Peptide transporter function and prolidase activities in Caco-2 cells: a lack of coordinated expression

New methods to evaluate intestinal drug absorption mediated by oligopeptide transporter from in vitro study using Caco-2 cells

The aim of the present work is to develop a convenient and rapid screening system in vitro for intestinal drug absorption mediated by oligopeptide transporter (PepT1). In this study, (1) Transports of cephalexin (CEX) and L-phenylalanine (L-Phe) across Caco-2 monolayers were measured and compared with those of passively transported drugs, (2) Inhibitory effects of various drugs on the transport of [(14)C]glycylsarcosine (Gly-Sar) across Caco-2 monolayers were measured and correlated with their in vivo permeability to rat small intestine, (3) Intracellular pH-change induced by co-transport of drugs with proton into Caco-2 cells was monitored by using Fluorometric Imaging Plate Reader (FLIPR, Molecular Devices Corp.). Concentration-dependent transport was observed in Caco-2 monolayers for CEX and L-Phe, although their permeability was relatively low compared to those of passively transported drugs. Inhibitory effects of various drugs including beta-lactam antibiotics and angiotensin converting enzyme-inhibitors on the transport of Gly-Sar correlated well with their in vivo permeability to rat small intestine. It was demonstrated that CEX, but not cefazolin, induced gradual decrease in the intracellular pH of Caco-2 cells. The degree of intracellular pH-change caused by various drugs showed a sigmoidal or saturable relationship with their permeability to rat small intestine. These in vitro approaches with Caco-2 cells should be useful to evaluate in vivo intestinal permeability of drugs mediated by PepT1, suggesting a possibility of high throughput screening of drug absorption.

Metabolism of flavonoids via enteric recycling: mechanistic studies of disposition of apigenin in the Caco-2 cell culture model

The purpose of this study was to determine the mechanisms responsible for intestinal disposition of apigenin in the human Caco-2 cell culture model. The results indicated that most of the absorbed apigenin (10 microM) were conjugated and only a small fraction was transported intact. The amounts of conjugates excreted, especially that of the sulfate, were dependent on days-post-seeding. Apical efflux of apigenin sulfate did not change with concentration of apigenin (4 to 40 microM), whereas its basolateral efflux increased (p < 0.01) with concentration and plateaued at about 25 microM. In contrast, sulfate formation rates in cell lysate increased with concentration and plateaued at 25 microM and were 4 to 6 times faster than the corresponding excretion rates. Formation and polarized excretion rates of glucuronidated apigenin increased with apigenin concentration but formation rates were usually 2.5 to 6 times faster than the corresponding excretion rates. Inhibitors of multidrug resistance-related proteins (MRPs) such as leukotriene C4 and MK-571, which inhibited glucuronidation of apigenin at a high concentration (>or=25 microM), significantly decreased excretion of both apigenin conjugates, and higher concentrations of MK-571 increased the extent of inhibition. In contrast, an organic anion transporter (OAT) inhibitor estrone sulfate only inhibited excretion of apigenin sulfate. In conclusion, we have shown for the first time that intestinal efflux is the rate-limiting step in the intestinal excretion of phase II conjugates of flavones. Furthermore, MRP and OAT are involved in the intestinal efflux of these hydrophilic phase II conjugates.

Functional and molecular characterization of rat intestinal prolidase

Genetic deficiency of prolidase can lead to severe problems in child development, including mental retardation. However, the exact pathogenesis of the disease is unclear. To understand the enzyme's physiologic functions, we studied the regulation of rat intestinal prolidase. The results indicated that 1) the activities of intestinal prolidase and its kinetic parameters (Km and Vmax) are site-dependent; 2) the jejunal prolidase activity was the most sensitive to the dietary restriction, and the duodenal and jejunal but not colonic kinetic parameters changed with dietary restriction; 3) the pH activity profile of jejunal prolidase at 24 h postfeeding was different from that at 48 h postfeeding, whereas the inhibition profiles of prolidase were qualitatively independent of dietary restriction; and 4) old-aged rats have lower prolidase activities in the small intestine. We also purified rat intestinal prolidase I to homogeneity. The characterization study indicated that the purified rat intestinal prolidase I is fairly similar to prolidase I from other species with a molecular weight of 116,000, which consisted of two monomers, 58,000 D each. The purified prolidase I has a Km value of 178 microM and a Vmax value of 601 micromol x min-1. mg protein-1. Screening of a rat intestinal cDNA library produced a 1.8-kb fragment that encodes the rat intestinal prolidase. This enzyme has 494 deduced amino acid sequence, which is 96% or 86% identical to mouse or human erythrocyte prolidase I. This represents the first report of a successful attempt to purify and clone an intestinal prolidase and of investigation to study prolidase regulation by diet.

Transport and metabolic characterization of Caco-2 cells expressing CYP3A4 and CYP3A4 plus oxidoreductase

PURPOSE

To further characterize CYP3A4-transfected Caco-2 cells with regard to morphological, transport, and metabolic properties, and to evaluate a different Caco-2 cell strain transfected with both CYP3A4 and oxidoreductase (OR).

METHODS

Transfected Caco-2 cells, Caco-2 TC7 cells, and wild-type Caco-2 cells grown onto Millicell were used. We determined the morphological characteristics of transfected cell monolayers using light and transmission electron microscope. We determined the transport and metabolic capabilities of the transfected cells, TC7 cells, and wild-type cells with a variety of drugs, nutrients, and marker compounds.

RESULTS

The transfected Caco-2 cells formed a tight monolayer with TEER values and mannitol transport similar to the untransfected parent cell strain (wild type). However, the transfected cells (grown onto Millicell) reached maturity approximately 33% faster than the wild-type cells. Permeabilities of propranolol, nifedipine, testosterone, linopirdine, mannitol, and cephalexin were similar in transfected and wild-type Caco-2 cells. On the other hand, the transfected cells of early passages were much more metabolically active, and metabolized standard CYP3A4 substrates (e.g., testosterone and nifedipine) as much as 100 times faster than untransfected cells. In addition, metabolism of standard substrates was inhibitable by ketoconazole and TAO. Using comparable data, the transfected cells metabolized testosterone the fastest, followed by linopirdine and nifedipine (approximate ratio: 10:6:2). The metabolites of standard substrates were generally preferably excreted to the apical membrane.

CONCLUSION

The monolayers of newly transfected cells (CYP3A4 + OR) have a significantly increased level of CYP3A4 activities compared to untransfected cells. These cell monolayers also have desirable morphological and transport characteristics that are similar to untransfected cells.

In vitro permeability across Caco-2 cells (colonic) can predict in vivo (small intestinal) absorption in man--fact or myth

PURPOSE

To evaluate and optimize the use of Caco-2 cell monolayers to predict the in vivo absorption of a broad range of compounds in man.

METHODS

Caco-2 cells are derived from human adenocarcinoma colon cells and spontaneously differentiate when grown on porous polyethylene terephthalate membranes (PETP) in a 12 well format to form monolayers of polarized cells possessing function similar to intestinal enterocytes. Transport experiments were conducted using 21 day cultured cells in a shaking water bath at 37 degrees C. Radiolabeled mannitol was used to determine monolayer integrity. Apparent permeability coefficient (Papp) was calculated from the appearance of drug in the receiver side.

RESULTS

A strong correlation was observed between in vivo human absorption and in vitro Papp for a variety of compounds (R = 0.95, N = 35). For compounds that are substrates of p-glycoprotein (Pgp), use of a Pgp inhibitor resulted in a better estimate of absorption in humans. The results of this study suggest that the overall ranking of compounds with Papp < 1 x 10(-6) cm/sec, between 1-10 x 10(-5) cm/ sec, and > 10 x 10(-6) cm/sec can be classified as poorly (0-20%), moderately (20-70%) and well (70-100%) absorbed compounds, respectively.

CONCLUSIONS

These data suggest that Caco-2 cells developed under the culturing and transport conditions defined herein can be used to predict in vivo human absorption of compounds regardless of transport mechanism, viz., transcellular, paracellular and carrier-mediated.

Development of Caco-2 cells expressing high levels of cDNA-derived cytochrome P4503A4

PURPOSE

To develop Caco-2 cell derivatives expressing high levels of human cytochrome P450 drug metabolizing enzymes.

METHODS

The cDNAs for two cytochrome P450 forms, CYP2A6 and CYP3A4, were introduced into an extrachromosomal vector under control of the cytomegalovirus early intermediate promoter. Vector-bearing cells were selected via resistance to hygromycin B.

RESULTS

Transfected cells exhibited high levels of cDNA-derived protein as measured by Western blot, spectrophotometric P450 determination and/or cytochrome P450 form-selective enzyme assay. CYP3A4 and CYP2A6 catalytic activities were about 100 fold higher than in control cells. cDNA-expressing cells were found to form tight monolayers and were suitable for study of xenobiotic transport and metabolism. The permeabilities of cephalexin, phenylalanine, mannitol and propranolol across transfected monolayers were found to be similar to those across untransfected monolayers. The appropriate transfected monolayers metabolized the CYP2A6 substrate coumarin and the CYP3A4 substrates testosterone and nifedipine.

CONCLUSIONS

A Caco-2 cell system to simultaneously study drug transport and metabolism has been developed.