Apo2L/TRAIL is actively investigated as a novel targeted agent to directly induce apoptosis of susceptible
cancer cells. Apo2L/TRAIL-refractory cells can be sensitized to the cytotoxic effect of this
ligand by cytotoxic chemotherapeutics. The aim of this study was to evaluate the in vitro tumoricidal activity of the Apo2L/TRAIL +
Trichostatin A in cultured thoracic
cancer cells and to elucidate the molecular basis of the synergistic cytotoxicity of this combination. Concurrent exposure of cultured
cancer cells to sublethal concentrations of Apo2L/TRAIL and
Trichostatin A resulted in profound enhancement of Apo2L/TRAIL-mediated cytotoxicity in all cell lines regardless of their intrinsic susceptibility to this
ligand. This combination was not toxic to primary normal cells. While Apo2L/TRAIL alone or
Trichostatin A alone mediated < 20% cell death, 60 to 90% of
cancer cells were apoptotic following treatment with
TSA + Apo2L/TRAIL combinations. Complete translocation of Bax from the cytosol to the mitochondria compartment was mainly observed in combination-treated cells and this was correlated with robust elevation of
caspase 9 proteolytic activity indicative of activation of the mitochondria apoptogenic effect. Profound
TSA + Apo2L/TRAIL-mediated cytotoxicity and apoptosis were completely abrogated by either Bcl2 over-expression or by the selective
caspase 9 inhibitor, highlighting the essential role of mitochondria-dependent apoptosis signaling cascade in this process. Moreover, increased
caspase 8 activity observed in cells treated with the
TSA + Apo2L/TRAIL combination was completely suppressed by Bcl-2 over-expression or by the selective
caspase 9 inhibitor indicating that the elevated
caspase 8 activity in combination-treated cells was secondary to a mitochondria-mediated amplification feedback loop of
caspase activation. These finding form the basis for further development of
HDAC inhibitors + Apo2L/TRAIL combination as novel targeted
therapy for thoracic
malignancies.