Attenuation of Nrf2 expression and antioxidant response by ochratoxin A - the impact on the profibrotic effect

Ochratoxin A (OTA), a nephrotoxic mycotoxin, is considered as a causal agent of Balkan endemic nephropathy (BEN), a chronic kidney disease. Generation of oxidative stress by OTA has been suggested as one of the mechanisms implicated in its genotoxicity and cytotoxicity. Oxidative stress in cells activates expression of superoxide dismutase (SOD), glutathione-S-transferase (GST) and heme oxygenase (HO), the protective enzymes, expression of which is regulated by nuclear factor – erythroid 2 related factor (Nrf2) transcription factor. In cultured kidney tubulus cells (LLC-PK1) OTA diminished antioxidant response via attenuation of Nrf2, SOD, GST and HO expression. Moreover, this mycotoxin potently elevated expression of profibrotic agents: transforming growth factors-b (TGFb1 and TGFb2) with concomitant decrease in expression of proangiogenic genes, like vascular endothelial growth factor (VEGF) and erythropoietin (Epo). Adenoviral vectors were used to investigate the role of Nrf2 and HO-1 in OTA-induced toxicity. Overexpression of Nrf2 as well as application of pharmacological inducer of Nrf2 (PGJ2) led to attenuation of OTA-elevated TGFb expression and caused increase of OTA-diminished HO-1 and Epo mRNA level. However, genetic overexpression of HO1 did not prevent OTAinduced alterations. The phosphorylation of MAP kinases – JNK and ERK42/44 after OTA treatment was observed, and pharmacological inhibition of ERK42/44 compensated the effect of OTA on TGFb2 expression. Present data indicate the attenuation of antioxidant response as one of the mechanisms of OTA action. Nrf2 overexpression can compensate some but not all of OTAevoked alterations. Nevertheless, Nrf2 may be considered as one of the important factors in antifibrotic therapy.