Oxidative stress induced by potassium bromate exposure results in altered tight junction protein expression in renal proximal tubule cells

Transcriptomic changes and mitochondrial toxicity in response to acute and repeat dose treatment with brequinar in human liver and kidney in vitro models

The potent dihydroorotate dehydrogenase (DHODH) inhibitor brequinar has been investigated as an anticancer, immunosuppressive, and antiviral pharmaceutical agent. However, its toxicity is still poorly understood. We investigated the cellular responses of primary human hepatocytes (PHH) and telomerase-immortalised human renal proximal tubular epithelial cells (RPTEC/TERT1) after a single 24-h exposure up to 100 μM brequinar. Additionally, RPTEC/TERT1 cells underwent repeated daily exposure for five consecutive days at 0.3, 3, and 20 μM. Transcriptomic analysis revealed that PHH were less sensitive to brequinar treatment than RPTEC/TERT1 cells. Upregulation of various phase I and II drug-metabolising enzymes, particularly Cytochrome P450 (CYP) 1 A and 3 A enzymes, in PHH suggests potential detoxification. Furthermore, brequinar exposure led to a significant upregulation of several stress response pathways in PHH and RPTEC/TERT1 cells, including the unfolded protein response, Nrf2, p53, and inflammatory responses. RPTEC/TERT1 cells exhibited greater sensitivity to brequinar at 0.3 μM with repeated exposure compared to a single exposure. Furthermore, brequinar could impair the mitochondrial respiration of RPTEC/TERT1 cells after 24 h. This study provides new insights into the differential responses of PHH and RPTEC/TERT1 cells in response to brequinar exposure and highlights the biological relevance of implementing repeated dosing regimens in in vitro studies.

Human and rat renal proximal tubule in vitro models for ADME applications

The kidney is a major organ dictating excretion rates of chemicals and their metabolites from the body and thus renal clearance is frequently a major component of pharmaco-(toxico)-kinetic profiles. Within the nephron, the proximal tubule is the major site for xenobiotic reabsorption from glomerular filtrate and xenobiotic secretion from the blood into the lumen via the expression of multiple inward (lumen to interstitium) and outward transport systems (interstitium to lumen). While there exist several human proximal tubular cell culture options that could be utilized for modelling the proximal tubule component of renal clearance, they do not necessarily represent the full complement of xenobiotic transport processes of their in vivo counterparts. Here, we review available human and rat renal proximal tubule in vitro models, including subcellular fractions, immortalized cell lines, primary cell cultures, induced pluripotent stem cell (iPSC)-derived models and also consider more organotypic cell culture environments such as microporous growth supports, organoids and microfluidic systems. This review focuses on expression levels and function of human and rat renal transporters and phase I and II metabolizing enzymes in these models in order to critically assess their usefulness and to identify potential solutions to overcome identified limitations.

Crocetin attenuating Urinary tract Infection and adherence of uropathogenic E. coli in NRK-52E cells via an inflammatory pathway

BACKGROUND AND OBJECTIVE

Urinary tract infections (UTI) are commonly treated with broad-spectrum antibiotics, but treatment has limitation due to causes of nephrotoxicity in uroepithelial cells. Recently, the researcher focuses their research on alternative therapy for the treatment of UTI. This study evaluated the anti-infectious effect of crocetin against adherence of pathogenic [2-¹⁴ C]-acetate labeled Escherichia coli (MTCC-729) to rat proximal renal tubular cells (NRK-52E cells) and explores the possible mechanism of action.

MATERIALS AND METHODS

In vitro cytotoxicity and radio acetate labeled tests were performed on NRK-52E cells. The rats were divided into five different groups as follows: normal control (NC), disease control (DC), and various doses of crocetin (1.25, 2.5, and 5 mg/kg) treated group rats. White blood cells in blood, urine, and bacterial colony counts were estimated at regular intervals. Pro-inflammatory cytokines, such as interleukin-6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), interleukin-10 (IL-10), and interleukin-8 (IL-8), were also estimated. In the current study, we estimated the mRNA expression of toll-like receptor-4 (TLR-4) and toll-like receptor-2 (TLR-2) in the renal and bladder tissues.

RESULTS

Crocetin significantly (p < .05) inhibited the adherence of E. Coli in NRK-52E cells. Crocetin suppresses the lipid peroxidation (LPO) 42% in cells treated with H₂ O₂ cells without crocetin. The white blood cells (WBC) count in blood and urine were augmented and crocetin treatment significantly (p < .05) reduced the WBC in urine and blood. The pro-inflammatory cytokines, such as IL-6, MCP-1, IL-10, and IL-8, significantly (p < .05) increased in the DC group and crocetin significantly (p < .05) reduced the pro-inflammatory cytokines. Dose-dependent treatment of crocetin significantly reduced the mRNA expression of TLR2 and TLR4 in the renal and bladder tissues.

CONCLUSION

Crocetin considerably reduced the bacterial adherence to NRK-52E cells, attenuated the H₂ O₂ induced toxicity in NRK-52E cells and also improved the renal tubular function, and reduced the inflammatory response via altering the inflammatory and antioxidant markers.

PRACTICAL APPLICATION

As we all know that urinary tract infection is the most common disease worldwide. In this study, we scrutinized the protective effect of crocetin against urinary tract infection. Crocetin treatment considerably reduced the zone of inhibition and improved radioactivity. Crocetin significantly reduced the levels of cytokines and inflammatory mediators. Crocetin can be used as a protective drug in the treatment of urinary tract infections.

A read-across case study on chronic toxicity of branched carboxylic acids (1): Integration of mechanistic evidence from new approach methodologies (NAMs) to explore a common mode of action

This read-across case study characterises thirteen, structurally similar carboxylic acids demonstrating the application of in vitro and in silico human-based new approach methods, to determine biological similarity. Based on data from in vivo animal studies, the read-across hypothesis is that all analogues are steatotic and so should be considered hazardous. Transcriptomic analysis to determine differentially expressed genes (DEGs) in hepatocytes served as first tier testing to confirm a common mode-of-action and identify differences in the potency of the analogues. An adverse outcome pathway (AOP) network for hepatic steatosis, informed the design of an in vitro testing battery, targeting AOP relevant MIEs and KEs, and Dempster-Shafer decision theory was used to systematically quantify uncertainty and to define the minimal testing scope. The case study shows that the read-across hypothesis is the critical core to designing a robust, NAM-based testing strategy. By summarising the current mechanistic understanding, an AOP enables the selection of NAMs covering MIEs, early KEs, and late KEs. Experimental coverage of the AOP in this way is vital since MIEs and early KEs alone are not confirmatory of progression to the AO. This strategy exemplifies the workflow previously published by the EUTOXRISK project driving a paradigm shift towards NAM-based NGRA.

The effect of chronic dosing and p53 status on the genotoxicity of pro-oxidant chemicals in vitro

In this study, we have studied the cytotoxicity and genotoxic potency of 3 pro-oxidants; H2O2, menadione and KBrO3 in different dosing scenarios, namely acute (1-day dosing) and chronic (5-days). For this purpose, relative population doubling (RPD%) and mononucleated micronucleus (MN) test were used. TK6 cells and NH32 were employed in in vitro experiments. In the study, the total acute dose was divided into 5 days for each prooxidant chemicals by dose fractionation (1/5th per day) method. Acute dosing was compared to chronic dosing. The oxidative stress caused by the exposure of cells with pro-oxidant chemicals to the cells was determined by an optimized 2',7'-dichlorofluorescein diacetate (DCFHDA) test method. The antioxidant levels of the cell lines were altered with buthionine sulfoxide (BSO) and N-acetyl cysteine (NAC), and the effect of antioxidant capacity on the MN formation in the cells was observed with this method. In the case of H2O2 and menadione, fractional dosing has been observed to result in lower toxicity and lower genotoxicity. But in the case of KBrO3, unlike the other 2 pro-oxidants, higher MN induction was observed with fractionated doses. DCFHDA test clearly demonstrated ROS induction with H2O2 and menadione but not with KBrO3. Unexpectedly, DCFHDA test demonstrated that KBrO3 did not cause an increase ROS levels in both acute and chronic dosing, suggesting an alternative ROS induction mechanism. It was also observed that, treatment with BSO and NAC, caused increasing and decreasing of MN fold change respectively, allowing further ROS specific mechanisms to be explored. Hence, dose fractionation expectedly caused less MN, cytotoxicity and ROS formation with H2O2 and menadione exposure, but not with KBrO3. This implies a unique mechanism of action for KBrO3 induced genotoxicity. Chronic dosing in vitro may be a valuable approach allowing better understanding of how chemicals damage DNA and pose human hazards.

PETER-assay: Combined Impedimetric Detection of Permeability (PE) and Resistance (TER) of Barrier-Forming Cell Layers

Epithelial and endothelial barrier function is typically studied in vitro by growing the cells of interest on permeable supports that are sandwiched between two fluid compartments. This setup mimics the physiological situation with the cell layer as the diffusion barrier at the interface between two chemically distinct fluids. Routinely, the barrier function is quantitatively described by two key parameters: (i) the transepithelial or transendothelial electrical resistance (TER) as a measure of the permeability for small inorganic ions and (ii) the permeability coefficient (P_E) as a descriptor of the permeability for molecular tracers. So far the two parameters have been determined in separate experiments. This study introduces a device that allows for simultaneous detection of P_E and TER of the very same cell monolayer in one single experiment (P_ETER-assay). The novel approach is entirely based on AC impedance measurements in two different modes, so that TER and P_E become available in real time. The new approach is demonstrated for three epithelial cell lines derived from the kidney (MDCK-I, MDCK-II, NRK) with very different barrier properties under stationary conditions and when challenged by barrier-breaking fungal toxin cytochalasin D. P_ETER provides an excellent time-resolution and completely automated data collection.

Persistence of Epigenomic Effects After Recovery From Repeated Treatment With Two Nephrocarcinogens

The discovery of the epigenetic regulation of transcription has provided a new source of mechanistic understanding to long lasting effects of chemicals. However, this information is still seldom exploited in a toxicological context and studies of chemical effect after washout remain rare. Here we studied the effects of two nephrocarcinogens on the human proximal tubule cell line RPTEC/TERT1 using high-content mRNA microarrays coupled with miRNA, histone acetylation (HA) and DNA methylation (DM) arrays and metabolomics during a 5-day repeat-dose exposure and 3 days after washout. The mycotoxin ochratoxin A (OTA) was chosen as a model compound for its known impact on HA and DM. The foremost effect observed was the modulation of thousands of mRNAs and histones by OTA during and after exposure. In comparison, the oxidant potassium bromate (KBrO₃) had a milder impact on gene expression and epigenetics. However, there was no strong correlation between epigenetic modifications and mRNA changes with OTA while with KBrO₃ the gene expression data correlated better with HA for both up- and down-regulated genes. Even when focusing on the genes with persistent epigenetic modifications after washout, only half were coupled to matching changes in gene expression induced by OTA, suggesting that while OTA causes a major effect on the two epigenetic mechanisms studied, these alone cannot explain its impact on gene expression. Mechanistic analysis confirmed the known activation of Nrf2 and p53 by KBrO₃, while OTA inhibited most of the same genes, and genes involved in the unfolded protein response. A few miRNAs could be linked to these effects of OTA, albeit without clear contribution of epigenetics to the modulation of the pathways at large. Metabolomics revealed disturbances in amino acid balance, energy catabolism, nucleotide metabolism and polyamine metabolism with both chemicals. In conclusion, the large impact of OTA on transcription was confirmed at the mRNA level but also with two high-content epigenomic methodologies. Transcriptomic data confirmed the previously reported activation (by KBrO₃) and inhibition (by OTA) of protective pathways. However, the integration of omic datasets suggested that HA and DM were not driving forces in the gene expression changes induced by either chemical.

OTA induces intestinal epithelial barrier dysfunction and tight junction disruption in IPEC-J2 cells through ROS/Ca2+-mediated MLCK activation

Ochratoxin A (OTA) is a frequent contaminant of feed and food worldwide. The toxicity of OTA on intestinal barrier was investigated in porcine intestinal epithelial cells (IPEC-J2). We observed that OTA induced intestinal barrier dysfunction as indicated by the reduction in transepithelial electrical resistance (TEER) and elevation in paracellular permeability to 4 kDa dextran. The barrier dysfunction was accompanied with tight junction disruption including a down-regulation in ZO-1 expression and redistribution of Occludin and ZO-1. Moreover, OTA exposure increased reactive oxygen species (ROS) generation, elevated the intracellular calcium level ([Ca²⁺]_c) and activated myosin light chain kinase (MLCK). Simultaneously, NAC, a ROS scavenger, blocked OTA-induced ROS generation, [Ca²⁺]_c elevation, barrier dysfunction and tight junction disruption, suggesting that OTA-induced ROS generation may act as a trigger. Next, we found that OTA-induced MLCK activation was inhibited by BAPTA-AM, the cytosolic Ca²⁺ chelator, demonstrating that OTA-induced MLCK activation is dependent on [Ca²⁺]_c elevation. Furthermore, inhibition of MLCK with ML-7 or inhibition of [Ca²⁺]_c elevation with BAPTA-AM markedly prevented OTA-induced barrier dysfunction and tight junction disruption. Taken together, our results indicated that OTA induces ROS generation, and then elevates the [Ca²⁺]_c and MLCK activity in turn, which finally induces barrier dysfunction and disrupts tight junction in IPEC-J2 cell monolayers.

Investigation of Nrf2, AhR and ATF4 Activation in Toxicogenomic Databases

Toxicological responses to chemical insult are largely regulated by transcriptionally activated pathways that may be independent, correlated and partially or fully overlapping. Investigating the dynamics of the interactions between stress responsive transcription factors from toxicogenomic data and defining the signature of each of them is an additional step toward a system level understanding of perturbation driven mechanisms. To this end, we investigated the segregation of the genes belonging to the three following transcriptionally regulated pathways: the AhR pathway, the Nrf2 pathway and the ATF4 pathway. Toxicogenomic datasets from three projects (carcinoGENOMICS, Predict-IV and TG-GATEs) obtained in various experimental conditions (in human and rat in vitro liver and kidney models and rat in vivo, with bolus administration and with repeated doses) were combined and consolidated where overlaps between datasets existed. A bioinformatic analysis was performed to refine pathways' signatures and to create chemical activation capacity scores to classify chemicals by their potency and selectivity of activation of each pathway. With some refinement such an approach may improve chemical safety classification and allow biological read across on a pathway level.

Comparison of base-line and chemical-induced transcriptomic responses in HepaRG and RPTEC/TERT1 cells using TempO-Seq

The utilisation of genome-wide transcriptomics has played a pivotal role in advancing the field of toxicology, allowing the mapping of transcriptional signatures to chemical exposures. These activities have uncovered several transcriptionally regulated pathways that can be utilised for assessing the perturbation impact of a chemical and also the identification of toxic mode of action. However, current transcriptomic platforms are not very amenable to high-throughput workflows due to, high cost, complexities in sample preparation and relatively complex bioinformatic analysis. Thus, transcriptomic investigations are usually limited in dose and time dimensions and are, therefore, not optimal for implementation in risk assessment workflows. In this study, we investigated a new cost-effective, transcriptomic assay, TempO-Seq, which alleviates the aforementioned limitations. This technique was evaluated in a 6-compound screen, utilising differentiated kidney (RPTEC/TERT1) and liver (HepaRG) cells and compared to non-transcriptomic label-free sensitive endpoints of chemical-induced disturbances, namely phase contrast morphology, xCELLigence and glycolysis. Non-proliferating cell monolayers were exposed to six sub-lethal concentrations of each compound for 24 h. The results show that utilising a 2839 gene panel, it is possible to discriminate basal tissue-specific signatures, generate dose-response relationships and to discriminate compound-specific and cell type-specific responses. This study also reiterates previous findings that chemical-induced transcriptomic alterations occur prior to cytotoxicity and that transcriptomics provides in depth mechanistic information of the effects of chemicals on cellular transcriptional responses. TempO-Seq is a robust transcriptomic platform that is well suited for in vitro toxicity experiments.

A perfusion chamber for monitoring transepithelial NaCl transport in an in vitro model of the renal tubule

Transepithelial electrical measurements in the renal tubule have provided a better understanding of how kidney regulates electrolyte and water homeostasis through the reabsorption of molecules and ions (e.g., H₂ O and NaCl). While experiments and measurement techniques using native tissue are difficult to prepare and to reproduce, cell cultures conducted largely with the Ussing chamber lack the effect of fluid shear stress which is a key physiological stimulus in the renal tubule. To overcome these limitations, we present a modular perfusion chamber for long-term culture of renal epithelial cells under flow that allows the continuous and simultaneous monitoring of both transepithelial electrical parameters and transepithelial NaCl transport. The latter is obtained from electrical conductivity measurements since Na⁺ and Cl^- are the ions that contribute most to the electrical conductivity of a standard physiological solution. The system was validated with epithelial monolayers of raTAL and NRK-52E cells that were characterized electrophysiologically for 5 days under different flow conditions (i.e., apical perfusion, basal, or both). In addition, apical to basal chemical gradients of NaCl (140/70 and 70/140 mM) were imposed in order to demonstrate the feasibility of this methodology for quantifying and monitoring in real time the transepithelial reabsorption of NaCl, which is a primary function of the renal tubule.

Identification of urinary exosomal noncoding RNAs as novel biomarkers in chronic kidney disease

In chronic kidney disease (CKD), the decline in the glomerular filtration rate is associated with increased morbidity and mortality and thus poses a major challenge for healthcare systems. While the contribution of tissue-derived miRNAs and mRNAs to CKD progression has been extensively studied, little is known about the role of urinary exosomes and their association with CKD. Exosomes are small, membrane-derived endocytic vesicles that contribute to cell-to-cell communication and are present in various body fluids, such as blood or urine. Next-generation sequencing approaches have revealed that exosomes are enriched in noncoding RNAs and thus exhibit great potential for sensitive nucleic acid biomarkers in various human diseases. Therefore, in this study we aimed to identify urinary exosomal ncRNAs as novel biomarkers for diagnosis of CKD. Since up to now most approaches have focused on the class of miRNAs, we extended our analysis to several other noncoding RNA classes, such as tRNAs, tRNA fragments (tRFs), mitochondrial tRNAs, or lincRNAs. For their computational identification from RNA-seq data, we developed a novel computational pipeline, designated as ncRNASeqScan. By these analyses, in CKD patients we identified 30 differentially expressed ncRNAs, derived from urinary exosomes, as suitable biomarkers for early diagnosis. Thereby, miRNA-181a appeared as the most robust and stable potential biomarker, being significantly decreased by about 200-fold in exosomes of CKD patients compared to healthy controls. Using a cell culture system for CKD indicated that urinary exosomes might indeed originate from renal proximal tubular epithelial cells.

Arsenic downregulates tight junction claudin proteins through p38 and NF-κB in intestinal epithelial cell line, HT-29

Arsenic is a naturally occurring metalloid that often is found in foods and drinking water. Human exposure to arsenic is associated with the development of gastrointestinal problems such as fluid loss, diarrhea and gastritis. Arsenic is also known to induce toxic responses including oxidative stress in cells of the gastrointestinal track. Tight junctions (TJs) regulate paracellular permeability and play a barrier role by inhibiting the movement of water, solutes and microorganisms in the paracellular space. Since oxidative stress and TJ damage are known to be associated, we examined whether arsenic produces TJ damage such as downregulation of claudins in the human colorectal cell line, HT-29. To confirm the importance of oxidative stress in arsenic-induced TJ damage, effects of the antioxidant compound (e.g., N-acetylcysteine (NAC)) were also determined in cells. HT-29 cells were treated with arsenic trioxide (40μM, 12h) to observe the modified expression of TJ proteins. Arsenic decreased expression of TJ proteins (i.e., claudin-1 and claudin-5) and transepithelial electrical resistance (TEER) whereas pretreatment of NAC (5-10mM, 1h) attenuated the observed claudins downregulation and TEER. Arsenic treatment produced cellular oxidative stress via superoxide generation and lowering glutathione (GSH) levels, while NAC restored cellular GSH levels and decreased oxidative stress. Arsenic increased phosphorylation of p38 and nuclear translocation of nuclear factor-kappa B (NF-κB) p65, while NAC attenuated these intracellular events. Results demonstrated that arsenic can damage intestinal epithelial cells by proinflammatory process (oxidative stress, p38 and NF-κB) which resulted in the downregulation of claudins and NAC can protect intestinal TJs from arsenic toxicity.

Evaluation of renal in vitro models used in ochratoxin research

Ochratoxin A (OTA) induces renal carcinomas in rodents with a specific localisation in the S3 segment of proximal tubules and distinct early severe tissue alterations, which have been observed also in other species. Pronounced species- and sex-specific differences in toxicity occur and similar effects cannot be excluded in humans, however precise mechanism(s) remain elusive until today. In such cases, the use of in vitro models for mechanistic investigations can be very useful; in particular if a non-genotoxic mechanism of cancer formation is assumed which include cytotoxic effects. However, potential genotoxic mechanisms can also be investigated in vitro. A crucial issue of in vitro research is the choice of the appropriate cell model. Apparently, the cellular target of OTA is the renal proximal tubular cell; therefore cells from this tissue area are the most reasonable model. Furthermore, cells from affected species should be used and can be compared to cells of human origin. Another important parameter is whether to use primary cultures or to choose a cell line from the huge variety of cell lines available. In any case, important characteristics and quality controls need to be verified beforehand. Therefore, this review discusses the renal in vitro models that have been used for the investigation of renal ochratoxin toxicity. In particular, we discuss the choice of the models and the essential parameters making them suitable models for ochratoxin research together with exemplary results from this research. Furthermore, new promising models such as hTERT-immortalised cells and 3D-cultures are briefly discussed.

Inter-laboratory study of human in vitro toxicogenomics-based tests as alternative methods for evaluating chemical carcinogenicity: a bioinformatics perspective

The assessment of the carcinogenic potential of chemicals with alternative, human-based in vitro systems has become a major goal of toxicogenomics. The central read-out of these assays is the transcriptome, and while many studies exist that explored the gene expression responses of such systems, reports on robustness and reproducibility, when testing them independently in different laboratories, are still uncommon. Furthermore, there is limited knowledge about variability induced by the data analysis protocols. We have conducted an inter-laboratory study for testing chemical carcinogenicity evaluating two human in vitro assays: hepatoma-derived cells and hTERT-immortalized renal proximal tubule epithelial cells, representing liver and kidney as major target organs. Cellular systems were initially challenged with thirty compounds, genome-wide gene expression was measured with microarrays, and hazard classifiers were built from this training set. Subsequently, each system was independently established in three different laboratories, and gene expression measurements were conducted using anonymized compounds. Data analysis was performed independently by two separate groups applying different protocols for the assessment of inter-laboratory reproducibility and for the prediction of carcinogenic hazard. As a result, both workflows came to very similar conclusions with respect to (1) identification of experimental outliers, (2) overall assessment of robustness and inter-laboratory reproducibility and (3) re-classification of the unknown compounds to the respective toxicity classes. In summary, the developed bioinformatics workflows deliver accurate measures for inter-laboratory comparison studies, and the study can be used as guidance for validation of future carcinogenicity assays in order to implement testing of human in vitro alternatives to animal testing.

Chemoprotective effect of taurine on potassium bromate-induced DNA damage, DNA-protein cross-linking and oxidative stress in rat intestine

Potassium bromate (KBrO3) is widely used as a food additive and is a major water disinfection by-product. It induces multiple organ toxicity in humans and experimental animals and is a probable human carcinogen. The present study reports the protective effect of dietary antioxidant taurine on KBrO3-induced damage to the rat intestine. Animals were randomly divided into four groups: control, KBrO3 alone, taurine alone and taurine+ KBrO3. Administration of KBrO3 alone led to decrease in the activities of intestinal brush border membrane enzymes while those of antioxidant defence and carbohydrate metabolism were also severely altered. There was increase in DNA damage and DNA-protein cross-linking. Treatment with taurine, prior to administration of KBrO3, resulted in significant attenuation in all these parameters but the administration of taurine alone had no effect. Histological studies supported these biochemical results showing extensive intestinal damage in KBrO3-treated animals and greatly reduced tissue injury in the taurine+ KBrO3 group. These results show that taurine ameliorates bromate induced tissue toxicity and oxidative damage by improving the antioxidant defence, tissue integrity and energy metabolism. Taurine can, therefore, be potentially used as a therapeutic/protective agent against toxicity of KBrO3 and related compounds.

Expression of xenobiotic transporters in the human renal proximal tubule cell line RPTEC/TERT1

The kidney is a major target for drug-induced injury, primarily due the fact that it transports a wide variety of chemical entities into and out of the tubular lumen. Here, we investigated the expression of the main xenobiotic transporters in the human renal proximal tubule cell line RPTEC/TERT1 at an mRNA and/or protein level. RPTEC/TERT1 cells expressed OCT2, OCT3, OCTN2, MATE1, MATE2, OAT1, OAT3 and OAT4. The functionality of the OCTs was demonstrated by directional transport of the fluorescent dye 4-Di-1-ASP. In addition, P-glycoprotein activity in RPTEC/TERT1 cells was verified by fluorescent dye retention in presence of various P-glycoprotein inhibitors. In comparison to proliferating cells, contact inhibited RPTEC/TERT1 cells expressed increased mRNA levels of several ABC transporter family members and were less sensitive to cyclosporine A. We conclude that differentiated RPTEC/TERT1 cells are well suited for utilisation in xenobiotic transport and pharmacokinetic studies.

Application of RPTEC/TERT1 cells for investigation of repeat dose nephrotoxicity: A transcriptomic study

The kidney is a major target organ for toxicity. Incidence of chronic kidney disease (CKD) is increasing at an alarming rate due to factors such as increasing population age and increased prevalence of heart disease and diabetes. There is a major effort ongoing to develop superior predictive models of renal injury and early renal biomarkers that can predict onset of CKD. In the EU FP7 funded project, Predict-IV, we investigated the human renal proximal tubule cells line, RPTEC/TERT1 for their applicability to long term nephrotoxic mechanistic studies. To this end, we used a tiered strategy to optimise dosing regimes for 9 nephrotoxins. Our final testing protocol utilised differentiated RPTEC/TERT1 cells cultured on filter inserts treated with compounds at both the apical and basolateral side, at concentrations not exceeding IC10, for 14 days in a 24 h repeat application. Transepithelial electrical resistance and supernatant lactate were measured over the duration of the experiments and genome wide transcriptomic profiles were assayed at day 1, 3 and 14. The effect of hypoxia was investigated for a subset of compounds. The transcriptomic data were analysed to investigate compound-specific effects, global responses and mechanistically informative signatures. In addition, several potential clinically useful renal injury biomarkers were identified.

Mechanism of cisplatin proximal tubule toxicity revealed by integrating transcriptomics, proteomics, metabolomics and biokinetics

Cisplatin is one of the most widely used chemotherapeutic agents for the treatment of solid tumours. The major dose-limiting factor is nephrotoxicity, in particular in the proximal tubule. Here, we use an integrated omics approach, including transcriptomics, proteomics and metabolomics coupled to biokinetics to identify cell stress response pathways induced by cisplatin. The human renal proximal tubular cell line RPTEC/TERT1 was treated with sub-cytotoxic concentrations of cisplatin (0.5 and 2 μM) in a daily repeat dose treating regime for up to 14 days. Biokinetic analysis showed that cisplatin was taken up from the basolateral compartment, transported to the apical compartment, and accumulated in cells over time. This is in line with basolateral uptake of cisplatin via organic cation transporter 2 and bioactivation via gamma-glutamyl transpeptidase located on the apical side of proximal tubular cells. Cisplatin affected several pathways including, p53 signalling, Nrf2 mediated oxidative stress response, mitochondrial processes, mTOR and AMPK signalling. In addition, we identified novel pathways changed by cisplatin, including eIF2 signalling, actin nucleation via the ARP/WASP complex and regulation of cell polarization. In conclusion, using an integrated omic approach together with biokinetics we have identified both novel and established mechanisms of cisplatin toxicity.

Systems biology modeling of omics data: effect of cyclosporine a on the Nrf2 pathway in human renal cells

Background

Incorporation of omic data streams for building improved systems biology models has great potential for improving their predictions of biological outcomes. We have recently shown that cyclosporine A (CsA) strongly activates the nuclear factor (erythroid-derived 2)-like 2 pathway (Nrf2) in renal proximal tubular epithelial cells (RPTECs) exposed in vitro. We present here a quantitative calibration of a differential equation model of the Nrf2 pathway with a subset of the omics data we collected.

Results

In vitro pharmacokinetic data on CsA exchange between cells, culture medium and vial walls, and data on the time course of omics markers in response to CsA exposure were reasonably well fitted with a coupled PK-systems biology model. Posterior statistical distributions of the model parameter values were obtained by Markov chain Monte Carlo sampling in a Bayesian framework. A complex cyclic pattern of ROS production and control emerged at 5 μM CsA repeated exposure. Plateau responses were found at 15 μM exposures. Shortly above those exposure levels, the model predicts a disproportionate increase in cellular ROS quantity which is consistent with an in vitro EC₅₀ of about 40 μM for CsA in RPTECs.

Conclusions

The model proposed can be used to analyze and predict cellular response to oxidative stress, provided sufficient data to set its parameters to cell-specific values. Omics data can be used to that effect in a Bayesian statistical framework which retains prior information about the likely parameter values.

Drug-induced nephrotoxicity: clinical impact and preclinical in vitro models

The kidney is a major target for drug-induced toxicity. Drug-induced nephrotoxicity remains a major problem in the clinical setting, where the use of nephrotoxic drugs is often unavoidable. This leads frequently to acute kidney injury, and current problems are discussed. One strategy to avoid such problems would be the development of drugs with decreased nephrotoxic potential. However, the prediction of nephrotoxicity during preclinical drug development is difficult and nephrotoxicity is typically detected only late. Also, the nephrotoxic potential of newly approved drugs is often underestimated. Regulatory approved or validated in vitro models for the prediction of nephrotoxicity are currently not available. Here, we will review current approaches on the development of such models. This includes a discussion of three-dimensional and microfluidic models and recently developed stem cell based approaches. Most in vitro models have been tested with a limited number of compounds and are of unclear predictivity. However, some studies have tested larger numbers of compounds and the predictivity of the respective in vitro model had been determined. The results showed that high predictivity can be obtained by using primary or stem cell derived human renal cells in combination with appropriate end points.

A review of the evidence that ochratoxin A is an Nrf2 inhibitor: implications for nephrotoxicity and renal carcinogenicity

Several studies have demonstrated that ochratoxin A (OTA) inhibits the nuclear factor, erythroid 2-like 2 (Nrf2) oxidative stress response pathway. At the cellular level this would attenuate (i) glutathione synthesis; (ii) recycling of oxidised glutathione; (iii) activity of oxidoreductases; and (iv) phase II metabolism inducibility. The effects combined would render the cell and tissue more vulnerable to oxidative stress. Indeed, Nrf2 knock out animals exhibit increased susceptibility to various types of chemical-induced injury. Several studies have shown that OTA exposure can inhibit Nrf2 responses. Such an action would initially lead to increased susceptibility to both physiological and chemical-induced cell stress. However, chronic exposure to OTA may also act as a selective pressure for somatic mutations in Nrf2 or its inhibitor Keap-1, leading to constitutive Nrf2 activation. Nrf2 overexpression confers a survival advantage and is often associated with cancer cell survival. Here we review the evidence for OTA’s role as an Nrf2 inhibitor and discuss the implications of this mechanism in nephrotoxicity and carcinogenicity.

Delineation of the key aspects in the regulation of epithelial monolayer formation

The formation, maintenance, and repair of epithelial barriers are of critical importance for whole-body homeostasis. However, the molecular events involved in epithelial tissue maturation are not fully established. To this end, we investigated the molecular processes involved in renal epithelial proximal-tubule monolayer maturation utilizing transcriptomic, metabolomic, and functional parameters. We uncovered profound dynamic alterations in transcriptional regulation, energy metabolism, and nutrient utilization over the maturation process. Proliferating cells exhibited high glycolytic rates and high transcript levels for fatty acid synthesis genes (FASN), whereas matured cells had low glycolytic rates, increased oxidative capacity, and preferentially expressed genes for beta oxidation. There were dynamic alterations in the expression and localization of several adherens (CDH1, -4, and -16) and tight junction (TJP3 and CLDN2 and -10) proteins. Genes involved in differentiated proximal-tubule function, cilium biogenesis (BBS1), and transport (ATP1A1 and ATP1B1) exhibited increased expression during epithelial maturation. Using TransAM transcription factor activity assays, we could demonstrate that p53 and FOXO1 were highly active in matured cells, whereas HIF1A and c-MYC were highly active in proliferating cells. The data presented here will be invaluable in the further delineation of the complex dynamic cellular processes involved in epithelial cell regulation.