Raf/MEK/MAPK Signaling Stimulates the Nuclear Translocation and Transactivating Activity of FOXM1

The proliferation-associated transcription factor FOXM1 is essential for cell cycle progression into mitosis. Using synchronized human fibroblasts we detected, by immunostaining, that FOXM1 is localized predominantly in the cytoplasm in cells at late-G1 and S phases. Nuclear translocation occurs just before progression into the G2/M phase of the cell cycle and requires activity of the Raf/MEK/MAPK signaling pathway. Using pharmacological modulators, we demonstrated that activity of the Raf/MEK/MAPK pathway is both necessary and sufficient for the nuclear translocation of FOXM1. Consistent with FoxM1c being the major isoform expressed in proliferating fibroblasts, constitutively active MEK1 enhances the transactivating effect of FOXM1c, but not FOXM1b, on the cyclin B1 promoter in transient reporter assays. Here, we describe in detail the methods involved in generating these findings, which support the notion that FOXM1 is an effector of Raf/MEK/MAPK signaling in G2/M regulation.