Abstract 545: Laminin induces tamoxifen resistance through alfa 6 integrin in breast cancer cells

Abstract

Abstract Even though it is now well established that the tumor microenvironment plays a central role in modulating tumor progression and response to therapy, its function as a regulator of endocrine resistance in breast cancer has not yet been thoroughly explored. We previously showed that fibronectin through beta 1 integrin induces tamoxifen resistance in breast cancer cells. The aim of this study was to determine whether other extracellular matrix components induce tamoxifen resistance and what mechanisms are involved. We found that laminin, but not collagen IV, induced tamoxifen resistance in both estrogen receptor positive (ER+) human MCF-7 and mouse LM05-E breast cancer cells (P<0.001). To understand what signaling pathways were involved, cells were treated for 48 hours with estradiol (10−8M) and 4-OH-tamoxifen (10−6M) with the addition of either the MAPK/ERK1/2 inhibitor PD98059 (10µM) or the PI3K/AKT inhibitor LY294002 (10µM). Only LY294002 reversed the protective effect of laminin (P<0.001). Next to determine what integrin was mediating endocrine resistance, cells were pre-incubated with either an alfa 6 blocking antibody (GoH3, 0.15µM) or a beta 1 integrin blocking antibody (AIIB2, 0.15µM). The alfa 6 blocking antibody interfered with tamoxifen resistance induced by laminin (P<0.01), but not the beta 1 integrin blocking antibody. To establish whether laminin or fibronectin regulated the transcriptional activity of ER we carried out luciferase reporter assays by transiently transfecting the cells with 0.2µg of a pSV40-ERE-Luc plasmid together with 0.04µg pRL-TK-Renilla as a control. As expected, estradiol 10−8M induced a significant stimulation of ER activity (P<0.001) which was counteracted by the addition of 10−6M 4-OH-tamoxifen (P<0.001). Cells were then additionally treated with either laminin or fibronectin (30µg/ml) to test whether on their own, or in the presence of estradiol and/or tamoxifen they could regulate ER activity. No regulation of the transcriptional activity of ER was found, even though we have previously shown that fibronectin stimulates the phosphorylation of ER-α in serine-118. Our results show that laminin, like fibronectin, induces tamoxifen resistance in breast cancer cells through the specific binding to alfa 6 integrin, and that the PI3K/AKT pathway mediates the protective effect downstream. However, the mechanism does not seem to involve regulation of the transcriptional activity of ER. These results suggest that integrin-matrix interactions could be a valuable molecular target for breast cancer patients that develop tamoxifen resistance. This work is supported by grants from the Susan G. Komen Foundation for the Cure (BCTR0600341), and FONCYT (PICT 2008-0325) to M.S. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 545. doi:10.1158/1538-7445.AM2011-545