Role of Organic Anion Transporter NPT4 in Renal Handling of Uremic Toxin 3-indoxyl Sulfate

Sodium-phosphate transporter NPT4 (SLC17A3) is a membrane transporter for organic anionic compounds localized on the apical membranes of kidney proximal tubular epithelial cells and plays a role in the urinary excretion of organic anionic compounds. However, its physiological role has not been sufficiently elucidated because its substrate specificity is yet to be determined. The present study aimed to comprehensively explore the physiological substrates of NPT4 in newly developed Slc17a3-/- mice using a metabolomic approach. Metabolomic analysis showed that the plasma concentrations of 11 biological substances, including 3-indoxyl sulfate, were more than two-fold higher in Slc17a3-/- mice than in wild-type mice. Moreover, urinary excretion of 3-indoxyl sulfate was reduced in Slc17a3-/- mice compared to that in wild-type mice. The uptake of 3-indoxyl sulfate by NPT4-expressing Xenopus oocytes was significantly higher than that by water-injected oocytes. The calculated Km and Vmax values for NPT4-mediated 3-indoxyl sulfate uptake were 4.52 ± 1.18 mM and 1.45 ± 0.14 nmol/oocyte/90 min, respectively. In conclusion, the present study revealed that 3-indoxyl sulfate is a novel substrate of NPT4 based on the metabolomic analysis of Slc17a3-/- mice, suggesting that NPT4 regulates systemic exposure to 3-indoxyl sulfate by regulating its urinary excretion.

Pharmacokinetic interactions of niclosamide in rats: Involvement of organic anion transporters 1 and 3 and organic cation transporter 2

Niclosamide is an anthelmintic drug with a long history of use and is generally safe and well tolerated in humans. As the conventional dose of niclosamide results in a low but certain level in systemic circulation, drug interactions with concomitant drugs should be considered. We aimed to investigate the interaction between niclosamide and drug transporters, as such information is currently limited. Niclosamide inhibited the transport activity of OATP1B1, OATP1B3, OAT1, OAT3, and OCT2 in vitro. Among them, the inhibitory effects on OAT1, OAT3, and OCT2 were strong, with IC50 values of less than 1 μM. When 3 mg/kg of niclosamide was co-administered to rats, systemic exposure to furosemide (a substrate of OAT1/3) and metformin (a substrate of OCT2) increased, and the renal clearance (CLr) of the drugs significantly decreased. These results suggest that niclosamide inhibits renal transporters, OAT1/3 and OCT2, not only in vitro but also in vivo, resulting in increased systemic exposure to the substrates of the transporters by strongly blocking the urinary elimination pathway in rats. The findings of this study will support a meticulous understanding of the transporter-mediated drug interactions of niclosamide and consequently aid in effective and safe use of niclosamide.

Pancreatic Hormone Insulin Modulates Organic Anion Transporter 1 in the Kidney: Regulation via Remote Sensing and Signaling Network

Organic anion transporter 1 (OAT1) expressed in the kidney plays an important role in the elimination of numerous anionic drugs used in the clinic. We report here that insulin, a pancreas-secreted hormone, regulated the expression and activity of kidney-specific OAT1 both in cultured cells and in rats. We showed that treatment of OAT1-expressing cells with insulin led to an increase in OAT1 expression, transport activity, and SUMOylation. Such insulin-induced increase was blocked by afuresertib, a specific inhibitor for protein kinase B (PKB), suggesting insulin regulates OAT1 through PKB signaling pathway. Furthermore, insulin stimulated transport activity and SUMOylation of endogenously expressed OAT1 in rat kidneys. In conclusion, our data support a remote sensing and signaling model, in which OAT1 plays an essential role in intercellular and inter-organ communication and in maintaining local and whole-body homeostasis. Such complex and dedicated communication is carried out by insulin, and PKB signaling and membrane sorting.

Changes in the expression of drug-metabolising enzymes and drug transporters in mice with collagen antibody-induced arthritis

1. We investigated the changes in the expression of drug-metabolising enzymes and drug transporters in the liver, small intestine and kidney of mice with collagen antibody-induced arthritis (CAIA) to determine whether changes in these expressions affect pharmacokinetics of drugs in patients with rheumatoid arthritis.2. mRNA expression levels of cytochrome P450 (Cyp) 2b10, Cyp2c29 and Cyp3a11 were observed to be lower in the liver and small intestine of CAIA mice than in control mice. Compared with control mice, mRNA expression levels of multidrug resistance 1 b, peptide transporter 2 and organic anion transporter (Oat) 2 were high in the liver of CAIA mice. Changes in these expression levels were different among organs. However, elevated expression of Oat2 mRNA was not associated with an increase in protein expression and transport activity evaluated using [³H]cGMP as a substrate.3. These results suggest that arthritis can change the expression of pharmacokinetics-related genes, but the changes may not necessarily be linked to the pharmacokinetics in patients with rheumatoid arthritis. On the other hand, we found Oat2 mRNA expression level was positively correlated with plasma interleukin-6 level, indicating that transcriptional activation of Oat2 may occur in inflammatory state.

Inhibitory effects of indoxyl sulfate and creatinine on the renal transport of meropenem and biapenem in rats

Carbapenem antibiotics are excreted preferentially in the urine after intravenous administration, with organic anion transporters (OATs) known to be involved in the renal tubular secretion of carbapenem antibiotics. Various uremic toxins (UTs) accumulate in the blood of patients with end-stage renal failure, and some UTs such as indoxyl sulfate (IS) and creatinine (Cr) are excreted in the urine via OATs. However, information about the possible interactions between these UTs and carbapenems in the renal secretion remains limited. In this study, we investigated the effects of IS and Cr on the renal transport of anionic meropenem and zwitterionic biapenem by using rat renal cortical slices. The uptake of meropenem and biapenem in the renal cortical slices was significantly decreased in the presence of 0.1 mM IS or 1 mM Cr. When biapenem and Cr were co-administered to rats intravenously, biapenem clearance from the plasma was clearly retarded, reflecting the current in vitro results. However, IS and Cr exerted no inhibitory effect on the uptake of metformin, a substrate of renal organic cation transporter (OCT) 2, in the renal cortical slices. Thus, our findings indicate that IS and Cr interfere with the renal secretion of carbapenem antibiotics by preferentially inhibiting OATs.

Uptake of gadoxetic acid in hepatobiliary phase magnetic resonance imaging and transporter expression in hypovascular hepatocellular nodules

AIMS

To evaluate the association between contrast patterns on gadoxetic acid-enhanced hepatobiliary phase (HBP) MR images and transporter expression in surgically resected hypovascular hepatocellular nodules including early hepatocellular carcinomas (HCCs).

METHODS

Forty-two hypovascular hepatic nodules and 43 hypervascular HCCs as a control were included in this retrospective study. Contrast of the nodules on HBP images was graded as hypo-, iso-, or hyperintense. Histopathological assessment was performed in the context of multistep hepatocarcinogenesis. Immunohistochemical staining of organic anion transporter 1B3 (OATP1B3) and multidrug resistance protein 2 (MRP2) was performed. Cramer's coefficient was used to determine the linear relationship between contrast grades and transporter expression, and the Cochran-Armitage trend test was used to determine the relationship between transporter expression and progression of multistep hepatocarcinogenesis.

RESULTS

Moderate linear relationships between contrast grades and OATP1B3 expression were observed for both hypo- and hypervascular nodules. OATP1B3 expression was negatively correlated with the progression of multistep hepatocarcinogenesis. MRP2 expression was not associated with the contrast grades or histopathological results.

CONCLUSION

OATP1B3 expression was associated with contrast grades of hepatocellular nodules observed in HBP image of gadoxetic acid-enhanced MRI in the hypovascular hepatocellular nodules and was negatively correlated with hepatocarcinogenesis.

Effects of rhein and Rheum palmatum L. extract on the pharmacokinetics and tissue distribution of aristolochic acid I and its demethylated metabolite in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Aristolochic acid nephropathy (AAN) is a kidney disease caused by the administration of plants containing aristolochic acids (AAs). Aristolochic acid I (AAI) is the main toxic component in AAs. Organic anion transporters (OATs) 1 and 3 mediate the renal uptake of AAI, which is related to AAN. In our previous study, we found that anthraquinones derived from the herbal medicine Rheum palmatum L. (RP) inhibited both OAT1 and OAT3, with rhein exhibiting the greatest potency among the components.

AIM OF THE STUDY

This study aimed to investigate the effects of rhein and RP extract on the pharmacokinetics and tissue distribution of AAI and its demethylated metabolite (8-hydroxy-aristolochic acid I [AAIa]) in rats.

MATERIALS AND METHODS

Rhein and RP extract were used as OAT inhibitors, and AAI was used as the toxic substrate. The pharmacokinetics and tissue distribution of AAI and AAIa in rats following the intravenous injection of AAI (10 mg/kg) in the presence and absence of rhein (100 mg/kg) or RP extract (5 g crude drug/kg) were investigated.

RESULTS

Co-administration with rhein increased AUC0-∞ of AAI and AAIa by 39 and 44%, respectively. However, the renal level of AAI was decreased to 50, 42, and 58% of those in rats treated with AAI alone at 5, 10, and 20 min after treatment, respectively, and the renal level of AAIa was decreased to 58, 57, and 61% of the level in rats treated with AAI alone, respectively, at these time points. In the RP extract co-administration group, AAI and AAIa plasma exposure was not significantly increased, but renal accumulation of AAI was decreased to 63, 58, and 68% of that in rats treated with AAI alone at 5, 10, and 20 min after treatment, respectively. In addition, renal accumulation of AAIa was decreased to 74, 70, and 70% of that in rats treated with AAI alone at 5, 10, and 20 min after treatment, respectively.

CONCLUSIONS

This study indicated that co-administration with rhein significantly increased the plasma exposure of AAI and AAIa while decreased their renal accumulation in rats. RP extract reduced the renal accumulation of AAI and AAIa, but have no significant effect on their plasma exposure levels in rats.

Substrate recognition of renally eliminated angiotensin II receptor blockers by organic anion transporter 4

Organic anion transporter (OAT) 4, which is localized at the apical membrane of human renal proximal tubules, transports olmesartan, an angiotensin II receptor blocker (ARB). Many ARBs, including olmesartan, undergo partial tubular secretion as active forms, and inhibit OAT4-mediated uptake activity. Here, we examined the substrate recognition of various ARBs by OAT4 in order to assess whether OAT4 might be involved in the renal handling of ARBs. Concentration-dependent OAT4-mediated uptake of azilsartan, candesartan, carboxylosartan, losartan, and valsartan was observed with K_m values of 6.6, 31, 7.2, 13, and 1.7 μM, respectively, in the absence of extracellular Cl^-. In the presence of extracellular Cl^-, OAT4-mediated uptake of dianionic ARBs (azilsartan, candesartan, carboxylosartan, and valsartan) was lower and reached a steady state faster than in the absence of extracellular Cl^-. Thus, OAT4 is proposed to use extracellular Cl^- as a counterpart for anion efflux. Our results suggest that OAT4 may play a role in the excretion of azilsartan, candesartan, carboxylosartan, and valsartan, as well as olmesartan. In contrast, OAT4-mediated uptake of losartan, a monoanionic ARB, was little affected by extracellular Cl^-, suggesting that only OAT4-mediated dianion transport is Cl^--sensitive.

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.

Organic anion transporter 1 and 3 contribute to traditional Chinese medicine-induced nephrotoxicity

With the internationally growing popularity of traditional Chinese medicine (TCM), TCM-induced nephropathy has attracted public attention. Minimizing this toxicity is an important issue for future research. Typical nephrotoxic TCM drugs such as Aristolochic acid, Tripterygium wilfordii Hook. f, Rheum officinale Baill, and cinnabar mainly damage renal proximal tubules or cause interstitial nephritis. Transporters in renal proximal tubule are believed to be critical in the disposition of xenobiotics. In this review, we provide information on the alteration of renal transporters by nephrotoxic TCMs, which may be helpful for understanding the nephrotoxic mechanism of TCMs and reducing adverse effects. Studies have proven that when administering nephrotoxic TCMs, the expression or function of renal transporters is altered, especially organic anion transporter 1 and 3. The alteration of these transporters may enhance the accumulation of toxic drugs or the dysfunction of endogenous toxins and subsequently sensitize the kidney to injury. Transporters-related drug combination and clinical biomarkers supervision to avoid the risk of future toxicity are proposed.

Piperacillin enhances the inhibitory effect of tazobactam on β-lactamase through inhibition of organic anion transporter 1/3 in rats

To assess the mechanism of the pharmacokinetic interaction between piperacillin and tazobactam, renal excretion and pharmacokinetic studies of piperacillin/tazobactam were investigated in normal and bacteremia rats. A bacteremia model was established to investigate the pharmacokinetic properties of piperacillin and tazobactam under different conditions. Renal slices were taken to examine the uptake of piperacillin and tazobactam. Pharmacokinetic studies of β-lactamase in rats were performed to study the contribution of rOat1/3 to the inhibition of tazobactam on β-lactamase. The AUC (from 2.93 ± 0.58 to 6.52 ± 1.44 mg·min/ml) and the plasma clearance (CL_P ) (from 2.41 ± 1.20 to 0.961 ± 0.212 ml/min/kg) of tazobactam were both altered after the intravenous coadministration of piperacillin and tazobactam in the bacteremia rats. The renal clearance (CL_R ) of tazobactam decreased from 1.30 ± 0.50 to 0.361 ± 0.043 ml/min/kg. In summary, there was a beneficial interaction between piperacillin and tazobactam mediated by rOat1 and rOat3. Piperacillin enhances the inhibitory effect of tazobactam on β-lactamase through the inhibition of rOat1 and rOat3 in rats. The contribution rate of rOat1/3 for the synergistic effect was 20% when the two drugs were coadministered.

Nicotinic acid transport into human liver involves organic anion transporter 2 (SLC22A7)

Nicotinic acid (NA) and nicotinamide (NAM) are biosynthetic precursors of nicotinamide adenine dinucleotide (NAD⁺) - a physiologically important coenzyme that maintains the redox state of cells. Mechanisms driving their entry into cells are not well understood. Here we evaluated the hepatic uptake mechanism(s) of NA and NAM using transporter-transfected cell systems and primary human hepatocytes. NA showed robust organic anion transporter (OAT)2-mediated transport with an uptake ratio (i.e., ratio of accumulation in transfect cells to wild-type cells) of 9.7 ± 0.3, and a Michaelis-Menten constant (K_m) of 13.5 ± 3.3 µM. However, no transport was apparent via other major hepatic uptake and renal secretory transporters, including OAT1/3/4, organic anion transporting polypeptide (OATP)1B1/1B3/2B1, sodium-taurocholate co-transporting polypeptide, organ cation transporter 1/2/3. OAT2-specific transport of NA was inhibited by ketoprofen and indomethacin (known OAT2 inhibitors) in a concentration-dependent manner. Similarly, NA uptake into primary human hepatocytes showed pH- and concentration-dependence and was subject to inhibition by specific OAT2 inhibitors. Unlike NA, NAM was not transported by the hepatic and renal solute carriers upon assessment in transfected cells, although its uptake into human hepatocytes was significantly inhibited by excess unlabelled NAM and a pan-SLC inhibitor (rifamycin SV 1 mM). In conclusion, these studies demonstrate, for the first time, a specific transport mechanism for NA uptake in the human liver and suggest that OAT2 (SLC22A7) has a critical role in its physiological and pharmacological functions.

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.

Estrogen receptor α (ERα) indirectly induces transcription of human renal organic anion transporter 1 (OAT1)

Organic anion transporter 1 (OAT1) is a polyspecific transport protein located in the basolateral membrane of renal proximal tubule cells. OAT1 plays a pivotal role in drug clearance. Adverse drug reactions (ADR) are observed more frequently in women than in men, especially ADR are higher in women for drugs which are known interactors of OAT1. Sex-dependent expression of Oat1 has been observed in rodents with a tendency to male-dominant expression. This study aims at elucidating the transcriptional regulation of human OAT1 and tests the effect of estrogen receptor α (ERα). Promoter activation of OAT1 was assessed by luciferase assays carried out by Opossum kidney (OK) cells, transiently transfected with promoter constructs of human OAT1 and expression vectors for ERα and exposed to 100 nmol/L 17β-estradiol. Furthermore, a transcription factor array and proteomic analysis was performed to identify estrogen-induced transcription factors. Human OAT1 was significantly activated by ligand activated ERα. However, activation occurred without a direct interaction of ERα with the OAT1 promoter. Our data rather show an activation of the transcription factors CCAAT-box-binding transcription factor (CBF) and heterogeneous nuclear ribonucleoprotein K (HNRNPK) by ERα, which in turn bind and initiate OAT1 promoter activity. Herewith, we provide novel evidence of estrogen-dependent, transcriptional regulation of polyspecific drug transporters including the estrogen-induced transcription factors CBF and HNRNPK.

Influence of organic anion transporter 1/3 on the pharmacokinetics and renal excretion of ginkgolides and bilobalide

ETHNOPHARMACOLOGICAL RELEVANCE

The major terpene lactones of ginkgo biloba extract (GBE) include ginkgolide A, B, C and bilobalide are used for the protection of cardiovascular, cerebrovascular and neurodegenerative diseases. Terpene lactones are orally bioavailable and predominantly eliminated via the renal pathway. However, information on the transporters involved in the pharmacokinetics (PK) and renal excretion of terpene lactones is limited.

AIM OF THE STUDY

The objective of this study is to assess the role of OAT1/3 which are important transporters in the human kidney in the PK and renal excretion ginkgolide A, B, C and bilobalide.

MATERIALS AND METHODS

Uptake of ginkgolide A, B, C and bilobalide in Madin-Darby Canine Kidney (MDCK) and human embryonic kidney 293 (HEK293) cells overexpressing OAT1 or OAT3, respectively were studied. To verify the result from in vitro cell models, the studies on PK, kidney accumulation and urinary excretion of ginkgolide A, B, C and bilobalide were carried out in rats.

RESULTS

The result showed that ginkgolide A, B, C and bilobalide are low-affinity substrates of OAT1/3. Following co-administration with probenecid, a typical inhibitor of OAT1/3, the rat plasma concentrations of ginkgolide A, B, C and bilobalide increased significantly. AUC showed a significant increase in the probenecid-treated rats compared to control rats (893.48 vs. 1123.85, 314.91 vs. 505.74, and 2724.97 vs. 3096.40 μg/L*h for ginkgolide A, B and bilobalide, respectively), while the clearance of these compounds significantly decreased. The accumulation of ginkgolide A, B and bilobalide in the kidney of the probenecid-treated rats was reduced by 1.8, 2.4, and 1.5-fold, respectively; further reducing the cumulative urinary recovery of these compounds.

CONCLUSION

The findings indicated that ginkgolide A, B and bilobalide are excreted via OAT1/3-mediated transport in the kidney and OAT1/3 inhibitor significantly influence the PK ginkgolides and bilobalide.

[Gadoxetic disodium-enhanced MRI to characterize T1 relaxation values and expression level of organic anion transporters and multidrug resistance protein on hepatocyte surface membrane of normal C57BL/6 mice]

Objective: The characteristics of T1 relaxation values and the expression levels of organic anion transport system (OATP) and multidrug resistance protein carrier (MRP) on hepatocyte surface membrane were quantitatively studied to evaluate liver function in normal C57BL/6 mice with gadoxetic disodium-enhanced MRI. Methods: Ten 6-weeks-old, normal C57BL/6 mice were included in this study. Gadoxetic disodium- enhanced MRI examination was performed. Longitudinal relaxation time images before and 20 min after contrast injection (hepatobiliary-specific phase) were acquired. T1-relaxation time, T1 relaxation time decline rate (△T) and rapid initial enhancement slope percentage in the first-pass study of the liver parenchyma before and after administration of gadoxetate disodium were measured. Liver parenchyma specimens were detected by Western blotting and the values ​​of OATP1, MRP2, and MRP3 were recorded. Statistical results were expressed in mean. Results: The mean T1 relaxation time of 10 normal C57BL/6 mice before and after enhancement was 659.13 ± 24.07, and 408.87 ± 27.21 ms. The mean T1 relaxation time decline rate and rapid initial enhancement slope percentage in the first-pass study was 37.12% ± 4.95% and 4.14% ± 0.96% ms. Furthermore, the mean value of OATP1, MRP2 and MRP3 were 29 952.1 ± 11 475.2, 34 376.4 ± 33 228.4 and 357 308.9 ± 64 646.5. Conclusion: T1-relaxation values, T1 relaxation time decline rate and rapid initial enhancement slope percentage in the first-pass study before and after gadoxetic disodium-enhanced MRI were determined in normal C57BL/6 mice as well as quantitative values of OATP1, MRP2 and MRP3 at the molecular level on the hepatocyte surface membrane were helpful for liver injury model with control study.

Effects of antibiotics on the pharmacokinetics of indoxyl sulfate, a nephro-cardiovascular toxin

Indoxyl sulfate (IS), a highly protein-bound nephro-cardiovascular toxin, was poorly removed by hemodialysis. IS exists as anions in the body and the renal excretion is mediated by organic anion transporter 1 (OAT1) and OAT3. Acidic antibiotics such as cephalosporins and fluoroquinolones were putative substrates/inhibitors of OATs. We hypothesized that cephalosporins and fluoroquinolones might compete with IS for OAT1- and/or OAT3-mediated renal excretions.This study investigated the effects of ciprofloxacin, cefuroxime, cefotaxime, cefazolin and ofloxacin on the intravenous pharmacokinetics of IS in rats. IS was intravenously injected with and without each individual antibiotics, and the concentrations of IS in serum and lysate were determined by HPLC.The results showed that ciprofloxacin significantly increased AUC_0-t and T_1/2 of IS by 272% and 491%, respectively, and decreased the clearance by 71%. However, ofloxacin, cefuroxime, cefotaxime and cefazolin did not alter the pharmacokinetics of IS. Furthermore, cell line study showed that ciprofloxacin inhibited the OAT3-mediated transport of IS.This study indicates 30 mg/kg of ciprofloxacin decreased the clearance of IS through inhibition on the OAT3-mediated transport, whereas 50 mg/kg of ofloxacin, cefuroxime, cefotaxime and cefazolin did not show significant influence.

Biflavonoids from Juniperus oblonga inhibit organic anion transporter 3

Organic anion transporters (OATs in humans, Oats in rodents) play an important role in the distribution and excretion of numerous endogenous metabolic products and exogenous organic anions, including a host of widely prescribed drugs. Their ligand recognition is also important for drug therapy and development. In this study, the n-butanol and dichloromethane soluble fractions of Juniperus oblonga were found to inhibit OAT3 in vitro and three biflavonoids were found to be responsible for this activity. One of these compounds, amentoflavone exhibited stronger inhibition than probenecid, a known strong inhibitor of OAT3. Biological characterization of amentoflavone in vivo also showed inhibition of Oat3. Preliminary observations of structure-activity relationships suggest that the biflavonoids are more potent inhibitors of this transporter than their corresponding monomer, and that methylation of even a single hydroxyl group results in a substantial decrease in activity. This greater potency of the biflavonoids may indicate the need for a more in-depth investigation of the distribution of biflavonoids in plants used as foodstuffs and herbal medicines, due to their potential for causing interactions with OAT3 substrate drugs.

Investigation of the differential transport mechanism of cinnabar and mercury containing compounds

BACKGROUND

Cinnabar has a long history of uses in Chinese traditional medicines as an ingredient in various remedies. However, the detailed mechanism of cinnabar in medication remains unclear, and the toxicity of cinnabar has been a debate due to its containing mercury sulfide. This study was designed to investigate the differential transport mechanism of cinnabar and other Hg-containing compounds HgCl₂, MeHg and HgS, and to determine if organic anion transporters OAT1 and OAT3 were involved in the differential transport mechanism.

MATERIALS AND METHODS

The 293T cells were employed to investigate and compare the differential transport mechanism of cinnabar and HgCl₂, MeHg and HgS. Cells were incubated with a low dose (5 μM HgCl₂ and MeHg, 200 μM HgS and cinnabar), medium dose (10 μM HgCl₂ and MeHg, 400 μM HgS and cinnabar), and high dose (20 μM HgCl₂ and MeHg, 800 μM HgS and cinnabar) of HgCl₂, MeHg, HgS and cinnabar for 24 h. Following treatment, the cells were collected and the cell viability was determined by MTT assay. The intracellular mercury content was measured at 1, 4, and 24 h after treatment with 10 μM of the tested agents by an atomic fluorescence spectrophotometer. The effect of these tested agents on mitochondrial respiration was determined in a high-resolution oxygraphyat 24 h following treatment. Furthermore, the effect of modulation of expression of transporters OAT1 and OAT3 on the transport and cytotoxicity of the tested agents was evaluated. The up and down regulation of OAT1 and OAT3 were achieved by overexpression and siRNA transfection, respectively.

RESULTS

Compared with HgCl₂ and MeHg, the cytotoxicity of cinnabar and HgS was lower, with cell viability at the high dose cinnabar and HgS being about 65%, while MeHg and HgCl₂ were 40% and 20%, respectively. The intracellular mercury accumulation was time-dependent. At 24 h the intracellular concentrations of HgCl₂ and MeHg were about 7 and 5 times higher, respectively, than that of cinnabar. No significant difference was found in the intracellular mercury content in cells treated with cinnabar compared to HgS. The knockdown and overexpression of the transporter OAT1 resulted in significant reduction and increase, respectively, in mercury accumulation in HgCl₂ -treated cells in relative to control cells, while no significant changes were observed in cells treated with cinnabar, MeHg, and HgS. In addition, the knockdown and overexpression of the transporter OAT3 caused significant reduction and increase, respectively, in mercury accumulation in both HgCl₂ and MeHg-treated cells in relative to control cells, while no significant changes were observed in cells treated with cinnabar and HgS. Furthermore, it was found that cells transfected with siOAT1 caused significant resistance to the cytotoxicity induced by HgCl₂, while no noticeable changes in cell viability were observed in cells treated with other tested agents. Additionally, cells transfected with OAT3 did not change cell sensitivity to cytotoxicity induced by all of the four tested agents.

CONCLUSION

This study demonstrates that differential transport and accumulation of mercury in 293T cells exists among cinnabar and the three mercury-containing compounds HgCl₂, MeHg and HgS, leading to distinct sensitivity to mercury induced cytotoxicity. The kidney organic anion transporters OAT1 and OAT3 are partially involved in the regulation of the transport of HgCl₂ and MeHg, but not in the regulation of the transport of cinnabar.

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.

Organic anion transporter 1 (OAT1/SLC22A6) enhances bioluminescence based on d-luciferin-luciferase reaction in living cells by facilitating the intracellular accumulation of d-luciferin

Bioluminescence (BL) imaging based on d-luciferin (d-luc)-luciferase reaction allows noninvasive and real-time monitoring of luciferase-expressing cells. Because BL intensity depends on photons generated through the d-luc-luciferase reaction, an approach to increase intracellular levels of d-luc could improve the detection sensitivity. In the present study, we showed that organic anion transporter 1 (OAT1) is useful, as a d-luc transporter, in boosting the BL intensity in luciferase-expressing cells. Functional screening of several transporters showed that the expression of OAT1 in HEK293 cells stably expressing Pyrearinus termitilluminans luciferase (HEK293/eLuc) markedly enhanced BL intensity in the presence of d-luc. When OAT1 was transiently expressed in HEK293 cells, intracellular accumulation of d-luc was higher than that in control cells, and the specific d-luc uptake mediated by OAT1 was saturable with a Michaelis constant (K_m) of 0.23 μM. The interaction between OAT1 and d-luc was verified using 6-carboxyfluorescein, a typical substrate of OAT1, which showed that d-luc inhibited the uptake of 6-carboxyfluorescein mediated by OAT1. BL intensity was concentration-dependent at steady states in HEK293/eLuc cells stably expressing OAT1, and followed Michaelis-Menten kinetics with an apparent K_m of 0.36 μM. In addition, the enhanced BL was significantly inhibited by OAT1-specific inhibitors. Thus, OAT1-mediated transport of d-luc could be a rate-limiting step in the d-luc-luciferase reaction. Furthermore, we found that expressing OAT1 in HEK293/eLuc cells implanted subcutaneously in mice also significantly increased the BL after intraperitoneal injection of d-luc. Our findings suggest that because OAT1 is capable of transporting d-luc, it can also be used to improve visualization and monitoring of luciferase-expressing cells.

Organic anion transporters, OAT1 and OAT3, are crucial biopterin transporters involved in bodily distribution of tetrahydrobiopterin and exclusion of its excess

Tetrahydrobiopterin (BH4) is a common coenzyme of phenylalanine-, tyrosine-, and tryptophan hydroxylases, alkylglycerol monooxygenase, and NO synthases (NOS). Synthetic BH4 is used medicinally for BH4-responsive phenylketonuria and inherited BH4 deficiency. BH4 supplementation has also drawn attention as a therapy for various NOS-related cardio-vascular diseases, but its use has met with limited success in decreasing BH2, the oxidized form of BH4. An increase in the BH2/BH4 ratio leads to NOS dysfunction. Previous studies revealed that BH4 supplementation caused a rapid urinary loss of BH4 accompanied by an increase in the blood BH2/BH4 ratio and an involvement of probenecid-sensitive but unknown transporters was strongly suggested in these processes. Here we show that OAT1 and OAT3 enabled cells to take up BP (BH4 and/or BH2) in a probenecid-sensitive manner using rat kidney slices and transporter-expressing cell systems, LLC-PK1 cells and Xenopus oocytes. Both OAT1 and OAT3 preferred BH2 and sepiapterin as their substrate roughly 5- to 10-fold more than BH4. Administration of probenecid acutely reduced the urinary exclusion of endogenous BP accompanied by a rise in blood BP in vivo. These results indicated that OAT1 and OAT3 played crucial roles: (1) in determining baseline levels of blood BP by excluding endogenous BP through the urine, (2) in the rapid distribution to organs of exogenous BH4 and the exclusion to urine of a BH4 excess, particularly when BH4 was administered, and (3) in scavenging blood BH2 by cellular uptake as the gateway to the salvage pathway of BH4, which reduces BH2 back to BH4.

Interactions of 172 plant extracts with human organic anion transporter 1 (SLC22A6) and 3 (SLC22A8): a study on herb-drug interactions

Background

Herb-drug interactions (HDIs) resulting from concomitant use of herbal products with clinical drugs may cause adverse reactions. Organic anion transporter 1 (OAT1) and 3 (OAT3) are highly expressed in the kidney and play a key role in the renal elimination of substrate drugs. So far, little is known about the herbal extracts that could modulate OAT1 and OAT3 activities.

Methods

HEK293 cells stably expressing human OAT1 (HEK-OAT1) and OAT3 (HEK-OAT3) were established and characterized. One hundred seventy-two extracts from 37 medicinal and economic plants were prepared. An initial concentration of 5 µg/ml for each extract was used to evaluate their effects on 6-carboxylfluorescein (6-CF) uptake in HEK-OAT1 and HEK-OAT3 cells. Concentration-dependent inhibition studies were conducted for those extracts with more than 50% inhibition to OAT1 and OAT3. The extract of Juncus effusus, a well-known traditional Chinese medicine, was assessed for its effect on the in vivo pharmacokinetic parameters of furosemide, a diuretic drug which is a known substrate of both OAT1 and OAT3.

Results

More than 30% of the plant extracts at the concentration of 5 µg/ml showed strong inhibitory effect on the 6-CF uptake mediated by OAT1 (61 extracts) and OAT3 (55 extracts). Among them, three extracts for OAT1 and fourteen extracts for OAT3 were identified as strong inhibitors with IC₅₀ values being <5 µg/ml. Juncus effusus showed a strong inhibition to OAT3 in vitro, and markedly altered the in vivo pharmacokinetic parameters of furosemide in rats.

Conclusion

The present study identified the potential interactions of medicinal and economic plants with human OAT1 and OAT3, which is helpful to predict and to avoid potential OAT1- and OAT3-mediated HDIs.

Serum- and glucocorticoid-inducible kinase sgk2 stimulates the transport activity of human organic anion transporters 1 by enhancing the stability of the transporter

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-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT1 is abundantly expressed in the kidney and brain. In the current study, we examined the regulation of hOAT1 by serum- and glucocorticoid-inducible kinase 2 (sgk2) in the kidney COS-7 cells. We showed that sgk2 stimulated hOAT1 transport activity. Such stimulation mainly resulted from an increased cell surface expression of the transporter, kinetically revealed as an increased maximal transport velocity V max without significant change in substrate-binding affinity K m. We further showed that stimulation of hOAT1 activity by sgk2 was achieved by preventing hOAT1 degradation. Our co-immunoprecipitation experiment revealed that the effect of sgk2 on hOAT1 was through a direct interaction between these two proteins. In conclusion, our study demonstrated that sgk2 stimulates hOAT1 transport activity by enhancing the stability of the transporter. This study provides the insights into sgk2 regulation of hOAT1-mediated transport in normal physiology and disease.

Inhibitory effects of chemotherapeutics on human organic anion transporter hOAT4

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters which play critical roles in the body disposition of clinically important drugs. hOAT4 is expressed in the kidney and placenta. In the current study, we examined the inhibitory effects of 101 anticancer drugs from a clinical drug library on hOAT4 transport activity. The studies were carried out in hOAT4-expressing human kidney HEK-293 cells and human placenta BeWo cells. Among these drugs, only chlorambucil and cabazitaxel demonstrated more than 50% cis-inhibitory effect on hOAT4-mediated uptake of (3)H-labeled estrone sulfate, a prototypical substrate for the transporter. The IC50 values for chlorambucil and cabazitaxel were 44.28 and 3.5 µM respectively. Dixon plot analysis revealed that inhibition by chlorambucil was competitive with a Ki = 55.73 µM whereas inhibition by cabazitaxel was non-competitive with a Ki = 1.78 µM. Our results demonstrated that chlorambucil and cabazitaxel were inhibitors of hOAT4. Furthermore, by comparing our data with clinically relevant exposures of these drugs, we conclude that the propensity for chlorambucil and cabazitaxel to cause drug-drug interaction through inhibition of hOAT4 is low.

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.

In vitro interaction of clopidogrel and its hydrolysate with OCT1, OCT2 and OAT1

Clopidogrel (CP) is metabolized by CYPs to the active metabolite, or hydrolyzed by esterase to clopidogrel carboxylate (CPC) in liver, and CPC is partly excreted from urine. Therefore, the objective of the present study was to evaluate the interactions of CP and CPC with organic cation transporter 1 (OCT1) (in liver), and CPC with organic cation transporter 2 (OCT2) and organic anion transporter 1 (OAT1) (in kidney). Both CP and CPC inhibited the uptake of 1-methyl-4-phenylpyridinium (MPP(+)) and metformin, typical substrates of OCT1, in MDCK-hOCT1 cells with low IC₅₀ (0.307-14.0 μM). CPC (100 μM) reduced the uptake of MPP(+) and metformin mediated by OCT2 in MDCK-hOCT2 cells to 60.8% and 33.6% of the control, CPC (500 μM) decreased the uptake of 6-carboxyfluorescein (6-CFL) and para-aminohippuric acid (PAH), substrates of OAT1, in MDCK-hOAT1 cells to 64.6% and 79.4% of the control. CP and CPC were also found to inhibit other drugs of OCT1 substrates, such as lamivudine and amantadine, in MDCK-hOCT1 cells with the IC₅₀ of 1.97-4.15μM, except CPC on amantadine (IC₅₀>100 μM). The inhibition of CP and CPC on lamivudine uptake in primary rat hepatocytes was also confirmed with the IC₅₀ of 2.91 and 1.25μM, respectively. Additionally, CP and CPC were not substrates of OCT1 and OCT2, whereas CPC was a substrate of OAT1 with the Km of 5.61 μM. In conclusion, CP and CPC are strong inhibitors of OCT1, but weak inhibitors of OCT2 and OAT1, and CPC is a high affinity substrate of OAT1.

Differential functional rescue of Lys(513) and Lys(516) processing mutants of MRP1 (ABCC1) by chemical chaperones reveals different domain-domain interactions of the transporter

Multidrug resistance protein 1 (MRP1) extrudes drugs as well as pharmacologically and physiologically important organic anions across the plasma membrane in an ATP-dependent manner. We previously showed that Ala substitutions of Lys(513) and Lys(516) in the cytoplasmic loop (CL5) connecting transmembrane helix 9 (TM9) to TM10 cause misfolding of MRP1, abrogating its expression at the plasma membrane in transfected human embryonic kidney (HEK) cells. Exposure of HEK cells to the chemical chaperones glycerol, DMSO, polyethylene glycol (PEG) and 4-aminobutyric acid (4-PBA) improved levels of K513A to wild-type MRP1 levels but transport activity was only fully restored by 4-PBA or DMSO treatments. Tryptic fragmentation patterns and conformation-dependent antibody immunoreactivity of the transport-deficient PEG- and glycerol-rescued K513A proteins indicated that the second nucleotide binding domain (NBD2) had adopted a more open conformation than in wild-type MRP1. This structural change was accompanied by differences in ATP binding and hydrolysis but no changes in substrate Km. In contrast to K513A, K516A levels in HEK cells were not significantly enhanced by chemical chaperones. In more permissive insect cells, however, K516A levels were comparable to wild-type MRP1. Nevertheless, organic anion transport by K516A in insect cell membranes was reduced by >80% due to reduced substrate Km. Tryptic fragmentation patterns indicated a more open conformation of the third membrane spanning domain of MRP1. Thus, despite their close proximity to one another in CL5, Lys(513) and Lys(516) participate in different interdomain interactions crucial for the proper folding and assembly of MRP1.

Effects of Increased Uric Acid Intake on the Abundance of Urate-anion exchanger and Organic Anion Transporter Proteins in the Rat Kidney

Renal handling of uric acid mainly occurs in the proximal tubule, and bidirectional transport of urate may involve apical absorption via the urate-anion exchanger (URAT1) and basolateral uptake via organic anion transporters (OAT1 and OAT3). In rat kidneys, we investigated whether the protein abundance of URAT1, OAT1, and OAT3 is affected by the increase in uric acid intake. Male Sprague-Dawley rats were randomly divided into control and uric acid-supplemented groups, and uric acid-supplemented rats were given 0.75 g of uric acid per 180 g body weight per day for 8 days. After the animal experiment, kidneys were harvested and semi-quantitative immunoblotting was carried out from cortical homogenates using polyclonal peptide-derived antibodies to URAT1, OAT1, and OAT3. Serum uric acid level showed an increasing tendency in the uric acid-supplemented rats compared with control rats, whereas urinary uric acid excretion was not significantly different between the uric acid-supplemented rats and control rats. URAT1 protein abundance in cortical homogenates was not significantly different between the uric acid-supplemented rats and control rats. However, OAT1 protein abundance was significantly increased in the uric acid-supplemented rats compared with the control rats. OAT3 protein abundance was not significantly different between the uric acid-supplemented rats and control rats. In conclusion, OAT1 may have a regulatory role in response to the increase in uric acid intake in the rat kidney. The up-regulation of OAT1 would exert stimulation of urinary uric acid excretion and might contribute to protection from hyperuricemia.