Histone deacetylase inhibitors enhance Ad5-TRAIL killing of TRAIL-resistant prostate tumor cells through increased caspase-2 activity

Interest in TNF-related apoptosis-inducing ligand (TRAIL) as a cancer therapeutic has been high since its first description. Recently, the use of histone deacetylase inhibitors (HDACi) to treat cancer has progressed from the laboratory to the clinic, and the combination of HDACi and TRAIL is very powerful in killing human tumors. Using a panel of prostate tumor cell lines (ALVA-31, DU-145, and LNCaP) with varying TRAIL sensitivity, we examined their sensitization to a recombinant adenovirus encoding TRAIL (Ad5-TRAIL) by sodium butyrate and trichostatin A. HDACi treatment increased coxsackie-adenovirus receptor (CAR) expression, resulting in increased adenoviral infection, and increased TRAIL-mediated killing. In TRAIL-resistant DU-145 cells, HDAC inhibition also decreased protein kinase casein kinase (PKCK) 2 activity, leading to caspase-2 activation. The importance of PKCK2 and caspase-2 in DU-145 sensitization was demonstrated with the PKCK-2-specific inhibitor, which enhanced Ad5-TRAIL-induced death, or the caspase-2-specific inhibitor, zVDVAD, which blocked Ad5-TRAIL-induced death. Thus, our data highlight the connection between HDAC inhibition of PKCK2 activity and tumor cell sensitivity to TRAIL-induced apoptosis. Specifically, HDAC inhibition leads to decreased PCKC2 activity, which is followed by caspase-2 activation and partial cleavage of caspase-8 that sensitizes the tumor cell to TRAIL.