Chinese hamster ovary mRNA-dependent, Na(+)-independent L-leucine transport in Xenopus laevis oocytes

Intestinal gaboxadol absorption via PAT1 (SLC36A1): modified absorption in vivo following co-administration of L-tryptophan

BACKGROUND AND PURPOSE

Gaboxadol has been in development for treatment of chronic pain and insomnia. The clinical use of gaboxadol has revealed that adverse effects seem related to peak serum concentrations. The aim of this study was to investigate the mechanism of intestinal absorption of gaboxadol in vitro and in vivo.

EXPERIMENTAL APPROACH

In vitro transport investigations were performed in Caco-2 cell monolayers. In vivo pharmacokinetic investigations were conducted in beagle dogs. Gaboxadol doses of 2.5 mg.kg(-1) were given either as an intravenous injection (1.0 mL.kg(-1)) or as an oral solution (5.0 mL.kg(-1)).

KEY RESULTS

Gaboxadol may be a substrate of the human proton-coupled amino acid transporter, hPAT1 and it inhibited the hPAT1-mediated L-[(3)H]proline uptake in Caco-2 cell monolayers with an inhibition constant K(i) of 6.6 mmol.L(-1). The transepithelial transport of gaboxadol was polarized in the apical to basolateral direction, and was dependent on gaboxadol concentration and pH of the apical buffer solution. In beagle dogs, the absorption of gaboxadol was almost complete (absolute bioavailability, F(a), of 85.3%) and T(max) was 0.46 h. Oral co-administration with 2.5-150 mg.kg(-1) of the PAT1 inhibitor, L-tryptophan, significantly decreased the absorption rate constant, k(a), and C(max), and increased T(max) of gaboxadol, whereas the area under the curve and clearance of gaboxadol were constant.

CONCLUSIONS AND IMPLICATIONS

The absorption of gaboxadol across the luminal membrane of the small intestinal enterocytes is probably mediated by PAT1. This knowledge is useful for reducing gaboxadol absorption rates in order to decrease peak plasma concentrations.

Transport of d-[1-14C]-amino acids into Chinese hamster ovary (CHO-K1) cells: implications for use of labeled d-amino acids as molecular imaging agents

INTRODUCTION

The fact that d-amino acids have been found in various tissues and are involved in various functions is a clue to how to develop new imaging agents. We examined d-amino acid transport mechanisms in Chinese hamster ovary (CHO-K1) cells because CHO-K1 cells are widely used in biomedical studies and are thought to be useful for expression of genes involved in metabolism of D-amino acids.

METHODS

Uptake experiments were performed. CHO-K1 cells cultured in 60-mm plastic culture dishes under ordinary culture conditions were incubated with 18.5 kBq of radiolabeled amino acid in 2 ml of phosphate-buffered-saline-based uptake solution at 37 degrees C. The following radiolabeled amino acid tracers were used: D-[1-(14)C]-alanine, L-[1-(14)C]-alanine, D-[1-(14)C]-serine, L-[1-(14)C]-serine, D-[1-(14)C]-methionine, L-[1-(14)C]-methionine, D-[1-(14)C]-phenylalanine, L-[1-(14)C]-phenylalanine, D-[1-(14)C]-leucine, L-[1-(14)C]-leucine, D-[1-(14)C]-valine, L-[1-(14)C]-valine, D-[1-(14)C]-tyrosine, L-[1-(14)C]-tyrosine, D-[1-(14)C]-glutamic acid, L-[1-(14)C]-glutamic acid, D-[1-(14)C]-lysine, L-[1-(14)C]-lysine, D-[1-(14)C]-arginine and L-[L-(14)C]-arginine. We tested the inhibitory effects of the following compounds (1.0 mM) on transport: 2-(methylamino)isobutyric acid (a specific inhibitor of system A, in Na(+)-containing uptake solution) and 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid (a specific inhibitor of system L, in Na(+)-free uptake solution).

RESULTS

D-[1-(14)C]-methionine, D-[1-(14)C]-phenylalanine and D-[1-(14)C]-tyrosine accumulated mainly via system L. D-[1-(14)C]-alanine and D-[1-(14)C]-serine accumulated primarily via system ASC. High uptake of D-[1-(14)C]-alanine, D-[1-(14)C]-methionine, D-[1-(14)C]-phenylalanine and D-[1-(14)C]-leucine was observed. The uptake of radiolabeled serine, valine, tyrosine, glutamic acid and arginine into CHO-K1 was highly stereoselective for l-isomers.

CONCLUSIONS

We observed high uptake of D-[1-(14)C]-alanine via system ASC (most likely alanine-serine-cysteine-selective amino acid transporter-1) and high uptake of D-[1-(14)C]-methionine and D-[1-(14)C]-phenylalanine via system L (most likely L-type amino acid transporter-1).

Mediation of Highly Concentrative Uptake of Pregabalin by L-Type Amino Acid Transport in Chinese Hamster Ovary and Caco-2 Cells

Pregabalin (PGB) is a novel drug under development for the treatment of epilepsy, neuropathic pain, fibromyalgia, and generalized anxiety disorder. In this study, we investigated PGB transport in rats, mammalian cell lines, and Xenopus laevis oocytes. In contrast to gabapentin (GBP), PGB absorption in rats showed unique linear pharmacokinetics. PGB entered CHO and Caco-2 cells predominately via Na(+)-independent processes. Uptake of PGB was mutually exclusive with leucine, GBP and 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid, the substrates preferential for system L. The preloaded PGB in CHO cells was exchangeable with leucine, but at a lower exchange rate than that of leucine and GBP. Dixon plots showed competitive inhibition of leucine uptake by PGB, with a K(i) value very close to the K(m) value for PGB uptake (377 versus 363 microM). At an extracellular concentration of 300 microM, the intracellular PGB concentration in CHO cells reached 1.5- and 23-fold higher than that of GBP and leucine, respectively. In contrast, at clinically relevant concentrations, PGB seemed not to interact with GABA transport in GAT1, GAT2, and GAT3 cell lines, system y(+), b(0,+), B(0,+), and B(0) transport activities in Caco-2 and NBL-1 cells, and the b(0,+)-like transport activity in rBAT cRNA-injected X. laevis oocytes. Taken together, these results suggest that L-type transport is the major transport route for PGB and GBP uptake in mammalian cells. The differential affinity of PGB and GBP at L-type system leads to more concentrative accumulation of PGB than GBP, which may facilitate PGB transmembrane absorption in vivo.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Evidence for a regulatory element controlling amino acid transport system L in Chinese hamster ovary cells

We have used the technique of somatic cell hybridization to study the regulation of the neutral amino acid transport system L in Chinese hamster ovary (CHO) cells. The cell line CHO-ts025C1 has a temperature-sensitive mutation in leucyl-tRNA synthetase. At the nonpermissive temperature of 39 degrees C, CHO-ts025C1 cells are unable to charge leucyl-tRNA and behave as though starved for leucine by increasing their system L transport activity two- to fourfold. From the temperature-sensitive cell line, we have isolated a regulatory mutant cell line, CHO-C11B6, that has constitutively elevated system L transport activity. The CHO-C11B6 cell line retains the temperature-sensitive leucyl-tRNA synthetase mutation, but growth of this cell line is temperature resistant because its increased system L transport activity leads to increased intracellular leucine levels, which compensate for the defective synthetase. Hybrid cells formed by fusion of the temperature-sensitive CHO-ts025C1 cells and the temperature-resistant CHO-C11B6 cells show temperature-sensitive growth and temperature-dependent regulation of leucine transport activity. These data suggest that the system L activity of CHO cells is regulated by a dominant-acting element that is defective or absent in the regulatory mutant CHO-C11B6 cell line.

Expression of Na+-independent isoleucine transport activity from rat brain in Xenopus laevis oocytes

Poly(A)+ RNA from C6-BU-1 rat glioma cells and rat astroglial cells induced isoleucine transport activity when injected into Xenopus laevis oocytes. The Na+-independent component of isoleucine transport was inhibited by leucine, phenylalanine and 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) but neither by methylaminoisobutyric acid (MeAIB) nor lysine. A Km value of approx. 100 microM was determined for the Na+-independent transport of isoleucine. These data are in accordance with expression of a system L like transporter. By injection of size fractionated poly(A)+ RNA a length of approx. 1.9 kb was determined for the pertinent mRNA.