Epidermal growth factor decreases PEPT2 transport capacity and expression in the rat kidney proximal tubule cell line SKPT0193 cl.2

Biology of Peptide Transporter 2 in Mammals: New Insights into Its Function, Structure and Regulation

Peptide transporter 2 (PepT2) in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also of significant medical and pharmacological significance in the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. In this review, we provide a comprehensive overview of the structure, substrate preferences and localization of PepT2 in mammals. As PepT2 is expressed in various organs, its function in the liver, kidney, brain, heart, lung and mammary gland has also been addressed. Finally, the regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed.

Substrates of the human oligopeptide transporter hPEPT2

Oligopeptide transporters serve important functions in nutrition and pharmacology. In particular, these transporters help maintain the homeostasis of peptides. The peptide-transporter PEPT2 is a high-affinity and low-capacity type oligopeptide transporter from the proton-coupled oligopeptide transporter family. PEPT2 has recently received attention because of its potential application in targeted drug delivery. PEPT2 is widely distributed in kidney, central nervous system, and lung of organisms. In general, all dipeptides, tripeptides, and peptide-like drugs such as β-lactam antibiotics and angiotensin-converting enzyme inhibitors could be mediated and transported as a substrate of PEPT2. The design of many extant drugs and prodrugs is based on the substrate structure of PEPT2 to accelerate absorption via peptide transporters. Thus, this paper summarizes the substrate features of PEPT2 to promote the rational design of drugs and prodrugs that target peptide transporters.

Expression and regulation of the proton-coupled oligopeptide transporter PhT2 by LPS in macrophages and mouse spleen

Membrane transporter PhT2 (SLC15A3), which belongs to the proton-coupled oligopeptide transporter family, mediates the transport of di/tripeptides and histidine utilizing an inwardly directed proton gradient and negative membrane potential. The aim of this study was to elucidate the molecular expression of PhT2 in macrophages and mouse tissues and to explore the regulation of PhT2 by lipopolysaccharide (LPS). The results showed relatively high expression of PhT2 in J774A.1 and THP-1 macrophage cells, mouse spleen, and lung. Using an LPS-induced inflammatory cell model, we found that hPhT2 mRNA expression was up-regulated in THP-1 cells and that the up-regulation was suppressed by pyrrolidine dithiocarbamate, a specific inhibitor of NF-κB. Similar results were observed in mouse spleen during LPS-induced acute inflammation. Using dual-labeling immunofluorescence and confocal laser scanning microscopy, we confirmed that mPhT2 was colocalizing with lysosome-associated membrane protein 1 in transfected HEK293 cells. These results suggested that PhT2, a lysosomal membrane transporter, was up-regulated by LPS via the NF-κB signaling pathway.

Calu-3 cells grown under AIC and LCC conditions: implications for dipeptide uptake and transepithelial transport of substances

The aim of the present study was to investigate whether Calu-3 cell culture conditions influence drug and nutrient transport known to occur via carriers or transporters. Calu-3 cell layers, an in vitro model of the lung epithelium, were cultured using air interfaced culture (AIC) or liquid covered culture (LCC) on either polycarbonate or polyester as filter support material. We found that the development of the Calu-3 cell layer barrier function did not depend on the filter material but rather on the culture conditions as follows: (i) the apical uptake of Gly-Sar was significantly larger for cells grown in AIC compared to LCC, (ii) the TEER values for cells grown in LCC were approximately three times larger than for cells grown in AIC, (iii) the transepithelial transport in both AIC and LCC Calu-3 cells was polarized in the apical-basolateral direction of proline, glycine, α-methyl-d-glucoside, glipizide, taurocholic acid and estrone-3-sulfate, whereas inulin, mannitol and Gly-Sar showed no polarized transport. Etoposide showed polarized efflux (basolateral to apical transport) in AIC and LCC Calu-3 layers. These findings provide information about nutrient and drug transport in Calu-3 cells, and this may have implications for selecting culture conditions for transport studies in this in vitro model of the lung epithelium.

Effects of tripeptides and lactogenic hormones on oligopeptide transporter 2 in bovine mammary gland

This study was conducted to investigate the expression of oligopeptide transporter 2 (PepT2) and its potential function in bovine mammary gland. First, the PepT2 mRNA and protein were determined in cultured mammary epithelial cells. Then the effects of lactogenic hormones (prolactin, hydrocortisone or insulin) and substrate (threonyl-phenylalanyl-phenylalanine) on PepT2 were investigated. The PepT2 mRNA and protein were successfully detected in bovine mammary epithelial cells. PepT2 gene expression was enhanced by the addition of 50, 500 and 5000 ng/ml prolactin, 10 and 100 ng/ml hydrocortisone, and 50, 500, 5000 and 50,000 ng/ml insulin. PepT2 mRNA abundance was increased when 5, 10 and 15% of threonyl-phenylalanyl-phenylalanine was included. Responses of PepT2 to lactogenic hormones and oligopeptide inferred that it may play an important role in bovine mammary gland.

U373-MG cells express PepT2 and accumulate the fluorescently tagged dipeptide-derivative β-Ala-Lys-N(ε)-AMCA

Aim of this study was to examine the dipeptide transport of β-Ala-Lys-N(ɛ)-AMCA in the human glioma cell line U373-MG and its potential regulation by diverse hormones and culture media. A mixed glial primary cell culture of the newborn rat served as reference cell system. β-Ala-Lys-N(ɛ)-AMCA (β-Ala-Lys-N(ɛ)-7-amino-4-methyl-coumarin-3-acetic acid) is a highly specific reporter substrate to investigate the dipeptide transport system PepT2. We were able to demonstrate that U373-MG cells express PepT2-mRNA and translocate β-Ala-Lys-N(ɛ)-AMCA via PepT2 into the cytoplasm. Previous results demonstrated that β-Ala-Lys-N(ɛ)-AMCA specifically accumulates in differentiated and dedifferentiated astrocytes but neither in differentiated nor dedifferentiated oligodendrocytes and in neurons. U373-MG cells were incubated with estradiol, testosterone, thyronine, dexamethasone, dibutyryl cyclic adenosine monophosphate and tetradecanoylphorbol acetate in order to detect potential substance-dependent changes in dipeptide uptake. There was no significant increase or decrease of β-Ala-Lys-N(ɛ)-AMCA-uptake after stimulation. Northern blot analyses confirmed that PepT2-mRNA is expressed in U373-MG and glial cells but showed no regulation of PepT2-mRNA expression in both cell types. Future investigations might offer the opportunity of an anti-tumor therapy with cytotoxic agents linked to a dipeptide-derivative such as β-Ala-Lys.

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.

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.

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.

Gene expression and regulation of drug transporters in the intestine and kidney

Intestinal absorption and renal secretion of ionic drugs are controlled by a number of drug transporters expressed at the brush-border and basolateral membranes of epithelial cells. Over the last several years, considerable progress has been made regarding the molecular identification and functional characterization of drug transporters. Under some physiological and pathophysiological conditions, the expression and transport activity of drug transporters are changed, affecting the pharmacokinetics of substrate drugs. The regulation of transport activity in response to endogenous and exogenous signals can occur at various levels such as transcription, mRNA stability, translation, and posttranslational modification. Transcriptional regulation is of particular interest, because changes in transport activity are dynamically regulated by increases or decreases in levels of mRNA expression. The tissue-specific expression of drug transporters is also under transcriptional control, and recent studies using clinical samples from human tissues have revealed the expression profiles of drug transporters in the human body. The purpose of this research updates is to review the recent progress in the study of the gene expression and regulation of intestinal and renal drug transporters.

The PDZ domain protein PDZK1 interacts with human peptide transporter PEPT2 and enhances its transport activity

The proton-coupled peptide transporter PEPT2 (SLC15A2) mediates the high-affinity low-capacity transport of small peptides as well as various oral peptide-like drugs in the kidney. In contrast to its well-characterized transport properties, there is less information available on its regulatory mechanism, although the interaction of PEPT2 to the PDZ (PSD-95, DglA, and ZO-1)-domain protein PDZK1 has been preliminarily reported. To examine whether PDZK1 is a physiological partner of PEPT2 in kidneys, we started from a yeast two-hybrid screen of a human kidney cDNA library with the C-terminus of PEPT2 (PEPT2 C-terminus (PEPT2-CT)) as bait. We could identify PDZK1 as one of the positive clones. This interaction requires the PDZ motif of PEPT2-CT detected by a yeast two-hybrid assay, in vitro binding assay and co-immunoprecipitation. The binding affinities of second and third PDZ domains of PDZK1 to PEPT2-CT were measured by surface plasmon resonance. Co-immunoprecipitation using human kidney membrane fraction and localization of PEPT2 in renal apical proximal tubules revealed the physiological meaning of this interaction in kidneys. Furthermore, we clarified the mechanism of enhanced glycylsarcosine (Gly-Sar) transport activity in PEPT2-expressing HEK293 cells after the PDZK1 coexpression. This augmentation was accompanied by a significant increase in the V(max) of Gly-Sar transport via PEPT2 and it was also associated with the increased surface expression level of PEPT2. These results indicate that the PEPT2-PDZK1 interaction thus plays a physiologically important role in both oligopeptide handling as well as peptide-like drug transport in the human kidney.

Characterization of rPEPT2-Mediated Gly-Sar Transport Parameters in the Rat Kidney Proximal Tubule Cell Line SKPT-0193 cl.2 Cultured in Basic Growth Media

The rat proximal kidney tubule cell line SKPT-0193 cl.2 (SKPT) expresses the di-/tripeptide transporter PEPT2 (rPEPT2) and has been used to study PEPT2-mediated transport. Traditionally, SKPT cells have been cultured in growth media supplemented with epidermal growth factor (EGF), apotransferrin, dexamethasone, and insulin. It was recently demonstrated that omission of EGF from the culture media caused a drastic increase in the expression of rPEPT2. The hypothesis was therefore that the SKPT cell line might be able to differentiate and express rPEPT2 in the absence of the four agonists traditionally added. The aim of the study was thus to characterize Gly-Sar transport parameters in SKPT cells cultured in basic growth media (conventional media without added agonists). Morphology was studied using confocal laser scanning microscopy (CLSM) and immunohistochemistry. Monolayer integrity was evaluated using transepithelial electrical resistance (TEER) measurements and [(3)H]-mannitol permeabilities. Di-/tripeptide transporter activity was studied using [(14)C]-glycylsarcosine ([(14)C]-Gly-Sar). SKPT cells grown in basic media for 4 days formed confluent monolayers with a TEER of 5.03 +/- 0.33 kOmega.cm(2) (n = 5). Apical Gly-Sar uptake peaked after 3-6 days in culture. Uptake at day 4 was 5.89 +/- 0.30 pmol.cm(-2).min(-1) (n = 3). Di-/tripeptide uptake displayed an optimum at approximately pH 6. Affinity values for cephalexin, kyotorphin, and delta-aminolevulinic acid were comparable to those obtained in other PEPT2-expressing model systems. It can be concluded that SKPT cells grown in the absence of the agonists traditionally added to the culture media retain all necessary properties for PEPT2-mediated peptide uptake studies. Furthermore, the absence of the agonists might facilitate studies of hormonal regulation of PEPT2 expression and transport activity.