Functional expression of the intestinal peptide-proton co-transporter in Xenopus laevis oocytes

Enhanced Permeability of Phenylalanyl-glycine (Phe-Gly) Across the Intestinal Membranes by Chemical Modification with Various Fatty Acids

We synthesized four novel lipophilic derivatives of phenylalanyl-glycine (Phe-Gly), C4-Phe-Gly, Phe-Gly-C4, C6-Phe-Gly and C8-Phe-Gly by chemical modification with butyric acid (C4), caproic acid (C6) and octanoic acid (C8). The effect of the acylation on the stability, permeability and accumulation of Phe-Gly in the intestine was investigated by in vitro studies. The stability of Phe-Gly in homogenates of duodenal and colonic membranes was low, but was significantly improved by the acylation except for Phe-Gly-C4. In the transport studies, a modified Ussing chamber was used for the intestinal permeability experiments with Phe-Gly and its acyl derivatives. The permeability of native Phe-Gly and Phe-Gly-C4 across the intestinal membrane was not observed during the transport studies. However, the permeability of Phe-Gly was much improved by chemical modification with various fatty acids to its N-terminal portion. The permeability of acyl-Phe-Gly derivatives across the intestinal membrane decreased with increasing the chain length of fatty acids. In addition, the intestinal tissue accumulation of acyl-Phe-Gly derivatives at the end of the transport studies was much higher than that of native Phe-Gly. The intestinal tissue accumulation of acyl-Phe-Gly in the duodenum increased as the chain length of fatty acids increased. Furthermore, intestinal permeability of C4-Phe-Gly was slightly inhibited in the presence of 5 mM ceftibuten and was significantly reduced under low temperature condition. We observed a directional difference in the transport of C4-Phe-Gly (the mucosal to serosal transport of C4-Phe-Gly was higher than its serosal to mucosal transport) suggesting that C4-Phe-Gly might be transported by a carrier-mediated process as well as other dipeptides. These findings indicate that acylation might be useful approach to enhance the transport of Phe-Gly, a model dipeptide, transported by a carrier-mediated process.

Poly(A)(+) RNA encoding proteins capable of transporting L-methionine and/or DL-2-hydroxy-4-(methylthio) butanoic acid are present in the intestinal mucosa of broilers

To investigate the presence of poly(A)(+) RNA that encode proteins capable of transporting L-methionine (L-Met) and/or DL-2-hydroxy-4-(methylthio) butanoic acid (HMB), Xenopus oocytes were injected with poly(A)(+) RNA isolated from broiler intestinal mucosa. Healthy oocytes at stage V or VI were collected from Xenopus laevis and microinjected with water, poly(A)(+) RNA or size-fractioned poly(A)(+) RNA. The ability of the injected oocytes to take up either L-Met or HMB was examined by incubating oocytes with [methyl-(3)H]-L-Met or [5-(14)C]-HMB. A greater uptake of L-Met (P < 0.01) and HMB (P < 0.05) by oocytes injected with poly(A)(+) RNA from the duodenum, jejunum and ileum of the small intestine was observed compared with water-injected oocytes. The greatest (P < 0.05) uptake occurred when poly(A)(+) RNA from the jejunum or ileum was injected. Injections from four different pools of sucrose gradient--fractionated poly(A)(+) RNA from all three intestinal segments induced (P < 0.01) L-Met uptake. There were three to four different pools of sucrose gradient--fractionated poly(A)(+) RNA from the duodenum, jejunum and ileum that induced (P < 0.05) HMB uptake. Uptake of HMB was greater at pH 5.5 than at pH 7.5 and was independent of Na(+). Uptake of L-Met induced by all four poly(A)(+) RNA pools decreased dramatically when Na(+) was removed from the uptake buffer, which indicated that the majority of L-Met uptake was Na(+)-dependent. These results indicate that there are multiple sized poly(A)(+) RNA that encode proteins capable of mediated transport of L-Met and/or HMB present in broiler intestinal mucosa.

Enhancement of the small intestinal uptake of phenylalanylglycine via a H+/oligopeptide transport system by chemical modification with fatty acids

The transport characteristics of chemically modified phenylalanylglycine (Phe-Gly) with butyric acid (C4-Phe-Gly) and caproic acid (C6-Phe-Gly) were examined using rabbit intestinal brush-border membrane vesicles (BBMVs). In the presence of an inwardly H+ gradient (pH 7.5 inside, pH 6.0 outside), the uptake of Phe-Gly via BBMVs was significantly enhanced by the covalent attachment of butyric or caproic acid to the N-terminal of Phe-Gly. Moreover, C4-Phe-Gly uptake was stimulated by the trans-stimulation effect of some dipeptides and cefadroxil, and was inhibited by other dipeptides and cefadroxil. These results indicate that N-terminal modified Phe-Gly with fatty acids are transported into BBMVs via an oligopeptide transporter. Therefore, chemical modification of dipeptides with fatty acids can enhance the intestinal absorption of dipeptide by a carrier-mediated transport via an oligopeptide transporter.

The predominant contribution of oligopeptide transporter PepT1 to intestinal absorption of beta-lactam antibiotics in the rat small intestine

Although recent evidence suggests that certain beta-lactam antibiotics are absorbed via a specific transport mechanism, its nature is unclear. To confirm whether peptide transport in the rat can be largely ascribed to the intestinal oligopeptide transporter PepT1, the transporter has been functionally characterized and its significance in the intestinal absorption of beta-lactam antibiotics was evaluated. For evaluation of transport activity complementary RNA (cRNA) of rat PepT1 was synthesized in-vitro and expressed in Xenopus laevis oocytes. cRNA induced uptake of several beta-lactam antibiotics and the dipeptide [14C]glycylsarcosine; this was specifically inhibited by various dipeptides and tripeptides but not by their constituent amino acids or by tetra- or pentapeptides. The transport activity of PepT1 for beta-lactam antibiotics correlated well with their in-vivo intestinal transport and absorption. Furthermore, mutual inhibitory effects on uptake were observed between glyclsarcosine and beta-lactam antibiotics. Hybrid depletion of the functional expression of rat PepT1 in oocytes injected with rat intestinal epithelial total mRNA was studied using an antisense oligonucleotide corresponding to the 5'-coding region of PepT1. In oocytes injected with rat mRNA pre-hybridized with the antisense oligonucleotide against rat PepT1, the uptake of [14C]glycylsarcosine was almost completely abolished, whereas its uptake was not influenced by a sense oligonucleotide for the same region of PepT1. Similarly, the uptake of beta-lactam antibiotics was also reduced by the antisense oligonucleotide against rat PepT1. These results demonstrate that the intestinal proton-coupled oligopeptide transporter PepT1 plays a predominant role in the carrier-mediated intestinal absorption of beta-lactam antibiotics and native oligopeptides in the rat.

Molecular mechanism for the relative binding affinity to the intestinal peptide carrier. Comparison of three ACE-inhibitors: enalapril, enalaprilat, and lisinopril

The affinity of three substrates for the intestinal peptide carrier is explained based on their three-dimensional (3D) structural data. The kinetic transport parameters of three ACE-inhibitors, enalapril, enalaprilat, and lisinopril, have been determined in an in vivo system using rat intestine. The observed kinetic transport parameters (+/- asymptotic standard error) of enalapril are: 0.81 (+/- 0.23) mM, 0.58 (+/- 0.37) mumol/h per cm2, and 0.56 (+/- 0.04) cm/h for the half-maximal transport concentration (KT), the maximal transport flux (Jmax) and the passive permeability constant (Pm). Enalaprilat was transported by passive diffusional with a Pm of 0.51 (+/- 0.04) cm/h. For lisinopril the kinetic transport parameters were 0.38 (+/- 0.19) mM, 0.12 (+/- 0.07) mumol/h per cm2, and 0.18 (+/- 0.02) cm/h for KT, Jmax, and Pm, respectively. The affinity of the ACE-inhibitors for the intestinal peptide carrier has been evaluated based on their ability to inhibit the transport rate of cephalexin. The inhibition constants (Ki) of enalapril, enalaprilat and lisinopril were 0.15, 0.28 and 0.39 mM, respectively. 3D structural analysis of lisinopril using molecular modelling techniques reveals that intramolecular hydrogen bond formation is responsible for decreased carrier affinity.

Human intestinal H+/peptide cotransporter. Cloning, functional expression, and chromosomal localization

In mammalian small intestine, a H(+)-coupled peptide transporter is responsible for the absorption of small peptides arising from digestion of dietary proteins. Recently a cDNA clone encoding a H+/peptide cotransporter has been isolated from a rabbit intestinal cDNA library (Fei, Y.J., Kanai, Y., Nussberger, S., Ganapathy, V., Leibach, F.H., Romero, M.F., Singh, S.K., Boron, W. F., and Hediger, M. A. (1994) Nature 368, 563-566). Screening of a human intestinal cDNA library with a probe derived from the rabbit H+/peptide cotransporter cDNA resulted in the identification of a cDNA which when expressed in HeLa cells or in Xenopus laevis oocytes induced H(+)-dependent peptide transport activity. The predicted protein consists of 708 amino acids with 12 membrane-spanning domains and two putative sites for protein kinase C-dependent phosphorylation. The cDNA-induced transport process accepts dipeptides, tripeptides, and amino beta-lactam antibiotics but not free amino acids as substrates. The human H+/peptide cotransporter exhibits a high degree of homology (81% identity and 92% similarity) to the rabbit H+/peptide cotransporter. But surprisingly these transporters show only a weak homology to the H(+)-coupled peptide transport proteins present in bacteria and yeast. Chromosomal assignment studies with somatic cell hybrid analysis and in situ hybridization have located the gene encoding the cloned human H+/peptide cotransporter to chromosome 13 q33-->q34.

Functional expression of intestinal dipeptide/beta-lactam antibiotic transporter in Xenopus laevis oocytes

An intestinal active transport system specific to small peptides and peptide-like drugs such as beta-lactam antibiotics was functionally expressed in Xenopus laevis oocytes after microinjection of messenger RNA (mRNA) derived from rat intestinal mucosal cells. The transport activity was evaluated by measuring the uptake of a tripeptide-like cephalosporin antibiotic, ceftibuten, which has high affinity for the intestinal peptide/H+ co-transporter and is resistant to peptidases. Ceftibuten transport in mRNA-injected oocytes was pH dependent (a proton gradient is the driving force), stereo selective (uptake of the cis-isomer of ceftibuten was about 4-fold higher than that of the trans-isomer), saturable and temperature dependent. Furthermore, various dipeptides showed cis-inhibitory and trans-stimulatory effects on the uptake of ceftibuten by mRNA-injected oocytes, suggesting that ceftibuten and dipeptides are transported by a common carrier protein. These results are in accordance with the functional properties of native proton-coupled peptide transporter previously clarified by studies with isolated intestinal brush-border membrane vesicles and other experimental systems. A protein with a molecular mass of about 130 kDa expressed in the membrane of mRNA-injected oocytes was identified as the transport protein by specific labeling with a photoreactive beta-lactam antibiotic, [3H]benzylpenicillin, followed by SDS-PAGE analysis of the radiolabeled protein. Furthermore, an experiment with mRNA size-fractionated by sucrose density gradient centrifugation indicated that the peptide transporter is encoded by mRNA of between 1.8 and 3.6 kb. These results, obtained using a heterologous gene expression technique, confirm that intestinal absorption of beta-lactam antibiotics occurs through a carrier-mediated mechanism and show that biologically stable beta-lactam antibiotics can be useful probes for molecular analysis of intestinal peptide transporter.

Human intestinal cell line Caco-2: a useful model for studying cellular and molecular regulation of biotin uptake

The mechanisms of enterocyte and molecular regulation of biotin uptake are poorly understood. An intestinal cell line processing the transport characteristics of native intestinal cells is highly desirable to investigate the finer details of the cellular processing and molecular regulation of biotin transport. In the present study, we investigated the uptake of the water-soluble vitamin biotin by a human intestinal cell line Caco-2. Uptake of both low (4 nM) and high (20 microM) concentrations of biotin by confluent monolayers of Caco-2 cells was appreciable and linear for up to 10 min of incubation. Replacement of Na+ in the incubation medium with other monovalent cations--K+, choline, Li+ and NH4(+)--caused a significant inhibition of biotin uptake; a relatively lesser inhibition was seen with Li+. Initial rate of uptake of biotin was temperature-dependent and saturable as a function of concentration at 37 degrees C but not at 4 degrees C. The Vmax and apparent Km of the temperature-dependent saturable process were 520 pmol/mg protein per min and 9.5 microM, respectively. The addition of unlabeled biotin and the structural analogue desthiobiotin to the incubation media caused a significant inhibition of the uptake of [3H]biotin. The inhibitory effect of desthiobiotin was competitive in nature with an inhibition constant (Ki) of 41 microM. Biocytin, on the other hand, was a weak inhibitor and biotin methyl ester and diaminobiotin did not have any effect. Pretreatment of Caco-2 cells with the monovalent cation ionophore gramicidin and the Na+, K+(-)ATPase inhibitor ouabain caused significant inhibition of biotin uptake. Pretreatment with the K+ ionophore valinomycin did not affect biotin uptake. Using the 'Activation Method', the stoichiometric ratio of biotin- to Na+ coupling was found to be 1:1. Growing confluent Caco-2 cells in a biotin-deficient environment resulted in rapid up-regulation of biotin transport with a marked increase (258%) in the Vmax of biotin uptake. These findings demonstrate that biotin uptake by Caco-2 cells is via a carrier-mediated system. This system is temperature-dependent, driven by Na(+)-gradient and is regulated by the substrate level. These in-vitro findings are very similar to and further confirm previous findings in human and animal studies and dispute other findings previously reported for Caco-2 cells; the present study also demonstrates the suitability of this system for further characterization of the cellular and molecular regulation of biotin uptake.

The transport mechanisms of organic cations and their zwitterionic derivatives across rat intestinal brush-border membrane. 1. Binding characteristics to the bio- and lipid-membranes

The uptake mechanisms of organic cations such as tryptamine, tyramine, 5-benzyloxytryptamine (BOTA) and their zwitterionic derivatives (tyrosine, tryptophan, 5-benzyloxytryptophan (BOTP)) by rat intestinal brush-border membrane vesicles and liposome containing phosphatidylserine were studied and compared. As compared to their zwitterionic derivatives, uptake rates by rat intestinal brush-border membrane of these three cations were far superior. The binding of cationic compounds to the brush-border membrane was also higher than those of their zwitterionic derivatives. Furthermore, the binding behaviour of BOTA and tryptamine to phospholipid liposome clearly amplified with increasing amounts of phosphatidylserine. In contrast, the contents of phosphatidylserine, a negatively charged phospholipid, exhibited no effects on the binding of zwitterionic derivatives (tryptophan and BOTP). The double-reciprocal plot of tryptamine binding with BOTA to liposome showed competitive inhibition. These results suggest that the binding of organic cations to the membrane lipid has a relatively high specificity despite the absence of membrane protein such as a transport-carrier in the liposome, and that the binding of cationic compounds play an important role in the uptake to the cell membrane systems.

Expression of human intestinal dipeptide transporter in Xenopus laevis oocytes

The human colon adenocarcinoma cell line Caco-2 retains the H+/dipeptide cotransporter. To identify the structure of the human dipeptide transporter, we have examined the expression of the transporter in Xenopus laevis oocytes injected with Caco-2 poly(A)+RNA, by monitoring the uptake of bestatin, a dipeptide-like anticancer agent. The bestatin uptake in the poly(A)+RNA-injected oocytes was inhibited by excess glycyl-L-leucine, and showed pH dependence (optimal pH of 5.5-6.0). These observations suggest that the human intestinal dipeptide transporter can be expressed functionally in Xenopus oocytes.