Notch Inhibition of E47 Supports the Existence of a Novel Signaling Pathway

Abstract

E47 is a widely expressed transcription factor that activates B-cell-specific immunoglobulin gene transcription and is required for early B-cell development. In an effort to identify processes that regulate E47, and potentially B-cell development, we found that activated Notch1 and Notch2 effectively inhibit E47 activity. Only the intact E47 protein was inhibited by Notch-fusion proteins containing isolated DNA binding and activation domains were unaffected-suggesting that Notch targets an atypical E47 cofactor. Although overexpression of the coactivator p300 partially reversed E47 inhibition, results of several assays indicated that p300/CBP is not a general target of Notch. Notch inhibition of E47 did not correlate with its ability to activate CBF1/RBP-J, the mammalian homolog of Suppressor of Hairless, a protein that associates physically with Notch and defines the only known Notch signaling pathway in drosophila. Importantly, E47 was inhibited independently of CBF1/RPB-J by Deltex, a second Notch-interacting protein. We provide evidence that Notch and Deltex may act on E47 by inhibiting signaling through Ras because (i) full E47 activity was found to be dependent on Ras and (ii) both Notch and Deltex inhibited GAL4-Jun, a hybrid transcription factor whose activity is dependent on signaling from Ras to SAPK/JNK.

The Notch signaling pathway: Transcriptional regulation at Notch target genes

“…Other Notch target genes are NFkB2 [29], Ifi-202, Ifi-204, Ifi-D3, and ADAM19 [19]. A number of other genes have been reported including Notch1 itself and Notch3 [16], bcl-2 [30] and E2A [31] and HoxA5, 9 and 10 [32].…”

Notch1 confers thymocytes a resistance to GC-induced apoptosis through Deltex1 by blocking the recruitment of p300 to the SRG3 promoter

“…25 We thus investigated the regulation of the SRG3 expression by Deltex1. By using real-time PCR analysis on 16610D9 cells infected with either retrovirus expressing Deltex1 (S003-Deltex1) or control vector (S003), we found that endogenous SRG3 mRNA was downregulated by Deltex1 (approximately 47% decrease) (Figure 4a).…”

“…26 Moreover, p300 can associate with the bHLH region and the amino-terminal region of E2A proteins. 21,25 Furthermore, Deltex1 was reported to bind directly to p300, thereby counteracting p300-dependent transcriptional activation by bHLH protein. 21 Therefore, we next examined whether both NotchIC and Deltex1 signaling sequesters the E47 away from coactivators such as p300 stimulating E2A transcriptional activation on the SRG3 promoter.…”

“…24,25 It has been shown that the inhibition of transcriptional activity of bHLH MASH1 and HNF4 is mediated by the direct physical interaction between Deltex1 and coactivator p300. 21 In addition, the interaction mechanism between E2A and p300/ CBP is reported in insulin gene transcription and in pre-B cells, respectively.…”

The Notch-Hes pathway in mammalian neural development

“…However, some Hes1-Hes5 double-null cells are still inhibited by caNotch-AP infection from differentiating as mature neurons [18], suggesting that Notch can still partially function independently of Hes1 and Hes5. The molecular nature of Hes1-Hes5-independent Notch pathway is a current hot topic [20,21] and remains to be analyzed.…”