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The Estrogen Receptor Pathway in Rhabdomyosarcoma: A Role for Estrogen Receptor-β in Proliferation and Response to the Antiestrogen 4'OH-Tamoxifen

¹ Division of Hematology/Oncology and ² General Surgery, The Hospital for Sick Children; and ³ The Institute of Medical Science and Departments of ⁴ Pediatrics, ⁵ Medical Biophysics, and ⁶ Surgery, University of Toronto, Toronto, Ontario, Canada; and ⁷ The Department of Pathology, The Norwegian Radiumhospital, Oslo, Norway

Requests for reprints: David Malkin, Division of Haematology/Oncology, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8. Phone: 416-813-5348; Fax: 416-813-5327; E-mail: david.malkin{at}sickkids.ca.

Key Words: estrogen receptor • rhabdomyosarcoma • hormone • tamoxifen • apoptosis

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children. Highly malignant, RMS frequently fails to respond to conventional aggressive multimodal radiation, surgery, and chemotherapy treatment protocols that also cause significant sequelae in the growing child. Other tumors of mesenchymal origin, such as locally aggressive fibromatoses and desmoid tumors, have been successfully treated with a selective estrogen receptor (ER) modulator, tamoxifen. In an effort to identify new targets for RMS therapy, our group investigated the previously uncharacterized ER pathway in RMS cell culture and primary tumors. We detected ER isoform β (ERβ), but not isoform , RNA, and protein in five RMS cell lines. Immunohistochemical staining of primary RMS tumor sections confirmed high levels of ERβ but not ER protein. RMS cell growth was dramatically inhibited in steroid-free conditions, and this growth inhibition was rescued with 17-β-estradiol (E2) supplementation. Exposure of RMS cells to 4'OH-tamoxifen (4OHT) decreased cell viability and inhibited colony formation as detected by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and colony-forming assays. 4OHT also induced apoptotic signaling in RMS cells as detected by cleavage of caspase-3 and poly(ADP)ribose polymerase. This effect increased 3- to 8-fold in steroid-deprived conditions but was rescued by supplementation with E2. Immunofluorescence studies detected a change in the subcellular localization of ERβ in response to 4OHT. Together, these data suggest an active ERβ-mediated signal transduction pathway in RMS. The ability of 4OHT to induce apoptotic signaling and disrupt estradiol-mediated proliferation provides a rationale to explore a role for selective ER modulators in the treatment of RMS. [Cancer Res 2008;68(9):3476–85]