Integrin-Linked Kinase Controls Notch1 Signaling by Down-Regulation of Protein Stability through Fbw7 Ubiquitin Ligase

Abstract

Integrin-linked kinase (ILK) is a scaffold and protein kinase that acts as a pivotal effector in integrin signaling for various cellular functions. In this study, we found that ILK remarkably reduced the protein stability of Notch1 through Fbw7. The kinase activity of ILK was essential for the inhibition of Notch1 signaling. Notably, the protein level and transcriptional activity of the endogenous Notch1 intracellular domain (Notch1-IC) were higher in ILK-null cells than in ILK wild-type cells, and the level of endogenous Notch1-IC was increased by the blocking of the proteasome, suggesting that ILK enhances the proteasomal degradation of Notch1-IC. ILK directly bound and phosphorylated Notch1-IC, thereby facilitating proteasomal protein degradation through Fbw7. Furthermore, we found down-regulation of Notch1-IC and up-regulation of ILK in basal cell carcinoma and melanoma patients but not in squamous cell carcinoma patients. These results suggest that ILK down-regulated the protein stability of Notch1-IC through the ubiquitin-proteasome pathway by means of Fbw7.

We thank R. Kopan (Washington University Medical School, St. Louis, MO), S. Dedhar (British Columbia Cancer Agency, Canada), K. A Jones (Salk Institute), and B. E. Clurman (Fred Hutchinson Cancer Research Center) for providing Notch, ILK, GST-Notch1-IC, and Fbw7 clones. GSK-3β-null cells were kindly provided by J. R. Woodgett (Ontario Cancer Institute, Canada).

This work was supported by a grant from the Brain Research Center of the 21st Century Frontier Research Program, funded by the Ministry of Science and Technology (to H.-S. Park), and by a Korea Research Foundation grant (MOEHRD, Basic Research Promotion Fund) (KRF-2005-070-C00105) (to H.-S. Park) from the Republic of Korea.