The Novel Cancer Drug Seliciclib Engages the Mitochondrial Apoptosis Pathway Via the Mcl-1/Noxa Axis in CLL.

Seliciclib (cyc202, R-roscovitine) is a novel cyclin dependent kinase inhibitor currently in pre-clinical testing. It indirectly inhibits RNA polymerase II and has been shown to be effective against B-cell malignancies by inducing apoptosis and Mcl-1 degradation. The aim of this study was to dissect the underlying apoptotic pathway(s) of seliciclib in chronic lymphocytic leukemia (B-CLL). Treatment of CLL cells with >10 μM seliciclib in vitro induced apoptosis within 24 hours, irrespective of IgV_H mutation status (n=20). Gene profiling by means of RT-MLPA, did not reveal involvement of p53 as indicated by the absence of Puma upregulation. None of the >30 other apoptosis genes tested were induced; instead signals for certain labile mRNAs (Mcl-1, A1/Bfl-1, PI-9) were clearly decreased upon seliciclib. Detailed investigation of B cell lines overexpressing various anti-apoptotic proteins (Bcl-2, caspase-9-DN, Flip, FADD-DN), indicated that seliciclib activated the mitochondrial but not the death receptor pathway. Neither mitochondrial activation nor the rapid degradation of Mcl-1 protein could be prevented by caspase inhibition (zVAD) or overexpression of inactive caspase-9 (DN). This indicated that Mcl-1 decline is an upstream, caspase-independent event. Immuno-precipitation demonstrated that pro-survival Mcl-1 is engaged by pro-apoptotic Noxa and Bim. The specific contribution of Noxa was confirmed by RNAi, resulting in inhibition of apoptosis after seliciclib, but not after staurosporin, CD95- or BCR triggering.

These findings demonstrate that seliciclib induces rapid, p53-independent apoptosis via Mcl-1 degradation and mitochondrial activation. The involvement of Noxa, which is highly expressed in CLL, suggests this BH3-only protein may be a novel therapeutic target.