Transport characteristics of L-carnosine and the anticancer derivative 4-toluenesulfonylureido-carnosine in a human epithelial cell line

Computational approaches in the rational design of improved carbonyl quenchers: focus on histidine containing dipeptides

AIM

The inhibition of protein carbonylation can play therapeutic roles in several oxidative-based diseases and direct carbonyl quenching appears the most effective inhibition strategies. l-carnosine derivatives are effective and selective quenchers toward 4-hydroxy-2-nonenal even though their activity was never investigated in a fully comparable way.

RESULTS

The reported results revealed that anserine, homocarnosine and carnosinamide retain a remarkable quenching activity combined with a satisfactory selectivity. In silico analyses confirmed the key role of flexibility, lipophilicity and nucleophilicity parameters in rationalizing the measured reactivity.

CONCLUSION

This study confirms that in silico approaches can be successfully used in the rational design of improved carbonyl quenchers. Physicochemical and stereoelectronic descriptors appear really informative especially when explored by their corresponding property spaces.

Carnosinases, their substrates and diseases

Carnosinases are Xaa-His dipeptidases that play diverse functions throughout all kingdoms of life. Human isoforms of carnosinase (CN1 and CN2) under appropriate conditions catalyze the hydrolysis of the dipeptides carnosine (β-alanyl-l-histidine) and homocarnosine (γ-aminobutyryl-l-histidine). Alterations of serum carnosinase (CN1) activity has been associated with several pathological conditions, such as neurological disorders, chronic diseases and cancer. For this reason the use of carnosinase levels as a biomarker in cerebrospinal fluid (CSF) has been questioned. The hydrolysis of imidazole-related dipeptides in prokaryotes and eukaryotes is also catalyzed by aminoacyl-histidine dipeptidases like PepD (EC 3.4.13.3), PepV (EC 3.4.13.19) and anserinase (EC 3.4.13.5). The review deals with the structure and function of this class of enzymes in physiological and pathological conditions. The main substrates of these enzymes, i.e., carnosine, homocarnosine and anserine (β-alanyl-3-methyl-l-histidine) will also be described.

PepT1 expressed in immune cells has an important role in promoting the immune response during experimentally induced colitis

We and others have shown that the dipeptide cotransporter PepT1 is expressed in immune cells, including macrophages that are in close contact with the lamina propria of the small and large intestines. In the present study, we used PepT1-knockout (KO) mice to explore the role played by PepT1 in immune cells during dextran sodium sulfate (DSS)-induced colitis. DSS treatment caused less severe body weight loss, diminished rectal bleeding, and less diarrhea in PepT1-KO mice than in wild-type (WT) animals. A histological examination of colonic sections revealed that the colonic architecture was less disrupted and the extent of immune cell infiltration into the mucosa and submucosa following DSS treatment was reduced in PepT1-KO mice compared with WT animals. Consistent with these results, the DSS-induced colitis increase in colonic myeloperoxidase activity was significantly less in PepT1-KO mice than in WT littermates. The colonic levels of mRNAs encoding the inflammatory cytokines CXCL1, interleukin (IL)-6, monocyte chemotactic protein-1, IL-12, and interferon-γ were significantly lower in DSS-treated PepT1-KO mice than in DSS-treated WT animals. Colonic immune cells from WT had significantly higher level of proinflammatory cytokines then PepT1 KO. In addition, we observed that knocking down the PepT1 expression decreases chemotaxis of immune cells recruited during intestinal inflammation. Antibiotic treatment before DSS-induced colitis eliminated the differential expression of inflammatory cytokines between WT and PepT1-KO mice. In conclusion, PepT1 in immune cells regulates the secretion of proinflammatory cytokines triggered by bacteria and/or bacterial products, and thus has an important role in the induction of colitis. PepT1 may transport small bacterial products, such as muramyl dipeptide and the tripeptide L-Ala-gamma-D-Glu-meso-DAP, into macrophages. These materials may be sensed by members of the nucleotide-binding site-leucine-rich repeat family of intracellular receptors, ultimately resulting in altered homeostasis of the intestinal microbiota.

Carrier-mediated γ-aminobutyric acid transport across the basolateral membrane of human intestinal Caco-2 cell monolayers

The aim of the present study was to investigate the transport of γ-aminobutyric acid (GABA) across the basolateral membrane of intestinal cells. The proton-coupled amino acid transporter, hPAT1, mediates the influx of GABA and GABA mimetic drug substances such as vigabatrin and gaboxadol and the anticancer prodrug δ-aminolevulinic acid across the apical membrane of small intestinal enterocytes. Little is however known about the basolateral transport of these substances. We investigated basolateral transport of GABA in mature Caco-2 cell monolayers using isotope studies. Here we report that, at least two transporters seem to be involved in the basolateral transport of GABA. The basolateral uptake consisted of a high-affinity system with a K(m) of 290 μM and V(max) of 75 pmol cm(-2) min(-1) and a low affinity system with a K(m) of approximately 64 mM and V(max) of 1.6 nmol cm(-2) min(-1). The high-affinity transporter is Na(+) and Cl(-) dependent. The substrate specificity of the high-affinity transporter was further studied and Gly-Sar, Leucine, gaboxadol, sarcosine, lysine, betaine, 5-hydroxythryptophan, proline and glycine reduced the GABA uptake to approximately 44-70% of the GABA uptake in the absence of inhibitor. Other substances such as β-alanine, GABA, 5-aminovaleric acid, taurine and δ-aminolevulinic acid reduced the basolateral GABA uptake to 6-25% of the uptake in the absence of inhibitor. Our results indicate that the distance between the charged amino- and acid-groups is particular important for inhibition of basolateral GABA uptake. Thus, there seems to be a partial substrate overlap between the basolateral GABA transporter and hPAT1, which may prove important for understanding drug interactions at the level of intestinal transport.

The PepT1-NOD2 signaling pathway aggravates induced colitis in mice

BACKGROUND & AIMS

The human di/tripeptide transporter human intestinal H-coupled oligonucleotide transporter (hPepT1) is abnormally expressed in colons of patients with inflammatory bowel disease, although its exact role in pathogenesis is unclear. We investigated the contribution of PepT1 to intestinal inflammation in mouse models of colitis and the involvement of the nucleotide-binding oligomerization domain 2 (NOD2) signaling pathway in the pathogenic activity of colonic epithelial hPepT1.

METHODS

Transgenic mice were generated in which hPepT1 expression was regulated by the β-actin or villin promoters; colitis was induced using 2,4,6-trinitrobenzene sulfonic acid (TNBS) or dextran sodium sulfate (DSS) and the inflammatory responses were assessed. The effects of NOD2 deletion in the hPepT1 transgenic mice also was studied to determine the involvement of the PepT1-NOD2 signaling pathway.

RESULTS

TNBS and DSS induced more severe levels of inflammation in β-actin-hPepT1 transgenic mice than wild-type littermates. Intestinal epithelial cell-specific hPepT1 overexpression in villin-hPepT1 transgenic mice increased the severity of inflammation induced by DSS, but not TNBS. Bone marrow transplantation studies showed that hPepT1 expression in intestinal epithelial cells and immune cells has an important role in the proinflammatory response. Antibiotics abolished the effect of hPepT1 overexpression on the inflammatory response in DSS-induced colitis in β-actin-hPepT1 and villin-hPepT1 transgenic mice, indicating that commensal bacteria are required to aggravate intestinal inflammation. Nod2-/-, β-actin-hPepT1 transgenic/Nod2-/-, and villin-hPepT1 transgenic/Nod2-/- littermates had similar levels of susceptibility to DSS-induced colitis, indicating that hPepT1 overexpression increased intestinal inflammation in a NOD2-dependent manner.

CONCLUSIONS

The PepT1-NOD2 signaling pathway is involved in aggravation of DSS-induced colitis in mice.

Ibuprofen is a non-competitive inhibitor of the peptide transporter hPEPT1 (SLC15A1): possible interactions between hPEPT1 substrates and ibuprofen

BACKGROUND AND PURPOSE

Recently, we identified etodolac as a possible ligand for the human intestinal proton-couple peptide transporter (hPEPT1). This raised the possibility that other non-steroidal anti-inflammatory drugs, and especially ibuprofen, could also interact with hPEPT1. Here, we have assessed the interactions of ibuprofen with hPEPT1.

EXPERIMENTAL APPROACH

The uptake of [(14)C]Gly-Sar, [(3)H]Ibuprofen and other radio-labelled compounds were investigated in Madin-Darby canine kidney cells (MDCK)/hPEPT1, MDCK/Mock, LLC-PK(1) or Caco-2 cells. The transepithelial transport of ibuprofen and hPEPT1 substrates was investigated in Caco-2 cell monolayers.

KEY RESULTS

Ibuprofen concentration dependently inhibited hPEPT1-mediated uptake of Gly-Sar in MDCK/hPEPT1 cells (K(i)(app) = 0.4 mM) but uptake of ibuprofen in Caco-2 cells and MDCK/hPEPT1 cells was not inhibited by hPEPT1 substrates. The maximum uptake rate for Gly-Sar uptake was reduced from 522 pmol·min(-1)·cm(-2) to 181 pmol·min(-1)·cm(-2) and 78 pmol·min(-1)·cm(-2) in the presence of 0.5 mM and 1 mM ibuprofen, respectively. The interaction between ibuprofen and hPEPT1 was thus non-competitive. In LLC-PK1 cells, ibuprofen (1 mM) did not influence the transporter-mediated uptake of glycine or α-methyl-D-glycopyranoside. In Caco-2 cell monolayers the absorptive transport of δ-aminolevulinic acid was reduced by 23% and 48% by ibuprofen (1 and 10 mM), respectively. Likewise the transport of Gly-Sar was reduced by 23% in the presence of ibuprofen (1 mM).

CONCLUSIONS AND IMPLICATIONS

Ibuprofen is a non-competitive inhibitor of hPEPT1. As ibuprofen reduced the transepithelial transport of δ-aminolevulinic acid, drug-drug interactions between ibuprofen and hPEPT1 drug substrates at their site of absorption are possible if administered together.

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.

Design, synthesis, and evaluation of carnosine derivatives as selective and efficient sequestering agents of cytotoxic reactive carbonyl species

Carnosine aryl derivatives as sequestering agents of RCS: Reactive carbonyl species (RCS) are cytotoxic mediators representing a novel drug target, as they are presumed to play a pathogenic role in several diseases. Carnosine is a selective RCS-sequestering agent, but is rapidly hydrolyzed by serum carnosinase. Herein we describe the in silico design, synthesis, and evaluation of a set of carnosine aryl derivatives.Reactive carbonyl species (RCS) are important cytotoxic mediators generated by lipid oxidation of polyunsaturated fatty acids (PUFAs) and represent a novel drug target, as they are presumed to play a pathogenic role in several diseases. L-Carnosine (L-CAR, beta-alanyl-L-histidine) is a specific detoxifying agent of RCS, but is rapidly hydrolyzed in human serum by carnosinase, a specific dipeptidase. Herein we describe the in silico design, synthesis, and biological evaluation of carnosine derivatives that are resistant to carnosinase and that have increased quenching efficacy. Stability against carnosinase-mediated turnover was achieved by isomerization of the histidine residue, leading to D-carnosine (D-CAR, beta-alanyl-D-histidine), which maintains the same quenching activity of L-carnosine. A molecular modeling approach was then used to design derivatives characterized by an increased quenching efficacy. The most promising candidates were synthesized, and their stability and quenching activity were evaluated. This study describes a set of aryl derivatives that are characterized by high stability in human plasma and a quenching activity toward 4-hydroxy-trans-2-nonenal (HNE), chosen as a model of RCS, up to threefold greater than D-carnosine.

Transport mechanisms of carnosine in SKPT cells: contribution of apical and basolateral membrane transporters

PURPOSE

The aim of this study was to investigate the transport properties of carnosine in kidney using SKPT cell cultures as a model of proximal tubular transport, and to isolate the functional activities of renal apical and basolateral transporters in this process.

METHODS

The membrane transport kinetics of 10 microM [3H]carnosine was studied in SKPT cells as a function of time, pH, potential inhibitors and substrate concentration. A cellular compartment model was constructed in which the influx, efflux and transepithelial clearances of carnosine were determined. Peptide transporter expression was probed by RT-PCR.

RESULTS

Carnosine uptake was 15-fold greater from the apical than basolateral surface of SKPT cells. However, the apical-to-basolateral transepithelial transport of carnosine was severely rate-limited by its cellular efflux across the basolateral membrane. The high-affinity, proton-dependence, concentration-dependence and inhibitor specificity of carnosine supports the contention that PEPT2 is responsible for its apical uptake. In contrast, the basolateral transporter is saturable, inhibited by PEPT2 substrates but non-concentrative, thereby, suggesting a facilitative carrier.

CONCLUSIONS

Carnosine is expected to have a substantial cellular accumulation in kidney but minimal tubular reabsorption in blood because of its high influx clearance across apical membranes by PEPT2 and very low efflux clearance across basolateral membranes.

β- and γ-Di- and Tripeptides as Potential Substrates for the Oligopeptide Transporter hPepT1

The hPepT1-mediated transport properties of a series of 11 synthesized beta- and gamma-peptides have been studied in Caco-2 cells. The results show that several of the compounds interact with the peptide transporter, but only two beta-dipeptides act as substrates and are transported across the cell monolayers. These two are less-efficient substrates than alpha-peptides. Larger derivatives than beta-dipeptides do not act as hPepT1 substrates, but instead, they appear to be substrates for P-glycoprotein efflux.

The oligopeptide transporter hPepT1: gateway to the innate immune response

Bacterial products that are normally present in the lumen of the colon, such as N-formylated peptides and muramyl-dipeptide, are important for inducing the development of mucosal inflammation. The intestinal dipeptide transporter, hPepT1, which is expressed in inflamed but not in noninflamed colonic epithelial cells, mediates the transport of these bacterial products into the cytosol of colonic epithelial cells. The small bacterial peptides subsequently induce an inflammatory response, including the induction of MHC class I molecules expression and cytokines secretion, via the activation of nucleotide-binding site and leucine-rich repeat (NBS-LRR) proteins, for example NOD2, and activation of NF-kappaB. Subsequent secretion of chemoattractants by colonic epithelial cells induces the movement of neutrophils through the underlying matrix, as well as across the epithelium. These bacterial products can also reach the lamina propria through the paracellular pathway and across the basolateral membrane of epithelial cells. As a consequence, small formylated peptides can interact directly with immune cells through specific membrane receptors. Since immune cells, including macrophages, also express hPepT1, they can transport small bacterial peptides into the cytosol where these may interact with the NBS-LRR family of intracellular receptors. As in intestinal epithelial cells, the presence of these small bacterial peptides in immune cells may trigger immune response activation.

Carbonic anhydrase inhibitors: Transepithelial transport of thioureido sulfonamide inhibitors of the cancer-associated isozyme IX is dependent on efflux transporters

Sulfonamides and their derivatives inhibit the catalytic activity of carbonic anhydrases (CA, EC 4.2.1.1). Isozyme IX (CA IX) is a transmembrane isoform with the active site oriented toward the extracellular space. CA IX was recently shown to be a drug target, and it is highly overexpressed in hypoxic tumors with limited distribution in normal tissues. The present report deals with the drug design, synthesis, and biological investigation of a group of thioureido sulfonamides, which have been obtained by reaction of isothiocyanate-substituted aromatic sulfonamides with amines. These compounds have potent inhibitory properties against CA IX with K(I) values in the range of 10-37 nM and P(app)values > 0.34 x 10(-6) cm/s for the absorptive transepithelial transport in Caco-2 cells. In Caco-2 cells, one of these compounds (A6) was shown to be a substrate for efflux transporters such as P-glycoprotein (P-gp). P-gp activity is not likely to be rate-limiting for intestinal absorption, but might be useful when targeting hypoxic tumors expressing both P-gp and CA IX.

Intestinal absorption of drugs mediated by drug transporters: mechanisms and regulation

The absorption of drugs from the gastrointestinal tract is one of the important determinants for oral bioavailability. Development of in vitro experimental techniques such as isolated membrane vesicles and cell culture systems has allowed us to elucidate the transport mechanisms of various drugs across the plasma membrane. Recent introduction of molecular biological techniques resulted in the successful identification of drug transporters responsible for the intestinal absorption of a wide variety of drugs. Each transporter exhibits its own substrate specificity, though it usually shows broad substrate specificity. In this review, we first summarize the recent advances in the characterization of drug transporters in the small intestine, classified into peptide transporters, organic cation transporters and organic anion transporters. In particular, peptide transporter (PEPT1) is the best-characterized drug transporter in the small intestine, and therefore its utilization to improve the oral absorption of poorly absorbed drugs is briefly described. In addition, regulation of the activity and expression levels of drug transporters seems to be an important aspect, because alterations in the functional characteristics and/or expression levels of drug transporters in the small intestine could be responsible for the intra- and interindividual variability of oral bioavailability of drugs. As an example, regulation of the activity and expression of PEPT1 is summarized.

Phe-Gly Dipeptidomimetics Designed for the Di-/Tripeptide Transporters PEPT1 and PEPT2: Synthesis and Biological Investigations

A series of five Phe-Gly dipeptidomimetics containing different amide bond replacements have been synthesized in a facile way from the readily available unsaturated ketoester 1, and their affinities for the di-/tripeptide transporters hPEPT1 (Caco-2 cells) and rPEPT2 (SKPT cells) were tested. The compounds contained the amide bond isosteres ketomethylene (2a), (R)- and (S)-hydroxyethylidene (3a and 4a), and (R)- and (S)-hydroxyethylene (5a and 6a) to provide information on the conformational and stereochemical requirements for hPEPT1 and rPEPT2 affinity. The affinity studies showed that for rPEPT2 there is no significant difference in affinity between the ketomethylene isostere 2a and the natural substrate Phe-Gly (K(i) values of 18.8 and 14.6 microM, respectively). Also the affinities for hPEPT1 are in the same range (K(i) values of 0.40 and 0.20 mM, respectively). This corroborates earlier findings that the amide bond as such is not essential for binding to PEPTX, but the results also reveal possible differences in the binding of ketomethylene isosteres to hPEPT1 and rPEPT2. The trans-hydroxyethylidene and hydroxyethylene isosteres proved to be poor substrates for PEPTX. These results provide new information about the importance of flexibility and of the stereochemistry at the C(4)-position for this class of compounds. Furthermore, the intracellular uptake of 2a-4a in Caco-2 cells was investigated, showing a 3-fold reduction of the uptake of 2a in the presence of the competetive inhibitor Gly-Pro, indicating contribution from an active transport component. No active uptake of 3a and 4a was observed. Transepithelial transport studies also indicated active transport of 2a across Caco-2 monolayers.

The intestinal H+/peptide symporter PEPT1: structure–affinity relationships

Peptide transporter 1, PEPT1, of the mammalian enterocyte is presently under intense investigation in many laboratories because of its nutritional importance in the absorption of protein hydrolysis products and because more recent studies have shown that many drugs and prodrugs gain entry into the systemic circulation via PEPT1. Until the exact structural features of the substrate binding site of PEPT1 become available, for example by X-ray crystallography, determination of affinities followed by proof of actual membrane translocation will have to suffice when testing for possible new substrates for PEPT1. Affinity constants reflect the strength of their interaction with the binding site of the transporter. A review of the literature shows a wide range of affinity constants between 2 microM and 30 mM. We consider affinity constants for substrates or inhibitors of PEPT1 lower than 0.5 mM as high affinity, between 0.5 and 5.0 mM as medium affinity and above 5 mM as low affinity. Values above 15 mM we consider with great caution. In this mini-review we discuss affinities and structural determinants which affect affinities of a variety of substrates for PEPT1.