Insulin-like Growth Factor Binding Protein 2 (IGFBP-2) Promotes Growth and Survival of Breast Epithelial Cells: Novel Regulation of the Estrogen Receptor

In breast tumours insulin-like growth factor binding protein-2 (IGFBP-2) is elevated and the presence of IGFBP-2 has been shown to correlate with malignancy. However, how IGFBP-2 contributes to the malignant state is still unclear. Silencing IGFBP-2 blocked cell proliferation and in MCF-7 cells increased cell death indicating that IGFBP-2 was acting in both a mitogenic and a survival capacity. Exogenous IGFBP-2 acting via integrin receptors to reduce PTEN levels protected these cells against death induced by various chemotherapeutic agents. This was dependent upon a functional ER-α as silencing ER-α blocked the ability of IGFBP-2 to confer cell survival. Loss of IGFBP-2 increased levels of PTEN and improved chemo-sensitivity of the cells confirming its role as a survival factor. Silencing IGFBP-2 had no effect on the response to IGF-II but responses to estrogen and Tamoxifen were no longer observed due to loss of ER-α, which could be prevented by inhibition of PTEN. Conversely, exogenous IGFBP-2 increased ER-α mRNA and protein in both normal and cancer cells via its interaction with integrin receptors. These actions of IGFBP-2 on ER-α involved the IGF-I receptor and activation of PI3K in the cancer cells but were independent of this in normal breast cells. The production of IGFBP-2 by breast cancer cells enhances their proliferative potential, increases their survival and protects them against chemotherapy-induced death. IGFBP-2 not only modulates IGFs and directly regulates PTEN but also has a role in maintaining ER-α expression.