Rhabdomyosarcoma (RMS) is a malignant tumor thought to arise from immature precursors of skeletal muscle cells. In children, RMS accounts for approximately 50% of soft tissue sarcomas and 10% of solid tumors. There are two major histologic subtypes, embryonal and alveolar, both of which exhibit metastatic potential. Despite multimodal therapy (surgery, radiation, and chemotherapy), the failure-free survival of patients with metastatic RMS is only 25%. Currently there is no molecularly targeted therapy for RMS, but several independent clinical observations point to tamoxifen (a selective estrogen receptor modulator) as a promising drug to explore, including the efficacy of tamoxifen against desmoid tumors (which share a similar ontogeny as RMS) and the tendency of RMS to arise during periods of elevated constitutive estrogen exposure. This led our group to investigate estrogen receptor (ER) signaling in RMS. Previous work in our lab has shown that RMS primary tumors and cell lines express ERβ, and that estrogen stimulates RMS cell growth in vitro. In addition, exposure of RMS cells to 4-hydroxytamoxifen (4OHT), an active metabolite of tamoxifen, leads to decreased cell viability and increased apoptotic signaling, accompanied by MAPK activation. These findings suggest that an active ER pathway play a role in RMS, and demonstrate the antagonistic effect of 4OHT in RMS in vitro. To expand on this work, we conducted studies to: 1) further define the molecular mechanism of 4OHT-induced apoptosis in RMS; and 2) test the efficacy of tamoxifen in combination with other chemotherapeutic drugs commonly used to treat RMS (vincristine and actinomycin D). We found that 4OHT (10 μM)-induced apoptosis can be blocked by inhibition of ER and c-Jun N-terminase kinase (JNK). In addition, ER inhibition blocked 4OHT (10 μM)-associated MAPK phosphorylation. Interestingly, long-term treatment (8 weeks) of RMS cells with 4OHT (4 μM) inhibited cell growth as determined by measuring cell population doublings. Using an MTS assay, we found that tamoxifen in combination with vincristine and actinomycin D significantly reduced RMS cell viability compared to vehicle and the drugs alone (p<0.05). These results show that: a) 4OHT-induced apoptosis may be mediated by a signaling sequence from ERs to MAPKs to apoptosis, b) the late growth- inhibitory effect of 4OHT may be caused by both early/transient and late/continuous genes and c) combining tamoxifen with vincristine and actinomycin D is effective against RMS cells in vitro. These findings will guide future gene expression profiling studies, provide a list of specific therapeutic targets that can be tested in vivo, and may find application in clinical protocols in the treatment of RMS.

Citation Format: Zainab A. Motala, Kevin Chen, David Malkin. The role of estrogen receptor signaling in rhabdomyosarcoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5029. doi:10.1158/1538-7445.AM2013-5029