Review
Proton-coupled oligopeptide transporter family SLC15: Physiological, pharmacological and pathological implications

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Abstract

Mammalian members of the proton-coupled oligopeptide transporter family (SLC15) are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs. The driving force for uphill electrogenic symport is the chemical gradient and membrane potential which favors proton uptake into the cell along with the peptide/mimetic substrate. The peptide transporters are responsible for the absorption and conservation of dietary protein digestion products in the intestine and kidney, respectively, and in maintaining homeostasis of neuropeptides in the brain. They are also responsible for the absorption and disposition of a number of pharmacologically important compounds including some aminocephalosporins, angiotensin-converting enzyme inhibitors, antiviral prodrugs, and others. In this review, we provide updated information on the structure–function of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4) and PhT2 (SLC15A3), and their expression and localization in key tissues. Moreover, mammalian peptide transporters are discussed in regard to pharmacogenomic and regulatory implications on host pharmacology and disease, and as potential targets for drug delivery. Significant emphasis is placed on the evolving role of these peptide transporters as elucidated by studies using genetically modified animals. Whenever possible, the relevance of drug–drug interactions and regulatory mechanisms are evaluated using in vivo studies.

Introduction

The acceptance of mammalian peptide transporters, as distinct from amino acid transporters, for the intestinal absorption and renal reabsorption of nutritional nitrogen was pushed to the forefront by studies in brush border membranes vesicles (BBMV) prepared from intestine and kidney (Ganapathy and Leibach, 1985, Ganapathy and Leibach, 1986). The BBMV studies clearly demonstrated that dipeptides, tripeptides and peptide-like drugs were actively transported into vesicles by a process that was coupled to the movement of protons down an electrochemical proton gradient (Fig. 1). With the advent of expression cloning techniques, the molecular basis for peptide transporters was first identified in mammals by Hediger and co-workers (Fei et al., 1994) and Daniel and coworkers (Boll et al., 1994) in which the cloning of Slc15a1 from a rabbit intestinal cDNA library and functional characterization of the encoded PepT1 was evaluated. Orthologs of PepT1 were soon to follow in other mammalian species as were PepT1 paralogs such as PepT2 (encoded by SLC15A2), PhT1 (encoded by SLC15A4) and PhT2 (encoded by SLC15A3). These proton-coupled oligopeptide transporters of the SLC15 family were shown to be phylogenetically conserved integral membrane proteins (Wang et al., 2010) and, as demonstrated in cell cultures, Xenopus oocytes and other heterologous expression systems (Brandsch et al., 2008, Rubio-Aliaga and Daniel, 2008), were responsible for the symport of protons and peptides/mimetics across biological membranes. Many studies have addressed the expression, localization and structure–function of the high-capacity, low-affinity “professed” intestinal peptide transporter PepT1 and the low-capacity, high-affinity “professed” renal peptide transporter PepT2. In contrast, much less is known about the peptide/histidine transporters PhT1 and PhT2. The proton-coupled oligopeptide transporter family (SLC15) belongs to the major facilitator superfamily (MFS) as detailed in the “transporter classification system” operated by the Saier Lab Bioinformatics Group (http://www.tcdb.org).

Section snippets

Structure–function

The oligopeptide transporters PepT1 and PepT2 mediate proton-coupled active transport of a broad range of dipeptides and tripeptides, including zwitterionic, anionic and cationic peptides as well as a variety of peptide-like drugs (e.g., cefadroxil, enalapril, valacyclovir). Numerous studies aimed at investigating the influence of charge of the dipeptide on PepT1 function. Thus, the parameters voltage, ion-dependence and proton-coupling were investigated. Kinetics of transport were extensively

Localization-expression

The physiological, pharmacological and pathological functions of mammalian SLC15 family members are determined, to a large extent, by their expression levels and cellular localization, especially in regard to intestine, kidney and brain. In the intestine, PepT1 protein is abundantly expressed at the apical membrane of enterocytes in mouse and human duodenum, jejunum and ileum, with little or no expression in normal colon (Groneberg et al., 2001, Jappar et al., 2010, Walker et al., 1998).

Pharmacogenomics

Many genetic variants have been reported for the SLC15 family members PepT1 and PepT2, encoded by SLC15A1 and SLC15A2, in humans (Brandsch et al., 2008, Zair et al., 2008; UCSF Pharmacogenetics of Membrane Transporters Database, http://pharmacogenetics.ucsf.edu). With respect to PepT1, only two variants are worth noting, both rare, in their ability to alter transporter activity. The first one, a P586L variant, demonstrated a significantly reduced transport capacity of GlySar in transfected HeLa

Studies in wild-type and genetically-modified mice

Major advances have been made in the structure–function, tissue and cellular localization, and regulatory properties of SLC15 family members using cellular, molecular and biochemical methodologies. However, these experimental approaches are often limited because they lack an intact blood supply, appropriate residence times of substrate at biological membranes, and the ability to study a particular gene product under physiological conditions. The development of genetically-modified mice has

Regulation

Epithelial SLC15s can be regulated by signals emanating from both endogenous and exogenous sources. These signals can be the result of drugs and hormones, diet and nutritional status, disease states, circadian rhythm, and developmental biology. In particular, the underlying mechanisms can be nonspecific, such as changes in membrane surface area, or specific for a certain carrier. Specific regulation can be brought about by alterations in the electrogenic driving force for transport, by changes

Human PEPT1-RF: an alternative splice variant?

Saito and co-workers (Saito et al. 1997) isolated cDNA encoding a pH-sensing regulatory factor of oligopeptide transporter (human PEPT1-RF) from the human duodenum library. They suggested this sequence to be a possible alternative splice variant of SLC15A1 gene, a finding supported by Urtti et al. (2001). However, in the NCBI database the “Gene” human PEPT1-RF (Accession AB001328) is not listed as splice variant of SLC15A1, which calls into question this designation. Indeed, this transcript

Conclusions and perspective

Substantial progress has been made in understanding the role and relevance of SLC15 mammalian transporters since the previous review by Daniel and Kottra (2004), as part of the mini-review series covering all SLC families (Hediger et al., 2004). In particular, in vivo studies have validated many of the prior in vitro and in situ results demonstrating the importance of PepT1 and PepT2 in nutrition, pharmacologic response, and disease progression. Moreover, our knowledge of PepT1 and PepT2

Acknowledgements

This work was supported by the National Institutes of Health National Institute of General Medical Sciences (Grant R01-GM035498) (to D.E.S.).

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    Publication in part sponsored by the Swiss National Science Foundation through the National Center of Competence in Research (NCCR) TransCure, University of Bern, Switzerland; Director Matthias A. Hediger; Web: http://www.transcure.ch.

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