Proteomics Demonstration That Normal Breast Epithelial Cells Can Induce Apoptosis of Breast Cancer Cells through Insulin-like Growth

Normal breast epithelial cells are known to exert an apo-ptotic effect on breast cancer cells, resulting in a potential paracrine inhibition of breast tumor development. In this study we purified and characterized the apoptosis-induc-ing factors secreted by normal breast epithelial cells. Conditioned medium was concentrated by ultrafiltration and separated on reverse phase Sep-Pak C18 and HPLC. The proapoptotic activity of eluted fractions was tested on MCF-7 breast cancer cells, and nano-LC-nano-ESI-MS/MS allowed the identification of insulin-like growth factor-binding protein-3 (IGFBP-3) and maspin as the pro-apoptotic factors produced by normal breast epithelial cells. Western blot analysis of conditioned media con-firmed the specific secretion of IGFBP-3 and maspin by normal cells but not by breast cancer cells. Immunodeple-tion of IGFBP-3 and maspin completely abolished the normal cell-induced apoptosis of cancer cells, and re-combinant proteins reproduced the effect of normal cell-conditioned medium on apoptosis of breast cancer cells. Together our results indicated that normal breast epithelial cells can induce apoptosis of breast cancer cells through IGFBP-3 and maspin. These findings pro-vide a molecular hypothesis for the long observed inhib-itory effect of normal surrounding cells on breast cance