Evidence for a role of claudin 2 as a proximal tubular stress responsive paracellular water channel

Characterization of Organic Anion and Cation Transport in Three Human Renal Proximal Tubular Epithelial Models

The polarised expression of specific transporters in proximal tubular epithelial cells is important for the renal clearance of many endogenous and exogenous compounds. Thus, ideally, the in vitro tools utilised for predictions would have a similar expression of apical and basolateral xenobiotic transporters as in vivo. Here, we assessed the functionality of organic cation and anion transporters in proximal tubular-like cells (PTL) differentiated from human induced pluripotent stem cells (iPSC), primary human proximal tubular epithelial cells (PTEC), and telomerase-immortalised human renal proximal tubular epithelial cells (RPTEC/TERT1). Organic cation and anion transport were studied using the fluorescent substrates 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP) and 6-carboxyfluorescein (6-CF), respectively. The level and rate of intracellular ASP accumulation in PTL following basolateral application were slightly lower but within a 3-fold range compared to primary PTEC and RPTEC/TERT1 cells. The basolateral uptake of ASP and its subsequent apical efflux could be inhibited by basolateral exposure to quinidine in all models. Of the three models, only PTL showed a modest preferential basolateral-to-apical 6-CF transfer. These results show that organic cation transport could be demonstrated in all three models, but more research is needed to improve and optimise organic anion transporter expression and functionality.

The role of claudins in homeostasis

Sequential expression of claudins, a family of tight junction proteins, along the nephron mirrors the sequential expression of ion channels and transporters. Only by the interplay of transcellular and paracellular transport can the kidney efficiently maintain electrolyte and water homeostasis in an organism. Although channel and transporter defects have long been known to perturb homeostasis, the contribution of individual tight junction proteins has been less clear. Over the past two decades, the regulation and dysregulation of claudins have been intensively studied in the gastrointestinal tract. Claudin expression patterns have, for instance, been found to be affected in infection and inflammation, or in cancer. In the kidney, a deeper understanding of the causes as well as the effects of claudin expression alterations is only just emerging. Little is known about hormonal control of the paracellular pathway along the nephron, effects of cytokines on renal claudin expression or relevance of changes in paracellular permeability to the outcome in any of the major kidney diseases. By summarizing current findings on the role of specific claudins in maintaining electrolyte and water homeostasis, this Review aims to stimulate investigations on claudins as prognostic markers or as druggable targets in kidney disease.

Claudin-2 in hyperproliferative migrating keratinocytes and migration inhibition via siRNA knockdown

Claudin-2 is a tight junction protein found in various tissues including the epidermis of the skin. Intracellular signalling via claudin-2 may have an effect on cell proliferation and migration. While the role of claudin-2 in the epidermis has not been established, here we show an increase in claudin-2 expression in hyperproliferative archival skin samples. To further examine the role of claudin-2 in cell migration we examined its expression in cultured keratinocytes and found it was increased in wound margins in an in vitro scratch test assay. We then used a claudin-2 knockdown assay using small interfering ribonucleic acid (siRNA) with a 77% transfection efficiency and decrease in claudin-2 protein via Western blot analysis to examine cell migration, which was inhibited following claudin-2 knockdown over a 5-day period. Cells transfected with claudin-2 siRNA also showed a decreased size compared to controls and a more diffuse staining pattern. Lastly we examined claudin-2 expression in migrating keratinocytes by Western blot analysis and found a significant decrease in protein staining in scratch-test assay cultures after 4 h, followed by a significant increase in claudin-2 protein after 24 h. Taken together these results indicate a role for claudin-2 signalling in proliferation and cell migration in the epidermis of the skin.

Differentiation and Subculturing of Renal Proximal Tubular-like Cells Derived from Human iPSC

Recently, we have developed a protocol to differentiate human induced pluripotent stem cells (iPSC) into proximal tubular-like cells (PTL) (Chandrasekaran et al., 2021). These cells express proximal tubular-specific markers, including megalin, and form a polarized monolayer expressing tight junction proteins, including ZO-3 and occludin. Furthermore, PTL display functional properties, including megalin-facilitated endocytosis, P-glycoprotein (ABCB1) efflux, and respond to parathyroid hormone. Here, we report step-by-step protocols to culture iPSC prior to differentiation (Basic Protocol 1), to differentiate PTL from iPSC (Basic Protocol 2), and to passage and freeze-thaw PTL (Basic Protocol 3). Additionally, we provide a protocol (Basic Protocol 4) to culture PTL on microporous growth supports (transwells). Immunofluorescence stainings for characteristic markers, including megalin, are shown for unpassaged (Basic Protocol 2) and passaged (Basic Protocol 3) PTL. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: iPSC culture Basic Protocol 2: iPSC-derived PTL differentiation Basic Protocol 3: PTL passaging, culturing, and freezing Basic Protocol 4: PTL culturing on transwells Support Protocol 1: Preparation of Geltrex-coated cell culture plates Support Protocol 2: Preparation of RPTEC/TERT1 or fHDF/TERT166-ECM-coated cell culture plates Support Protocol 3: Preparation of human collagen IV-coated cell culture plates Support Protocol 4: Immunofluorescence staining.

Absorption and metabolism of 3-MCPD in hepatic and renal cell lines

3-Monochloropropane-1,2-diol (3-MCPD) fatty acid esters are process contaminants mainly formed during the refinement of vegetable oils. Gastrointestinal hydrolysis yields free 3-MCPD, which is resorbed into the body. In long-term rat studies, 3-MCPD caused renal and testicular neoplasms. 3-MCPD metabolism via β-chlorolactic acid has been postulated to underlie the toxic effects of 3-MCPD. Various efforts are ongoing to characterize the toxicological mode of action of 3-MCPD using in vitro systems. Published results suggest a very low sensitivity of cell cultures in vitro, as compared to 3-MCPD levels causing toxic effects in vivo. The insensitivity of in vitro systems raises the question to which extent 3-MCPD is absorbed and metabolized in vitro. We therefore analyzed cytotoxicity, absorption and metabolism of 3-MCPD and its metabolite β-chlorolactic acid in renal and hepatic cells. Cytotoxicity tests using up to 100 mM 3-MCPD confirmed the low sensitivity of human and rat cell lines towards 3-MCPD toxicity. Furthermore, absorption and metabolism of 3-MCPD examined via GC-MS and LC-MS/MS were only observed to a minor degree, and 3-MCPD was also not converted by a metabolizing system (S9 fraction). In conclusion, our data indicate that current in vitro models are not well suited for studying 3-MCPD metabolism and toxicity.

Involvement of claudin-5 in H2S-induced acute lung injury

Acute exposure to hydrogen sulfide (H₂S) can cause fatal acute lung injury (ALI). However, the mechanisms of H₂S-induced ALI are still not fully understood. This study aims to investigate the role of the tight junction protein claudin-5 in H₂S-induced ALI. In our study, Sprague-Dawley (SD) rats were exposed to H₂S to establish the ALI model, and in parallel, human pulmonary microvascular endothelial cells (HPMECs) were incubated with NaHS (a H₂S donor) to establish a cell model. Lung immunohistochemistry and electron microscopy assays were used to identify H₂S-induced ALI, and the expression of claudin-5, p-AKT/t-AKT and p-FoxO1/t-FoxO1 was detected. Our results show that H₂S promoted the formation of ALI by morphological investigation and decreased claudin-5 expression. Dexamethasone (Dex) could partly attenuate NaHS-mediated claudin-5 downregulation, and the protective effects of Dex could be partially blocked by LY294002, a PI3K/AKT/FoxO1 pathway antagonist. Moreover, as a consequence of the altered phosphorylation of AKT and FoxO1, a change in claudin-5 with the same trend was observed. Therefore, the tight junction protein claudin-5 might be considered a therapeutic target for the treatment of ALI induced by H₂S and other hazardous gases.

Chronic stress and intestinal permeability: Lubiprostone regulates glucocorticoid receptor-mediated changes in colon epithelial tight junction proteins, barrier function, and visceral pain in the rodent and human

BACKGROUND

Chronic psychological stress is associated with increased intestinal epithelial permeability and visceral hyperalgesia. Lubiprostone, an agonist for chloride channel-2, promotes secretion and accelerates restoration of injury-induced epithelial barrier dysfunction. The mechanisms underlying how lubiprostone regulates colon epithelial barrier function and visceral hyperalgesia in chronic stress remain unknown.

METHODS

Male rats were subjected to water avoidance stress for 10 consecutive days. Lubiprostone was administered daily during the stress phase. Visceromotor response to colorectal distension was measured. Human colon crypts and cell lines were treated with cortisol and lubiprostone. The transepithelial electrical resistance and FITC-dextran permeability were assayed. Chromatin immunoprecipitation was conducted to assess glucocorticoid receptor binding at tight junction gene promoters.

KEY RESULTS

Lubiprostone significantly decreased chronic stress-induced visceral hyperalgesia in the rat (P < 0.05; n = 6). WA stress decreased occludin and claudin-1 and increased claudin-2 in rat colon crypts, which was prevented by lubiprostone. Cortisol treatment induced similar alterations of tight junction protein expression in Caco-2/BBE cells (P < 0.05) and significantly changed paracellular permeability in monolayers (P < 0.01). These changes were blocked by lubiprostone. Glucocorticoid receptor and its binding at occludin promoter region were decreased in cortisol-treated cells and human colon crypts, which was largely reversed by lubiprostone. In rat colonic cells, glucocorticoid receptor and its co-chaperone proteins were down-regulated after corticosterone treatment and lubiprostone reversed these changes.

CONCLUSIONS & INFERENCES

Lubiprostone preferentially prevents chronic stress-induced alterations of intestinal epithelial tight junctions, barrier function, and visceral hyperalgesia that was associated with modulation of glucocorticoid receptor expression and function.

Urate Transport via Paracellular Route across Epithelial Cells

Urate is the final oxidation product of purine metabolism in humans. We have recently reported that the paracellular route is the major urate transport pathway across the blood-placental barrier. In this study, the mechanism of urate paracellular transport was investigated in several epithelial cell lines including Madin-Darby canine kidney (MDCK) type I, Lilly Laboratories cell-porcine kidney 1 (LLC-PK1) and Caco-2 cells. Very little urate passed through MDCK and LLC-PK1 cell layers. In contrast, one of the Caco-2 cell lines was found to be urate-permeable. This urate paracellular movement across Caco-2 cell layer was not inhibited by the urate transporter inhibitor benzbromarone but was partially inhibited by 4,4'-diisothiocyanato-2,2'-stilbenedisulfonic acid (DIDS), which inhibits chloride transport. Detection and quantification of claudin proteins that are important for paracellular transport of ions were performed by LC/MS. Claudins 1, 3, 4, 6, 7 and 12 were detected in urate-permeable cell lines, BeWo cells and Caco-2 cells. We compared claudin expression patterns in urate-permeable and urate-non-permeable Caco-2 cells by LC/MS and found that claudin 12 had a higher expression level in urate-permeable Caco-2 cells. Overexpression of these claudins in MDCK cells did not increase urate paracellular transport. Although there were differences in claudin expression pattern between urate-permeable and non-permeable cells, increased expression of single claudin alone did not explain paracellular permeability of urate.

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.

Acute and chronic exposure to high levels of glucose modulates tight junction-associated epithelial barrier function in a renal tubular cell line

AIMS

Type 2 diabetes mellitus (T2DM) is one of the most prevalent diseases worldwide. Diabetic nephropathy (DN) is a complication of diabetes and the mechanisms underlying onset and progression of this disease are not fully understood. It has been shown that hyperglycemia is an independent factor to predict the development of DN in individuals with T2DM, however, a link between high plasma glucose levels and renal tubular injuries in DN remains unknown. In this study, we investigated the effect of high levels of glucose (i.e. 180 or 360mg/dL) for up to 24h (acute) or over 72h (chronic) upon tight junction (TJ)-mediated epithelial barrier integrity of the kidney tubular cell line, MDCK.

METHODS/KEY FINDINGS

High levels of glucose (180 and 360mg/dL) induced a decrease in transepithelial electrical resistance associated with an increase in TJ cation selectivity at 24h or in TJ permeability to a paracellular marker, Lucifer Yellow, at 72h-exposure when compared to control group (exposed to 100mg/dL glucose). Immunofluorescence analyses showed that glucose treatment induced a significant decrease in the tight junctional content of claudins-1 and -3 as well as a significant increase in claudin-2 (particularly at 24h-exposure) and a time-dependent change in occludin/ZO-1 junctional content. The analyses of total cell content of these junctional proteins by Western blot did not reveal significant changes, except in claudin-2 expression.

SIGNIFICANCE

Our data suggest that high levels of glucose induce time-dependence changes in TJ structure in MDCK monolayers, suggesting a possible link between hyperglycemia-induced tubular epithelial barrier disruption and diabetic nephropathy.

Water channels and barriers formed by claudins

Physiological studies in leaky epithelia, like kidney proximal tubules and the small intestine, have documented water transport via both transcellular and paracellular pathways. The discovery of aquaporin water channels provided a molecular basis for transcellular water movement. In contrast, the contribution, or even existence, of a specific paracellular water pathway has been disputed for a long time, until the cation channel-forming tight junction protein claudin-2 was shown to also permit the paracellular passage of water through its pore. In proximal kidney tubules, claudin-2-based water transport contributes 23-30% of the total water transport. Other paracellular ion channels (claudin-10a, -10b, and -17) proved to be impermeable to water, although their pore size would be sufficient for water molecules to pass. Studies of barrier-forming claudins, like claudin-1 and claudin-3, which tighten the paracellular pathway against ions and larger solutes, indicate that changes in the expression of these sealing claudins do not influence transepithelial water permeability. The present genetic, molecular, computational, and physiological studies are just now beginning to probe the mechanisms and regulation of paracellular permeation.

Systems Toxicology: Real World Applications and Opportunities

Systems Toxicology aims to change the basis of how adverse biological effects of xenobiotics are characterized from empirical end points to describing modes of action as adverse outcome pathways and perturbed networks. Toward this aim, Systems Toxicology entails the integration of in vitro and in vivo toxicity data with computational modeling. This evolving approach depends critically on data reliability and relevance, which in turn depends on the quality of experimental models and bioanalysis techniques used to generate toxicological data. Systems Toxicology involves the use of large-scale data streams ("big data"), such as those derived from omics measurements that require computational means for obtaining informative results. Thus, integrative analysis of multiple molecular measurements, particularly acquired by omics strategies, is a key approach in Systems Toxicology. In recent years, there have been significant advances centered on in vitro test systems and bioanalytical strategies, yet a frontier challenge concerns linking observed network perturbations to phenotypes, which will require understanding pathways and networks that give rise to adverse responses. This summary perspective from a 2016 Systems Toxicology meeting, an international conference held in the Alps of Switzerland, describes the limitations and opportunities of selected emerging applications in this rapidly advancing field. Systems Toxicology aims to change the basis of how adverse biological effects of xenobiotics are characterized, from empirical end points to pathways of toxicity. This requires the integration of in vitro and in vivo data with computational modeling. Test systems and bioanalytical technologies have made significant advances, but ensuring data reliability and relevance is an ongoing concern. The major challenge facing the new pathway approach is determining how to link observed network perturbations to phenotypic toxicity.

Physiological roles of claudins in kidney tubule paracellular transport

The paracellular pathways in renal tubular epithelia such as the proximal tubules, which reabsorb the largest fraction of filtered solutes and water and are leaky epithelia, are important routes for transepithelial transport of solutes and water. Movement occurs passively via an extracellular route through the tight junction between cells. The characteristics of paracellular transport vary among different nephron segments with leaky or tighter epithelia. Claudins expressed at tight junctions form pores and barriers for paracellular transport. Claudins are from a multigene family, comprising at least 27 members in mammals. Multiple claudins are expressed at tight junctions of individual nephron segments in a nephron segment-specific manner. Over the last decade, there have been advances in our understanding of the structure and functions of claudins. This paper is a review of our current knowledge of claudins, with special emphasis on their physiological roles in proximal tubule paracellular solute and water transport.

Tight junctions in pulmonary epithelia during lung inflammation

Inflammatory lung diseases like asthma bronchiale, chronic obstructive pulmonary disease and allergic airway inflammation are widespread public diseases that constitute an enormous burden to the health systems. Mainly classified as inflammatory diseases, the treatment focuses on strategies interfering with local inflammatory responses by the immune system. Inflammatory lung diseases predispose patients to severe lung failures like alveolar oedema, respiratory distress syndrome and acute lung injury. These life-threatening syndromes are caused by increased permeability of the alveolar and airway epithelium and exudate formation. However, the mechanism underlying epithelium barrier breakdown in the lung during inflammation is elusive. This review emphasises the role of the tight junction of the airway epithelium as the predominating structure conferring epithelial tightness and preventing exudate formation and the impact of inflammatory perturbations on their function.

Development of a chitosan-based tissue-engineered renal proximal tubule conduit

Renal proximal tubule cells (RPTCs) are responsible for glomerular filtration and maintenance of water/electrolyte balance. To regenerate a proximal tubule, sufficient cell numbers and normal cell function are requisite. Collagen has been routinely used as a substrate for culturing human RPTCs (HRPTCs); however, the role of biomaterials has not been thoroughly explored. In this study, RPTCs retrieved from human nephrectomy/nephroureterectomy specimens were cultivated on chitosan as a substrate in serum-free condition for up to 150 days. HRPTCs could maintain a typical epithelial morphology and the specific differentiation feature of transporting epithelia after such long-term culture. As compared with HRPTCs cultivated on collagen, those cultivated on chitosan showed more dome formation, higher Na⁺ -K⁺ ATPase expression, lower vimentin expression, and lower transepithelial electrical resistance, indicating that HRPTCs cultivated on chitosan presented better differentiation status and would be more functional with better active transportation. Thus, the current study indicates greater scope for the use of chitosan as a biomaterial for preparing a HRPTC-coated chitosan conduit, which might be useful for the scaffold design of tissue-engineered proximal tubules. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 9-20, 2018.

Placental claudin expression and its regulation by endogenous sex steroid hormones

Tight junctions (TJs) form continuous intercellular contacts controlling the paracellular transportation across the cell-to-cell junction. TJ components include the peripheral protein zonula occludens-1 (ZO-1), junctional adhesion molecules (JAMs), and integral proteins such as occludin and claudins. Among the junction proteins, claudins play a major role in regulation of paracellular electrolyte transportation. This study explores the expression and distribution of tight junctions and their regulation during pregnancy. To study the regulation of claudin family, we examined expression of mouse placental tight junction proteins, including claudin-1 to -24, with real-time PCR and Western blotting and distribution of tight junction proteins with immunohistochemistry. Pregnant C57/BL6 mice were used in this study. The pregnant mice were divided into three groups depending on pregnant day (on days 12, 16, and 20 of gestation). Regarding the transcription levels, claudin-1, claudin-2, claudin-4, and claudin-5 expression levels were relatively high compared to other claudin family in all periods of pregnancy. Claudin-4 and 5 expressions, which reduce ion permeability, were increased over a period of time. However, claudin-2 expression, that is the responsive protein for a decrease in paracellular conductance, was decreased. Following this modulation of expression during mid-term pregnancy, we identified endogenous hormonal modulation of claudin family using estrogen receptor antagonist ICI 182,780 and progesterone receptor antagonist RU-486. After administration of ICI and RU-486, expression of claudin-4 mRNA and protein was increased. In addition, immunohistochemistry was performed to identify their localization for inferring permeability in placenta. Due to the function of claudins as effectors of ion transport at the end of regulatory pathways, they must be transducing proteins that modulate the function of claudins and thus link the physiologic inputs to the final effectors. This study will provide the claudin expressions and their localization in the mouse placenta, and their regulation by endogenous hormones. Taken together, the results of this study may contribute to assuming the roles and regulatory mechanism of these tight junction genes regarding maternal-fetal ion transportation in the placenta.

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.

"The future of in vitro toxicology"

In 1959 Russell and Burch stated that "Mammalian tissue cultures have become one of the most important replacement techniques". Although they would likely be disappointed at the slow progress towards the 3Rs, I am sure they would be amazed and excited about the advances made. For example, omic technologies now provide an unbiased holistic view of cellular events and help the development of new mechanistic biomarker sets. New techniques to immortalise cells, such as telomerase overexpression, allow us to create human cell lines without introducing viral oncogenes and thereby provide a source of cells with normal phenotypes. In addition, the truly ground-breaking discovery of inducible pluripotent stem cells allows the possibility of generating target tissues from any individual. There has also been great advances in material science and bio-engineering, creating exciting new microfluidic devices which are well placed for the development of in vitro multi-organ systems. All of these advances have allowed us to better understand the mechanisms of how cells deal with chemical induced cellular perturbations. Utilising this knowledge and these tools, together with biokinetics we can create strategies that will be useful for regulatory purposes. Only in such an integrated interdisciplinary way will we ensure that human in vitro systems become critical toxicological tools, eventually replacing animal models altogether.