Mitogen-activated protein kinases control p27/Kip1 expression and growth of human melanoma cells

The mitogen-activated protein kinases (MAPKs) extracellular signal-regulate d protein kinase (ERK)1 and ERK2, involved in regulating cell growth and di fferentiation, are constitutively active in A375 and WM239 human melanoma c ells. Using PD098059. an inhibitor of MAPK kinase (MEK), we investigated th e role of persistently activated ERK1/2 in cell growth. The inhibition of M APK activity induced a dose-dependent growth arrest in G(0)/G(1) phase. Cor respondingly, we observed the upregulation of the cyclin-dependent kinase ( Cdk) inhibitor p27/Kip1 and hypophosphorylation of the retinoblastoma prote in. Further studies showed that PD098059 treatment significantly decreased Cdk2 kinase activity, most probably owing to an augmented level of p27/Kip1 associated with cyclin E-Cdk2 complexes. The accumulation of p27/Kip1 prot ein in A375 cells was attributed to its increased stability. Our findings s uggest that constitutively active ERK1/2 kinases contribute to the growth o f melanoma cells by negative regulation of the p27/Kip 1 inhibitor.