Chemosensitization of rhabdomyosarcoma cells by the histone deacetylase inhibitor SAHA

• SAHA and chemotherapeutics synergize to trigger apoptosis in RMS.
• SAHA and Doxorubicin change expression of pro- and antiapoptotic Bcl-2 proteins.
• SAHA and Doxorubicin cooperate to activate Bax and Bak.
• Bcl-2 overexpression inhibits SAHA/Doxorubicin-induced apoptosis.

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma in childhood with a dismal prognosis, highlighting the need for novel treatment strategies. Here, we identify a novel synthetic lethal interaction between the histone deacetylase inhibitor (HDACI) SAHA and anticancer drugs in RMS cells. Importantly, SAHA significantly increases chemotherapeutic drug-induced apoptosis in both embryonal and alveolar RMS cell lines, including several anticancer agents that are used in the clinic for the treatment of RMS such as Doxorubicin, Etoposide, Vincristine and Cyclophosphamide. Calculation of combination index (CI) reveals that the interaction of SAHA and Doxorubicin or Etoposide is synergistic. Mechanistically, SAHA causes acetylation of histone H3 protein in RMS cells, indicating that SAHA alters the chromatin context. Also, cotreatment with SAHA and Doxorubicin changes the ratio of pro- and antiapoptotic Bcl-2 proteins with downregulation of Mcl-1 and Bcl-xL, dephosphorylation of Bcl-2 and upregulation of BimEL, thus shifting the balance towards apoptosis. Consistently, SAHA and Doxorubicin cooperate to stimulate activation of Bax and Bak, caspase activation and caspase-dependent apoptosis. Overexpression of Bcl-2 significantly rescues SAHA/Doxorubicin-mediated apoptosis, underscoring the requirement of the mitochondrial apoptotic pathway for the synergistic induction of apoptosis by SAHA and Doxorubicin. Caspase-dependent apoptotic cell death is confirmed by the use of the broad-range caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD.fmk), which significantly decreases SAHA/Doxorubicin-triggered apoptosis. In conclusion, these findings demonstrate that the HDACI SAHA represents a promising strategy to prime RMS cells for chemotherapy-induced apoptosis and warrants further investigation in combination regimens.