ETO, but Not Leukemogenic Fusion Protein AML1/ETO, Augments RBP-Jκ/SHARP-Mediated Repression of Notch Target Genes

Abstract

Notch is a transmembrane receptor that determines cell fates and pattern formation in all animal species. After specific ligand binding, the intracellular part of Notch is cleaved off and translocates to the nucleus, where it targets the DNA binding protein RBP-J. In the absence of Notch, RBP-J represses Notch target genes by recruiting a corepressor complex. We and others have previously identified SHARP as one component of this complex. Here, we show that the corepressor ETO as well as the leukemogenic fusion protein AML1/ ETO directly interacts with SHARP, that ETO is part of the endogenous RBP-J-containing corepressor complex, and that ETO is found at Notch target gene promoters. In functional assays, corepressor ETO, but not AML1/ETO, augments SHARP-mediated repression in an histone deacetylase-dependent manner. Furthermore, either the knockdown of ETO or the overexpression of AML1/ETO activates Notch target genes. Therefore, we propose that AML1/ETO can disturb the normal, repressive function of ETO at Notch target genes. This activating (or derepressing) effect of AML1/ETO may contribute to its oncogenic potential in myeloid leukemia.

The Notch signaling pathway: Transcriptional regulation at Notch target genes

“…Moreover, we most recently demonstrated that ETO is an additional component of the SHARP corepressor complex [78], (Fig. 1B).…”

“…It is a member of the heterogeneous SPEN-homology (Split-ends) domain family implicated in biological processes from embryogenesis to ageing. SHARP contains RRMs (RNA Recognition Motifs) in the amino-terminal region and a highly conserved SPOC-domain at the very C-terminus [76] and reviewed in [77]; this domain has been crystallized and contains a highly conserved positively charged patch which is crucial for the interaction with corepressors SMRT/NCoR [78,79]. The very last 34 amino acids of the SPOC-domain of SHARP are required for binding to corepressors CtIP/CtBP [36] and ETO (Eight-twenty-one) (Fig.…”

“…The very last 34 amino acids of the SPOC-domain of SHARP are required for binding to corepressors CtIP/CtBP [36] and ETO (Eight-twenty-one) (Fig. 1) [78,79]. In addition, the SPOC-domain of SHARP can be phosphorylated by Pak1 (p21-activated kinase-1), and hence enhances SHARP corepressor function.…”

Mtg16/Eto2 Contributes to Murine T-Cell Development

“…Upon ligand binding, the Notch receptor is cleaved and the intracellular domain (ICD) of Notch moves to the nucleus and binds the transcription factor CBF1-Suppressor of Hairless-Lag1 (CSL) to activate transcription (26). MTGs appear to act as corepressors for CSL, and independent of CSL, Mtg16 also associates with the Notch ICD, suggesting that Mtg16 mediates some aspects of Notch functions (14,38).…”

“…The action of E proteins and Notch signaling are critical to T-cell development, and a potential role for Mtg16 in lymphopoiesis was further suggested by the identification of an association between MTGs and these pathways (9,14,19,38,41,48). Upon ligand binding, the Notch receptor is cleaved and the intracellular domain (ICD) of Notch moves to the nucleus and binds the transcription factor CBF1-Suppressor of Hairless-Lag1 (CSL) to activate transcription (26).…”