Introduction. Breast tumours frequently develop resistance to DNA-damaging therapy, and epidermal growth factor receptor (EGFR) may contribute to this process. EGFR can modulate the repair of DNA double strand breaks (DSB), forming nuclear complexes that include the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). Insulin-like growth factor binding protein-3 (IGFBP-3) is highly expressed in many triple-negative breast cancers (TNBC) and is associated with decreased disease-free survival. Our recent studies (Lin et al., Oncogene 33:85-96, 2014) indicated that DNA-damaging agents promote the interaction of IGFBP-3 with EGFR-DNA-PKcs complexes as part of the activation of DNA repair signalling. The aim of this study was to establish an in vivo model to study IGFBP-3 effects in DNA damage signalling, and to further determine the role of IGFBP-3 in regulating the breast cancer response to DNA-damaging therapies, leading to potential approaches for molecular targeted therapy of breast cancer.
Results. Stable knockdown of IGFBP-3 expression by shRNA in the TNBC cell line Hs578T inhibited tumour growth in vitro and more significantly in an orthotopic NOD/SCID xenograft mouse model (P=0.001), indicating strong growth stimulation by IGFBP-3. In both MDA-MB-468 and Hs578T cells, the activation of DNA-PK, ATM and other DNA repair signalling, induced by DNA-damaging agents etoposide, doxorubicin or neocarzinostatin, was decreased by IGFBP-3 silencing. Analysis of xenograft tumour tissues indicated that stable IGFBP-3 silencing also down-regulated total DNA-PKcs expression in Hs578T tumours.
Conclusion. The interaction of IGFBP-3 with EGFR and DNA-PKcs and its stimulation of breast tumour growth support its previously unrecognized involvement in facilitating DNA DSB repair in response to DNA damage. These novel findings suggest the possibility of a therapeutic approach for sensitizing triple-negative breast cancers to chemo- or radiotherapy by targeting the DNA repair function of IGFBP-3. (Supported by Australian Research Council (RCB) and Northern Translational Cancer Research Unit (MZL).