Indirubin enhances tumor necrosis factor-induced apoptosis through modulation of nuclear factor-κB signaling pathway

Although indirubin is known to exhibit anti-cancer and antiinflammatory 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-kappa B pathway. As examined by the DNA binding of NF-kappa B, we found that indirubin suppressed tumor necrosis factor (TNF)-induced NF-kappa B activation in a dose-and time-dependent manner. Indirubin also suppressed the NF-kappa B activation induced by various inflammatory agents and carcinogens. Further studies showed that indirubin blocked the phosphorylation and degradation of I kappa B alpha through the inhibition of activation of I kappa B alpha kinase and phosphorylation and nuclear translocation of p65. NF-kappa B reporter activity induced by TNFR1, TNF receptor-associated death domain, TRAF2, TAK1, NF-kappa B-inducing kinase, and IKK beta was inhibited by indirubin but not that induced by p65 transfection. We also found that indirubin inhibited the expression of NF-kappa B-regulated gene products involved in antiapoptosis (IAP1, IAP2, Bcl-2, Bcl-x(L), 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-kappa B activation pathway.