Abstract. Rhabdomyosarcomas (RMS) are soft tissue tumours treated with a combination of surgery and chemotherapy. However, mortality rates remain high in case of recurrences and metastatic disease due to drug resistance and failure to undergo apoptosis. Therefore, innovative approaches targeting specific signalling pathways are urgently needed. We analysed the impact of different hedgehog (Hh) pathway inhibitors on growth and survival of six RMS cell lines using MTS assay, colony formation assay, 3D spheroid cultures, flow cytometry and western blotting. Especially the glioma-associated oncogene family (GLI) inhibitor arsenic trioxide (ATO) effectively reduced viability as well as clonal growth and induced cell death in RMS cell lines of embryonal, alveolar and sclerosing, spindle cell subtype, whereas normal skeletal muscle cells were hardly compromised by ATO. Combination of ATO with itraconazole potentiated the reduction of colony formation and spheroid size. These results show that ATO is a promising substance for treatment of relapsed and refractory RMS by directly targeting GLI transcription factors. The combination with itraconazole or other chemotherapeutic drugs has the opportunity to enforce the treatment efficiency of resistant and recurrent RMS. IntroductionRhabdomyosarcomas (RMS) are the most common paediatric soft tissue tumours, accounting for ~5% of all cancers at that age. Several histologic RMS variants can be differentiated with two thirds of RMS belonging to the embryonal subtype (ERMS) usually diagnosed in younger children below the age of 6 years. The more aggressive alveolar subtype (ARMS), which is more common in adolescents and young adults, makes up ~20% of new cases. ARMS are characterised by the reciprocal translocations t(2;13)(p35;q14) or t(1;13)(p36;q14) leading to the expression of fusion proteins consisting of the DNA-binding domains of PAX3 or PAX7 and the transactivation domain of FOXO1 (1,2). ERMS often exhibit loss of heterozygosity (LOH) at the chromosomal band 11p15.5 affecting the expression of the tumour suppressor genes H19 and CDKN1C (2). There are additional subtypes as the high grade pleomorphic RMS with a very unfavourable prognosis (3) and the sclerosing, spindle cell variant. In children this subtype often has a favourable outcome, whereas in adults it is highly aggressive (4,5).The standard therapy of RMS is surgery combined with a first line chemotherapy composed of vincristine, actinomycin D and cyclophosphamide (6). However, mortality rates remain high in case of recurrences and metastatic disease. Chemotherapy resistance and the failure of RMS cells to undergo apoptosis often occurs during disease progression (7). Therefore, new innovative approaches targeting specific signalling pathways are urgently needed.The hedgehog (Hh) pathway is involved in development, tissue regeneration but also several kinds of cancer including basal cell carcinoma, medulloblastoma, osteosarcoma, Ewing sarcoma and RMS (8-11). Ligand-dependent activation of Hh signalling occurs via ...

Rhabdomyosarcomas (RMS) are the most prevalent soft tissue sarcomas affecting children and adolescents. Despite intensive treatment consisting of multimodal chemotherapy and surgery RMS patients diagnosed with metastatic disease expect long term survival rates of only 20%. Often multidrug resistance arises upon initial response emphasizing the need for new therapeutic drugs to improve treatment efficiency. Previously, we demonstrated the efficacy of the FDA approved drug arsenic trioxide (ATO) specifically inhibiting viability and clonal growth as well as inducing cell death in human RMS cell lines of different subtypes. In this study, we combined low dose ATO with lithium chloride (LiCl), which is approved as mood stabilizer for the treatment of bipolar disorder, but also inhibits growth and survival of different cancer cell types in pre-clinical research. Indeed, we could show additive effects of LiCl and ATO on viability reduction, decrease of colony formation as well as cell death induction. In the course of this, LiCl induced inhibitory glycogen synthase kinase-3β (GSK-3β) serine 9 phosphorylation, whereas glioma associated oncogene family 1 (GLI1) protein expression was particularly reduced by combined ATO and LiCl treatment in RD and RH-30 cell lines, showing high rates of apoptotic cell death. These results imply that combination of ATO with LiCl or another drug targeting GSK-3 is a promising strategy to enforce the treatment efficiency in resistant and recurrent RMS.

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