RESUMEN
Tumorigenesis is frequently accompanied by enhanced rRNA transcription, but the signaling mechanisms responsible for such enhancement remain unclear. Here, we report evidence suggesting a novel link between deregulated Hedgehog signaling and the augmented rRNA transcription in cancer. Aberrant activation of the Hedgehog pathway in keratinocytes is a hallmark of basal cell carcinoma (BCC), the most common cancer in light-skinned individuals. We show that Gli proteins, downstream effectors of the Hedgehog pathway, increase expression of a novel rRNA gene (rDNA) transcription factor, basonuclin, whose expression is markedly elevated in BCCs. The promoter of the human basonuclin gene contains a Gli-binding site, which is required for Gli protein binding and transcriptional activation. We show also that the level of 47S pre-rRNA is much higher in BCCs than in normal epidermis, suggesting an accelerated rRNA transcription in the neoplastic cells. Within BCC, those cells expressing the highest level of basonuclin also exhibit the greatest increase in 47S pre-rRNA, consistent with a role for basonuclin in increasing rRNA transcription in these cells. Our data suggest that Hedgehog-Gli pathway enhances rRNA transcription in BCC by increasing basonuclin gene expression.
Asunto(s)
Carcinoma Basocelular/genética , Carcinoma Basocelular/metabolismo , Regulación Neoplásica de la Expresión Génica/genética , Proteínas Oncogénicas/genética , Biosíntesis de Proteínas , Neoplasias Cutáneas/genética , Neoplasias Cutáneas/metabolismo , Factores de Transcripción/genética , Secuencia de Bases , Sitios de Unión , Proteínas de Unión al ADN , Células HeLa , Humanos , Datos de Secuencia Molecular , Proteínas Oncogénicas/biosíntesis , Fosfoproteínas , Regiones Promotoras Genéticas , Proteínas/genética , Proteínas/metabolismo , ARN Ribosómico/genética , ARN Ribosómico/metabolismo , Transactivadores , Factores de Transcripción/biosíntesis , Activación Transcripcional , Transfección , Regulación hacia Arriba , Proteína con Dedos de Zinc GLI1RESUMEN
The Gli family of zinc finger transcription factors are mediators of Shh signalling in vertebrates. In previous studies, we showed that Shh signalling, via an essential Gli-binding site in the Myf5 epaxial somite (ES) enhancer, is required for the specification of epaxial muscle progenitor cells. Shh signalling is also required for the normal mediolateral patterning of myogenic cells within the somite. In this study, we investigate the role and the transcriptional activities of Gli proteins during somite myogenesis in the mouse embryo. We report that Gli genes are differentially expressed in the mouse somite. Gli2 and Gli3 are essential for Gli1 expression in somites, establishing Gli2 and Gli3 as primary mediators and Gli1 as a secondary mediator of Shh signalling. Combining genetic studies with the use of a transgenic mouse line expressing a reporter gene under the control of the Myf5 epaxial somite enhancer, we show that Gli2 or Gli3 is required for Myf5 activation in the epaxial muscle progenitor cells. Furthermore, Gli3, but not Gli2 represses Myf5 transcription in a dose-dependent manner in the absence of Shh. Finally, we provide evidence that hypaxial and myotomal gene expression is mispatterned in Gli2-/-Gli3-/- and Gli3-/-Shh-/- somites. Together, our data demonstrate both positive and negative regulatory functions for Gli2 and Gli3 in the control of Myf5 activation in the epaxial muscle progenitor cells and in dorsoventral and mediolateral patterning of the somite.