Restored expression and activity of organic ion transporters rOAT1, rOAT3 and rOCT2 after hyperuricemia in the rat kidney

We previously reported that in hyperuricemic rats, renal impairment occurred and organic ion transport activity decreased, accompanied with a specific decrease in the expression of rat organic anion transporters, rOAT1 and rOAT3, and organic cation transporter, rOCT2. In the present study, we investigated the reversibility of the organic ion transport activity and expression of organic ion transporters (slc22a) during recovery from hyperuricemia. Hyperuricemia was induced by the administration of a chow containing uric acid and oxonic acid, an inhibitor of uric acid metabolism. Four days after discontinuance of the chow, the plasma uric acid concentration returned to the normal level, and renal functions such as creatinine clearance and BUN levels were restored, although the recovery of tubulointerstitial injury was varied in sites of the kidney. Basolateral uptake of p-aminohippurate (PAH) and tetraethylammonium (TEA), and both protein and mRNA levels of rOAT1, rOAT3 and rOCT2 in the kidney gradually improved during 14 days of recovery from hyperuricemia. Basolateral PAH transport showed a higher correlation with the protein level of rOAT1 (r(2)=0.80) than rOAT3 (r(2)=0.34), whereas basolateral TEA transport showed a strong correlation with rOCT2 protein (r(2)=0.91). The plasma testosterone concentration, which is a dominant factor in the regulation of rOCT2, was gradually restored during the recovery from hyperuricemia, but the correlation between the plasma testosterone level and rOCT2 protein expression in the kidney was not significant. These results suggest that the regulation of organic ion transporters, rOAT1, rOAT3 and rOCT2, by hyperuricemia is reversible, and the organic ion transport activity restores according to the expression levels of these transporters.     

Physiology,structure, and regulation of the cloned organic anion transporters

1.?The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity.

2.?This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression.

3.?Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues — drug–drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites — are discussed in the concluding Perspectives section.     

Association of brominated proteins and changes in protein expression in the rat kidney with subcarcinogenic to carcinogenic doses of bromate

The water disinfection byproduct bromate (BrO₃⁻) produces cytotoxic and carcinogenic effects in rat kidneys. Our previous studies demonstrated that BrO₃⁻ caused sex-dependent differences in renal gene and protein expression in rats and the elimination of brominated organic carbon in their urine. The present study examined changes in renal cell apoptosis and protein expression in male and female F344 rats treated with BrO₃⁻ and associated these changes with accumulation of 3-bromotyrosine (3-BT)-modified proteins. Rats were treated with 0, 11.5, 46 and 308 mg/L BrO₃⁻ in drinking water for 28 days and renal sections were prepared and examined for apoptosis (TUNEL-staining), 8-oxo-deoxyguanosine (8-oxoG), 3-BT, osteopontin, Kim-1, clusterin, and p-21 expression. TUNEL-staining in renal proximal tubules increased in a dose-related manner beginning at 11.5 mg BrO₃⁻/L in female rats and 46 mg/L in males. Increased 8-oxoG staining was observed at doses as low as 46 mg/L. Osteopontin expression also increased in a dose-related manner after treatment with 46 mg/L, in males only. In contrast, Kim-1 expression increased in a dose-related manner in both sexes, although to a greater extent in females at the highest dose. Clusterin and p21 expression also increased in a dose-related manner in both sexes. The expression of 3-BT-modified proteins only increased in male rats, following a pattern previously reported for accumulation of α-2_u-globulin. Increases in apoptosis in renal proximal tubules of male and female rats at the lowest doses suggest a common mode of action for renal carcinogenesis for the two sexes that is independent of α-2_u-globulin nephropathy.     

Decreased activity of basolateral organic ion transports in hyperuricemic rat kidney: roles of organic ion transporters,rOAT1, rOAT3 and rOCT2

We investigated organic anion and cation transport activity and the expression of several organic ion transporters in hyperuricemic rat kidney. Feeding oxonic acid, an inhibitor of uric acid metabolism, and uric acid for 10 days significantly increased plasma uric acid level. Plasma creatinine and blood urea nitrogen concentrations also increased in hyperuricemic rats, indicating impaired renal function. The accumulation of organic anions, p-aminohippurate (PAH) and methotrexate, and cations, tetraethylammonium (TEA) and cimetidine, into renal slices was markedly decreased, suggesting decreased transport activity for organic anions and cations at the basolateral membrane in the kidney. The expression levels of basolateral organic anion transporters rOAT1 and rOAT3, and organic cation transporter, rOCT2, significantly decreased in hyperuricemic rat kidney as assessed by mRNA and protein levels. In contrast, the expression of rOCT1 was unaltered by hyperuricemia at both mRNA and protein levels. Moreover, the mRNA expression of kidney-specific organic anion transporters, OAT-K1 and OAT-K2, and organic anion transporting polypeptide (oatp) 1, which localize at the brush-border membrane in the kidney, was unchanged in hyperuricemic rats. In conclusion, we showed decreased basolateral organic anion and cation transport activity, accompanied by a specific decrease in rOAT1, rOAT3 and rOCT2 expression in hyperuricemic rat kidney. These phenomena partly contribute to the changed renal disposition of organic anions and cations in hyperuricemia.     

Interaction of six protoberberine alkaloids with human organic cation transporters 1, 2 and 3

1.?Organic cation transporters (OCTs) play an important role in drug safety and efficacy. Protoberberine alkaloids are ubiquitous organic cations or weak bases with remarkable biological actives. This study was to elucidate the potential interaction of alkaloids (coptisine, jatrorrhizine, epiberberine, berberrubine, palmatine and corydaline) with OCTs using Madin–Darby canine kidney (MDCK) cells stably expressing human OCT1, OCT2 and OCT3.

2.?All the tested alkaloids significantly inhibited the uptake of MPP⁺, a model OCT substrate, in MDCK-hOCTs cells with the IC₅₀ of 0.931–9.65?μM. Additionally, coptisine, jatrorrhizine and epiberberine were substrates of all the hOCTs with the K_m of 0.273–5.80?μM, whereas berberrubine was a substrate for hOCT1 and hOCT2, but not for hOCT3, the K_m values were 1.27 and 1.66?μM, respectively. The transport capacity of coptisine in MDCK cells expressing the variants of hOCT1-P341L or hOCT2-A270S was significantly higher than that in wild-type (WT) cells with the Cl_int (V_max/K_m) of 379?±?7.4 and 433?±?5.7?μl/mg protein/min, respectively.

3.?The above data indicate that the tested alkaloids are potent inhibitors, and coptisine, jatrorrhizine, epiberberine and berberrubine are substrates of hOCT1, hOCT2 and/or hOCT3 with high affinity. In addition, the variants (OCT1-P341L and OCT2-A270S) possess higher transport capacity to coptisine than WT hOCTs.     

Differential contribution of organic cation transporters, OCT2 and MATE1, in platinum agent-induced nephrotoxicity

The mechanism of severe nephrotoxicity caused by cisplatin, but not carboplatin, oxaliplatin, and nedaplatin, is not fully understood. The renal accumulation and subsequent nephrotoxicity of platinum agents were examined in rats. Among these four drugs, only cisplatin induced nephrotoxicity at 2 days after its intraperitoneal administration. The urinary activity of N-acetyl-beta-D-glucosaminidase and expression of kidney injury molecule-1 mRNA and osteopontin were markedly enhanced in the cisplatin-treated rats. Although some markers were affected in the rats administered nedaplatin, only minor histological change was observed. The renal accumulation of cisplatin was much greater than that of the other drugs. In the in vitro study, the cellular accumulation of cisplatin and oxaliplatin was stimulated by the expression of rat (r) OCT2. Oxaliplatin was also transported by rOCT3. A luminal H(+)/organic cation antiporter, rMATE1 (multidrug and toxin extrusion) as well as human (h) MATE1 and hMATE2-K, stimulated the H(+)-gradient-dependent antiport of oxaliplatin, but not of cisplatin. Carboplatin and nedaplatin were not transported by these transporters. In conclusion, the nephrotoxicity of platinum agents was closely associated with their renal accumulation, which is determined by the substrate specificity of the OCT and MATE families.     

Association between tubular toxicity of cisplatin and expression of organic cation transporter rOCT2 (Slc22a2) in the rat

Cisplatin is an effective anticancer drug, but has its severe adverse effects, especially nephrotoxicity. The molecular mechanism of cisplatin-induced nephrotoxicity is still not clear. In the present study, we examined the role of rat (r)OCT2, an organic cation transporter predominantly expressed in the kidney, in the tubular toxicity of cisplatin. Using HEK293 cells stably expressing rOCT2 (HEK-rOCT2), we evaluated the cisplatin-induced release of lactate dehydrogenase and the uptake of cisplatin. The release of lactate dehydrogenase and the accumulation of platinum were greater in HEK-rOCT2 cells treated with cisplatin than in mock-transfected cells. Moreover, cimetidine and corticosterone, OCT2 inhibitors, inhibited the cytotoxicity and the transport of cisplatin in HEK-rOCT2 cells. Pharmacokinetics of cisplatin was investigated in male and female rats because the renal expression level of rOCT2 was higher in male than female rats. The renal uptake clearance of cisplatin was greater in male than female rats, while the hepatic uptake clearance was similar between the sexes. In addition, glomerular filtration rate and liver function were unchanged, but N-acetyl-β-d-glucosaminidase activity in the bladder urine and the urine volume were markedly increased 2 days after the administration of 2 mg/kg of cisplatin in male rats. Moreover, cisplatin did not induce the elevation of urinary N-acetyl-β-d-glucosaminidase activity in the castrated male rats whose renal rOCT2 level was lower than that of the sham-operated rats. In conclusion, the present results indicated that renal rOCT2 expression was the major determinant of cisplatin-induced tubular toxicity.     

Aristolochic acid-induced destruction of organic ion transporters and fatty acid metabolic disorder in the kidney of rats

Aristolochic acid (AA) nephropathy exhibits early proximal tubular injury and fatty acid metabolic disorder. In order to study the unrecognized abnormalities of organic ion transporters and fatty acid metabolism indicators in AA nephropathy, Wistar rats were orally administrated with vehicle, 10 and 20 mg/kg AA once daily for 7 days, respectively. At day 8, significant reduction of body weight and right kidney weight, as well as elevation of plasma blood urea nitrogen (BUN) levels, renal long-chain fatty acids (LCFAs), non-esterified fatty acids (NEFA) and triglycerides (TG) contents were observed in AA-treated rats, accompanying with down-regulation of renal rOAT1/3, rOCT1/2 and rOCTN1/2 expressions. OCTN2 particularly transports l-carnitine through cell membrane. AA treatment also induced a significant decrease of l-carnitine levels in renal cortex of rats. Down-regulation of peroxisome proliferator-activated receptor alpha (rPPARα) and carnitine acyltransferase 1 (rCPT1), and up-regulation of acetyl coenzyme A carboxylase 1/2 (rACC1/2) in renal cortex were detected in AA-treated rats. These results indicate that alterations of organic ion transportation and fatty acid metabolism are part of AA-induced nephropathy (AAN), contribute to the altered urinary metabolic profile and may lead to further proximal tubule injury in rats.     

Nitric oxide down-regulates the expression of organic cation transporters (OCT) 1 and 2 in rat kidney during endotoxemia

In the kidney, P-glycoprotein (Abcb1), an ATP-driven drug efflux pump, plays an important role in the detoxification of proximal tubule cells through the excretion of cationic and amphipathic organic compounds. We recently found that NO, produced by renal inducible NO synthase (iNOS), is involved in an up-regulation of P-glycoprotein during endotoxemia in rats. In the present study, we investigated the functional consequences of endotoxemia on the renal handling of rhodamine 123 by using isolated perfused rat kidneys. Wistar Hannover rats were injected intraperitoneally with 5 mg/kg body weight lipopolysaccharide (LPS) or with both LPS and the iNOS inhibitor, aminoguanidine. Despite an increased P-glycoprotein expression, we found a diminished urinary rhodamine 123 clearance 12 h after LPS (P<0.001). In addition, we found a diminished perfusate clearance (P<0.05) for rhodamine 123 after LPS treatment, suggesting a predominant role of influx carriers in urinary rhodamine 123 excretion. We examined the expression levels of organic cation transporter 1 (Slc22a1/Oct1) and Slc22a2/Oct2. Both appeared to be down-regulated at the mRNA and protein level, 12 h after LPS. Co-administration of aminoguanidine attenuated the down-regulation of both Oct1 and Oct2 protein expression and reversed the decrease in rhodamine 123 clearance (P<0.001). These findings indicate that NO, produced by iNOS, is responsible for a down-regulation of the influx carriers, Oct1 and Oct2.     

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

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

Downregulated expression of organic anion transporting polypeptide (Oatp) 2b1 in the small intestine of rats with acute kidney injury

The expression of transporters on the apical and basal membranes of renal tubular cells is modulated under acute kidney injury (AKI). However, little is known about alterations in non-renal transporters in the tissues other than the kidney under AKI situation. This study aimed to assess the modulation of organic anion transporting polypeptide (Oatp) 1a2 and Oatp2b1 expression/function in the small intestine of rats with drug-induced AKI. AKI was induced by intraperitoneal administration of cisplatin at a dose of 5 mg/kg. On day 3 after cisplatin administration, morphological changes in the small intestine, Oatp1a2 and Oatp2b1 expression, and absorption of pravastatin and theophylline were evaluated. Non-negligible atrophy was observed in the jejunum and ileum of the AKI rats. However, the absorption of theophylline was not affected. While intestinal Oatp2b1 expression was markedly decreased in the AKI rats, no alteration was observed in Oatp1a2 expression. The plasma levels of pravastatin after intraluminal administration declined significantly in the AKI rats. However, no such decline was observed after intravenous administration. This study suggested that the responses of intestinal Oatps to experimentally induced AKI was not unidirectional and that pravastatin absorption was governed more potently by Oatp2b1 than by Oatp1a2 in the rat intestine.     

High doses of simvastatin upregulate dopamine D1 and D2 receptor expression in the rat prefrontal cortex: possible involvement of endothelial nitric oxide synthase

This study aims to investigate whether or not long-term statin treatment causes upregulation of D1 and D2 receptor gene expression with concomitant increase in endothelial nitric oxide synthase (eNOS) expression in Sprague-Dawley rats. Serum triglyceride levels were dose dependently reduced in the simvastatin-treated rats reaching statistical significance at the highest dose (49% reduction), while pravastatin caused similar effects (52%) at the same dose. Cholesterol levels remained unchanged in both groups at all doses. Simvastatin, 10 or 30 mg kg(-1) day(-1), increased D1 and D2 receptor expressions in the prefrontal cortex. Similar upregulation was observed neither with simvastatin in the striatum nor with pravastatin in both brain regions. Simvastatin (10 mg kg(-1) day(-1)) also increased eNOS expression in the prefrontal cortex but not neuronal NOS or inducible NOS. D1 receptor activation by chloro-APB (5 microM) increased cAMP levels in synaptosomes prepared from the prefrontal cortex of control and simvastatin-treated rats by 88 and 285%, respectively. This effect was markedly attenuated by the selective D1 antagonist SCH-23390 (25 microM). D2 receptor activation by quinpirole (5 microM) had no effect on the basal cAMP levels in synaptosomes prepared from the prefrontal cortex of control and simvastatin-treated rats, while the same concentration of quinpirole completely abolished the D1 receptor-mediated increase. These results suggest that lipophilic statins can alter dopaminergic functions in the prefrontal cortex possibly via a central mechanism. The possibility of a nitric oxide mechanism involving eNOS requires further investigation.     

Effect of hyperosmotic conditions on flavin-containing monooxygenase activity,protein and mRNA expression in rat kidney

Flavin-containing monooxigenases (FMOs) are a polymorphic family of drug and pesticide metabolizing enzymes, found in the smooth endoplasmatic reticulum that catalyze the oxidation of soft nucleophilic heteroatom substances to their respective oxides. Previous studies in euryhaline fishes have indicated induction of FMO expression and activity in vivo under hyperosmotic conditions. In this study we evaluated the effect of hypersaline conditions in rat kidney. Male Sprague–Dawley rats were injected intraperitoneal with 3.5 M NaCl at a doses ranging from 0.3 cm³/100 g to 0.6 cm³/100 g in two separate treatments. Three hours after injection, FMO activities and FMO1 protein was examined in the first experiment, and the expression of FMO1 mRNA was measured in the second experiment from kidneys after treatment with NaCl. A positive significant correlation was found between FMO1 protein expression and plasma osmolarity (p < 0.05, r = 0.6193). Methyl-p-tolyl sulfide oxidase showed a statistically significant increase in FMO activity, and a positive correlation was observed between plasma osmolarity and production of FMO1-derived (R)-methyl-p-tolyl sulfoxide (p < 0.05, r = 0.6736). Expression of FMO1 mRNA was also positively correlated with plasma osmolality (p < 0.05, r = 0.8428). Similar to studies in fish, these results suggest that expression and activities of FMOs may be influenced by hyperosmotic conditions in the kidney of rats.     

Renal organic anion transporters OAT1 and OAT3 mediate the cellular accumulation of 5-sulfooxymethylfurfural, a reactive, nephrotoxic metabolite of the Maillard product 5-hydroxymethylfurfural

5-Hydroxymethylfurfural (HMF) is formed when sugars are acidified or heated. It is present at high levels in numerous foods. HMF is inactive in standard genotoxicity tests, but can be metabolized to a chemically reactive intermediate, 5-sulfooxymethylfurfural (SMF), which is mutagenic and carcinogenic. We recently found that direct parental administration of SMF to mice leads to abundant acute necrosis and proteinaceous casts in the proximal tubules as the dominating toxicological effect. Since proximal tubule cells actively mediate the excretion of many organic anions, we hypothesized that transporter-mediated uptake of SMF into the cells could be the reason for this selective organotoxicity. To test this hypothesis, we used human embryonic kidney (HEK293) cells stably expressing human (h) OAT1 or OAT3. SMF was a competitive inhibitor of p-aminohippurate uptake by hOAT1 and estrone sulfate uptake by hOAT3 with K_i values of 225 μM and 1.5 mM, respectively. Moreover, the initial rates of SMF uptake were 5.2- and 3.1-fold higher in cells expressing hOAT1 and hOAT3, respectively, than in control HEK293 cells. Likewise, the sensitivity of hOAT1- and hOAT3-expressing cells to SMF cytotoxicity was significantly higher than that of control cells, and was reduced by addition of probenecid, an inhibitor of OATs. Taken together, these results indicate that OAT1 and OAT3 mediate the uptake of SMF into proximal tubule cells and thereby may be involved in SMF-induced nephrotoxicity.     

Transport of the dopamine D2 agonist pramipexole by rat organic cation transporters OCT1 and OCT2 in kidney.

We investigated the mechanism of renal tubular secretion of the dopamine D2 receptor agonist pramipexole in rats, focusing on organic cation transporters 1 and 2. The uptake of [14C]pramipexole by Xenopus oocytes injected with complementary RNA of either rat organic cation transporter (rOCT) 1 or rOCT2 was significantly higher than that by water-injected oocytes: the kinetic parameters, K(m) and V(max), of pramipexole uptake were 49.5 muM and 234 pmol/60 min/oocyte for rOCT1, and 16.9 microM and 12.8 pmol/60 min/oocyte for rOCT2. Pramipexole was taken up into kidney slices in a time- and concentration-dependent manner, and Eadie-Hofstee plots revealed the involvement of two saturable components. The kinetic parameters, K(m1) and V(max1), of the high-affinity component were 12.9 microM and 10.7 nmol/15 min/g kidney, respectively. The uptake of [14C]pramipexole by rOCT1, rOCT2, and kidney slices was inhibited by procainamide and corticosterone, which are selective inhibitors of rOCT1 and rOCT2, respectively. The IC50 values of procainamide and corticosterone for the uptake of [14C]pramipexole by rOCT1, rOCT2, and kidney slices were 7.7, 167.0, and 47.0 microM and 163.7, 10.7, and 47.7 microM, respectively. These results demonstrate that both rOCT1 and rOCT2 are involved in the renal uptake of pramipexole across the basolateral membrane of the proximal tubular epithelial cells.     

Effects of 1α,25‐dihydroxyvitamin D3 on transporters and enzymes of the rat intestine and kidney in vivo

1α,25‐Dihydroxyvitamin D₃ (1,25(OH)₂D₃), the natural ligand of the vitamin D receptor (VDR), was found to regulate bile acid related transporters and enzymes directly and indirectly in the rat intestine and liver in vivo. The kidney is another VDR‐rich target organ in which VDR regulation on xenobiotic transporters and enzymes is ill‐defined. Hence, changes in protein and mRNA expression of nuclear receptors, transporters and enzymes of the rat intestine and kidney in response to 1,25(OH)₂D₃ treatment (0 to 2.56 nmol/kg/day intraperitoneally in corn oil for 4 days) were studied. In the intestine, protein and not mRNA levels of Mrp2, Mrp3, Mrp4 and PepT1 in the duodenum and proximal jejunum were induced, whereas Oat1 and Oat3 mRNA were decreased in the ileum after 1,25(OH)₂D₃ treatment. In the kidney, VDR, Cyp24, Asbt and Mdr1a mRNA and protein expression increased significantly (2‐ to 20‐fold) in 1,25(OH)₂D₃‐treated rats, and a 28‐fold increase of Cyp3a9 mRNA but not of total Cy3a protein nor Cyp3a1 and Cyp3a2 mRNA was observed, implicating that VDR played a significant, renal‐specific role in Cyp3a9 induction. Additionally, renal mRNA levels of PepT1, Oat1, Oat3, Ostα, and Mrp4, and protein levels of PepT1 and Oat1 were decreased in a dose‐dependent manner, and the ～50% concomitant reduction in FXR, SHP, HNF‐1α and HNF‐4α mRNA expression suggests the possibility of cross‐talk among the nuclear receptors. It is concluded that the effects of 1,25(OH)₂D₃ changes are tissue‐specific, differing between the intestine and kidney which are VDR‐rich organs. Copyright © 2009 John Wiley & Sons, Ltd.     

Functional characteristics and pharmacokinetic significance of kidney-specific organic anion transporters, OAT-K1 and OAT-K2, in the urinary excretion of anionic drugs

During the last decade, cDNA cloning has identified various gene families of drug transporters, and pharmacokinetic studies of drugs based on the molecular characteristics of transporters have advanced. We cloned and characterized two organic anion transporters OAT-K1 and OAT-K2 from the rat kidney. The expression of both transporters was limited to the kidney, especially the brush-border membranes of proximal tubules, with an apparent molecular mass of 40 kDa. Using MDCK or LLC-PK1 cells stably expressing OAT-K1, posttranslational cleavage was suggested to affect the membrane localization and functional characteristics; 50 kDa with multispecificity in the apical membrane of MDCK cells and 70 kDa with methotrexate specific transport in the basolateral membrane of LLC-PK1 cells. A wide variety of anionic compounds including methotrexate are bidirectionally transported via OAT-K1 and OAT-K2 across the apical membrane in the MDCK-transfectants. The urinary secretion of methotrexate was depressed in 5/6 nephrectomized rats in association with the selective loss of OAT-K1 and OAT-K2 expression, and both transporters were suggested to be target molecules for methotrexate-folinic acid rescue. In this review, recent advances in the study of OAT-K1 and OAT-K2 were summarized in comparison with other transporters.     

Effects of the diuretics furosemide,ethacrynic acid,and chlorothiazide on gluconeogenesis from various substrates in rat kidney cortex slices

Summary The effects of wide concentration ranges of furosemide, ethacrynic acid, and chlorothiazide on glucose formation by rat kidney cortex slices were investigated from fructose, pyruvate, lactate, and -ketoglutarate and partly at two different pH.Under control conditions without the drugs, more glucose was synthetized in moles/g dry weight · hour at the lower pH of 7.21 than at pH 7.49 without substrate as well as with the 5 substrates studied.Furosemide stimulated the rate of gluconeogenesis concentration dependent if fructose, pyruvate, or lactate were present as substrates. The percent increase was higher at pH 7.49 but the rates achieved with the maximal effective concentrations were about the same at both pH. The rate of glucose formation was only slightly enhanced from the substrates malate and -ketoglutarate. Ethacrynic acid enhanced the rate of glucose synthesis from fructose, pyruvate, and lactate, too, but had no stimulating effects, if malate or -ketoglutarate were the substrates. It exerted its effects at even lower concentrations than furosemide, but the maximal rates which could be observed were in the same range. Gluconeogenesis was not stimulated by chlorothiazide.Various and different sites of action on enzyme activities in the Embden-Meyerhof pathway are discussed for the two drugs influenced either directly by them or indirectly by changing concentrations of ATP or cyclic AMP.Preliminary reports of the results were presented in part at the joint meeting of the German and the Scandinavian Pharmacological Societies in Copenhagen 1971 and at the meeting of the Gesellschaft für Nephrologie in Aachen 1971.Supported by Deutsche Forschungsgemeinschaft Fu 45/4.     

Coordinate regulation of UDP-glucuronosyltransferase UGT1A6 induction by 3-methylcholanthrene and multidrug resistance protein MRP2 expression by dexamethasone in primary rat hepatocytes

Concentration-dependent regulation of 3-methylcholanthrene (MC) inducibility of UDP-glucuronosyltransferase UGT1A6 by the synthetic glucocorticoid, dexamethasone (DEX) was studied. Treatment of cultured rat hepatocytes with MC, 0.1, 1, and 10 microM DEX, and MC combined with DEX, resulted in different induction patterns measured in the intact cells compared to that observed in the microsomes prepared from the same cells. DEX treatment in various concentrations caused a concentration-dependent increase in p-nitrophenol (p-NP) conjugation in intact cells (3-, 4-, and 5-fold over control, respectively), and it positively regulated MC induction (4-, 5-, and 6-fold over control, respectively). In contrast, DEX had smaller effect on microsomal p-NP conjugation (115, 200, 220% of control, respectively) and although MC induction was increased significantly by 0.1 microM DEX (520% of control), but higher concentrations of DEX (10 microM) decreased the degree of induction to 410%. Similar results obtained from in vivo experiments showed that at high DEX concentration (100mg/kg), the rate of MC induction (540%) decreased (420%). Permeabilization of the plasma membrane resulted in a 15-fold increase of p-NP conjugation indicating the importance of transport in the rate of overall p-NP elimination, and the induction pattern was similar to that observed in microsomes isolated from cells. Hyper-osmolarity (405 mOsmol/L) led to a 3-fold decrease of p-NP conjugation, the loss of DEX inducibility and reduction of the MRP2 protein level. Our results suggest coordinated regulation of UGT1A6 inducibility and substrate or product transport by DEX.