Denbinobin, a phenanthraquinone derivative, was shown to exert antitumor activities in several types of
cancer cell lines. However, the precise mechanism underlying
denbinobin-induced cell death remains unclear. In this study, we investigated the apoptotic signaling cascade elicited by
denbinobin in human
glioblastoma multiforme (GBM) cells.
Denbinobin concentration-dependently caused a decrease in the cell viability of GBM cells. A flow cytometric analysis of
propidium iodide (PI)-stained cells demonstrated that
denbinobin induced GBM cell apoptosis.
Denbinobin evoked
caspase-3 activation and degradation of
poly (ADP-ribose) polymerase (PARP) and N-
benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (
zVAD-fmk), a broad-spectrum
caspase inhibitor that prevented
denbinobin-induced cell death. In addition,
denbinobin-induced cell death was diminished by the transfection of wild-type (WT) Akt or IκB
kinase (IKK) into GBM cells.
Denbinobin reduced IKK phosphorylation in a time-dependent manner, and
denbinobin-dephosphorylated IKK was accompanied by a decrease in Akt phosphorylation. The phosphorylation status of forkhead in
rhabdomyosarcoma (FKHR), a downstream signal molecule of Akt, was also diminished by the presence of
denbinobin. Furthermore, transfection of GBM cells with WT IKKα markedly suppressed the decreases in Akt and FKHR phosphorylation caused by
denbinobin. In contrast, transfection with WT IKKβ only slightly affected
denbinobin's action against IKK, Akt, and FKHR. These results suggest that IKKα inactivation, followed by Akt and FKHR dephosphorylation and
caspase-3 activation, contributes to
denbinobin-induced GBM cell apoptosis.