Malignant melanoma is a deadly skin cancer with poor prognosis if not diagnosed early and treated before metastasis occurs. Chemotherapy is often ineffective primarily due to the cancer cell's intrinsic resistance to current therapeutic agents like Dacarbazine and BCNU. Hence, it is important to identify new agents that would be effective in killing malignant melanoma cells. The naturally occurring small molecule, OSW-1 ((3 beta, 16 beta, 17 alpha-trihydroxycholest-5-en-22 - one 16-O-(2-O-4-methoxybenzoyl-beta-D-xylopyranosyl)-(1→3)-(2-O-acetyl-alpha-L arabinopyranoside) isolated from the bulbs of ivory coast lily (O. saudersiae) has been shown to be highly cytotoxic in numerous cancer cell lines with yet undefined mechanisms of action. Herein, we report results of our study on the anticancer activity and selectivity of OSW-1 in malignant melanoma cells and its potential mechanism of action. OSW-1 was highly antiproliferative in malignant melanoma cells, and its cytotoxic effect was time- and concentration-dependent, with the IC50 value below 1 nM. Importantly, OSW-1 preferentially killed melanoma cells and exerted much lower toxicity to normal melanocytes in culture. Also, this inhibitory activity was more pronounced in highly metastatic melanoma cells compared to the less invasive ones. Using an isogenic pair of melanoma cells with different Akt activation states, we observed that cells with higher Akt expression (metastatic cells) exhibited less spontaneous cell death in culture, but were significantly more sensitive to OSW-1 than cells with lower basal levels of phosphorylated Akt (noninvasive cells). Biochemical analysis with Rhodamine-123 indicated that OSW-1 stimulated cell death by damaging mitochondrial membrane integrity, leading to a decrease in transmembrane potential and subsequently initiating cell death, apparently through autophagy. This was confirmed by western blot analysis of the lipidation of LC3 after drug treatment. Interestingly, a combination of OSW-1 and chloroquine, a lysosomotropic agent known to promote rapid apoptosis in autophagic cells, resulted in a synergistic cytotoxic effect with a two-fold increase in cell death, which occurred much faster than in cells treated with either compound alone. The mode of cell death appeared to be shifted toward apoptosis. Western blots from cells treated with the drug combination showed an increase in the levels of caspase-3 activation. Further biochemical study suggested that OSW-1 might cause a significant disturbance of cellular calcium homeostasis, leading to aberrant calcium-mediated processes including mitochondrial impairment. We postulate that an abnormally high level of phosphorylated Akt could lead to mitochondrial dysfunction and deregulation of calcium homeostasis in malignant melanoma cells, and thus render them more vulnerable to OSW-1. Further work is needed to elucidate the detailed signaling pathways and molecular mechanisms.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3567.