Abstract:
Although tumor necrosis factor-related apoptosis-inducing
ligand (TRAIL) has shown efficacy in a phase 2 clinical trial,
development of resistance to TRAIL by tumor cells is a major
roadblock. We investigated whether azadirone, a limonoidal
tetranortriterpene, can sensitize human tumor cells to TRAIL.
Results indicate that azadirone sensitized cancer cells to TRAIL.
The limonoid induced expression of death receptor (DR) 5 and
DR4but did not affect expression of decoy receptors in cancer cells.
The induction of DRs was mediated through activation ofERKand
through up-regulation of a transcription factorCCAATenhancerbinding
protein homologous protein (CHOP) as silencing of these
signaling molecules abrogated the effect of azadirone. These
effects of azadirone were cancer cell-specific. The CHOP binding
site on the DR5 gene was required for induction of DR5 by
azadirone. Up-regulation of DRs was mediated through the generation
of reactive oxygen species (ROS) as ROS scavengers reduced
the effect of azadirone on ERK activation, CHOP up-regulation,
DR induction, and TRAIL sensitization. The induction of DRs by
this limonoid was independent of p53, but sensitization to TRAIL
was p53-dependent. The limonoid down-regulated the expression
of cell survival proteins and up-regulated the proapoptotic proteins.
The combination of azadirone with TRAIL was found to be
additive at concentrations lower than IC50, whereas at higher
concentrations, the combination was synergistic. Overall, this
study indicates that azadirone can sensitize cancer cells to
TRAIL through ROS-ERK-CHOP-mediated up-regulation of
DR5 and DR4 signaling, down-regulation of cell survival proteins,
and up-regulation of proapoptotic proteins.
