Nutrient transporter function studied in heterologous expression systems

Measurement of electric current evoked by substrate transport via bi-directional H+/oligopeptide transporter over-expressed in HeLa cells: Electrogenic efflux and existence of a newly observed channel-like state

In the present study, we measured an electric current induced by substrate transport in a HeLa cell over-expressing a human intestinal di/tri-peptide transporter using the whole-cell patch-clamp technique. Gly-Sar, a typical substrate, induced an inward current associated with its uptake, which showed concentration-dependency following Michaelis-Menten-type kinetics with an apparent K(0.5) of 1.3mM as well as voltage-dependency. An outward current accompanying the efflux of Gly-Sar was also observed after washing out the cell. This outward current was voltage-dependent and was reduced by the inward proton gradient. In the case of hydrophobic dipeptides such as Gly-Phe and Gly-Leu, a distinctive current was observed: after washing out the cells, no outward current was observed, but rather, an 'inward leak' current was sustained in spite of the absence of transportable substrate. This leaky current was abolished by the perfusion of Gly-Sar and subsequent washing. It is considered that the hydrophobic substrate sticks within the substrate-binding site and causes the newly observed state, or the 'inward leak' current.

The Extracellular pH Dependency of Transport Activity by Human Oligopeptide Transporter 1 (hPEPT1) Expressed Stably in Chinese Hamster Ovary (CHO) Cells: A Reason for the Bell-Shaped Activity versus pH

Human oligopeptide transporter (hPEPT1) translocates di/tri-peptide by coupling to movement of proton down the electrochemical gradient. This transporter has the characteristics that the pH-profile of neutral dipeptide transport shows a bell-shaped curve with an optimal pH of 5.5. In the present study, we examined the reason for the decrease in the acidic region with hPEPT1-transfected CHO cells stably oeverexpressing hPEPT1 (CHO/hPEPT1). The pH profile of the transport activity vs. pH was measured in the presence of nigericin/monensin. Under this condition, the inwardly directed proton concentration gradient was dissipated while the membrane potential remained. As pH increased the activity increased, and the Henderson-Hasselbalch equation with a single pKa was fitted well to the activity curve. The pKa value was estimated to be 6.7+/-0.2. This value strongly suggests that there is a key amino acid residue, which is involved in pH regulation of transport activity. To identify the key amino acid residue, we examined the effects of various chemical modifications on pH-profile of the transport activity. Modification of carboxyl groups or hydroxyl groups had no significant influence on the pH-profile, whereas a chemical modification of histidine residue with diethylpyrocarbonate (DEPC) completely abolished the transport activity in CHO/hPEPT1 cells. On the other hand, this abolishment was almost prevented by the presence of 10 mM Gly-Sar. This protection was observed only in the presence of the substrate of hPEPT1, indicating that the histidine residue is located at the substrate recognition site. The pH-profile of the transport activity in CHO/hPEPT1 cells treated with DEPC in the presence of 10 mM Gly-Sar also showed a bell-shape similar to that in non-treated CHO/hPEPT1 cells. These data stressed that the histidine residue located at or near the substrate binding site is involved in the pH regulation of transport activity.

ATP-dependent transport of organic anions into isolated basolateral membrane vesicles from rat intestine

The mechanism for the cellular extrusion of organic anions across the intestinal basolateral membrane was examined using isolated membrane vesicles from rat jejunum, ileum, and colon. It was found that 17beta-estradiol 17beta-D-glucuronide (E217betaG) is taken up in an ATP-dependent manner into the basolateral membrane vesicles (BLMVs) but not into the brush-border or microsomal counterparts. The ATP-dependent uptake of E217betaG into BLMVs from jejunum and ileum was described by a single component with a Km value of 23.5 and 8.31 microM, respectively, whereas that into the BLMVs from colon was described by assuming the presence of high (Km=0.82 microM)- and low-affinity (Km=35.4 microM) components. Taurocholate, 6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridylmethyl) benzothiazole glucuronide and taurolithocholate sulfate, but not leukotriene C4, were significantly taken up by the BLMVs. In addition to such substrate specificity, the inhibitor sensitivity of the ATP-dependent transport in BLMVs was similar to that of rat multidrug resistance-associated protein 3 (Mrp3), which is located on the basolateral membrane of enterocytes. Together with the fact that the rank order of the extent of the expression of Mrp3 (jejunum < ileum << colon) is in parallel with that of the extent of the transport of ligands, these results suggest that the ATP-dependent uptake of organic anions into isolated intestinal BLMVs is at least partly mediated by Mrp3.

Molecular mechanisms of pulmonary peptidomimetic drug and peptide transport

The aerosolic administration of peptidomimetic drugs could play a major role in the future treatment of various pulmonary and systemic diseases, because rational drug design offers the potential to specifically generate compounds that are transported efficiently into the epithelium by distinct carrier proteins such as the peptide transporters. From the two presently known peptide transporters, PEPT1 and PEPT2, which have been cloned from human tissues, the high-affinity transporter PEPT2 is expressed in the respiratory tract epithelium. The transporter is an integral membrane protein with 12 membrane-spanning domains and mediates electrogenic uphill peptide and peptidomimetic drug transport by coupling of substrate translocation to a transmembrane electrochemical proton gradient serving as driving force. In human airways, PEPT2 is localized to bronchial epithelium and alveolar type II pneumocytes, and transport studies revealed that both peptides and peptidomimetic drugs such as antibiotic, antiviral, and antineoplastic drugs are carried by the system. PEPT2 is also responsible for the transport of delta-aminolevulinic acid, which is used for photodynamic therapy and the diagnostics of pulmonary neoplasms. Based on the recent progress in understanding the structural requirements for substrate binding and transport, PEPT2 becomes a target for a rational drug design that may lead to a new generation of respiratory drugs and prodrugs that can be delivered to the airways via the peptide transporter.

Enhanced Intestinal Absorption of Drugs by Activation of Peptide Transporter PEPT1 Using Proton‐Releasing Polymer

Utilization of carrier-mediated transport systems in the gastrointestinal tract to increase the bioavailability of drugs is of great interest. In the present study, an increased supply of the driving force for peptide transporter PEPT1 by utilizing a proton-releasing polymer, Eudragit L100-55, was employed to increase the intestinal transport activity. Intestinal absorption of zwitterionic cefadroxil and dianionic cefixime was studied in rats by using in situ ileal closed loops and by in vivo oral administration of the drugs concomitantly with Eudragit L100-55. The results showed that Eudragit L100-55 decreased the pH in the intestinal loops, and increased the disappearance of both cefadroxil and cefixime from the loops. In rats, the plasma concentration after oral administration was increased significantly by coadministration of Eudragit L100-55, whereas a proton-nonreleasing analogous polymer, Eudragit RSPO, did not have any effect. Furthermore, the increased absorption of cefixime caused by Eudragit L100-55 was blocked by simultaneous administration of cefadroxil, a PEPT1 substrate/inhibitor, in a concentration-dependent manner. These results demonstrate that improvement of intestinal absorption of peptide-mimetics via a peptide transporter is possible by optimizing the transporter activity through coadministration of a proton-releasing polymer that supplies the driving force for the transporter.

Methods used to study intestinal nutrient transport: past and present

BACKGROUND

Vitally important to the future of surgical care is the study of nutrition and nutrient uptake. Advances in this field of research have become increasingly dependent upon the disciplines of immunology, histology, and molecular biology. The fusion of these sciences has deepened our insight into the relationship between molecular structure and physiologic function. The ability to apply new technologies to this endeavor will enable the surgeon-investigator to further widen our understanding of nutrient transport.

MATERIALS AND METHODS

Medline and current literature review.

RESULTS AND CONCLUSIONS

We summarize many of the methods used to measure the uptake of nutrients by the intestinal epithelium, providing a historical perspective.

Enhanced delivery of drugs to the liver by adenovirus-mediated heterologous expression of the human oligopeptide transporter PEPT1

To explore the feasibility of drug delivery to the liver by the use of adenovirus-mediated human oligopeptide transporter (hPEPT1) gene transfer, we examined the accumulation of L-[(3)H]carnosine in the hepatoma cell line (HepG2 and WIFB9) and mouse liver. We constructed a recombinant adenovirus encoding hPEPT1-enhanced yellow fluorescent protein (EYFP) fusion gene (AdhPEPT1-EYFP). In vitro uptake of L-[(3)H]carnosine was determined in HepG2 and WIFB9 cells transduced with AdhPEPT1-EYFP. In vivo, the accumulation of L-[(3)H]carnosine in mouse liver was evaluated after transduction of AdhPEPT1-EYFP. At pH 6.0, the uptake of L-[(3)H]carnosine by HepG2 and WIFB9 cells transduced with AdhPEPT1-EYFP was increased 15- and 2-fold, respectively, compared with the cells without transduction. At pH 7.4, uptake of L-[(3)H]carnosine in AdhPEPT1-EYFP transduced HepG2 cells was 3 times greater than that of nontransduced cells. In the presence of carnosine or glycylsarcosine as an inhibitor at 20 mM, the uptake of L-[(3)H]carnosine was reduced to a level comparable to that of nontransduced cells. At 30 min after intravenous administration of L-[(3)H]carnosine to mice transduced with AdhPEPT1-EYFP at 1 x 10(10) plaque-forming units/mouse, the tissue-to-plasma concentration ratio (K(p)) of L-[(3)H]carnosine in the liver was significantly increased to 7 times that of nontransduced mice. In contrast, the K(p) value of [(14)C]inulin, a marker for extracellular fluid space, remained unchanged after adenoviral transduction suggesting minimal pathological damage of tissues. hPEPT1-EYFP was localized at both the basolateral and apical membranes in HepG2 cells, WIFB9 cells, and mouse liver. In conclusion, our results suggest that delivery of oligopeptide to the liver by adenovirus-mediated heterologous expression of hPEPT1 in vivo is feasible.