Inhibition of cyclin-dependent kinase 1 (CDK1) by indirubin derivatives in human tumour cells.

The bisindole indirubin has been described, more than 30 years ago, as being clinically active in the treatment of human chronic myelocytic leukaemia. However, the underlying mechanism of action has remained unclear. We have reported previously that indirubin and its analogues are potent and selective inhibitors of cyclin-dependent kinases (CDK). In this study, we investigated the influence of indirubin and derivatives on CDK1/cyclin B kinase in human tumour cells at concentrations known to induce growth inhibition. Cells of the mammary carcinoma cell line MCF-7, synchronized by serum deprivation, after serum repletion stay arrested in the G(1)/G(0)phase of the cell cycle in the presence of 2 microM indirubin-3'-monoxime. At higher drug concentrations (> or = 5 microM) an increase of the cell population in the G(2)/M phase is additionally observed. Cells synchronized in G(2)/M phase by nocodazole remain arrested in the G(2)/M phase after release, in the presence of indirubin-3'-monoxime (> or =5 microM). After 24 h treatment with 10 microM indirubin-3'-monoxime a sub-G(2)peak appears, indicative for the onset of apoptotic cell death. Treatment of MCF-7 cells with growth inhibitory concentrations of indirubin-3'-monoxime induces dose-dependent inhibition of the CDK1 activity in the cell. After 24 h treatment, a strong decrease of the CDK1 protein level along with a reduction of cyclin B in complex with CDK1 is observed. Taken together, the results of this study strongly suggest that inhibition of CDK activity in human tumour cells is a major mechanism by which indirubin derivatives exert their potent antitumour efficacy.