Journal
ENDOCRINE-RELATED CANCER
Volume 15, Issue 4, Pages 985-1002Publisher
BIOSCIENTIFICA LTD
DOI: 10.1677/ERC-07-0240
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Funding
- Mary-Jean Mitchell Green Foundation
- Breakthrough Breast Cancer [BBC024]
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Acquired resistance to endocrine therapies remains a major clinical obstacle in hormone-sensitive breast tumors. We used an MCF-7 breast tumor cell line (Tam(R)-1) resistant to tamoxifen to investigate this mechanism. We demonstrate that Tam(R)-1 express elevated levels of phosphorylated AKT and MAPK3/1-activated RPS6KA2 compared with the parental MCF-7 cell line (MCF-7). There was no change in the level of total ESR between the two cell lines; however, the TaMR-1 cells had increased phosphorylation of ESR1 ser(167). SiRNA blockade of AKT or MAPK3/1 had little effecton ESR1 ser(167) phosphorylation, but a combination of the two siRNAs abrogated this. Co-localization studies revealed an association between ERBB2 and ESR1 in the Tam(R)-1 but not MCF-7 cells. ESR1 was redistributed to extranuclear sites in TaMR-1 and was less transcriptionally competent compared with MCF-7 suggesting that nuclear ESR1 activity was suppressed in TaMR-1. Tamoxifen resistance in the TaMR-1 cells could be partially overcome by the ERBB2 inhibitor AG825 in combination with tamoxifen, and this was associated with re-localization of ESR1 to the nucleus. These data demonstrate that tamoxifen-resistant cells have the ability to switch between ERBB2 or ESR1 pathways promoting cell growth and that pharmacological inhibition of ERBB2 may be a therapeutic strategy for overcoming tamoxifen resistance.
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