Rhabdomyosarcoma is the most frequent
soft tissue sarcoma in children and adolescents, with a high rate of relapse that dramatically affects the clinical outcome. Multiagent
chemotherapy, in combination with surgery and/or
radiation therapy, is the treatment of choice. However, the relapse rate is disappointingly high and identification of new therapeutic tools is urgently needed. Under this respect, the selective block of key features of cancer stem cells (CSC) appears particularly promising. In this study, we isolated
rhabdomyosarcoma CSC with stem-like features (high expression of NANOG and OCT3/4, self-renewal ability, multipotency).
Rhabdomyosarcoma CSC showed higher invasive ability and a reduced cytotoxicity to
doxorubicin in comparison to native cells, through a mechanism unrelated to the classical multidrug resistance process. This was dependent on a high level of lysosome acidity mediated by a high expression of
vacuolar ATPase (V-
ATPase). Since it was not associated with other paediatric
cancers, like
Ewing's sarcoma and
neuroblastoma, V-
ATPase higher expression in CSC was
rhabdomyosarcoma specific. Inhibition of lysosomal acidification by the V-
ATPase inhibitor
omeprazole, or by specific
siRNA silencing, significantly enhanced
doxorubicin cytoxicity. Unexpectedly, lysosomal targeting also blocked cell growth and reduced the invasive potential of
rhabdomyosarcoma CSC, even at very low doses of
omeprazole (10 and 50 µM, respectively). Based on these observations, we propose lysosome acidity as a valuable target to enhance chemosensitivity of
rhabdomyosarcoma CSC, and suggest the use of anti-V-
ATPase agents in combination with standard regimens as a promising tool for the eradication of
minimal residual disease or the prevention of metastatic disease.