E2F1 represses beta-catenin transcription and is antagonized by both pRB and CDK8

The E2F1 transcription factor can promote proliferation or apoptosis when activated, and is a key downstream target of the retinoblastoma tumour suppressor protein ( pRB). Here we show that E2F1 is a potent and specific inhibitor of beta-catenin/T-cell factor (TCF)- dependent transcription, and that this function contributes to E2F1- induced apoptosis. E2F1 deregulation suppresses beta-catenin activity in an adenomatous polyposis coli ( APC)/glycogen synthase kinase- 3 ( GSK3)- independent manner, reducing the expression of key beta-catenin targets including c- MYC. This interaction explains why colorectal tumours, which depend on beta- catenin transcription for their abnormal proliferation, keep RB1 intact. Remarkably, E2F1 activity is also repressed by cyclin- dependent kinase- 8 ( CDK8), a colorectal oncoprotein(1). Elevated levels of CDK8 protect beta-catenin/TCF-dependent transcription from inhibition by E2F1. Thus, by retaining RB1 and amplifying CDK8, colorectal tumour cells select conditions that collectively suppress E2F1 and enhance the activity of beta-catenin.