RESUMEN
The E2F transcription factors mediate the activation or repression of key cell cycle regulatory genes under the control of the retinoblastoma protein (pRB) tumor suppressor and its relatives, p107 and p130. Here we investigate how E2F4, the major "repressive" E2F, contributes to pRB's tumor-suppressive properties. Remarkably, E2F4 loss suppresses the development of both pituitary and thyroid tumors in Rb(+/-) mice. Importantly, E2F4 loss also suppresses the inappropriate gene expression and proliferation of pRB-deficient cells. Biochemical analyses suggest that this tumor suppression occurs via a novel mechanism: E2F4 loss allows p107 and p130 to regulate the pRB-specific, activator E2Fs. We also detect these novel E2F complexes in pRB-deficient cells, suggesting that they play a significant role in the regulation of tumorigenesis in vivo.
Asunto(s)
Proteínas de Unión al ADN/deficiencia , Proteínas de Unión al ADN/genética , Regulación Neoplásica de la Expresión Génica , Proteínas , Proteína de Retinoblastoma/deficiencia , Proteína de Retinoblastoma/genética , Factores de Transcripción/deficiencia , Factores de Transcripción/genética , Animales , Western Blotting , Transformación Celular Neoplásica/genética , Células Cultivadas , Ciclina E/biosíntesis , Proteínas de Unión al ADN/metabolismo , Factor de Transcripción E2F4 , Fibroblastos/metabolismo , Ratones , Ratones Mutantes , Mutación , Proteínas Nucleares/metabolismo , Fosfoproteínas/metabolismo , Neoplasias Hipofisarias/genética , Proteína p107 Similar a la del Retinoblastoma , Proteína p130 Similar a la del Retinoblastoma , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Neoplasias de la Tiroides/genética , Factores de Transcripción/metabolismoRESUMEN
The E2F transcription factor family is known to play a key role in the timely expression of genes required for cell cycle progression and proliferation, but only a few E2F target genes have been identified. We explored the possibility that E2F regulators play a broader role by identifying additional genes bound by E2F in living human cells. A protocol was developed to identify genomic binding sites for DNA-binding factors in mammalian cells that combines immunoprecipitation of cross-linked protein-DNA complexes with DNA microarray analysis. Among approximately 1200 genes expressed during cell cycle entry, we found that the promoters of 127 were bound by the E2F4 transcription factor in primary fibroblasts. A subset of these targets was also bound by E2F1. Most previously identified target genes known to have roles in DNA replication and cell cycle control and represented on the microarray were confirmed by this analysis. We also identified a remarkable cadre of genes with no previous connection to E2F regulation, including genes that encode components of the DNA damage checkpoint and repair pathways, as well as factors involved in chromatin assembly/condensation, chromosome segregation, and the mitotic spindle checkpoint. Our data indicate that E2F directly links cell cycle progression with the coordinate regulation of genes essential for both the synthesis of DNA as well as its surveillance.