Indirubin-3′-monoxime, a derivative of a Chinese anti-leukemia medicine, inhibits Notch1 signaling

Notch proteins perform a critical function in cell-fate decisions and in differentiation. In this study, we determined that indirubin-3′-monoxime reduced Notch1 signaling to a remarkable extent. Indirubin-3′-monoxime has been shown to inhibit both constitutive active mutants of Notch1 and Notch1-IC-mediated transactivation activity. However, in such cases, neither the Notch cleavage pattern nor the protein stability of Notch1-IC was determined to have been significantly altered. Indirubin-3′-monoxime suppresses Notch1 transcriptional activity via the dissociation of the Notch1-IC–RBP-Jk complex. Notably, the transcriptional activity of Notch1-IC was not suppressed significantly in the GSK-3β null cells by indirubin-3′-monoxime as compared to what was observed with GSK-3β wild-type cells. In the previous study, we synthesized a series of indirubin derivatives. Interestingly, some of these indirubin derivatives were characterized as potent inhibitors of Notch1 signaling. Taken together, the results of this study indicate that indirubin-3′-monoxime downregulated Notch1 signaling in a GSK-3β-dependent and proteosomal degradation-independent manner.