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Molecular and Cellular Biology, May 2002, p. 3129-3139, Vol. 22, No. 9
0270-7306/02/$04.00+0     DOI: 10.1128/MCB.22.9.3129-3139.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Notch Signaling Induces Rapid Degradation of Achaete-Scute Homolog 1

Virote Sriuranpong,1,2 Michael W. Borges,2 Christopher L. Strock,2 Eric K. Nakakura,2,3 D. Neil Watkins,2 Christine M. Blaumueller,4 Barry D. Nelkin,2 and Douglas W. Ball2,5*

Program in Cellular and Molecular Medicine,1 Oncology Center,2 Department of Medicine,5 Department of Surgery, Johns Hopkins University School of Medicine, Baltimore, Maryland 21231,3 European Molecular Biology Organization, Heidelberg, Germany4

Received 16 February 2001/ Returned for modification 17 November 2001/ Accepted 12 December 2001

In neural development, Notch signaling plays a key role in restricting neuronal differentiation, promoting the maintenance of progenitor cells. Classically, Notch signaling causes transactivation of Hairy-enhancer of Split (HES) genes which leads to transcriptional repression of neural determination and differentiation genes. We now report that in addition to its known transcriptional mechanism, Notch signaling also leads to rapid degradation of the basic helix-loop-helix (bHLH) transcription factor human achaete-scute homolog 1 (hASH1). Using recombinant adenoviruses expressing active Notch1 in small-cell lung cancer cells, we showed that the initial appearance of Notch1 coincided with the loss of hASH1 protein, preceding the full decay of hASH1 mRNA. Overexpression of HES1 alone was capable of down-regulating hASH1 mRNA but could not replicate the acute reduction of hASH1 protein induced by Notch1. When adenoviral hASH1 was coinfected with Notch1, we still observed a dramatic and abrupt loss of the exogenous hASH1 protein, despite high levels of ongoing hASH1 RNA expression. Notch1 treatment decreased the apparent half-life of the adenoviral hASH1 protein and increased the fraction of hASH1 which was polyubiquitinylated. The proteasome inhibitor MG132 reversed the Notch1-induced degradation. The Notch RAM domain was dispensable but a lack of the OPA and PEST domains inactivated this Notch1 action. Overexpression of the hASH1-dimerizing partner E12 could protect hASH1 from degradation. This novel function of activated Notch to rapidly degrade a class II bHLH protein may prove to be important in many contexts in development and in cancer.

* Corresponding author. Mailing address: Johns Hopkins Oncology Center, 1650 Orleans St., Room 553, Baltimore, MD 21231. Phone: (410) 955-8506. Fax: (410) 614-9884. E-mail: dball{at}jhu.edu.

Molecular and Cellular Biology, May 2002, p. 3129-3139, Vol. 22, No. 9
0022-538X/02/$04.00+0     DOI: 10.1128/MCB.22.9.3129-3139.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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