Regulation of ErbB Signaling in Breast Cancer Epithelial Cells by Cb1 Proto-Oncogene Product

摘要

A large body of evidence has shown that members of the epidermal growth factor (EUF) family play critical roles in the proliferation and differentiation of normal breast epithelial cells and are important for cell proliferation in breast cancer. The response to EGF is mediated by the transmembrane receptor tyrosine ErbB 1, or epidermal growth factor receptor (EGF-R). This receptor plays a key role in the induction of proliferation in mammary epithelial cells (MECs). Overexpression of EGF-R due to gene amplification or increased transcription is seen in a third or more of all breast cancers. Receptor overexpression may be causally linked to oncogenesis, and correlates with development and progression of tumors that are refractile to standard therapeutic approaches. Thus, EGF-R is a potential target for therapeutic intervention in the subset of women with worst prognosis on conventional treatment. Understanding and manipulating the biochemical mechanisms regulating signal transduction through EGF-R therefore represent important goals in breast cancer research, and are the focus of this study. We have proposed the hypothesis that the proto-oncoprotein Cbl provides ligand- specific negative regulation of EGF-R in epithelial cells. We and others have shown that Cbl effects EGF- mediated down-regulation, ubiquitination, and degradation of EOF-R. In the 8.5 months of this reporting period (following reactivation of the award at the place of the P.I.'s independent appointment), we have tested whether Cbl's recruitment to the receptor and its ability to ubiquitinate EGF-R depends on the nature of the stimulating ligand. In vivo, EGF stimulation of EGF-R leads to receptor downregulation and the termination of signaling, but TGF-alpha stimulation of EUF-R leads to receptor recycling and prolonged signaling. We have found that EGF-activated EUF-R binds to Cbl and undergoes polyubiquitination, but TGF-alpha-activated EGF-Rdoes the same.