Alsterpaullone, a novel cyclin-dependent kinase inhibitor, induces apoptosis by activation of caspase-9 due to perturbation in mitochondrial membrane potential

The majority of human neoplasms have aberrations in the retinoblastoma pathway due to hyperactivation of cyclin-dependent kinases (CDK). Based on this observation, novel small molecules, such as flavopiridol and UCN-01, are being developed and are currently being tested in the clinic. Efforts to develop CDK modulators led us to the discovery of a novel class of CDK inhibitors, the paullones [Cancer Res 1999;59:2566]. Initial studies demonstrated that paullones inhibit CDKs in vitro, thereby blocking cell-cycle progression. However, the exact mechanism for the antiproliferative effects of paullones was never explored. In this report, we demonstrate for the first time that the most potent paullone, alsterpaullone (Alp), induced Apoptosis and promoted loss in clonogenicity in the Jurkat cell line. Alp caused early activation of both Caspase-8 and -9, leading to cleavage of Caspase-3 and poly(ADP-ribose) polymerase (PARP). Moreover, Apoptosis by Alp was not associated with loss in anti-apoptotic proteins such as XIAP or Bcl-xL. Pre-incubation with cell-permeable inhibitors z-Asp(OMe)-Glu(OMe)-Val-Asp(Ome)-fluoromethylketone and benzyloxycarbonyl-Val-Ala-Asp (OMe)-fluoromethylketone (ZVAD) blocked Alp-induced Apoptosis. Moreover, the general Caspase Inhibitor ZVAD blocked the cleavage and activation of most caspases tested except caspase-9. Studies of mitochondrial membrane potential also demonstrated that Alp is able to disrupt mitochondrial potential in the presence of ZVAD, suggesting that the activation of caspase-9 by Alp follows mitochondrial perturbation. Pre-incubation of Jurkat cells with ZVAD did not prevent the depletion of cyclin D3, loss of CDK, or cell-cycle arrest by Alp. In summary, these experiments suggest that Alp activates caspase-9 via mitochondrial perturbation. Active caspase-9 cleaves and activates Caspase-8 and Caspase-3, leading to Apoptosis. In the presence of the general Caspase Inhibitor ZVAD, the cell-cycle effects of Alp are unaltered while Apoptosis is blocked, suggesting that the CDK effects of Alp are not sufficient for Alp-induced Apoptosis. Additional studies with paullones are warranted to further characterize their preclinical effects and to explore their potential use in the clinical setting.