Inhibition of the mitogen activated protein kinase ERK1/2 amplifies ochratoxin A toxicity in the proximal tubule of the kidney

Ochratoxin A (OTA) is a mycotoxin showing nephrotoxic properties. OTA activates the mitogen activated protein kinases ERK, JNK and p38 in renal epithelial cells. In brief, activation of ERK supports mitosis, growth and differentiation, whereas JNK and p38 are considered to induce the opposite effects. The balance of the mentioned key protein kinases decides the further fate of the cell. In renal disease, the proximal tubule of the nephron often is affected first. Thus, we investigated the effect of OTA incubation (24 or 48 hours) on proximal tubular OK cells (oppossum) and/or NRK-52E cells (rat) in presence of an inhibitor of ERK1/2 activation (U0126). U0126 (25 μM) completely abolished ERK1/2 activation induced by OTA. Parameters indicating necrosis, apoptosis, epithelial tightness, fibrosis, dedifferentiation and inflammation were determined. In presence of U0126, OTA led to a decrease of cell number as compared to OTA alone. U0126 in presence of OTA increased LDH release as compared to OTA alone. OTA alone did not change epithelial integrity, whereas OTA in presence of U0126 reduced epithelial tightness. 100 nM OTA alone did not increase apoptosis, while addition of U0126 to OTA induced apoptotis. U0126 stimulated the basolateral deposition of collagen induced by OTA. Furthermore, as investigated by RT-PCR, the effect of OTA on markers of inflammation (NF-κB) and dedifferentiation (α-smooth muscle actin) was also more pronounced when ERK1/2 was inhibited. ERK1/2 inhibition enhanced the effects of OTA. Thus, activation of ERK1/2 after OTA is a protective mechanism. We conclude that ERK1/2 not only acts anti-apoptotic but also is beneficial on cell viability, epithelial tightness, interstitial fibrosis, inflammation and trans-differentiation. We further conclude that ERK1/2 is a key protection factor in the proximal tubule. However, long term OTA exposition could lead to clonal selection of kidney cells overexpressing ERK1/2. As strong expression of ERK1/2 is found in various tumours not only of the kidney, we hypothesize that the mentioned clonal selection could be a mechanism inducing the cancerogenic action discussed for OTA. Presented at the 25^th Mykotoxin Workshop in Giessen, Germany, May 19–21, 2003