Sensitization of mesothelioma to TRAIL apoptosis by inhibition of histone deacetylase: role of Bcl-xL down-regulation

The TNF-related apoptosis-inducing ligand (TRAIL) is an immunological inducer of apoptosis selectively killing many, but not all, cancer cells. Malignant mesothelioma (MM) is fatal neoplasia with no current treatment, most likely due to high resistance of MM cells towards inducers of apoptosis, including TRAIL. We studied whether inhibition of histone deacetylase (HDAC), recently shown to sensitize malignant cells to a variety of apoptogenic substances, renders MM cells susceptible to TRAIL. Indeed, sub-apoptotic doses of the HDAC inhibitor suberohydroxamic acid (SBHA) sensitized MM cells to TRAIL apoptosis. Of the apoptotic mediators tested, the anti-apoptotic protein Bcl-x(L) was strongly down-regulated by combined treatment of the cells with SBHA and TRAIL but not by the HDAC inhibitor alone, while little or no change in the expression of other Bcl-2 family members highly expressed in MM cells, including Mcl-1 and Bax, was observed. Our data suggest a cross-talk between HDAC inhibition and TRAIL that results in modulation of expression of specific apoptotic mediators, and point to the potential of their combinatorial use in treatment of TRAIL-resistant neoplastic disease.