Rhabdomyosarcomas (RMS) represent the most common childhood soft-tissue sarcoma. Over the past few decades outcomes for low and intermediate risk RMS patients have slowly improved while patients with metastatic or relapsed RMS still face a grim prognosis. New chemotherapeutic agents or combinations of chemotherapies have largely failed to improve the outcome. Based on the identification of novel molecular targets, potential therapeutic approaches in RMS may offer a decreased reliance on conventional chemotherapy. Thus, identification of effective therapeutic agents that specifically target relevant pathways may be particularly beneficial for patients with metastatic and refractory RMS. The PI3K/AKT/mTOR pathway has been found to be a potentially attractive target in RMS therapy. In this study, we provide evidence that rapamycin (sirolimus) abrogates growth of RMS development in a RMS xenograft mouse model. As compared to a vehicle-treated control group, more than 95% inhibition in tumor growth was observed in mice receiving parenteral administration of rapamycin. The residual tumors in rapamycin-treated group showed significant reduction in the expression of biomarkers indicative of proliferation and tumor invasiveness. These tumors also showed enhanced apoptosis. Interestingly, the mechanism by which rapamycin diminished RMS tumor growth involved simultaneous inhibition of mTOR and hedgehog (Hh) pathways. Diminution in these pathways in this model of RMS also inhibited epithelial mesenchymal transition (EMT) which then dampened the invasiveness of these tumors. Our data provide bases for using rapamycin either alone or in combination with traditional chemotherapeutic drugs to block the pathogenesis of high risk RMS.
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