N-(4-Hydroxyphenyl) Retinamide Potentiated Anti-Tumor Efficacy of Genistein in Human Ewing’s Sarcoma Xenografts

BACKGROUND: Ewing's sarcoma is a pediatric tumor that mainly occurs in soft tissues and bones. New therapeutic strategies are urgently needed for treatment of Ewing's sarcoma. We examined for the first time the efficacy of N-(4-hydroxyphenyl) retinamide (4-HPR) and genistein (GST) alone and also in combination for controlling growth of human Ewing's sarcoma SK-N-MC and RD-ES xenografts. METHODS: Efficacy of combination therapy was evaluated using histopathological parameters. Molecular mechanisms of combination therapy were detected using Western blotting and immunofluorescence microscopy. RESULTS: Histopathological examination of tumor sections showed that control group maintained characteristic growth of tumors, 4-HPR alone caused differentiation of tumor cells, GST alone induced apoptosis to some extent, and combination of 4-HPR and GST significantly induced apoptosis in both Ewing's sarcoma xenografts. Time-dependent reductions in body weight, tumor volume, and tumor weight were also found. Combination therapy increased Bax:Bcl-2 ratio to trigger mitochondrial release of Smac/Diablo into the cytosol to down regulate the baculovirus inhibitor-of-apoptosis repeat containing (BIRC) proteins such as BIRC-2 and BIRC-3 and thereby promote apoptosis. Activation of caspase-3 and mitochondrial release of apoptosis-inducing factor (AIF) occurred in course of apoptosis. Down regulation of the survival factor NF-κB and the angiogenic factors VEGF and FGF2 and increase in caspase-3 activity controlled tumor growth. In situ immunofluorescent labelings showed overexpression of calpain, caspase-12, and caspase-3, and AIF in xenografts, indicating induction of cysteine proteases and AIF for apoptosis. CONCLUSIONS: Results revealed that combination of 4-HPR and GST could be highly effective treatment for inhibiting Ewing's sarcomas in vivo.