5,6-dichloro-ribifuranosylbenzimidazole- and apigenin-induced sensitization of colon cancer cells to TNF-α-mediated apoptosis

Tumor necrosis factor-alpha (TNF-alpha) is a multifunctional cytokine involved in the expression of many genes integral to the inflammatory response. In addition, it activates both apoptotic and survival pathways, the latter being mediated through the activation of the transcription factor nuclear factor-kappaB (NF-kappaB). Protein kinase CK2, a serine-threonine kinase that is universally upregulated in human malignancies, may be involved at multiple levels in this process. However, its role in mediating a survival response within colon cancer cells remains incompletely understood. Here we report that inhibition of CK2 in HCT-116 and HT-29 cells with the use of two specific CK2 inhibitors, 5,6-dichlororibifuranosylbenzimidazole (DRB) and apigenin, effected a synergistic reduction in cell survival when used in conjunction with TNF-alpha. Furthermore, there was a demonstrable synergistic reduction in colony formation in soft agar with the use of the same combinations. Western blot analysis showed that poly-ADP ribose polymerase and procaspase-3 cleavage complemented the fluorescence-activated cell sorter analysis findings of significantly increased subdiploid DNA-containing cell populations using these conditions. Remarkably, these events occurred in the absence of any reduction in the expression of the Bcl-2 family members Bcl-2, Mcl-1, and Bcl-x(L) or any change in the proapoptotic molecules Bad or Bax. One-hybrid NF-kappaB promoter assays utilizing a Gal4-p65 transactivation domain construct revealed that the TNF-induced transactivation was inhibited by both DRB and apigenin. This was associated with a concomitant reduction in the expression of a recognized anti-apoptotic NF-kappaB target, manganese superoxide dismutase, demonstrated by Q-PCR. Our findings indicate a potentially novel strategy for the treatment of colon cancer, one that targets CK2 simultaneous with TNF-alpha administration.