RESUMEN
E2F transcription factors play a pivotal role in the regulation of cellular proliferation and can be subdivided into activating and repressing family members [1]. Like other E2Fs, E2F6 binds to E2F consensus sites, but in contrast to E2F1-5, it lacks an Rb binding domain and functions as an Rb-independent transcriptional repressor [2, 3, 4 and 5]. Instead, E2F6 has been shown to complex with Polycomb (PcG) group proteins [6 and 7], which have a well-established role in gene silencing. Here, we show that E2F6 plays an unexpected and essential role in the tissue specificity of gene expression. E2F6-deficient mice ubiquitously express the alpha-tubulin 3 and 7 genes, which are expressed strictly testis-specifically in control mice. Like an additional E2F6 target gene, Tex12, that we identified, tubulin 3 and 7 are normally expressed in male germ cells only. The promoters of the alpha-tubulin and Tex12 genes share a perfectly conserved E2F site, which E2F6 binds to. Mechanistically, E2F6-mediated repression involves CpG hypermethylation locking target promoters in an inactive state. Thus, E2F6 is essential for the long-term somatic silencing of certain male-germ-cell-specific genes, but it is dispensable for cell-cycle regulation.
Asunto(s)
Metilación de ADN , Regulación de la Expresión Génica , Testículo/metabolismo , Factores de Transcripción/genética , Factores de Transcripción/metabolismo , Tubulina (Proteína)/metabolismo , Animales , Secuencia de Bases , Sitios de Unión , Inmunoprecipitación de Cromatina , Cromatografía de Afinidad , Huella de ADN , Cartilla de ADN , Factor de Transcripción E2F6 , Ensayo de Cambio de Movilidad Electroforética , Masculino , Ratones , Ratones Noqueados , Análisis por Micromatrices , Datos de Secuencia Molecular , Regiones Promotoras Genéticas/genética , Alineación de Secuencia , Análisis de Secuencia de ADNRESUMEN
Resistance to the anti-neoplastic drug paclitaxel is frequent in breast cancer patients. Most studies of paclitaxel resistance have focused on pathways that elicit cellular response, while little is known about players involved in the acquirement of taxane resistance. By screening a cohort of breast cancer cell lines, we observed a correlation between level of protein Daxx and response to paclitaxel. Cells lines expressing increased level of Daxx displayed a robust paclitaxel response with nearly all cells undergoing micronucleation, while cell lines with low amount of Daxx showed a decrease in micronucleation, and accumulation in mitosis. At used paclitaxel concentrations, apoptotic levels were negligible in all cell lines tested. Human cell lines expressing anti-Daxx siRNA as well as Daxx-/- mouse fibroblasts showed similar cellular response to paclitaxel. Importantly, absence or depletion of Daxx resulted in cell survival after paclitaxel treatment, as measured by colony formation assay. We conclude that Daxx may be an important predictive factor in cellular response to paclitaxel, which emphasizes a critical but unknown function of this protein in mitotic progression, which, when disabled, leads to survival advantages upon paclitaxel treatment.