Blockade of the extracellular signal-regulated kinase pathway induces marked G(1) cell cycle arrest and apoptosis in tumor cells in which the pathwayis constitutively activated - Up-regulation of p27(Kip1)

Constitutive activation of the ERK pathway is associated with the neoplasti c phenotype of a relatively large number of human tumor cells. Blockade of the ERK pathway by treatment with PD98059, a specific inhibitor of mitogen- activated protein (MAP) kinase/ERK kinase (MEK), completely suppressed the growth of tumor cells in which the pathway is constitutively activated (RPM I-SE and HT1080 cells). Consistent with its prominent antiproliferative eff ect, PD98059 induced a remarkable G(1) cell cycle arrest, followed by a mod est apoptotic response, in these tumor cells. Selective up-regulation of p2 7(Kip1) was observed after PD98059 treatment of RPMI-SE and HT1080 cells. O verexpression in RPMI-SE cells of either a kinase-negative form of MEK1 or wild-type MAP kinase phosphatase-3 also induced up-regulation of p27(Kip1), The up-regulation of p27(Kip1) correlated with increased association of p2 7(Kip1) With cyclin E-cyclin-dependent kinase (CDK) 2 complexes, a concomit ant inhibition of cyclin E-CDK2 kinase activity, and a consequent decrease in the phosphorylation state of retinoblastoma protein, which would culmina te in the marked G(1), cell cycle arrest observed in these tumor cells. The se results suggest that the complete growth suppression that follows specif ic blockade of the ERK pathway in tumor cells in which the pathway is const itutively activated is mediated by up-regulation of p27(Kip1).