Recent advances in molecular mechanisms of nephrotoxicity

Recent Advances in Synthetic Drugs and Natural Actives Interacting with OAT3

Organic anion transporter 3 (OAT3) is predominantly expressed in the kidney and plays a vital role in drug clearance. Consequently, co-ingestion of two OAT3 substrates may alter the pharmacokinetics of the substrate. This review summarizes drug-drug interactions (DDIs) and herbal-drug interactions (HDIs) mediated by OAT3, and inhibitors of OAT3 in natural active compounds in the past decade. This provides a valuable reference for the combined use of substrate drugs/herbs for OAT3 in clinical practice in the future and for the screening of OAT3 inhibitors to avoid harmful interactions.

Interactions of human organic anion transporters 1-4 and human organic cation transporters 1-3 with the stimulant drug methamphetamine and amphetamine

Drug membrane transport system proteins, namely, drug transporters, are expressed in the kidney and liver and play a crucial role in the excretion process. This study aimed to elucidate the interactions of the drug transporters human organic anion transporters 1, 2, 3, 4 (hOAT1, 2, 3, 4) and human organic cation transporters 1, 2, 3 (hOCT1, 2, 3), which are expressed primarily in human kidney, liver, and brain, with the stimulants methamphetamine (METH) and amphetamine (AMP). The results of an inhibition study using representative substrates of hOATs and hOCTs showed that METH and AMP significantly inhibited (by >50%) uptake of the hOCT1 and hOCT3 representative substrate 1-methy1-4-phenylpyridinium ion (MPP+) and hOCT2 representative substrate tetraethyl ammonium (TEA). However, METH and AMP did not inhibit uptake of the representative substrates of hOAT1, hOAT2, hOAT3, and hOAT4, (i.e., p-aminohippuric (PAH) acid, prostaglandin F2α (PGF2α), estron sulfate (ES), and ES respectively). Kinetic analyses revealed that METH competitively inhibited hOCT1-mediated MPP+ and hOCT2-mediated TEA uptake (Ki, 16.9 and 78.6 µM, respectively). Similarly, AMP exhibited competitive inhibition, with Ki values of 78.6 and 42.8 µM, respectively. In contrast, hOCT3 exhibited mixed inhibition of representative substrate uptake; hence, calculating Ki values was not possible. Herein, we reveal that hOCTs mediate the inhibition of METH and AMP. The results of this uptake study suggest that METH and AMP bind specifically to hOCT1 and hOCT2 without passing through the cell membrane, with subsequent passage of METH and AMP via hOCT3.

Introduction of Organic Anion Transporters (SLC22A) and a Regulatory Mechanism by Caveolins

The kidney is an important organ for controlling the volume of body fluids, electrolytic balance and excretion/reabsorption of endogenous and exogenous compounds. Among these renal functions, excretion/reabsorption of endogenous and exogenous substance is very important for the maintenance of physiological homeostasis in the body. Recently discovered organic anion transporters (OAT or SLC22A) have important roles for renal functions. It is well known as drug transporter. Several isoforms belong to SLC22A family. They showed different transport substrate spectrums and different localizations within the kidney. Their gene expressions are changed by some stimulus. The functional transport properties are regulated by protein kinase C. In addition, the function of organic anion transporters are also regulated by protein-protein interaction, such as caveolin which is compositional protein of caveolae structure. In this review, we will give an introduction of organic anion transporters and its regulatory mechanisms.

Effect of lycopene against cisplatin-induced acute renal injury in rats: organic anion and cation transporters evaluation

In the present study, we investigated the effects of lycopene on the expression of organic anion transporters (OATs), organic cation transporters (OCTs), and multidrug resistance-associated proteins (MRPs) of cisplatin-induced nephrotoxicity in rats. Twenty-eight 8-week-old Wistar rats were divided into four groups: control, lycopene-treated (6 mg/kg BW by oral gavage), cisplatin-treated (7 mg/kg BW, IP), and lycopene in combination with cisplatin-treated groups. In the presence of cisplatin, serum urea nitrogen (urea-N) (48.5 vs. 124.3 mg/dl) and creatinine (0.29 vs. 1.37 mg/dl) levels and the kidney efflux transporters MRP2 and MRP4 levels were significantly increased, whereas OAT1, OAT3, OCT1, and OCT2 levels in kidney were decreased in the treated rats compared with normal control rats. However, administration of lycopene in combination with cisplatin resulted in a reduction in the serum urea-N (124.3 vs. 62.4) and creatinine (1.37 vs. 0.40) levels and the kidney efflux transporters MRP2 and MRP4 proteins in the kidneys. Administration of lycopene to acute renal injury-induced rats largely upregulated the organic anion transporters (OAT1 and 3) and organic cation transporters (OCT1 and 2) to decrease the side effects of cisplatin. The present study suggests that lycopene synergizes with its nephroprotective effect against cisplatin-induced acute kidney injury in rats.

Mixed organic solvents induce renal injury in rats

To investigate the injury effects of organic solvents on kidney, an animal model of Sprague-Dawley (SD) rats treated with mixed organic solvents via inhalation was generated and characterized. The mixed organic solvents consisted of gasoline, dimethylbenzene and formaldehyde (GDF) in the ratio of 2∶2:1, and were used at 12,000 PPM to treat the rats twice a day, each for 3 hours. Proteinuria appeared in the rats after exposure for 5–6 weeks. The incidences of proteinuria in male and female rats after exposure for 12 weeks were 43.8% (7/16) and 25% (4/16), respectively. Urinary N-Acetyl-β-(D)-Glucosaminidase (NAG) activity was increased significantly after exposure for 4 weeks. Histological examination revealed remarkable injuries in the proximal renal tubules, including tubular epithelial cell detachment, cloud swelling and vacuole formation in the proximal tubular cells, as well as proliferation of parietal epithelium and tubular reflux in glomeruli. Ultrastructural examination found that brush border and cytoplasm of tubular epithelial cell were dropped, that tubular epithelial cells were partially disintegrated, and that the mitochondria of tubular epithelial cells were degenerated and lost. In addition to tubular lesions, glomerular damages were also observed, including segmental foot process fusion and loss of foot process covering on glomerular basement membrane (GBM). Immunofluorescence staining indicated that the expression of nephrin and podocin were both decreased after exposure of GDF. In contrast, increased expression of desmin, a marker of podocyte injury, was found in some areas of a glomerulus. TUNEL staining showed that GDF induced apoptosis in tubular cells and glomerular cells. These studies demonstrate that GDF can induce both severe proximal tubular damage and podocyte injury in rats, and the tubular lesions appear earlier than that of glomeruli.

Deletion of multispecific organic anion transporter Oat1/Slc22a6 protects against mercury-induced kidney injury

The primary site of mercury-induced injury is the kidney due to uptake of the reactive Hg(2+)-conjugated organic anions in the proximal tubule. Here, we investigated the in vivo role of Oat1 (organic anion transporter 1; originally NKT (Lopez-Nieto, C. E., You, G., Bush, K. T., Barros, E. J., Beier, D. R., and Nigam, S. K. (1997) J. Biol. Chem. 272, 6471-6478)) in handling of known nephrotoxic doses of HgCl(2). Oat1 (Slc22a6) is a multispecific organic anion drug transporter that is expressed on the basolateral aspects of renal proximal tubule cells and that mediates the initial steps of elimination of a broad range of endogenous metabolites and commonly prescribed pharmaceuticals. Mercury-induced nephrotoxicity was observed in a wild-type model. We then used the Oat1 knock-out to determine in vivo whether the renal injury effects of mercury are mediated by Oat1. Most of the renal injury (both histologically and biochemically as measured by blood urea nitrogen and creatinine) was abolished following HgCl(2) treatment of Oat1 knock-outs. Thus, acute kidney injury by HgCl(2) was found to be mediated mainly by Oat1. Our findings raise the possibility that pharmacological modulation of the expression and/or function of Oat1 might be an effective therapeutic strategy for reducing renal injury by mercury. This is one of the most striking phenotypes so far identified in the Oat1 knock-out. (Eraly, S. A., Vallon, V., Vaughn, D. A., Gangoiti, J. A., Richter, K., Nagle, M., Monte, J. C., Rieg, T., Truong, D. M., Long, J. M., Barshop, B. A., Kaler, G., and Nigam, S. K. (2006) J. Biol. Chem. 281, 5072-5083).

Renal expression of organic anion transporter OAT2 in rats and mice is regulated by sex hormones

The renal reabsorption and/or excretion of various organic anions is mediated by specific organic anion transporters (OATs). OAT2 (Slc22a7) has been identified in rat kidney, where its mRNA expression exhibits gender differences [females (F) > males (M)]. The exact localization of OAT2 protein in the mammalian kidney has not been reported. Here we studied the expression of OAT2 mRNA by RT-PCR and its protein by Western blotting (WB) and immunocytochemistry (IC) in kidneys of adult intact and gonadectomized M and F, sex hormone-treated castrated M, and prepubertal M and F rats, and the protein in adult M and F mice. In adult rats, the expression of OAT2 mRNA was predominant in the outer stripe (OS) tissue, exhibiting 1) gender dependency (F > M), 2) upregulation by castration and downregulation by ovariectomy, and 3) strong downregulation by testosterone and weak upregulation by estradiol and progesterone treatment. A polyclonal antibody against rat OAT2 on WB of isolated renal membranes labeled a approximately 66-kDa protein band that was stronger in F. By IC, the antibody exclusively stained brush border (BB) of the proximal tubule S3 segment (S3) in the OS and medullary rays (F > M). In variously treated rats, the pattern of 66-kDa band density in the OS membranes and the staining intensity of BB in S3 matched the mRNA expression. The expression of OAT2 protein in prepubertal rats was low and gender independent. In mice, the expression pattern largely resembled that in rats. Therefore, OAT2 in rat (and mouse) kidney is localized to the BB of S3, exhibiting gender differences (F > M) that appear in puberty and are caused by strong androgen inhibition and weak estrogen and progesterone stimulation.

The multispecific organic anion transporter family: properties and pharmacological significance

Physiological and pharmacological studies indicate that the renal and hepatic organic anion transport systems are responsible for the elimination of numerous compounds, such as drugs, environmental substances and metabolites of both endogenous and exogenous origins. Recently, the molecular identity of the organic anion transport system, the OAT family, was revealed. To date, six OAT members have been identified and shown to have important roles not only in detoxification in the kidneys, liver and brain, but also in the reabsorption of essential compounds such as urate. The OAT family members are closely associated with the pharmacokinetics, drug-drug interactions and toxicity of anionic substances such as nephrotoxic drugs and uremic toxins. The molecular characterization of the OAT family encoded by SLC22A will be discussed.

Towards an understanding of organic anion transporters: Structure-function relationships

Organic anion transporters (OAT) play essential roles in the body disposition of clinically important anionic drugs, including anti-viral drugs, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories. The activities of OATs are directly linked to drug toxicity and drug-drug interactions. So far, four members of the OAT family have been identified: OAT1, OAT2, OAT3, and OAT4. These transporters share several common structural features including 12 transmembrane domains, multiple glycosylation sites localized in the first extracellular loop between transmembrane domains 1 and 2, and multiple phosphorylation sites present in the intracellular loop between transmembrane domains 6 and 7, and in the carboxyl terminus. The impact of these structural features on the function of these transporters has just begun to be explored. In the present review, the author will summarize recent progress made from her laboratory as well as from others, on the molecular characterization of the structure-function relationships of OATs, including particular amino acid residues/regions of the transporter protein ("molecular domains") that potentially determine transport characteristics.

Structure, function, and regulation of renal organic anion transporters

Renal elimination of anionic drugs, xenobiotics, and toxins is necessary for the survival of mammalian species. This process is mediated by vectorial transport from blood to urine through the cooperative functions of specific transporters in the basolateral and apical membranes of the proximal tubule epithelium. The first step of this process is the extraction of organic anions from the peritubular blood plasma into proximal tubule cells largely through the organic anion transporter (OAT) pathway. Therefore, the OAT pathway is one of the major sites for body drug clearance/detoxification. As a result, it is also the site for drug-drug interaction and drug-induced nephrotoxicity. To maximize therapeutic efficacy and minimize toxicity, the structure-function relationships of OATs and their regulation must be defined. The recent cloning and identification of OATs have paved the way for such investigations. This review summarizes the available data on the general properties of OATs, focusing in particular on the recent progress made from the author's laboratory as well as from other's, on the molecular characterization of the structure-function relationships of OATs and their regulatory mechanisms.

Species- and sex-specific variations in binding of ochratoxin A by renal proteins in vitro

The mycotoxin ochratoxin A (OTA) is a potent renal carcinogen in rodents and induces renal fibrosis in pigs. Furthermore, OTA has been associated with the development of renal tumors and nephropathies in humans. Large species- and sex-differences are observed in sensitivity toward OTA-mediated toxicity and carcinogenicity, yet neither the mechanism(s) resulting in OTA toxicity nor the reasons for the observed species- and sex-specificities are known. This paper investigated variations in OTA handling viz binding to renal proteins which could possibly explain the observed differences in OTA susceptibility in vivo and in vitro. The results obtained via a modification of a standard receptor-binding assay demonstrated the presence of at least one homogeneous binding component in renal cortical homogenates from pig, mouse, rat and humans. This component was shown to bind OTA in a specific and saturable manner. A range of compounds selected for their affinity for steroid receptors and/or for various known organic anion transporters were employed in a competition assay to answer the question whether this homogenous OTA binding component represents a steroid-like receptor component or one of the known organic anion transporters of the kidney. Although many of the compounds were able to compete with OTA for protein-binding, the competition patterns displayed a distinct species specificity and did not correspond to the competition patterns associated with presently known organic anion transporters of the kidney in the mouse, rat or human. The data thus suggests the presence of a new organic anion transporter or more likely, a cytosolic binding component of unknown function with high affinity and capacity for OTA binding in humans, rats, mice and possibly pigs.

Immunohistochemical evidence for the expression and induction of paraoxonase in rat liver, kidney, lung and brain tissue. Implications for its physiological role

Studies on the localization of paraoxonases (PON's) are of interest because of its involvement in both the detoxication of activated organophosphorus pesticides and in the prevention of peroxidative damage to phospholipids and cholesteryl-esters in LDL and HDL particles and cell membranes during the atherogenic process. In the present study, we have investigated the cellular localization of PON1 by immunohistochemistry in different rat tissues. The protein was mainly detected in the endothelial lining of every tissue studied (liver, kidney, lung and brain). Besides, it was found in hepatocytes from the centrolobular region of the liver, in the glomeruli and basal pole of the proximal convoluted tubule of the kidney, in cells from bronchiolar epithelium and type I pneumocytes of the lung, and in leptomeningeal cells, ependymal cells and ventricular side of choroid plexus cells of the brain. However, neurons and glia lacked immunostaining. After 3-methylcholanthrene induction an increase in the intensity of immunostaining was observed in the same areas, as well as an additional staining in midzonal hepatocytes. On the basis of the tissue distribution observed for PON1, it is proposed that this enzyme might have a function related to the inactivation of oxidative stress by-products (either at a cellular level or blood-vessel wall) and other environmental chemicals. At present it has not yet been established whether the paraoxonase detected in the various tissues is truly a product of the PON1 gene or could represent products of the PON2 or PON3 genes.

Intracellular glutathione regulates taurocholate transport in HepG2 cells

The hepatic organic anion transporter 1, Oatp1, was recently demonstrated to function as a GSH exchanger, indicating that hepatic uptake of drugs and xenobiotics may be sensitive to intracellular GSH levels. The present study characterized taurocholate uptake and efflux mechanisms in HepG2 cells and the effects of intracellular GSH on these transport processes. Taurocholate uptake into HepG2 cells was Na(+)-independent, saturable ( K(m) = 82 +/- 16 microM), and was cis-inhibited by bromosulfophthalein and some bile acids. Intracellular GSH depletion inhibited 3H-taurocholate uptake, and, conversely, the release of GSH from HepG2 cells was stimulated in the presence of extracellular taurocholate and other bile acids, consistent with a role for intracellular GSH in stimulating organic anion uptake. Interestingly, efflux of 3H-taurocholate from HepG2 cells was also sensitive to intracellular GSH concentration: efflux was inhibited in cells with lower intracellular GSH and stimulated in cells with higher GSH. RT-PCR analysis revealed that OATP-A, OATP-D, OATP-E, OATP-8, MRP1, MRP2, and MRP3 are expressed in HepG2 cells but that their expression is not altered by the maneuvers used to lower or raise intracellular GSH. These results provide direct evidence that intracellular GSH levels modulate both uptake and efflux of taurocholate and suggest that GSH plays a regulatory role in the hepatobiliary transport of potentially toxic organic compounds.

[Molecular identification of the multispecific organic anion transporter family (the OAT family): the role in the pharmacokinetics and toxicokinetics]

The multispecific organic anion transporters have been indicated to be involved in the transmembrane transport of various anionic substances. The kidney and liver possess the distinct organic anion transport pathways for the elimination of potentially toxic anionic drugs and metabolites. In the kidney, proximal tubular cells actively excrete organic anions of both endogenous and exogenous origin. We have isolated the renal multispecific organic anion transporter, OAT1 (organic anion transporter 1), from the rat kidney. OAT1 is a 551-amino acid residue protein with 12 putative membrane spanning domains. OAT1 mediates sodium-independent, anion exchange for a variety of organic anions including p-aminohippurate, cyclic nucleotides, prostanoides, dicarboxylates, and anionic drugs including beta-lactams, non-steroidal antiinflammatory drugs, diuretics and antiviral drugs. So far, three other isoforms have been identified. OATs comprise a new family of multispecific organic anion transporter, i.e., the OAT family. OATs show weak structural similarity to organic cation transporters (OCTs) and OCTN/carnitine transporters. All of the members of the OAT family are commonly expressed in the kidney, suggesting its significance in the renal organic anion excretion. In addition, OAT members appear to be responsible for the distribution/elimination of water soluble anionic drugs into/from the liver, brain and fetus.

Mechanism of electrogenic cation transport by the cloned organic cation transporter 2 from rat

The organic cation transporter 2 (OCT2) is expressed in plasma membranes of kidney and brain. Its transport mechanism and substrates are debated. We studied substrate-induced changes of electrical current with the patch clamp technique after expression of rat OCT2 in oocytes. Activation of current, corresponding to efflux, was observed for small organic cations, e.g. choline. In contrast, the bigger cations quinine and tetrabutylammonium elicited no change in current. However, transport of choline could be inhibited by applying quinine or tetrabutylammonium to the cytoplasmic side. Inhibition of organic cation efflux by quinine was competitive with substrates. Quinine at the inside also inhibited substrate influx from the outside. Current-voltage analysis showed that both maximal turnover and apparent affinity to substrates are voltage-dependent. Substrate-induced currents with organic cations on both membrane sides reversed as predicted from the Nernst potential. Our results clearly identify the electrochemical potential as driving force for transport at neutral pH and exclude an electroneutral H(+)/organic cation(+) exchange. We suggest the existence of an electroneutral organic cation(+) exchange and propose a model for a carrier-type transport mechanism.