Purpose: c-Src is an important adapter protein with oestrogen receptor (ER) and human epidermal growth factor receptor 2 (HER2), which validates it as an attractive target for the treatment of breast cancer. A specific c-Src inhibitor, 4-amino-5-(4-chlorophenyl)-7(t-butyl)pyrazolo[3,4-d]pyrinidine (PP2), was utilised to block c-Src activity to identify targeted vulnerabilities affected by ER and HER2 in a panel of breast cancer cell lines.
Methods: ER, growth factor receptors and signalling pathways were detected by Western-blot. The DNA content of the cells was determined by using a DNA fluorescence quantitation kit. Cell cycles were analysed by flow cytometry.
Results: The antiproliferative effect of PP2 closely correlated with the inhibition of c-Src mediated extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) and/or phosphoinositide 3-kinase (PI3K)/Akt growth pathways. Inhibition of c-Src tyrosine kinase predominantly blocked ER negative breast cancer cell growth, particularly the triple (i.e. ER, progesterone receptor (PR), and HER2) negative cells. In contrast, ER negative Sk-Br-3 cells with highest HER2 phosphorylation were resistant to PP2, in which hyper-activated HER2 directly regulated growth pathways. However, blocking c-Src recovered ER expression and down-regulated HER2 which made Sk-Br-3 cells regain responsiveness to 4-hydroxytamoxifen. The majority of ER positive cells were not sensitive to PP2 regardless of wild-type or endocrine resistant cell lines.
Conclusions: c-Src mediates the essential role of growth pathways in ER negative breast cancer cells. The ER positive and HER2 over-activation are two important predictive biomarkers for the resistance to a c-Src inhibitor. These data provided an important therapeutic rationale for patient selection in clinical trials with c-Src inhibitors in breast cancer.
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