transporters in rat rectal colon. The application of 30 mM butyrate shifted I_SC in a negative direction, but did not attenuate the activity of epithelial Na⁺ channels (ENaC). The application of bumetanide, a Na⁺-K⁺-2Cl⁻ cotransporter inhibitor, to the basolateral side reduced the negative I_SC shift induced by butyrate. The application of XE991, a KCNQ-type K⁺ channel inhibitor, to the apical side decreased the I_SC shift induced by butyrate in a dose-dependent manner. The I_SC shift was independent of HCO₃⁻ and insensitive to ibuprofen, an SMCT1 inhibitor. The mucosa from rat rectal colon expressed mRNAs of H⁺-coupled monocarboxylate transporters (MCT1, MCT4, and MCT5, also referred to as SLC16A1, SLC16A3, and SLC16A4, respectively). RT-PCR and immunofluorescence analyses demonstrated that KCNQ2 and KCNQ4 localized to the apical membrane of surface cells in rat rectal colon. These results indicate that butyrate, which may be transported by H⁺-coupled monocarboxylate transporters, activates K⁺ secretion through KCNQ-type K⁺ channels on the apical membrane in rat rectal colon. KCNQ-type K⁺ channels may play a role in intestinal secretion and defense mechanisms in the gastrointestinal tract.

Identification of a selective inhibitor of human monocarboxylate transporter 4

The human monocarboxylate transporters (hMCTs/SLC16As) mediate the uptake of various monocarboxylates. Several isoforms of hMCTs are expressed in cancerous tissue as well as in normal tissue. In cancerous tissue, hypoxia induces the expression of hMCT4, which transports the energetic metabolite l-lactate across the plasma membrane. Since hMCT4 is involved in pH regulation and the transport of l-lactate in cancer cells, an hMCT4 inhibitor could function as an anticancer agent. Although several non specific hMCT inhibitors have been developed, a selective hMCT4 inhibitor has not yet been identified. The aim of this study was therefore to identify a selective hMCT4 inhibitor for use as a pharmacological tool for studying hMCT4. The heterologous expression system of the Xenopus oocyte was used to assess the effects of test compounds on hMCT4, whereupon isobutyrate derivatives, fibrates, and bindarit (2-[(1-benzyl-1H-indazol-3-yl)methoxy]-2-methylpropanoic acid) were demonstrated to exhibit selective inhibitory effects against this transporter. It is suggested that the structure formed from the joining of an isobutyrate moiety and two aromatic rings by appropriate linkers is important for acquiring the selective hMCT4-inhibiting activity. These findings provide novel insights into the ligand recognition of hMCT4, and contribute to the development of novel anticancer agents.

A pharmacokinetic evaluation and metabolite identification of the GHB receptor antagonist NCS-382 in mouse informs novel therapeutic strategies for the treatment of GHB intoxication

Gamma‐aminobutyric acid (GABA) is an endogenous inhibitory neurotransmitter and precursor of gamma‐hydroxybutyric acid (GHB). NCS‐382 (6,7,8,9‐tetrahydro‐5‐hydroxy‐5H‐benzo‐cyclohept‐6‐ylideneacetic acid), a known GHB receptor antagonist, has shown significant efficacy in a murine model of succinic semialdehyde dehydrogenase deficiency (SSADHD), a heritable neurological disorder featuring chronic elevation of GHB that blocks the final step of GABA degradation. NCS‐382 exposures and elimination pathways remain unknown; therefore, the goal of the present work was to obtain in vivo pharmacokinetic data in a murine model and to identify the NCS‐382 metabolites formed by mouse and human. NCS‐382 single‐dose mouse pharmacokinetics were established following an intraperitoneal injection (100, 300, and 500 mg/kg body weight) and metabolite identification was conducted using HPLC‐MS/MS. Kinetic enzyme assays employed mouse and human liver microsomes. Upon gaining an understanding of the NCS‐382 clearance mechanisms, a chemical inhibitor was used to increase NCS‐382 brain exposure in a pharmacokinetic/pharmacodynamic study. Two major metabolic pathways of NCS‐382 were identified as dehydrogenation and glucuronidation. The K_m for the dehydrogenation pathway was determined in mouse (K_m = 29.5 ± 10.0 μmol/L) and human (K_m = 12.7 ± 4.8 μmol/L) liver microsomes. Comparable parameters for glucuronidation were >100 μmol/L in both species. Inhibition of NCS‐382 glucuronidation, in vivo, by diclofenac resulted in increased NCS‐382 brain concentrations and protective effects in gamma‐butyrolactone‐treated mice. These initial evaluations of NCS‐382 pharmacokinetics and metabolism inform the development of NCS‐382 as a potential therapy for conditions of GHB elevation (including acute intoxication & SSADHD).

Monocarboxylate transporter 1 is up-regulated in Caco-2 cells by the methionine precursor DL-2-hydroxy-(4-methylthio)butanoic acid

The methionine precursor, DL-2-hydroxy-(4-methylthio)butanoic acid (HMTBA), is a synthetic source of dietary methionine, which is widely used as a poultry nutritional supplement. In the intestinal epithelium, HMTBA transport across the apical membrane is mediated by monocarboxylate transporter 1 (MCT1). The first step in biological utilisation of this methionine precursor is the stereospecific conversion of HMTBA to the corresponding keto acid. In the present study, the regulation of trans-epithelial HMTBA transport was investigated in Caco-2 cell monolayers. Differentiated Caco-2 cells were maintained under control conditions (apical compartment: 0.2 mmol/L L-methionine) or in a HMTBA-enriched medium (2 mmol/L HMTBA). The effect of culture on HMTBA transport was evaluated from apical and basolateral kinetic parameters. MCT1 and MCT4 immuno-localisation and gene expression were investigated by confocal microscopy and real-time quantitative RT-PCR, respectively. The results indicated that apical MCT1 was up-regulated by exposure to HMTBA (1.4-fold increase in Vmax without changes in Km). Moreover, total monolayer MCT1 immunoreactivity increased 1.8-fold in HMTBA-supplemented cultures, this effect mainly being localised at the apical membrane. Functional and immuno-localisation data suggest involvement of MCT1 and MCT4 in basolateral HMTBA transport, although, in this case, no effect was observed for HMTBA-enrichment. Molecular analysis confirmed MCT1 mRNA up-regulation (1.8-fold), with no effect on MCT4 mRNA expression. Thus, exposure to HMTBA up-regulates the trans-epithelial transport of this methionine precursor by increasing the expression and the transport capacity of apical MCT1.

Monocarboxylate 4 mediated butyrate transport in a rat intestinal epithelial cell line

BACKGROUND

Short chain fatty acids (SCFA) are absorbed by carrier mediated uptake in the small intestine by pH-dependent SCFA/HCO3 (-) exchangers on the apical membrane of epithelial cells. Conventional assumption is that MCT1 mediates SCFA/HCO3 (-) exchange in the intestine. Further, due to the presence of multiple such anion exchangers, the identity of the intestinal SCFA/HCO3 (-) has been controversial.

AIMS

The aim of this study was to determine the identities of the butyrate transporter in the intestinal epithelial cells (IEC-18).

METHODS

IEC-18 cells were treated with specific siRNAs for MCT1 and MCT4, and butyrate and lactate uptake studies were performed.

RESULTS

Alpha-cyano-4-hydroxycinnamic acid inhibited lactate uptake but not butyrate uptake in IEC-18 cells, indicating that these two substrates are transported via two different transporter systems. MCT1 siRNA treatment abolished both MCT1 mRNA by more than 95 % and protein expression by 83 % as evidenced by RTQ-PCR and western blotting experiments. However, MCT1 siRNA treatment inhibited butyrate uptake upto 24 %, whereas it inhibited lactate uptake significantly by 70 %. Treatment with MCT4 siRNA inhibited MCT4 mRNA expression by 75 % and protein expression by 85 % in these cells. MCT4 siRNA inhibited butyrate uptake by 40 %. Further, several non-steroidal anti-inflammatory drugs (NSAIDs) are transported by the butyrate transporter. Finally, MCT4 siRNA inhibited salicylate uptake by 27 % indicating direct evidence for the transport of salicylate by MCT4.

CONCLUSIONS

These data indicate that MCT1 is the high affinity lactate transporter and MCT4 is the high affinity butyrate transporter in the intestinal epithelial cell line IEC-18.

Oral drug delivery utilizing intestinal OATP transporters

Transporters play important roles in tissue distribution and urinary- and biliary-excretion of drugs and transporter molecules involved in those processes have been elucidated well. Furthermore, an involvement of efflux transporters such as P-glycoproteins, multidrug resistance associated protein 2, and breast cancer resistance protein as the intestinal absorption barrier and/or intestinal luminal secretion mechanisms has been demonstrated. However, although there are many suggestions for the contribution of uptake/influx transporters in intestinal absorption of drugs, information on the transporter molecules responsible for the intestinal absorptive process is limited. Among them, most studied absorptive drug transporter is peptide transporter PEPT1. However, utilization of PEPT1 for oral delivery of drugs may not be high due to the chemical structural requirement of PEPT1 limited to peptide-mimetics. Recently, organic anion transporting polypeptide (OATP) family such as OATP1A2 and OATP2B1 has been suggested to mediate intestinal absorption of several drugs. Since OATPs exhibit species difference in expressed tissues and functional properties between human and animals, human studies are essential to clarify the intestinal absorption mechanisms of drugs via OATPs. Recent pharmacogenomic studies demonstrated that OATP2B1 is involved in the drug absorption in human. In addition, information of drug-juice interaction in the intestine also uncovered the contribution of OATP1A2 and OATP2B1 in drug absorption. Since OATP1A2 and OATP2B1 exhibit broader substrate selectivity compared with PEPT1, their potential to be applied for oral delivery should be high. In this review, current understanding of characteristics and contribution as the absorptive transporters of OATPs in small intestine in human is described. Now, it is getting clearer that OATPs have significant roles in intestinal absorption of drugs, therefore, there are higher possibility to utilize OATPs as the tools for oral delivery.

The SLC16A family of monocarboxylate transporters (MCTs)--physiology and function in cellular metabolism, pH homeostasis, and fluid transport

The SLC16A family of monocarboxylate transporters (MCTs) is composed of 14 members. MCT1 through MCT4 (MCTs 1-4) are H(+)-coupled monocarboxylate transporters, MCT8 and MCT10 transport thyroid hormone and aromatic amino acids, while the substrate specificity and function of other MCTs have yet to be determined. The focus of this review is on MCTs 1-4 because their role in lactate transport is intrinsically linked to cellular metabolism in various biological systems, including skeletal muscle, brain, retina, and testis. Although MCTs 1-4 all transport lactate, they differ in their transport kinetics and vary in tissue and subcellular distribution, where they facilitate "lactate-shuttling" between glycolytic and oxidative cells within tissues and across blood-tissue barriers. However, the role of MCTs 1-4 is not confined to cellular metabolism. By interacting with bicarbonate transport proteins and carbonic anhydrases, MCTs participate in the regulation of pH homeostasis and fluid transport in renal proximal tubule and corneal endothelium, respectively. Here, we provide a comprehensive review of MCTs 1-4, linking their cellular distribution to their functions in various parts of the human body, so that we can better understand the physiological roles of MCTs at the systemic level.

Nicotinate uptake by two kinetically distinct Na÷-dependent carrier-mediated transport systems in the rat small intestine

Recent studies have identified monocarboxylate transporter 1 (MCT1), sodium-coupled monocarboxylate transporter 1 (SMCT1) and SMCT2 as those that may be involved in the carrier-mediated intestinal absorption of nicotinate, but their roles have not been fully clarified yet. To address the issue, we examined the uptake of nicotinate in the rat small intestine by using everted tissue sacs. The uptake of nicotinate was Na⁺-dependent and saturable at pH 7.4 in both the jejunum and ileum. The saturable transport consisted of a single component with the Michaelis constant (K(m)) of 1.18 mM in the jejunum, while in the ileum it consisted of the high and the low affinity components with the K(m) values of 8.62 µM and 2.36 mM, respectively, and the latter was prevailing in transport capacity and similar to the jejunal transport component. Nicotinate uptake activity attributable to a H⁺-dependent transporter like MCT1 was, however, only minimal in the two intestinal sites. These results suggest that a low affinity type of SMCT2-like transporter would be in operation with high capacity throughout the small intestine, playing the role as the major intestinal nicotinate uptake transporter, and a high affinity type of SMCT1-like transporter would be additionally in operation in the ileum.

Hijacking solute carriers for proton-coupled drug transport

The physiological role of mammalian solute carrier (SLC) proteins is to mediate transmembrane movement of electrolytes, nutrients, micronutrients, vitamins, and endogenous metabolites from one cellular compartment to another. Many transporters in the small intestine, kidney, and solid tumors are H(+)-coupled, driven by local H(+)-electrochemical gradients, and transport numerous drugs. These transporters include PepT1 and PepT2 (SLC15A1/2), PCFT (SLC46A1), PAT1 (SLC36A1), OAT10 (SLC22A13), OATP2B1 (SLCO2B1), MCT1 (SLC16A1), and MATE1 and MATE2-K (SLC47A1/2).

Physiological and cellular aspects of phytotoxicity tolerance in plants: the role of membrane transporters and implications for crop breeding for waterlogging tolerance

Waterlogging affects large areas of agricultural land, resulting in severe economic penalties because of massive losses in crop production. Traditionally, plant breeding for waterlogging tolerance has been based on the field assessment of a range of agronomic and morphological characteristics. This review argues for a need to move towards more physiologically based approaches by targeting specific cellular mechanisms underling key components of waterlogging tolerance in plants. Also, while the main focus of researchers was predominantly on plant anoxia tolerance, less attention was given to plant tolerance to phytotoxins under waterlogged conditions. This paper reviews the production of major elemental and organic phytotoxins in waterlogged soils and describes their adverse effects on plant performance. The critical role of plasma membrane transporters in plant tolerance to secondary metabolite toxicity is highlighted, and ionic mechanisms mediating the this tolerance are discussed. A causal link between the secondary metabolite-induced disturbances to cell ionic homeostasis and programmed cell death is discussed, and a new ethylene-independent pathway for aerenchyma formation is put forward. It is concluded that plant breeding for waterlogging tolerance may significantly benefit from targeting mechanisms of tolerance to phytotoxins.

Transepithelial transport and intraepithelial metabolism of short-chain fatty acids (SCFA) in the porcine proximal colon are influenced by SCFA concentration and luminal pH

Short-chain fatty acids (SCFA) are end products of bacterial fermentation in the colon and cecum of monogastric animals. As SCFA serve as relevant energy suppliers for colonocytes and various tissues, it is important to reveal fundamental mechanistic characteristics of their transepithelial transport subjected to transient variations of fermentations rates. We performed Ussing chamber studies with porcine (Sus scrofa) colon epithelium under physiological conditions and examined individual mucosal disappearance, metabolized loss, tissue concentrations and serosal release of acetate, propionate and butyrate by gas chromatography. Reduction of initial SCFA concentrations from 80 to 40 mmol/L resulted in diminished absolute flux rates, but the relative proportions of mucosal disappearance and intracellular metabolization of individual SCFA were slightly enhanced. Simulation of high fermentation rates by lowering the mucosal pH induced an increase in mucosal disappearance and serosal release of all SCFA, while their tissue contents trended to lower levels. With respect to the metabolization at lowered pH we found increased acetate concentrations and a decrease of propionate and butyrate. Our data indicate that the colon epithelium possesses a high adaptive capacity to ensure its energetic maintenance under various intraluminal fermentation rates by utilizing the unique features of individual SCFA as energy sources.

Use of the Øie-Tozer model in understanding mechanisms and determinants of drug distribution

Volume of distribution (VD) is a key pharmacokinetic property that together with clearance determines the half-life or residence time of drug in the body. It is commonly expressed as steady-state volume of distribution VD(ss) with a physiological basis for its understanding developed by Øie and Tozer in 1979. The Øie-Tozer equation uses terms for plasma protein binding (f(up)), tissue binding (f(ut)), and the extravascular/intravascular ratio of albumin as well as constants for the volumes of plasma, extracellular fluid, and tissue. We explored this model using a data set of 553 drugs for which VD(ss) and plasma protein binding were available in humans. Eighteen percent of cases (102 compounds) did not obey the Øie-Tozer model, with the rearranged equation giving an aberrant f(ut) value (f(ut) < 0 or f(ut) > 1), in particular for compounds with VD(ss) < 0.6 l/kg and f(up) > 0.1. Further analysis of this group of compounds revealed patterns in physicochemical attributes with a high proportion exemplified by logP less than 0 (i.e., very hydrophilic), polar surface area >150 A(2), and a difference between logP and logD >2.5. In addition there was a high representation of certain drug classes including anti-infectives as well as neuromuscular blockers and contrast agents. The majority of compounds were also found to have literature evidence, implicating active transport processes in their disposition. This analysis provides some important insights for pharmacokinetic optimization in this particular chemical space, as well as in the application of the Øie-Tozer model for predicting volume of distribution in humans.

Butyrate utilization by the colonic mucosa in inflammatory bowel diseases: a transport deficiency

The short-chain fatty acid butyrate, which is mainly produced in the lumen of the large intestine by the fermentation of dietary fibers, plays a major role in the physiology of the colonic mucosa. It is also the major energy source for the colonocyte. Numerous studies have reported that butyrate metabolism is impaired in intestinal inflamed mucosa of patients with inflammatory bowel disease (IBD). The data of butyrate oxidation in normal and inflamed colonic tissues depend on several factors, such as the methodology or the models used or the intensity of inflammation. The putative mechanisms involved in butyrate oxidation impairment may include a defect in beta oxidation, luminal compounds interfering with butyrate metabolism, changes in luminal butyrate concentrations or pH, and a defect in butyrate transport. Recent data show that butyrate deficiency results from the reduction of butyrate uptake by the inflamed mucosa through downregulation of the monocarboxylate transporter MCT1. The concomitant induction of the glucose transporter GLUT1 suggests that inflammation could induce a metabolic switch from butyrate to glucose oxidation. Butyrate transport deficiency is expected to have clinical consequences. Particularly, the reduction of the intracellular availability of butyrate in colonocytes may decrease its protective effects toward cancer in IBD patients.

Monocarboxylate transporter-mediated transport of gamma-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 gamma-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: K(m) = 17.6 +/- 10.5 mM, V(max) = 17.3 +/- 11.7 nmol/min/mg, and P = 0.38 +/- 0.15 microl/min/mg; and d-lactate: K(m) = 6.0 +/- 2.9 mM, V(max) = 35.0 +/- 18.4 nmol/min/mg, and P = 1.3 +/- 0.6 microl/min/mg). The uptake of GHB and d-lactate was significantly decreased by the known MCT inhibitor alpha-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.

Formed and preformed metabolites: facts and comparisons

The administration of metabolites arising from new drug entities is often employed in drug discovery to investigate their associated toxicity. It is expected that administration of metabolites can predict the exposure of metabolites originating from the administration of precursor drug. Whether exact and meaningful information can be obtained from this has been a topic of debate. This communication summarizes observations and theoretical relationships based on physiological modelling for the liver, kidney and intestine, three major eliminating organs/tissues. Theoretical solutions based on physiological modelling of organs were solved, and the results suggest that deviations are expected. Here, examples of metabolite kinetics observed mostly in perfused organs that did not match predictions are provided. For the liver, discrepancies in fate between formed and preformed metabolites may be explained by the heterogeneity of enzymes, the presence of membrane barriers and whether transporters are involved. For the kidney, differences have been attributed to glomerular filtration of the preformed but not the formed metabolite. For the intestine, the complexity of segregated flows to the enterocyte and serosal layers and differences in metabolism due to the route of administration are addressed. Administration of the metabolite may or may not directly reflect the toxicity associated with drug use. However, kinetic data on the preformed metabolite will be extremely useful to develop a sound model for modelling and simulations; in-vitro evidence on metabolite handling at the target organ is also paramount. Subsequent modelling and simulation of metabolite data arising from a combined model based on both drug and preformed metabolite data are needed to improve predictions on the behaviours of formed metabolites.

Dietary pectin up-regulates monocaboxylate transporter 1 in the rat gastrointestinal tract

This work was undertaken to study the effect of pectin feeding on the expression level, cellular localization and functional activity of monocarboxylate transporter 1 (MCT1) in the gastrointestinal tract of rats. The results indicated that MCT1 protein level was significantly increased along the entire length of the gastrointestinal tract of pectin-fed rats in comparison with control animals. Immunohistochemical analysis revealed an increase in MCT1 in the stratified squamous epithelia of the forestomach as well as in the basolateral membranes of the cells lining the gastric pit of the glandular stomach of pectin-fed rats when compared with control animals. The parietal cells, which showed barely any or no detectable MCT1 in the control group, exhibited a strong intensity of MCT1 on the basolateral membranes in pectin-fed rats. In the small intestine of pectin-fed rats, strong immunopositivity for MCT1 was detected in the brush border and basolateral membranes of the absorptive enterocytes lining the entire villi, while in control rats, weak reactivity was detected on the brush border membrane in a few absorptive enterocytes in the villus tip. In the large intestine of control animals, MCT1 was detected on the basolateral membranes of the epithelia lining the caecum and colon. This staining intensity was markedly increased in pectin-fed rats, along with the appearance of strong reactivity for MCT1 on the apical membranes of the surface and crypt epithelia of caecum and colon. Our results also showed that MCT1 co-localizes with its chaperone, basigin (CD147), in the rat gastrointestinal tract, and that the pectin feeding increased the expression of CD147. In vivo functional studies revealed an enhanced acetate absorption in the colon of pectin-fed in comparison with control animals. We conclude that MCT1 is up-regulated along the gastrointestinal tract of pectin-fed rats, which might represent an adaptive response to the increased availability of its substrates.

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.

Safety testing of metabolites: Expectations and outcomes

Metabolites arising from chemical entities, old or new, are often mediators of toxicity. Frequently, metabolites are investigated in test animals, with the expectation that the resultant toxicity or activity will mimic the exposure of their formed counterparts. This communication described observations that showed discrepant kinetics between formed and preformed metabolites in the liver, intestine, and kidney, major drug removal organs. Differences in the observed areas under the curve (AUCs) or the extraction ratios (Es) of formed and preformed metabolites in the liver had been attributed to zonal, enzyme heterogeneity, membrane barriers, or transporters. Preformed and formed metabolite also differed in their handling by the kidney; only the preformed and not the formed metabolite would be filtered. In the intestine, differences in the absorption of the precursor and the metabolite and the flow pattern in the intestine would bring about discrepancy in the time-courses of the formed vs. preformed metabolites. Analytical solutions of the AUCs of the metabolites and extraction ratios, based on physiological modeling of the liver, kidney, and intestine, showed that the AUC of the preformed, administered metabolite was dependent only on metabolite parameters, whereas the AUC of the formed metabolite was modulated additionally by the metabolic, secretory and intestinal absorptive intrinsic clearances of the precursor drug. Hence, administration of the synthetic metabolite would not reflect the toxicity associated with the metabolite formed via bioactivation. However, data on preformed metabolite may be used for simultaneous fitting by a combined model of drug and metabolite. Such a strategy is shown to be successful in risk assessment of environmental chemicals. Upon refinement of the resultant model with data on metabolite transport and handling by modeling and simulations, the resultant model would be more robust to provide improved predictions on metabolite toxicity pursuant to drug administration.

Pharmacokinetic interaction between the flavonoid luteolin and gamma-hydroxybutyrate in rats: potential involvement of monocarboxylate transporters

Monocarboxylate transporter 1 (MCT1) has been previously reported as an important determinant of the renal reabsorption of the drug of abuse, gamma-hydroxybutyrate (GHB). Luteolin is a potent MCT1 inhibitor, inhibiting the uptake of GHB with an IC(50) of 0.41 microM in MCT1-transfected MDA-MB231 cells. The objectives of this study were to characterize the effects of luteolin on GHB pharmacokinetics and pharmacodynamics in rats, and to investigate the mechanism of the interaction using model-fitting methods. GHB (400 and 1,000 mg/kg) and luteolin (0, 4 and 10 mg/kg) were administered to rats via iv bolus doses. The plasma or urine concentrations of luteolin and GHB were determined by HPLC and LC/MS/MS, respectively. The pharmacodynamic parameter sleep time in rats after GHB administration was recorded. A pharmacokinetic model containing capacity-limited renal reabsorption and metabolic clearance was constructed to characterize the in vivo interaction. Luteolin significantly decreased the plasma concentration and AUC, and increased the total and renal clearances of GHB. Moreover, luteolin significantly shortened the duration of GHB (1,000 mg/kg)-induced sleep in rats (161 +/- 16, 131 +/- 14 and 121 +/- 5 min for control, luteolin 4 and 10 mg/kg groups, respectively, p < 0.01). An uncompetitive inhibition model, with an inhibition constant of 1.1 microM, best described the in vivo pharmacokinetic interaction. The results of this study indicated that luteolin significantly altered the pharmacokinetics of GHB by inhibiting its MCT1-mediated transport. The interaction between luteolin and GHB may offer a potential clinical detoxification strategy to treat GHB overdoses.

Involvement of organic anion transport system in transdermal absorption of flurbiprofen

We previously demonstrated that transdermal permeation of flurbiprofen is mediated by a nonlinear transport mechanism(s). Here, we aimed to characterize this transport mechanism by employing an Ussing-type chamber method with tape-stripped hairless mouse skin. Transdermal permeation of [(3)H]flurbipofen was vectorial, saturable and energy-dependent, suggesting the involvement of a carrier-mediated transport system. Transdermal permeation and uptake from the epidermal side of [(3)H]flurbiprofen were inhibited by various nonsteroidal anti-inflammatory drugs (NSAIDs). The inhibitory potency did not correlate well with lipophilicity; anionic NSAIDs tended to be more potent inhibitors than non-anionic NSAIDs. The inhibition profile of both [(3)H]flurbiprofen permeation and uptake, and the Michaelis constants, were similar for a given anionic compound. These results suggest that an organic anion transport system is involved in flurbiprofen uptake from the epidermal side during the process of transdermal absorption. Efflux of [(3)H]flurbiprofen from the skin to the epidermal side, but not to the hypodermal side, increased in the presence of flurbiprofen or several anionic compounds. Such trans-stimulation may suggest the involvement of an organic anion exchanger system. Organic anion transporter 2 (OAT2) is a candidate for the exchanger involved in uptake and/or efflux of flurbiprofen in the skin.

Expression of monocarboxylate transporter 1 (MCT1) in the dog intestine

In this study, the expression and distribution of monocarboxyolate transporter 1 (MCT1) along the intestines (duodenum, jejunum, ileum, cecum, colon and rectum) of dogs were investigated at both the mRNA and protein levels. The expression of MCT1 protein and its distribution were confirmed by Western blotting and immunohistochemical staining using the antibody for MCT1. We identified mRNA coding for MCT1 and a 43-kDa band of MCT1 protein in all regions from the duodenum to the rectum. Immunoreactive staining for MCT1 was also observed in epithelial cells throughout the intestines. MCT1 immunoreactivity was greater in the large intestine than in the small intestine. MCT1 protein was predominantly expressed on the basolateral membranes along intestinal epithelial cells, suggesting that MCT1 may play an important role in lactate efflux and transport of short-chain fatty acids (SCFAs) to the bloodstream across the basolateral membranes of the dog intestine.

H+-coupled nutrient, micronutrient and drug transporters in the mammalian small intestine

The H(+)-electrochemical gradient was originally considered as a driving force for solute transport only across cellular membranes of bacteria, plants and yeast. However, in the mammalian small intestine, a H(+)-electrochemical gradient is present at the epithelial brush-border membrane in the form of an acid microclimate. Over recent years, a large number of H(+)-coupled cotransport mechanisms have been identified at the luminal membrane of the mammalian small intestine. These transporters are responsible for the initial stage in absorption of a remarkable variety of essential and non-essential nutrients and micronutrients, including protein digestion products (di/tripeptides and amino acids), vitamins, short-chain fatty acids and divalent metal ions. Proton-coupled cotransporters expressed at the mammalian small intestinal brush-border membrane include: the di/tripeptide transporter PepT1 (SLC15A1); the proton-coupled amino-acid transporter PAT1 (SLC36A1); the divalent metal transporter DMT1 (SLC11A2); the organic anion transporting polypeptide OATP2B1 (SLC02B1); the monocarboxylate transporter MCT1 (SLC16A1); the proton-coupled folate transporter PCFT (SLC46A1); the sodium-glucose linked cotransporter SGLT1 (SLC5A1); and the excitatory amino acid carrier EAAC1 (SLC1A1). Emerging research demonstrates that the optimal intestinal absorptive capacity of certain H(+)-coupled cotransporters (PepT1 and PAT1) is dependent upon function of the brush-border Na(+)-H(+) exchanger NHE3 (SLC9A3). The high oral bioavailability of a large number of pharmaceutical compounds results, in part, from absorptive transport via the same H(+)-coupled cotransporters. Drugs undergoing H(+)-coupled cotransport across the intestinal brush-border membrane include those used to treat bacterial infections, hypercholesterolaemia, hypertension, hyperglycaemia, viral infections, allergies, epilepsy, schizophrenia, rheumatoid arthritis and cancer.

Monocarboxylate transporter (MCT) mediates the transport of gamma-hydroxybutyrate in human kidney HK-2 cells

PURPOSE

Previous studies in our laboratory have suggested that GHB may undergo renal reabsorption mediated by monocarboxylic acid transporters (MCT). The objectives of this study were to characterize the renal transport of GHB using HK-2 cells and the role of MCT in the renal transport of GHB.

MATERIALS AND METHODS

Western blot was used to detect the protein expression of MCT1, 2, and 4. Cellular uptake and directional flux studies were conducted to investigate the transport of GHB and L-lactate. RNA interference assay was used to investigate the involvement of MCT isoforms in the transport of GHB.

RESULTS

MCT1, 2 and 4 were present in HK-2 cells. The cellular uptake of L-lactate and GHB exhibited pH- and concentration-dependence (L-lactate: K (m) of 6.5 +/- 1.1 mM and V (max) of 340 +/- 60 nmol mg(-1)min(-1); GHB: K (m) of 2.07 +/- 0.79 mM, V (max) of 27.6 +/- 9.3 nmol mg(-1)min(-1), and a diffusional clearance of 0.54 +/- 0.15 microl mg(-1)min(-1)), but not sodium-dependence. alpha-Cyano-4-hydroxycinnamate (CHC) competitively inhibited the uptake of GHB and L-lactate with inhibition constants (K (i)) of 0.28 +/- 0.1 mM, and 0.19 +/- 0.03 mM, respectively. Using small-interference RNA (siRNA) for MCT1, the protein expression of MCT1 and the uptake of L-lactate and GHB were significantly decreased. The siRNA treatment of MCT2 in HK-2 cells inhibited the uptake of GHB by 17%, and the siRNA treatment of MCT4 demonstrated no inhibition of GHB uptake. GHB exhibited a directional flux across HK-2 monolayer from apical to basal chambers in the presence of a pH gradient of pH 6.0 to pH 7.4.

CONCLUSION

These data suggest that MCT1 represents an important transporter for GHB transport in renal tubule cells, responsible for the reabsorption of GHB in the kidney.

Cellular expression of monocarboxylate transporters (MCT) in the digestive tract of the mouse, rat, and humans, with special reference to slc5a8

Short-chain fatty acids (SCFA) are monocarboxylates produced by bacterial fermentation that play a crucial role in maintaining homeostasis in the large intestine. Two major transporters for SCFA, monocarboxylate transporter (MCT) and slc5a8 (or SMCT), exist in the digestive tract. The present histochemical study using in situ hybridization and immunohistochemistry revealed the distribution and subcellular localization of the MCT family in the digestive tract of mice, rats, and humans, comparing these with that of slc5a8. The expression of mucosal MCT1 in the mouse and rat was most intense in the cecum, followed by the colon, but low in the stomach and small intestine. Among other MCT subtypes, only MCT2 was detected in the parietal cell region of the gastric mucosa. Slc5a8 had predominant expression sites in the distal half of the large bowel and in the most terminal ileum. The mucosal MCT1 was localized in the basolateral membrane of enterocytes, while slc5a8 was restricted to the apical cell membrane, suggesting the involvement of slc5a8 in the uptake of luminal SCFA, and of MCT1 in the efflux of SCFA and monocarboxylate metabolites towards blood circulation. The large intestine expressed both types of the transporter, but their distribution patterns differed along the longitudinal axis of the intestine and along the perpendicular axis of the mucosa.

Monocarboxylate transporter 1 mediates DL-2-Hydroxy-(4-methylthio)butanoic acid transport across the apical membrane of Caco-2 cell monolayers

The methionine hydroxy analogue DL-2-hydroxy-(4-methylthio)butanoic acid (DL-HMB) is a supplementary source of methionine commonly added to commercial animal diets to satisfy the total sulfur amino acid requirement. In this study, we characterized DL-HMB transport across the apical membrane of Caco-2 cells to identify the transport mechanism involved in the intestinal absorption of this methionine source. DL-HMB transport induced a significant decrease in intracellular pH (pH(i)) and was inhibited in the presence of the protonophore carbonyl cyanide 4-(trifluoromethoxy)-phenylhydrazone. Moreover, both Na(+) removal and 5-(N-ethyl-N-isopropyl)amiloride, an inhibitor of apical Na(+)/H(+) exchanger (NHE3), significantly reduced substrate uptake and pH(i) recovery, suggesting cooperation between H(+)-dependent DL-HMB transport and NHE3 activity. cis-Inhibition experiments with L-Ala, beta-Ala, D-Pro, betaine, or glycyl-sarcosine excluded the participation of systems proton amino acid transporter 1 and peptide transporter 1. In contrast, alpha-cyano-4-hydroxycinnamate, phloretin, L-lactate, beta-hydroxybutyrate, butyrate, and pyruvate, inhibitors and substrates of monocarboxylate transporter 1 (MCT1), significantly reduced DL-HMB uptake. Dixon plot analysis of L-lactate transport in the presence of DL-HMB revealed a competitive interaction (inhibition constant, 17.5 +/- 0.11 mmol/L), confirming the participation of system MCT1. The kinetics of DL-HMB uptake was described by a model involving passive diffusion and a single low-affinity, high-capacity transport mechanism (K(D), 1.9 nL/microg protein; K(m), 13.1 +/- 0.04 mmol/L; and V(max), 43.6 +/- 0.14 pmol/microg protein) compatible with MCT1 kinetic characteristics. In conclusion, the methionine hydroxy analogue is transported in Caco-2 cell apical membrane by a transport mechanism with functional characteristics similar to those of MCT1.

Characterization of the uptake mechanism for a novel loop diuretic, M17055, in Caco-2 cells: involvement of organic anion transporting polypeptide (OATP)-B

PURPOSE

M17055 is under development as a novel loop diuretic for oral administration. To investigate the molecular mechanism of its gastrointestinal absorption, we initially aimed to clarify the mechanism of uptake of M17055 by Caco-2 cells, focusing on possible involvement of OATP-B (SLCO2B1), which is localized in the apical membranes of human intestinal epithelial cells.

MATERIALS AND METHODS

The uptake of [14C]M17055 by Caco-2 cells cultured on multi-well dishes was measured after cultivation for 14 days. Uptake of [14C]M17055 by HEK293 cells stably expressing OATP-B (HEK293/OATP-B cells) was also examined.

RESULTS

M17055 uptake by Caco-2 cells was saturable, and was inhibited by various organic anions, including other loop diuretics, and several bile acids. Uptake of M17055 by HEK293/OATP-B cells was much higher than that by mock cells. The inhibitory profiles of various organic anions and the estimated Km values for M17055 uptake were similar in Caco-2 and HEK293/OATP-B cells. Moreover, the values of inhibition constants of several inhibitors for M17055 uptake were comparable in the two cell lines.

CONCLUSION

Our data suggest that OATP-B plays a major role in the uptake of the novel loop diuretic M17055 from apical membranes in Caco-2 cells.

Monocarboxylate transporter 1 (MCT1) plays a direct role in short-chain fatty acids absorption in caprine rumen

Despite the importance of short-chain fatty acids (SCFA) in maintaining the ruminant physiology, the mechanism of SCFA absorption is still not fully studied. The goal of this study was to elucidate the possible involvement of monocarboxylate transporter 1 (MCT1) in the mechanism of SCFA transport in the caprine rumen, and to delineate the precise cellular localization and the level of MCT1 protein along the entire caprine gastrointestinal tract. RT-PCR revealed the presence of mRNA encoding for MCT1 in all regions of the caprine gastrointestinal tract. Quantitative Western blot analysis showed that the level of MCT1 protein was in the order of rumen >/= reticulum > omasum > caecum > proximal colon > distal colon > abomasum > small intestine. Immunohistochemistry and immunofluorescence confocal analyses revealed widespread immunoreactive positivities for MCT1 in the caprine stomach and large intestine. Amongst the stratified squamous epithelial cells of the forestomach, MCT1 was predominantly expressed on the cell boundaries of the stratum basale and stratum spinosum. Double-immunofluorescence confocal laser-scanning microscopy confirmed the co-localization of MCT1 with its ancillary protein, CD147 in the caprine gastrointestinal tract. In vivo and in vitro functional studies, under the influence of the MCT1 inhibitors, p-chloromercuribenzoate (pCMB) and p-chloromercuribenzoic acid (pCMBA), demonstrated significant inhibitory effect on acetate and propionate transport in the rumen. This study provides evidence, for the first time in ruminants, that MCT1 has a direct role in the transepithelial transport and efflux of the SCFA across the stratum spinosum and stratum basale of the forestomach toward the blood side.

Transport of gamma-hydroxybutyrate in rat kidney membrane vesicles: Role of monocarboxylate transporters

Intoxication with gamma-hydroxybutyrate (GHB) is associated with coma, seizure, and death; treatment of overdoses is symptomatic. Previous studies in our laboratory have demonstrated that L-lactate and pyruvate treatment can increase the renal clearance of GHB and increase its elimination in rats, suggesting that GHB may undergo renal reabsorption mediated by monocarboxylic acid transporters (MCTs). The goals of this study were to characterize the renal transport of GHB in rats and to determine the role of MCT in its renal transport. Brush-border membrane (BBM) and basolateral membrane (BLM) vesicles were isolated from rat kidney cortex, and the uptake of L-lactate and GHB was characterized. L-Lactate and GHB undergo both pH- and sodium-dependent transport in BBM vesicles and pH-dependent transport in BLM vesicles. A simple Michaelis-Menten equation best described the pH-dependent uptake of GHB in BBM (Km, 8.0 +/- 1.8 mM; Vmax, 838 +/- 45 pmol/mg/s) and in BLM vesicles (Km, 10.5 +/- 2.6 mM; Vmax, 806 +/- 253 pmol/mg/s). mRNA of MCT1 and MCT2 was determined in rat kidney cortex using reverse transcriptase-polymerase chain reaction; using Western blot, the protein expression of MCT1 was present mainly in BLM vesicles, with weak expression in BBM vesicles, whereas that of MCT2 was exclusively in BLM vesicles. Studies with rat MCT1 gene-transfected MDA-MB231 cells demonstrated that GHB was a substrate of MCT1. The data suggest that rat MCT1 may represent an important transporter for GHB in renal tubule cells. This investigation provides evidence for the importance of MCTs in the reabsorption of the monocarboxylic acids l-lactate and GHB in the kidney.

Predominant contribution of organic anion transporting polypeptide OATP-B (OATP2B1) to apical uptake of estrone-3-sulfate by human intestinal Caco-2 cells

Human organic anion transporting polypeptide OATP-B (OATP2B1) is a pH-sensitive transporter expressed in the apical membranes of small intestinal epithelial cells. In this study, we have examined the contribution of OATP-B to the uptake of [3H]estrone-3-sulfate in Caco-2 cells in comparison with those of its homologs OATP-D (OATP3A1) and OATP-E (OATP4A1). Immunocytochemical study revealed that OATP-B is expressed in the apical membranes of Caco-2 cells. The uptake of [3H]estrone-3-sulfate by Caco-2 cells was Na+-independent and inhibited by several organic anions. It showed biphasic saturation kinetics with Km values of 1.81 microM and 1.40 mM. The uptake of [3H]estrone-3-sulfate by human embryonic kidney (HEK) 293 cells stably expressing OATP-B (HEK293/OATP-B) was also Na+-independent and inhibited by several organic anions. The Km value for estrone-3-sulfate uptake by OATP-B (1.56 microM) was close to that for the high-affinity component observed in Caco-2 cells. The mRNA expression level of OATP-B was higher than that of OATP-D or OATP-E in Caco-2 cells and in human jejunum biopsies from healthy volunteers. The values of [3H]estrone-3-sulfate uptake normalized to OATP-B mRNA expression were similar in Caco-2 cells and HEK293/OATP-B cells. The specific activity of OATP-B per mRNA expression was much higher than that of OATP-D and OATP-E. [3H]Estrone-3-sulfate uptake by membrane vesicles prepared from HEK293/OATP-B cells exhibited an overshoot phenomenon in the presence of an inwardly directed H+ gradient, suggesting that an H+ gradient is the driving force of estrone-3-sulfate transport by OATP-B. These results suggest that OATP-B is predominantly responsible for the apical uptake of estrone-3-sulfate in Caco-2 cells.

Monocarboxylate transporter 1 (MCT1) mediates transport of short-chain fatty acids in bovine caecum

The present study was undertaken to investigate the functional role of monocarboxylate transporter 1 (MCT1) in the ruminant large intestine. Messenger RNA encoding for MCT1 was verified by reverse transcriptase-polymerase chain reaction in caecum, proximal colon and distal colon of adult cattle. Both immunohistochemistry and confocal laser microscopy verified that the MCT1 protein was abundant in the surface epithelium of the large intestine, and the amount decreased from the opening of the crypt to its base. In the immunopositive cells, MCT1 was primarily localized in the basolateral membranes of epithelium lining the large intestine. Western blotting indicated that the levels of MCT1 protein were highest in the caecum, followed by proximal colon and then distal colon. In vitro studies were conducted to elucidate the possible involvement of MCT1 in the transport of short-chain fatty acids (SCFA) across the isolated mucosal sheets of cattle caecum using the Ussing chamber technique. Acetate absorption was found to be pH dependent, and the rate of acetate absorption increased as pH decreased. The serosal application of the MCT1 inhibitor 'p-chloromercuribenzoic acid (pCMB)' significantly reduced the transport of acetate across the caecal epithelium of cows. In addition, the transport of acetate was significantly reduced in the presence of its analogue, propionate, indicating that acetate and propionate compete for binding to the same transporter. The results show that MCT1 is a major route for SCFA efflux across the basolateral membrane of bovine large intestine and that it could play a role in the regulation of intracellular pH.

Role of SLC xenobiotic transporters and their regulatory mechanisms PDZ proteins in drug delivery and disposition

Various types of xenobiotic (or drug) transporters have been recently identified to play important roles as barriers against toxic compounds and influx pumps to take up nutrients into the body. Since those xenobiotic transporters generally have wide range of recognition specificity and accept various types of compounds as substrates, localization and functional expression of such transporters could be one of the critical factors that affect the disposition and subsequent biological activity of therapeutic agents. Identification and characterization of drug transporters have given us a scientific basis for understanding drug delivery and disposition, as well as the molecular mechanisms of drug interaction and inter-individual/inter-species differences. To precisely understand pharmacological roles of the transporters in the body, it is also important to clarify molecular mechanisms involved in regulation of the transporters. As a first step to clarify the regulatory mechanisms that govern cell-surface expression and/or function of these transporters, recent researches have focused on PDZ (PSD-95/Discs-large/ZO-1) binding motif localized on carboxylic terminus of several types of xenobiotic transporters. Most of the transporters showing direct interaction potential with the PDZ domain-containing proteins are expressed on apical membranes in epithelial cells of kidney and/or small intestine, implying that such protein-protein interaction may play a role in apical localization of the transporters. In this mini-review article, we summarize importance of transporters and their regulatory mechanisms in drug delivery and disposition, focusing on several aspects of transporter-mediated drug targeting.

Cloning and functional identification of slc5a12 as a sodium-coupled low-affinity transporter for monocarboxylates (SMCT2)

We report in the present paper, on the isolation and functional characterization of slc5a12, the twelfth member of the SLC5 gene family, from mouse kidney. The slc5a12 cDNA codes for a protein of 619 amino acids. Heterologous expression of slc5a12 cDNA in mammalian cells induces Na+-dependent transport of lactate and nicotinate. Several other short-chain monocarboxylates compete with nicotinate for the cDNA-induced transport process. Expression of slc5a12 in Xenopus oocytes induces electrogenic and Na+-dependent transport of lactate, nicotinate, propionate and butyrate. The substrate specificity of slc5a12 is similar to that of slc5a8, an Na+-coupled transporter for monocarboxylates. However, the substrate affinities of slc5a12 were much lower than those of slc5a8. slc5a12 mRNA is expressed in kidney, small intestine and skeletal muscle. In situ hybridization with sagittal sections of mouse kidney showed predominant expression of slc5a12 in the outer cortex. This is in contrast with slc5a8, which is expressed in the cortex as well as in the medulla. The physiological function of slc5a12 in the kidney is likely to mediate the reabsorption of lactate. In the intestinal tract, slc5a12 is expressed in the proximal parts, whereas slc5a8 is expressed in the distal parts. The expression of slc5a12 in the proximal parts of the intestinal tract, where there is minimal bacterial colonization, suggests that the physiological function of slc5a12 is not to mediate the absorption of short-chain monocarboxylates derived from bacterial fermentation but rather to mediate the absorption of diet-derived short-chain monocarboxylates. Based on the functional and structural similarities between slc5a8 and slc5a12, we suggest that the two transporters be designated as SMCT1 (sodium-coupled monocarboxylate transporter 1) and SMCT2 respectively.

Characterization of the transdermal transport of flurbiprofen and indomethacin

Transdermal permeation of two types of NSAIDs, [(3)H] flurbiprofen and [(14)C] indomethacin, was examined by use of the Ussing-type chamber method. We found that the transdermal permeability in the absorptive direction (P(abs)) of [(3)H] flurbiprofen was significantly higher than that of [(14)C] indomethacin. A lower pH (5.0) on the epidermal side increased the accumulation and the P(abs) of [(3)H] flurbiprofen (18-fold and 50-fold, respectively) and [(14)C] indomethacin (18-fold and 22-fold, respectively), compared with pH 7.4. Coadministration of unlabeled flurbiprofen and indomethacin increased P(abs) of [(3)H] flurbiprofen and [(14)C] indomethacin, respectively, in a concentration-dependent manner. Similar high-affinity transport was also observed in the uptake of [(3)H] flurbiprofen by human epidermal keratinocytes (HEK001 cells). RT-PCR analysis revealed the expression of mRNA of numerous transporters including MRP, OATP, MCT and OCTN family members in hairless mouse skin, human skin and HEK001 cells. These findings support the novel hypothesis that transdermal permeation of NSAIDs is mediated by saturable transport mechanisms, which may be candidates as targets for transdermal delivery of drugs.

Identification and functional characterization of riboflavin transporter in human-derived retinoblastoma cell line (Y-79): mechanisms of cellular uptake and translocation

Drug delivery to the retina is a challenging task owing to its complex physiology and presence of the blood-retinal barrier (BRB), which regulates the permeation of substances from blood into the retina. Transporter-targeted drug delivery has become a clinically significant drug-delivery approach for enhancing the bioavailability of various drugs. Different nutrient transporters have been reported to be expressed on the retina. Riboflavin (vitamin B2), an essential nutritional vitamin for the development and maintenance of the surface structures and functions of epithelial cells of the ocular tissues, must be acquired from retinal or choroidal blood supply. The uptake mechanism, cellular translocation, and major regulatory pathways of riboflavin uptake into retina are poorly understood. Therefore, the aim of this study was to investigate the presence of a riboflavin transporter and delineate uptake and intracellular trafficking of riboflavin in the human-derived retinoblastoma cell line (Y-79), a model for neural retina. Uptake characteristics of [3H]riboflavin in Y-79 cells were found to be (1) linear with time over 10 min of incubation; (2) temperature- and energy-dependent; (3) sodium, chloride-, and pH-independent; (4) concentration dependence with an apparent K(m) of 19.21 +/- 0.37 nM and V(max) of 6.98 +/- 0.30 pmol/min/mg protein; (5) inhibited by the structural analogs (lumiflavin and lumichrome) but not by the structurally unrelated vitamins; and (6) uptake of [3H]riboflavin was trans-stimulated by the intracellular riboflavin. Neither protein kinase C- nor protein tyrosine kinase-mediated pathways were involved in regulating riboflavin uptake. However, protein kinase A pathway activators (IBMX and forskolin) and inhibitors (H-89) and Ca2+/calmodulin pathways appeared to play important roles in the regulation of riboflavin uptake in Y-79 cells through significant reduction in V(max) (39%) and significant increase in K(m) (112%) of the uptake process. These studies demonstrated, for the first time, the existence of a specialized carrier-mediated system for riboflavin uptake in human-derived retinoblastoma cells. The system appears to be regulated by protein kinase A and Ca2+/calmodulin pathways. Being a high-affinity low-capacity transport system, the presence of this transporter on the retina may be suitable for the design of transporter-targeted prodrugs to achieve enhanced permeability for highly potent, but poorly bioavailable, compounds where a small increase in the bioavailability could result in a significant increase in therapeutic response.