RESUMO
E2F transcription factors play an essential role in cell proliferation and apoptosis and their activity is frequently deregulated in human cancers. In a yeast two-hybrid screen we identified a novel E2F-binding protein. Due to its strong phosphorylation we named it EAPP (e2F-associated phosphoprotein). EAPP is localized in the nucleus and interacts with E2F-1, E2F-2, and E2F-3, but not with E2F-4. Examination of a number of human cell lines revealed that EAPP levels are elevated in most transformed cells. Moreover, EAPP mRNA was detected in all investigated human tissues in varying amounts. EAPP is present throughout the cell cycle but disappears during mitosis. In transfection assays with reporters controlled by either an artificial E2F-dependent promoter or the murine thymidine kinase promoter, EAPP increased the activation caused by E2F-1 but not by E2F-4. Surprisingly, the promoter of the p14(ARF) gene, which was also activated by E2F-1, became repressed by EAPP. Overexpression of EAPP in U2OS cells resulted in a significant increase of cells in S-phase, whereas RNAi-mediated knock down of EAPP reduced the fraction of cells in S-phase. Taken together, these data suggest that EAPP modulates E2F-regulated transcription, stimulates proliferation, and may be involved in the malignant transformation of cells.
Assuntos
Proteínas de Transporte/genética , Proteínas de Transporte/metabolismo , Proteínas de Ciclo Celular/metabolismo , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/metabolismo , Fosfoproteínas/genética , Fosfoproteínas/metabolismo , Fatores de Transcrição/genética , Fatores de Transcrição/metabolismo , Sequência de Aminoácidos , Animais , Sequência de Bases , Linhagem Celular , Transformação Celular Neoplásica , Clonagem Molecular , DNA Complementar/genética , Fatores de Transcrição E2F , Fator de Transcrição E2F1 , Fator de Transcrição E2F2 , Fator de Transcrição E2F3 , Fator de Transcrição E2F4 , Células HeLa , Humanos , Camundongos , Dados de Sequência Molecular , Interferência de RNA , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Fase S , Homologia de Sequência de Aminoácidos , Transcrição Gênica , Transfecção , Técnicas do Sistema de Duplo-HíbridoRESUMO
Expression of the trophoblast-specific subunit of human chorionic gonadotropin, CGbeta, is associated with fusion of cytotrophoblasts into a multinuclear syncytium. Here, we studied regulation of the CGbeta5 gene in trophoblasts undergoing in vitro syncytialization. Transfection of luciferase reporters harboring different lengths of the CGbeta5 upstream sequence revealed that the proximal promoter region (-345 to +114) is sufficient to govern differentiation-dependent induction. Mutational analyses suggested that two selective promoter factor (Sp) and three activating protein 2 (AP-2) recognition sequences are necessary for full activity of the promoter. During syncytialization these elements interacted with increasing amounts of the transcription factors Sp1, Sp3, and AP-2alpha in electrophoretic mobility shift assay, but only AP-2alpha binding rose upon elevation of cAMP levels with forskolin. Increasing expression of different isoforms of Sp1 and Sp3 could also be detected by Western blot analyses. Sp1/Sp3 localized to syncytial nuclei both in differentiated cultures and in term placental tissue, suggesting assembly of functional transcriptional complexes. Costaining of the transcription factors with E-cadherin on term placental sections revealed that 47 and 33% of cytotrophoblast nuclei were negative for Sp1 and Sp3, respectively. In contrast, immunohistochemistry of early tissue demonstrated expression of Sp1 in the majority of cyto- and syncytiotrophoblasts, whereas Sp3 was absent from the syncytium. Sp1 and Sp3 induced wild-type/mutant promoter constructs upon transfection in Sp-deficient SL-2 cells, indicating that the Sp elements function as activating sequences. The data suggest that increasing concentrations of Sp1, Sp3, and AP-2alpha enhance transcription of CGbeta in differentiating term trophoblasts, whereas a different combination of factors may control expression in early placentas.
