2-methoxyestradiol induces apoptosis in Ewing sarcoma cells through mitochondrial hydrogen peroxide production

The Ewing sarcoma is the second most common bone tumor in children and young adults. Despite the advances in therapy, the 5-year survival rate for patients with metastatic disease is poor, indicating the need for alternative treatments. Here, we report that 2-methoxy-estradiol (2-Me), a natural estrogen metabolite, induced a caspase-dependent apoptosis of Ewing sarcoma-derived cells independently of their p53 status. 2-Me-induced apoptosis occurred through the mitochondrial death pathway as evidenced by reduction of the mitochondrial transmembrane potential, cytochrome c release and caspase-9 activation. Treatment of cells with 2-Me resulted in generation of intracellular H2O2, which occurred earlier than caspase-9 activation. The H2O2-reducing agent Ebselen and the lipid peroxidation inhibitor vitamin E decreased both 2-Me-induced caspase-9 activation and cell death, thus providing evidence for a role of H2O2 and lipid peroxides in the initiation of this process. Rotenone, an inhibitor of the mitochondrial respiratory chain, abolished both apoptosis and H2O2 production, thereby identifying mitochondria as the source of H2O2. Moreover, we observed that treatment of cells with 2-Me or H2O2 induced activation of the c-Jun N-terminal kinase (JNK). Overexpression of a dominant-negative mutant of JNK1 reduced 2-Me-induced apoptosis indicating that JNK participates in this process. Altogether, our results provide evidence that 2-Me triggers apoptosis of Ewing sarcoma cells through induction of a mitochondria redox-dependent mechanism and suggest that this compound or other agents that selectively increase the level of reactive oxygen species may prove useful to the development of novel strategies for treatment of Ewing tumors.