Structure, function, and regulation of renal organic anion transporters

NEDD4L in human tumors: regulatory mechanisms and dual effects on anti-tumor and pro-tumor

Tumorigenesis and tumor development are closely related to the abnormal regulation of ubiquitination. Neural precursor cell expressed developmentally downregulated 4-like (NEDD4L), an E3 ubiquitin ligase critical to the ubiquitination process, plays key roles in the regulation of cancer stem cells, as well as tumor cell functions, including cell proliferation, apoptosis, cell cycle regulation, migration, invasion, epithelial-mesenchymal transition (EMT), and tumor drug resistance, by controlling subsequent protein degradation through ubiquitination. NEDD4L primarily functions as a tumor suppressor in several tumors but also plays an oncogenic role in certain tumors. In this review, we comprehensively summarize the relevant signaling pathways of NEDD4L in tumors, the regulatory mechanisms of its upstream regulatory molecules and downstream substrates, and the resulting functional alterations. Overall, therapeutic strategies targeting NEDD4L to treat cancer may be feasible.

Research Methods and New Advances in Drug-Drug Interactions Mediated by Renal Transporters

The kidney is critical in the human body's excretion of drugs and their metabolites. Renal transporters participate in actively secreting substances from the proximal tubular cells and reabsorbing them in the distal renal tubules. They can affect the clearance rates (CLr) of drugs and their metabolites, eventually influence the clinical efficiency and side effects of drugs, and may produce drug-drug interactions (DDIs) of clinical significance. Renal transporters and renal transporter-mediated DDIs have also been studied by many researchers. In this article, the main types of in vitro research models used for the study of renal transporter-mediated DDIs are membrane-based assays, cell-based assays, and the renal slice uptake model. In vivo research models include animal experiments, gene knockout animal models, positron emission tomography (PET) technology, and studies on human beings. In addition, in vitro-in vivo extrapolation (IVIVE), ex vivo kidney perfusion (EVKP) models, and, more recently, biomarker methods and in silico models are included. This article reviews the traditional research methods of renal transporter-mediated DDIs, updates the recent progress in the development of the methods, and then classifies and summarizes the advantages and disadvantages of each method. Through the sorting work conducted in this paper, it will be convenient for researchers at different learning stages to choose the best method for their own research based on their own subject's situation when they are going to study DDIs mediated by renal transporters.

Chloroquine and Hydroxychloroquine, as Proteasome Inhibitors, Upregulate the Expression and Activity of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3), at the basolateral membrane of kidney proximal tubule cells, facilitates the elimination of numerous widely used drugs. Earlier investigation from our laboratory revealed that ubiquitin conjugation to OAT3 leads to OAT3 internalization from the cell surface, followed by degradation in the proteasome. In the current study, we examined the roles of chloroquine (CQ) and hydroxychloroquine (HCQ), two well-known anti-malarial drugs, in their action as proteasome inhibitors and their effects on OAT3 ubiquitination, expression, and function. We showed that in cells treated with CQ and HCQ, the ubiquitinated OAT3 was considerably enhanced, which correlated well with a decrease in 20S proteasome activity. Furthermore, in CQ- and HCQ-treated cells, OAT3 expression and OAT3-mediated transport of estrone sulfate, a prototypical substrate, were significantly increased. Such increases in OAT3 expression and transport activity were accompanied by an increase in the maximum transport velocity and a decrease in the degradation rate of the transporter. In conclusion, this study unveiled a novel role of CQ and HCQ in enhancing OAT3 expression and transport activity by preventing the degradation of ubiquitinated OAT3 in proteasomes.

Peptide Hormone Insulin Regulates Function, Expression, and SUMOylation of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays an important role in the disposition of various anionic drugs which impacts the pharmacokinetics and pharmacodynamics of the therapeutics, thus influencing the pharmacological effects and toxicity of the drugs. In this study, we investigated the effect of insulin on the regulation of OAT3 function, expression, and SUMOylation. We demonstrated that insulin induced an increase in OAT3 transport activity through a dose- and time-dependent manner in COS-7 cells. The insulin-induced elevation in OAT3 function was blocked by PKA inhibitor H89, which correlated well with OAT3 protein expression. Moreover, both PKA activator Bt2-cAMP-induced increase and insulin-induced increase in OAT3 function were blocked by PKB inhibitor AKTi1/2. To further investigate the involvement of SUMOylation, we treated OAT3-expressing cells with insulin in presence or absence of H89 or AKTi1/2 followed by examining OAT3 SUMOylation. We showed that insulin enhanced OAT3 SUMOylation, and such enhancement was abrogated by H89 and AKTi1/2. Lastly, insulin increased OAT3 function and SUMOylation in rat kidney slice. In conclusion, our investigations demonstrated that insulin regulated OAT3 function, expression, and SUMOylation through PKA/PKB signaling pathway. Graphical abstract.

Oral Proteasomal Inhibitors Ixazomib, Oprozomib, and Delanzomib Upregulate the Function of Organic Anion Transporter 3 (OAT3): Implications in OAT3-Mediated Drug-Drug Interactions

Organic anion transporter 3 (OAT3) is mainly expressed at the basolateral membrane of kidney proximal tubules, and is involved in the renal elimination of various kinds of important drugs, potentially affecting drug efficacy or toxicity. Our laboratory previously reported that ubiquitin modification of OAT3 triggers the endocytosis of OAT3 from the plasma membrane to intracellular endosomes, followed by degradation. Oral anticancer drugs ixazomib, oprozomib, and delanzomib, as proteasomal inhibitors, target the ubiquitin-proteasome system in clinics. Therefore, this study investigated the effects of ixazomib, oprozomib, and delanzomib on the expression and transport activity of OAT3 and elucidated the underlying mechanisms. We showed that all three drugs significantly increased the accumulation of ubiquitinated OAT3, which was consistent with decreased intracellular 20S proteasomal activity; stimulated OAT3-mediated transport of estrone sulfate and p-aminohippuric acid; and increased OAT3 surface expression. The enhanced transport activity and OAT3 expression following drug treatment resulted from an increase in maximum transport velocity of OAT3 without altering the substrate binding affinity, and from a decreased OAT3 degradation. Together, our study discovered a novel role of anticancer agents ixazomib, oprozomib, and delanzomib in upregulating OAT3 function, unveiled the proteasome as a promising target for OAT3 regulation, and provided implication of OAT3-mediated drug-drug interactions, which should be warned against during combination therapies with proteasome inhibitor drugs.

Impaired Transport Activity of Human Organic Anion Transporters (OATs) and Organic Anion Transporting Polypeptides (OATPs) by Wnt Inhibitors

The Wnt/β-catenin signaling pathway is dysregulated in diseases and Wnt inhibitors like PRI-724 are in clinical development. This study evaluated the regulatory actions of PRI-724 and other Wnt inhibitors on the transport activity of human renal Organic anion transporters (OATs) and Organic anion transporting polypeptides (OATPs). The substrate uptake by OAT4 and OATP2B1 was markedly decreased by PRI-724 (V_max/K_m: ∼26% and ∼17% of corresponding control), with less pronounced decreases in OAT1, OAT3 and OAT1A2. PRI-724 decreased the plasma membrane expression of inhibited OATs/OATPs but didn't affect their total cellular expression. Two model Wnt inhibitors - FH535 and 21H7 - were also tested in comparative studies. Like PRI-724, they also strongly decreased the activities and membrane expression of multiple OATs/OATPs. In contrast, FH535 didn't affect the substrate uptake by organic cation transporters. In control studies, the EGFR inhibitor lapatinib did not inhibit the function of some OATs/OATPs. Together these findings suggest that Wnt inhibitors selectively modulate the function of multiple organic anions transporters, so their clinical use may have unanticipated effects on drug entry into cells. These findings are pertinent to current clinical trials that have been designed to understand the safety and efficacy of new Wnt inhibitor drugs.

Ubiquitin-specific peptidase 8 regulates the trafficking and stability of the human organic anion transporter 1

Background Organic anion transporter 1 (OAT1) plays a vital role in avoiding the potential toxicity of various anionic drugs through the involvement of kidney elimination. We previously demonstrated that ubiquitin conjugation to OAT1 led to OAT1 internalization from cell surface, followed by degradation. Ubiquitination is a dynamic process, where deubiquitination is catalyzed by a class of ubiquitin-specific peptidases. Methods The role of ubiquitin-specific peptidase 8 (USP8) in hOAT1 function, expression and ubiquitination was assessed by conducting transporter uptake assay, biotinylation assay and ubiquitination assay. Results We demonstrated that USP8 overexpression in hOAT1-expressing cells led to an increased hOAT1 transporter activity and expression, which correlated well with a reduced hOAT1 ubiquitination. Such phenomenon was not observed in inactive USP8 mutant-transfected cells. In addition, the knockdown of endogenous USP8 by USP8-specific siRNA resulted in an increased hOAT1 ubiquitination, which correlated well with a decrease in hOAT1 expression and transport activity. Biotinylation experiments demonstrated that USP8-induced increase in hOAT1 expression and transport activity occurred through a deceleration of the rates of hOAT1 internalization and degradation. Conclusions These results indicated the regulatory role of USP8 in OAT1 function, expression, trafficking, and stability. General significance USP8 could be a new target for modulating OAT1-mediated drug transport.

Regulation of organic anion transporters: Role in physiology, pathophysiology, and drug elimination

The members of the organic anion transporter (OAT) family are mainly expressed in kidney, liver, placenta, intestine, and brain. These transporters play important roles in the disposition of clinical drugs, pesticides, signaling molecules, heavy metal conjugates, components of phytomedicines, and toxins, and therefore critical for maintaining systemic homeostasis. Alterations in the expression and function of OATs contribute to the intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs, and to many pathophysiological conditions. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. This review will present an update on the recent advance in understanding the cellular and molecular mechanisms underlying the regulation of renal OATs, emphasizing on the post-translational modification (PTM), the crosstalk among these PTMs, and the remote sensing and signaling network of OATs. Such knowledge will provide significant insights into the roles of these transporters in health and disease.

Proteasome Inhibitors Bortezomib and Carfilzomib Stimulate the Transport Activity of Human Organic Anion Transporter 1

Organic anion transporter 1 (OAT1), expressed at the basolateral membrane of renal proximal tubule epithelial cells, mediates the renal excretion of many clinically important drugs. Previous study in our laboratory demonstrated that ubiquitin conjugation to OAT1 leads to OAT1 internalization from the cell surface and subsequent degradation. The current study showed that the ubiquitinated OAT1 accumulated in the presence of the proteasomal inhibitors MG132 and ALLN rather than the lysosomal inhibitors leupeptin and pepstatin A, suggesting that ubiquitinated OAT1 degrades through proteasomes. Anticancer drugs bortezomib and carfilzomib target the ubiquitin-proteasome pathway. We therefore investigate the roles of bortezomib and carfilzomib in reversing the ubiquitination-induced downregulation of OAT1 expression and transport activity. We showed that bortezomib and carfilzomib extremely increased the ubiquitinated OAT1, which correlated well with an enhanced OAT1-mediated transport of p-aminohippuric acid and an enhanced OAT1 surface expression. The augmented OAT1 expression and transport activity after the treatment with bortezomib and carfilzomib resulted from a reduced rate of OAT1 degradation. Consistent with this, we found decreased 20S proteasomal activity in cells that were exposed to bortezomib and carfilzomib. In conclusion, this study identified the pathway in which ubiquitinated OAT1 degrades and unveiled a novel role of anticancer drugs bortezomib and carfilzomib in their regulation of OAT1 expression and transport activity. SIGNIFICANCE STATEMENT: Bortezomib and carfilzomib are two Food and Drug Administration-approved anticancer drugs, and proteasome is the drug target. In this study, we unveiled a new role of bortezomib and carfilzomib in enhancing OAT1 expression and transport activity by preventing the degradation of ubiquitinated OAT1 in proteasomes. This finding provides a new strategy in regulating OAT1 function that can be used to accelerate the clearance of drugs, metabolites, or toxins and reverse the decreased expression under disease conditions.

Insulin-like growth factor 1 modulates the phosphorylation, expression, and activity of organic anion transporter 3 through protein kinase A signaling pathway

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad variety of anionic drugs from kidney, thus avoiding their possible toxicity in the body. In the current study, we investigated the role of insulin-like growth factor 1 (IGF-1) in the regulation of OAT3. We showed that IGF-1 induced a dose- and time-dependent increase in OAT3 transport activity, which correlated well with an increase in OAT3 expression. The IGF-1-induced increase in OAT3 expression was blocked by protein kinase A (PKA) inhibitor H89. Moreover, IGF-1 induced an increase in OAT3 phosphorylation, which was also blocked by H89. These data suggest that the IGF-1 modulation of OAT3 occurred through PKA signaling pathway. To further confirm the involvement of PKA, we treated OAT3-expressing cells with PKA activator Bt2-cAMP, followed by examining OAT activity and phosphorylation. We showed that OAT3 activity and phosphorylation were much enhanced in Bt2-cAMP-treated cells as compared to that in control cells. Finally, linsitinib, an anticancer drug that blocks the IGF-1 receptor, abrogated IGF-1-stimulated OAT3 transport activity. In conclusion, our study demonstrated that IGF-1 regulates OAT3 expression and transport activity through PKA signaling pathway, possibly by phosphorylating the transporter.

The SUMO-Specific Protease Senp2 Regulates SUMOylation, Expression and Function of Human Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad array of anionic drugs from kidney, thereby avoiding their possibly toxic side effects in the body. We earlier demonstrated that OAT3 is subjected to a specific type of post-translational modification called SUMOylation. SUMOylation is a dynamic event, where de-SUMOylation is catalyzed by a class of SUMO-specific proteases. In the present investigation, we assessed the role of SUMO-specific protease Senp2 in OAT3 SUMOylation, expression and function. We report here that overexpression of Senp2 in COS-7 cells led to a reduced OAT3 SUMOylation, which correlated well with a decreased OAT3 expression and transport activity. Such phenomenon was not observed in cells overexpressing an inactive mutant of Senp2. Furthermore, transfection of cells with Senp2-specific siRNA to knockdown the endogenous Senp2 resulted in an increased OAT3 SUMOylation, which correlated well with an enhanced OAT3 expression and transport activity. Coimmunoprecipitation experiments showed that Senp2 directly interacted with OAT3 in the kidneys of rats. Together these results provided first demonstration that Senp2 is a significant regulator for OAT3-mediated organic anion/drug transport.

Interaction of Anticancer Drugs with Human Organic Anion Transporter hOAT4

Human organic anion transporter 4 (hOAT4) belongs to a family of multispecific organic anion transporters that play critical roles in the disposition of numerous drugs and therefore are the major sites for drug-drug interaction. Drug-drug interactions contribute significantly to the individual variation in drug response. hOAT4 is expressed in the kidney and placenta. In the current study, we examined the interaction of 36 anticancer drugs with hOAT4 in kidney COS-7 cells and placenta BeWo cells. Among the drugs tested, only epirubicin hydrochloride and dabrafenib mesylate exhibited > 50% cis-inhibitory effect, in COS-7 cells, on hOAT4-mediated uptake of estrone sulfate, a prototypical substrate for the transporter. The IC₅₀ values for epirubicin hydrochloride and dabrafenib mesylate were 5.24±0.95 μM and 8.30±3.30 μM, respectively. Dixon plot analysis revealed that inhibition by epirubicin hydrochloride was noncompetitive with a K_i = 3 μM whereas inhibition by dabrafenib mesylate was competitive with a K_i = 4.26 μM. Our results established that epirubicin hydrochloride and dabrafenib mesylate are inhibitors of hOAT4. Furthermore, by comparing our data with clinically relevant exposures of these drugs, we conclude that although the tendency for dabrafenib mesylate to cause drug-drug interaction through hOAT4 is insignificant in the kidney, the propensity for epirubicin hydrochloride to cause drug-drug interaction is high.

Activation of Protein Kinase A Stimulates SUMOylation, Expression, and Transport Activity of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad variety of anionic drugs from kidney, thus avoiding their possible toxicity in the body. We earlier established that activation of protein kinase C (PKC) enhances OAT3 ubiquitination, which promotes OAT3 internalization from the cell plasma membrane to intracellular endosomes and consequent degradation. As a result, OAT3 expression and transport activity are reduced. In the current study, we discovered that protein kinase A (PKA) had an opposite effect to PKC on the regulation of OAT3. We showed that activation of PKA by Bt2-cAMP stimulated OAT3 transport activity, which was largely caused by an enhanced plasma membrane expression of the transporter, kinetically reflected as an augmented maximal transport velocity V_max without notable alteration in substrate-binding affinity K_m. Additionally, we showed that PKA activation accelerated the rate of OAT3 recycling from intracellular compartments to the plasma membrane and decelerated the rate of OAT3 degradation. We further showed that OAT3 is subjected to post-translational modification by SUMO-2 and SUMO-3 not by SUMO-1. PKA activation enhanced OAT3 SUMOylation, which was accompanied by a reduced OAT3 ubiquitination. Finally, insulin-like growth factor 1 significantly stimulated OAT3 transport activity and SUMOylation through PKA signaling pathway. In conclusion, this is the first demonstration that PKA stimulated OAT3 expression and transport activity by altering the trafficking kinetics of OAT3 possibly through the crosstalk between SUMOylation and ubiquitination. Our studies are consistent with a remote sensing and signaling model for transporters (Wu et al. in Mol Pharmacol. 79(5):795-805, 2011).

The mechanistic links between insulin and human organic anion transporter 4

Human organic anion transporter 4 (hOAT4) belongs to a class of organic anion transporters that exert critical function in the secretion, absorption, and distribution of numerous drugs in the body, such as anti-viral drugs, anti-cancer therapeutics, antibiotics, antihypertensive medicine, and anti-inflammatory drugs. hOAT4 is richly existent in the kidney and placenta. We previously established that serum- and glucocorticoid-inducible kinases (sgk) stimulate hOAT4 expression and transport activity by abrogating the inhibitory effect of a ubiquitin ligase Nedd4-2. Insulin is one of the upstream signaling molecules for sgk. We therefore investigated the effect of insulin on hOAT4 function. We showed that insulin stimulated hOAT4 expression and transport activity, and the action of insulin was abolished in cells overexpressing Nedd4-2-specific siRNA to knockdown the endogenous Nedd4-2. We further showed that insulin phosphorylated serine 327 on Nedd4-2 and weakened the interaction between hOAT4 and Nedd4-2. Interestingly, in cells overexpressing sgk2, the stimulatory effect of insulin on hOAT4 was diminished. In addition, the stimulatory effect of insulin on hOAT4 was blocked by wortmannin and buparlisib, two PI3K inhibitors. In conclusion, our study demonstrated that insulin stimulates hOAT4 expression and transport activity by abrogating the inhibition effect of Nedd4-2 on the transporter. Moreover, insulin regulates hOAT4 by competing with sgk2 rather than through sgk2.

AG490, a JAK2-specific inhibitor, downregulates the expression and activity of organic anion transporter-3

Human organic anion transporter-3 (hOAT3) is richly expressed in the kidney, where it plays critical roles in the secretion of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. In the current study, we examined the role of AG490, a specific inhibitor of the Janus tyrosine kinase 2 (JAK2), in hOAT3 transport activity in the kidney COS-7 cells. AG490 induced a time- and concentration-dependent inhibition of hOAT3-mediated uptake of estrone sulfate, a prototypical substrate for the transporter. The inhibitory effect of AG490 correlated with a reduced expression of hOAT3 at the cell surface. Our lab previously demonstrated that Nedd4-2, a ubiquitin ligase, down regulates OAT expression and transport activity by enhancing OAT ubiquitination, which leads to an internalization of OAT from cell surface to intracellular compartments and subsequent degradation. In the current study, we showed that treatment of hOAT3-expressing cells with AG490 resulted in an enhanced hOAT3 ubiquitination and degradation, which was accompanied by a strengthened association of Nedd4-2 with hOAT3 and a reduction in Nedd4-2 phosphorylation. SiRNA knockdown of endogenous Nedd4-2 abrogated the effects of AG490 on hOAT3. In summary, our study demonstrated that AG490 regulates hOAT3 expression and transport activity through the modulation of Nedd4-2.

The activity of organic anion transporter-3: Role of dexamethasone

Human organic anion transporter-3 (hOAT3) is richly expressed in the kidney, where it plays critical roles in the secretion, from the blood to urine, of clinically important drugs, such as anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. In the current study, we examined the role of dexamethasone in hOAT3 transport activity in the kidney HEK293 cells. Cis-inhibition study showed that dexamethasone exhibited a concentration-dependent inhibition of hOAT3-mediated uptake of estrone sulfate, a prototypical substrate for the transporter, with IC₅₀ value of 49.91 μM. Dixon plot analysis revealed that inhibition by dexamethasone was competitive with a Ki = 47.08 μM. In contrast to the cis-inhibition effect of dexamethasone, prolonged incubation (6 h) of hOAT3-expressing cells with dexamethasone resulted in an upregulation of hOAT3 expression and transport activity, kinetically revealed as an increase in the maximum transport velocity V_max without meaningful alteration in substrate-binding affinity K_m. Such upregulation was abrogated by GSK650394, a specific inhibitor for serum- and glucocorticoid-inducible kinases (sgk). Dexamethasone also enhanced sgk1 phosphorylation. Our study demonstrated that dexamethasone exhibits dual effects on hOAT3: it is a competitive inhibitor for hOAT3-mediated transport, and interestingly, when entering the cells, it stimulates hOAT3 expression and transport activity through sgk1.

SGK1/Nedd4-2 signaling pathway regulates the activity of human organic anion transporters 3

Human organic anion transporter 3 (hOAT3) is localized at the basolateral membrane of renal proximal tubule cells and facilitates the renal secretion of numerous clinical drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, antihypertension drugs and anti-inflammatories. The present study explored the role of serum and glucocorticoid-inducible kinase 1 (sgk1) in the regulation of hOAT3. It was shown that over-expression of sgk1 in hOAT3-expressing cells stimulated hOAT3 transport activity by enhancing the transporter expression at the plasma membrane, kinetically reflected as an increased maximal transport velocity V_max without substantial change in the substrate-binding affinity K_m . In contrast, treatment of cells with the sgk-specific inhibitor GSK650394 resulted in a dose-dependent inhibition of hOAT3 transport activity. Evidence was further provided that sgk1 regulation of hOAT3 activity was mediated by ubiquitin ligase Nedd4-2, an enzyme previously shown to have an inhibitory effect on hOAT3. It was shown that sgk1 phosphorylated Nedd4-2, weakened the association between Nedd4-2 and hOAT3, and decreased hOAT3 ubiquitination. Functionally, the sgk1-stimulated hOAT3 transport activity was attenuated in the presence of a ligase-dead mutant of Nedd4-2. In summary, the investigation established for the first time that sgk1 stimulates hOAT3 transport activity by interfering with the inhibitory effect of Nedd4-2 on the transporter.

Loops and layers of post-translational modifications of drug transporters

Drug transporters encoded by solute carrier (SLC) family are distributed in multiple organs including kidney, liver, placenta, brain, and intestine, where they mediate the absorption, distribution, and excretion of a diverse array of environmental toxins and clinically important drugs. Alterations in the expression and function of these transporters play important roles in intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. While it is clear that the regulation of these transporters tightly depends on genetic mechanisms, many studies have demonstrated that these transporters are the target of various post-translational modifications. This review article summarizes the recent advances in identifying the posttranslational modifications underlying the regulation of the drug transporters of SLC family. Such mechanisms are pivotal not only in physiological conditions, but also in diseases.

PKC/Nedd4-2 Signaling Pathway Regulates the Cell Surface Expression of Drug Transporter hOAT1

Human organic anion transporter-1 (hOAT1) regulates the absorption, distribution, and excretion of a wide range of clinically important drugs. Our previous work demonstrated that hOAT1 is a dynamic membrane transporter, constitutively internalizing from and recycling back to the cell plasma membrane. Short-term activation (<30 minutes) of protein kinase C (PKC) promotes the attachment of a lysine 48-linked polyubiquitin chain to hOAT1, a process catalyzed by ubiquitin ligase neural precursor cell expressed developmentally down-regulated 4-2 (Nedd4-2). The ubiquitination of hOAT1 then triggers an accelerated endocytosis of the transporter from plasma membrane, which results in reduced hOAT1 expression at the cell surface and decreased hOAT1 transport activity. In the present study, we investigated the long-term effect of PKC on hOAT1. We showed that long-term activation (>2 hours) of PKC significantly enhanced hOAT1 degradation, and such action was partially blocked by ubiquitin mutant Ub-K48R, which has its lysine (K) 48 mutated to arginine (R) and is incapable of forming a K48-linked polyubiquitin chain. The ubiquitin ligase Nedd4-2 was also found to augment hOAT1 degradation. These results suggest that PKC-regulated and Nedd4-2-catalyzed attachment of a lysine 48-linked polyubiquitin chain to hOAT1 is important for hOAT1 stability. We further showed through coimmunoprecipitation experiments that there was a direct association between hOAT1 and Nedd4-2, and such interaction was weakened when the WW3 and WW4 domains of the ligase were mutated. Mutating WW3 and WW4 domains of the ligase also impaired its ability to ubiquitinate hOAT1. Therefore, WW3 and WW4 domains of Nedd4-2 are critical for its association with and modulation of the transporter.

Effects of nonsteroidal anti-inflammatory drugs on the renal excretion of indoxyl sulfate, a nephro-cardiovascular toxin, in rats

Chronic kidney disease (CKD) is a health problem worldwide. Indoxyl sulfate (IS) is a nephro-cardiovascular toxin accumulated in CKD patients and cannot be removed through hemodialysis. The renal excretion of IS was mediated by organic anion transporters (OATs) OAT 1 and OAT 3. Because a number of nonsteroidal anti-inflammatory drugs (NSAIDs) have been reported to inhibit OATs, we hypothesize that NSAIDs might inhibit the renal excretion of IS. Rats were intravenously injected IS with and without diclofenac, ketoprofen or salicylic acid, individually. Blood samples were collected at predetermined time points and the concentrations of IS were determined by HPLC method. The results showed that diclofenac and ketoprofen at 10.0mg/kg significantly decreased the systemic clearance of IS by 71% and 82%, and increased the MRT of IS by 106% and 105%, respectively, whereas salicylic acid did not exhibit significant effects. Cell studies indicated that diclofenac and ketoprofen inhibited the uptake of IS mediated by OAT 1 and OAT 3. In conclusion, diclofenac and ketoprofen inhibited the excretion of IS through inhibition on OAT 1 and OAT 3.

Riceberry bran extract prevents renal dysfunction and impaired renal organic anion transporter 3 (Oat3) function by modulating the PKC/Nrf2 pathway in gentamicin-induced nephrotoxicity in rats

PURPOSE

This study investigated the protective effects of Riceberry bran extract (RBBE) on renal function, and the function and expression of renal organic anion transporter 3 (Oat3) in gentamicin-induced nephrotoxicity in rats and explored the mechanisms for its protective effects.

MATERIAL AND METHODS

Male Sprague Dawley rats (n= 42) were divided into six groups to receive normal saline, gentamicin (100mg/kg), co-treatment of gentamicin and RBBE (at dose of 250, 500 and 1000mg/kg), and RBBE (at dose of 1000mg/kg) only, for consecutive fifteen days. Renal function, oxidative and antioxidative markers, the function and expression of Oat3 and histological changes in the kidney were evaluated.

RESULTS

Elevation of BUN, serum creatinine levels and reduction in urine creatinine and creatinine clearance indicated decreased renal function in the gentamicin-treated rats. The decrease of [³H]ES uptake in the renal cortical slices of these rats, reflecting the attenuation of Oat3 transport function that was accompanied by decreased expression of Oat3. Moreover, increased MDA level and reduced superoxide dismutase (SOD) and glutathione (GSH) activities were found in gentamicin-treated rats compared to the control group. These changes were associated with the upregulated PKCα, Nrf-2, Keap 1, NQO-1 and HO-1 expressions in kidneys. RBBE treatment improved the renal function and Oat3 transport function and expression in gentamicin-treated rats. The oxidative status was also restored by RBBE treatment.

CONCLUSION

RBBE protects kidney injury by its antioxidant effect, subsequently leading to modulation of the PKC/Nrf2 antioxidant defense pathway.

Posttranslational Regulation of Organic Anion Transporters by Ubiquitination: Known and Novel

Organic anion transporters (OATs) encoded by solute carrier 22 family are localized in the epithelia of multiple organs, where they mediate the absorption, distribution, and excretion of a diverse array of negatively charged environmental toxins and clinically important drugs. Alterations in the expression and function of OATs play important roles in intra- and interindividual 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. The regulation of OAT transport activity in response to various stimuli can occur at several levels such as transcription, translation, and posttranslational modification. Posttranslational regulation is of particular interest, because it usually happens within a very short period of time (minutes to hours) when the body has to deal with rapidly changing amounts of substances as a consequence of variable intake of drugs, fluids, or meals as well as metabolic activity. This review article highlights the recent advances from our laboratory in uncovering several posttranslational mechanisms underlying OAT regulation. These advances offer the promise of identifying targets for novel strategies that will maximize therapeutic efficacy in drug development.

The role of Nedd4-1 WW domains in binding and regulating human organic anion transporter 1

Human organic anion transporter 1 (hOAT1), expressed at the basolateral membrane of kidney proximal tubule cells, mediates the active renal secretion of a diverse array of clinically important drugs, including anti-human immunodeficiency virus therapeutics, antitumor drugs, antibiotics, antihypertensives, and anti-inflammatories. We have previously demonstrated that posttranslational modification of hOAT1 by ubiquitination is an important mechanism for the regulation of this transporter. The present study aimed at identifying the ubiquitin ligase for hOAT1 and its mechanism of action. We showed that overexpression of neural precursor cell expressed, developmentally downregulated (Nedd)4-1, an E3 ubiquitin ligase, enhanced hOAT1 ubiquitination, decreased hOAT1 expression at the cell surface, and inhibited hOAT1 transport activity. In contrast, overexpression of the ubiquitin ligase-dead mutant Nedd4-1/C867S was without effects on hOAT1. Furthermore, knockdown of endogenously expressed Nedd4-1 by Nedd4-1-specific small interfering RNA reduced hOAT1 ubiquitination. Immunoprecipitation experiments in cultured cells and rat kidney slices and immunofluorescence experiments in rat kidney slices showed that there was a physical interaction between OAT1 and Nedd4-1. Nedd4-1 contains four protein-protein interacting WW domains. When these WW domains were inactivated by mutating two amino acid residues in each of the four WW domains (Mut-WW1: V210W/H212G, Mut-WW2: V367W/H369G, Mut-WW3: I440W/H442G, and Mut-WW4: I492W/H494G, respectively), only Mut-WW2 and Mut-WW3 significantly lost their ability to bind and to ubiquitinate hOAT1. As a result, Mut-WW2 and Mut-WW3 were unable to suppress hOAT1-mediated transport as effectively as wild-type Nedd4-1. In conclusion, this is the first demonstration that Nedd4-1 regulates hOAT1 ubiquitination, expression, and transport activity through its WW2 and WW3 domains.

Serum- and glucocorticoid-inducible kinase SGK2 regulates human organic anion transporters 4 via ubiquitin ligase Nedd4-2

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT4 is abundantly expressed in the kidney and placenta. In the current study, we examined the regulation of hOAT4 by serum- and glucocorticoid-inducible kinase 2 (sgk2) in the kidney COS-7 cells. We showed that sgk2 stimulated hOAT4 transport activity. Such stimulation mainly resulted from an increased cell surface expression of the transporter, kinetically revealed as an increased maximal transport velocity Vmax without significant change in substrate-binding affinity Km. We further showed that regulation of hOAT4 activity by sgk2 was mediated by ubiquitin ligase Nedd4-2. Overexpression of Nedd4-2 enhanced hOAT4 ubiquitination, and inhibited hOAT4 transport activity, whereas overexpression of ubiquitin ligase-dead mutant Nedd4-2/C821A or siRNA knockdown of endogenous Nedd4-2 had opposite effects on hOAT4. Our co-immunoprecipitation experiment revealed that sgk2 weakened the association between hOAT4 and Nedd4-2. In conclusion, our study demonstrated for the first time that sgk2 stimulated hOAT4 transport activity by abrogating the inhibitory effect of Nedd4-2 on the transporter.

Nedd4-2 but not Nedd4-1 is critical for protein kinase C-regulated ubiquitination, expression, and transport activity of human organic anion transporter 1

Human organic anion transporter 1 (hOAT1) expressed at the membrane of the kidney proximal tubule cells mediates the body disposition of a diverse array of clinically important drugs, including anti-HIV therapeutics, antitumor drugs, antibiotics, antihypertensives, and antiinflammatories. Therefore, understanding the regulation of hOAT1 will provide significant insights into kidney function and dysfunction. We previously established that hOAT1 transport activity is inhibited by activation of protein kinase C (PKC) through accelerating hOAT1 internalization from cell surface into intracellular endosomes and subsequent degradation. We further established that PKC-induced hOAT1 ubiquitination is an important step preceding hOAT1 internalization. In the current study, we identified two closely related E3 ubiquitin ligases, neural precursor cell expressed, developmentally downregulated 4-1 and 4-2 (Nedd4-1 and Nedd4-2), as important regulators for hOAT1: overexpression of Nedd4-1 or Nedd4-2 enhanced hOAT1 ubiquitination, reduced the hOAT1 amount at the cell surface, and suppressed hOAT1 transport activity. In further exploring the relationship among PKC, Nedd4-1, and Nedd4-2, we discovered that PKC-dependent changes in hOAT1 ubiquitination, expression, and transport activity were significantly blocked in cells transfected with the ligase-dead mutant of Nedd4-2 (Nedd4-2/C821A) or with Nedd4-2-specific siRNA to knockdown endogenous Nedd4-2 but not in cells transfected with the ligase-dead mutant of Nedd4-1 (Nedd4-1/C867S) or with Nedd4-1-specific siRNA to knockdown endogenous Nedd4-1. In conclusion, this is the first demonstration that both Nedd4-1 and Nedd4-2 are important regulators for hOAT1 ubiquitination, expression, and function. Yet they play distinct roles, as Nedd4-2 but not Nedd4-1 is a critical mediator for PKC-regulated hOAT1 ubiquitination, expression, and transport activity.

An Essential Role of Nedd4-2 in the Ubiquitination, Expression, and Function of Organic Anion Transporter-3

Organic anion transporter-3 (OAT3) is a member of the organic anion transporter family that mediates the body disposition of a diverse array of clinically important drugs. We previously demonstrated that activation of protein kinase C (PKC) inhibits OAT3 transport activity by accelerating OAT3 internalization from cell surface into intracellular compartments. In the current study, we established that PKC-induced inhibition of OAT3 transport activity occurred through an enhanced OAT3 ubiquitination, a process catalyzed by an E3 ubiquitin-protein ligase Nedd4-2 (neural precursor cell expressed, developmentally downregulated 4-2). Overexpression of Nedd4-2 enhanced OAT3 ubiquitination, decreased OAT3 expression at the cell surface, and inhibited OAT3 transport activity. In contrast, overexpression of the ubiquitin ligase-dead mutant Nedd4-2/C821A or siRNA knockdown of endogenous Nedd4-2 had opposite effects on OAT3. Furthermore, immunoprecipitation experiments conducted both in culture cells and with rat kidney slices showed that there was a physical interaction between OAT3 and Nedd4-2. In conclusion, our results provided the first evidence that Nedd4-2 is an important regulator for OAT3 ubiquitination, expression, and transport activity.

Flavonoids are inhibitors of human organic anion transporter 1 (OAT1)-mediated transport

Organic anion transporter 1 (OAT1) has been reported to be involved in the nephrotoxicity of many anionic xenobiotics. As current clinically used OAT1 inhibitors are often associated with safety issues, identifying potent OAT1 inhibitors with little toxicity is of great value in reducing OAT1-mediated drug nephrotoxicity. Flavonoids are a class of polyphenolic compounds with exceptional safety records. Our objective was to evaluate the effects of 18 naturally occurring flavonoids, and some of their glycosides, on the uptake of para-aminohippuric acid (PAH) in both OAT1-expressing and OAT1-negative LLC-PK1 cells. Most flavonoid aglycones produced substantial decreases in PAH uptake in OAT1-expressing cells. Among the flavonoids screened, fisetin, luteolin, morin, and quercetin exhibited the strongest effect and produced complete inhibition of OAT1-mediated PAH uptake at a concentration of 50 μM. Further concentration-dependent studies revealed that both morin and luteolin are potent OAT1 inhibitors, with IC50 values of <0.3 and 0.47 μM, respectively. In contrast to the tested flavonoid aglycones, all flavonoid glycosides had negligible or small effects on OAT1. In addition, the role of OAT1 in the uptake of fisetin, luteolin, morin, and quercetin was investigated and fisetin was found to be a substrate of OAT1. Taken together, our results indicate that flavonoids are a novel class of OAT1 modulators. Considering the high consumption of flavonoids in the diet and in herbal products, OAT1-mediated flavonoid-drug interactions may be clinically relevant. Further investigation is warranted to evaluate the nephroprotective role of flavonoids in relation to drug-induced nephrotoxicity mediated by the OAT1 pathway.

Three ubiquitination sites of organic anion transporter-1 synergistically mediate protein kinase C-dependent endocytosis of the transporter

Organic anion transporter-1 (OAT1) mediates the body disposition of a diverse array of clinically important drugs, including anti-HIV therapeutics, antitumor drugs, antibiotics, antihypertensives, and anti-inflammatories. Therefore, understanding the regulation of OAT1 has profound clinical significance. We previously established that OAT1 constitutively internalizes from and recycles back to cell surface and that activation of protein kinase C (PKC) inhibits OAT1 activity by promoting ubiquitination of the transporter, which then leads to an accelerated internalization of the transporter from cell surface to intracellular compartments. In the current study, we showed that PKC isoform PKCα was responsible for OAT1 ubiquitination. To directly address the role of OAT1 ubiquitination, we then generated two OAT1 mutants, each having multiple lysines (K) simultaneously mutated to arginine (R). One mutant K163/297/303/315/321R lost sensitivities to PKC-induced inhibition of transport activity, to PKC-induced ubiquitination, and to PKC-induced acceleration of transporter internalization. Further dissecting each lysine in this mutant, we identified Lys297, Lys303, and Lys315 as being the ubiquitin conjugation sites. Of interest, mutating any one of the three lysines prevented the ubiquitin conjugation to the other two lysines, suggesting that Lys297, Lys303, and Lys315 may form an optimal structure to interact with ubiquitination machineries. This is the first demonstration that Lys297, Lys303, and Lys315 play a synergistic role in PKC-regulated OAT1 ubiquitination, trafficking, and transport activity.

Pharmacokinetics and Relative Bioavailability of Flavonoids between Two Dosage Forms of Gegen-Qinlian-Tang in Rats

Gegen-Qinlian-Tang (GQT), a popular Chinese medicine prescription, consists of Puerariae Radix, Scutellariae Radix, Coptidis Rhizoma, and Glycyrrhizae Radix. This study investigated the pharmacokinetics of GQT in rats and compared the bioavailability between two dosage forms, that is, traditional decoction (TD) and concentrated powder (CP). Rats were given TD and CP of GQT in a crossover design, and blood samples were withdrawn at predetermined time points. The quantitation methods of ten constituents in two dosage forms of GQT and in serum specimen using HPLC were developed and validated in this study. The pharmacokinetic parameters were calculated using noncompartment model. The results showed that daidzein, baicalein, wogonin, berberine, palmatine, and coptisine were not found in the circulation, whereas the sulfates/glucuronides of daidzein, baicalein, and wogonin were the major forms after oral administration of GQT. Comparison between two dosage forms indicated that the AUC_0–t of daidzein sulfates/glucuronides after administration of CP was significantly lower than that of TD by 28.9%, whereas the bioavailabilities of baicalin/baicalein and wogonoside/wogonin were comparable between two dosage forms. In conclusion, the major flavonoids of GQT were extensively metabolized into sulfates/glucuronides and present as the major molecules in the circulation. TD of GQT revealed higher bioavailability of daidzin/daidzein than CP.

Lysine 48-linked polyubiquitination of organic anion transporter-1 is essential for its protein kinase C-regulated endocytosis

Organic anion transporter-1 (OAT1) mediates the body's disposition of a diverse array of environmental toxins and clinically important drugs. Therefore, understanding the regulation of this transporter has profound clinical significance. We had previously established that OAT1 undergoes constitutive internalization from and recycling back to the cell surface and that acute activation of protein kinase C (PKC) inhibits OAT1 activity by reducing OAT1 cell-surface expression through accelerating its internalization from cell surface to intracellular compartments. However, the underlying mechanisms are poorly understood. In the current study, we provide novel evidence that acute activation of PKC significantly enhances OAT1 ubiquitination both in vitro and ex vivo. We further show that ubiquitination of cell-surface OAT1 increases in cells transfected with dominant negative mutant of dynamin-2, a maneuver blocking OAT1 internalization, which suggests that OAT1 ubiquitination proceeds before OAT1 internalization. Mass spectroscopy has revealed that ubiquitination of OAT1 consists of polyubiquitin chains, primarily through lysine 48 linkage. Transfection of cells with the dominant negative mutant of ubiquitin Ub-K48R, which prevents the formation of Lys48-linked polyubiquitin chains, abolishes PKC-stimulated OAT1 ubiquitination and internalization. Together, our findings demonstrate for the first time that Lys48-linked polyubiquitination is essential for PKC-regulated OAT1 trafficking.

Potent inhibitors of human organic anion transporters 1 and 3 from clinical drug libraries: discovery and molecular characterization

Transporter-mediated drug-drug interactions in the kidney dramatically influence the pharmacokinetics and other clinical effects of drugs. Human organic anion transporters 1 (hOAT1) and 3 (hOAT3) are the major transporters in the basolateral membrane of kidney proximal tubules, mediating the rate-limiting step in the elimination of a broad spectrum of drugs. In the present study, we screened two clinical drug libraries against hOAT1 and hOAT3. Of the 727 compounds screened, 92 compounds inhibited hOAT1 and 262 compounds inhibited hOAT3. When prioritized based on the peak unbound plasma concentrations of these compounds, three inhibitors for hOAT1 and seven inhibitors for hOAT3 were subsequently identified with high inhibitory potency (>95%). Computational analyses revealed that inhibitors and noninhibitors can be differentiated from each other on the basis of several physicochemical features, including number of hydrogen-bond donors, number of rotatable bonds, and topological polar surface area (TPSA) for hOAT1; and molecular weight, number of hydrogen-bond donors and acceptors, TPSA, partition coefficient (log P(7.4)), and polarizability for hOAT3. Pharmacophore modeling identified two common structural features associated with inhibitors for hOAT1 and hOAT3, viz., an anionic hydrogen-bond acceptor atom, and an aromatic center separated by ∼5.7 Å. Such model provides mechanistic insights for predicting new OAT inhibitors.

Conformational changes of the multispecific transporter organic anion transporter 1 (OAT1/SLC22A6) suggests a molecular mechanism for initial stages of drug and metabolite transport

The solute carrier (SLC) family of transporters play key roles in the movement of charged organic ions across the blood-urine, blood-cerebrospinal fluid, and blood-brain barriers and thus mediate the absorption, disposition, and elimination of many common pharmaceuticals (i.e., nonsteroidal anti-inflammatory drug (NSAIDs), antibiotics, and diuretics). They have also been proposed to participate in a remote sensing and signaling network involving small molecules. Nevertheless, other than possessing a 12-transmembrane α-helical topology comprised of two six-helix hemidomains interacting through a long loop, the structural and mechanistic details for these transporters remains unclear. Recent crystallographic studies of bacterial homologs support the idea of a "switching" mechanism, which allows for periodic changes in the overall transporter configuration and cyclic opening of the transporter to the extracellular or cytoplasmic sides of the membrane. To investigate this, computational modeling based on our recent study of glycerol-3-phosphate transporter (GlpT) (Tsigelny et al. J Bioinform Comput Biol 6:885-904, 2008) was performed for organic anion transporter 1 (OAT1/SLC22A6, originally identified as NKT), the prototypical member of this family. OAT1 was inserted into an artificial phospholipid bilayer and the positional change of the six-helix hemidomains relative to each other was followed for 100 ns. The hemidomains were found to tilt relative to each other while their configuration is mostly inflexible. Since the modeling was performed for about 100 ns, the data suggest that this tilting mechanism might explain the early steps in the transport of organic anionic metabolites, drugs, and toxins by this clinically important transporter.

Linkage of organic anion transporter-1 to metabolic pathways through integrated "omics"-driven network and functional analysis

The main kidney transporter of many commonly prescribed drugs (e.g. penicillins, diuretics, antivirals, methotrexate, and non-steroidal anti-inflammatory drugs) is organic anion transporter-1 (OAT1), originally identified as NKT (Lopez-Nieto, C. E., You, G., Bush, K. T., Barros, E. J., Beier, D. R., and Nigam, S. K. (1997) J. Biol. Chem. 272, 6471-6478). Targeted metabolomics in knockouts have shown that OAT1 mediates the secretion or reabsorption of many important metabolites, including intermediates in carbohydrate, fatty acid, and amino acid metabolism. This observation raises the possibility that OAT1 helps regulate broader metabolic activities. We therefore examined the potential roles of OAT1 in metabolic pathways using Recon 1, a functionally tested genome-scale reconstruction of human metabolism. A computational approach was used to analyze in vivo metabolomic as well as transcriptomic data from wild-type and OAT1 knock-out animals, resulting in the implication of several metabolic pathways, including the citric acid cycle, polyamine, and fatty acid metabolism. Validation by in vitro and ex vivo analysis using Xenopus oocyte, cell culture, and kidney tissue assays demonstrated interactions between OAT1 and key intermediates in these metabolic pathways, including previously unknown substrates, such as polyamines (e.g. spermine and spermidine). A genome-scale metabolic network reconstruction generated some experimentally supported predictions for metabolic pathways linked to OAT1-related transport. The data support the possibility that the SLC22 and other families of transporters, known to be expressed in many tissues and primarily known for drug and toxin clearance, are integral to a number of endogenous pathways and may be involved in a larger remote sensing and signaling system (Ahn, S. Y., and Nigam, S. K. (2009) Mol. Pharmacol. 76, 481-490, and Wu, W., Dnyanmote, A. V., and Nigam, S. K. (2011) Mol. Pharmacol. 79, 795-805). Drugs may alter metabolism by competing for OAT1 binding of metabolites.

Transmembrane peptide as potent inhibitor of oligomerization and function of human organic anion transporter 1

Human organic anion transporter 1 (hOAT1) plays a critical role in the body disposition of environmental toxins and clinically important drugs, including anti-HIV therapeutics, antitumor drugs, antibiotics, antihypertensives, and anti-inflammatories. We have demonstrated previously that hOAT1 forms homo-oligomers in cultured cells and in rat kidney. However, the functional consequence of such oligomerization has never been elucidated. In the current study, we used a novel approach by examining the effects of short hydrophobic peptides corresponding to transmembrane domains (TMDs) 1 to 12 of hOAT1 on the oligomerization and function of the transporter. We constructed expression vectors encoding short fusion peptides corresponding to TMDs 1 to 12 of hOAT1. These peptides were transfected into hOAT1-expressing COS-7 cells. Our results showed that among all 12 peptides examined, only the peptide corresponding to TMD 6 of hOAT1 significantly disrupted hOAT1 oligomerization demonstrated by cross-linking and coimmunoprecipitation experiments. The same peptide also caused a reduced expression of hOAT1 at the cell surface. As a result, hOAT1-mediated transport activity was compromised. Our data suggest that the peptide corresponding to TMD 6 of hOAT1 is a potent inhibitor of hOAT1 oligomerization and that oligomerization of hOAT1 is critical for the expression of the transporter at the cell surface and consequently for the proper function of the transporter.

Regulation of human organic anion transporter 3 by peptide hormone bradykinin

Human organic anion transporter (hOAT) 3 belongs to a family of organic anion transporters that play critical roles in the body disposition of numerous clinically important drugs. In the current study, we examined the regulation of hOAT3 by peptide hormone bradykinin (BK) in COS-7 cells. BK (<or=500 nM) induced a concentration- and time-dependent stimulation of hOAT3 activity, kinetically revealed as an increased V(max). Such an increase in V(max) resulted from an increased cell surface expression without a change in total cell expression of the transporter. BK-induced stimulation of hOAT3 activity could be prevented by treating hOAT3-expressing cells with staurosporine, a general inhibitor for protein kinase C (PKC). To obtain further information on which PKC isoform mediates BK regulation of hOAT3 activity, cellular distribution of various PKC isoforms was examined in cells treated with BK. We showed that BK treatment resulted in a significant translocation of PKCdelta, PKCepsilon, and PKCzeta from cytosol to membrane. We further showed that BK treatment enhanced association of hOAT3 with PKCdelta, PKCepsilon, and PKCzeta and that isoform-specific inhibitor for PKCdelta, PKCepsilon, and PKCzeta reversed BK effect on hOAT3 activity. We therefore concluded that BK stimulated hOAT3 activity through activation of PKCdelta, PKCepsilon, and PKCzeta, which then led to the redistribution of hOAT3 from the intracellular compartments to the cell surface and to the up-regulation of hOAT3 activity.

Organic cation transporters and their roles in antiretroviral drug disposition

BACKGROUND

Understanding metabolising processes and drug-transporter interactions is particularly crucial to the management of patients with HIV infection, given the several antiretroviral drugs that must be taken lifelong and the use of other medications for HIV-related and non-HIV-related conditions. Several interactions of antiretroviral drugs with metabolising enzymes, especially cytochrome P450 and ATP-dependent transporter P-glycoprotein, have been described but the role of the organic cation transporters (OCTs) is less clearly defined.

OBJECTIVE

To review the relevance of the OCTs for antiretroviral drug disposition.

METHODS

Interactions of OCTs with antiretroviral drugs and evidence for clinical relevance are discussed.

RESULTS/CONCLUSION

Several antiretroviral drugs show relevant interactions with the OCTs in cell-based experiments and the OCTs are highly upregulated in HIV-infected patients. For evaluating the clinical significance, interaction studies in HIV patients and reliable in vitro models for delineation of in vivo effects are needed.

Functional characterization of nonsynonymous single nucleotide polymorphisms in the human organic anion transporter 4 (hOAT4)

BACKGROUND AND PURPOSE

The human organic anion transporter (hOAT) family of transmembrane carrier proteins mediate the cellular flux of anionic substances, including certain hormones and anti-cancer drugs. hOAT4 is highly expressed at the apical membrane of the renal tubular cell and facilitates drug re-absorption in the kidney. In the present study, the impact of 10 nonsynonymous single nucleotide polymorphisms (SNPs) of hOAT4 on transport function in COS-7 cells was characterized.

EXPERIMENTAL APPROACH

Transport uptake assay was used to assess the function of the variant transporters. Cell surface biotinylation and western blot analysis were used to investigate the expression characteristics of the transporter proteins. Comparative modelling was used to interpret the influence of nonsynonymous changes in terms of hOAT4 structure.

KEY RESULTS

Four naturally occurring hOAT4 variants (L29P, R48Y, V155G and T392I) exhibited a significant loss of function. Substitution of leucine-29, which is a conserved residue in OATs, with a proline residue, impaired the synthesis or the apparent stability of the transporter and membrane insertion was disrupted in the R48Y variant. In the case of the V155G and T392I variants, impaired function was due to decreased affinity of the transporter for oestrone sulphate and impaired transporter-substrate turnover respectively. The T392I variant was inhibited more extensively than the wild-type transporter by the cationic substrate tetraethyl ammonium.

CONCLUSIONS AND IMPLICATIONS

Several naturally occurring SNPs encode variant hOAT4s that may impair the renal tubular re-absorption of important drug substrates.

Functional characterization of nonsynonymous single nucleotide polymorphisms in the human organic anion transporter 4 (hOAT4)

BACKGROUND AND PURPOSE

The human organic anion transporter (hOAT) family of transmembrane carrier proteins mediate the cellular flux of anionic substances, including certain hormones and anti-cancer drugs. hOAT4 is highly expressed at the apical membrane of the renal tubular cell and facilitates drug re-absorption in the kidney. In the present study, the impact of 10 nonsynonymous single nucleotide polymorphisms (SNPs) of hOAT4 on transport function in COS-7 cells was characterized.

EXPERIMENTAL APPROACH

Transport uptake assay was used to assess the function of the variant transporters. Cell surface biotinylation and western blot analysis were used to investigate the expression characteristics of the transporter proteins. Comparative modelling was used to interpret the influence of nonsynonymous changes in terms of hOAT4 structure.

KEY RESULTS

Four naturally occurring hOAT4 variants (L29P, R48Y, V155G and T392I) exhibited a significant loss of function. Substitution of leucine-29, which is a conserved residue in OATs, with a proline residue, impaired the synthesis or the apparent stability of the transporter and membrane insertion was disrupted in the R48Y variant. In the case of the V155G and T392I variants, impaired function was due to decreased affinity of the transporter for oestrone sulphate and impaired transporter-substrate turnover respectively. The T392I variant was inhibited more extensively than the wild-type transporter by the cationic substrate tetraethyl ammonium.

CONCLUSIONS AND IMPLICATIONS

Several naturally occurring SNPs encode variant hOAT4s that may impair the renal tubular re-absorption of important drug substrates.

Identification of genetic polymorphisms of human OAT1 and OAT2 genes and their relationship to hOAT2 expression in human liver

BACKGROUND

Organic anion transporters (OATs) play an essential role in the disposition of numerous organic anions. To clarify the interindividual variation in the function of OATs, genetic polymorphisms of the SLC22A6 and SLC22A7 in Korean subjects were investigated and associated with hepatic hOAT2 expression and the SLC22A7 genotype.

METHODS

The genetic variants and their frequencies for the SLC22A6 and SLC22A7 genes in 50 Korean subjects were investigated by direct sequencing. The expression of hOAT2 protein from 34 human liver samples was examined by western blot analysis.

RESULTS

Eight SNPs including 2 novel SNPs were identified in the SLC22A6 gene and eight SNPs including 4 novel SNPs were identified in the SLC22A7 gene. No amino acid alteration was found. Linkage disequilibrium (LD) analysis revealed that the SLC22A6 and SLC22A7 genes have separated single LD blocks and consist of a limited number of haplotypes (14 haplotypes for SLC22A6 and 5 haplotypes for SLC22A7). The expression of the hOAT2 protein varied 10-fold among 34 human livers but was not associated with the SLC22A7 genotype (p=0.16).

CONCLUSIONS

The SLC22A7 genomic sequences showed low variability. A 10-fold variation in hOAT2 protein expression in the liver specimens was not correlated with SLC22A7 genotypes. These results suggest that genetic polymorphisms may not be a significant contributing factor to variations in the hOAT2 expression or hOAT2 transport activity.

Toward a systems level understanding of organic anion and other multispecific drug transporters: a remote sensing and signaling hypothesis

Organic anion transporters (Oats) are located in the barrier epithelia of diverse organs, where they mediate the absorption and excretion of a wide range of metabolites, signaling molecules, and xenobiotics. Although their interactions with a broad group of substrates have been extensively studied and described, the primary physiological role of Oats remains elusive. The presence of overlapping substrate specificities among the different Oat isoforms, together with recent metabolomic data from the Oat1, Oat3, and renal-specific transporter (RST/URAT1) knockout mice, suggests a possible role in remote signaling wherein substrates excreted through one Oat isoform in one organ are taken up by another Oat isoform located in a different organ, thereby mediating communication between different organ systems, or even between different organisms. Here we further develop this "remote sensing and signaling hypothesis" and suggest how the regulation of SLC22 subfamily members (including those of the organic cation, organic carnitine, and unknown substrate transporter subfamilies) can be better understood by considering the organism's broader need to communicate between epithelial and other tissues by simultaneous regulation of transport of metabolites, signaling molecules, drugs, and toxins. This systems biology perspective of remote signaling (sensing) could help reconcile an enormous array of tissue-specific data for various SLC22 family genes and, possibly, other multispecific transporters, such as those of the organic anion transporting polypeptide (OATP, SLC21) and multidrug resistance-associated protein (MRP) families.

Novobiocin is a potent inhibitor for human organic anion transporters

Organic anion transporters (OATs) mediate the body disposition of a diverse array of environmental toxins and clinically important drugs. Previous studies have shown that novobiocin, an inhibitor for breast cancer resistance proteins (BCRP), inhibited organic anion transport. However, its interactions with specific OATs are unknown. In the current study, we characterized the inhibitory effects of novobiocin on the function of human OATs (hOAT)1, hOAT3, and hOAT4. Kinetic study revealed that novobiocin inhibited OAT-mediated uptake in a competitive manner, with K(i) of 14.87 +/- 0.40 microM for hOAT1, K(i) of 4.77 +/- 1.12 microM for hOAT3, and K(i) of 90.50 +/- 7.50 microM for hOAT4. Furthermore, the cis- and trans-inhibition feature of novobiocin demonstrated that novobiocin was a potent inhibitor but not a substrate for hOAT1 (IC(50) = 34.76 +/- 0.31 microM), hOAT3 (IC(50) = 4.987 +/- 0.35 microM), and hOAT4 (IC(50) = 92.68 +/- 0.34 microM). We further showed that the effects of novobiocin on OATs were not mediated through a change in transporter protein abundance on the plasma membrane. Taken together, we conclude that novobiocin seems to interact with the substrate-binding sites of OATs from both the intracellular and the extracellular sides, and this interaction interferes with the substrate-binding site(s) on respective carriers, leading to an apparent reduction in carriers available for the substrates. Because BCRP is often expressed in the same tissue where multiple OATs are identified such as liver, kidney and placenta, when dissecting the contribution of BCRP to drug disposition using novobiocin as an inhibitor, its inhibitory effect to OATs has to be taken into consideration.

Regulation of human organic anion transporter 1 by ANG II: involvement of protein kinase Calpha

Human organic anion transporter 1 (hOAT1) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-human immunodeficiency virus therapeutics, anti-tumor drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT1 is abundantly expressed in the kidney. In the current study, we examined the regulation of hOAT1 by ANG II in kidney COS-7 cells. ANG II induced a concentration- and time-dependent inhibition of hOAT1 transport activity. Such inhibition mainly resulted from a decreased cell surface expression without a change in total cell expression of the transporter, kinetically revealed as a decreased maximal velocity without significant change in Michaelis constant. ANG II-induced inhibition of hOAT1 activity could be prevented by treating hOAT1-expressing cells with staurosporine, a general protein kinase C (PKC) inhibitor. To obtain further information on which PKC isoform mediates ANG II regulation of hOAT1 activity, cellular distribution of various PKC isoforms was examined in cells treated with or without ANG II. We showed that ANG II treatment resulted in a significant translocation of PKCalpha from cytosol to membrane, and such translocation was blocked by treating hOAT1-expressing cells with Gö-6976, a PKCalpha-specific inhibitor. We further showed that ANG II-induced inhibition of hOAT1 activity and retrieval of hOAT1 from the cell surface could also be prevented by treating hOAT1-expressing cells with Gö-6976. We concluded that ANG II inhibited hOAT1 activity through activation of PKCalpha, which led to the redistribution of the transporter from the cell surface to the intracellular compartments.

Organic anion transporter OAT1 undergoes constitutive and protein kinase C-regulated trafficking through a dynamin- and clathrin-dependent pathway

Organic anion transporter 1 (OAT1) mediates the body disposition of a diverse array of environmental toxins and clinically important drugs. Therefore, understanding the regulation of this transporter has profound clinical significance. We previously demonstrate that OAT1 activity was down-regulated by activation of protein kinase C (PKC), kinetically revealed as a decrease in the maximum transport velocity V(max) without significant change in the substrate affinity K(m) of the transporter. In the current study, we showed that OAT1 constitutively internalized from and recycled back to the plasma membrane, and PKC activation accelerated OAT1 internalization without affecting OAT1 recycling. We further showed that treatment of OAT1-expressing cells with concanavalin A, depletion of K(+) from the cells, or transfection of dominant negative mutants of dynamin-2 or Eps15 into the cells, all of which block the clathrin-dependent endocytotic pathway, significantly blocked constitutive and PKC-regulated OAT1 internalization. We finally showed that OAT1 colocalized with transferrin, a marker for clathrin-dependent endocytosis, at the cell surface and in the EEA1-positive early endosomes. Together, our findings demonstrated for the first time that (i) OAT1 constitutively traffics between plasma membrane and recycling endosomes, (ii) PKC activation down-regulates OAT1 activity by altering already existent OAT1 trafficking, and (iii) OAT1 internalization occurs partly through a dynamin- and clathrin-dependent pathway.

Drug and toxicant handling by the OAT organic anion transporters in the kidney and other tissues

Organic anion transporters (OATs) translocate drugs as well as endogenous substances and toxins. The prototype, OAT1 (SLC22A6), first identified as NKT in 1996, is the best-studied member of the OAT subgroup of the SLC22 transporter family, which also includes OCTs (organic cation transporters), OCTNs (organic cation transporters of carnitine) and Flipts (fly-like putative transporters). The SLC22 family is evolutionarily conserved, with members expressed in fly and worm. An unusual feature of many SLC22A genes is a tendency to exist in pairs or clusters in the genome. Much of the early research in the field focused on the role of OATs and other SLC22 family members in renal drug transport. OATs have now been localized to other epithelial tissues, including placenta (OAT4) and mouse olfactory mucosa (Oat6). Although findings from in vivo physiological studies in mice lacking OATs (e.g. Oat1 and Oat3) have generally been consistent with in vitro transport data from Xenopus oocytes and transfected cells, these in vivo data are helping to clarify the relative contributions of individual OATs to the renal excretion of particular organic anions and drugs. Moreover, in mutant mice, certain endogenous anions accumulate, suggesting the physiological roles of the proteins encoded by the mutant genes. It has been proposed that the presence of OATs and other SLC22-family members in multiple tissue compartments might enable a 'remote sensing' mechanism by allowing communication between organs, and possibly individuals, through organic ions. Variability of human drug responses and susceptibility to drug toxicity might, in part, be explained by variations in the coding and promoter regions of these genes. Computational biological studies are likely to not only shed light on molecular mechanisms of transport for compounds of clinical and toxicological interest, but also aid in drug design.

Effect of dronedarone on renal function in healthy subjects

AIMS

To assess the effects of dronedarone on renal function and tubular cation handling.

METHODS

Twelve healthy males were enrolled in a randomized, cross-over, placebo-controlled, double-blind study. They received 400 mg dronedarone or placebo twice daily for 7 days. Baseline and on-treatment renal function tests were performed under strict standardization of intakes, by assessing creatinine, sinistrin, para-amino-hippurate (PAH) and N-methylnicotinamide (NMN) CLs, and electrolyte excretion.

RESULTS

Compared with placebo, dronedarone significantly decreased renal creatinine CL (mean 138-119 ml min(-1) after dronedarone vs. 142-149 ml min(-1) after placebo) and NMN CL (448-368 ml min(-1)vs. 435-430 ml min(-1)), but did not alter renal sinistrin CL, PAH CL and other renal parameters.

CONCLUSIONS

Dronedarone reduces renal creatinine and NMN clearance by about 18%, without evidence of an effect on GFR, renal plasma flow or electrolyte exchanges. This suggests a specific partial inhibition of tubular organic cation transporters (OCT). A limited increase in serum creatinine is therefore expected with dronedarone treatment, but does not mean there is a decline in renal function.

Regulation of human organic anion transporter 4 by progesterone and protein kinase C in human placental BeWo cells

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-human immunodeficiency virus therapeutics, anti-tumor drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT4 is abundantly expressed in the placenta. In the current study, we examined the regulation of hOAT4 by pregnancy-specific hormones progesterone (P(4)) and 17beta-estradiol (E(2)) and by protein kinase C (PKC) in human placental BeWo cells. P(4) induced a time- and concentration-dependent downregulation of hOAT4 transport activity, whereas E(2) had no effect on hOAT4 function. The downregulation of hOAT4 activity by P(4) mainly resulted from a decreased cell surface expression without a change in total cell expression of the transporter, kinetically revealed as a decreased V(max) without significant change in K(m). Activation of PKC by phorbol 12,13-dibutyrate also resulted in an inhibition of hOAT4 activity through a decreased cell surface expression of the transporter. However, P(4)-induced downregulation of hOAT4 activity could not be prevented by treating hOAT4-expressing cells with the PKC inhibitor staurosporine. We concluded that both P(4) and activation of PKC inhibited hOAT4 activity through redistribution of the transporter from cell surface to the intracellular compartments. However, P(4) regulates hOAT4 activity by mechanisms independent of PKC pathway.

Comparison of the Interaction of Human Organic Anion Transporter hOAT4 with PDZ Proteins between Kidney Cells and Placental Cells

PURPOSE

To compare the interaction of human organic anion transporter hOAT4 with PDZ proteins between kidney cells and placental cells.

MATERIALS AND METHODS

PDZ proteins PDZK1 and NHERF1 were transfected into kidney LLC-PK1 cells and placental BeWo cells expressing hOAT4 or hOAT4-Delta, which lacks the PDZ consensus binding site. The interaction of PDZK1 and NHERF1 with hOAT4 and hOAT4-Delta was investigated by measurement of [3H] estrone sulfate uptake, cell surface and total cell expression of hOAT4.

RESULTS

PDZK1 and NHERF1 enhanced hOAT4 activity in LLC-PK1 cells by increasing the cell surface expression of the transporter. In contrasts, these two PDZ proteins had no effect on hOAT4 activity in BeWo cells.

CONCLUSION

The interaction of PDZ proteins with hOAT4 may be cell-specific. In placenta, a different set of interacting proteins from PDZK1 and NHERF1 may be required to modulate hOAT4 activity.

Downregulation of organic anion transporters OAT1 and OAT3 correlates with impaired secretion ofpara-aminohippurate after ischemic acute renal failure in rats

Ischemic acute renal failure (iARF) was described to reduce renal extraction of the organic anion para-aminohippurate (PAH) in humans. The rate-limiting step of renal organic anion secretion is its basolateral uptake into proximal tubular cells. This process is mediated by the organic anion transporters OAT1 and OAT3, which both have a broad spectrum of substrates including a variety of pharmaceutics and toxins. Using a rat model of iARF, we investigated whether impairing the secretion of the organic anion PAH might be associated with downregulation of OAT1 or OAT3. Inulin and PAH clearance was determined starting from 6 up to 336 h after ischemia-reperfusion (I/R) injury. Net secretion of PAH was calculated and OAT1 as well as OAT3 expression was analyzed by RT-PCR and Western blotting. Inulin and PAH clearance along with PAH net secretion were initially diminished after I/R injury with a gradual recovery during follow-up. This initial impairment after iARF was accompanied by decreased mRNA and protein levels of OAT1 and OAT3 in clamped animals compared with sham-operated controls. In correlation to the improvement of kidney function, both mRNA and protein levels of OAT1 and OAT3 were upregulated during the follow-up. Thus decreased expression of OAT1 and OAT3 is sufficient to explain the decline of PAH secretion after iARF. As a result, this may have substantial impact on excretion kinetics and half-life of organic anions. As a consequence, the biological effects of a variety of organic anions may be affected after iARF.

Alteration in Renal Organic Anion Transporter 1 After Ischemia/Reperfusion in Cadaveric Renal Allografts

We have previously shown that postischemic injury to renal allografts results in profound impairment of p-aminohippuric acid (PAH) extraction. To elucidate the cellular integrity of the human organic anion transporter 1 (hOAT1) in postischemic acute renal failure (ARF), immunohistochemical analysis of hOAT1 was performed in cadaveric renal allografts using confocal microscopy for three-dimensional reconstruction of serial optical images. Biopsy samples were obtained from 10 cadaveric renal allografts 1 hr after reperfusion during transplant operation. Control tissues were obtained from four living donors of healthy kidneys immediately before an arterial clamp was applied to the renal artery. Control tissues demonstrated hOAT1 distributed to basolateral membrane of proximal tubule cells. In contrast, maldistribution of hOAT1 to cytoplasm and/or diminution of the protein was noted in cadaveric allografts. Characteristics of maldistribution were variable: disappearance of lateral distribution, diffuse cytoplasmic aggregates, apical cytoplasmic aggregates, and disappearance of the staining. In addition, iothalamate and PAH clearances were performed on posttransplant days 3–7 in 18 recipients of a cadaveric renal allograft. PAH clearance was depressed <250 ml/min in all but three subjects. We conclude that reperfused, transplanted kidneys exhibit maldistribution of hOAT1 in proximal tubule cells, resulting in impairment of PAH clearance. This manuscript contains online supplemental material at http://www.jhc.org . Please visit this article online to view these materials.

Determination of the external loops and the cellular orientation of the N- and the C-termini of the human organic anion transporter hOAT1

The OAT (organic anion transporter) family mediates the absorption, distribution and excretion of a diverse array of environmental toxins and clinically important drugs. OAT dysfunction significantly contributes to renal, hepatic, neurological and fetal toxicity and disease. As a first step to establish the topological model of hOAT1 (human OAT1), we investigated the external loops and the cellular orientation of the N- and the C-termini of this transporter. Combined approaches of immunofluorescence studies and site-directed chemical labelling were used for such purpose. Immunofluorescence microscopy of Myc-tagged hOAT1 expressed in cultured cells identified that both the N- and the C-termini of the transporter were located in the cytoplasm. Replacement of Lys59 in the predicted extracellular loop I with arginine resulted in a mutant (K59R), which was largely inaccessible for labelling by membrane-impermeable NHS (N-hydroxysuccinimido)-SS (dithio)-biotin present in the extracellular medium. This result suggests that loop I faces outside of the cell membrane. A single lysine residue introduced into putative extracellular loops III, V and VI of mutant K59R, which is devoid of extracellular lysine, reacted readily with membrane-impermeable NHS-SS-biotin, suggesting that these putative extracellular loops are in the extracellular domains of the protein. These studies provided the first experimental evidence on the extracellular loops and the cellular orientation of the N- and the C-termini of hOAT1.