1. ¹C.N.R.S., Cell Cycle Group and UPS2682, Station Biologique, Bretagne, France
2. ²Molecular Pharmacology Laboratory, DCMB, School of Medicine, University of Lleida, Lleida, Catalunya, Spain
3. ³ProQinase GmbH, Freiburg, Germany
4. ⁴INSERM U 482, 184, rue du Faubourg Saint Antoine, Paris Cedex, France
5. ⁵Department of Pharmacy, Division of Pharmacognosy and Natural Products Chemistry, University of Athens, Panepistimiopolis Zografou, Athens, Greece

Correspondence: Dr L Meijer, C.N.R.S., Cell Cycle Group and UPS2682, Station Biologique, B.P. 74, 29682 Roscoff cedex, Bretagne, France. E-mail: meijer@sb-roscoff.fr

⁶These two authors contributed equally to this work.

Received 7 December 2005; Revised 27 March 2006; Accepted 27 March 2006; Published online 15 May 2006.

Abstract

Indirubin, an isomer of indigo, is a reported inhibitor of cyclin-dependent kinases (CDKs) and glycogen synthase kinase-3 (GSK-3) as well as an agonist of the aryl hydrocarbon receptor (AhR). Indirubin is the active ingredient of a traditional Chinese medicinal recipe used against chronic myelocytic leukemia. Numerous indirubin analogs have been synthesized to optimize this promising kinase inhibitor scaffold. We report here on the cellular effects of 7-bromoindirubin-3'-oxime (7BIO). In contrast to its 5-bromo- and 6-bromo- isomers, and to indirubin-3'-oxime, 7BIO has only a marginal inhibitory activity towards CDKs and GSK-3. Unexpectedly, 7BIO triggers a rapid cell death process distinct from apoptosis. 7-Bromoindirubin-3'-oxime induces the appearance of large pycnotic nuclei, without classical features of apoptosis such as chromatin condensation and nuclear fragmentation. 7-Bromoindirubin-3'-oxime-induced cell death is not accompanied by cytochrome c release neither by any measurable effector caspase activation. Furthermore, the death process is not altered either by the presence of Q-VD-OPh, a broad-spectrum caspase inhibitor, or the overexpression of Bcl-2 and Bcl-XL proteins. Neither AhR nor p53 is required during 7BIO-induced cell death. Thus, in contrast to previously described indirubins, 7BIO triggers the activation of non-apoptotic cell death, possibly through necroptosis or autophagy. Although their molecular targets remain to be identified, 7-substituted indirubins may constitute a new class of potential antitumor compounds that would retain their activity in cells refractory to apoptosis.