Molecular mechanism of inositol hexaphosphate-mediated apoptosis in human malignant glioblastoma T98G cells

Abstract

Glioblastoma is the deadliest brain tumor in humans. Current therapies are mostly ineffective and new agents need to be explored for controlling this devastating disease. Inositol hexaphosphate (IP6) is a phytochemical that is widely found in corns, cereals, nuts, and high fiber-content foods. Previous studies demonstrated anti-cancer properties of IP6 in several in vitro and in vivo tumor models. However, therapeutic efficacy of IP6 has not yet been evaluated in glioblastoma. Here, we explored the molecular mechanism of action of IP6 in human malignant glioblastoma T98G cells. The viability of T98G cells decreased following treatment with increasing doses of IP6. T98G cells exposed to 0.25, 0.5, and 1 mM IP6 for 24 h showed morphological and biochemical features of apoptosis. Western blotting indicated changes in expression of Bax and Bcl-2 proteins resulting in an increase in Bax:Bcl-2 ratio and upregulation of cytosolic levels of cytochrome c and Smac/Diablo, suggesting involvement of mitochondria-dependent caspase cascade in apoptosis. IP6 downregulated cell survival factors such as baculovirus inhibitor-of-apoptosis repeat containing-2 (BIRC-2) protein and telomerase to promote apoptosis. Upregulation of calpain and caspase-9 occurred in course of apoptosis. Increased activities of calpain and caspase-3 cleaved 270 kD alpha-spectrin at specific sites generating 145 kD spectrin break down product (SBDP) and 120 kD SBDP, respectively. Increased caspase-3 activity also cleaved inhibitor of caspase-3-activated DNase and poly(ADP-ribose) polymerase. Collectively, our results demonstrated that IP6 down regulated the survival factors BIRC-2 and telomerase and upregulated calpain and caspase-3 activities for apoptosis in T98G cells.