Differential expression and activation of MAP kinases in dedifferentiated MDCK-focus cells

Mitogen-activated protein kinases (MAPK) play a key role in the regulation of cellular processes such as cell growth, cell differentiation, and apoptosis. However, the specific function of single isoforms of the MAPK family in renal epithelial cell differentiation and/or proliferation has not been investigated so far. We now report stable reduction of extracellular signal-regulated kinase 1 (ERK1) protein expression and lack of serum-induced ERK1 activation in alkali-dedifferentiated Madin-Darby canine kidney-C7 focus (MDCK-C7F) cells compared with their parental epithelial MDCK-C7 cells. The changes in ERK1 protein expression and activation were accompanied by a small rise in c-jun NH2-terminal kinase 1 (JNK1) protein expression but slightly decreased basal and anisomycin-stimulated JNK1 activity. In contrast, ERK2 phosphorylation, as assessed by using an antibody which detects phosphorylated tyrosine 204 of both ERK1 and ERK2, as well as enzymatic ERK2 activity, was substantially increased in untreated and fetal calf serum-treated MDCK-C7F cells, although ERK2 protein expression remained unchanged. Differential expression and activation of ERK1, ERK2, and JNK1 were accompanied by an inhibition of serum-induced MDCK-C7F cell proliferation. Together, our results demonstrate an association between changes in the activation of certain MAPK and alkali-induced stable MDCK-C7 cell dedifferentiation. Moreover, these data provide evidence for distinct signaling functions of ERK1 and ERK2 in these cells.