Characterization of an ochratoxin-A-dedifferentiated and cloned renal epithelial cell line

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.

Ochratoxin A at low concentrations inhibits in vitro growth of canine umbilical cord matrix mesenchymal stem cells through oxidative chromatin and DNA damage

Ochratoxin A (OTA) exposure during pregnancy in laboratory animals induces delayed/abnormal embryo development. Foetal adnexa-derived mesenchymal stem cells (MSCs) could help evaluate the developmental risk of exposure to chemicals in advanced gestational age. We tested the effects of OTA at concentrations ranging from 2.5×10(-4) to 25nM on growth parameters of canine umbilical cord matrix (UCM)-derived MSCs. The hypothesis that oxidative chromatin and DNA damage could underlie OTA-mediated cell toxicity was also investigated. After in vitro exposure, OTA significantly decreased cell density and increased doubling time in a passage- and concentration-dependent manner and no exposed cells survived beyond passage 5. Significantly higher rates of cells showed condensed and fragmented chromatin and oxidized DNA, as assessed by OxyDNA assay. These findings showed that in vitro exposure to OTA, at picomolar levels, perturbs UCM-MSC growth parameters through oxidative chromatin and DNA damage, suggesting possible consequences on canine foetal development.

The food contaminant and nephrotoxin ochratoxin A enhances Wnt1 inducible signaling protein 1 and tumor necrosis factor-α expression in human primary proximal tubule cells

SCOPE

The underlying molecular mechanisms of nanomolar ochratoxin A (OTA) concentrations, especially those on pathophysiological relevant gene expression in target tissue and underlying signaling mechanisms are unknown.

METHODS AND RESULTS

qPCR arrays showed that 14 days exposure of human primary proximal tubule cells to 10 nM OTA influences the expression of genes that are related to inflammation, malignant transformation, and epithelial-to-mesenchymal transition. Wnt1 inducible signaling protein 1 (WISP1), an oncogenic, and profibrotic growth factor, turned out to be the gene with the strongest upregulation. Its expression, and that of TNF-α, an important inflammatory mediator, was further investigated in human renal cells and in primary human lung fibroblasts. OTA-induced upregulation of WISP1 and TNF-α occurs only in renal cells. Inhibition of ERK1/2 activation reverses the effect of OTA on WISP1 and TNF-α expression. Wnt or other signaling pathways were not involved. Upregulation of WISP1 and TNF-α occured independently of each other.

CONCLUSION

Long-term exposure of human kidney cells with OTA concentrations expectable in renal tissue due to average dietary intake leads in an ERK1/2-dependent manner to pathogenetic alterations of gene expression, notably WISP1 and TNF-α. Renal long-term risk by OTA is actually not excludable and argues for low but rational safety levels.

Ochratoxin A: An overview on toxicity and carcinogenicity in animals and humans

Ochratoxin A (OTA) is a ubiquitous mycotoxin produced by fungi of improperly stored food products. OTA is nephrotoxic and is suspected of being the main etiological agent responsible for human Balkan endemic nephropathy (BEN) and associated urinary tract tumours. Striking similarities between OTA-induced porcine nephropathy in pigs and BEN in humans are observed. International Agency for Research on Cancer (IARC) has classified OTA as a possible human carcinogen (group 2B). Currently, the mode of carcinogenic action by OTA is unknown. OTA is genotoxic following oxidative metabolism. This activity is thought to play a central role in OTA-mediated carcinogenesis and may be divided into direct (covalent DNA adduction) and indirect (oxidative DNA damage) mechanisms of action. Evidence for a direct mode of genotoxicity has been derived from the sensitive 32P-postlabelling assay. OTA facilitates guanine-specific DNA adducts in vitro and in rat and pig kidney orally dosed, one adduct comigrates with a synthetic carbon (C)-bonded C8-dG OTA adduct standard. In this paper, our current understanding of OTA toxicity and carcinogenicity are reviewed. The available evidence suggests that OTA is a genotoxic carcinogen by induction of oxidative DNA lesions coupled with direct DNA adducts via quinone formation. This mechanism of action should be used to establish acceptable intake levels of OTA from human food sources.

Toxicokinetics and toxicodynamics of ochratoxin A, an update

Ochratoxin A (OTA) is a mycotoxin produced by fungi of two genera: Penicillium and Aspergillus. OTA has been shown to be nephrotoxic, hepatotoxic, teratogenic and immunotoxic to several species of animals and to cause kidney and liver tumours in mice and rats. Because of differences in the physiology of animal species, wide variations are seen in the toxicokinetic patterns of absorption, distribution and elimination of the toxin. Biotransformation of OTA has not been entirely elucidated. At present, data regarding OTA metabolism are controversial. Several metabolites have been characterized in vitro and/or in vivo, whereas other metabolites remain to be characterized. Several major mechanisms have been shown as involved in the toxicity of OTA: inhibition of protein synthesis, promotion of membrane peroxidation, disruption of calcium homeostasis, inhibition of mitochondrial respiration and DNA damage. The contribution of metabolites in OTA genotoxicity and carcinogenicity is still unclear. The genotoxic status of OTA is still controversial because contradictory results were obtained in various microbial and mammalian tests, notably regarding the formation of DNA adducts. More recent studies are focused on the OTA ability to disturb cellular signalling and regulation, to modulate physiological signals and thereby to influence cells viability and proliferation. The present paper offers an update on these different issues. In addition since humans and animals are likely to be simultaneously exposed to several mycotoxins, especially through their diet, the little information available on the combined effects of OTA and other mycotoxins has also been reviewed.

Ochratoxin A at nanomolar concentrations: A signal modulator in renal cells

Ochratoxin A (OTA) is a ubiquitous fungal metabolite with nephrotoxic, carcinogenic, and apoptotic potential. Toxicokinetics make the kidney the primary target organ for OTA. Due to its widespread occurrence in improperly stored foodstuff the complete and safe avoidance of OTA for humans is impossible. There are several reports showing a significant correlation between OTA exposure and certain forms of nephropathies. At nanomolar concentrations OTA leads to specific changes of function and phenotype in renal cells. The toxin interacts with certain cellular "key-molecules" (e. g., mitogen-activated protein (MAP) kinases, Ca2+), thereby disturbing cellular signalling and regulation events as well as mitochondrial function. Moreover, OTA has the ability to modulate physiological signals (e. g., angiotensin II or TNFalpha) and thereby influences cell function and cell growth and may even stable re-program the cells (e. g., altered distribution of chromosomes). This review concentrates on the effects of OTA in the nanomolar range and its interactions with cellular signalling networks in different renal cells proposing OTA to act as a signal modulator.

Effect of the mycotoxin, ochratoxin A, on hormone-stimulated ion transport in a cultured cell model of the renal principal cell

The mycotoxin ochratoxin A (OTA) is a common contaminant of many foodstuffs and, consequently, is present in a large proportion of tested populations of humans and commercial animals. The predominant effects of OTA are manifested in the kidney where the severity varies from salt wasting to renal carcinoma formation in a concentration-dependent fashion. The MDCK-C7 renal cell culture model responds to various hormones known to regulate electrolyte and fluid balance and was used as a model to study the chronic effects of an acute exposure to low dose OTA. The natriferic hormones aldosterone and insulin-like growth factor 1 (IGF1) both stimulate Na(+) flux in a reabsorptive direction via activation of the epithelial Na(+) channel (ENaC). In contrast, anti-diuretic hormone (ADH) stimulates three separate and temporally distinct ion transport responses, one of which is Na(+) reabsorption. Treatment of MDCK-C7 cells with OTA (100 nM) for 48 h selectively and irreversibly inhibits hormone-stimulated Na(+) reabsorption via ENaC. This effect was retained for 48 cell passages after the removal of the toxin and mimics the OTA-induced salt-wasting that has been documented in clinical studies. These studies indicate that the effect of the toxin is genomic and therefore, likely to be long lasting in exposed animals and humans.

Evidence for epidermal growth factor receptor as negative-feedback control in aldosterone-induced Na+ reabsorption

Aldosterone enhances Na+ reabsorption via epithelial Na+ channels (ENaC). Aldosterone also stimulates the protein kinase ERK1/2- and the epidermal growth factor (EGF) receptor (EGFR)-signaling pathway. Yet EGF and ERK1/2 are known inhibitors of ENaC-mediated Na+ reabsorption. In the present study, using the well-established Madin-Darby canine kidney C7 cell line, we tested the hypothesis that EGFR represents a negative-feedback control for chronic aldosterone-induced Na+ reabsorption [amiloride-inhibitable short-circuit current ( Isc)]. Mineralocorticoid receptor expression was confirmed by RT-PCR and Western blot analysis. Aldosterone enhanced ERK1/2 phosphorylation in an EGFR-dependent way. Furthermore, aldosterone stimulated EGFR expression. Aldosterone (10 nmol/l) induced a small transient increase in Isc under control conditions. Inhibition of ERK1/2 phosphorylation with U-0126 (10 μmol/l) stimulated Isc, indicating constitutive ENaC inhibition. Aldosterone exerted a significantly larger effect in the presence of U-0126 than without U-0126. EGF (10 μg/l) inhibited Isc, whereas inhibition of EGFR kinase by tyrphostin AG-1478 (100 nmol/l) enhanced Isc. Aldosterone was more effective in the presence of AG-1478 than without AG-1478. In summary, we propose that the EGFR-signaling cascade can serve as a negative-feedback control to limit the effect of aldosterone-induced Na+ reabsorption.

Inhibitory effects of ochratoxin A on nerve growth factor-induced neurite extension through downregulation of p38 MAP kinase and AP-1 activation in cultured pheochromocytoma cells

Ochratoxin A (OTA) induces microcephaly in animals and in vitro cultured whole embryos. Inhibition of neuronal cell differentiation was proposed as underlying mechanisms responsible for OTA-induced microcephaly. Previously it was found that OTA inhibited differentiation of cultured rat embryonic midbrain cells into neurons. In this study, the influence of OTA on differentiation in PC-12 cells, a widely accepted model cells for study of neuronal differentiation was examined. Cell differentiation was assessed by measurement of neurite extension and quantified by the number of neurites extended. OTA decreased serum and nerve growth factor (NGF)-induced neurite extension in a concentration-dependent manner. Since MAP kinase and transcription factors have been implicated in cell differentiation of neuronal cells, and our previous study demonstrated that p38 MAP kinase and AP-1 are activated during PC 12 cell differentiation, the effect of OTA on NGF-induced p38 MAP kinase and transcription factor activation was examined. Co-treatment of OTA with NGF resulted in inhibition of NGF-induced p38 MAP kinase and AP-1 activation. Moreover, SB203580, a specific inhibitor of p38 MAP kinase blocked p38 MAP kinase and AP-1 activation accompanied by further inhibition of neurite extension. The present study shows that OTA inhibited cell differentiation of PC-12 cells, and this inhibitory effect may be related to inhibition of the activation of the p38 MAP kinase in conjunction with transcription factors AP-1. This finding suggests that the inhibitory effect on neuronal cell differentiation by OTA might be a mechanism responsible for OTA-induced microcephaly.

In vitro DNA and dGMP adducts formation caused by ochratoxin A

Ochratoxin A (OTA), a nephrotoxic and nephrocarcinogenic mycotoxin, leads to the formation of DNA adducts after administration to animals. This could be due to an epigenetic effect. In vitro assays can exclude an indirect effect, where the xenobiotic can generate, in vivo, endogenous reactive compounds which give adducts on DNA. Microsomes prepared from mice or rabbit kidney and liver, used as metabolic activators, were incubated in the presence of commercial salmon testes DNA and OTA, with NADPH or arachidonic acid used as cofactors. Upto 126 DNA adducts for 10(9) nucleotides were detected using the 32P postlabeling method after incubation with the mouse kidney system. Similar results were obtained with rabbit kidney microsomes. Using liver microsomes, the number of DNA adducts detected was much lower. When NADPH was used as a cosubstrate (to explore the cytochrome P450 metabolic pathways), with mice kidney microsomes, the adduct level was only 44% of the one obtained with arachidonic acid. These results lend support to the hypothesis of the preferential activation of OTA by the peroxidase activity of prostaglandin synthases and/or lipoxygenases to direct genotoxic metabolites, and are in agreement with the previously obtained results after in vivo treatment of mice. In order to identify the nucleotides of DNA modified by the OTA metabolites, dAMP, dGMP, dTMP and dCMP were used as substrates under the same conditions as with DNA. The adducts were found only on dGMP. The total adduct level was of 344 adducts per 10(9) nucleotides with the appearance of three major adducts in the presence of arachidonic acid. With NADPH, 271 adducts were obtained per 10(9) nucleotides, with again three major adducts, but only two of them were similar to two adducts obtained in the presence of arachidonic acid. Desferal (desferrioxamine B methanesulphonate), at a 50 microM concentration, did not reduce the adduct level. Adducts were also obtained when polydG, polydC and dG-p-dG were used as alternative substrates, whereas no adducts were obtained with polydA, polydT and polydC. The major adduct obtained after incubation of DNA with OTA, comigrated with the major adduct obtained with dGMP, in two chromatographic solvents. These results show that OTA is metabolized to genotoxic metabolite(s) which interact with the guanine residues of DNA.