Abstract 5335: The histone deacetylase inhibitor MS-275 sensitizes osteosarcoma cells and osteosarcoma lung metastases to FasL-induced cell death by the downregulation c-FLIP

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Abstract The purpose of this study was to determine the effects of the histone deacetylase inhibitor, MS-275, on the Fas signaling pathway and susceptibility of osteosarcoma (OS) to FasL-induced cell death. OS metastasizes almost exclusively to the lungs. We have shown that Fas expression by OS cells inversely correlates with their metastatic potential. Fas+ cells are rapidly eliminated when they enter the lungs via interaction with FasL, which is constitutively expressed in the lungs. OS cells that are Fas− escape this FasL-induced apoptosis and survive in the lung microenvironment. Moreover, upregulation of Fas expression on established OS lung metastases results in tumor regression. Therefore, agents that upregulate Fas or activate the Fas signaling pathway may have therapeutic potential. These investigations utilized the Fas− metastatic osteosarcoma cell lines LM7 and CCH-OS-D. Treatment of OS cells with 2µM MS-275 sensitized cells to FasL-induced cell death in vitro, as quantified by MTT, trypan blue and clonogenic assays. We have demonstrated that blocking Fas signaling by transfecting cells with a FADD dominant-negative (FDN) plasmid, inhibited MS-275-induced sensitization to FasL-mediated cell death, suggesting the involvement of the Fas pathway in this mechanism. We found that treatment of OS cells in vitro did not alter the expression of Fas on the cell surface, rather it resulted in the downregulation of the anti-apoptotic protein, c-FLIP (cellular FLICE-inhibitory protein), which was determined to be a result of MS-275-induced downregulation of FLIP mRNA. Downregulation of c-FLIP correlated with caspase activation and induction of apoptosis. Treatment of nu/nu-mice with established OS lung metastases with oral MS-275 resulted in tumor regression, increased apoptosis and a significant inhibition of c-FLIP expression in tumors. Histopathological examination of mice receiving oral MS-275 showed no evidence of toxicity. Overall, these data suggest that the mechanism by which MS-275 sensitizes OS cells and OS lung metastases to FasL-induced cell death may be by a direct reduction in the expression of c-FLIP. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5335. doi:10.1158/1538-7445.AM2011-5335