Hyperoside ameliorates cisplatin-induced acute kidney injury by regulating the expression and function of Oat1

Cisplatin is a widely used chemotherapeutic agent to treat solid tumours in clinics. However, cisplatin-induced acute kidney injury (AKI) limits its clinical application. This study investigated the effect of hyperoside (a flavonol glycoside compound) on regulating AKI.The model of cisplatin-induced AKI was established, and hyperoside was preadministered to investigate its effect on improving kidney injury.Hyperoside ameliorated renal pathological damage, reduced the accumulation of SCr, BUN, Kim-1 and indoxyl sulphate in vivo, increased the excretion of indoxyl sulphate into the urine, and upregulated the expression of renal organic anion transporter 1 (Oat1). Moreover, evaluation of rat kidney slices demonstrated that hyperoside promoted the uptake of PAH (p-aminohippurate, the Oat1 substrate), which was confirmed by transient over-expression of OAT1 in HEK-293T cells. Additionally, hyperoside upregulated the mRNA expression of Oat1 upstream regulators hepatocyte nuclear factor-1α (HNF-1α) and pregnane X receptor (PXR).These findings indicated hyperoside could protect against cisplatin-induced AKI by promoting indoxyl sulphate excretion through regulating the expression and function of Oat1, suggesting hyperoside may offer a potential tactic for cisplatin-induced AKI treatment.

Renal Organic Anion Transporters 1 and 3 In Vitro: Gone but Not Forgotten

Organic anion transporters 1 and 3 (OAT1 and OAT3) play a crucial role in kidney function by regulating the secretion of multiple renally cleared small molecules and toxic metabolic by-products. Assessing the activity of these transporters is essential for drug development purposes as they can significantly impact drug disposition and safety. OAT1 and OAT3 are amongst the most abundant drug transporters expressed in human renal proximal tubules. However, their expression is lost when cells are isolated and cultured in vitro, which is a persistent issue across all human and animal renal proximal tubule cell models, including primary cells and cell lines. Although it is well known that the overall expression of drug transporters is affected in vitro, the underlying reasons for the loss of OAT1 and OAT3 are still not fully understood. Nonetheless, research into the regulatory mechanisms of these transporters has provided insights into the molecular pathways underlying their expression and activity. In this review, we explore the regulatory mechanisms that govern the expression and activity of OAT1 and OAT3 and investigate the physiological changes that proximal tubule cells undergo and that potentially result in the loss of these transporters. A better understanding of the regulation of these transporters could aid in the development of strategies, such as introducing microfluidic conditions or epigenetic modification inhibitors, to improve their expression and activity in vitro and to create more physiologically relevant models. Consequently, this will enable more accurate assessment for drug development and safety applications.

Honey Mushroom, Armillaria mellea (Agaricomycetes) and Its Fermentation Products Target Regulation of OAT1/OAT3 Proteins to Reduce Hyperuricemia in Mice

BACKGROUND

Disorders of purine metabolism are the main cause of hyperuricemia. Current drugs for the treatment of hyperuricemia usually cause a degree of cardiovascular damage.

METHODS

This study aimed to investigate the therapeutic effects of Armillaria mellea fruiting body (AFB), Armillaria rhizomorph (AR) and Armillaria mellea fermentation product (after rhizomorphs removal) (AFP) on hyperuricemic mice. The hyperuricemia mouse model was established by oral administration of potassium oxonate 0.9 g⋅kg-1 and hypoxanthine 0.5 g⋅kg-1 for two weeks. Starting from the third week, the intragastric administration of the intervention drug group was as follows: Allopurinol 0.013 g⋅kg-1, AFB (3.9 and 7.8 g⋅kg-1), AR (3.9 and 7.8 g⋅kg-1), AFP (1.95 and 3.9 g⋅kg-1) once daily for 14 days.

RESULTS

Results showed that AFB, AR, and AFP reduced the contents of serum uric acid, serum creatinine, and blood urea nitrogen in hyperuricemic mice and the mechanism of action might be through up-regulation of the expression levels of organic anion transporter 1/organic anion transporter 3 proteins in kidney tissue. AR and AFP both exhibited better uric acid-lowering effects than AFB, which may be due to the higher purine content of AFB.

CONCLUSIONS

Armillaria mellea and its fermentation products can treat hyperuricemia by up-regulating OAT1 protein and OAT3 protein, reducing uric acid content in mice.

Improvement of Protein Expression Profile in Three-Dimensional Renal Proximal Tubular Epithelial Cell Spheroids Selected Based on OAT1 Gene Expression: A Potential In Vitro Tool for Evaluating Human Renal Proximal Tubular Toxicity and Drug Disposition

The proximal tubule plays an important role in the kidney and is a major site of drug interaction and toxicity. Analysis of kidney toxicity via in vitro assays is challenging, because only a few assays that reflect functions of drug transporters in renal proximal tubular epithelial cells (RPTECs) are available. In this study, we aimed to develop a simple and reproducible method for culturing RPTECs by monitoring organic anion transporter 1 (OAT1) as a selection marker. Culturing RPTECs in spherical cellular aggregates increased OAT1 protein expression, which was low in the conventional two-dimensional (2D) culture, to a level similar to that in human renal cortices. By proteome analysis, it was revealed that the expression of representative two proximal tubule markers was maintained and 3D spheroid culture improved the protein expression of approximately 7% of the 139 transporter proteins detected, and the expression of 2.3% of the 4,800 proteins detected increased by approximately fivefold that in human renal cortices. Furthermore, the expression levels of approximately 4,800 proteins in three-dimensional (3D) RPTEC spheroids (for 12 days) were maintained for over 20 days. Cisplatin and adefovir exhibited transporter-dependent ATP decreases in 3D RPTEC spheroids. These results indicate that the 3D RPTEC spheroids developed by monitoring OAT1 gene expression are a simple and reproducible in vitro experimental system with improved gene and protein expressions compared with 2D RPTECs and were more similar to that in human kidney cortices. Therefore, it can potentially be used for evaluating human renal proximal tubular toxicity and drug disposition. SIGNIFICANCE STATEMENT: This study developed a simple and reproducible spheroidal culture method with acceptable throughput using commercially available RPTECs by monitoring OAT1 gene expression. RPTECs cultured using this new method showed improved mRNA/protein expression profiles to those in 2D RPTECs and were more similar to those of human kidney cortices. This study provides a potential in vitro proximal tubule system for pharmacokinetic and toxicological evaluations during drug development.

Berberrubine attenuates potassium oxonate- and hypoxanthine-induced hyperuricemia by regulating urate transporters and JAK2/STAT3 signaling pathway

Phellodendri Chinensis Cortex (PC) is a traditional medicinal material used to treat gout and hyperuricemia (HUA) in China. Berberine (BBR), the main component of PC, possesses anti-hyperuricemic and anti-gout effects. However, BBR exhibits low bioavailability due to its extensive metabolism and limited absorption. Thus, the metabolites of BBR are believed to be the potential active forms responsible for its in vivo biological activities. Berberrubine (BRB), one of the major metabolites of BBR, exhibits appreciable biological activities even superior to BBR. In this work, the anti-hyperuricemic efficacy of BRB was investigated in HUA model mice induced by co-administration with intraperitoneal potassium oxonate (PO) and oral hypoxanthine (HX) for 7 days. Results showed that administration with BRB (6.25, 12.5, and 25.0 mg/kg) significantly decreased the serum levels of uric acid (UA) by 49.70%, 75.35%, and 75.96% respectively, when compared to the HUA group. In addition, BRB sharply decreased the levels of blood urea nitrogen (BUN) (by 19.62%, 28.98%, and 38.72%, respectively) and serum creatinine (CRE) (by 16.19%, 25.07%, and 52.08%, respectively) and reversed the PO/HX-induced renal histopathological damage dose-dependently. Additionally, BRB lowered the hepatic XOD activity, downregulated the expressions of glucose transporter 9 (GLUT9) and urate transporter 1 (URAT1), upregulated expressions of organic anion transporter 1/3 (OAT1/3) and ATP-binding cassette transporter subfamily G member 2 (ABCG2) at both protein and mRNA levels, and suppressed the activation of the JAK2/STAT3 signaling pathway. In addition, BRB significantly decreased the levels of inflammatory mediators (IL-1β, IL-6, and TNF-α). In conclusion, our study indicated that BRB exerted anti-hyperuricemic effect, at least in part, via regulating the urate transporter expressions and suppressing the JAK2/STAT3 signaling pathway. BRB was believed to be promising for further development into a potential therapeutic agent for HUA treatment.

Ameliorative effect and mechanism of Yi-Suan-Cha against hyperuricemia in rats

BACKGROUND

This study aimed to evaluate the urate-lowering effects of Yi-Suan-Cha and explore its underlying mechanisms in experimental hyperuricemia induced in rats.

METHODS

Forty-eight male SD rats were randomly allocated into normal control, model, allopurinol, benzbromarone, low-dose Yi-Suan-Cha (0.2 g/ml), and high-dose Yi-Suan-Cha (0.4 g/ml) groups (n = 8 rats per group). Rat models of hyperuricemia were established through intragastric administration of adenine 25 mg/kg + potassium oxalate 300 mg/kg for 3 weeks. After the last administration, serum uric acid, creatinine, and urea nitrogen levels were measured. Renal histopathology was observed by hematoxylin-eosin staining. Xanthine oxidase level in serum and liver homogenates was measured by ELISA. The protein and mRNA expression of URAT1, ABCG2, OAT1, and GLUT9 in the kidney was detected by Western blotting and RT-PCR, respectively.

RESULTS

The serum uric acid levels were significantly lowered in all medication groups than in the model group. The benzbromarone and both Yi-Suan-Cha groups showed clear kidney structures with no obvious abnormalities. Compared with the normal control group, the model group showed increased URAT1/GLUT9 protein expression and decreased ABCG2/OAT1 protein expression. Compared with the model group, both Yi-Suan-Cha groups showed decreased URAT1/GLUT9 protein expression and increased ABCG2/OAT1 protein expression. Compared with that in the normal control group, URAT1/GLUT9 mRNA expression increased in the model group. Compared with the model group, the low-dose and high-dose Yi-Suan-Cha groups showed decreased URAT1/GLUT9 mRNA expression and increased ABCG2/OAT1 mRNA expression.

CONCLUSION

Yi-Suan-Cha may lower uric acid level by downregulating URAT1/GLUT9 expression and upregulating ABCG2/OAT1 expression.

A key role for the transporter OAT1 in systemic lipid metabolism

Organic anion transporter 1 (OAT1/SLC22A6) is a drug transporter with numerous xenobiotic and endogenous substrates. The Remote Sensing and Signaling Theory suggests that drug transporters with compatible ligand preferences can play a role in “organ crosstalk,” mediating overall organismal communication. Other drug transporters are well known to transport lipids, but surprisingly little is known about the role of OAT1 in lipid metabolism. To explore this subject, we constructed a genome-scale metabolic model using omics data from the Oat1 knockout mouse. The model implicated OAT1 in the regulation of many classes of lipids, including fatty acids, bile acids, and prostaglandins. Accordingly, serum metabolomics of Oat1 knockout mice revealed increased polyunsaturated fatty acids, diacylglycerols, and long-chain fatty acids and decreased ceramides and bile acids when compared with wildtype controls. Some aged knockout mice also displayed increased lipid droplets in the liver when compared with wildtype mice. Chemoinformatics and machine learning analyses of these altered lipids defined molecular properties that form the structural basis for lipid-transporter interactions, including the number of rings, positive charge/volume, and complexity of the lipids. Finally, we obtained targeted serum metabolomics data after short-term treatment of rodents with the OAT-inhibiting drug probenecid to identify potential drug–metabolite interactions. The treatment resulted in alterations in eicosanoids and fatty acids, further supporting our metabolic reconstruction predictions. Consistent with the Remote Sensing and Signaling Theory, the data support a role of OAT1 in systemic lipid metabolism.

CP-25 improves nephropathy in collagen-induced arthritis rats by inhibiting the renal inflammatory response

Paeoniflorin-6'-O-benzene sulfonate (CP-25) is a derivative of paeoniflorin. We previously confirmed that CP-25 inhibits inflammatory responses in several arthritis animal models. The aim of the present study was to investigate the beneficial effects of CP-25 on renal damage in rats with collagen-induced arthritis (CIA). CIA was induced in rats, which were orally administered CP-25 (25, 50 and 100 mg/kg/day) for 24 days. The levels of plasma blood urea nitrogen (BUN) and urine protein in CIA rats were measured. Pathological changes in renal tissues and joints were observed, and inflammatory cell infiltration was evaluated by immunohistochemistry. Moreover, renal inflammatory mediators and transporters were measured by western blotting. We found that CP-25 not only inhibited arthritis manifestations but also improved renal pathological manifestations and kidney injury by decreasing serum BUN and urine protein levels. Further study revealed that CP-25 treatment reduced the number of renal CD68+ cells and downregulated the levels of MCP-1, TNF-α and IL-6 in CIA rats. On the other hand, we noted that CP-25 decreased the ratios of phosphorylated NF-κB p65 (p-p65) to total p65 and p-IκBα to total IκBα in CIA rats, suggesting that CP-25 blocked NF-κB activation. Finally, we observed that CP-25 restored the abnormal expression of OAT1 and OCT1 in the renal tissues of CIA rat. Our data indicate that CP-25 ameliorates kidney damage in CIA rats, and this beneficial effect is closely related to inhibiting renal inflammation and the abnormal expression of transporters.

Nano Ellagic Acid Counteracts Cisplatin-Induced Upregulation in OAT1 and OAT3: A Possible Nephroprotection Mechanism

Cisplatin is an anticancer drug commonly used for solid tumors. However, it causes nephrotoxicity. OAT1 and OAT3 are organic anion transporters known to contribute to the uptake of cisplatin into renal tubular cells. The present study was designed to examine the protective role of ellagic acid nanoformulation (ellagic acid nano) on cisplatin-induced nephrotoxicity in rats, and the role of OAT1/OAT3 in this effect. Four groups of male Wistar rats were used (n = 6): (1) control, (2) cisplatin (7.5 mg/kg single dose, intraperitoneal), (3) cisplatin + ellagic acid nano (1 mg/kg), and (4) cisplatin + ellagic acid nano (2 mg/kg). Nephrotoxic rats treated with ellagic acid nano exhibited a significant reduction in elevated serum creatinine, urea, and oxidative stress marker, malondialdehyde (MDA). Additionally, ellagic acid nano restored renal glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). Ellagic acid nano improved the histopathological changes induced by cisplatin, such as tubular dilatation, necrosis, and degeneration. Interestingly, OAT1 and OAT3 showed significantly lower expression at both mRNA and protein levels following ellagic acid nano treatment relative to the cisplatin-exposed group. These findings reveal a potential inhibitory role of ellagic acid antioxidant on OAT1 and OAT3 expression and thus explains its nephroprotective effect against cisplatin nephrotoxicity.

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.

Andrographolide is neither a human organic anion transporter 1 (hOAT1) substrate nor inhibitor

Andrographolide, a major bioactive compound isolated from Andrographis paniculata (Burm. F.) Nees, was evaluated for its effects on the hOAT1 membrane transporter. Substrate determination and inhibition of hOAT1-mediated uptake transport assay was carried out using recombinant CHO-hOAT1 cells. The results showed that the uptake ratio of andrographolide was less than 2.0 at all concentrations tested, indicating that andrographolide is not a hOAT1 substrate. Andrographolide has no significant effects on the p-aminohippuric acid uptake and on the mRNA and protein expression of hOAT1. In conclusion, andrographolide may not pose a drug-herb interaction risk related to hOAT1.

The anti-hyperuricemic effect of four astilbin stereoisomers in Smilax glabra on hyperuricemic mice

ETHNOPHARMACOLOGICAL RELEVANCE

Smilax glabra Roxb. (SG), a Chinese medicinal herb which called "tufuling", is believed to be effective in treating hyperuricemia and gout symptoms. But the active substance and pharmacological mechanism of reducing uric acid remain unknown. This study aimed to obtain the total flavonoids including four astilbin stereoisomers and to examine their effects on reducing uric acid content in hyperuricemic mice.

MATERIALS AND METHODS

The total flavonoids of S. glabra (TFSG) were purified and then analysed by HPLC-PDA-MS. The effect of TFSG on the content of serum uric acid (S_UA), Serum creatinine (S_Cr), blood urea nitrogen (BUN) and the activities of xanthine oxidase (XOD) in hyperuricemic mouse model induced by potassium oxonate were examined. Western blot and PCR method were also used to investigate whether TFSG have effect on renal transport protein organic anion transporter 1 (OAT1), organic cation/carnitine transporter 2 (OCTN2) and their mRNA in hypeuricemic mice.

RESULT

Total flavonoids were obtained from EtOAc soluble portion of S. glabra. Four compounds were identified as neoastilbin, astilbin, neoisoastilbin and isoastilbin, which accounted for 55.6% of total flavonoids. TFSG could significantly reduce the serum uric acid content in hyperuricemic mouse (p < 0.01 or p < 0.05). The activities of hepatic XOD have been reduced in hyperuricemic mouse administered with 125 mg/kg TFSG (p < 0.05). The expressions of renal OAT1, OCTN2 and their mRNA have been up-regulated in hyperuricemic mice administered with TFSG (250, 125 mg/kg) (p < 0.01or p < 0.05). TFSG (62.5 mg/kg) could also elevated the expression of renal OCTN2 (p < 0.05).

CONCLUSION

A novel and simple method for preparative separation of astilbin stereoisomers from S. glabra was developed. It was the first time to obtain total flavonoids (including four marker compounds) of S. glabra, and the total content was up to 55.6%. The results suggested TFSG has significant effect on reducing uric acid in hyperuricemic mice by inhibiting the XOD activities and up-regulating the expression of OAT1, OCTN2 and their mRNA in kidney tissue.

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.

From the Cover: Identification of Natural Products as Inhibitors of Human Organic Anion Transporters (OAT1 and OAT3) and Their Protective Effect on Mercury-Induced Toxicity

Mercury accumulates in kidneys and produces acute kidney injury. Semen cassiae (SC), a widely consumed tea and herbal medicine in Eastern Asia, has been reported to have protective effects on kidneys. In this study, SC extract was shown to almost abolish the histological alterations induced by mercuric chloride in rat kidneys. A total of 22 compounds were isolated from SC, and 1,7,8-methoxyl-2-hydroxyl-3-methyl-anthraquinone was detected in SC for the first time. Among the eight compounds identified in the blood of rats after SC treatment, six were strong inhibitors of human organic anion transporter 1 and 3 (OAT1 and OAT3). Inhibitory studies revealed that OAT1 and OAT3 were inhibited by SC constituents, in both a competitive and noncompetitive manner. Both OAT1- and OAT3-overexpressing cells were susceptible to the cytotoxicity of the cysteine-mercury conjugate, but only OAT1-overexpressing cells could be protected by 200 μM probenecid or 10 μM of the eight inhibitors in SC, suggesting that OAT1 is the major determinant in the cellular uptake of mercury. To facilitate the identification of inhibitors of OAT1 and OAT3, models of OAT1 and OAT3 were constructed using recently determined protein templates. By combining in silico and in vitro methods, inhibitors of OAT1 and OAT3 were predicted and validated from SC constituents. Collectively, the present study suggests that additional inhibitors of OAT1 and OAT3 can be predicted and validated from natural products by combining docking and in vitro screening, and could be a source of pharmaceutical compounds for developing treatments for mercury-induced kidney injury.

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.

Osteomalacia induced by long-term low-dose adefovir dipivoxil: Clinical characteristics and genetic predictors

CONTEXT

Adefovir dipivoxil (ADV) was an important cause of adult-onset hypophosphatemic osteomalacia. However, its clinical characteristics and mechanisms have not been well defined.

OBJECTIVE

The objective of the study was to summarize the clinical characteristics of ADV-induced osteomalacia and to explore the association between ADV-associated tubulopathy and polymorphisms in genes encoding drug transporters.

DESIGN, SETTING, PATIENTS, AND MAIN OUTCOME MEASURE

Seventy-six affected patients were clinically studied. The SLC22A6 and ABCC2 genes were screened and compared with healthy people from the HapMap.

RESULTS

Hypophosphatemia, high serum alkaline phosphatase (ALP) levels, hypouricemia, nondiabetic glycosuria, proteinuria, metabolic acidosis and high bone turnover markers were the main metabolic characteristics. Fractures and pseudofractures occurred in 39 patients. Stopping ADV administration, supplementing calcitriol and calcium was effective during the follow-up period. Single SNP analysis revealed a higher percentage of the G/A genotype at c.2934 in exon 22 of the ABCC2 gene (rs3740070) in patients than in healthy people (12% [7 of 58 patients] vs. 0% [0 of 45 patients]; P=0.017), while there was no subject with homozygosity for the A allele at c.2934.

CONCLUSIONS

ADV can be nephrotoxic at a conventional dosage. The G/A genotype at c.2934 of the ABCC2 gene may be a predictor of patients at greater risk for developing ADV-associated tubulopathy. Larger case-control studies are needed to further verify this finding.

Anti-hyperuricemia effects of allopurinol are improved by Smilax riparia, a traditional Chinese herbal medicine

ETHNOPHARMACOLOGICAL RELEVANCE

The roots and rhizomes of Smilax riparia are called "Niu-Wei-Cai" in traditional Chinese medicine (TCM). This botanical has been used in treating the symptoms of gout and other hyperuricemic-related conditions in TCM. Allopurinol is a commonly used medication to treat hyperuricemia and its complications. In this study, we evaluated whether Smilax riparia could enhance allopurinol׳s effects by decreasing the serum uric acid level in a hyperuricemic mouse model induced by potassium oxonate.

MATERIALS AND METHODS

We examined the effects of allopurinol (5mg/kg) administration alone or in combination with Smilax riparia saponins (SRS, 500 mg/kg) on the serum uric acid (SUA), serum creatinine (SCr) and blood urea nitrogen (BUN) levels in a hyperuricemic mouse model. The effects of allopurinol alone or those of allopurinol plus SRS on the XOD activities were measured. Western blot analysis was used to measure the levels of mURAT1, mGLUT9 and mOTA1 in the mice.

RESULTS

Compared with allopurinol alone, the combination of allopurinol and SRS significantly decreased the serum uric acid level and increased the urine uric acid level (both P<0.05), leading to the normalized serum and urine uric acid concentrations. Data on serum and urine creatinine and BUN supported these observations. The attenuation of hyperuricemia-induced renal dysfunction was linked to the inhibition of both serum and hepatic xanthine oxidase (XOD), the down-regulation of renal mURAT1 and mGLUT9, and the up-regulation of mOAT1.

CONCLUSION

The anti-hyperuricemia effects of allopurinol are improved by Smilax riparia co-administration. The results were supported by the measurement of uric acid, creatinine, BUN, XOD, mURAT1, mGLUT9 and mOAT1. Our data may have a potential value in clinical practice in the treatment of gout and other hyperuricemic conditions.

Predictors of kidney tubular dysfunction induced by adefovir treatment for chronic hepatitis B

AIM: To investigate the predictors of proximal kidney tubular dysfunction (PKTD) induced by adefovir dipivoxil (ADV) treatment for chronic hepatitis B.

METHODS: Seventy-nine patients (age at the evaluation of PKTD: 56.9 ± 10.7 years) with chronic hepatitis B undergoing long-term oral antiviral nucleos(t)ide analogue treatment were consecutively recruited. PKTD was defined by the presence of at least two of the following five abnormalities: phosphate diabetes, nondiabetic glucosuria, metabolic acidosis, β₂-microglobulinuria, or renal hypouricemia. The single-nucleotide polymorphisms (SNPs) in the SLC22A6 gene encoding human organic anion transporter 1 (hOAT1) and ABCC2 encoding multidrug resistance protein 2 (MRP2) were analyzed using the TaqMan Allelic Discrimination Demonstration Kit.

RESULTS: Nine (30.0%) of the 30 ADV-treated patients were diagnosed with PKTD, while no patients without ADV developed PKTD (P < 0.001). Three patients with ADV were diagnosed with symptomatic osteomalacia. Among the patients who took ADV, those with PKTD were of higher age at initiation, had significantly longer treatment duration, and had a significantly lower body mass index than those without PKTD. The incidence of PKTD dramatically increased after 96 mo from the start of ADV administration. In contrast, the SNPs were not correlated with PKTD. Logistic regression analysis extracted older age at initiation (OR = 5.0, 95%CI: 1.1-23.4; P = 0.040) and longer treatment duration (OR = 3.2, 95%CI: 1.2-8.6; P = 0.020) as significant factors associated with PKTD.

CONCLUSION: Our results suggest that the tubular function of the kidney of older patients undergoing long-term ADV treatment should be carefully evaluated.

[Effect of Zhusha Anshen pill, cinnabar, HgS, HgCl2 and MeHg on gene expression of renal transporters in mice]

OBJECTIVE

To study the effect of Zhusha Anshen pill, cinnabar, HgS, HgCl2 and MeHg on the gene expression of renal transporters in mice.

METHOD

Healthy male mice were given equivalent physiological saline, Zhusha Anshen pill (1.8 g · kg(-1), containing 0.17 g · kg(-1) of mercury), cinnabar (0.2 g · kg(-1), containing 1.7 g · kg(-1) of mercury), high dose cinnabar (2 g · kg(-1), containing 1.7 g · kg(-1) of mercury), HgS (0.2 g · kg(-1), containing 0.17 g · kg(-1) of mercury), HgCl2 (0.032 g · kg(-1), containing 0. 024 g · kg(-1) of mercury), MeHg (0.026 g · kg(-1), containing 0.024 g · kg(-1) of mercury), once daily, for 30 d, measuring body mass gain. 30 days later, the mice were sacrificed. The mercury accumulation in kidneys was detected with atomic fluorescence spectrometer. Expressions of Oat1, Oat2, Oat3, Mrp2, Mrp4, Urat1 were detected with RT-PCR.

RESULT

Compared with the normal control group, a significant accumulation of Hg in kidney in HgCl2 and MeHg groups was observed (P <0.05), but these changes were not found in other groups. Compared with normal control group, mRNA expressions of Oat1 and Oat2 were evidently lower in HgCl2 and MeHg groups, but mRNA expressions of Mrp2 were apparently higher in HgCl2 group (P <0.05), mRNA expression of Mrp4 was significant higher in HgCl2 and MeHg groups, and mRNA expression of Urat1 was apparently lower in MeHg group.

CONCLUSION

HgCl2 and MeHg groups show significant difference from the normal group in mercury accumulation in kidneys and gene expression of kidney transporters, but with no difference between other groups and the normal group. Compared with HgCl2 and MeHg, cinnabar and its compounds could cause lower renal toxicity to mice.

Total saponins from Discorea nipponica ameliorate urate excretion in hyperuricemic mice

Rhizoma Dioscoreae Nipponicae, from Discorea nipponica, is a widely used traditional Chinese herb. It is used to treat arthroncus, arthrodynia, and arthritis. Hyperuricemia is an important foundation of gouty arthritis. The current study was aimed at investigating whether the effects of total saponins from Rhizoma Dioscoreae Nipponicae on hyperuricemia were due to renal organic ion transporters in potassium oxonate-induced hyperuricemia mice. Hyperplasia of synovial cells prepared from Wistar rats was induced by IL-1β (1 × 10(4) µg/mL). MTT was used and to screen active components in the inhibition of hyperplasia by total saponins from Rhizoma Dioscoreae Nipponica, individual pure compounds, and different combinations of these compounds. Sixty Kun Ming mice were randomly divided into six groups: normal, model, allopurinol (40 mg/kg), and three total saponins groups receiving dose (600 mg/kg), middle (300 mg/kg), and low doses (60 mg/kg). Hyperuricemic mice were induced with potassium oxonate (300 mg/kg) intragastrically. The total saponins were given six days and the positive drug allopurinol was given one day before inducing hyperuricemia. The serum and urine levels of uric acid and creatinine and the fractional excretion of uric acid were measured in normal and hyperuricemic mice treated with Rhizoma Dioscoreae Nipponicae and allopurinol. The mRNA and protein levels of the mouse urate transporter 1, glucose transporter 9, organic anion transporter 1, and organic anion transporter 3 were analyzed by real-time-PCR and Western blotting methods, respectively. Total saponins from Rhizoma Dioscoreae Nipponicae could effectively reverse potassium oxonate-induced alterations in renal mouse urate transporter 1, glucose transporter 9, organic anion transporter 1, and organic anion transporter 3 mRNA and protein levels, resulting in enhancement of renal urate excretion in mice. These findings suggested that the total saponins from Rhizoma Dioscoreae Nipponicae had a uricosuric effect on the regulation of renal organic ion transporters in hyperuricemic animals.

Differential effect of COX1 and COX2 inhibitors on renal outcomes following ischemic acute kidney injury

BACKGROUND/AIMS

We have previously shown that 1 mg/kg indomethacin improves expression and functionality of renal organic anion transporters Oat1 and Oat3 after renal ischemia and furthermore improves renal outcome after ischemia. As we detected differential effects of COX1 or COX2 inhibitors on organic anion transport after ischemia and reperfusion in culture, we investigated the effect of the SC560 (COX1 inhibitor) and SC58125 (COX2 inhibitor) on expression of Oat1/3 and renal outcome after ischemic acute kidney injury (iAKI).

METHODS

iAKI was induced in rats by bilateral clamping of renal arteries for 45 min. SC560 or SC58125 (1 mg/kg each) were given intraperitoneally as soon as reperfusion started. Sham-treated animals served as controls. Oat1/3 were determined by qPCR and Western blot. Glomerular filtration rate (GFR), p-aminohippurate (PAH) clearance and PAH extraction ratio was determined. All parameters were detected 24 h after ischemia. Renal plasma flow was calculated.

RESULTS

In clamped animals SC560 (COX1 inhibitor) restored expression of Oat1/3, as well as renal perfusion. Additionally, SC560 substantially improved kidney function as measured by GFR. Application of the COX2 inhibitor SC58125 did not exert these beneficial effects.

CONCLUSION

Our study indicates that COX1 inhibitor SC560 applied after ischemia prevents ischemia-induced downregulation of Oat1/3 during reperfusion and has a substantial protective effect on kidney function. Whether and to what particular extent this apparent improvement of function is mechanistically due to beneficial effects on tubular function, renal perfusion or glomerular filtration will be the scope of future studies.

Effect of JBP485 on obstructive jaundice is related to regulation of renal Oat1, Oat3 and Mrp2 expression in ANIT-treated rats

The objective was to determine whether protective effects of JBP485 on biliary obstruction induced by alpha-naphthylisothiocyanate (ANIT) are mediated by the organic anion transporters Oat1, Oat3 and the multidrug resistance-associated protein Mrp2. The ANIT-induced increases in bilirubin (BIL), alanine aminotransferase (ALT) and aspartate transaminase (AST) in rat serum were inhibited significantly by oral administration of JBP485. The plasma concentration of JBP485 which is the substrate of Oat1 and Oat3 determined by LC-MS/MS was markedly increased after intravenous administration in ANIT-treated rats, whereas cumulative urinary excretion of JBP485 in vivo and the uptake of JBP485 in kidney slices were decreased remarkably. RT-PCR and Western blot showed the decreased expression of Oat1 and Oat3, increased expression of Mrp2 in ANIT-induced rats, meanwhile, the expression levels of Mrp2 and Oat1 were up-regulated after administration of JBP485. The up-regulation of Mrp2 and Oat1 was associated with a concomitant increase in urinary BIL after treatment with JBP485 in ANIT-treated rats. The mechanism for JBP485 to restore liver function might be related to improvement of the expression and function for Oat1 and Mrp2 as well as facilitation of urinary excretion for hepatoxic substance.

Male-dominant activation of rat renal organic anion transporter 1 (Oat1) and 3 (Oat3) expression by transcription factor BCL6

Background

Organic anion transporters 1 (Oat1) and 3 (Oat3) mediate the transport of organic anions, including frequently prescribed drugs, across cell membranes in kidney proximal tubule cells. In rats, these transporters are known to be male-dominant and testosterone-dependently expressed. The molecular mechanisms that are involved in the sex-dependent expression are unknown. Our aim was to identify genes that show a sex-dependent expression and could be involved in male-dominant regulation of Oat1 and Oat3.

Methodology/Principal Findings

Promoter activities of Oat1 and Oat3 were analyzed using luciferase assays. Expression profiling was done using a SurePrint G3 rat GE 8×60K microarray. RNA was isolated from renal cortical slices of four adult rats per sex. To filter the achieved microarray data for genes expressed in proximal tubule cells, transcription database alignment was carried out. We demonstrate that predicted androgen response elements in the promoters of Oat1 and Oat3 are not functional when the promoters were expressed in OK cells. Using microarray analyses we analyzed 17,406 different genes. Out of these genes, 56 exhibit a sex-dependent expression in rat proximal tubule cells. As genes potentially involved in the regulation of Oat1 and Oat3 expression, we identified, amongst others, the male-dominant hydroxysteroid (17-beta) dehydrogenase 1 (Hsd17b1), B-cell CLL/lymphoma 6 (BCL6), and polymerase (RNA) III (DNA directed) polypeptide G (Polr3g). Moreover, our results revealed that the transcription factor BCL6 activates promoter constructs of Oat1 and Oat3.

Conclusion

The results indicate that the male-dominant expression of both transporters, Oat1 and Oat3, is possibly not directly regulated by the classical androgen receptor mediated transcriptional pathway but appears to be regulated by the transcription factor BCL6.

Elucidation of common pharmacophores from analysis of targeted metabolites transported by the multispecific drug transporter-Organic anion transporter1 (Oat1)

Organic anion transporter 1 (Oat1), first identified as NKT, is a multispecific transporter responsible for the handling of drugs and toxins in the kidney and choroid plexus, but its normal physiological role appears to be in small molecule metabolite regulation. Metabolites transported by Oat1 and which are altered in the blood and urine of the murine Oat1 knockout, may serve as templates for further drug design. This may lead to better tissue targeting of drugs or design of Oat1 inhibitors that prolong the half-life of current drugs. Due to the multispecificity of the transporter, 19 of known targeted metabolites have different chemical structures and properties that make constructing a common pharmacophore model difficult. Here we propose an approach that clustered the metabolites into four distinct groups which allowed for the construction of a consensus pharmacophore for each cluster. The screening of commercial molecular databases determined the top candidates whose interaction with Oat1 was confirmed in an experimental model of organic anion transport. Thus, these candidate selections represent potential molecules for further drug design.

[Mangiferin promotes uric acid excretion and kidney function improvement and modulates related renal transporters in hyperuricemic mice]

The effects of mangiferin on uric acid excretion, kidney function and related renal transporters were investigated in hyperuricemic mice induced by potassium oxonate. Mice were divided into normal control group, and 5 hyperuricemic groups with model control, 50, 100, and 200 mg x kg(-1) mangiferin, and 5 mg x kg(-1) allopurinol. Mice were administered by gavage once daily with 250 mg x kg(-1) potassium oxonate for seven consecutive days to create the model. And 3 doses of mangiferin were orally initiated on the day 1 h after potassium oxonate was given, separately. Serum uric acid, creatinine and urea nitrogon levels, as well as urinary uric acid creatinine levels were measured. Mouse uromodulin (mUMOD) levels in serum, urine and kidney were determined by ELISA method. The mRNA and protein levels of related renal transporters were assayed by RT-PCR and Western blotting methods, respectively. Compared to model group, mangiferin significantly reduced serum uric acid, creatinine and urea nitrogon levels, increased 24 h uric acid and creatinine excretion, and fractional excretion of uric acid in hyperuricemic mice, exhibiting uric acid excretion enhancement and kidney function improvement. Mangiferin was found to down-regulate mRNA and protein levels of urate transporter 1 (mURAT1) and glucose transporter 9 (mGLUT9), as well as up-regulate organic anion transporter 1 (mOAT1) in the kidney of hyperuricemic mice. These findings suggested that mangiferin might enhance uric acid excretion and in turn reduce serum uric acid level through the decrease of uric acid reabsorption and the increase of uric acid secretion in hyperuricemic mice. Moreover, mangiferin remarkably up-regulated expression levels of renal organic cation and carnitine transporters (mOCT1, mOCT2, mOCTN1 and mOCTN2), increased urine mUMOD levels, as well as decreased serum and kidney mUMOD levels in hyperuricemic mice, which might be involved in mangiferin-mediated renal protective action.

Site-specific accumulation of the cancer preventive dietary polyphenol ellagic acid in epithelial cells of the aerodigestive tract

Ellagic acid (EA), a polyphenol present in berries, has been demonstrated to prevent oesophageal and colon cancer in animals. To better understand the site-specificity of these effects, we studied the accumulation and transport of [14C]EA in rat aerodigestive epithelial cells in-vivo and in cultured human cells. When [14C]EA was administered to rats by gavage, a high content of EA was found in the oesophagus and small intestine at 0.5h after oral administration and in the colon at 12h, with very low amounts in plasma and peripheral tissues. Studies in human intestinal Caco-2 and human oesophageal HET-1A cells found very limited transcellular transport (Caco-2) of EA but high accumulation (Caco-2 and HET-1A) in the cells. In more detailed studies in the Caco-2 cells, accumulation of EA displayed ATP- and Na+-dependency. Multiple interventions permitted the exclusion of a number of transporters as mediators of this uptake. A dramatically reduced transport of EA at low pH (5.5) compared with high pH (7.4) suggested an important role for the negative charge of EA. This was supported by the organic anion transport inhibitors 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid and bromosulfophthalein. The latter produced as much as 78% inhibition at the 100 μm concentration. Finally, Caco-2 cells were shown to express organic anion transporter 4 (OAT4) mRNA, as was the human large intestine. EA appears to be accumulated along the aerodigestive tract using OATlike transporters, one of which might be OAT4.

Identification and characterization of 3-iodothyronamine intracellular transport

3-Iodothyronamine (T(1)AM) is a naturally occurring thyroid hormone metabolite with distinct biological effects that are opposite those of thyroid hormone. The known molecular targets of T(1)AM include both plasma membrane and intracellular proteins, suggesting that intracellular transport of T(1)AM may be an important component of its action, although no uptake mechanism has yet been described. Using various human cell lines, we show that, indeed, cellular uptake of T(1)AM occurs in multiple cell types and that this process involves specific, saturable, and inhibitable transport mechanisms. These mechanisms are sodium and chloride independent, pH dependent, thyronamine specific, and do not involve the likely candidate transporters of other monoamines, organic cations, or thyroid hormones. A large-scale RNA interference screen targeting the entire solute carrier superfamily of transporter genes reveals that the transport of T(1)AM into cells involves multiple transporters, and we identify eight transporters that may contribute to the uptake of T(1)AM in HeLa cells. This type of transporter small interfering RNA screening approach can be used in general to identify the constellation of transporters that participate in the intracellular disposition of compounds.

Protective effect of morin on the imipenem-induced nephrotoxicity in rabbits

The present study investigated the protective effect of morin, a natural flavonoid, on the imipenem-induced nephrotoxicity in rabbits. Nephrotoxicity of imipenem was examined after the intravenous administrations of imipenem (200 mg/kg) to rabbits in the presence and the absence of morin (12, 25, 50 mg/kg, p.o.). Cytotoxicity of imipenem was also examined in the presence and the absence of morin (100 microM) by using MDCK cells overexpressing human organic anion transporter 1 and 3 (MDCK/hOAT1 or MDCK/hOAT3). Intravenous dosing of imipenem alone induced severe proximal tubular necrosis in rabbits, however, the concurrent use of morin (25 or 50 mg/kg, p.o.) significantly suppressed the histopathological damage in the kidney induced by imipenem. While imipenem was not cytotoxic in MDCK/hOAT1 cells over the tested concentrations up to 10 mM, it showed significant cellular toxicity with CC(50) of 0.77 mM in MDCK/hOAT3 cells, implying that OAT3 may involve more actively in the imipenem-induced nephrotoxicity. In addition, the cellular toxicity of imipenem decreased by approximately 20 folds in the presence of morin in MDCK/hOAT3 cells. In conclusion, the present study suggests that morin might be beneficial to reduce the nephrotoxicity of imipenem, at least in part, via the inhibition of OAT3-mediated renal excretion of imipenem.

Analysis of regulatory polymorphisms in organic ion transporter genes (SLC22A) in the kidney

Organic cation transporters (OCTs) and organic anion transporters (OATs) (SLC22A family) play crucial roles in the renal secretion of various drugs. Messengar ribonucleic acid (mRNA) expression of transporters can be a key factor regulating interindividual differences in drug pharmacokinetics. However, the source of variations in mRNA levels of transporters is unclear. In this study, we focused on single nucleotide polymorphisms (SNP) in the promoter region [regulatory SNPs (rSNPs)] as candidates for the factor regulating mRNA levels of SLC22A. We sequenced the promoter regions of OCT2 and OAT1-4 in 63 patients and investigated the effects of the identified rSNPs on transcriptional activities and mRNA expression. In the OCT2 promoter region, one deletion polymorphism (-578_-576delAAG) was identified; -578_-576delAAG significantly reduced OCT2 promoter activity (p < 0.05), and carriers of -578_-576delAAG tend to have lower OCT2 mRNA levels, but the difference is not significant. There was no rSNP in the OAT1 and OAT2 genes. The five rSNPs of OAT3 and one rSNP of OAT4 were unlikely to influence mRNA expression and promoter activity. This is the first study to investigate the influences of rSNPs on mRNA expression of SLC22A in the kidney and to identify a regulatory polymorphism affecting OCT2 promoter activity.

Interaction characteristics of flavonoids with human organic anion transporter 1 (hOAT1) and 3 (hOAT3)

The present study aimed to investigate the interaction characteristics of flavonoids with human organic anion transporter 1 (hOAT1) and 3 (hOAT3). Five flavonoids (morin, silybin, naringin, naringenin and quercetin) were selected and their interaction characteristics with hOAT1 and hOAT3 were examined in MDCK cells overexpressing hOAT1 or hOAT3. Among tested flavonoids, morin and silybin exhibited significant inhibition effects on the cellular uptake of [3H]-para-aminohippuric acid ([3H]-PAH) in MDCK-hOAT1 cells with Ki of 0.46 microM and 24 microM, respectively, while all the tested flavonoids appeared to be less interactive with hOAT3 compared to hOAT1. A kinetic study suggested that morin and silybin inhibited hOAT1-mediated cellular uptake of [3H]-PAH in a competitive manner. Furthermore, morin and silybin were translocated by hOAT1 across the cellular membrane. In conclusion, the present study identified some of flavonoids as a new class of hOAT1 inhibitors, suggesting a potential for flavonoid-drug interactions via the modulation of hOAT1 activity.

Cl−-dependent upregulation of human organic anion transporters: different effects on transport kinetics between hOAT1 and hOAT3

Chloride ion has a stimulatory effect on the transport of organic anions across renal basolateral membranes. However, the exact mechanisms at molecular levels have been unclear as of yet. Human organic anion transporters hOAT1 and hOAT3 play important roles in renal basolateral membranes. In this study, the effects of Cl− on the activities of these transporters were evaluated by using HEK293 cells stably expressing hOAT1 or hOAT3 (HEK-hOAT1 or HEK-hOAT3). The uptake of p-[14C]aminohippurate by HEK-hOAT1 and [3H]estrone sulfate by HEK-hOAT3 was greater in the presence of Cl− than in the presence of SO42− or gluconate. Additionally, the uptake of various compounds by HEK-hOAT1 and HEK-hOAT3 was significantly higher in the Cl−-containing medium than the gluconate-containing medium, suggesting that the influences of Cl− are not dependent on substrate and that Cl− directly stimulates the functions of hOAT1 and hOAT3. The substitution of gluconate with Cl− did not change the Km value for the uptake of p-[14C]aminohippurate by HEK-hOAT1 but caused an approximately threefold increase in the maximal uptake rate (Vmax) value. On the other hand, replacement of gluconate with Cl− decreased the Km value for the uptake of [3H]estrone sulfate and cefotiam by HEK-hOAT3 to about one-third, while it did not change the Vmax value. In summary, Cl− upregulates the activities of both hOAT1 and hOAT3, but its effects on transport kinetics differ between these transporters. It was suggested that Cl− participates in the trans-location process for hOAT1, and the substrate recognition process for hOAT3.

The effect of lopinavir/ritonavir on the renal clearance of tenofovir in HIV-infected patients

We determined the effects of lopinavir/ritonavir on tenofovir renal clearance. Human immunodeficiency virus-infected subjects taking tenofovir disoproxil fumarate (TDF) were matched on age, race, and gender and were enrolled into one of the following two groups: group 1: subjects taking TDF plus lopinavir/ritonavir plus other nucleoside reverse transcriptase inhibitors (NRTIs); group 2: subjects taking TDF plus NRTIs and/or non-NRTIs but no protease inhibitors. Twenty-four-hour blood and urine collections were carried out in subjects for tenofovir quantification. Drug transporter genotype associations with tenofovir pharmacokinetics were examined. In 30 subjects, median (range) tenofovir apparent oral clearance, renal clearance, and fraction excreted in urine were 34.6 l/h (20.6-89.5), 11.3 l/h (6.2-22.6), and 0.33 (0.23-0.5), respectively. After adjusting for renal function, tenofovir renal clearance was 17.5% slower (P=0.04) in subjects taking lopinavir/ritonavir versus those not taking a protease inhibitor, consistent with a renal interaction between these drugs. Future studies should clarify the exact mechanism and whether there is an increased risk of nephrotoxicity.

The Contribution of Organic Anion Transporters OAT1 and OAT3 to the Renal Uptake of Rosuvastatin

Rosuvastatin is a potent inhibitor of 3-hydroxy-3-methylglutaryl-CoA reductase and has been shown to be highly effective in reducing low-density lipoprotein cholesterol. Clinical trials have demonstrated that renal excretion and, in particular, tubular secretion, plays a role in rosuvastatin clearance. The aim of this study was to determine the involvement of the basolateral organic anion transporters, OAT1 and OAT3, in the renal uptake of rosuvastatin. Expression of human (h) OAT3 in Xenopus oocytes significantly increased the uptake of rosuvastatin above control levels (K(m) = 7.4 microM). In contrast hOAT1 did not mediate rosuvastatin uptake. Furthermore, hOAT3-mediated estrone-3-sulfate uptake could be inhibited, with a rank order of potency, by atorvastatin, rosuvastatin, simvastatin, and pravastatin, whereas hOAT1-mediated PAH uptake was only significantly inhibited by simvastatin. To estimate the contribution of hOAT3 to the overall renal uptake of rosuvastatin, a series of experiments were conducted using rat kidney slices. Rosuvastatin uptake in rat renal slices was abolished in the presence of the rat (r) Oat3-specific inhibitor benzylpenicillin, suggesting that rOat3 is responsible for the majority of rosuvastatin uptake across the basolateral membrane in rat kidney. From these findings, we can suggest that hOAT3 contributes to the renal uptake of rosuvastatin in humans.

Human Organic Anion Transporters 1 (hOAT1/SLC22A6) and 3 (hOAT3/SLC22A8) Transport Edaravone (MCI-186; 3-methyl-1-phenyl-2-pyrazolin-5-one) and Its Sulfate Conjugate

3-Methyl-1-phenyl-2-pyrazolin-5-one (MCI-186; edaravone), a novel free radical scavenger, is used for the treatment of acute cerebral infarction. After marketing, a few cases of acute renal failure were reported in patients following treatment with this drug. Because edaravone is mainly excreted into the urine following conjugation to glucuronide or sulfate, the renal excretion mechanisms of edaravone should help provide important information when considering the clinical cases. We examined the transport of edaravone and its sulfate and glucuronide conjugates via human organic anion transporter 1 (hOAT1) and 3 (hOAT3), expressed on the basolateral membranes of proximal tubules. The hOAT1- and hOAT3-transfected human embryonic kidney (HEK)-293 cells exhibited a markedly higher uptake of edaravone sulfate and a slightly higher uptake of edaravone than vector-transfected cells. The K(m) values of edaravone sulfate uptake by hOAT1 and hOAT3 were 11 and 15 microM, respectively. Estimation of the relative contribution of hOAT1 and hOAT3 using reference compounds suggested that hOAT1 and hOAT3 might contribute to the renal uptake of edaravone sulfate to the same extent. However, edaravone and its sulfate showed no cytotoxicity toward both hOAT1-HEK and control cells, suggesting that higher uptake in hOAT1-HEK did not associate with cytotoxicity of these compounds. In conclusion, our results suggest that both hOAT1 and hOAT3 are responsible for the basolateral uptake of edaravone sulfate in the kidney.

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.

An oral adsorbent, AST-120 protects against the progression of oxidative stress by reducing the accumulation of indoxyl sulfate in the systemic circulation in renal failure

PURPOSE

The effect of AST-120, an oral adsorbent, on oxidative stress in the systemic circulation in chronic renal failure (CRF) was examined and the potential role of indoxyl sulfate (IS), an uremic toxin adsorbed by AST-120, in inducing the formation of reactive oxygen species (ROS) in the vascular system was studied, in vitro and in vivo.

MATERIALS AND METHODS

The level of oxidized albumin, a marker for oxidative stress in the systemic circulation was determined by HPLC, as previously reported. The mRNA levels of TGF-beta (1) and Oat1 were measured by quantitative RT-PCR. The IS induced ROS generation in cultured human umbilical vein endothelial cells (HUVECs) was estimated using a fluorescence microplate reader.

RESULTS

An increase in the ratio of oxidized to unoxidized albumin was determined using 5/6 nephrectomized rats (CRF rats) compared to a control group. The ratio was significantly reduced in the group that received AST-120 of 4 weeks, suggesting that AST-120 inhibits oxidative stress in CRF. An anti-oxidative effect of AST-120 was also observed in CRF rats with a similar renal function. The ratio of oxidized albumin was correlated with serum IS levels in vivo. The same relationship was also observed in CRF rats with the continued administration of IS. In addition, IS dramatically increased the generation of ROS in both a dose- and time- dependent manner in HUVEC, suggesting that accumulated IS may play an important role in enhancing intravascular oxidative stress.

CONCLUSION

We propose that AST-120 reduces IS concentrations in the blood that induces ROS production in endothelial cells, thereby inhibiting the subsequent occurrence of oxidative stress in the systemic circulation in renal failure.

The chloride dependence of the human organic anion transporter 1 (hOAT1) is blunted by mutation of a single amino acid

Organic anion transporter 1 (OAT1) is key for the secretion of organic anions in renal proximal tubules. These organic anions comprise endogenous as well as exogenous compounds including frequently used drugs of various chemical structures. The molecular basis for the polyspecificity of OAT1 is not known. Here we mutated a conserved positively charged arginine residue (Arg(466)) in the 11(th) transmembrane helix of human OAT1. The replacement by the positively charged lysine (R466K) did not impair expression of hOAT1 at the plasma membrane of Xenopus laevis oocytes but decreased the transport of p-aminohippurate (PAH) considerably. Extracellular glutarate inhibited and intracellular glutarate trans-stimulated wild type and mutated OAT1, suggesting for the mutant R466K an unimpaired interaction with dicarboxylates. However, when Arg(466) was replaced by the negatively charged aspartate (R466D), glutarate no longer interacted with the mutant. PAH uptake by wild type hOAT1 was stimulated in the presence of chloride, whereas the R466K mutant was chloride-insensitive. Likewise, the uptake of labeled glutarate or ochratoxin A was chloride-dependent in the wild type but not in R466K. Kinetic experiments revealed that chloride did not alter the apparent K(m) for PAH but influenced V(max) in wild type OAT1-expressing oocytes. In R466K mutants the apparent K(m) for PAH was similar to that of the wild type, but V(max) was not changed by chloride removal. We conclude that Arg(466) influences the binding of glutarate, but not interaction with PAH, and interacts with chloride, which is a major determinant in substrate translocation.

Hepatocyte nuclear factor-4{alpha} regulates the human organic anion transporter 1 gene in the kidney

Human organic anion transporter 1 (OAT1, SLC22A6), which is localized to the basolateral membranes of renal tubular epithelial cells, plays a critical role in the excretion of anionic compounds. OAT1 is regulated by various pathophysiological conditions, but little is known about the molecular mechanisms regulating the expression of OAT1. In the present study, we investigated the transcriptional regulation of OAT1 and found that hepatocyte nuclear factor (HNF)-4alpha markedly transactivated the OAT1 promoter. A deletion analysis of the OAT1 promoter suggested that the regions spanning -1191 to -700 base pairs (bp) and -140 to -79 bp were essential for the transactivation by HNF-4alpha. These regions contained a direct repeat separated by two nucleotides (DR-2), which is one of the consensus sequences binding to HNF-4alpha, and an inverted repeat separated by eight nucleotides (IR-8), which was recently identified as a novel element for HNF-4alpha, respectively. An electrophoretic mobility shift assay showed that HNF-4alpha bound to DR-2 and IR-8 under the conditions of HNF-4alpha overexpression. Furthermore, under normal conditions, HNF-4alpha bound to IR-8, and a mutation in IR-8 markedly reduced the OAT1 promoter activity, indicating that HNF-4alpha regulates the basal transcription of OAT1 via IR-8. This paper reports the first characterization of the human OAT1 promoter and the first gene in the kidney whose promoter activity is regulated by HNF-4alpha.

Downregulation of organic anion transporters in rat kidney under ischemia/reperfusion-induced qacute renal failure

The effect of acute renal failure (ARF) induced by ischemia/reperfusion (I/R) of rat kidney on the expression of organic anion transporters (OATs) was examined. The level of serum indoxyl sulfate (IS), a uremic toxin and substrate of OATs in renal tubules, shows a marked increase with the progression of ARF. However, this increase was significantly attenuated by ingestion of cobalt. The level of mRNA and protein of both rOAT1 and rOAT3 were markedly depressed in the ischemic kidney. The uptake of p-aminohippuric acid (PAH) and estrone sulfate (ES) by renal slices of ischemic rats was significantly reduced compared to control rats. Renal slices taken from ischemic rats treated with cobalt displayed significantly elevated levels of ES uptake. Cobalt intake did not affect PAH uptake, indicating the functional restoration of rOAT3 but not rOAT1. The expression of Na(+)/K(+)-ATPase was markedly depressed in the ischemic kidney, suggesting that the inward Na(+) gradient in renal tubular cells had collapsed, thereby reducing the outward gradient of alpha-ketoglutarate, a driving force of both rOATs. The decreased expression of Na(+)/K(+)-ATPase was significantly restored by cobalt treatment. Our results suggest that the downregulation of renal rOAT1 and rOAT3 could be responsible for the increase in serum IS level of ischemic rats. Cobalt treatment has a significant protective effect on ischemia-induced ARF, being accompanied by the restoration of rOAT3 and/or Na(+)/K(+)-ATPase function.

The influence of macrolide antibiotics on the uptake of organic anions and drugs mediated by OATP1B1 and OATP1B3

Macrolides may cause severe drug interactions due to the inhibition of metabolizing enzymes. Transporter-mediated uptake of drugs into cells [e.g., by members of the human organic anion transporting polypeptide (OATP) family] is a determinant of drug disposition and a prerequisite for subsequent metabolism. However whether macrolides are also inhibitors of uptake transporters, thereby providing an additional mechanism of drug interactions, has not been systematically studied. The human OATP family members OATP1B1 and OATP1B3 mediate the uptake of endogenous substances and drugs such as antibiotics and HMG-CoA reductase inhibitors (statins) into hepatocytes. In this study we investigated the potential role of these uptake transporters on macrolide-induced drug interactions. By using sulfobromophthalein (BSP) and the HMG-CoA reductase inhibitor pravastatin as substrates, the effects of the macrolides azithromycin, clarithromycin, erythromycin, and roxithromycin and of the ketolide telithromycin on the OATP1B1- and OATP1B3-mediated uptake were analyzed. These experiments demonstrated that the OATP1B1- and OATP1B3-mediated uptake of BSP and pravastatin can be inhibited by increasing concentrations of all macrolides except azithromycin. The IC50 values for the inhibition of OATP1B3-mediated BSP uptake were 11 microM for telithromycin, 32 microM for clarithromycin, 34 microM for erythromycin, and 37 microM for roxithromycin. These IC50 values were lower than the IC50 values for inhibition of OATP1B1-mediated BSP uptake (96-217 microM). These macrolides also inhibited in a concentration-dependent manner the OATP1B1- and OATP1B3-mediated uptake of pravastatin. In summary, these results indicate that alterations of uptake transporter function by certain macrolides/ketolides have to be considered as a potential additional mechanism underlying drug-drug interactions.

Evidence of Oatp and Mdr1 in cryopreserved rat hepatocytes

Transport proteins control uptake of drugs into the liver (e.g., organic anion transporting polypeptide (Oatp)) and excretion of drugs from the liver (e.g., multidrug resistance protein 1 (Mdr1)). In this study, cryopreserved rat hepatocytes were used to investigate the effect of different culture conditions (suspension, conventional culture and sandwich culture) on the uptake of [(3)H]-taurocholate+/-probenecid and the efflux of [(3)H]-vinblastine+/-ketoconazole; mRNA levels of Oatp1a1, Oatp1a4, Mdr1a and Mdr1b were determined using real-time reverse transcription polymerase chain reaction (RT-PCR) and protein expression of Mdr was assessed by immunocytochemistry. The uptake of [(3)H]-taurocholate was higher in cryopreserved rat hepatocytes maintained in suspension as compared to hepatocytes in culture. A significant time dependent decline in the uptake of [(3)H]-taurocholate was noticed from day 2 to day 4 in conventional and sandwich cultures. [(3)H]-taurocholate uptake was significantly reduced using the inhibitor probenecid. Oatp mRNA expression in hepatocytes in suspension was similar to that of liver, whereas much lower levels were detected in the cultures; this was in accordance with [(3)H]-taurocholate uptake results. Mdr1 activity was assessed by accumulation of the Mdr1 selective substrate, [(3)H]-vinblastine, in hepatocytes using ketoconazole as an inhibitor. The results showed Mdr1 activity in cryopreserved rat hepatocytes in conventional and sandwich cultures. A time dependent increase in Mdr1 activity was noticed from day 2 to day 4. Mdr1 activity was not found using hepatocytes in suspension. Mdr1 mRNA expression was high in cryopreserved hepatocytes from both culture systems. Immunocytochemistry showed the Mdr protein in membranes of hepatocytes in culture as well as in that of hepatocytes in liver sections. In conclusion, the present study showed that cryopreserved rat hepatocytes maintained canalicular transport activity (Mdr1) and basolateral transport activity. Hepatocytes in suspension had a higher uptake of taurocholate with a high Oatp (1a1 and 1a4) mRNA expression as compared to hepatocytes in culture. The presence of Mdr1 in both conventional and sandwich culture was confirmed at mRNA level, by protein expression as well as transport activity.

Slamf1, the NKT Cell Control Gene Nkt1

Invariant NKT cells play a critical role in controlling the strength and character of adaptive immune responses. We have previously reported deficiencies in the numbers and function of NKT cells in the NOD mouse strain, which is a well-validated model of type 1 diabetes and systemic lupus erythematosus. Genetic control of thymic NKT cell numbers was mapped to two linkage regions: Nkt1 on distal chromosome 1 and Nkt2 on chromosome 2. In this study, we report the production and characterization of a NOD.Nkrp1(b).Nkt1(b) congenic mouse strain, apply microarray expression analyses to limit candidate genes within the 95% confidence region, identify Slamf1 (encoding signaling lymphocyte activation molecule) and Slamf6 (encoding Ly108) as potential candidates, and demonstrate retarded signaling lymphocyte activation molecule expression during T cell development of NOD mice, resulting in reduced expression at the CD4(+)CD8(+) stage, which is consistent with decreased NKT cell production and deranged tolerance induction in NOD mice.

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.

The putative transmembrane segment 7 of human organic anion transporter hOAT1 dictates transporter substrate binding and stability

Human organic anion transporter hOAT1 plays a critical role in the body disposition of clinically important drugs. We examined the role of the putative transmembrane segment (TM) 7 in the function of hOAT1. Each residue within putative TM7 was replaced by alanine, and the uptake of para-aminohippurate was studied in cells expressing the mutants. We discovered four critical amino acid residues: Trp-346, Thr-349, Tyr-353, and Tyr-354. Substitution of Tyr-353 and Tyr-354 with alanine led to the loss of transport activity without affecting the surface expression of the transporter, whereas substitution of Trp-346 and Thr-349 with alanine lead to the loss of the total expression of the transporter. The effect of side chains of Tyr-353 and Tyr-354 on transporter functions were further evaluated by replacing these residues with Phe or Trp. Among all the mutants studied (Y353W, Y353F, Y354W, and Y354F), only mutant Y353F regained 30% transport activity, which was lost from replacement of Tyr-353 with alanine, suggesting that both the -OH group and the size of the side chain at positions 353 and 354 are critical for maintaining the full transport activity. To investigate the mechanisms underlying the loss of total protein expression when Trp-346 and Thr-349 were replaced with alanine, mutant-expressing cells were treated with lysosomal or proteasomal inhibitors. Our results showed that only proteasomal inhibitors resulted in the accumulation of mutant proteins, indicating that proteasome is involved in the degradation of the mutant transporters. Therefore, Trp-346 and Thr-349 are critically involved in the stability of the transporter.

Intra- and interpopulation genotype reconstruction from tagging SNPs

The optimal method to be used for tSNP selection, the applicability of a reference LD map to unassayed populations, and the scalability of these methods to genome-wide analysis, all remain subjects of debate. We propose novel, scalable matrix algorithms that address these issues and we evaluate them on genotypic data from 38 populations and four genomic regions (248 SNPs typed for approximately 2000 individuals). We also evaluate these algorithms on a second data set consisting of genotypes available from the HapMap database (1336 SNPs for four populations) over the same genomic regions. Furthermore, we test these methods in the setting of a real association study using a publicly available family data set. The algorithms we use for tSNP selection and unassayed SNP reconstruction do not require haplotype inference and they are, in principle, scalable even to genome-wide analysis. Moreover, they are greedy variants of recently developed matrix algorithms with provable performance guarantees. Using a small set of carefully selected tSNPs, we achieve very good reconstruction accuracy of "untyped" genotypes for most of the populations studied. Additionally, we demonstrate in a quantitative manner that the chosen tSNPs exhibit substantial transferability, both within and across different geographic regions. Finally, we show that reconstruction can be applied to retrieve significant SNP associations with disease, with important genotyping savings.

A three-dimensional model of human organic anion transporter 1: aromatic amino acids required for substrate transport

Organic anion transporters (OATs) play a critical role in the handling of endogenous and exogenous organic anions by excretory and barrier tissues. Little is known about the OAT three-dimensional structure or substrate/protein interactions involved in transport. In this investigation, a theoretical three-dimensional model was generated for human OAT1 (hOAT1) based on fold recognition to the crystal structure of the glycerol 3-phosphate transporter (GlpT) from Escherichia coli. GlpT and hOAT1 share several sequence motifs as major facilitator superfamily members. The structural hOAT1 model shows that helices 5, 7, 8, 10, and 11 surround an electronegative putative active site ( approximately 830A(3)). The site opens to the cytoplasm and is surrounded by three residues not previously examined for function (Tyr(230) (domain 5) and Lys(431) and Phe(438) (domain 10)). Effects of these residues on p-aminohippurate (PAH) and cidofovir transport were assessed by point mutations in a Xenopus oocyte expression system. Membrane protein expression was severely limited for the Y230A mutant. For the K431A and F438A mutants, [(3)H]PAH uptake was less than 30% of wild-type hOAT1 uptake after protein expression correction. Reduced V(max) values for the F438A mutant confirmed lower protein expression. In addition, the F438A mutant exhibited an increased affinity for cidofovir but was not significantly different for PAH. Differences in handling of PAH and cidofovir were also observed for the Y230F mutant. Little uptake was determined for cidofovir, whereas PAH uptake was similar to wild-type hOAT1. Therefore, the hOAT1 structural model has identified two new residues, Tyr(230) and Phe(438), which are important for substrate/protein interactions.

The flounder organic anion transporter fOat has sequence, function, and substrate specificity similarity to both mammalian Oat1 and Oat3

The flounder renal organic anion transporter (fOat) has substantial sequence homology to mammalian basolateral organic anion transporter orthologs (OAT1/Oat1 and OAT3/Oat3), suggesting that fOat may have functional properties of both mammalian forms. We therefore compared uptake of various substrates by rat Oat1 and Oat3 and human OAT1 and OAT3 with the fOat clone expressed in Xenopus oocytes. These data confirm that estrone sulfate is an excellent substrate for mammalian OAT3/Oat3 transporters but not for OAT1/Oat1 transporters. In contrast, 2,4-dichlorophenoxyacetic acid and adefovir are better transported by mammalian OAT1/Oat1 than by the OAT3/Oat3 clones. All three substrates were well transported by fOat-expressing Xenopus oocytes. fOat K(m) values were comparable to those obtained for mammalian OAT/Oat1/3 clones. We also characterized the ability of these substrates to inhibit uptake of the fluorescent substrate fluorescein in intact teleost proximal tubules isolated from the winter flounder (Pseudopleuronectes americanus) and killifish (Fundulus heteroclitus). The rank order of the IC(50) values for inhibition of cellular fluorescein accumulation was similar to that for the K(m) values obtained in fOat-expressing oocytes, suggesting that fOat may be the primary teleost renal basolateral Oat. Assessment of the zebrafish (Danio rerio) genome indicated the presence of a single Oat (zfOat) with similarity to both mammalian OAT1/Oat1 and OAT3/Oat3. The puffer fish (Takifugu rubripes) also has an Oat (pfOat) similar to mammalian OAT1/Oat1 and OAT3/Oat3 members. Furthermore, phylogenetic analyses argue that the teleost Oat1/3-like genes diverged from a common ancestral gene in advance of the divergence of the mammalian OAT1/Oat1, OAT3/Oat3, and, possibly, Oat6 genes.

Relative contribution of OAT1 and OAT3 transport activities in isolated perfused rabbit renal proximal tubules

The expression of both OAT1 and OAT3 along the isolated rabbit renal proximal tubule (RPT) was determined using RT-PCR. They were found to be very strong in S2 segment and weak in S1 and S3 segments. We further examined the relative transport activity of these transporters in isolated perfused rabbit RPT using [(3)H]para-aminohippurate ([(3)H]PAH), and estrone sulfate ([(3)H]ES) as specific substrates for rbOAT1 and rbOAT3, respectively. The transport activity of OAT1 was in the order S2>S1=S3 segments and that of OAT3 was in the order S1=S2>>S3 segments. The addition of alpha-ketoglutarate (100 muM) in the bathing medium increased both OAT1 and OAT3 transport activities in all segments of proximal tubule. The kinetics of [(3)H]succinic acid transport, used to measure the activity of sodium dicarboxylate transporter 3 (NaDC3), were examined. The J(max) for succinic acid was in the order S2>S3 and unmeasurable in the S1 segment. Our data indicate that both OAT1 and OAT3 play quantitatively significant roles in the renal transport of organic anions along the proximal tubule but predominately in S2 segment. The relative contribution of both transporters depends on their relative expression levels and may possibly be affected by the activity of NaDC3 in RPT.