Rabbit Renal Proximal Tubule Research Articles

Abstract 4364: Oncometabolite L-2-hydroxyglutarate blocks differentiation of renal proximal tubule cells in matrigel

Abstract Previously, the Sudarshan laboratory has identified elevations in the level of the oncometabolite L-2-hydroxyglutarate (L-2HG) in human renal cell carcinomas (RCCs). Of particular interest in these regards, both L-2HG inhibits 2-oxoglutarate-dependent dioxygenases (2-OGDs), including histone demethylases, which regulate the epigenetic landscape of cells, and cellular differentiation. Thus, it was of interest to determine whether the accumulation of L-2HG alters the differentiation of the normal renal proximal tubule (RPT), ultimately affecting the phenotype of the RCC. Towards these ends, L-2HG dehydrogenase (L2HGDH) was knocked down in primary cultures of normal rabbit RPT cells, and their capacity for cellular differentiation was examined. Initially, we examined the ability of primary RPT cells to form tubules in matrigel following an L2HGDH knockdown (KD). While tubulogenesis was stimulated by Epidermal Growth Factor (EGF) in primary RPT cells transduced with a control lentiviral vector, tubulogenesis in matrigel was dramatically impaired in primary RPT cells when L2HGDH was knocked down by lentiviral L2HGDH shRNA. In order to determine whether the expression of differentiated transport functions was affected, RealTime PCR (RTPCR) was conducted. The results indicated that an L2HGDH knockdown (80%) resulted in a reduction in the expression of the Na+/Pi cotransporter NaPi2a (81%), the Na+/glucose cotransporter SGLT2 (88%), the water transporter Aquaporin 1 (AQP1) (95%), and the Na,K-ATPase ß1 subunit (atp1b1) (43%), whereas the expression of the p-Aminohippurate transporter OAT1 was not significantly affected. Similar results were obtained when using L2HGDH siRNA and lentiviral shRNA. Not only is tissue architecture critical for the maintenance of functional differentiation, but alterations in tissue architecture are a necessary component of tumor formation. Thus, we are examining the underlying causes for the reduced tubulogenesis in matrigel cultures with an L2HGDH KD. In our initial studies, we examined the expression of differentiated transport functions in matrigel vs. monolayer cultures. Notably, while the expression such transporters as AQP2 increased dramatically in matrigel (as opposed to monolayer cultures), the expression of other transporters such as NaPi2a, SGLT2 and atp1b1 was affected to a much smaller extent. Of particular interest in these regards, is the known role of AQP1 in the migration of renal proximal tubule cells, tumor spread, and wound healing. We are currently studying the effects of an L2HGDH KD on the expression of such genes as AQP1 during tubulogenesis in matrigel.This work has been funded by Grant # 1RO1CA200653-01A1 to Dr. Sunil Sudarshan (PI) with a subaward to Dr. Mary Taub. Citation Format: Mary L. Taub, Sunil Sudarshan. Oncometabolite L-2-hydroxyglutarate blocks differentiation of renal proximal tubule cells in matrigel [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4364.

Far infrared radiation promotes rabbit renal proximal tubule cell proliferation and functional characteristics, and protects against cisplatin-induced nephrotoxicity.

Far infrared radiation, a subdivision of the electromagnetic spectrum, is beneficial for long-term tissue healing, anti-inflammatory effects, growth promotion, sleep modulation, acceleration of microcirculation, and pain relief. We investigated if far infrared radiation is beneficial for renal proximal tubule cell cultivation and renal tissue engineering. We observed the effects of far infrared radiation on renal proximal tubules cells, including its effects on cell proliferation, gene and protein expression, and viability. We also examined the protective effects of far infrared radiation against cisplatin, a nephrotoxic agent, using the human proximal tubule cell line HK-2. We found that daily exposure to far infrared radiation for 30 min significantly increased rabbit renal proximal tubule cell proliferation in vitro, as assessed by MTT assay. Far infrared radiation was not only beneficial to renal proximal tubule cell proliferation, it also increased the expression of ATPase Na+/K+ subunit alpha 1 and glucose transporter 1, as determined by western blotting. Using quantitative polymerase chain reaction, we found that far infrared radiation enhanced CDK5R1, GNAS, NPPB, and TEK expression. In the proximal tubule cell line HK-2, far infrared radiation protected against cisplatin-mediated nephrotoxicity by reducing apoptosis. Renal proximal tubule cell cultivation with far infrared radiation exposure resulted in better cell proliferation, significantly higher ATPase Na+/K+ subunit alpha 1 and glucose transporter 1 expression, and significantly enhanced expression of CDK5R1, GNAS, NPPB, and TEK. These results suggest that far infrared radiation improves cell proliferation and differentiation. In HK-2 cells, far infrared radiation mediated protective effects against cisplatin-induced nephrotoxicity by reducing apoptosis, as indicated by flow cytometry and caspase-3 assay.

In vitro studies with renal proximal tubule cells show direct cytotoxicity of Androctonus australis hector scorpion venom triggered by oxidative stress, caspase activation and apoptosis

Scorpion envenomation injures a number of organs, including the kidney. Mechanisms proposed to explain the renal tubule injury include direct effects of venom on tubule epithelial cells, as well as indirect effects of the autonomic nervous system, and inflammation. Here, we report direct effects of Androctonus australis hector (Aah) scorpion venom on the viability of Renal Proximal Tubule (RPT) cells in vitro, unlike distal tubule and collecting duct cells. Extensive NucGreen nuclear staining was observed in immortalized rabbit RPT cells following treatment with Aah venom, consistent with cytotoxicity. The involvement of oxidative stress is supported by the observations that 1) anti-oxidants mitigated the Aah venom-induced decrease in the number of viable RPT cells, and 2) Aah venom-treated RPT cells were intensively stained with the CellROX® Deep Red reagent, an indicator of Reactive Oxygen Species (ROS). Relevance to normal RPT cells is supported by the red fluorescence observed in Aah venom treated primary rabbit RPT cell cultures following their incubation with the Flica reagent (indicative of caspase activation and apoptosis), and the green fluorescence of Sytox Green (indicative of dead cells).

The multidrug transporter MATE1 sequesters OCs within an intracellular compartment that has no influence on OC secretion in renal proximal tubules.

Secretion of organic cations (OCs) across renal proximal tubules (RPTs) involves basolateral OC transporter (OCT)2-mediated uptake from the blood followed by apical multidrug and toxin extruder (MATE)1/2-mediated efflux into the tubule filtrate. Whereas OCT2 supports electrogenic OC uniport, MATE is an OC/H(+) exchanger. As assessed by epifluorescence microscopy, cultured Chinese hamster ovary (CHO) cells that stably expressed human MATE1 accumulated the fluorescent OC N,N,N-trimethyl-2-[methyl(7-nitrobenzo[c][l,2,5]oxadiazol-4-yl)amino]ethanaminium (NBD-MTMA) in the cytoplasm and in a smaller, punctate compartment; accumulation in human OCT2-expressing cells was largely restricted to the cytoplasm. A second intracellular compartment was also evident in the multicompartmental kinetics of efflux of the prototypic OC [(3)H]1-methyl-4-phenylpyridinium (MPP) from MATE1-expressing CHO cells. Punctate accumulation of NBD-MTMA was markedly reduced by coexposure of MATE1-expressing cells with 5 μM bafilomycin (BAF), an inhibitor of V-type H(+)-ATPase, and accumulation of [(3)H]MPP and [(3)H]NBD-MTMA was reduced by >30% by coexposure with 5 μM BAF. BAF had no effect on the initial rate of MATE1-mediated uptake of NBD-MTMA, suggesting that the influence of BAF was a secondary effect involving inhibition of V-type H(+)-ATPase. The accumulation of [(3)H]MPP by isolated single nonperfused rabbit RPTs was also reduced >30% by coexposure to 5 μM BAF, suggesting that the native expression in RPTs of MATE protein within endosomes can increase steady-state OC accumulation. However, the rate of [(3)H]MPP secretion by isolated single perfused rabbit RPTs was not affected by 5 μM BAF, suggesting that vesicles loaded with OCs(+) are not likely to recycle into the apical plasma membrane at a rate sufficient to provide a parallel pathway for OC secretion.

MATE1 sequesters organic cations within an intracellular, bafilomycin‐sensitive compartment (892.39)

OC secretion across renal proximal tubules (RPTs) involves basolateral OCT2‐mediated uptake from the blood, followed by apical MATE1/2‐mediated efflux into the tubule filtrate. Whereas OCT2 is an electrogenic OC uniporter, MATE is an OC/H exchanger. Epifluorescence microscopy of hMATE1‐expressing CHO cells revealed accumulation of the fluorescent OC, N,N,N‐trimethyl‐2‐[methyl(7‐nitrobenzo[c][l,2,5]oxadiazol‐4‐yl)amino]ethanaminium (NBD‐MTMA) in the cytoplasm and in a smaller, punctate compartment; accumulation in OCT2 expressing cells was restricted to the cytoplasm. A second intracellular compartment was also evident in the multicompartmental kinetics of NBD‐MTMA efflux from MATE1‐CHO cells. Punctate accumulation (20 min) was markedly reduced by coexposure of MATE1‐CHO cells with 1 μM bafilomycin (BAF) and eliminated with 10 μM BAF. BAF had no effect on the initial rate of MATE1‐mediated uptake of NBD‐MTMA (10‐120 sec) suggesting that its effect involved collapse of endosomal pH gradients via inhibition of the V‐type H‐ATPase. The 15 min accumulation of the prototypic OC, [³H]TEA by isolated single non‐perfused rabbit RPTs was also reduced 40% by coexposure to 1 μM BAF. Thus, the native expression in RPTs of MATE protein within endosomes can increase steady‐state OC accumulation. Normal rates of apical endosome turnover may provide a significant parallel pathway for apical OC effluxGrant Funding Source: NIH grant 5R01DK080801

Effect of acute acid-base disturbances on ErbB1/2 tyrosine phosphorylation in rabbit renal proximal tubules

The renal proximal tubule (PT) is a major site for maintaining whole body pH homeostasis and is responsible for reabsorbing ∼80% of filtered HCO3(-), the major plasma buffer, into the blood. The PT adapts its rate of HCO3(-) reabsorption (JHCO3(-)) in response to acute acid-base disturbances. Our laboratory previously showed that single isolated perfused PTs adapt JHCO3(-) in response to isolated changes in basolateral (i.e., blood side) CO2 and HCO3(-) concentrations but, surprisingly, not to pH. The response to CO2 concentration can be blocked by the ErbB family tyrosine kinase inhibitor PD-168393. In the present study, we exposed enriched rabbit PT suspensions to five acute acid-base disturbances for 5 and 20 min using a panel of phosphotyrosine (pY)-specific antibodies to determine the influence of each disturbance on pan-pY, ErbB1-specific pY (four sites), and ErbB2-specific pY (two sites). We found that each acid-base treatment generated a distinct temporal pY pattern. For example, the summated responses of the individual ErbB1/2-pY sites to each disturbance showed that metabolic acidosis (normal CO2 concentration and reduced HCO3(-) concentration) produced a transient summated pY decrease (5 vs. 20 min), whereas metabolic alkalosis produced a transient increase. Respiratory acidosis (normal HCO3(-) concentration and elevated CO2 concentration) had little effect on summated pY at 5 min but produced an elevation at 20 min, whereas respiratory alkalosis produced a reduction at 20 min. Our data show that ErbB1 and ErbB2 in the PT respond to acute acid-base disturbances, consistent with the hypothesis that they are part of the signaling cascade.

Activation of AMP kinase plays a role in the increased apoptosis in the renal proximal tubule in cystinosis

In cystinosis, renal proximal tubule (RPT) function is compromised, due to mutations in ctns, which encodes for the transporter cystinosin, which removes cystine from lysosomes. Altered RPT function in cystinosis has been attributed to decreased ATP, as well as increased apoptosis. In this report, the role of AMPK was examined. AMPK was activated in primary rabbit RPT cells with a cystinosin knockdown, using cystinosin siRNA. The activation of AMPK was associated with a 50% decrease in ATP and a 1.7‐fold increase in the ADP/ATP level. Cisplatin‐induced apoptosis also increased in primary RPT cells with a cystinosin knockdown. The role of AMPK in the increased sensitivity to cisplatin was examined. The increased sensitivity to cisplatin was prevented in primary RPT cells with a cystinosin knockdown by the AMPK inhibitor Compound C. The effect of siRNAs against AMPKα1 and AMPKα2 was also studied. The siRNAs knocked down AMPKα, and prevented AMPKα activation by 5‐aminoimidazole‐4‐carboxamide‐1‐β‐d‐ribofuranoside (AICAR). The siRNAs against AMPKα1 and AMPKα2 also prevented the increased sensitivity to cisplatin in the primary RPT cells with a cystinosin knockdown. These results suggest that signaling through AMPK plays a role in the enhanced apoptosis in the RPT in cystinosis. Funded by the Cystinosis Research Foundation to MLT.

Activation of AMP kinase plays a role in the increased apoptosis in the renal proximal tubule in cystinosis

In cystinosis, renal proximal tubule (RPT) function is compromised, due to mutations in ctns, which encodes for the transporter cystinosin, which removes cystine from lysosomes. Altered RPT function in cystinosis has been attributed to decreased ATP, as well as increased apoptosis. In this report, the role of AMPK was examined. AMPK was activated in primary rabbit RPT cells with a cystinosin knockdown, using cystinosin siRNA. The activation of AMPK was associated with a 50% decrease in ATP and a 1.7-fold increase in the ADP/ATP level. Cisplatin-induced apoptosis also increased in primary RPT cells with a cystinosin knockdown. The role of AMPK in the increased sensitivity to cisplatin was examined. The increased sensitivity to cisplatin was prevented in primary RPT cells with a cystinosin knockdown by the AMPK inhibitor Compound C. The effect of siRNAs against AMPKα1 and AMPKα2 was also studied. The siRNAs knocked down AMPKα, and prevented AMPKα activation by 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR). The siRNAs against AMPKα1 and AMPKα2 also prevented the increased sensitivity to cisplatin in the primary RPT cells with a cystinosin knockdown. These results suggest that signaling through AMPK plays a role in the enhanced apoptosis in the RPT in cystinosis.

Luminal transport of thiol S-conjugates of methylmercury in isolated perfused rabbit renal proximal tubules

Lumen-to-cell transport, cellular accumulation, and toxicity of l-cysteine (Cys), glutathione (GSH) and N-acetylcysteine (NAC) S-conjugates of methylmercury (CH3Hg+) were evaluated in isolated, perfused rabbit proximal tubular segments. When these conjugates were perfused individually through the lumen of S2 segments of the proximal tubule it was found that Cys-S-CH3Hg and GSH-S-CH3Hg were transported avidly, while NAC-S-CH3Hg was transported minimally. In addition, 95% of the 203Hg taken up by the tubular cells was associated with precipitable proteins of the tubule, while very little was found in the acid-soluble cytosol. No visual cellular pathological changes were observed during 30min of study. Luminal uptake of Cys-S-CH3Hg was temperature-dependent and inhibited significantly by the amino acids l-methionine and l-cystine. Rates of luminal uptake of GSH-S-CH3Hg were twice as great as that of Cys-S-CH3Hg and uptake was inhibited significantly (74%) by the presence of acivicin. When 2,3-bis(sulfanyl)propane-1-sulfonate (DMPS) was added to the bathing or luminal fluid, luminal uptake of Cys-S-CH3Hg was diminished significantly. Overall, our data indicate that Cys-S-CH3Hg is likely a transportable substrate of one or more amino acid transporters (such as system B0,+ and system b0,+) involved in luminal absorption of l-methionine and l-cystine along the renal proximal tubule. In addition, GSH-S-CH3Hg appears to be degraded enzymatically to Cys-S-CH3Hg, which can then be taken up at the luminal membrane. By contrast NAC-S-CH3Hg and Cys-S-CH3Hg (in the presence of DMPS) are not taken up avidly at the luminal membrane of proximal tubular cells, thus promoting the excretion of CH3Hg+ into the urine.

Evidence for Activation of AMP Kinase (AMPK) in Nephropathic Cystinosis

The objective of this study was to investigate the involvement of AMPK in the altered signaling that results in nephropathic cystinosis. Cystinosis is characterized by cysteine accumulation, due to mutations in CTNS, encoding for cystinosin, a lysosomal cystine transporter. Patients develop the Fanconi Syndrome, due to defective reabsorption by apical Na+/solute cotransport systems in the renal proximal tubule (RPT). In this study, cystinosin was knocked down >80% in primary rabbit RPT cells using cystinosin siRNA. This reduction in cystinosin strongly inhibited Na+ dependent Phosphate (Pi) uptake (70%). Intracellular Na+and Na,K‐ATPase were unaffected. However, ATP dropped 52%, the ADP/ATP ratio increased 300%, and apical NaPi2a transporters were reduced. Because increases in ADP result in AMPK phosphorylation and activation, the phosphorylated AMPKα subunit (pAMPKα) was examined. In primary RPT cells with a cystinosin knockdown the level of pAMPKα was elevated (8.0 +/− 4.2 fold relative to the control), and was unaffected by AICAR (5‐Aminoimidazole‐4‐Carboxamide‐1‐β‐D‐ribofuranoside), an AMPK activator (unlike control cells). Further studies are in progress to determine whether AMPK activation is responsible for the reduced growth, increased apoptotis, as well as reduced transport in primary RPT cells with a cystinosin knockdown. The funding was from the Cystinosis Research Foundation.

Abstract 3523: C7, C9-dimethylifosfamide analogs evaluation for antitumor activity and toxicity: Detoxification pathways might explain nephrotoxicity

Abstract Ifosfamide (IFO) is an anticancer drug, widely used against various types of human solid tumors. This prodrug produced active alkylating isophosphoramide mustard. But its side-chain oxidative metabolism is associated with toxicities in high-dose chemotherapy. As acrolein urotoxicity is limited by coadministration of mesna, chloroacetaldehyde (CAA) is responsible for neurotoxicity and nephrotoxicity. C7, C9 dimethyl-ifosfamide analogs have been synthesized to prevent these side effects. In vitro metabolism and anticancer activity investigations demonstrated that the generated dimethylated mustards were faster alkylating agents (28 fold faster) than isophosphoramide mustard and IC50 values on 9L glioblastoma cells showed higher cytotoxicity of analogs, especially the 7S,9S enantiomer (1). This later was selected for in vivo investigations. As these analogs were still metabolized through the same N-deschloroalkylation pathway, monoochloroacetone (MCA) is produced instead of CAA. In a first step, in vitro/ex vivo experiments were conducted to investigate its toxicity. The toxicity of both metabolites was compared by evaluation of cell viability using canine kidney cell lines (MDCK) and rabbit renal proximal tubules (RRPT). Cell viability experiments showed that similar IC50 values were obtained for CAA and MCA (21 and 23 µM) on MDCK. To determine detoxification pathways, several experiments were conducted on RRPT. Whereas these compounds could react directly with glutathione (GSH), identification of GSH and its conjugates (GSR) was performed by LC/MS after incubation of MDCK and RRPT with 200 µM CAA and MCA. Both enzymatic reduction and GSH conjugation pathways occurred. Conjugates were then quantified by LC-MS/MS over 1-650 µM for GSH and over 1-200 µM for GSR from 20 to 120 min. The results demonstrated a rapid depletion of GSH and few differences in detoxification's kinetics for CAA and MCA. As MCA in vivo generated levels are still unknown, 7S,9S dimethyl-IFO analog is currently under investigation on human tumor cells xenografted in mice in order to confirm its antitumor activity and to evaluate its toxicity. (1) Storme T, Deroussent A, Mercier L, Prost E, Re M, Munier F, Martens T, Bourget P, Vassal G, Royer J and Paci A. J Pharmacol Exp Ther. 2009, 328(2):598-609. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3523.

In isolated rabbit renal proximal tubules, the antibiotic cephaloridine does not inhibit the mitochondrial uptake of succinate, a biomarker of its nephrotoxicity: A cellular metabolomic study with carbon 13 NMR

Cephaloridine is a model nephrotoxicant. In isolated rabbit renal mitochondria, cephaloridine (i) inhibits succinate uptake and (ii) reduces respiration. We have re‐examined this problem using isolated proximal tubules. Tubules were incubated in Krebs‐Henseleit buffer containing either 1,4‐13C‐succinate, or 2,3‐13C‐succinate or succinate + 13C‐bicarbonate without or with cephaloridine. Substrate removal and product formation were measured by enzymatic and carbon 13 NMR methods. Enzymatic fluxes were calculated by combining these results with an original mathematical model of renal succinate metabolism. Succinate was converted mainly into fumarate, malate, glucose and CO2. Cephaloridine induced an increase in the accumulation of fumarate, malate and a decrease in the production of glucose, CO2 and the level of ATP. Surprisingly, it did not change the removal of succinate. Fluxes through malate dehydrogenase, glucose 6‐phosphatase and citrate synthase were also diminished by cephaloridine.In conclusion, the absence of inhibition of mitochondrial uptake of succinate in intact renal proximal tubules strongly suggests that results obtained with isolated mitochondria do not necessarily apply to mitochondria in intact cells and, therefore, should be interpreted with great caution. These results also suggest that mitochondrial functions should be studied in intact cells.

Knockdown of Lysosomal Cystine Transporter Causes the Fanconi Syndrome in Primary Renal Cells

The autosomal recessive disorder cystinosis is due to a defect in cystinosin, a lysosomal cystine transporter. The Fanconi Syndrome emerges in the kidney in this disorder, due to reduced reabsorption of phosphate (Pi) and other solutes by the renal proximal tubule (RPT). The hypothesis was examined that in RPT cells the defect in cystinosin not only causes a reduction in the activity of apical Na+ solute/cotransporters, but also results in increased apoptosis. In order to examine this hypothesis, siRNA was used to knock‐down cystinosin in primary rabbit RPT cells. Na+ dependent Pi and alpha methyl‐D‐glucoside uptake was reduced by more than 60% following a 3‐day incubation with cystinosin siRNA (and a 80% knock‐down of cystinosin) vs. scrambled (scr) controls. Na+ independent 3‐0‐methyl‐D‐glucoside uptake was unaffected. The sensitivity of primary RPT cells to apoptotic stimuli was examined. TNFα caused a 10.8 +/− 0.7 fold increase in caspase 3 activity in RPTs treated with cystinosin siRNA (2.4 fold higher than scr controls), and 20 μM cisplatin resulted in the death of 31 +/− 3% primary RPTs treated with cystinosin siRNA after 12 hr, a value 2.2 fold higher than in scr controls. Cystinosin siRNA also reduced cell growth by 45 +/− 10%. Further studies are in progress to determine whether these alterations can be explained by a change in the cellular redox state caused by reduced transport of cystine from lysosomes.

Regulation of renal proximal tubule Na-K-ATPase by prostaglandins

Prostaglandins (PGs) play a number of roles in the kidney, including regulation of salt and water reabsorption. In this report, evidence was obtained for stimulatory effects of PGs on Na-K-ATPase in primary cultures of rabbit renal proximal tubule (RPT) cells. The results of our real-time PCR studies indicate that in primary RPTs the effects of PGE(2), the major renal PG, are mediated by four classes of PGE (EP) receptors. The role of these EP receptors in the regulation of Na-K-ATPase was examined at the transcriptional level. Na-K-ATPase consists of a catalytic α-subunit encoded by the ATP1A1 gene, as well as a β-subunit encoded by the ATP1B1 gene. Transient transfection studies conducted with pHβ1-1141 Luc, a human ATP1B1 promoter/luciferase construct, indicate that both PGE(1) and PGE(2) are stimulatory. The evidence for the involvement of both the cAMP and Ca(2+) signaling pathways includes the inhibitory effects of the myristolylated PKA inhibitor PKI, the adenylate cyclase (AC) inhibitor SQ22536, and the PKC inhibitors Gö 6976 and Ro-32-0432 on the PGE(1) stimulation. Other effectors that similarly act through cAMP and PKC were also stimulatory to transcription, including norepinephrine and dopamine. In addition to its effects on transcription, a chronic incubation with PGE(1) was observed to result in an increase in Na-K-ATPase mRNA levels as well as an increase in Na-K-ATPase activity. An acute stimulatory effect of PGE(1) on Na-K-ATPase was observed and was associated with an increase in the level of Na-K-ATPase in the basolateral membrane.

Organic anion transporter 3 (OAT3) and renal transport of the metal chelator 2,3-dimercapto-1-propanesulfonic acid (DMPS)

Recent investigations involving intact rabbit renal proximal tubules indicated that organic anion transporter 3 (OAT3) may be involved in the transport of 2,3-dimercapto-1-propanesulfonic acid (DMPS). Therefore, we evaluated the interaction of OAT3 with DMPS to determine the effect of OAT3 on basolateral DMPS uptake. We used stably transfected HEK293 cells expressing human and rabbit orthologs of the exchanger OAT1 and OAT3. Using 6-carboxyfluorescein (6-CF) as a substrate, the IC50 determinations for reduced DMPS (DMPSH) revealed a stronger interaction with OAT1 than with OAT3 (rbOAT1, 123.3 +/- 13.7; hOAT1, 85.1 +/- 8.8; rbOAT3, 171.7 +/- 22.3; and hOAT3, 172.2 +/- 36.4 micromol/L). However, inhibition of 6-CF uptake by the oxidized form of DMPS (DMPSS), the main form of DMPS in the blood, showed a greater affinity for OAT3 (rbOAT1, 237.4 +/- 23; hOAT1, 104.6 +/- 13.1; rbOAT3, 52.4 +/- 7.6; and hOAT3, 31.6 +/- 6.6 micromol/L). To determine whether DMPSH and DMPSS are substrates for OAT3, we performed efflux studies with [14C]glutarate and inwardly directed gradients of glutarate. The inhibitors trans-stimulated the efflux of [14C]glutarate, suggesting that OAT3 may be able to transport both forms of DMPS. On the basis of the substantial interaction of OAT3 with DMPSS, we conclude that OAT3 represents the dominant basolateral player in renal detoxification processes resulting from use of DMPS.

Potential mechanisms involved in the absorptive transport of cadmium in isolated perfused rabbit renal proximal tubules

Lumen-to-cell transport, cellular accumulation, and toxicity of cadmium as ionic cadmium (Cd2+) or as the l-cysteine (Cys) or d,l-homocysteine (Hcy) S-conjugate of cadmium (Cys-S-Cd-S-Cys, Hcy-S-Cd-S-Hcy) were studied in isolated, perfused rabbit proximal tubular segments. When Cd2+ (0.73μM) or Cys-S-Cd-S-Cys (0.73μM) was perfused through the lumen of S2 segments of the proximal tubule, no visual evidence of cellular pathological changes was detected during 30min of study. Cd2+-transport was temperature-dependent and was inhibited by Fe2+, Zn2+, and elevated concentrations of Ca2+. Luminal uptake of Cys-S-Cd-S-Cys was also temperature-dependent and was inhibited by the amino acids l-cystine and l-arginine, while stimulated by l-methionine. Neither l-aspartate, l-glutamate, the synthetic dipeptide, Gly-Sar nor Zn2+ had any effect on the rate of Cys-S-Cd-S-Cys transport. Conclusions: When delivered to the luminal compartment, Cd2+ appears to be capable of utilizing certain transporter(s) of Zn2+ and some transport systems sensitive to Ca2+ and Fe2+. In addition, Cys-S-Cd-S-Cys and Hcy-S-Cd-S-Hcy appear to be transportable substrates of one or more amino acid transporters participating in luminal absorption of the amino acid l-cystine (such as system b0,+). These findings indicate that multiple mechanisms could be involved in the luminal absorption of cadmium (Cd) in proximal tubular segments depending on its form. These findings provide a focus for future studies of Cd absorption in the proximal tubule.

Cellular effect evaluation of micropollutants using transporter functions of renal proximal tubule cells

Issues pertaining to the effects of micropollutants in reclaimed water are arising in terms of their effect on human health. However, current cellular methodologies face some difficulties to detect subtle effects of waterborne micropollutants at environmental concentrations (ng L −1–μg L −1) on human and animal cells. In this study, an appropriate cellular model capable of detecting the subtle effects of aquatic micropollutants at environmental concentrations using the functions of primary cultured rabbit renal proximal tubule cells (PTCs) is proposed. Tris-(2-chloroethyl)-phosphate (TCEP) was chosen as the representative micropollutant from eight typical micropollutants via lactate dehydrogenase assay. TCEP significantly decreased not only ion (sodium, calcium, and phosphate) uptake from 10 −2 mg L −1 (64.8–82.5%, 60.4–68.8%, and 91.9–93.8% of the control, respectively), but also the expression of ion transporters (NHE-3 and L-type Ca channel) from 10 −2 mg L −1 (53.9–87.4% and 38.6–63.6% of the control, respectively). Moreover, TCEP significantly decreased both the non-ion (glucose, fructose, and l-arginine) uptake and the expression of non-ion transporters (SGLT 1, GLUT 5, and rBAT) from 10 −2 mg L −1. Therefore, the results demonstrated that the function of PTCs as a cellular model can be used to determine subtle effects of environmental micropollutants at low concentrations.

In isolated rabbit renal proximal tubules, the beta-lactam antibiotic cephaloridine does not inhibit the mitochondrial uptake of succinate, a biomarker of its nephrotoxicity: A cellular metabolomic study with carbon 13 NMR

In isolated rabbit renal proximal tubules, the beta-lactam antibiotic cephaloridine does not inhibit the mitochondrial uptake of succinate, a biomarker of its nephrotoxicity: A cellular metabolomic study with carbon 13 NMR

Regulation of Transcription of Renal Proximal Tubule Na,K‐ATPase by G‐Protein Coupled Receptors

Our previous studies indicate that transcription of the Na,K‐ATPase beta subunit gene in MDCK cells is regulated by Prostaglandins, via 2 Prostaglandin Regulatory Elements (PGREs). In order to determine whether 1) our studies with MDCK have a broader physiologic significance, and 2) similar regulation occurs in the renal proximal tubule (RPT), transient transfection studies were conducted with primary rabbit RPT cells. When transfecting using pH ?1‐1141Luc, a beta subunit promoter/luciferase construct, we observed a stimulation by PGE1 and PGE2 , as well as the Gs‐coupled EP2 and EP4 receptor agonists butaprost and PGE1‐OH (respectively). The PGE1 stimulation was inhibited by the antagonists SC51089 and AH6909 (which block Gq coupled EP1 receptors).Na,K‐ATPase in the renal proximal tubule is regulated by epinephrine and dopamine, which, like PGE, act via G‐ Protein Coupled Receptors (GPCRs). Indeed, 1 µM epinephrine stimulated transcription, as well as the alpha 1 and beta agonists phenylephrine and isoprenaline, respectively. Similarly, dopamine was stimulatory, an effect which was inhibited both by the D1 and D2 receptor antagonists, SCH23390 and L741, 626, respectively. These results are consistent with previously reported chronic stimulatory effects of dopamine on renal Na,K‐ATPase (in addition to acute inhibition), and may be mediated transcriptionally via PGREs.

Activation of Protein Kinase Cζ Increases OAT1 (SLC22A6)- and OAT3 (SLC22A8)-mediated Transport

Organic anion transporters (OATs) play a pivotal role in the clearance of small organic anions by the kidney, yet little is known about how their activity is regulated. A yeast two-hybrid assay was used to identify putative OAT3-associated proteins in the kidney. Atypical protein kinase Czeta (PKCzeta) was shown to bind to OAT3. Binding was confirmed in immunoprecipitation assays. The OAT3/PKCzeta interaction was investigated in rodent renal cortical slices from fasted animals. Insulin, an upstream activator of PKCzeta, increased both OAT3-mediated uptake of estrone sulfate (ES) and PKCzeta activity. Both effects were abolished by a PKCzeta-specific pseudosubstrate inhibitor. Increased ES transport was not observed in renal slices from OAT3-null mice. Transport of the shared OAT1/OAT3 substrate, rho-aminohippurate, behaved similarly, except that stimulation was reduced, not abolished, in the OAT3-null mice. This suggested that OAT1 activity was also modified by PKCzeta, subsequently confirmed using an OAT1-specific substrate, adefovir. Inhibition of PKCzeta also blocked the increase in ES uptake seen in response to epidermal growth factor and to activation of protein kinase A. Thus, PKCzeta acted downstream of the epidermal growth factor to protein kinase A signaling pathway. Activation of transport was accompanied by an increase in V(max) and was blocked by microtubule disruption, indicating that activation may result from trafficking of OAT3 into the plasma membrane. These data demonstrate that PKCzeta activation up-regulates OAT1 and OAT3 function, and that protein-protein interactions play a central role controlling these two important renal drug transporters.