Insulin-like growth factors (IGFs) regulate normal growth and development. In breast cancer, they stimulate cell proliferation, cell migration and inhibit apoptosis. The IGF signal transduction pathway is, therefore, a potential therapeutic target in the treatment of breast cancer [1, 2]. Inhibitors of the IGF pathway may be effective in the treatment of breast cancer with de novo or acquired endocrine resistance. We have studied IGF signalling in oestrogen nonresponsive MDA-MB-231, HBL-100 and BT-20 breast cancer cell lines as models of endocrine resistant breast cancer. Oestrogen responsive MCF-7 cells were also studied.

Components of the IGF signalling pathway, type I IGF Receptor (IGF1R), IRS-1, IRS-2, and the three Shc isoforms, were expressed at varying levels, demonstrating a range of phenotypes in the breast cancer cells. IRS-1 is expressed in a truncated form in the BT-20 cells as an antibody to the C-terminus is unable to detect the protein.

IGF-1 activated IGF1R, IRS-1, MAP kinase and Akt in the MCF-7, MDA-MB-231 and HBL-100 cell lines. IGF-1 stimulated phosphorylation of IGF1R in BT-20 cells but did not alter the level of activation of IRS-1, MAP kinase or Akt. The MEK1/2 inhibitor (PD 98059) and the PI-3 kinase inhibitor (LY 294002) decreased the level of phosphorylation of MAP kinase and Akt in BT-20 cells. A phosphospecific antibody to tyrosine 896, the Grb2 SH2 binding site, shows that IRS-1 is constitutively phosphorylated in BT-20 cells.

IGF-1 inhibited staurosporine-induced apoptosis in MCF-7, MDA-MB-231 and HBL-100 cells but not in BT-20 cells. Inhibition of the IGF signalling pathways with PD 98059 and LY 294002 sensitise MDA-MB-231 cells to staurosporine-induced apoptosis. IGF-1 stimulated growth in MCF-7 and MDA-MB-231 cells but not in BT-20 cells.

Expression and activation of IGF signalling proteins vary among the oestrogen nonresponsive cells. These differences will affect the response of breast cancer cells to IGF targeted therapy. BT-20 cells provide a useful model for constitutive IRS-1 phosphorylation which is reported to occur in breast tumours [3].