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

The bioavailable octapeptide Gly-Ala-Hyp-Gly-Leu-Hyp-Gly-Pro stimulates nitric oxide synthesis in vascular endothelial cells

Gly-Ala-Hyp-Gly-Leu-Hyp-Gly-Pro (GAXGLXGP, X: Hyp), an octapeptide contained in chicken collagen hydrolysate, inhibits angiotensin I-converting enzyme activity in vitro. Intestinal Caco-2 and bovine aortic endothelial cells (BAECs) were used to investigate whether the transported GAXGLXGP improves vascular function. When GAXGLXGP was added to the apical side of Caco-2 monolayers, the intact form of GAXGLXGP was released to the basolateral side without incorporation into the cells. This transport was energy-independent but was associated with tight junction permeability. GAXGLXGP was then added to BAECs, and endothelial nitric oxide (NO) synthase (eNOS) activation was examined. GAXGLXGP at a concentration of 10 microM stimulated production of NO during a 1 h incubation. This event involved phosphorylation of eNOS at Ser(1179) without a change in the total eNOS protein level. These findings indicate that GAXGLXGP absorbed intact through the intestinal epithelium has direct effects on eNOS activity in vascular endothelial cells, leading to NO synthesis, thereby suggesting the potential for improvement in vascular function.

Computational tools for the interactive exploration of proteomic and structural data

Linking proteomics and structural data is critical to our understanding of cellular processes, and interactive exploration of these complementary data sets can be extremely valuable for developing or confirming hypotheses in silico. However, few computational tools facilitate linking these types of data interactively. In addition, the tools that do exist are neither well understood nor widely used by the proteomics or structural biology communities. We briefly describe several relevant tools, and then, using three scenarios, we present in depth two tools for the integrated exploration of proteomics and structural data.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Bioavailability through PepT1: the role of computer modelling in intelligent drug design

In addition to being responsible for the majority of absorption of dietary nitrogen, the mammalian proton-coupled di- and tri-peptide transporter PepT1 is also recognised as a major route of drug delivery for several important classes of compound, including beta-lactam antibiotics and angiotensin-converting enzyme inhibitors. Thus there is considerable interest in the PepT1 protein and especially its substrate binding site. In the absence of a crystal structure, computer modelling has been used to try to understand the relationship between PepT1 3D structure and function. Two basic approaches have been taken: modelling the transporter protein, and modelling the substrate. For the former, computer modelling has evolved from early interpretations of the twelve transmembrane domain structure to more recent homology modelling based on recently crystallised bacterial members of the major facilitator superfamily (MFS). Substrate modelling has involved the proposal of a substrate binding template, to which all substrates must conform and from which the affinity of a substrate can be estimated relatively accurately, and identification of points of potential interaction of the substrate with the protein by developing a pharmacophore model of the substrates. Most recently, these two approaches have moved closer together, with the attempted docking of a substrate library onto a homology model of the human PepT1 protein. This article will review these two approaches in which computers have been applied to peptide transport and suggest how such computer modelling could affect drug design and delivery through PepT1.

Direct evidence for efficient transport and minimal metabolism of L-cephalexin by oligopeptide transporter 1 in budded baculovirus fraction

The oligopeptide transporter PEPT1 (SLC15A1) is responsible for absorption of peptidic nutrients in the small intestine. Although the L-diastereomer of the beta-lactam antibiotic cephalexin (L-cephalexin) is likely to be transported by PEPT1, there has been no direct demonstration of PEPT1-mediated L-cephalexin transport. Indeed, after the incubation with L-cephalexin, the intact form of L-cephalexin has not been identified inside vesicles/proteoliposomes prepared from brush border membrane of intestinal epithelial cells or cultured cell lines exogenously transfected with PEPT1 gene. Thus, it appears that L-cephalexin is rapidly metabolized by PEPT1 or PEPT1-associated proteins. Here, we attempted to verify whether L-cephalexin is transported by PEPT1 and whether it is hydrolyzed by PEPT1 itself, by using budded baculovirus expressing PEPT1 protein. Marked uptake of L-cephalexin in PEPT1-expressing budded baculovirus, compared with wild-type virus, indicated that L-cephalexin is a substrate for PEPT1. The uptake was found to be pH sensitive, and was strongly inhibited by the D-diastereomer of cephalexin and glycylsarcosine, but not by glycine. Thus, L-cephalexin is transported by PEPT1 itself. Upon the transport of both L- and D-cephalexin by PEPT1, dose-dependent membrane depolarization was observed; the EC(50) values of 0.18 and 2.9 mM, respectively, indicate that the affinity of L-cephalexin for PEPT1-mediated transport is much higher than that of the D-diastereomer. On the other hand, the L-cephalexin metabolite 7-aminodesacetoxycephalosporanic acid was not detected in PEPT1-expressing or wild-type virus at either pH 6.0 or 7.4. We conclude that L-cephalexin is transported by PEPT1 with high affinity, but is not metabolized by PEPT1 itself.

Pharmaceutical and pharmacological importance of peptide transporters

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many β-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

In silico prediction of human oral absorption based on QSAR analyses of PAMPA permeability

The parallel artificial membrane permeation assay (PAMPA) was developed as a model for the prediction of transcellular permeation in the process of drug absorption. Our research group has measured the PAMPA permeability of peptide-related compounds, diverse drugs, and agrochemicals. This work led to a classical quantitative structure-activity relationship (QSAR) equation for PAMPA permeability coefficients of structurally diverse compounds based on simple physicochemical parameters such as lipophilicity at a particular pH (log P(oct) and |pKa-pH|), H-bond acceptor ability (SA(HA)), and H-bond donor ability (SA(HD)). Since the PAMPA permeability of lipophilic compounds decreased with their apparent lipophilicity due to the unstirred water layer (UWL) barrier on membrane surfaces and to membrane retention, a bilinear QSAR model was introduced to explain the permeability of a broader set of compounds using the same physicochemical parameters as those used for the linear model. We also compared PAMPA and Caco-2 cell permeability coefficients of compounds transported by various absorption mechanisms. The compounds were classified according to their absorption pathway (passively transported compounds, actively transported compounds, and compounds excreted by efflux systems) in the plot of Caco-2 vs. PAMPA permeability. Finally, based on the QSAR analyses of PAMPA permeability, an in silico prediction model of human oral absorption for possibly transported compounds was proposed, and the usefulness of the model was examined.

Comparative analysis of vertebrate PEPT1 and PEPT2 genes

The plasma membrane transport proteins belong to SoLute Carrier 15 (SLC15) family and two members of this family have been characterized extensively in higher vertebrates, namely PEPT1 and PEPT2. Despite many efforts have made to define a pharmacophore model for efficient binding and transporting of substrates, there is not a comprehensive study performed to elucidate the evolutionary mechanisms among the SLC15 family members and to statistically evaluate sequence conservation and functional divergence between members. In this study, we compared and contrasted the rates and patterns of molecular evolution of 2 PEPT genes. Phylogenetic tree assembly with all available vertebrate PEPTs suggests that the PEPTs originated by duplications and diverged from a common protein at the base of the eukaryotic tree. Topological structure demonstrates both members share the similar hydrophobic domains (TMDs), which have been constrained by purifying selection. Although both genes show qualitatively similar patterns, their rates of evolution differ significantly due to an increased rate of synonymous substitutions in the structural domains in one copy, suggesting substantial differences in functional constraint on each gene. Site-specific profiles were established by posterior probability analysis revealing significantly divergent regions mainly locate at the hydrophobic region between predicted transmembrane domains 9 and 10 of the proteins. Thus, these results provide the evidence that several amino acid residues with reduced selective constraints are largely responsible for functional divergence between the paralogous PEPTs. These findings may provide a starting point for further experimental verifications.

Mutagenesis and cysteine scanning of transmembrane domain 10 of the human dipeptide transporter

PURPOSE

The human dipeptide transporter (hPEPT1) facilitates transport of dipeptides and drugs from the intestine into the circulation. The role of transmembrane domain 10 (TMD10) of hPEPT1 in substrate translocation was investigated using cysteine-scanning mutagenesis with 2-Trimethylammonioethyl methanethiosulfonate (MTSET).

METHODS

Each amino acid in TMD10 was mutated individually to cysteine, and transport of [(3)H]Gly-Sar was evaluated with and without MTSET following transfection of each mutant in HEK293 cells. Similar localization and expression levels of wild type (WT) hPEPT1 and all mutants were confirmed by immunostaining and biotinylation followed by western blot analysis.

RESULTS

E595C- and G594C-hPEPT1 showed negligible Gly-Sar uptake. E595D-hPEPT1 showed similar uptake to WT-hPEPT1, but E595K- and E595R-hPEPT1 did not transport Gly-Sar. Double mutations E595K/R282E and E595R/R282E did not restore uptake. G594A-hPEPT1 showed similar uptake to WT-hPEPT1, but G594V-hPEPT1 eliminated uptake. Y588C-hPEPT1 showed uptake of 20% that of WT-hPEPT1. MTSET modification supported a model of TMD10 with an amphipathic helix from I585 to V600 and increased solvent accessibility from T601 to F605.

CONCLUSIONS

Our results suggest that G594 and E595 in TMD10 of hPEPT1 have key roles in substrate transport and that Y588 may have an important secondary mechanistic role.

Oral administration of Excitin-1 (beta-alanyl-L-leucine) alters behavior and brain monoamine and amino acid concentrations in rats

We previously demonstrated that beta-alanyl-branched chain amino acids have excitatory effects. Therefore, we named beta-alanyl-L-leucine, beta-alanyl-L-isoleucine and beta-alanyl-L-valine as Excitin-1, -2, and -3 , respectively. Since there is little known about the effects of Excitins, we clarified whether oral administration of Excitin-1 affects behavior in rats, alters the monoamine and amino acid levels in the central nervous system, whether Excitin-1 is incorporated into the brain, and how long it remains in the blood. Excitin-1 increased motor behavior, increasing the distance of path and number of rearings in the open field. Excitin-1 influenced some monoamine and amino acid levels in the cerebral cortex and hypothalamus. Following oral administration, Excitin-1 was detected in the cerebral cortex, hypothalamus, hippocampus and olfactory bulb. In the plasma, Excitin-1 and its metabolites beta-alanine and L-leucine were recorded. The present study demonstrated that Excitin-1 was incorporated in the brain and promoted behavioral changes in rats.

Review. The mammalian proton-coupled peptide cotransporter PepT1: sitting on the transporter-channel fence?

The proton-coupled di- and tripeptide transporter PepT1 (SLC15a1) is the major route by which dietary nitrogen is taken up from the small intestine, as well as being the route of entry for important therapeutic (pro)drugs such as the beta-lactam antibiotics, angiotensin-converting enzyme inhibitors and antiviral and anti-cancer agents. PepT1 is a member of the major facilitator superfamily of 12 transmembrane domain transporter proteins. Expression studies in Xenopus laevis on rabbit PepT1 that had undergone site-directed mutagenesis of a conserved arginine residue (arginine282 in transmembrane domain 7) to a glutamate revealed that this residue played a role in the coupling of proton and peptide transport and prevented the movement of non-coupled ions during the transporter cycle. Mutations of arginine282 to other non-positive residues did not uncouple proton-peptide cotransport, but did allow additional ion movements when substrate was added. By contrast, mutations to positive residues appeared to function the same as wild-type. These findings are discussed in relation to the functional role that arginine282 may play in the way PepT1 operates, together with structural information from the homology model of PepT1 based on the Escherichia coli lactose permease crystal structure.

Modeling of the intestinal peptide transporter hPepT1 and analysis of its transport capacities by docking and pharmacophore mapping

An early pharmacokinetic screen for peptidomimetic drugs should have the ability to predict molecules with high affinity for intestinal transporters, as peptide-like derivatives are seldom absorbed passively. Hence, the first objective of this study was to generate a reliable model for the structure of the hPepT1 protein, which is the main intestinal transporter involved in the absorption of both dietary peptides and peptidomimetics. The modeling was based on the resolved structure of the homologous bacterial lactose permease LacY using a fragmental strategy. The interaction capacities of the hPepT1 model were explored by docking a set of 50 known ligands. Despite the known predilection of hPepT1 for hydrophobic ligands, docking results unveiled the key role of the polar interactions stabilized by charged termini, especially concerning the ammonium head group. The docking results were further verified by developing a pharmacophore model that confirmed the key features required for optimal hPepT1 affinity. The consistency of the docking results and the agreement with the pharmacophore model afford an encouraging validation for the proposed model and suggest that it can be exploited to design peptide-like molecules with an improved affinity for such a transporter.

Exploring and dissecting genome-wide gene expression responses of Penicillium chrysogenum to phenylacetic acid consumption and penicillinG production

BACKGROUND

Since the discovery of the antibacterial activity of penicillin by Fleming 80 years ago, improvements of penicillin titer were essentially achieved by classical strain improvement through mutagenesis and screening. The recent sequencing of Penicillium chrysogenum strain Wisconsin1255-54 and the availability of genomics tools such as DNA-microarray offer new perspective.

RESULTS

In studies on beta-lactam production by P. chrysogenum, addition and omission of a side-chain precursor is commonly used to generate producing and non-producing scenarios. To dissect effects of penicillinG production and of its side-chain precursor phenylacetic acid (PAA), a derivative of a penicillinG high-producing strain without a functional penicillin-biosynthesis gene cluster was constructed. In glucose-limited chemostat cultures of the high-producing and cluster-free strains, PAA addition caused a small reduction of the biomass yield, consistent with PAA acting as a weak-organic-acid uncoupler. Microarray-based analysis on chemostat cultures of the high-producing and cluster-free strains, grown in the presence and absence of PAA, showed that: (i) Absence of a penicillin gene cluster resulted in transcriptional upregulation of a gene cluster putatively involved in production of the secondary metabolite aristolochene and its derivatives, (ii) The homogentisate pathway for PAA catabolism is strongly transcriptionally upregulated in PAA-supplemented cultures (iii) Several genes involved in nitrogen and sulfur metabolism were transcriptionally upregulated under penicillinG producing conditions only, suggesting a drain of amino-acid precursor pools. Furthermore, the number of candidate genes for penicillin transporters was strongly reduced, thus enabling a focusing of functional analysis studies.

CONCLUSION

This study demonstrates the usefulness of combinatorial transcriptome analysis in chemostat cultures to dissect effects of biological and process parameters on gene expression regulation. This study provides for the first time clear-cut target genes for metabolic engineering, beyond the three genes of the beta-lactam pathway.

Multidrug resistant Acinetobacter baumannii--the role of AdeABC (RND family) efflux pump in resistance to antibiotics

Acinetobacter baumannii is an opportunistic pathogen which play the more and more greater role in the pathogenicity of the human. It is often attached with the hospital environment, in which is able easily to survive for many days even in adverse conditions. Acinetobacter baumannii is the species responsible for a serious nosocomial infections, especially in the intensive care units. Option of surviving in natural niches, and in the hospital environment could also be associated with the efflux pump mechanisms. Mechanisms of efflux universally appear in all cells (eukaryotic and prokaryotic) and play the physiological important role. In prokaryote, the main functions are evasion of such naturally produced molecules, removal of metabolic products and toxins. These pumps could also be involved in an early stage of infection, such as adhesion to host cells and the colonization. Importantly, they remove commonly used antibiotics from the cell in therapy of infections caused by these bacteria. Efflux pumps exemplify a unique phenomenon in drug resistance: a single mechanism causing resistance against several different classes of antibiotics. In Acinetobacter baumannii, the AdeABC efflux pump, a member of the resistance-nodulation-cell division family (RND), has been well characterized. Aminoglicosides, tetracyclines, erythromycin, chloramphenicol, trimethoprim, fluoroquinolones, some beta-lactams, and also recently tigecycline, were found to be substrates for this pump. Drugs, as substrates for the AdeABC pump, can increase the expression of the AdeABC genes, leading to multidrug resistance (MDR). From this reason, treatment failure and death caused by Acinetobacter baumannii infections or underlying diseases are common. Because the AdeABC pump is widespread in Acinetobacter baumannii, similarly to other pumps in Gram-negative and Gram-positive bacteria, exists a need of searching a new therapeutic solutions. Specific efflux inhibitors of pumps (EPIs), including AdeABC inhibitors, could be suppress the activity of pumps and restore the sensitivity of such important bacteria as Acinetobacter baumannii to commonly used antibiotic.

Peptide transporters and their roles in physiological processes and drug disposition

1. The peptide transporters belong to the peptide transporter (PTR) family and serve as integral membrane proteins for the cellular uptake of di- and tripeptides in the organism. By their ability also to transport peptidomimetics and other substrates with therapeutic activities or precursors of pharmacologically active agents, they are of considerable importance in pharmacology. 2. PEPT1 is the low-affinity, high-capacity transporter and is mainly expressed in the small intestine, whereas PEPT2 is the high-affinity, low-capacity transporter and has a broader distribution in the organism. 3. Targeted mouse models have revealed PEPT2 to be the dominant transporter for the reabsorption of di- and tripeptides and its pharmacological substrates in the organism, and for the removal of these substrates from the cerebrospinal fluid. Moreover, the peptide transporters undergo physiological and pharmacological regulation and, of great interest, are present in disease states where PEPT1 exhibits ectopic expression in colonic inflammation. 4. The paper reviews the structural characteristics of the peptide transporters, the structural requirements for substrates, the distribution of the peptide transporters in the organism, and finally their regulation in the organism in healthy and pathological situations.

An energy supply network of nutrient absorption coordinated by calcium and T1R taste receptors in rat small intestine

T1R taste receptors are present throughout the gastrointestinal tract. Glucose absorption comprises active absorption via SGLT1 and facilitated absorption via GLUT2 in the apical membrane. Trafficking of apical GLUT2 is rapidly up-regulated by glucose and artificial sweeteners, which act through T1R2 + T1R3/alpha-gustducin to activate PLC beta2 and PKC betaII. We therefore investigated whether non-sugar nutrients are regulated by taste receptors using perfused rat jejunum in vivo. Under different conditions, we observed a Ca(2+)-dependent reciprocal relationship between the H(+)/oligopeptide transporter PepT1 and apical GLUT2, reflecting the fact that trafficking of PepT1 and GLUT2 to the apical membrane is inhibited and activated by PKC betaII, respectively. Addition of L-glutamate or sucralose to a perfusate containing low glucose (20 mM) each activated PKC betaII and decreased apical PepT1 levels and absorption of the hydrolysis-resistant dipeptide L-Phe(PsiS)-L-Ala (1 mM), while increasing apical GLUT2 and glucose absorption within minutes. Switching perfusion from mannitol to glucose (75 mM) exerted similar effects. c-glutamate induced rapid GPCR internalization of T1R1, T1R3 and transducin, whereas sucralose internalized T1R2, T1R3 and alpha-gustducin. We conclude that L-glutamate acts via amino acid and glucose via sweet taste receptors to coordinate regulation of PepT1 and apical GLUT2 reciprocally through a common enterocytic pool of PKC betaII. These data suggest the existence of a wider Ca(2+) and taste receptor-coordinated transport network incorporating other nutrients and/or other stimuli capable of activating PKC betaII and additional transporters, such as the aspartate/glutamate transporter, EAAC1, whose level was doubled by L-glutamate. The network may control energy supply.

Targeted disruption of peptide transporter Pept1 gene in mice significantly reduces dipeptide absorption in intestine

PEPT1 is a high-capacity, low-affinity peptide transporter that mediates the uptake of di- and tripeptides in the intestine and kidney. PEPT1 also has significance in its ability to transport therapeutic agents and because of its potential as a target for anti-inflammatory therapies. To further understand the relevance of specific peptide transporters in intestinal physiology, pharmacology and pathophysiology, we have generated Pept1 null mice by targeted gene disruption. The Pept1 gene was disrupted by insertion of a lacZ reporter gene under the control of the endogenous Pept1 promoter. Phenotypic profiling of wild-type and Pept1 null mice was then performed, along with in vitro intestinal uptake, in situ intestinal perfusion and in vivo pharmacokinetic studies of glycylsarcosine (GlySar). Pept1 null mice lacked expression of PEPT1 protein in the intestine and kidney, tissues in which this peptide transporter is normally expressed. Pept1-deficient mice were found to be viable, fertile, grew to normal size and weight, and were without any obvious abnormalities. Nevertheless, Pept1 deletion dramatically reduced the intestinal uptake and effective permeability of the model dipeptide GlySar (i.e., by at least 80%), and its oral absorption following gastric gavage (i.e., by about 50%). In contrast, the plasma profiles of GlySar were almost superimposable between wild-type and Pept1 null animals after intravenous dosing. These novel findings provide strong evidence that PEPT1 has a major role in the in vivo oral absorption of dipeptides.

Pharmacokinetics of amino acid ester prodrugs of acyclovir after oral administration: interaction with the transporters on Caco-2 cells

In vivo systemic absorption of the amino acid prodrugs of acyclovir (ACV) after oral administration was evaluated in rats. Stability of the prodrugs, L-alanine-ACV (AACV), L-serine-ACV (SACV), L-isoleucine-ACV (IACV), gamma-glutamate-ACV (EACV) and L-valine-ACV (VACV) was evaluated in various tissues. Interaction of these prodrugs with the transporters on Caco-2 cells was studied. In vivo systemic bioavailability of these prodrugs upon oral administration was evaluated in jugular vein cannulated rats. The amino acid ester prodrugs showed affinity towards various amino acid transporters as well as the peptide transporter on the Caco-2 cells. In terms of stability, EACV was most enzymatically stable compared to other prodrugs especially in liver homogenate. In oral absorption studies, ACV and AACV showed high terminal elimination rate constants (lambda(z)). SACV and VACV exhibited approximately five-fold increase in area under the curve (AUC) values relative to ACV (p<0.05). C(max(T)) (maximum concentration) of SACV was observed to be 39+/-22 microM in plasma which is 2 times better than VACV and 15 times better than ACV. C(last(T)) (concentration at the last time point) of SACV was observed to be 0.18+/-0.06 microM in plasma which is two times better than VACV and three times better than ACV. Amino acid ester prodrugs of ACV were absorbed at varying amounts (C(max)) and eliminated at varying rates (lambda(z)) thereby leading to varying extents (AUC). The amino acid ester prodrug SACV owing to its enhanced stability, higher AUC and better concentration at last time point seems to be a promising candidate for the oral treatment of herpes infections.

Pharmacokinetic analysis of polyamide nucleic-acid-cell penetrating peptide conjugates targeted against HIV-1 transactivation response element

We have demonstrated that polyamide nucleic acids complementary to the transactivation response (TAR) element of HIV-1 LTR inhibit HIV-1 production when transfected in HIV-1 infected cells. We have further shown that anti-TAR PNA (PNA(TAR)) conjugated with cell-penetrating peptide (CPP) is rapidly taken up by cells and exhibits strong antiviral and anti-HIV-1 virucidal activities. Here, we pharmacokinetically analyzed (125)I-labeled PNA(TAR) conjugated with two CPPs: a 16-mer penetratin derived from antennapedia and a 13-mer Tat peptide derived from HIV-1 Tat. We administered the (125)I-labeled PNA(TAR)-CPP conjugates to male Balb/C mice through intraperitoneal or gavage routes. The naked (125)I-labeled PNA(TAR) was used as a control. Following a single administration of the labeled compounds, their distribution and retention in various organs were monitored at various time points. Regardless of the administration route, a significant accumulation of each PNA(TAR)-CPP conjugate was found in different mouse organs and tissues. The clearance profile of the accumulated radioactivity from different organs displayed a biphasic exponential pathway whereby part of the radioactivity cleared rapidly, but a significant portion of it was slowly released over a prolonged period. The kinetics of clearance of individual PNA(TAR)-CPP conjugates slightly varied in different organs, while the overall biphasic clearance pattern remained unaltered regardless of the administration route. Surprisingly, unconjugated naked PNA(TAR) displayed a similar distribution and clearance profile in most organs studied although extent of its uptake was lower than the PNA(TAR)-CPP conjugates.

Alpha-melanocyte-stimulating hormone and related tripeptides: biochemistry, antiinflammatory and protective effects in vitro and in vivo, and future perspectives for the treatment of immune-mediated inflammatory diseases

Alpha-MSH is a tridecapeptide derived from proopiomelanocortin. Many studies over the last few years have provided evidence that alpha-MSH has potent protective and antiinflammatory effects. These effects can be elicited via centrally expressed melanocortin receptors that orchestrate descending neurogenic antiinflammatory pathways. alpha-MSH can also exert antiinflammatory and protective effects on cells of the immune system and on peripheral nonimmune cell types expressing melanocortin receptors. At the molecular level, alpha-MSH affects various pathways implicated in regulation of inflammation and protection, i.e., nuclear factor-kappaB activation, expression of adhesion molecules and chemokine receptors, production of proinflammatory cytokines and mediators, IL-10 synthesis, T cell proliferation and activity, inflammatory cell migration, expression of antioxidative enzymes, and apoptosis. The antiinflammatory effects of alpha-MSH have been validated in animal models of experimentally induced fever; irritant and allergic contact dermatitis, vasculitis, and fibrosis; ocular, gastrointestinal, brain, and allergic airway inflammation; and arthritis, but also in models of organ injury. One obstacle limiting the use of alpha-MSH in inflammatory disorders is its pigmentary effect. Due to its preserved antiinflammatory effect but lack of pigmentary action, the C-terminal tripeptide of alpha-MSH, KPV, has been delineated as an alternative for antiinflammatory therapy. KdPT, a derivative of KPV corresponding to amino acids 193-195 of IL-1beta, is also emerging as a tripeptide with antiinflammatory effects. The physiochemical properties and expected low costs of production render both agents suitable for the future treatment of immune-mediated inflammatory skin and bowel disease, fibrosis, allergic and inflammatory lung disease, ocular inflammation, and arthritis.

Butyrate transcriptionally enhances peptide transporter PepT1 expression and activity

Background

PepT1, an intestinal epithelial apical di/tripeptide transporter, is normally expressed in the small intestine and induced in colon during chronic inflammation. This study aimed at investigating PepT1 regulation by butyrate, a short-chain fatty acid produced by commensal bacteria and accumulated inside inflamed colonocyte.

Results

We found that butyrate treatment of human intestinal epithelial Caco2-BBE cells increased human PepT1 (hPepT1) promoter activity in a dose- and time-dependent manner, with maximal activity observed in cells treated with 5 mM butyrate for 24 h. Under this condition, hPepT1 promoter activity, mRNA and protein expression levels were increased as assessed by luciferase assay, real-time RT-PCR and Western blot, respectively. hPepT1 transport activity was accordingly increased by ∼2.5-fold. Butyrate did not alter hPepT1 mRNA half-life indicating that butyrate acts at the transcriptional level. Molecular analyses revealed that Cdx2 is the most important transcription factor for butyrate-induced increase of hPepT1 expression and activity in Caco2-BBE cells. Butyrate-activated Cdx2 binding to hPepT1 promoter was confirmed by gel shift and chromatin immunoprecipitation. Moreover, Caco2-BBE cells overexpressing Cdx2 exhibited greater hPepT1 expression level than wild-type cells. Finally, treatment of mice with 5 mM butyrate added to drinking water for 24 h increased colonic PepT1 mRNA and protein expression levels, as well as enhanced PepT1 transport activity in colonic apical membranes vesicles.

Conclusions

Collectively, our results demonstrate that butyrate increases PepT1 expression and activity in colonic epithelial cells, which provides a new understanding of PepT1 regulation during chronic inflammation.

Ethanol inhibits functional activity of the human intestinal dipeptide transporter hPepT1 expressed in Xenopus oocytes

BACKGROUND

The pathological effects of high alcohol (ethanol) consumption on gastrointestinal and hepatic systems are well recognized. However, the effects of ethanol intake on gastric and intestinal absorption and transport systems remain unclear. The present study investigates the effects of ethanol on the human peptide transporter 1 (hPepT1) which mediates the transport of di-and tripeptides as well as several orally administered peptidomimetic drugs such as beta-lactam antibiotics (e.g., penicillin), angiotensin-converting enzyme inhibitors, the anti-neoplastic agent bestatin, and prodrugs of acyclovir.

METHODS

Xenopus oocytes were injected with hPepT1 cRNA and incubated for 3 to 10 days. Currents induced by glycyl-sarcosine (Gly-Sar), Ala-Ala (dipeptides), penicillin and enalapril measured in the presence or absence of ethanol were determined using an 8-channel 2-electrode voltage clamp system, with a membrane potential of -70 mV and 11 voltage steps of 100 milliseconds (from +50 mV to -150 mV in -20 mV increments).

RESULTS

Ethanol (200 mM) inhibited Gly-Sar and Ala-Ala currents by 42 and 30%, respectively, with IC(50)s of 184 and 371 mM, respectively. Ethanol reduced maximal transport capacity (I(max)) of hPepT1 for Gly-Sar without affecting Gly-Sar binding affinity (K(0.5) and Hill coefficient). Penicillin- and enalapril-induced currents were significantly less than those induced by dipeptides and were not inhibited by ethanol.

CONCLUSION

Ethanol significantly reduced transport of dipeptides via a reduction in transport capacity, rather than competing for binding sites in hPepT1. Ethanol inhibition or alteration of transport function may be a primary causative factor contributing to both the nutritional deficits as well as the immunological deficiencies that many alcoholics experience including alcohol liver disease and brain damage.

Board-invited review: Peptide absorption and utilization: Implications for animal nutrition and health

Over the last 50 yr, the study of intestinal peptide transport has rapidly evolved into a field with exciting nutritional and biomedical applications. In this review, we describe from a historical and current perspective intestinal peptide transport, the importance of peptides to whole-body nutrition, and the cloning and characterization of the intestinal peptide transporter, PepT1. We focus on the nutritional significance of peptide transport and relate these findings to livestock and poultry. Amino acids are transported into the enterocyte as free AA by a variety of AA transporters that vary in substrate specificity or as di- and tripeptides by the peptide transporter, PepT1. Expression of PepT1 is largely restricted to the small intestine in most species; however, in ruminants, peptide transport and activity is observed in the rumen and omasum. The extent to which peptides are absorbed and utilized is still unclear. In ruminants, peptides make a contribution to the portal-drained visceral flux of total AA and are detected in circulating plasma. Peptides can be utilized by the mammary gland for milk protein synthesis and by a variety of other tissues. We discuss the factors known to regulate expression of PepT1 including development, diet, hormones, diurnal rhythm, and disease. Expression of PepT1 is detected during embryological stages in both birds and mammals and increases with age, a strategic event that allows for the immediate uptake of nutrients after hatch or birth. Both increasing levels of protein in the diet and dietary protein deficiencies are found to upregulate the peptide transporter. We also include in this review a discussion of the use of dietary peptides and potential alternate routes of nutrient delivery to the cell. Our goal is to impart to the reader the nutritional implications of peptide transport and dietary peptides and share discoveries that shed light on various biological processes, including rapid establishment of intestinal function in early neonates and maintenance of intestinal function during fasting, starvation, and disease states.

Oligopeptide transporter PepT1 in Atlantic cod (Gadus morhua L.): cloning, tissue expression and comparative aspects

A novel full-length cDNA that encodes for the Atlantic cod (Gadus morhua L.) PepT1-type oligopeptide transporter has been cloned. This cDNA (named codPepT1) was 2,838 bp long, with an open reading frame of 2,190 bp encoding a putative protein of 729 amino acids. Comparison of the predicted Atlantic cod PepT1 protein with zebrafish, bird and mammalian orthologs allowed detection of many structural features that are highly conserved among all the vertebrate proteins analysed, including (1) a larger than expected area of hydrophobic amino acids in close proximity to the N terminus; (2) a single highly conserved cAMP/cGMP-dependent protein kinase phosphorylation motif; (3) a large N-glycosylation-rich region within the large extracellular loop; and (4) a conserved and previously undescribed stretch of 8-12 amino acid residues within the large extracellular loop. Expression analysis at the mRNA level indicated that Atlantic cod PepT1 is mainly expressed at intestinal level, but that it is also present in kidney and spleen. Analysis of its regional distribution along the intestinal tract of the fish revealed that PepT1 is ubiquitously expressed in all segments beyond the stomach, including the pyloric caeca, and through the whole midgut. Only in the last segment, which included the hindgut, was there a lower expression. Atlantic cod PepT1, the second teleost fish PepT1-type transporter documented to date, will contribute to the elucidation of the evolutionary and functional relationships among vertebrate peptide transporters. Moreover, it can represent a useful tool for the study of gut functional regionalization, as well as a marker for the analysis of temporal and spatial expression during ontogeny.

Cancer detection using a PET tracer, 11C-glycylsarcosine, targeted to H+/peptide transporter

H+/peptide transporter, PEPT1, is functionally expressed in some human cancer cell lines and might be a candidate molecular target for detection of cancers in vivo using PET. The aim of the present study was to establish a novel tumor-imaging technology using a PET tracer targeted to H+/peptide transporter(s). We also compared the tracer with 18F-FDG, focusing on the specificity of their accumulation between tumor and inflammatory tissues.

METHODS

A dipeptide PET tracer, 11C-glycylsarcosine (11C-Gly-Sar), was injected intravenously into athymic mice transplanted with human pancreatic, prostate, and gastric cancer cells. The distribution patterns of 11C-Gly-Sar and 18F-FDG in the tumor-bearing mice, and in mice with inflammatory tissue, were assessed by imaging with a positron planar imaging system (PPIS). Tissue distributions of tracer radioactivity were also measured. The expression levels of PEPT1 and PEPT2 (PEPTs) proteins in tumor xenografts and inflammatory tissue were examined by immunohistochemical analysis. The messenger RNA expression levels of PEPTs in 58 available cancer cell lines were quantified by means of real-time polymerase chain reaction.

RESULTS

All 3 tumor xenografts were well visualized with the PPIS after injection of 11C-Gly-Sar. Expression of PEPTs in those xenografts was confirmed by immunohistochemical analysis. Tumor-to-blood concentration ratios of 11C-Gly-Sar increased in a time-dependent manner and were much higher than unity. Most of the radioactivity found in the tumor tissue was recovered as the intact tracer. These results indicated that 11C-Gly-Sar was taken up by the PEPTs in tumor xenografts. It is noteworthy that 11C-Gly-Sar was minimally present in inflammatory tissues that expressed no PEPT1 or PEPT2 protein, whereas 18F-FDG was highly accumulated, with the values of the selectivity index being >25.1 and 0.72 for 11C-Gly-Sar and 18F-FDG, respectively. The mRNAs of PEPT1 and PEPT2 were expressed in 27.6% and 93.1%, respectively, of the cancer cell lines examined in the present study.

CONCLUSION

The present study indicates that 11C-Gly-Sar is a promising tumor-imaging agent and is superior to 18F-FDG for distinguishing between tumors and inflammatory tissue. Because PEPTs were ubiquitously expressed in various types of tumor cells examined, 11C-Gly-Sar could be useful for the detection of many types of cancers.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

PepT1-mediated tripeptide KPV uptake reduces intestinal inflammation

BACKGROUND & AIMS

KPV is a tripeptide (Lys-Pro-Val), which possesses anti-inflammatory properties; however, its mechanisms of action still remain unknown. PepT1 is a di/tripeptide transporter normally expressed in the small intestine and induced in colon during inflammatory bowel disease (IBD). The aim of this study was to 1) investigate whether the KPV anti-inflammatory effect is PepT1-mediated in intestinal epithelian and immune cells, and 2) examine the anti-inflammatory effects in two models of mice colitis.

METHODS

Human intestinal epithelial cells Caco2-BBE, HT29-Cl.19A, and human T cells (Jurkat) were stimulated with pro-inflammatory cytokines in the present or absence of KPV. KPV anti-inflammatory effect was assessed using a NF-kappaB luciferase gene reporter, Western blot, real-time RT-PCR and ELISA. Uptake experiments were performed using cold KPV as a competitor for PepT1 radiolabelled substrate or using [(3)H]KPV to determine kinetic characteristics of KPV uptake. Anti-inflammatory effect of KPV was also investigated in DSS- and TNBS-induced colitis in mice. KPV was added to drinking water and inflammation was assessed at the histologic level and by proinflammatory cytokine mRNA expression.

RESULTS

Nanomolar concentrations of KPV inhibit the activation of NF-kappaB and MAP kinase inflammatory signaling pathways, and reduce pro-inflammatory cytokine secretion. We found that KPV acts via PepT1 expressed in immune and intestinal epithelial cells. Furthermore, oral administration of KPV reduces the incidence of DSS- and TNBS-induced colitis indicated by a decrease in pro-inflammatory cytokine expression.

CONCLUSIONS

This study indicates tht KPV is transported into cells by PepT1 and might be a new therapeutic agent for IBD.

Role and relevance of PEPT2 in drug disposition, dynamics, and toxicity

Pept2 knockout mice are an important tool to evaluate the evolving role and relevance of this proton-coupled oligopeptide transporter beyond drug disposition, where the transporter also modulates the pharmacodynamic and toxicodynamic effects of drug substrates. Our in vivo studies with glycylsarcosine in Pept2 knockout mice have established "proof of concept" that PEPT2 can have a significant effect on dipeptide disposition. Subsequent studies with the aminocephalosporin antibiotic cefadroxil have shown relevance to pharmacology and infectious disease. Finally, studies with the endogenous peptidomimetic 5-aminolevulinic acid have demonstrated relevance to toxicology in the framework of porphyria- and lead-induced neurotoxicity. These studies have consistently demonstrated the dual action of PEPT2 with respect to its apical localization in choroid plexus epithelium and kidney in: 1) effluxing substrates from CSF into choroid plexus, thereby affecting regional pharmacokinetics in brain; and 2) reabsorbing substrates from renal tubular fluid into proximal tubules, thereby affecting systemic pharmacokinetics and exposure. Moreover, these studies have shown that the regional effect of PEPT2 in limiting substrate concentrations in the CSF is more dramatic than its effect in increasing systemic exposure. In the case of 5-aminolevulinic acid, such regional modulation of drug disposition translates directly into significant changes in neurotoxicity.

hPEPT1 is responsible for uptake and transport of Gly-Sar in the human bronchial airway epithelial cell-line Calu-3

The purpose of this work was to investigate the apical uptake and transepithelial transport of Gly-Sar along with the expression of the di-/tripeptide transporters hPEPT1 and hPEPT2 in human Calu-3 bronchial epithelial cells. The apical Gly-Sar uptake rate in Calu-3 cells followed Michaelis-Menten kinetics with a Km value of 1.3 +/- 0.3 mM and a Vmax value of 0.60 +/- 0.06 nmol cm(-2) min(-1). Transepithelial apical to basolateral transport of 50 microM [3H]-labelled Gly-Sar across the Calu-3 cell monolayer was pH-dependent. The Gly-Sar flux was significantly reduced in the presence of delta-aminolevulinic acid (2.5 mM), cephalexin (25 mM), and captopril (25 mM; p < 0.05, n = 3). Reverse transcriptase polymerase chain reaction (RT-PCR) revealed the presence of both hPEPT1 and hPEPT2 mRNA in the Calu-3 cells. These findings were confirmed in healthy human bronchial cDNA. Restriction-endonuclease analysis identified hPEPT2 in Calu-3 cells to be the hPEPT2*1 haplotype. Western blotting demonstrated expression of the hPEPT1 protein (approximately 80 kDa), and the immunolabel was mainly localized in the apical membrane as judged by immunolocalization studies using confocal laser scanning microscopy (CLSM). This work presents for the first time hPEPT1 and hPEPT2*1 expression in human Calu-3 cells. Surprisingly, the results indicate that Gly-Sar uptake and transport in Calu-3 cells are hPEPT1-mediated rather than hPEPT2-mediated.

Association of PepT1 with lipid rafts differently modulates its transport activity in polarized and nonpolarized cells

The transporter PepT1, apically expressed in intestinal epithelial cells, is responsible for the uptake of di/tripeptides. PepT1 is also expressed in nonpolarized immune cells. Here we investigated the localization of PepT1 in lipid rafts in small intestinal brush border membranes (BBMs) and polarized and nonpolarized cells, as well as functional consequences of the association of PepT1 with lipid rafts. Immunoblot analysis showed the presence of PepT1 in low-density fractions isolated from mouse intestinal BBMs, polarized intestinal Caco2-BBE cells, and nonpolarized Jurkat cells by solubilization in ice-cold 0.5% Triton X-100 and sucrose gradient fractionation. PepT1 colocalized with lipid raft markers GM1 and N-aminopeptidase in intestinal BBMs and Caco2-BBE cell membranes. Disruption of lipid rafts with methyl-beta-cyclodextrin (MbetaCD) shifted PepT1 from low- to high-density fractions. Remarkably, we found that MbetaCD treatment increased PepT1 transport activity in polarized intestinal epithelia but decreased that in intestinal BBM vesicles and nonpolarized immune cells. Mutational analysis showed that phenylalanine 293, phenylalanine 297, and threonine 281 in transmembrane segment 7 of the human di/tripeptide transporter, hPepT1, are important for the targeting to lipid rafts and transport activity of hPepT1. In conclusion, the association of PepT1 with lipid rafts differently modulates its transport activity in polarized and nonpolarized cells.

Is there a compromise between nutrient uptake and gas exchange in the gut of Misgurnus anguillicaudatus, an intestinal air-breathing fish?

The Asian weatherloach, Misgurnus anguillicaudatus (Cobitidae), is a facultative air-breathing teleost fish that makes use of its hindgut or intestine as an accessory air-breathing organ (ABO). The hindgut is highly modified, being well vascularized with intraepithelial capillaries, which makes it well suited for gas exchange. However, the consequences for nutrient uptake, the traditional function of the intestine are unknown. The alimentary canal was examined histologically to assess differences between the fore-, mid- and hindgut regions that have been considered as the digestive, spiral and respiratory zones, respectively. In order to characterise the potential digestive (absorptive) function of the respiratory zone we used semi-quantitative polymerase chain reaction (PCR) to detect the presence of the intestinal Na(+):glucose cotransporter (SGLT1; SLC5A1) and H(+):peptide cotransporter (PEPT1a; SLC15A1) and partially sequenced the SGLT1 and PEPT1a cDNAs. These two transporters play important roles in the absorption of carbohydrate and di-/tripeptides, respectively, in the gut of fishes and other vertebrates and were therefore used as markers for potential nutrient uptake function. We also determined their tissue distributions through semi-quantitative RT-PCR. The effects of diet composition (high protein or high carbohydrate) or fasting on gene expression were also examined. SGLT1 expression was found in kidney, liver, heart, as well as in the three zones of the gut except the most distal part of the hindgut. PEPT1a mRNA was found in heart, brain, liver, and fore- and midgut, but absent in the hindgut. Our results clearly show high expression of SGLT1 (both mRNA and protein by immunolocalization) and PEPT1a (mRNA) in the foregut and midgut correlated with the digestive region of the gut. Modulatory effects of diet on the gene expression for both SGLT1 and PEPT1a were not observed. The presence of SGLT1 transcripts in the respiratory zone of the intestine suggests an overlap in function. However, in the case of PEPT1a, the distal limit was the midgut. Thus, despite its highly modified structure, the hindgut of the loach retains some potential nutrient uptake function.

Tripeptides of RS1 (RSC1A1) inhibit a monosaccharide-dependent exocytotic pathway of Na+-D-glucose cotransporter SGLT1 with high affinity

The human gene RSC1A1 codes for a 67-kDa protein named RS1 that mediates transcriptional and post-transcriptional regulation of Na(+)-D-glucose cotransporter SGLT1. The post-transcriptional regulation occurs at the trans-Golgi network (TGN). We identified two tripeptides in human RS1 (Gln-Cys-Pro (QCP) and Gln-Ser-Pro (QSP)) that induce posttranscriptional down-regulation of SGLT1 at the TGN leading to 40-50% reduction of SGLT1 in plasma membrane. For effective intracellular concentrations IC(50) values of 2.0 nM (QCP) and 0.16 nm (QSP) were estimated. Down-regulation of SGLT1 by tripeptides was attenuated by intracellular monosaccharides including non-metabolized methyl-alpha-D-glucopyranoside and 2-deoxyglucose. In small intestine post-transcriptional regulation of SGLT1 may contribute to glucose-dependent regulation of liver metabolism and intestinal mobility. QCP and QSP are transported by the H(+)-peptide cotransporter PepT1 that is colocated with SGLT1 in small intestinal enterocytes. Using coexpression of SGLT1 and PepT1 in Xenopus oocytes or polarized Caco-2 cells that contain both transporters we demonstrated that the tripeptides were effective when applied to the extracellular compartment. After a 1-h perfusion of intact rat small intestine with QSP, glucose absorption was reduced by 30%. The data indicate that orally applied tripeptides can be used to down-regulate small intestinal glucose absorption, e.g. in diabetes mellitus.

Artificial neural network models for prediction of intestinal permeability of oligopeptides

BACKGROUND

Oral delivery is a highly desirable property for candidate drugs under development. Computational modeling could provide a quick and inexpensive way to assess the intestinal permeability of a molecule. Although there have been several studies aimed at predicting the intestinal absorption of chemical compounds, there have been no attempts to predict intestinal permeability on the basis of peptide sequence information. To develop models for predicting the intestinal permeability of peptides, we adopted an artificial neural network as a machine-learning algorithm. The positive control data consisted of intestinal barrier-permeable peptides obtained by the peroral phage display technique, and the negative control data were prepared from random sequences.

RESULTS

The capacity of our models to make appropriate predictions was validated by statistical indicators including sensitivity, specificity, enrichment curve, and the area under the receiver operating characteristic (ROC) curve (the ROC score). The training and test set statistics indicated that our models were of strikingly good quality and could discriminate between permeable and random sequences with a high level of confidence.

CONCLUSION

We developed artificial neural network models to predict the intestinal permeabilities of oligopeptides on the basis of peptide sequence information. Both binary and VHSE (principal components score Vectors of Hydrophobic, Steric and Electronic properties) descriptors produced statistically significant training models; the models with simple neural network architectures showed slightly greater predictive power than those with complex ones. We anticipate that our models will be applicable to the selection of intestinal barrier-permeable peptides for generating peptide drugs or peptidomimetics.

H+-coupled nutrient, micronutrient and drug transporters in the mammalian small intestine

The H(+)-electrochemical gradient was originally considered as a driving force for solute transport only across cellular membranes of bacteria, plants and yeast. However, in the mammalian small intestine, a H(+)-electrochemical gradient is present at the epithelial brush-border membrane in the form of an acid microclimate. Over recent years, a large number of H(+)-coupled cotransport mechanisms have been identified at the luminal membrane of the mammalian small intestine. These transporters are responsible for the initial stage in absorption of a remarkable variety of essential and non-essential nutrients and micronutrients, including protein digestion products (di/tripeptides and amino acids), vitamins, short-chain fatty acids and divalent metal ions. Proton-coupled cotransporters expressed at the mammalian small intestinal brush-border membrane include: the di/tripeptide transporter PepT1 (SLC15A1); the proton-coupled amino-acid transporter PAT1 (SLC36A1); the divalent metal transporter DMT1 (SLC11A2); the organic anion transporting polypeptide OATP2B1 (SLC02B1); the monocarboxylate transporter MCT1 (SLC16A1); the proton-coupled folate transporter PCFT (SLC46A1); the sodium-glucose linked cotransporter SGLT1 (SLC5A1); and the excitatory amino acid carrier EAAC1 (SLC1A1). Emerging research demonstrates that the optimal intestinal absorptive capacity of certain H(+)-coupled cotransporters (PepT1 and PAT1) is dependent upon function of the brush-border Na(+)-H(+) exchanger NHE3 (SLC9A3). The high oral bioavailability of a large number of pharmaceutical compounds results, in part, from absorptive transport via the same H(+)-coupled cotransporters. Drugs undergoing H(+)-coupled cotransport across the intestinal brush-border membrane include those used to treat bacterial infections, hypercholesterolaemia, hypertension, hyperglycaemia, viral infections, allergies, epilepsy, schizophrenia, rheumatoid arthritis and cancer.

Effect of l-phenylalanine supplementation and a high-protein diet on pharmacokinetics of cefdinir in healthy volunteers: an exploratory study

BACKGROUND

Upregulation of oligopeptide transport activity by dietary protein, certain dipeptides and amino acids has been reported in the rat intestine and a human intestinal cell line.

OBJECTIVE

In this study, the pharmacokinetics of cefdinir were investigated after L-phenylalanine supplementation and a high-protein diet (HPD) in humans to explore changes in the activities of intestinal and renal oligopeptide transporters.

METHODS

A normal-protein diet (NPD, 73.2 +/- 2.6 g/day), NPD + l-phenylalanine (7.5 g/day), or HPD (141.3 +/- 3.7 g/day) was given to six male healthy volunteers for 12 days followed by a single dose of cefdinir after an overnight fast in a randomized three-way crossover study with a 22-day washout. Blood and urine were collected over a 12-h period after administration of cefdinir. Concentrations of cefdinir in plasma and/or urine were measured by high-performance liquid chromatography.

RESULTS

Plasma concentrations and urinary excretion of the drug did not change throughout the study. Physiological variables and laboratory values did not reveal any differences between the three periods except for serum and urinary nitrogen levels and serum triglyceride.

DISCUSSION

A reason for the unchanged pharmacokinetics of cefdinir may be due to lower doses of L-phenylalanine and protein in humans than in animals when converting animal effective doses to humans.

CONCLUSION

In humans, L-phenylalanine supplementation and HPD do not seem to upregulate intestinal and renal oligopeptide transport in the ranges of duration and dose examined.

Design, Synthesis, and Evaluation of Tripeptidic Promoieties Targeting the Intestinal Peptide Transporter hPEPT1

The human intestinal proton coupled di/tri-peptide transporter hPEPT1 promotes the oral bioavailability of several drug compounds. The strategy behind the present work is that by linking a suitable di- or tripeptidic promoiety to a drug substance, by a hydrolysable ester bond, it may give rise to a prodrug that targets hPEPT1. 29 tripeptides were designed based on known structural requirements for substrates binding hPEPT1. Serine, homoserine, or threonine was incorporated in the tripeptide as hydroxy group donors in order for them to be linked to carboxylic drug substances. Optimisation of the promoiety included a study of 14 unnatural tripeptides whose diversity was expressed by VolSurf descriptors. A total of 29 tripeptides was synthesised by solid phase peptide synthesis and a standard Fmoc protocol. The affinity of the tripeptides to hPEPT1 was determined by measuring the inhibition of [(14)C]Gly-Sar in mature Caco-2 cell monolayers which resulted in K(i) values ranging from 0.22 to 25 mM or above. Translocation through the intestinal membrane, mediated by hPEPT1, was measured by recording the membrane potential relative to that induced by the known substrate Gly-Sar. The change in membrane potential is caused by influx of protons due to the co-transport of substrates and protons by hPEPT1 and is, as such, an indication of translocation. A K(i) value of 0.30 mM combined with efficient translocation indicated that H-Phe-Ser-Ala-OH is a suitable lead promoiety for targeted hPEPT1 prodrug design.

Regulation of intestinal hPepT1 (SLC15A1) activity by phosphodiesterase inhibitors is via inhibition of NHE3 (SLC9A3)

The H⁺-coupled transporter hPepT1 (SLC15A1) mediates the transport of di/tripeptides and many orally-active drugs across the brush-border membrane of the small intestinal epithelium. Incubation of Caco-2 cell monolayers (15 min) with the dietary phosphodiesterase inhibitors caffeine and theophylline inhibited Gly–Sar uptake across the apical membrane. Pentoxifylline, a phosphodiesterase inhibitor given orally to treat intermittent claudication, also decreased Gly–Sar uptake through a reduction in capacity (V_max) without any effect on affinity (K_m). The reduction in dipeptide transport was dependent upon both extracellular Na⁺ and apical pH but was not observed in the presence of the selective Na⁺/H⁺ exchanger NHE3 (SLC9A3) inhibitor S1611. Measurement of intracellular pH confirmed that caffeine was not directly inhibiting hPepT1 but rather having an indirect effect through inhibition of NHE3 activity. NHE3 maintains the H⁺-electrochemical gradient which, in turn, acts as the driving force for H⁺-coupled solute transport. Uptake of β-alanine, a substrate for the H⁺-coupled amino acid transporter hPAT1 (SLC36A1), was also inhibited by caffeine. The regulation of NHE3 by non-nutrient components of diet or orally-delivered drugs may alter the function of any solute carrier dependent upon the H⁺-electrochemical gradient and may, therefore, be a site for both nutrient–drug and drug–drug interactions in the small intestine.

Cysteine scanning of transmembrane domain three of the human dipeptide transporter: implications for substrate transport

The human intestinal dipeptide transporter (hPepT1) transports dipeptides and pharmacologically active drugs from the intestine to the blood. The role of transmembrane domain 3 (TMD3) of hPepT1 was studied using cysteine-scanning mutagenesis and methane thiosulfonate (MTS) cysteine modification. Each amino acid in TMD3 was individually mutated to a cysteine and Gly-Sar uptake by each mutated and modified transporter was determined relative to wild-type hPepT1. Uptake data for mutated transporters modified with the lipid-insoluble cysteine-modifying reagent MTSET suggested tilting of TMD3 relative to the substrate translocation pathway; the extracellular region of TMD3 showed little MTSET reactivity, indicative of solvent inaccessibility, whereas the intracellular part of TMD3 was relatively solvent accessible. Modification at 10 positions of TMD3 with MTSEA, a lipid-soluble cysteine-modifying reagent, gave unusual and statistically significant increases in Gly-Sar uptake relative to untreated mutants. We interpret these data in terms of the spatial properties of the hPepT1 substrate translocation channel and possible interactions of TMD3 with other transmembrane domains.

Functional expression of stereoselective metabolism of cephalexin by exogenous transfection of oligopeptide transporter PEPT1

Gastrointestinal absorption of the beta-lactam antibiotic cephalexin (CEX) is highly stereoselective: l- and d-CEX are both taken up by intestinal epithelial cells through the brush-border membrane, most likely via oligopeptide transporter PEPT1, but l-CEX is not found in serum or urine after administration p.o. because of its rapid intestinal metabolism, whereas d-CEX is well absorbed in the unchanged form. We examined the contribution of PEPT1 to the stereoselective uptake and metabolism of CEX. We observed stereoselective metabolism of CEX after exogenous transfection of PEPT1 alone into mammalian cell lines: l-CEX, but not d-CEX, was metabolized to 7-aminodesacetoxycephalosporanic acid (7-ADCA) in HeLa and human embryonic kidney 293 cells stably and transiently expressing human PEPT1, respectively, whereas such metabolism was minor in cells expressing the vector alone. The formation rate of 7-ADCA depended on the amount of PEPT1 cDNA transfected. l-CEX metabolism was rapid because only 7-ADCA was found inside and outside the cells during incubation with l-CEX. The characteristics of PEPT1-mediated metabolism of l-CEX were similar, but not identical, to those of PEPT1-mediated transport. PEPT1-mediated metabolism was also observed in permeabilized cells expressing PEPT1, in which PEPT1-mediated intracellular substrate accumulation was negligible, suggesting that the increase in l-CEX metabolism by PEPT1 transfection cannot be fully explained by an increase in uptake and subsequent exposure to intracellular hydrolases. The present findings show that stereoselectivity in CEX absorption can be fully explained in terms of PEPT1, implying that the l-CEX hydrolase is PEPT1 itself or is induced by PEPT1.

Distribution and abundance of nutrient transporter mRNA in the intestinal tract of the black bear, Ursus americanus

End products of digestion are absorbed by the body through the action of transporter proteins expressed on the apical membrane of intestinal epithelial cells. We investigated the mRNA abundance and distribution of a peptide transporter (PepT1), a glucose transporter (SGLT1), two amino acid transporters (NBAT and b(o,+)AT), and a digestive enzyme, aminopeptidase N (APN), in the intestinal tract of black bears (Ursus americanus). Intestinal total RNA was isolated from 10 bears and abundance of PepT1, SGLT1, NBAT, b(o,+)AT, and APN mRNA were determined by Northern blots. Abundance of PepT1 (P<0.05), APN (P<0.05), and SGLT1 (P<0.0001) changed quadratically from the proximal to distal intestine with abundance being greatest in the midregion. Abundance of b(o,+)AT mRNA increased linearly (P<0.05) from the proximal to distal intestine. The number of molecules of mRNA/ng of total RNA for each gene was determined using Real-Time PCR. PepT1 mRNA was present at 10-fold or greater levels than amino acid transporter mRNA in all segments of the intestine, suggesting that di- and tripeptides constitute a major form in which amino acids are absorbed in the black bear. The abundance of NBAT and b(o,+)AT mRNA was greater towards the distal intestine, suggesting a role in salvaging unabsorbed amino acids.

Transepithelial transport of zinc and L-histidine across perfused intestine of American lobster, Homarus americanus

The intestine of the American lobster, Homarus americanus, was isolated and perfused in vitro with a physiological saline, based on the ion composition of the blood, to characterize the mechanisms responsible for transmural transport of zinc and how the amino acid, L-histidine, affects the net movement of the metal across the tissue. Previous studies with this preparation, focusing on the characteristics of unidirectional mucosa to serosa (M to S) fluxes of (65)Zn(2+) and (3)H-L-histidine, indicated the presence of a brush border co-transport process responsible for simultaneously transferring the metal and amino acid across this tissue as an apparent bis-complex (Zn-[His](2)) using a PEPT-1-like dipeptide carrier mechanism. In addition, both zinc and L-histidine were also transferred toward the blood by separate transporters that were independent of the other substrate. The focus of the present study was to characterize the serosa to mucosa (S to M) flux of (65)Zn(2+) under a variety of conditions, and use these values in conjunction with those from the previous study, to assess the direction and magnitude of net metal movement across the tissue. Transmural S to M transport of (65)Zn(2+) was markedly reduced with the addition of the serosal inhibitors ouabain (32%), excess K(+) (25%), excess Ca(2+) (30%), Cu(2+) (38%), nifedipine (21%), and vanadate (53%). In contrast, this flux was markedly stimulated with the serosal addition of ATP (24%) and excess Na(+) (28%). These results suggest that S to M fluxes of zinc occurred by the combination of the basolateral Na/Ca exchanger (NCX), where zinc replaced calcium, and a basolateral nifedipine-sensitive calcium channel. Transmural M to S (65)Zn(2+) fluxes (5-100 microM) were threefold greater than S to M metal transport, and the addition of luminal L-histidine doubled the net M to S zinc flux over its rate in the absence of the amino acid. The results of this paper and those in its predecessor indicate that zinc transport by the lobster intestine is absorptive and significantly enhanced by luminal amino acids.

Abnormal expression of the peptide transporter PepT1 in the colon of massive bowel resection rat: a potential route for colonic mucosa damage by transport of fMLP

The aim of this study was to investigate the effect of abnormal intestinal oligopeptide transporter (PepT1) on rat colon inflammation by transportion of N-formyl-methionyl-leucyl-phenylalanine (fMLP). We induced upregulation of PepT1 in the colon of rats by 80% small bowel resection and examined colonic PepT1 di-tripeptide transport activity. By Western blot analysis, PepT1 was clearly detected in the colon of bowel resection rats, while it was absent from the colon of bowel transection and reanastomosis rats. The experiment with cephalexin colon perfusion showed that the arterial cephalexin concentration in resection rats was five to nine times that in transection rats. Inhibition of PepT1 by Gly-Gly completely abolished cephalexin absorption from the colon of resection rats. We found that 10 microM fMLP perfusion in the colon of resection rats for 4 hr significantly increased myeloperoxidase (MPO) activity and caused colon wall damage. In contrast, 10 muM fMLP perfusion in the colon of transection rats did not induce any inflammation. A 5 mM Gly-Gly perfusion completely inhibited the MPO activity and colonic wall damage induced by 10 muM fMLP. We conclude that colonic PepT1 induced by bowel resection may provide a mechanism for oligopeptide transport and may serve as a potential cause of colonic mucosa damage by transport of the bacterial product fMLP in rat colon.

A charge pair interaction between Arg282 in transmembrane segment 7 and Asp341 in transmembrane segment 8 of hPepT1

PURPOSE

To determine whether R282 in transmembrane segment 7 (TMS7) of hPepT1 forms a salt bridge with D341 in TMS8.

METHODS

Mutated hPepT1 transporters containing point mutations at R282 and/or D341 were transiently transfected into HEK293 cells. Their steady state expression and functional activity were measured using immunoprecipitation and 3H-gly-sar uptake, respectively. Gly-sar uptake by cysteine mutants (R282C and D341C) was also measured in the presence and absence of cysteine-modifying MTS reagents.

RESULTS

The reverse-charge mutants R282D-hPepT1 and D341R-hPepT1 showed significantly reduced gly-sar uptake, but the double mutant (R282D/D341R-hPepT1) has functionality comparable to that of wild-type hPepT1. Gly-sar uptake by R282C-hPepT1 is reduced, but pre-incubation with 1 mM MTSET, a positively charged cysteine-modifying reagent, restored function to wild-type levels. Similarly, pre-incubation of D341C-hPepT1 with 10 mM MTSES, a negatively charged cysteine-modifying reagent, increased gly-sar uptake compared to unmodified D341C-hPepT1. In contrast, MTSET modification of D341C-hPepT1 (giving a positive charge at position 341) resulted in significant reduction in gly-sar uptake, compared to D341C-hPepT1.

CONCLUSION

Our results are consistent with a salt bridge between R282 and D341 in hPepT1, and we use these and other data to propose a role for the R282-D341 charge pair in the hPepT1 translocation mechanism.

Leptin transcriptionally enhances peptide transporter (hPepT1) expression and activity via the cAMP-response element-binding protein and Cdx2 transcription factors

PepT1 is an intestinal epithelial apical membrane transporter that is expressed in the small intestine, with little or no expression in the normal colon. However, we previously demonstrated that colonic PepT1 may be expressed during chronic inflammation. To begin elucidating inflammatory hPepT1 signaling, we herein investigated the long term leptin treatments, on PepT1 expression and activity in Caco2-BBE cells, and began to reveal the involved signaling pathways. We successfully cloned the 723-bp hPepT1 promoter region and identified the human transcription initiation site 86 bp upstream from the translation start site. Leptin treatment dose- and time-dependently increased hPepT1 promoter and transport activities in Caco2-BBE cells, with maximal activity observed in cells treated with 100 nM leptin for 8 h. Under these conditions, we observed 2-fold increases in hPepT1 mRNA and protein expression, as well as increased transport activity. Our molecular analyses of possible signal-transduction pathways revealed that leptin treatment enhanced the intracellular levels of cAMP and phosphorylated cAMP-response element-binding protein (CREB) protein in Caco2-BBE cells, whereas our deletion, mutation, and CDX2 overexpression analyses demonstrated that interaction of the Cdx2 and phosphorylated CREB transcription factors was essential for leptin-induced hPepT1 transcription in Caco2-BBE cells. Our results indicate that leptin, which is increased in inflamed colonic mucosa, triggers colonic expression of hPepT1 via the CREB and Cdx2 transcription factors. These findings provide important new insights into the mechanisms of intestinal inflammation and may suggest new therapeutic modalities in the future.