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Background: Arnidiol is a pentacyclic triterpene diol that has multiple pharmacological activities. However, the apoptotic activities of arnidiol in human cancer cells have not yet been explored, nor has the mechanism by which arnidiol induces apoptosis been examined in depth. Methods: MDA-MB-231 cells and xenografted mice were treated with arnidiol. Mitochondrial fission and apoptosis were determined by immunofluorescence, flow cytometry and related molecular biological techniques. The interaction and colocalization of cofilin and Drp1 was determined by immunoprecipitation and immunofluorescence assays. Results: Arnidiol induces mitochondrial fission and apoptosis through mitochondrial translocation of Drp1 and cofilin. Importantly, the interaction of Drp1 and cofilin in mitochondria is involved in arnidiol-induced mitochondrial fission and apoptosis. Knockdown of either Drp1 or cofilin abrogated arnidiol-induced mitochondrial translocation, interaction of Drp1 and cofilin, mitochondrial fission and apoptosis. Only dephosphorylated Drp1 (Ser637) and cofilin (Ser3) were translocated to the mitochondria. Mutants of Drp1 S637A and cofilin S3A, which mimic the dephosphorylated forms, enhanced mitochondrial fission and apoptosis induced by arnidiol, whereas mutants of Drp1 S637D and cofilin S3E, which mimic the phosphorylated forms, suppressed mitochondrial fission and apoptosis induced by arnidiol. A mechanistic study revealed that ROCK1 activation plays an important role in the arnidiol-mediated Drp1 and cofilin dephosphorylation and mitochondrial translocation, mitochondrial fission, and apoptosis. Conclusions: Our data reveal a novel role of both Drp1 and cofilin in the regulation of mitochondrial fission and apoptosis and suggest that arnidiol could be developed as a potential agent for the treatment of human cancer. [ABSTRACT FROM AUTHOR]