Tubulotoxic mechanisms of ochratoxin A

Comparative Ochratoxin Toxicity: A Review of the Available Data

Ochratoxins are a group of mycotoxins produced by a variety of moulds. Ochratoxin A (OTA), the most prominent member of this toxin family, was first described by van der Merwe et al. in Nature in 1965. Dietary exposure to OTA represents a serious health issue and has been associated with several human and animal diseases including poultry ochratoxicosis, porcine nephropathy, human endemic nephropathies and urinary tract tumours in humans. More than 30 years ago, OTA was shown to be carcinogenic in rodents and since then extensive research has been performed in order to investigate its mode of action, however, this is still under debate. OTA is regarded as the most toxic family member, however, other ochratoxins or their metabolites and, in particular, ochratoxin mixtures or combinations with other mycotoxins may represent serious threats to human and animal health. This review summarises and evaluates current knowledge about the differential and comparative toxicity of the ochratoxin group.

Role of microRNA-29b in the ochratoxin A-induced enhanced collagen formation in human kidney cells

Ochratoxin A (OTA) is an ubiquitous mycotoxin suspected to cause fibrotic kidney diseases. The involvement of mircoRNAs in these processes is unknown. Here, we investigated the role of the anti-fibrotic miR-29b in OTA-induced alterations of cellular collagen homeostasis. OTA exposure of human embryonic kidney cells (HEK293) cells led to an increase of collagen I, III and IV protein amounts without changes in collagen mRNA expression levels, indicating post-transcriptionally mediated mechanisms potentially involving microRNAs and 3'UTRs of collagen mRNAs. This was confirmed by enhanced luciferase activity of a collagen1A1-3'UTR reporter plasmid after OTA exposure. OTA also enhanced the luciferase activity of a reporter plasmid containing the seed region of miR-29b showing that OTA diminishes miR-29b action. Additionally, OTA induced an altered intracellular distribution of miR-29b leading to decreased cytoplasmic abundance of miR-29b. Abundantly added miR-29b (miR-29b clamp) completely prevented OTA-induced collagen formation. In summary, we show that OTA has the potential to initiate or support the development of fibrotic kidney diseases by involving post-transcriptional regulation mechanisms comprising miR-29b. OTA reduces the impact of miR-29b and thus enhances collagen protein expression. These findings allow a new perspective on how the exposure to nanomolar OTA concentrations can lead to fibrotic tissue alterations.

A Journey Through Mitogen-Activated Protein Kinase and Ochratoxin A Interactions

Ochratoxin A (OTA) is a ubiquitous mycotoxin with potential nephrotoxic, carcinogenic, and cytotoxic action. It has been proposed that OTA might be involved in the development of Balkan endemic nephropathy, which is associated with an increased risk of urinary tract tumours, and of other forms of interstitial nephritis. Cell susceptibility to OTA mainly depends on mycotoxin concentrations, duration of exposure, and intracellular molecular and genetic context. OTA can affect a cell by stimulating or inhibiting certain signalling pathways such as mitogen-activated protein kinase (MAPK). Three major mammalian MAPKs have been described: extracellular signal-regulated protein kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK. All MAPKs regulate diverse cellular programmes, but in most cases ERKs have been linked to cell survival, while JNKs, and p38 MAPKs have been implicated in cell death by apoptosis. This review looks into OTA-mediated MAPK activation and its effects.

Identification and in vitro cytotoxicity of ochratoxin A degradation products formed during coffee roasting

The mycotoxin ochratoxin A is degraded by up to 90% during coffee roasting. In order to investigate this degradation, model heating experiments with ochratoxin A were carried out, and the reaction products were analyzed by HPLC-DAD and HPLC-MS/MS. Two ochratoxin A degradation products were identified, and their structure and absolute configuration were determined. As degradation reactions, the isomerization to 14-(R)-ochratoxin A and the decarboxylation to 14-decarboxy-ochratoxin A were identified. Subsequently, an analytical method for the determination of these compounds in roasted coffee was developed. Quantification was carried out by HPLC-MS/MS and the use of stable isotope dilution analysis. By using this method for the analysis of 15 coffee samples from the German market, it could be shown that, during coffee roasting, the ochratoxin A diastereomer 14-(R)-ochratoxin A was formed in amounts of up to 25.6% relative to ochratoxin A. The decarboxylation product was formed only in traces. For toxicity evaluations, first preliminary cell culture assays were performed with the two new substances. Both degradation products exhibited higher IC50 values and caused apoptotic effects with higher concentrations than ochratoxin A in cultured human kidney epithelial cells. Thus, these cell culture data suggest that the degradation products are less cytotoxic than ochratoxin A.

Long-term effects of ochratoxin A on fibrosis and cell death in human proximal tubule or fibroblast cells in primary culture

Ochratoxin A (OTA) is a mycotoxin produced by several fungi which grow on human food source material. Consumption of OTA is almost unavoidable. The consumption leads to low but detectable amounts of OTA in human blood. Risk assessment of OTA is based on studies performed either in animals or cultured cells. So far, mainly cell lines of different origin were used. To be as close as possible to the situation in humans with respect to the experimental setup, we studied the effect of OTA in human proximal tubule cells (RPTEC) and human fibroblasts in primary culture. OTA was administered at concentrations ranging from 0.3 nmol/l up to 10 micromol/l for time periods up to 14 days. Apoptotic and necrotic cell death, collagen I, III, IV and fibronectin secretion as well as NF-kappaB activation were studied. Under our experimental conditions OTA exerted comparable effects on caspase-3 activity and necrosis in both cell types, however RPTEC were more sensitive (order of 10). Surprisingly, very low concentrations of OTA (0.3-10nM) led to cell hypertrophy during prolonged exposure (14 days). RPTEC but not fibroblasts responded with an increase of NF-kappaB activity and collagen III as well as fibronectin secretion underlining the profibrotic action of OTA in the kidney. Collagen I and IV secretion was only slightly changed. The results presented here give good reasons to re-asses the risk of OTA consumption leading to low blood concentrations which have so far been considered harmless.

A rapid screening method to test apoptotic synergisms of ochratoxin A with other nephrotoxic substances

The body may be exposed simultaneously to more than one nephrotoxic substance and to measure the effects of the great number of possible combinations of nephrotoxins will rapidly become a great challenge when using the traditional methods. Therefore, we developed a rapid and cost-efficient method to screen the apoptotic potential of combinations of known cell- or nephrotoxic substances as ochratoxin A (OTA), cisplatin, cadmium, H(2)O(2), and amphotericin B on renal epithelial cell lines. The cells were seeded in 96-well plates and the apoptotic and necrotic potential of different combinations of nephrotoxins was determined. We found different results for the combinations used: depending on the concentrations of the various substances, antagonistic, additive, or potentiating effects on caspase-3 activity were found after co-exposure to OTA. We conclude that the co-exposure of renal cells to OTA with other substances can enhance or reduce the apoptotic potential of one substance alone depending on the substance, the concentration and on the cell line investigated. A "harmless" substance can thus convert to a potent cell toxic substance when combined with OTA or vice versa. The underlying mechanisms of the synergistic effects remain unknown.

The Nephrotoxin Ochratoxin A Induces Key Parameters of Chronic Interstitial Nephropathy in Renal Proximal Tubular Cells

Ochratoxin A (OTA) is a nephrotoxic and cancerogenic mycotoxin. There is epidemiological evidence that OTA exposition leads to cortical interstitial nephropathies in humans. However, virtually no data are available investigating the effect of OTA on renal cortical cells with respect to induction of nephropathy. Thus, we investigated whether OTA is able to induce changes of cellular properties potentially leading to interstitial nephropathy, using proximal tubular cell lines (OK, NRK-52E). OTA decreased cell number and cell protein time and dose dependently. Accordingly we investigated the effect of 100 nM or 1000 nM OTA. The decline of cell number after OTA exposure is due to necrosis and apoptosis, as measured by LDH release or DNA ladder formation and caspase-3 activation, respectively. OTA incubation of proximal tubular cells also resulted in a loss of epithelial tightness as determined by diffusion of FITC labeled inulin. Inflammation, fibrosis and epithelial-to-mesenchymal transition are described in chronic interstitial renal disease. Therefore, we also investigated the effect of OTA on NFkappaB activity, collagen secretion and generation of alpha smooth muscle actin. OTA alone was sufficient to induce the latter parameters in proximal tubular cells. Finally, OTA is a nephrotoxcic substance and elevated activity of mitogen activated protein kinases (MAPK) is described in nephropathies. As we investigated the effect of OTA on activity of ERK, JNK and p38 by ELISA, we found that OTA activates the MAPK measured dose dependently. In summary, OTA induced phenomena typical for chronic interstitial nephropathy, like loss of cells and epithelial tightness, necrosis and apoptosis as well as markers of inflammation, fibrosis and epithelial-to-mesenchymal transition in proximal tubular cells. Thus, we could show for the first time that OTA is able to induce key parameters of nephropathy in proximal tubular cells in culture. Moreover OTA interacts with MAPK and thus may exert its specific toxic actions.

Exposure to nephrotoxic Ochratoxin A enhances collagen secretion in human renal proximal tubular cells

Ochratoxin A (OTA) is a nephrotoxic mycotoxin. There is evidence that OTA leads to cortical interstitial nephropathies in humans, associated with fibrosis. No data are available on the effect of OTA-induced collagen secretion from renal cortical cells. As kidney cortex mainly consists of proximal tubules, we investigated the effect of OTA on particular collagens (I, III, IV) in a well-established proximal tubular cell line (opossum kidney (OK) cells) and in primary cultured human renal proximal tubular epithelial cells (RPTECs). In fibroblasts, OTA neither exerted toxic effects nor induced collagen secretion, most probably due to the absence of suitable uptake mechanisms. OTA exerted time- and dose-dependent toxicity in both OK cells and human RPTECs. Moreover, OTA induced collagen secretion in a time- and dose-dependent manner in both cell types. In opposite to transforming growth factor beta1 (TGF-beta1), OTA incubation induced increased apical secretion of the basement membrane collagen IV. This might be evidence for a loss of cellular polarity after OTA incubation. We conclude that in proximal tubular cells, OTA is able to induce extracellular matrix deposition. As collagen secretion was also inducible in primary cultured human RPTECs, we hypothesize that OTA-induced extracellular matrix deposition by proximal tubular cells may be of importance in generation of renal diseases in humans which are under suspicion of being induced by OTA.

Proximal Tubular Toxicity of Ochratoxin A Is Amplified by Simultaneous Inhibition of the Extracellular Signal-Regulated Kinases 1/2

Ochratoxin A (OTA) is a mycotoxin involved in the development of chronic nephropathies and a known carcinogen. As we have shown previously, OTA activates mitogen-activated protein kinases [extracellular signal-regulated kinase 1 and 2 (ERK1/2), c-jun amino-terminal kinase (JNK), and extracellular-regulated protein kinase 38 (p38)] in proximal tubular cells (opossum kidney and normal rat kidney epithelial). ERK1/2, JNK, or p38 are thought to mediate opposite action on apoptosis, fibrosis, and inflammation. As we have already shown, OTA activates the latter processes. Here, we investigated the effect of OTA in the absence or presence of the ERK1/2 inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4bis(2-aminophenylthio)-butadiene] to test whether OTA then will exert increased toxicity. In the presence of ERK1/2 inhibition, OTA decreased cell number and protein to a significantly larger extent compared with OTA alone. The same was true for epithelial tightness, apoptosis (caspase-3 activity), and necrosis (lactate dehydrogenase release). Furthermore, simultaneous inhibition of ERK1/2 amplified the effect of OTA on markers of inflammation (nuclear factor of the kappa-enhancer in B cells activity), fibrosis (collagen secretion), and epithelial mesenchymal transition (alpha smooth muscle actin). OTA induces phenomena typical for chronic interstitial nephropathy and activates ERK1/2, JNK, and p38 in proximal tubular cells. Inhibition of ERK1/2 aggravates the effects of OTA or even induces toxicity at normally nontoxic concentrations. This is highly likely due to activation of JNK and p38. Our data indicate a new mechanistic explanation for the toxic actions induced by OTA, and they are notable with respect to a possible coexposition of the kidney to OTA and naturally occurring ERK1/2 inhibitors. Finally, our data give rise to an attractive hypothesis on the coincidence of increased OTA exposition and urinary tract tumors in humans.

New insights into the mechanisms involved in renal proximal tubular damage induced in vitro by ochratoxin A

The mycotoxin ochratoxin A is a contaminant of human and animal food products. It is a potent nephrotoxin known to damage the proximal tubule. The aim of this work was to investigate the effects of ochratoxin A on a porcine renal proximal tubular epithelial cell line (LLC-PK1), and to identify sensitive endpoints revealing damage at the epithelial barrier level and at the molecular level. Cells exposed for 24 h to 5-10 microM ochratoxin indicated a clear damage to the intactness of the epithelial barrier, as shown by measurements of trans-epithelial resistance and zonula occludens-1 protein expression. At the mitochondrial level we observed alterations of the normal functions, such as an increase of the membrane potential, the formation of straight extensions, and the formation of giant mitochondria. At higher ochratoxin concentrations (50 microM), at which cytotoxicity assays revealed a significant toxicity, alterations of the cytoskeleton organization and induction of apoptosis were evident. In addition, we analyzed the expression of genes by using a cDNA macroarray. Our data indicate that ochratoxin-induced nephrotoxicity can be detected at the barrier and at the mitochondrial level at rather low concentrations, at which conventional cytotoxicity assays are unable to reveal toxic effects.

Apoptosis and oxidative stress induced by ochratoxin A in rat kidney

Ochratoxin A (OTA) is a widespread mycotoxin produced by several species of fungi. OTA induces a tubular-interstitial nephropathy in humans and in animals. It has been implicated as one of the aetiological agents involved in the development of endemic nephropathy. OTA-induced oxidative stress and apoptosis may play key roles in the development of chronic tubulointerstitial nephritis connected to the long-term exposure to this food contaminant. We studied the effects of low doses of OTA on kidney cells. Wistar rats were treated with 120 microg OTA/kg bodyweight daily, for 10, 30 or 60 days. Toxin concentration in kidney was proportional to the time of exposure, and amounted to 547.2, 752.5 and 930.3 ng OTA/g kidney tissue after 10, 30 and 60 days, respectively. OTA treatment caused an increased number of cells undergoing apoptosis in both proximal and distal epithelial kidney cells. The apoptotic cells were visualised using the TUNEL assay and staining with haematoxylin and eosin in situ. The number of apoptotic cells in rats treated for 10, 30 and 60 days increased by 5-, 6.4- and 12.7-fold, respectively, compared with the control cells. However, DNA electrophoresis did not show characteristic fragmentation (DNA laddering). The oxidative stress was evident via increased malondialdehyde formation. The concentration of lipid peroxides showed an increase (36%), but the activity of superoxide dismutase decreased (26%) in 60-day treated rats. In spite of the observed biochemical and morphological changes in the kidney cells, renal functional status was preserved to the end of experiment. This study demonstrates that a combination of morphologic and biochemical markers can be used to monitor early cell death in OTA-induced renal injury. We have shown that the exposure to the relatively low OTA concentrations has activated apoptotic processes and oxidative damage in kidney cells.

Effects of the mycotoxin ochratoxin A and some of its metabolites on human kidney cell lines

The modulations of complement-regulating surface proteins on a human embryonic and a renal carcinoma cell line are described regarding the effects of ochratoxin A and some of its metabolites on the surface markers CD46, CD55 and CD59. Membrane integrity, cell proliferation and metabolic activity were reduced to different extents, depending on the kind of mycotoxin and the dosage, which was ranging from 10 to 1000 ng/ml. The number of cells carrying surface markers was suppressed significantly at 1000 ng/ml, in some cases even at 100 ng/ml, whereas the intensity of receptor expression on the positive cells was found to be stimulated. The fraction RE2 (OTC) isolated from an OTA-containing crude toxin surpassed the effects of all other ochratoxin metabolites. Apart from well-known cytotoxic and genotoxic effects modulation of cell surface marker expression by low concentrations of OTA and OTC deserves more attention with regard to its immuno-pathogenic importance. Furthermore, occurrence and impact of the mycotoxin OTC should be studied more into detail.

Programmed cell death by interaction of ochratoxin A with other nephrotoxins

We determined the effects of co-exposure of OTA and other (potentially) nephrotoxic substances on renal tubule cells (IHKE, from the proximal tubule; MDCK-C7 cells, representing principal cells of the collecting duct) by measuring the activation of caspase-3, an enzyme with key roles in the process of programmed cell death or apoptosis.Substances used in combination with OTA included: cadmium, a known nephrotoxin with apoptotic effects, cisplatin (anticancer drug, also a known nephrotoxin), cyclosporin A (an immunosuppresive agent), H2O2 (generated during oxidative cell stress), amphotericin B (antifungal agent), and fumonisin B1. Because of the great number of possible combinations, a method based on the 96-well format was developed which allowed a fast and cost-efficient succesive measurement of caspase-3 activity and protein content in one well as well as necrotic effects in parallel.We found that cells responded differently to the various combinations: for example, 1 µM OTA combined with 100 µM cisplatin had an antagonizing effect on caspase-3 activation in IHKE cells but potentiating effects in MDCK-C7. DNA ladder formation confirmed the results. Necrotic effects were very small and additive. We conclude that the co-exposure of renal cells to OTA with other substances can enhance or reduce the apoptotic potential of one substance alone depending on the substance and on the cell line investigated. A "harmless" substance can thus convert to a potent cell toxic substance when combined with OTA or other mycotoxins.These first findings show the necessity to further investigate the combined effects of OTA or other mycotoxins with other substances or of mycotoxins with each other.

Interaction of ochratoxin A with human serum albumin. Binding sites localized by competitive interactions with the native protein and its recombinant fragments

Competitive interactions of ochratoxin A (OTA) and several other acidic compounds were utilized to gain insight into the localization of binding sites and the nature of binding interactions between anionic species and human serum albumin (HSA). Depolarization of OTA fluorescence in the presence of a competing anion was used to quantify ligand-protein interactions. The results obtained were rationalized in terms of OTA displacement from its major binding site. Based on their ability to displace OTA, two distinct groups of the anionic ligands were revealed. The first group contained structurally diverse compounds that shared a common binding site in subdomain IIA (Sudlow Site I). The second group consisted of three non-steroidal anti-inflammatory drugs, which showed much lower affinity to Site I than the OTA dianion. The major site for these drugs was located in domain III. Fluorescence spectroscopy measurements of OTA, warfarin (WAR) and naproxen (NAP) complexes with recombinant proteins corresponding to the domains of HSA (D1-D3) revealed binding to all domains but with different affinities. The binding constants for OTA and WAR decreased in the series D2z.Gt;D3>D1. In contrast, NAP showed the most favorable interaction with D3 and comparable affinities to the two remaining domains. The OTA binding constant for D2, 7.9 x 10(5) M(-1), was smaller than the largest constant for HSA by a factor of approximately 7. The binding constant for OTA with D3, 1.1 x 10(5) M(-1), was very close to that of the secondary binding site for HSA.

The mycotoxin ochratoxin A alters intestinal barrier and absorption functions but has no effect on chloride secretion

Ochratoxin A (OTA) is a mycotoxin that contaminates cereals and animal feed and causes nephropathy to a variety of animal species. OTA is also known as a potent immunotoxic, teratogenic, and carcinogenic mycotoxin. In addition, OTA ingestion induces intestinal injuries, including inflammation and diarrhea. With the aim to study the cellular mechanisms associated with the intestinal toxicity of OTA, two human epithelial intestinal cell lines (HT-29-D4 and Caco-2-14 cells), widely used as in vitro models for the intestinal epithelium, were incubated with OTA. The main effects of the mycotoxin were an inhibition of cellular growth and a dramatic decrease of transepithelial resistance in both cell lines. Since transepithelial resistance reflects the organization of tight junctions over the cell monolayer, these data may suggest that OTA could potentiate its own absorption through paracellular pathways. OTA induced a 60% decrease of sodium-dependent glucose absorption but increased the absorption of fructose and L-serine in HT-29-D4 cells. Moreover, the mycotoxin did not inhibit the cAMP-dependent chloride secretion through the cystic fibrosis transmembrane conductance regulator channel. The inhibitory effect of OTA on active glucose transport was partially antagonized by L-phenylalanine, but not by alpha-tocopherol, suggesting that the toxicity of OTA could result from an inhibition of protein synthesis, rather than an induction of lipid peroxidation. In particular, OTA affected the protein content of plasma membrane microdomains, which are known to regulate tight junction assembly and intestinal transport activity. Taken together, these data showed that OTA alters both barrier and absorption functions of the intestinal epithelium.