Abstract
The MEK1‐ERK1/2 signaling pathway has been implicated in the regulation of renal epithelial cell proliferation, epithelial‐to‐mesenchymal transition and the induction of an invasive cell phenotype. Much less information is available about the MEK5‐ERK5 module and its role in renal epithelial cell proliferation and differentiation. In the present study we have investigated the regulation of these two families of extracellular signal‐regulated kinases in epidermal growth factor (EGF)‐stimulated human kidney‐2 (HK‐2) cells and a possible interaction between ERK1/2 and ERK5. Here we report that 5 ng/ml EGF led to a strong stimulation of HK‐2 cell proliferation, which was largely U0126‐sensitive. Both synthetic MEK1/2 inhibitors U0126 and Cl‐1040, when used at 10 and 1 µM, respectively, inhibited basal and EGF‐induced ERK1/2 phosphorylation but not ERK5 phosphorylation. Long‐term inhibition of MEK1/2‐ERK1/2 signaling and/or vanadate‐sensitive protein phosphatases enhanced and prolonged EGF‐induced ERK5 phosphorylation, while transient expression of an adenoviral constitutively active MEK1 (Ad‐caMEK1) construct completely blocked EGF‐induced ERK5 phosphorylation. Expression of Ad‐caMEK1 in HK‐2 cells resulted in the upregulation of the dual‐specificity phosphatases MKP‐3/DUSP6, MKP‐1/DUSP1, and DUSP5. The EGF‐mediated time‐dependent induction of MKP‐3, MKP‐1 and DUSP5 mRNA levels was U0126‐sensitive at a concentration, which blocked EGF‐mediated ERK1/2 phosphorylation but not ERK5 phosphorylation. Furthermore, U0126 inhibited EGF‐induced MKP‐3 and MKP‐1 protein expression. Both MKP‐3 and MKP‐1 co‐immunoprecipitated with ERK5 in unstimulated as well as in EGF‐stimulated HK‐2 cells. These results suggest the existence of an ERK1/2‐driven negative feed‐back regulation of ERK5 signaling in EGF‐stimulated HK‐2 cells, which is mediated by MKP‐3, DUSP5 and/or MKP‐1. J. Cell. Physiol. 211: 88–100, 2007. © 2006 Wiley‐Liss, Inc.