Abstract 3388: DDX5 modulates Notch signaling via interaction with the MAML1 co-activator and is required to optimal Notch signaling in leukemic cells

Abstract

Abstract Notch receptor-mediated signaling is essential for embryonic development and adult tissue homeostasis by regulating cell proliferation, differentiation and survival. Aberrant Notch signaling is linked to many epithelial cancers and hematological malignancies. For example, activated NOTCH1 mutations are detected in more than 50% of T cell acute lymphoblastic leukemia (T-ALL) patients. Since the constitutive activation of Notch signaling results in leukemic cell transformation and is required for the maintenance of the leukemic phenotype, the Notch pathway is a promising and highly specific molecular therapeutic target for T-ALL. We have previously identified the family of the MAML transcriptional co-activators as essential components of the core Notch transcription activation complex. The prototypic member MAML1 was found to be the major co-activator that regulates NOTCH1 oncogenic activities in leukemic cells. Hence, the modulation of MAML1 expression levels or functional activities is expected to affect Notch-induced leukemic initiation and progression. However, it remains unclear how MAML1 function is regulated. To gain insights into the molecular basis underlying MAML1 function, we undertook the task of identifying cellular factors that interact with MAML1 and characterizing their roles in regulating Notch signaling and Notch-mediated T cell leukemia. Through proteomic studies, we identified DDX5, an ATP-dependent DEAD-box RNA helicase, as a component in the MAML1 protein complex. Given the role of DDX5 as a transcriptional modulator, we reasoned that DDX5 might be a critical regulator of MAML1-mediated Notch target genes transcription. We showed that DDX5 interacts with MAML1 in vitro and in vivo, and is associated with the endogenous Notch1 transcription activation complex through interaction with MAML1. Lentiviral-mediated shRNA knockdown of DDX5 resulted in decreased expression of Notch target signature, along with reduced cell proliferation and survival in leukemic cells that carry activated NOTCH1 mutations. Our findings thus revealed an essential role of DDX5 in promoting efficient Notch-mediated transcription in leukemic cells where the Notch pathway is constitutively activated, suggesting that DDX5 might be critical for Notch-mediated T-ALL pathogenesis and could be a potential new target for modulating the Notch signaling in leukemia. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3388. doi:10.1158/1538-7445.AM2011-3388