ErbB2 activation of Notch1 signaling in breast cancer

Abstract

3684

Notch signaling is evolutionarily conserved and plays a crucial role in regulating diverse cell fate decisions during development. Because malignant transformation is associated with an alteration in cell fate determination, changes in Notch signaling can potentially contribute to cancer development. The t(7:9)(q34;q34.4) translocation associated with T-cell leukemia encodes a constitutively active truncated Notch1 fragment, and gain-of-function mutations in Notch1 or Notch4 have been reported in breast cancer. In this study, we determined that Notch1 was constitutively active in invasive breast cancer and that Notch1 signaling interfaced with the initiation and maintenance of oncogenesis mediated by the ErbB2 signaling pathway, which is frequently activated in breast cancer. Constitutively active intracellular Notch1 was highly expressed in approximately 40% of invasive human breast cancers by tissue microarray analysis, and higher levels of activated Notch1 were also found in several breast cancer cell lines. In the latter instances, Notch activation was associated with either lower levels of Numb, a negative regulator or Notch, or with overexpression of ErbB2 and high levels of Musashi1, a negative regulator of Numb. Conditional expression of dominant-negative Notch1 in MDA-MB-231 cells inhibited growth and correlated with inhibition of Notch1-dependent transcription. Breast cancer cells expressing activated ErbB2/NeuT exhibited marked activation of Notch1-dependent reporter gene activity that was blocked by a selective ErbB2 antagonist, CP-654577. In addition, transformation of breast epithelial cells by NeuT was blocked by L-685,458, a γ-secretase inhibitor of Notch processing, which correlated with inhibition of Notch- and β-catenin/TCF-dependent reporter gene activity. These results suggest for the first that Notch1 is constitutively active in a high percentage of invasive breast cancers, and that Notch1 activity is linked to ErbB2 activation. These data provide a basis for determining whether inhibition of both ErbB2 activation and Notch processing may be an effective approach for breast cancer therapy.