title: Estrogen receptor-alpha 36 mediates mitogenic antiestrogen signaling in ER-negative breast cancer cells.
abstract: It is prevailingly thought that the antiestrogens tamoxifen and ICI 182, 780 are competitive antagonists of the estrogen-binding site of the estrogen receptor-alpha (ER-α). However, a plethora of evidence demonstrated both antiestrogens exhibit agonist activities in different systems such as activation of the membrane-initiated signaling pathways. The mechanisms by which antiestrogens mediate estrogen-like activities have not been fully established. Previously, a variant of ER-α, EP-α36, has been cloned and showed to mediate membrane-initiated estrogen and antiestrogen signaling in cells only expressing ER-α36. Here, we investigated the molecular mechanisms underlying the antiestrogen signaling in ER-negative breast cancer MDA-MB-231 and MDA-MB-436 cells that express high levels of endogenous ER-α36. We found that the effects of both 4-hydoxytamoxifen (4-OHT) and ICI 182, 780 (ICI) exhibited a non-monotonic, or biphasic dose response curve; antiestrogens at low concentrations, elicited a mitogenic signaling pathway to stimulate cell proliferation while at high concentrations, antiestrogens inhibited cell growth. Antiestrogens at l nM induced the phosphorylation of the Src-Y416 residue, an event to activate Src, while at 5 µM induced Src-Y527 phosphorylation that inactivates Src. Antiestrogens at 1 nM also induced phosphorylation of the MAPK/ERK and activated the Cyclin D1 promoter activity through the Src/EGFR/STAT5 pathways but not at 5 µM. Knock-down of ER-α36 abrogated the biphasic antiestrogen signaling in these cells. Our results thus indicated that ER-α36 mediates biphasic antiestrogen signaling in the ER-negative breast cancer cells and Src functions as a switch of antiestrogen signaling dependent on concentrations of antiestrogens through the EGFR/STAT5 pathway.
authors.affiliation: Department of Medical Microbiology and Immunology, Creighton University Medical School, Omaha, Nebraska, United States of America.
authors.forename: XinTian
authors.initials: X
authors.lastname: Zhang
authors.forename: Ling
authors.initials: L
authors.lastname: Ding
authors.forename: LianGuo
authors.initials: L
authors.lastname: Kang
authors.forename: Zhao-Yi
authors.initials: ZY
authors.lastname: Wang
mesh_terms: D015153:Blotting, Western; D001943:Breast Neoplasms / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D002867:Chromones / Q000494:pharmacology; D004958:Estradiol / Q000031:analogs & derivatives / Q000494:pharmacology; D020847:Estrogen Receptor Modulators / Q000494:pharmacology*; D047628:Estrogen Receptor alpha / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D005260:Female; D000077267:Fulvestrant; D006801:Humans; D006915:Hydroxytestosterones / Q000494:pharmacology; D015151:Immunoblotting; D047468:Immunoprecipitation; D009025:Morpholines / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D015398:Signal Transduction / Q000187:drug effects
chemical_list: D002867:Chromones; C506487:ESR1 protein, human; D020847:Estrogen Receptor Modulators; D047628:Estrogen Receptor alpha; D006915:Hydroxytestosterones; D009025:Morpholines; C032278:4,17 beta-dihydroxy-4-androstene-3-one; D000077267:Fulvestrant; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; D004958:Estradiol