Transport Mechanisms of the Imino Acid L-Proline in the Human Intestinal Epithelial Caco-2 Cell Line

Abstract

The intestinal transport of L-proline (L-Pro) has been investigated in various animal species with the use of different tissue preparations. Because major qualitative differences have been observed among the species, it is difficult to extent the results obtained with animal models to humans. In addition, studies on human tissue are lacking because of difficulties in obtaining material for experiments. To characterize the mechanisms involved in the intestinal absorption of L-Pro in humans, the transport of this nonessential imino acid was studied in monolayers of human intestinal Caco-2 cells that were cultivated on microporous membranes. In this model, L-Pro was transported selectively in the apical (AP)-to-basolateral (BL) direction. This transport was significantly reduced by metabolic inhibitors and by an incubation at 4 degrees C; it was Na(+) dependent and showed competition with (methylamino)-alpha-isobutyric acid and L-hydroxyproline. By contrast, transport in the BL-to-AP direction resulted to a large extent from passive movement (paracellular passage and transcellular diffusion). L-Pro accumulation by Caco-2 cells was significantly greater from the AP pole than from the BL pole. About 30-50% of the accumulated molecules were incorporated into newly synthesized proteins in a process inhibited by cycloheximide, whereas the remainder were extensively metabolized into non-amino acid compounds. L-Pro accumulations from the AP and BL poles were both Na(+) dependent, but they exhibited different characteristics. AP accumulation was inhibited by competition with (methylamino)-alpha-isobutyric acid, L-hydroxyproline and, to a lesser extent, D-Pro, whereas BL accumulation was inhibited by competition with L-hydroxyproline, (methylamino)-alpha-isobutyric acid, alpha-aminoisobutyric acid, L-histidine and small neutral amino acids. The results indicate that AP-to-BL transport and AP accumulation of L-Pro exhibited very different characteristics than BL-to-AP transport and BL accumulation.

An in vitro investigation of species-dependent intestinal transport of selenium and the impact of this process on selenium bioavailability

“…To allow for a better comparison between data obtained for Se, LY and mannitol, results of the present experiment were expressed as clearance values. These correspond to the volume of solution containing the compound of interest that passed across the cell monolayer per h (μl/h) ( 21 ) . Data presented in Table 1 show that the BL to AP passage was low and very similar for all Se species and control substances (LY and mannitol).…”

Transport of Pharmacologically Active Proline Derivatives by the Human Proton-Coupled Amino Acid Transporter hPAT1

“…Conflicting results reported in the literature concern the driving force (Na ϩ or H ϩ gradients), the localization of the transporters and the contribution of diverse amino acid transporters such as systems A, B, IMINO, and others to the overall proline uptake (for review and discussion, see Chen et al, 2003). Even among studies using one particular model, the Caco-2 cell, the results are on first sight incompatible (Nicklin et al, 1992;Thwaites et al, 1993b;Berger et al, 2000;Chen et al, 2003). Now that PAT1 has been cloned and studied functionally, the remaining problems will soon be resolved.…”

Amino Acid Transport Across Mammalian Intestinal and Renal Epithelia

“…Amino acid transport has also been studied in monolayers of Caco-2 cells. The results indicate that carrier-mediated transport of amino acids is significantly faster than their paracellular transport (141,148) or the transport of D-mannitol, which occurs entirely by the paracellular route (25,26). These studies used low concentrations of radiolabeled amino acids and therefore may underestimate paracellular transport at higher concentrations.…”