Short-term regulation of organic anion transporters

Abstract

Organic anion transporters (OATs), which belong to the superfamily SLC22A, are key determinants in the absorption, distribution, and excretion of a diverse array of environmental toxins, and clinically important drugs, and, therefore, are critical for the survival of mammalian species. Alteration in the function of these drug transporters plays important roles in intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs. As a result, the activity of OATs must be under tight regulation so as to carry out their normal functions. This review article highlights the recent advances from our laboratory as well as from others in delineating the short-term regulation of OATs. These advances provide important insights into strategies to maximize therapeutic efficacy in drug development.

Introduction

The organic anion transporter (OAT) family belongs to the amphiphilic solute carrier transporters family 22a (SLC22A), which transports a broad diversity of substrates including metabolites, toxins and clinical drugs such as β-lactam antibiotics, antivirals, ACE inhibitors, diuretics, and NSAIDs (You, 2004, Anzai et al., 2006, El-Sheikh et al., 2008, Srimaroeng et al., 2008). To date, ten members of the OAT family (OAT1-10) have been identified (Sweet et al., 1997, Sekine et al., 1997, Sekine et al., 1998, Kusuhara et al., 1999, Cha et al., 2000, Youngblood and Sweet, 2004, Schnabolk et al., 2006, Shin et al., 2007, Bahn et al., 2008, Yokoyama et al., 2008), which differ from each other by their localization, expression level, and substrate specificity. As the first cloned OAT, OAT1 is expressed predominantly at the basolateral membrane of renal proximal tubular cells (Sekine et al., 1997, Sweet et al., 1997, Lu et al., 1999, Race et al., 1999) and at low levels in the cerebral cortex and hippocampus as well as in the choroid plexus (Alebouyeh et al., 2003, Bahn et al., 2005). OAT2 is primarily expressed in the liver (Sekine et al., 1998) and kidney (Kojima et al., 2002). In the liver, OAT2 is expressed at the basolateral membrane of hepatocytes (Hilgendorf et al., 2007). In the kidney, however, the cellular location of OAT2 seems to be species-dependent. Human OAT2 is expressed similarly as hOAT1 at the basolateral membrane of the proximal tubule (Enomoto et al., 2002b), while in rat and mouse, OAT2 is located at the apical membrane of proximal tubular cells (Ljubojevic et al., 2007). OAT3 is highly expressed at the basolateral membrane of renal proximal tubular cells (Cha et al., 2001, Hasegawa et al., 2002, Kobayashi et al., 2004) and at the apical membrane of the choroid plexus (Sweet et al., 2002). OAT4 is expressed at the apical membrane of renal proximal tubular cells (Ekaratanawong et al., 2004), while in the placenta, OAT4 is expressed at the basal membrane of syncytiotrophoblast (Ugele et al., 2003). Expressed exclusively in kidneys, OAT5, similar to OAT4, is located at the apical membrane of renal proximal tubular cells (Youngblood & Sweet, 2004). The unique feature of OAT6 is its complete absence in the kidney and its expression in olfactory mucosa (Monte et al., 2004). Discovered as a liver-specific OAT, OAT7 localizes at the sinusoidal membrane of the hepatocyte (Shin et al., 2007). Recently, three other OATs were identified in the kidney. Rat OAT8 is expressed in rat renal collecting ducts and located at the apical membrane of renal intercalated cells (Yokoyama et al., 2008). OAT9 is expressed in the kidney and brain (Anzai et al., 2006). OAT10 is highly expressed at the apical side of renal proximal tubular cells (Bahn et al., 2008). The urate transporter URAT1 (SLC22A12), acting as an organic anion exchanger, is expressed at the apical membrane of the kidney proximal tubule and is believed to be responsible for renal urate reabsorption (Enomoto et al., 2002a).

In the kidney, OAT1 and OAT3 utilize a tertiary transport mechanism to move organic anions across the basolateral membrane into the proximal tubular cells for subsequent exit across the apical membrane into the urine for elimination (Fig. 1). Through this tertiary transport mechanism, Na⁺/K⁺-ATPase maintains an inwardly directed (blood-to-cell) Na⁺ gradient. The Na⁺ gradient then drives a sodium dicarboxylate cotransporter, sustaining an outwardly directed dicarboxylate gradient that is utilized by a dicarboxylate/organic anion exchanger, namely OAT, to move the organic anion substrate into the cell. This cascade of events indirectly links organic anion transport to metabolic energy and the Na⁺ gradient, allowing the entry of a negatively charged substrate against both its chemical concentration gradient and the electrical potential of the cell.

Structurally, all members of the OATs share some common features (Fig. 2) including 12 putative membrane-spanning segments, a cluster of potential glycosylation sites located in the first extracellular loop between transmembrane domains 1 and 2, and multiple presumptive phosphorylation sites in the intracellular loop between the sixth and the seventh transmembrane domains and in the carboxyl terminus.

One remarkable characteristic of OATs is their wide range of substrate recognition, from endogenous metabolites to xenobiotics and drugs including anti-HIV therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories (You, 2004, Enomoto and Endou, 2005, Anzai et al., 2006, Srimaroeng et al., 2008). Given the critical role of OATs in the control of distribution, elimination and retention of such a diverse array of chemicals within the body, the activity of these transporters must be under tight regulation in order to accomplish their normal task. This review article highlights the recent advances from our laboratory as well as from others in unveiling the regulation of OATs, especially the short-term regulation of these transporters, as well as their clinical implications.