Indirubin Enhances Tumor Necrosis Factor-induced Apoptosis through Modulation of Nuclear Factor-κB Signaling Pathway

Although indirubin is known to exhibit anti-cancer and anti-inflammatory activities, very little is known about its mechanism of action. In this study, we investigated whether indirubin mediates its effects through interference with the NF-κB pathway. As examined by the DNA binding of NF-κB, we found that indirubin suppressed tumor necrosis factor (TNF)-induced NF-κB activation in a dose- and time-dependent manner. Indirubin also suppressed the NF-κB activation induced by various inflammatory agents and carcinogens. Further studies showed that indirubin blocked the phosphorylation and degradation of IκBα through the inhibition of activation of IκBα kinase and phosphorylation and nuclear translocation of p65. NF-κB reporter activity induced by TNFR1, TNF receptor-associated death domain, TRAF2, TAK1, NF-κB-inducing kinase, and IKKβ was inhibited by indirubin but not that induced by p65 transfection. We also found that indirubin inhibited the expression of NF-κB-regulated gene products involved in antiapoptosis (IAP1, IAP2, Bcl-2, Bcl-xL, and TRAF1), proliferation (cyclin D1 and c-Myc), and invasion (COX-2 and MMP-9). This correlated with enhancement of the apoptosis induced by TNF and the chemotherapeutic agent taxol in human leukemic KBM-5 cells. Indirubin also suppressed cytokine-induced cellular invasion. Overall, our results indicate that anti-cancer and anti-inflammatory activities previously assigned to indirubin may be mediated in part through the suppression of the NF-κB activation pathway.