The development of tamoxifen resistance (TamR) in oestrogen receptor positive (ER+) breast cancer is a major therapeutic challenge. Mechanisms suggested to ac-count for this have mainly focussed on the activation of alternative growth factor pathways. The insulin-like growth factor (IGF) axis is a prime candidate for investiga-tion in this area and the use of anti-IGF strategies in the clinical setting of tamoxifen resistance is under investigation. However such strategies, usually targeted to block the IGF-1 receptor (IGF-1R) have proved disappointing. The IGF axis is a multicom-ponent molecular system and the activity of IGF is modulated by the presence of six soluble high affinity IGF binding proteins (IGFBP 1-6). Given the potential role of the IGF axis in the development of tamoxifen resistance it is important to investigate whether the IGFBP family may play a role in this process opening up a route for alter-native anti-IGF based therapies. Using the ER+ MCF-7 cell line we demonstrated that five IGF axis genes (IGF-IR, IGF-2R, IGFBP-2, IGFBP-4 and IGFBP-5) were ex-pressed by both parental wt and tamoxifen resistant (TamR) MCF-7 cells with the re-maining genes (IGF-1, IGF-2, IGFBP-1, IGFBP-3 and IGFBP-6) either not expressed or expressed only at a very low level. IGFBP-5 expression was down-regulated by approximately 7-fold while IGFBP-2 was up-regulated by approximately 2-fold in TamR versus wt cells. These alterations in IGFBP-2 and IGFBP-5 gene expression were mirrored in protein levels measured in a conditioned medium by ELISA, Western and Ligand blot. Significantly, a knockdown of IGFBP-2 in TamR cells restored sensi-tivity to 4-hydroxytamoxifen (4-HT), reduced ERα expression to 45 ± 11.9% and en-hanced cell migration. Knock down of IGFBP-5 in wt cells had no effect on sensitivity to 4-HT but enhanced cell migration. Exogenous IGFBP-2 had no effect on tamoxifen sensitivity which may suggest an intracellular mechanism of action for IGFBP-2. Im-munohistochemical analysis of breast cancer tissue microarrays (TMAs) indicated that expression of IGFBP-2 was significantly associated with survival advantage in tamoxifen resistant patients
