SUSTAINED ACTIVATION OF EXTRACELLULAR-SIGNAL-REGULATED KINASE-1 (ERK1) IS REQUIRED FOR THE CONTINUED EXPRESSION OF CYCLIN D1 IN G(1) PHASE

In Chinese hamster embryo fibroblasts (IIC9 cells), platelet-derived g rowth factor (PDGF) stimulated mitogen-activated protein kinase/extrac ellular-signal-regulated kinase (MAP kinase/ERK) activity, but not tha t of c-jun N-terminal kinase (JNK), and induced G(1) phase progression . ERK1 activation was biphasic and was sustained throughout the G(1) p hase of the cell cycle. PDGF induced cyclin DI protein and mRNA levels in a time-dependent manner. Inhibition of PDGF-induced ERK1 activity by the addition of a selective inhibitor of MEK1 (MAP kinase kinase/ER K kinase 1) activation, PD98059, or transfection with a dominant-negat ive ERK1 (dnERK(-)) was correlated with growth arrest. In contrast, gr owth was unaffected by expression of dominant-negative JNK (dnJNK(-)). Interestingly, addition of PD98059 or dnERK(-), but not dnJNK(-), res ulted in a dramatic decrease in cyclin D1 protein and mRNA levels, con comitant with a decrease in cyclin D1-cyclin-dependent kinase activity . To investigate the importance of sustained ERK1 activation, ERK1 act ivity was blocked by the addition of PD98059 throughout G(1). Addition of PD98059 up to 4 h after PDGF treatment decreased ERK1 activity to the levels found in growth-arrested IIC9 cells. Loss of cyclin D1 mRNA and protein expression was observed within 1 h after inhibition of th e second sustained phase of ERK1 activity. Disruption of sustained ERK 1 activity also resulted in G(1) growth arrest. These data provide evi dence for a role for sustained ERK activity in controlling G(1) progre ssion through positive regulation of the continued expression of cycli n D1, a protein known to positively regulate G(1) progression.