Possible role of anion exchanger AE2 as the intestinal monocarboxylic acid/anion antiporter

Reassessment of the Transport Mechanism of the Human Zinc Transporter SLC39A2

The human zinc transporter SLC39A2, also known as ZIP2, was shown to mediate zinc transport that could be inhibited at pH <7.0 and stimulated by HCO₃^-, suggesting a Zn²⁺/HCO₃^- cotransport mechanism [Gaither, L. A., and Eide, D. J. (2000) J. Biol. Chem. 275, 5560-5564]. In contrast, recent experiments in our laboratory indicated that the functional activity of ZIP2 increases at acidic pH [Franz, M. C., et al. (2014) J. Biomol. Screening 19, 909-916]. The study presented here was therefore designed to reexamine the findings about the pH dependence and to extend the functional characterization of ZIP2. Our current results show that ZIP2-mediated transport is modulated by extracellular pH but independent of the H⁺ driving force. Also, in our experiments, ZIP2-mediated transport is not modulated by extracellular HCO₃^-. Moreover, a high extracellular [K⁺], which induces depolarization, inhibited ZIP2-mediated transport, indicating that the transport mechanism is voltage-dependent. We also show that ZIP2 mediates the uptake of Cd²⁺ ( K_m ∼ 1.57 μM) in a pH-dependent manner ( K_H⁺ ∼ 66 nM). Cd²⁺ transport is inhibited by extracellular [Zn²⁺] (IC₅₀ ∼ 0.32 μM), [Cu²⁺] (IC₅₀ ∼ 1.81 μM), and to a lesser extent [Co²⁺], but not by [Mn²⁺] or [Ba²⁺]. Fe²⁺ is not transported by ZIP2. Accordingly, the substrate selectivity of ZIP2 decreases in the following order: Zn²⁺ > Cd²⁺ ≥ Cu²⁺ > Co²⁺. Altogether, we propose that ZIP2 is a facilitated divalent metal ion transporter that can be modulated by extracellular pH and membrane potential. Given that ZIP2 expression has been reported in acidic environments [Desouki, M. M., et al. (2007) Mol. Cancer 6, 37; Inoue, Y., et al. (2014) J. Biol. Chem. 289, 21451-21462; Tao, Y. T., et al. (2013) Mol. Biol. Rep. 40, 4979-4984], we suggest that the herein described H⁺-mediated regulatory mechanism might be important for determining the velocity and direction of the transport process.

CD38 produces nicotinic acid adenosine dinucleotide phosphate in the lysosome

Nicotinic acid adenosine dinucleotide phosphate (NAADP) is a Ca²⁺-mobilizing second messenger that regulates a wide range of biological activities. However, the mechanism of its biogenesis remains controversial. CD38 is the only enzyme known to catalyze NAADP synthesis from NADP and nicotinic acid. CD38-mediated catalysis requires an acidic pH, suggesting that NAADP may be produced in acidic endolysosomes, but this hypothesis is untested. In this study, using human cell lines, we specifically directed CD38 to the endolysosomal system and assessed cellular NAADP production. First, we found that nanobodies targeting various epitopes on the C-terminal domain of CD38 could bind to cell surface-localized CD38 and induce its endocytosis. We also found that CD38 internalization occurred via a clathrin-dependent pathway, delivered CD38 to the endolysosome, and elevated intracellular NAADP levels. We also created a CD38 variant for lysosome-specific expression, which not only withstood the degradative environment in the lysosome, but was also much more active than WT CD38 in elevating cellular NAADP levels. Supplementing CD38-expressing cells with nicotinic acid substantially increased cellular NAADP levels. These results demonstrate that endolysosomal CD38 can produce NAADP in human cells. They further suggest that CD38's compartmentalization to the lysosome may allow for its regulation via substrate access, rather than enzyme activation, thereby providing a reliable mechanism for regulating cellular NAADP production.

Capturing intracellular pH dynamics by coupling its molecular mechanisms within a fully tractable mathematical model

We describe the construction of a fully tractable mathematical model for intracellular pH. This work is based on coupling the kinetic equations depicting the molecular mechanisms for pumps, transporters and chemical reactions, which determine this parameter in eukaryotic cells. Thus, our system also calculates the membrane potential and the cytosolic ionic composition. Such a model required the development of a novel algebraic method that couples differential equations for slow relaxation processes to steady-state equations for fast chemical reactions. Compared to classical heuristic approaches based on fitted curves and ad hoc constants, this yields significant improvements. This model is mathematically self-consistent and allows for the first time to establish analytical solutions for steady-state pH and a reduced differential equation for pH regulation. Because of its modular structure, it can integrate any additional mechanism that will directly or indirectly affect pH. In addition, it provides mathematical clarifications for widely observed biological phenomena such as overshooting in regulatory loops. Finally, instead of including a limited set of experimental results to fit our model, we show examples of numerical calculations that are extremely consistent with the wide body of intracellular pH experimental measurements gathered by different groups in many different cellular systems.

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.

Identifying an uptake mechanism for the antiepileptic and bipolar disorder treatment valproic acid using the simple biomedical model Dictyostelium

Valproic acid (VPA) is the most highly prescribed epilepsy treatment worldwide and is also used to prevent bipolar disorder and migraine. Surprisingly, very little is known about its mechanisms of cellular uptake. Here, we employ a range of cellular, molecular and genetic approaches to characterize VPA uptake using a simple biomedical model, Dictyostelium discoideum. We show that VPA is taken up against an electrochemical gradient in a dose-dependent manner. Transport is protein-mediated, dependent on pH and the proton gradient and shows strong substrate structure specificity. Using a genetic screen, we identified a protein homologous to a mammalian solute carrier family 4 (SLC4) bicarbonate transporter that we show is involved in VPA uptake. Pharmacological and genetic ablation of this protein reduces the uptake of VPA and partially protects against VPA-dependent developmental effects, and extracellular bicarbonate competes for VPA uptake in Dictyostelium. We further show that this uptake mechanism is likely to be conserved in both zebrafish (Danio rerio) and Xenopus laevis model systems. These results implicate, for the first time, an uptake mechanism for VPA through SLC4-catalysed activity.

Kinetic analysis of butyrate transport in human colon adenocarcinoma cells reveals two different carrier-mediated mechanisms

Butyrate has antitumorigenic effects on colon cancer cells, inhibits cell growth and promotes differentiation and apoptosis. These effects depend on its intracellular concentration, which is regulated by its transport. We have analysed butyrate uptake kinetics in human colon adenocarcinoma cells sensitive to the apoptotic effects of butyrate (BCS-TC2, Caco-2 and HT-29), in butyrate-resistant cells (BCS-TC2.BR2) and in normal colonic cells (FHC). The properties of transport were analysed with structural analogues, specific inhibitors and different bicarbonate and sodium concentrations. Two carrier-mediated mechanisms were detected: a low-affinity/high-capacity (K(m)=109+/-16 mM in BCS-TC2 cells) anion exchanger and a high-affinity/low-capacity (K(m)=17.9+/-4.0 microM in BCS-TC2 cells) proton-monocarboxylate co-transporter that was energy-dependent and activated via PKCdelta (protein kinase Cdelta). All adenocarcinoma cells analysed express MCT (monocarboxylate transporter) 1, MCT4, ancillary protein CD147 and AE2 (anion exchanger 2). Silencing experiments show that MCT1, whose expression increases with butyrate treatment in butyrate-sensitive cells, plays a key role in high-affinity transport. Low-affinity uptake was mediated by a butyrate/bicarbonate antiporter along with a possible contribution of AE2 and MCT4. Butyrate treatment increased uptake in a time- and dose-dependent manner in butyrate-sensitive but not in butyrate-resistant cells. The two butyrate-uptake activities in human colon adenocarcinoma cells enable butyrate transport at different physiological conditions to maintain cell functionality. The high-affinity/low-capacity transport functions under low butyrate concentrations and may be relevant for the survival of carcinoma cells in tumour regions with low glucose and butyrate availability as well as for the normal physiology of colonocytes.

Is once-daily mesalazine equivalent to the currently used twice-daily regimen? A study performed in 30 healthy volunteers

A randomized, 2-way, crossover study was conducted in 30 volunteers to compare the pharmacokinetic profile of a new once-daily dosing regimen of mesalazine (1 x 4 g/d) with the current twice-daily dosage (2 x 2 g/d) used in many European countries. The 2 dosages were administrated orally for 8 days, separated by a 2-week washout. Plasma concentrations of mesalazine and N-acetyl-mesalazine were determined on days 1 and 8 by a validated high-performance liquid chromatography method and C(max), t(max), and AUCs calculated. The bioequivalence was obtained for a 90% confidence interval of the AUC(0-24h) ratio (test/reference) for mesalazine and N-acetyl-mesalazine on days 1 and 8, within the range of 0.80 to 1.25. The bioequivalence was demonstrated on day 1 for mesalazine and N-acetyl-mesalazine and on day 8 for mesalazine. As it is desirable to offer patients a preparation with a less frequent administration to enhance compliance, this once-daily regimen may be an attractive dosing option.

Tolbutamide uptake via pH- and membrane-potential-dependent transport mechanism in mouse brain capillary endothelial cell line

The purpose of this study was to investigate the transport mechanism of tolbutamide across the blood-brain barrier (BBB) using MBEC4 cells as an in vitro BBB model.

METHODS

The BBB transport of tolbutamide was studied by using a mouse brain capillary endothelial cell line, MBEC4, cultured on dishes with their luminal membrane facing the culture medium.

RESULTS

The uptake of [14C]tolbutamide by MBEC4 cells was dependent on temperature and energy. The uptake coefficient of [14C]tolbutamide increased markedly with decreasing pH of the external medium from neutral to acidic. Valinomycin and replacement of chloride with sulfate or gluconate significantly increased the initial uptake of [14C]tolbutamide, while replacement with nitrate significantly decreased it. The uptake was significantly reduced by a proton ionophore, FCCP, and an anion-exchange inhibitor, DIDS. The initial uptake of [14C]tolbutamide was saturable with Kt of 0.61+/-0.03 mM (pH 7.4) and 1.76+/-0.19 mM (pH 6.5). At pH 6.5, the initial uptake of [14C]tolbutamide was significantly reduced by several sulfa drugs, salicylic acid, valproic acid and probenecid, and was competitively inhibited by sulfaphenazole (Ki=3.47+/-0.50 mM) and valproic acid (Ki=2.29+/-0.43 mM).

CONCLUSION

These observations indicate the existence of a pH- and membrane-potential-dependent anion exchange and/or proton-cotransport system(s) for concentrative uptake of tolbutamide and sulfa drugs in MBEC4 cells.

Transepithelial Permeation of Tolbutamide across the Human Intestinal Cell Line, Caco-2

Sulfonylurea hypoglycemic agents have interindividual variability in the gastrointestinal absorption rate. However, the absorption mechanism at the intestinal epithelium has not yet been clarified. To elucidate contribution of the specific mechanism for transepithelial transport of sulfonylureas, the apical-to-basolateral and basolateral-to-apical transport studies of tolbutamide were carried out using Caco-2 cell monolayers cultured on the polycarbonate membrane. The transported amounts of the substrate were measured by HPLC to estimate the apparent permeability coefficients (P(app)). In the apical-to-basolateral flux, the transport activity of tolbutamide was facilitated when the pH of the apical medium was more acidic than the basolateral one. ATP-depletion decreased the P(app) of tolbutamide. The kinetic analysis of the permeation rate indicated that the saturable process largely contributed to the tolbutamide flux. The P(app) of tolbutamide was lowered by an ionophore and monocarboxylic acids, while dicarboxylic acids and the inhibitor for the anion exchanger had no effect. In addition, mutual inhibition with benzoic acid was observed in transepithelial transport of tolbutamide. On the other hand, the permeation rate of tolbutamide from the basolateral to apical side was concentration-independent and neither affected by metabolic inhibitors, probenecid nor inhibitors for P-glycoprotein. In conclusion, these results suggest that apical-to-basolateral transport of tolbutamide across the Caco-2 cell monolayers is mediated by the pH-dependent specific system, presumably shared with other organic anions such as benzoic acid.

Involvement of human organic anion transporting polypeptide OATP-B (SLC21A9) in pH-dependent transport across intestinal apical membrane

Some organic anions are absorbed from the gastrointestinal tract through carrier-mediated transport mechanism(s), which may include proton-coupled transport, anion exchange transport, and others. However, the molecular identity of the organic anion transporters localized at the apical membrane of human intestinal epithelial cells has not been clearly demonstrated. In the present study, we focused on human organic anion transporting polypeptide OATP-B and examined its subcellular localization and functionality in the small intestine. Localization of OATP-B was determined by immunohistochemical analysis. Transport properties of estrone-3-sulfate and the 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor pravastatin by OATP-B-transfected human embryonic kidney 293 cells were measured. OATP-B was immunohistochemically localized at the apical membrane of intestinal epithelial cells in humans. Uptake of [3H]estrone-3-sulfate and [14C]pravastatin by OATP-B at pH 5.5 was higher than that at pH 7.4. [3H]Estrone-3-sulfate transport was decreased by pravastatin, aromatic anion compounds, and the anion exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, but not by small anionic compounds, such as lactic acid and acetic acid. The inhibitory effect of pravastatin on the uptake of [3H]estrone-3-sulfate was concentration-dependent, and the IC50 value was 5.5 mM. The results suggested that OATP-B mediates absorption of anionic compounds and its activity may be optimum at the acidic surface microclimate pH of the small intestine. Accordingly, OATP-B plays a role in the absorption of anionic compounds across the apical membrane of human intestinal epithelial cells, although it cannot be decisively concluded that pH-dependent absorption of pravastatin is determined by OATP-B alone.

Search for carrier-mediated transport systems in the rat colon

Several nutrients and drugs, which are known to be absorbed by specific carrier-mediated transport systems in the small intestine, had their transport investigated in the rat colon, by measuring uptake into everted sacs, to find if carrier-mediated transport systems may also be present in the colon. Among those transported by Na+-dependent carriers in the small intestine, D-glucose and taurocholate were found to be transported in an Na+-dependent manner in the colon, while 5-fluorouracil and ascorbate were not. It was also found that the colonic transports of D-glucose and taurocholate were saturable. These results suggest the presence in the colon of Na+-dependent carrier-mediated transport systems for D-glucose and taurocholate, but not for 5-fluorouracil and ascorbate. For nicotinate and methotrexate, which are transported by H+-dependent carriers in the small intestine, their transport was elevated at a lower pH (5.0) than the pH 7.4 in the colon, but not saturable. Therefore, the elevated transport of these acidic compounds may be explained by an increase in passive flux due to an increase in the fraction of the unionized and/or neutral forms, without postulating the presence of H+-dependent carrier-mediated transport systems in the colon. The transport activity of the suggested colonic transport systems for D-glucose and taurocholate was much lower than those of their respective counterparts in the small intestine. However, it may be possible to use them for oral drug delivery via the colon. Their physiological roles would also be of interest.

[Biopharmaceutical studies on molecular mechanisms of membrane transport]

By incorporating the transporter-mediated or receptor-mediated transport process in physiologically based pharmacokinetic models, we succeeded in the quantitative prediction of plasma and tissue concentrations of beta-lactam antibiotics, insulin, pentazocine, quinolone antibacterial agents, and inaperizone and digoxin. The author's research on transporter-mediated pharmacokinetics focuses on the molecular and functional characteristics of drug transporters such as oligopeptide transporter, monocarboxylic acid transporter, anion antiporter, organic anion transporters, organic cation/carnitine transporters (OCTNs), and the ATP-binding cassette transporters P-glycoprotein and MRP2. We have successfully demonstrated that these transporters play important roles in the influxes and/or effluxes of drugs in intestinal and renal epithelial cells, hepatocytes, and brain capillary endothelial cells that form the blood-brain barrier. In the systemic carnitine deficiency (SCD) phenotype mouse model, juvenile visceral steatosis (jvs) mouse, a mutation in the OCTN2 gene was found. Furthermore, several types of mutation in human SCD patients were found, demonstrating that OCTN2 is a physiologically important carnitine transporter. Interestingly, OCTNs transport carnitine in a sodium-dependent manner and various cationic drugs transport it in a sodium-independent manner. OCTNs are thought to be multifunctional transporters for the uptake of carnitine into tissue cells and for the elimination of intracellular organic cationic drugs.

Studies on intestinal absorption of sulpiride (1): carrier-mediated uptake of sulpiride in the human intestinal cell line Caco-2

We investigated whether the uptake of a specific antipsychotic agent, sulpiride, in Caco-2 cells is mediated by a carrier-mediated system. Caco-2 cell monolayers were cultured in plastic culture dishes and uptake and efflux studies were conducted. The determination of sulpiride was performed by HPLC. At 37 degrees C, sulpiride uptake in pH 6.0 was twice as much as in pH 7.4. At 4 degrees C, however, no significant difference was observed between pH 6.0 and 7.4. The uptake at 4 degrees C was markedly lower than that obtained at 37 degrees C. The subtraction of the uptake at 4 degrees C from the uptake at 37 degrees C indicated a saturable process, and the result of the Eadie-Hofstee plot analysis indicated that the uptake consists of two or more saturable components. The uptake was significantly inhibited by uncoupler, protonophore, amino acid modifying agent and proteinase. Sulpiride efflux was temperature-dependent and was significantly inhibited by uncoupler and amino acid modifying agent. These findings indicate that sulpiride uptake and efflux in Caco-2 cells are carrier-mediated. Furthermore, the uptake was significantly decreased by some substrates and inhibitors of peptide transporter, PEPT1, and organic cation transporters, OCTN1 and OCTN2, and was significantly increased by preloading with them. The uptake was also significantly increased by a typical substrate of P-glycoprotein. From these findings, we presumed that peptide transporter PEPT1 and organic cation transporters OCTN1 and OCTN2 are involved with this uptake. P-glycoprotein may also contribute to the efflux of sulpiride.

Transporter-mediated Drug Interactions

Since 1994, researchers have isolated various genes encoding transporter proteins involved in drug uptake into and efflux from tissues that play key roles in the absorption, distribution and secretion of drugs in animals and humans. The pharmacokinetic characteristics of drugs that are substrates for these transporters are expected to be influenced by coadministered drugs that work as inhibitors or enhancers of the transporter function. This review deals with recent progress in molecular and functional research on drug transporters, and then with transporter-mediated drug interactions in absorption and secretion from the intestine, secretion from the kidney and liver, and transport across the blood-brain barrier in humans. Although the participation of the particular transporters in observed drug-drug interactions can be difficult to confirm in humans, this review focuses mainly on pharmacokinetic interactions of clinically important drugs.

Intestinal drug efflux: formulation and food effects

The intestine, primarily regarded as an absorptive organ, is also prepared for the elimination of certain organic acids, bases and neutral compounds depending on their affinity to intestinal carrier systems. Several of the transport systems known to mediate efflux in the major clearing organs--liver and kidney--are also expressed in the intestine. Examples of secretory transporters in the intestine are P-glycoprotein, members of the multidrug resistance associated protein family, breast cancer resistance protein, organic cation transporters and members of the organic anion polypeptide family. In this communication, the P-glycoprotein mediated intestinal secretion of talinolol, a model compound showing metabolic stability, has been investigated in the jejunum, ileum and colon of rat intestine by single-pass perfusion. A model has been developed which demonstrates an increase in carrier-mediated secretion in the order jejunum<ileum<colon. Furthermore, the potency of common excipients in peroral drug products towards inhibition of P-gp mediated secretion has been investigated using a radioligand-binding assay and transport studies in Caco-2 cell monolayers. Finally, evidence is provided which demonstrates that constituents of grapefruit juice not only may influence intestinal drug metabolism, but can also interfere with secretory transport systems, leading to a new and yet undescribed mechanism in drug-food interactions.

Uptake mechanism of valproic acid in human placental choriocarcinoma cell line (BeWo)

Valproic acid is an anticonvulsant widely used for the treatment of epilepsy. However, valproic acid is known to show fetal toxicity, including teratogenicity. In the present study, to elucidate the mechanisms of valproic acid transport across the blood-placental barrier, we carried out transcellular transport and uptake experiments with human placental choriocarcinoma epithelial cells (BeWo cells) in culture. The permeability coefficient of [3H]valproic acid in BeWo cells for the apical-to-basolateral flux was greater than that for the opposite flux, suggesting a higher unidirectional transport in the fetal direction. The uptake of [3H]valproic acid from the apical side was temperature-dependent and enhanced under acidic pH. In the presence of 50 microM carbonyl cyanide p-trifluoromethoxylhydrazone, the uptake of [3H]valproic acid was significantly reduced. A metabolic inhibitor, 10 mM sodium azide, also significantly reduced the uptake of [3H]valproic acid. Therefore, valproic acid is actively transported in a pH-dependent manner on the brush-border membrane of BeWo cells. Kinetic analysis of valproic acid uptake revealed the involvement of a non-saturable component and a saturable component. The Michaelis constant for the saturable transport (K(t)) was smaller under acidic pH, suggesting a proton-linked active transport mechanism for valproic acid in BeWo cells. In the inhibitory experiments, some short-chain fatty acids, such as acetic acid, lactic acid, propanoic acid and butyric acid, and medium-chain fatty acids, such as hexanoic acid and octanoic acid, inhibited the uptake of [3H]valproic acid. The uptake of [3H]valproic acid was also significantly decreased in the presence of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, salicylic acid and furosemide, which are well-known inhibitors of the anion exchange system. Moreover, p-aminohippuric acid significantly reduced the uptake of [3H]valproic acid. These results suggest that an active transport mechanism for valproic acid exists on the brush-border membrane of placental trophoblast cells and operates in a proton-linked manner.

Prodrug based optimal drug delivery via membrane transporter/receptor

The carrier-mediated absorption of drugs and prodrugs across epithelial and endothelial barriers is emerging as a novel trend in biotherapeutics. This review examines the important advances in this field in the past decade. The feasibility of drug absorption of the parent drug or the appropriately modified prodrug via these transporters is discussed in detail. Several successful examples of synthesis of prodrugs recognised by the targeted transporters are described. The applicability of this approach in translocating drugs across the almost impenetrable blood-brain barrier (BBB) has also been examined.

Mechanism(s) of butyrate transport in Caco-2 cells: role of monocarboxylate transporter 1

The short-chain fatty acid butyrate was readily taken up by Caco-2 cells. Transport exhibited saturation kinetics, was enhanced by low extracellular pH, and was Na(+) independent. Butyrate uptake was unaffected by DIDS; however, alpha-cyano-4-hydroxycinnamate and the butyrate analogs propionate and L-lactate significantly inhibited uptake. These results suggest that butyrate transport by Caco-2 cells is mediated by a transporter belonging to the monocarboxylate transporter family. We identified five isoforms of this transporter, MCT1, MCT3, MCT4, MCT5, and MCT6, in Caco-2 cells by PCR, and MCT1 was found to be the most abundant isoform by RNase protection assay. Transient transfection of MCT1, in the antisense orientation, resulted in significant inhibition of butyrate uptake. The cells fully recovered from this inhibition by 5 days after transfection. In conclusion, our data showed that the MCT1 transporter may play a major role in the transport of butyrate into Caco-2 cells.

Anion antiport mechanism is involved in transport of lactic acid across intestinal epithelial brush-border membrane

Intestinal epithelial membrane transport of L-lactic acid was characterized using rabbit jejunal brush-border membrane vesicles (BBMVs). The uptake of L-[(14)C]lactic acid by BBMVs showed an overshoot phenomenon in the presence of outward-directed bicarbonate and/or inward-directed proton gradients. Kinetic analysis of L-[(14)C]lactic acid uptake revealed the involvement of two saturable processes in the presence of both proton and bicarbonate gradients. An arginyl residue-modifying agent, phenylglyoxal, inhibited L-[(14)C]lactic acid transport by the proton cotransporter, but not by the anion antiporter. The initial uptakes of L-[(14)C]lactic acid which are driven by bicarbonate ion and proton gradients were inhibited commonly by monocarboxylic acids and selectively by anion exchange inhibitor 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid and protonophore carbonylcyanide p-trifluoromethoxyphenylhydrazone, respectively. These observations demonstrate that L-lactic acid is transported across the intestinal brush-border membrane by multiple mechanisms, including an anion antiporter and a previously known proton cotransporter.

Structural characterization of substrates for the anion exchange transporter in Caco-2 cells

The present study was conducted to characterize the structural specificity of an anion exchange transporter in intestinal epithelial cells. The transport of carboxylic acids with hydroxyl group(s) at the 2, 3, 4, and/or 5 positions with respect to carboxylate was examined by using Caco-2 cells in the presence of bicarbonate ions on the basolateral side to enhance the activity of the anion-exchange transporter. In the presence of the bicarbonate ion gradient, transport of L-lactic acid consisted of a saturable process and a nonsaturable process as judged from the Eadie-Hofstee plot. The transport of L-lactic acid at 1 microM was reduced by sodium azide, dinitrophenol, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). It was also reduced by 2-, 4-, and 5-hydroxycarboxylic acids such as hydroxyacetic acid, 4-hydroxybutyric acid, and 5-hydroxydecanoic acid, but not by 3-hydroxycarboxylic acids such as 3-hydroxypropionic acid and 3-hydroxybutyric acid. Transport of both 2- and 4-hydroxybutyric acids involved saturable and nonsaturable processes, whereas that of 3-hydroxybutyric acid was nonsaturable and was not inhibited by DIDS. These results indicate that 3-hydroxycarboxylic acids might not be substrates for this anion exchange transporter in intestinal epithelial cells, suggesting that the position of hydroxylation is significant for molecular recognition by the transporter.

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

The participation of the monocarboxylic acid transporter MCT1 in the intestinal absorption of weak organic acids has been clarified by functional characterization, by use of stably transfected cells, and by immunohistochemical location of the transporter in intestinal tissues. Immunohistochemical analysis by use of the anti-MCT1 antibody showed that MCT1 is distributed throughout the upper and lower intestines, especially in the basolateral membrane and, to a lesser extent, in the brush-border membrane. When the transporter gene rat MCT1 was transfected into MDA-MB231 cells, transport of benzoic acid, a model weak organic acid that has been generally believed to be transported across the cell membranes by passive diffusion, and lactic acid in rat MCT1-transfected cells was significantly increased compared with transport in cells transfected with the expression vector pRc-CMV alone (mock cells). The observed transport was pH-dependent and activity increased between pH 7.5 and pH 5.5, whereas pH-dependence in mock cells was moderate. Rat MCT1-mediated benzoic acid uptake was saturable, with an apparent Km value of 3.05 mM. In addition, MCT1 increased the efflux of [14C]benzoic acid from the cells. Several weak organic acids were also transported by rat MCT1. These results show that pH-dependent intestinal absorption of weak organic acids, previously explained in terms of passive diffusion according to the pH-partition hypothesis, is at least partially accounted for by MCT1-mediated transport energized at acidic pH by utilization of the proton gradient as a driving force.