211. Combination of Forced Transduction of P53 and an Agent That Blocks MDM2-p53 Interactions Produces Synergistic Cytotoxicity on Mesothelioma Defective of the INK4A/ARF Region

Abstract

A majority of malignant mesothelioma specimens possesses the wild-type p53 gene with a homologous deletion in the INK4A/ARF locus which encodes the p14ARF and the p16INK4A genes. Consequently, the p53 downstream pathways are functionally inactivated since the p14 defect augments activities of MDM2 which mediates p53 degradation through the ubiquitination processes. Furthermore, a loss of p16 induces pRb phosphorylation and uninhibited cell cycle progression through up-regulated CKD activities. We then examined possible anti-tumor effects of MDM2 inhibitors, Nutlin-3a and RITA, on human mesothelioma cells in combination of adenoviruses expressing the p53 gene (Ad-p53). Nutlin-3a, inhibiting the MDM2-p53 interactions, increased p53 phosphorylation levels and suppressed viability of mesothelioma cells in a p53-dependent manner. RITA, although blocked the MDM2-p53 interactions and phosphorylated p53, inhibited viability of mesothelioma cells independently of the p53 geneotype. Transduction of mesothelioma cells with Ad-p53 induced apoptotic cell death irrespective of the genotype. Moreover p21 induced by Ad-p53 dephosphorylated pRb and subsequently inhibited cell cycle progression. A combinatory use of Ad-p53 and Nutlin-3a or RITA, produced synergistic cytotoxicity of mesothelioma irrespective of the p53 geneotype. In addition, we demonstrated that the combination produced anti-tumor effects in an orthotopic animal model, mesothelioma developed in the pleural cavity in nude mice. In contrast, heat shock protein 90 inhibitors, blocking functions of MDM4, a homologous gene product of MDM2 without ubiquitination activities, did not produce the synergistic cytotoxicity with Ad-p53. These data collectively showed that MDM2 inhibitors enhanced expression levels of endogenous and exogenously transduced p53 through increased stability of the gene produce, and consequently activated the p53-mediated apoptosis. The p53-directed therapy is thus a possible therapeutic strategy for mesothelioma with the INK4A/ARF deletion.