A therapeutic target for hormone-independent estrogen receptor-positive breast cancers

Background: The action of the steroid hormone estradiol (E2) is mediated vi a interaction with a specific receptor (ER) that initiates a series of even ts downstream, leading to the modulation of hormone-responsive genes and ce ll proliferation. Antihormones also bind, but do not confer the active conf iguration to ER, thereby, blocking the transmission of E2-ER-initiated sign als for cell proliferation. Although these compounds qualify for successful therapy of ER-positive [ER (+)] breast cancer patients, only a fraction of patients responds to antihormone treatment. In this study, the functional status of ER is determined to identify alternative targets for therapy of a ntihormone-resistant ER (+) breast cancers. Method: The interaction of ER with a specific DNA sequence, designated as E 2 response element (ERE), was targeted to assess the functional state of ER . ER-ERE complex formation was measured by electrophoretic mobility shift a ssay (EMSA) and by a newly developed technique, based on the preferential b inding of DNA-protein complex to a nitrocellulose membrane (NMBA) that meas ures both total and functional fraction of ER. Results: The NMBA assay identified functional variants of ER among ER (+) b reast cancer cell lines and breast tumor biopsy specimens. ER of (21PT) cel ls did not bind E2 and these cells were tamoxifen (TAM) resistant. However 21PT cells were sensitive to a calmodulin (CaM) antagonist, W7, that blocke d ERE-ER complex formation. Conclusions: ER variants of the 21PT type were detected among breast cancer biopsy specimens, emphasizing the significance of an alternative therapeut ic target for TAM-resistant ER (+) human breast cancers with compounds such as W7.