Notch signaling is important for numerous cellular processes including cell proliferation, differentiation, and cell-fate decisions. It is also becoming increasingly apparent that the Notch pathway is significant in the pathogenesis of several human tumors. We have previously demonstrated the importance of Notch-1 in the biology of glioma cells, showing that Notch-1 and its ligands, Dll-1 and Jagged-1, are over-expressed in glioma cell lines and primary human gliomas and that siRNA-mediated down-regulation of Notch-1 impairs glioma cell survival and proliferation. In order to elucidate the mechanisms underlying the function of Notch-1 in human gliomas, we generated gene expression profiles from glioma cells transfected with Notch-1 siRNA. Analysis of this microarray data showed that transcription of the epidermal growth factor receptor (EGFR) gene is markedly decreased. Since EGFR is also commonly over-expressed and/or mutated in glioma cells and is important for various oncogenic processes including cellular proliferation, we investigated its possible relationship to Notch-1. Utilizing RT-PCR and Western blot analyses, we have shown that transfection of human glioma cells with Notch-1 siRNA lowers EGFR mRNA transcription and protein expression, respectively. Analysis of primary human glioblastoma samples has also revealed a positive correlation between the expression of Notch-1 and EGFR. In considering potential mediators of the relationship between Notch-1 and EGFR, we investigated prior observations demonstrating that the tumor suppressor protein p53 regulates EGFR expression. We have now shown that down-regulation of Notch-1 results in diminished p53 transcript and protein expression, as well as diminished p53 activity as assayed using a p53-luciferase reporter plasmid. Studies are ongoing to determine the significance of other potential mechanisms of Notch-1 regulation of EGFR expression. Various levels of crosstalk between the Notch and EGFR pathways have been described in a number of studies but none have demonstrated a role for Notch-1 in the regulation of EGFR expression. Given the significance of EGFR in various malignant phenotypes, this finding further illustrates the implications of increased Notch-1 activity in cancer and supports the potential utility of novel therapeutics designed to abrogate Notch-1 signaling as well as combination therapies with EGFR inhibitors.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]