title: Raf/MEK/MAPK signaling stimulates the nuclear translocation and transactivating activity of FOXM1c.
abstract: The forkhead box (FOX) transcription factor FOXM1 is ubiquitously expressed in proliferating cells. FOXM1 expression peaks at the G2/M phase of the cell cycle and its functional deficiency in mice leads to defects in mitosis. To investigate the role of FOXM1 in the cell cycle, we used synchronized hTERT-BJ1 fibroblasts to examine the cell cycle-dependent regulation of FOXM1 function. We observed that FOXM1 is localized mainly in the cytoplasm in cells at late-G1 and S phases. Nuclear translocation occurs just before entry into the G2/M phase and is associated with phosphorylation of FOXM1. Consistent with the dependency of FOXM1 function on mitogenic signals, nuclear translocation of FOXM1 requires activity of the Raf/MEK/MAPK signaling pathway and is enhanced by the MAPK activator aurintricarboxylic acid. This activating effect was suppressed by the MEK1/2 inhibitor U0126. In transient reporter assays, constitutively active MEK1 enhances the transactivating effect of FOXM1c, but not FOXM1b, on the cyclin B1 promoter. RT-PCR analysis confirmed that different cell lines and tissues predominantly express the FOXM1c transcript. Mutations of two ERK1/2 target sequences within FOXM1c completely abolish the MEK1 enhancing effect, suggesting a direct link between Raf/MEK/MAPK signaling and FOXM1 function. Importantly, inhibition of Raf/MEK/MAPK signaling by U0126 led to suppression of FOXM1 target gene expression and delayed progression through G2/M, verifying the functional relevance of FOXM1 activation by MEK1. In summary, we provide the first evidence that Raf/MEK/MAPK signaling exerts its G2/M regulatory effect via FOXM1c.
authors.affiliation: Department of Biochemistry, Faculty of Medicine, The University of Hong Kong, 3/F Laboratory Block, The Faculty of Medicine Building, 21 Sassoon Road, Pokfulam, Hong Kong, China.
authors.forename: Richard Y M
authors.initials: RY
authors.lastname: Ma
authors.forename: Tommy H K
authors.initials: TH
authors.lastname: Tong
authors.forename: Alice M S
authors.initials: AM
authors.lastname: Cheung
authors.forename: Anthony C C
authors.initials: AC
authors.lastname: Tsang
authors.forename: Wai Ying
authors.initials: WY
authors.lastname: Leung
authors.forename: Kwok-Ming
authors.initials: KM
authors.lastname: Yao
mesh_terms: D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D000072278:Forkhead Box Protein M1; D051858:Forkhead Transcription Factors; D006801:Humans; D020935:MAP Kinase Signaling System; D051379:Mice; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D008969:Molecular Sequence Data; D010766:Phosphorylation; D020033:Protein Isoforms / Q000378:metabolism; D015534:Trans-Activators / Q000737:chemistry / Q000378:metabolism*; D014157:Transcription Factors / Q000737:chemistry / Q000378:metabolism*; D015533:Transcriptional Activation; D048490:raf Kinases / Q000378:metabolism
chemical_list: C107585:FOXM1 protein, human; D000072278:Forkhead Box Protein M1; D051858:Forkhead Transcription Factors; D020033:Protein Isoforms; D015534:Trans-Activators; D014157:Transcription Factors; D048490:raf Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; D020929:Mitogen-Activated Protein Kinase Kinases