Potential of survivin as a new therapeutic target in diffuse malignant mesothelioma of the peritoneum

4613 Peritoneal diffuse malignant mesothelioma is a rare neoplasm characterized by a poor prognosis, which is not modified by conventional anticancer treatments. Since resistance to apoptosis is considered a major mechanism in tumor progression and drug resistance, we examined the expression of anti-apoptotic (Bcl-2, Bcl-X L , survivin, c-IAP 1 , c-IAP 2 and X-IAP) and pro-apoptotic (SMAC/Diablo) proteins by immunohistochemistry in 32 peritoneal diffuse malignant mesothelioma specimens. Overexpression of survivin and other IAP proteins was observed in a high percentage of tumors (range of positivity rate: 69-100%) and in an elevated fraction of tumor cells. Bcl-2 and Bcl-X L were also expressed in a high fraction of cases (66% and 82%, respectively), with high intensity immunostaining, whereas SMAC/Diablo immunostaining was detectable in only 34% of tumors. Accordingly, a low apoptotic index (median percentage of M30-positive cells, 0.45%; range, 0.01-5.8%) was consistently observed. To investigate whether anti-apoptotic proteins represent potential targets for new therapeutic interventions in this disease, we tested the effects of survivin knockdown accomplished through RNA interference in a peritoneal mesothelioma cell line. Cells were transfected with 100 nM of a 21-mer double-stranded small interfering RNA (siRNA) targeting survivin mRNA for 8 h, and the effects of cell transfection with siRNA on mesothelioma cell phenotype were evaluated at different time points. A strong inhibition (> 80%) of survivin expression at mRNA and protein levels was observed, which was followed by a time-dependent reduction of cell growth and a significant increase of caspase-9-mediated apoptotic rate. Moreover, sequential exposure of siRNA-transfected mesothelioma cells to anticancer drugs (cisplatin and doxorubicin) induced additive antiproliferative effects and markedly increased the apoptotic response to individual drug treatment. Overall, our results indicate that peritoneal diffuse malignant mesothelioma is characterized by deregulation of apoptosis pathways, in terms of increased expression of anti-apoptotic proteins, and suggest that strategies aimed at interfering with such proteins may provide a novel approach for the treatment of this malignancy.