LAUSR

The transcription factor YAP1 is a major effector of the tumor suppressive Hippo signaling pathway and is also necessary to maintain pluripotency in embryonic stem cells. Elevated levels of YAP1 expression antagonize the tumor suppressive effects of the Hippo pathway, that normally represses YAP1 function. High YAP1 expression is observed in several types of human cancers and is particularly prominent in cancer stem cells. The stem cell transcription factor Sox2, which marks and maintains cancer stem cells in osteosarcomas, promotes YAP1 expression by binding to an intronic enhancer element and YAP1 expression is also crucial for the maintainance of osteosarcoma stem cells. To further understand the regulation of YAP1 expression in osteosarcomas we subjected the YAP1 intronic enhancer to scanning mutagenesis to identify all DNA cis-elements critical for enhancer function. Through this approach we identified two novel transcription factors, GABP and MZF1, which are essential for basal YAP1 transcription. These factors are highly expressed in osteosarcomas and bind to distinct sites in the YAP1 enhancer. Depletion of either factor leads to drastically reduced YAP1 expression and thus a reversal of stem cell properties. We also found that YAP1 can regulate the expression of Sox2 by binding to two distinct DNA binding sites upstream and downstream of the Sox2 gene. Thus Sox2 and YAP1 reinforce each other expression to maintain stemness and tumorigenicity in osteosarcomas, but the activity of MZF1 and GABP is essential for YAP1 transcription. This article is protected by copyright. All rights reserved.