Associate editor: K. -I. Inui
Drug transport by Organic Anion Transporters (OATs)

https://doi.org/10.1016/j.pharmthera.2012.07.010Get rights and content

Abstract

Common to all so far functionally characterized Organic Anion Transporters (OATs) is their broad substrate specificity and their ability to exchange extracellular against intracellular organic anions. Many OATs occur in renal proximal tubules, the site of active drug secretion. Exceptions are murine Oat6 (nasal epithelium), human OAT7 (liver), and rat Oat8 (renal collecting ducts). In human kidneys, OAT1, OAT2, and OAT3 are localized in the basolateral membrane, and OAT4, OAT10, and URAT1 in the apical cell membrane of proximal tubule cells, respectively. In rats and mice, Oat1 and Oat3 are located basolaterally, and Oat2, Oat5, Oat9, Oat10, and Urat1 apically. Several classes of drugs interact with human OAT1-3, including ACE inhibitors, angiotensin II receptor antagonists, diuretics, HMG CoA reductase inhibitors, β-lactam antibiotics, antineoplastic and antiviral drugs, and uricosuric drugs. For most drugs, interaction was demonstrated in vitro by inhibition of OAT-mediated transport of model substrates; for some drugs, transport by OATs was directly proven. Based on IC50 values reported in the literature, OAT1 and OAT3 show comparable affinities for diuretics, cephalosporins, and nonsteroidal anti-inflammatory drugs whereas OAT2 has a lower affinity to most of these compounds. Drug–drug interactions at OAT1 and OAT3 may retard renal drug secretion and cause untoward effects. OAT4, OAT10, and URAT1 in the apical membrane contribute to proximal tubular urate absorption, and OAT10 to nicotinate absorption. OAT4 is in addition able to release drugs, e.g. diuretics, into the tubule lumen.

Introduction

Organic Anion Transporters (OATs) play a pivotal role in renal excretion of water-soluble, negatively charged organic compounds including endogenous waste products, numerous drugs and drug metabolites. OATs are located in the plasma membranes of epithelial cells of proximal tubules, the site of efficient renal organic anion secretion. Selected OATs are present also outside the kidneys, e.g. in liver, placenta, nasal epithelium, and liquor-brain barrier, where they serve special functions.

Together with transporters for organic cations and zwitterions, the OATs are members of the Solute Carrier Family 22 [SLC22; (Koepsell and Endou, 2004, Jacobsson et al., 2007)]. This review will deal only with those OATs for which transport of organic anions has been shown experimentally, i.e. OATs1–4, OAT7, OAT10, and URAT1 in humans (upper case lettering), as well as Oats1–3, Oat5, Oat6, Oat8, Oat9, and Urat1/Rst in rodents (lower case lettering). Table 1 provides a summary of the OATs dealt with here. For a complete survey over all presently known SLC22 family members in humans, rats and mice see Jacobsson et al. (2007).

For each characterized OAT/Oat, cloning and protein features, tissue distribution of mRNA, subcellular localization of the OAT/Oat protein, abundance of OAT/Oat protein, function and endogenous substrates, interaction with drug classes, drug–drug interactions and single nucleotide polymorphisms are described. The interaction of OATs/Oats with exogenous and endogenous toxins is not a subject of this review.

In Chapter #6, the interaction of OAT1, OAT2, and OAT3, all located in the basolateral membrane of human renal proximal tubule cells, with selected drug classes is compared to point out which of these three transporters may be involved in the handling of antihypertensive agents, cephalosporins and nonsteroidal anti-inflammatory drugs. Such a comparison is possible because these three drug classes have been tested under similar experimental conditions with heterologously expressed OATs. For other drugs, a straightforward comparison of literature data is not possible due to the use of different expression systems (oocytes, HEK cells, immortalized mouse S2 segments, and others) and different radiolabeled or fluorescent test anions.

Section snippets

Relationships

The genetic relationship between OATs1–10 and URAT1 is shown in Fig. 1. There are two major branches that divided up early in evolution with OAT1/Oat1 and OAT3/Oat3 on the one side (A in Fig. 1), and OAT6/Oat6 on the other (B). A third branch (C) diverged into 1) the group of OAT2/Oat2 and OAT10/Oat10 (D), 2) a cluster consisting of OATs5, 7, 8, and 9 (E), and 3) the OAT4 and URAT1/Urat1 group (F). The genes for human OAT1 (SLC22A6) and OAT3 (SLC22A8) are paired on chromosome 11q12.3, and those

Cloning and protein features

As shown in Table 1, OAT1/Oat1 was cloned and functionally characterized from man, monkey, pig, rabbit, rat, and mouse. Oat1 sequences derived from many more species are deposited in gene banks, but functional studies were not reported. Four splice variants of human OAT1 occur in kidneys, the longer two of them (OAT1-1; OAT1-2) having identical transport functions and the shorter two (OAT1-3; OAT1-4) lacking transport capability (Bahn et al., 2000, Bahn et al., 2004). Human OAT1-2 is composed

Cloning and protein features

As shown in Table 1, OAT2/Oat2 was cloned from man, rat and mouse (Simonson et al., 1994, Sekine et al., 1998, Sun et al., 2001, Kobayashi et al., 2002). OAT2/Oat2 proteins consist of 535 (rat), 540 (mouse) or 546/548 (man) amino acids. The shorter splice variant of human OAT2 (546 aa) lacking a serine and a glutamine in the large extracellular loop can be functionally expressed whereas the 548 aa variant turned out to be non-functional (Cropp et al., 2008). The gene for human OAT2 is located

Cloning and protein features

As shown in Table 1, OAT3/Oat3 was cloned from man, monkey, pig, rabbit, rat, and mouse. Functionally not yet characterized Oat3 sequences were deposited for additional species in the gene bank. The respective OAT3/Oat3 proteins have 536–542 amino acids. By site-directed mutagenesis of rat Oat3, amino acid residues in TMHs 7, 8, and 11 were found to be involved in substrate binding (Feng et al., 2001, Feng et al., 2002). The gene for human OAT3, SLC22A8, is located on chromosome 11q12.3, in

Interactions of OAT1, OAT2, and OAT3 with selected drug classes

In human kidneys, these three transporters are located in the basolateral membrane of proximal tubule cells and thus are likely instrumental in the uptake of drugs from the blood into proximal tubule cells. Drug uptake through OAT1 and OAT3 is driven by the efflux of α-ketoglutarate (dic2− in Fig. 2). In case of OAT2, succinate (or fumarate) drives drug uptake. α-Ketoglutarate and succinate are substrates of the sodium-dicarboxylate cotransporter 3 (NaDC3 or NaC3) and intermediate products of

Cloning and protein features

OAT4 was cloned from a human kidney library (Cha et al., 2000). There is no ortholog in rodents. A monkey OAT4 is deposited in the gene bank. The human OAT4 protein consists of 550 amino acids. N-linked glycosylation at residues in the large extracellular loop is required for targeting of OAT4 to the membrane and proper transport function (Zhou et al., 2005). Through its C-terminal PDZ motif OAT4 interacts with the scaffolding proteins PDZK1 and NHERF1, and expression of these proteins

Cloning and protein features of human OAT5

This transporter was cloned from human liver, the only organ in which OAT5 is expressed (Sun et al., 2001, Eraly and Nigam, 2002). The protein consists of 541 amino acids. The expression was trans-activated by HNF-1α and, in human liver biopsies, expression levels of HNF-1α and OAT5 correlated positively (Klein et al., 2010). The gene coding for human OAT5, SLC22A10, is located on chromosome 11q12.3 (Jacobsson et al., 2007). There is no rodent homologue of OAT5. The function of OAT5 is unknown.

Cloning and protein features of Oat5

As shown in Table 1, Oat5 was cloned from rat and mouse (Youngblood and Sweet, 2004, Anzai et al., 2005). Rat Oat5 has 551 amino acids. The gene, Slc22a19, is located on chromosome 1q43 in rat and on 19qA in mouse (Jacobsson et al., 2007). Rodent Oat5 (Slc22a19) is not homologous to human OAT5 (SLC22A10).

Tissue distribution and location of Oat5

In rats and mice, this transporter is restricted to the kidneys where it is expressed in the apical membrane of late proximal tubule cells (Youngblood and Sweet, 2004, Anzai et al., 2005, Kwak

Cloning and protein features

Oat6 was cloned from mouse (Monte et al., 2004, Schnabolk et al., 2006). The protein has 556 amino acids. The gene, Slc22a20, is located on chromosome 19qA. In rats, a homologous gene was found on chromosome 1q43 (Jacobsson et al., 2007). A human OAT6 is deposited in the gene bank, but has not been functionally characterized.

Tissue distribution and location of Oat6

Messenger RNA for murine Oat6 was detected in nasal olfactory mucosa and, to a smaller extent, in testis; no expression was found in kidneys and liver (Monte et al., 2004,

Cloning, protein features, and tissue distribution

Previously known as UST3 or OAT4, OAT7 has been cloned from a human liver library (Shin et al., 2007). OAT7 is a 553 amino acid protein and is restricted to the liver where it is located in the sinusoidal membrane. The expression of OAT7 was trans-activated by HNF-1α (Klein et al., 2010). The gene, SLC22A9, is located on human chromosome 11q12.3 (Jacobsson et al., 2007).

Function of OAT7 and endogenous substrates

OAT7 transported ES (Km 8.7 μM) and DHEAS (Km 2.2 μM) with high affinity. Unlike other OATs, OAT7 did not interact with

Cloning, protein features, and tissue distribution

Previously cloned as Ust1, functional expression revealed the ability to transport organic anions. Hence, the transporter was renamed as rat Oat8 (Yokoyama et al., 2008). Oat8 mRNA occurred in kidneys, but not in liver. Immunoreactivity was found in the intercalated cells of collecting ducts. In acid-secreting intercalated cells (type A), Oat8 was detected in the apical cell membrane. In acid-absorbing (bicarbonate-secreting) type B cells, Oat8 occurred at the basolateral cell side. Thus, Oat8

Cloning, protein features, and tissue distribution

Two clones, one coding for a long form (Oat9L) and one for a short form (Oat9S) of Oat9 were obtained from mouse. The long Oat9 protein consists of 551 amino acids with the usual 12 transmembrane helices. The short Oat9 protein is lacking transmembrane helices 3–6. The mRNA occurred predominantly in the kidneys and, more weakly, in the liver. Immunohistochemistry with antibodies reacting with both isoforms located Oat9 to the apical membrane of proximal tubule cells (segments S1 and S2) and to

Cloning, protein features, and tissue distribution

Previously cloned as “organic cation transporter like 3 (ORCTL3)” this transporter turned out to be an organic anion transporter and was renamed OAT10. The mRNA for full length OAT10 and splice variants was predominantly expressed in kidneys and – to a weaker extent – in brain, heart and colon. In the kidneys, the highest amount of mRNA was found in proximal tubules and less in collecting ducts. Western blot studies on isolated membranes indicated that the OAT10 protein is expressed in the

Cloning and protein features

Originally cloned as “renal specific transporter (Rst)”, the human homolog transported urate and therefore was named “Urate Transporter 1 (URAT1)” (Enomoto et al., 2002a). For the functional mouse clone (Hosoyamada et al., 2004, Imaoka et al., 2004), either Rst or Urat1 are used as names. URAT1 protein has 555 amino acids, Urat1/Rst 553 amino acids. The gene for human URAT1, SLC22A12, is located on chromosome 11q13.1, in close neighborhood to the gene SLC22A11 coding for OAT4 (Eraly et al.,

Pharmacological role of OATs 1–3

Organic Anion Transporters 1–3 in humans and Oat1 and Oat3 in rodents are located in the basolateral membrane of renal proximal tubule cells (Fig. 10). This in vivo location and the in vitro interaction of OATs1–3 with various drug classes strongly suggest their involvement in drug uptake from the blood into proximal tubule cells. Uptake across the basolateral membrane is the first step in transcellular drug secretion and is likely energized by the release of dicarboxylates from the cell to the

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