Protein kinase C iota as a therapeutic target in alveolar rhabdomyosarcoma

Alveolar rhabdomyosarcoma is an aggressive pediatric cancer exhibiting skeletal-muscle differentiation. New therapeutic targets are required to improve the dismal prognosis for invasive or metastatic alveolar rhabdomyosarcoma. Protein kinase C iota (PKC iota) has been shown to have an important role in tumorigenesis of many cancers, but little is known about its role in rhabdomyosarcoma. Our gene-expression studies in human tumor samples revealed overexpression of PRKCl. We confirmed overexpression of PKC iota at the mRNA and protein levels using our conditional mouse model that authentically recapitulates the progression of rhabdomyosarcoma in humans. Inhibition of Prkci by RNA interference resulted in a dramatic decrease in anchorage-independent colony formation. Interestingly, treatment of primary cell cultures using aurothiomalate (ATM), which is a gold-containing classical anti-rheumatic agent and a PKC iota-specific inhibitor, resulted in decreased interaction between PKC iota and Par6, decreased Rac1 activity and reduced cell viability at clinically relevant concentrations. Moreover, co-treatment with ATM and vincristine (VCR), a nnicrotubule inhibitor currently used in rhabdomyosarcoma treatment regimens, resulted in a combination index of 0.470-0.793 through cooperative accumulation of non-proliferative multinuclear cells in the G2/M phase, indicating that these two drugs synergize. For in vivo tumor growth inhibition studies, ATM demonstrated a trend toward enhanced VCR sensitivity. Overall, these results suggest that PKC iota is functionally important in alveolar rhabdomyosarcoma anchorage-independent growth and tumor-cell proliferation and that combination therapy with ATM and microtubule inhibitors holds promise for the treatment of alveolar rhabdomyosarcoma. Oncogene (2013) 32, 286-295; doi:10.1038/onc.2012.46; published online 20 February 2012