Carcinogenesis, Vol. 23, No. 12, 2031-2041,
December 2002
© 2002 Oxford University Press
CANCER BIOLOGY |
Inositol hexakisphosphate blocks tumor cell growth by activating apoptotic machinery as well as by inhibiting the Akt/NF
B-mediated cell survival pathway
1 Laboratory of Molecular and Cellular Biochemistry, Faculty of Dental Science and Station for Collaborative Research, Kyushu University, Fukuoka 812-8582, Japan and
2 Department of Immunology, Medical Institute of Bioregulation, Kyushu University, Fukuoka 812-8582, Japan
It has been reported that inositol hexakisphosphate (InsP6, phytic acid), a natural product, has an anticancer role. However, there is inadequate information regarding the mechanism by which InsP6 exerts anticancer actions, and the effect requires relatively high concentration of the agent, both of which hinders the usage of InsP6 as an anticancer drug. In the present study, we investigated the mechanism by which InsP6 acts as an anticancer agent, and tried to reduce the concentration of effective InsP6. Treatment of HeLa cells with InsP6 at 1 mM induced apoptosis, as assessed by counting the cell number, and by Hoechst and TUNEL staining. This is probably mediated by intracellular InsP6 itself and/or the dephosphorylated forms of metabolized InsP6, because incubation of HeLa cells with [3H]InsP6 produces dephosphorylated forms such as InsP4 and InsP5. Induction of apoptosis by InsP6 was examined in two ways: inhibition of cell survival signaling and direct induction of apoptosis. Treatment of HeLa cells with tumor necrosis factor (TNF) or insulin stimulated the Akt-nuclear factor 
B (NFB) pathway, a cell survival signal, which involves the phosphorylation of Akt and IB, nuclear translocation of NFB and NFB-luciferase transcription activity. InsP6 blocked all these cellular events, but phosphatidylinositol 3-kinase activity was not affected. As well as inhibiting the Akt-NFB pathway, InsP6 itself caused mitochondrial permeabilization, followed by cytochrome c release, which later caused activation of the apoptotic machinery, caspase 9, caspase 3 and poly (ADP-ribose) polymerase. When InsP6 was applied together with histone, the effective concentration to induce apoptosis was ~10-fold lower. These results revealed that extracellularly applied InsP6 directly activates the apoptotic machinery as well as inhibits the cell survival signaling, probably by the intracellular delivery followed by a dephosphorylation.
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