As breast cancer cells develop secondary resistance to estrogen deprivation therapy, they increase their utilization of non-genomic signaling pathways. Our prior work demonstrated that estradiol causes an association of ERα with Shc, Src and the IGF-1-R. In cells developing resistance to estrogen deprivation (surrogate for aromatase inhibition) and to the anti-estrogens tamoxifen, 4-OHtamoxifen, and fulvestrant, an increased association of ERα with c-Src and the EGF-R occurs. At the same time, there is a translocation of ERα out of the nucleus and into the cytoplasm and cell membrane. Blockade of cSrc with the Src kinase inhibitor, PP-2 causes relocation of ERα into the nucleus. While these changes are not identical in response to each anti-estrogen, ERα binding to the EGF-R is increased in response to 4-OH-Tamoxifen when compared with tamoxifen. The changes in EGF-R interactions with ERα impart an enhanced sensitivity of tamoxifen resistant cells to the inhibitory properties of the specific EGF-R tyrosine kinase inhibitor, AG 1478. However, with long term exposure of tamoxifen-resistant cells to AG 1478, the cells begin to re-grow but can now be inhibited by the IGF-R tyrosine kinase inhibitor, AG 1024. These data suggest that the IGF-R system becomes the predominant signaling mechanism as an adaptive response to the EGF-R inhibitor. Taken together, this information suggests that both the EGF-R and IGF-R pathways can mediate ERα signaling.To further examine the effects of fulvestrant on ERα function, we examined the acute effects of fulvestrant, on non-genomic functionality. Fulvestrant enhanced ERα association with the membrane IGF-1 receptor (IGF-1R). Using siRNA or expression vectors to knock-down or knock-in selective proteins, we further demonstrated that the ERα/IGF-1R association is Src-dependent. Fulvestrant rapidly induced IGF-1R and MAPK phosphorylation. The Src inhibitor PP2 and IGF-1R inhibitor AG1024 greatly blocked fulvestrant-induced ERα/IGF-1R interaction leading to a further depletion of total cellular ERα induced by fulvestrant and further enhanced fulvestrant-induced cell growth arrest. More dramatic was the translocation of ERα to the plasma membrane in combination with the IGF-1-R as shown by confocal microscopy. Taken in aggregate, these studies suggest that secondary resistance to hormonal therapy results in usage of both IGF-R and EGF-R for non-genomic signaling.