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The drug-dependent induction of premature senescence in neoplastic cells is considered per se an important tumor suppressive mechanism. DNA demethylating agents recently introduced in clinical trials, such as 5-aza-cytidine (Decitabine) and its derivatives, have been extensively characterized in recent years as antiproliferative compounds that act through multiple mechanisms, which have not yet been fully clarified. We recently analyzed the introduction of Decitabine in therapy for malignant pleural mesothelioma (MPM) observing that, despite the ability to induce profound biological effects in MPM cells, the drug failed to generate a massive apoptotic response. Since one of the most intriguing aspects of DNA demethylating agents is the possibility to accelerate the senescent response of tumor cells, we investigated the hypothesis of Decitabine inducing, in vitro, the premature aging of MPM cells.

Malignant pleural mesothelioma (MPM) is a locally aggressive neoplasm, principally linked to asbestos fibres exposure. Strong evidences associate this pollutant with induction of DNA breaks, aberrant chromosomes segregation and important chromosomal rearrangements, considered crucial events in malignant transformation. A considerable contribution to cellular transformation in MPM is also given by the presence of high genomic instability, as well as by the increased DNA methylation, and consequent decreased expression, of tumor-suppressor genes. In this study we first demonstrated that MPM cells are characterized by a decreased methylation level of pericentromeric DNA sequences which can justify, at least in part, the genomic instability observed in this neoplasia. Concomitantly, we found a paradoxical increased expression of DNMT1, the most expressed DNA methyltransferases in MPM cells, DNMT3a and all five isoforms of DNMT3b. Thus, we compared two experimental strategies, DNMT1 silencing and usage of a demethylating agent (5-aza-2'-deoxycytidine or Decitabine), both theoretically able to revert the locally hypermethylated phenotype and considered potential future therapeutic approaches for MPM. Interestingly, both strategies substantially decrease cell survival of MPM cells but the antitumor activity of Decitabine, differently from DNMT1 silencing, is mediated, at least in part, by a p53-independent p21 upregulation, and is characterized by the arrest of MPM cells at the G2/M phase of the cell cycle. These results indicate that the two approaches act probably through different mechanisms and, thus, that DNMT1 silencing can be considered an effective alternative to Decitabine for cancer treatment.