The Flavonoid Kaempferol Sensitizes Human Glioma Cells to TRAIL-mediated Apoptosis by Proteasomal Degradation of Survivin

Resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL/Apo2L) limits its potential as a drug for cancer therapy. Here, we report that kaempferol, a bioactive plant flavonoid, sensitizes U251 and U87 glioma cells to TRAIL-mediated apoptosis. In contrast, U373 cells are not affected by kaempferol treatment. Treatment of kaempferol alone for 24 h did not induce apoptosis in the cell lines. We provide evidence that TRAIL-induced apoptosis is partially driven by kaempferol-mediated reduction of survivin protein levels. On kaempferol treatment, proteasomal degradation of survivin was observed. Inhibition of proteasomal degradation with MG132 in kaempferol-treated cells restored survivin protein levels in both glial cell lines. Consequently, overexpression of survivin attenuated TRAIL-kaempferol-induced apoptosis. In addition, we show that kaempferol mediates down-regulation of phosphorylated Akt, thereby further reducing survivin protein level. Furthermore, the blockage of the serine/threonine kinase Akt activity by kaempferol is important for inhibition of survivin because active phosphorylated Akt enhances the stability of survivin. However, we also show that the combined treatment of TRAIL and kaempferol induces cleavage (activation) of caspase-8, thereby exerting a proapoptotic effect independent of survivin known not to inhibit caspase-8 activation. Other effects induced by kaempferol were suppression of X-linked inhibitor of apoptosis proteins as the antiapoptotic members of the Bcl-2 family, Bcl-2, Bcl-xL, and Mcl-1 in a concentration-dependent manner. In summary, we showed that suppression of survivin is an essential mechanism in TRAIL-kaempferol-mediated apoptosis.