Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.
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
Department of Medicine,5
Department of Surgery, Johns Hopkins University School of Medicine, Baltimore, Maryland 21231,3
European Molecular Biology Organization, Heidelberg, Germany4
Received for publication 16 February 2001.
Revision received 17 November 2001.
Accepted for publication 12 December 2001.
In neural development, Notch signaling plays a key role in restrictingneuronal differentiation, promoting the maintenance of progenitorcells. Classically, Notch signaling causes transactivation ofHairy-enhancer of Split (HES) genes which leads to transcriptionalrepression of neural determination and differentiation genes.We now report that in addition to its known transcriptionalmechanism, Notch signaling also leads to rapid degradation ofthe basic helix-loop-helix (bHLH) transcription factor humanachaete-scute homolog 1 (hASH1). Using recombinant adenovirusesexpressing active Notch1 in small-cell lung cancer cells, weshowed that the initial appearance of Notch1 coincided withthe loss of hASH1 protein, preceding the full decay of hASH1mRNA. Overexpression of HES1 alone was capable of down-regulatinghASH1 mRNA but could not replicate the acute reduction of hASH1protein induced by Notch1. When adenoviral hASH1 was coinfectedwith Notch1, we still observed a dramatic and abrupt loss ofthe exogenous hASH1 protein, despite high levels of ongoinghASH1 RNA expression. Notch1 treatment decreased the apparenthalf-life of the adenoviral hASH1 protein and increased thefraction of hASH1 which was polyubiquitinylated. The proteasomeinhibitor MG132 reversed the Notch1-induced degradation. TheNotch RAM domain was dispensable but a lack of the OPA and PESTdomains inactivated this Notch1 action. Overexpression of thehASH1-dimerizing partner E12 could protect hASH1 from degradation.This novel function of activated Notch to rapidly degrade aclass II bHLH protein may prove to be important in many contextsin development and in cancer.