Tumor necrosis factor-related apoptosis-inducing
ligand (TRAIL) induces apoptosis via the
death receptors DR4 and DR5 in transformed cells in vitro and exhibits potent antitumor activity in vivo with minor side effects.
Protein kinase casein kinase II (CK2) is increased in response to diverse growth stimuli and is aberrantly elevated in a variety of human
cancers.
Rhabdomyosarcoma tumors are the most common
soft-tissue sarcoma in childhood. In this investigation, we demonstrate that CK2 is a key survival factor that protects
tumor cells from TRAIL-induced apoptosis. We have demonstrated that inhibition of CK2 phosphorylation events by
5,6-dichlorobenzimidazole (
DRB) resulted in dramatic sensitization of
tumor cells to TRAIL-induced apoptosis. CK2 inhibition also induced rapid cleavage of
caspase-8, -9, and -3, as well as the
caspase substrate
poly(ADP-ribose) polymerase after TRAIL treatment. Overexpression of Bcl-2 protected cells from TRAIL-induced apoptosis in the presence of the CK2 inhibitor. Death signaling by TRAIL in these cells was Fas-associated death domain and
caspase dependent because dominant negative Fas-associated death domain or the
cowpox interleukin 1beta-converting
enzyme inhibitor protein cytokine response modifier A prevented apoptosis in the presence of
DRB. Analysis of death-inducing signaling complex (DISC) formation demonstrated that inhibition of CK2 by
DRB increased the level of recruitment of
procaspase-8 to the DISC and enhanced caspase-8-mediated cleavage of Bid, thereby increasing the release of the proapoptotic factors
cytochrome c, HtrA2/Omi, Smac/DIABLO, and
apoptosis inducing factor (AIF) from the mitochondria, with subsequent degradation of
X-linked inhibitor of apoptosis protein (XIAP). To further interfere with CK2 function, JR1 and Rh30 cells were transfected with either
short hairpin RNA targeted to CK2alpha or
kinase-inactive CK2alpha (K68M) or CK2alpha' (K69M). Data show that the CK2
kinase activity was abrogated and that TRAIL sensitivity in both cell lines was increased. Silencing of CK2alpha expression with
short hairpin RNA was also associated with degradation of XIAP. These findings suggest that CK2 regulates TRAIL signaling in
rhabdomyosarcoma by modulating TRAIL-induced DISC formation and XIAP expression.