Mechanisms underlying tumor sensitivity to the antitumor agent UCN-01 (7-hydroxystaurosporine) were examined in the nude mouse model using three human tumor xenografts, two pancreatic cancers (PAN-3-JCK and CRL 1420) and a breast cancer (MX-1). UCN-01 antitumor activity was evaluated in terms of relative tumor weights in treated and untreated mice bearing the tumor xenografts. The activity of cyclin-dependent kinase 2 (CDK2), levels of p21 and p27 proteins, pRb status and cell cycle were evaluated. Induction of p21 and apoptosis were also assessed immunohistochemically in CRL 1420. UCN-01 was administered intraperitoneally at a dose of either 5 or 10 mg/kg daily for 5 days followed by a further 5 injections after an interval of 2 days. UCN-01 significantly suppressed the growth of both pancreatic cancers, but was ineffective against MX-1. p21 protein expression was markedly induced in the UCN-01-sensitive pancreatic carcinoma xenografts at both doses, but p21 induction was only evident in the UCN-01-resistant MX-1 at 10 mg/kg. MX-1 exhibited CDK2 activity that was 6-fold higher than that of pancreatic cancer strains, which may explain the resistance of MX-1 to UCN-01 despite the induction of p21 at the dose of 10 mg/kg. The UCN-01-sensitive tumors exhibited G1 arrest and increased levels of apoptosis, changes not observed in resistant MX-1. In conclusion, it appears that a determining factor of in vivo UCN-01 sensitivity involves the balance of CDK2 kinase activity and p21 protein induction, resulting in augmented pRb phosphorylation, G1 cell cycle arrest and apoptosis.
Key words: UCN-01 -Human xenografts -In vivo sensitivityProtein phosphorylation is a major mediator of signal transduction during progression through the cell cycle. Members of the cyclin-dependent kinase (CDK) family and cyclins have important roles at different points in the cell cycle. For instance, cell cycle progression from early to mid-G1 phase is dependent on CDK4 and/or CDK6 activation by D-type cyclins whilst binding of CDK2 to cyclins A and E is necessary for the coordinated progression from mid-G1 to S phase.1) In addition to binding to the appropriate partner cyclin, CDKs also require the proper sequence of stimulation and phosphorylation by cyclin-activating kinases.2-4) Transition through G1 to S phase is regulated by activation of CDK2 by cyclin E, and the resultant cyclin/CDK complexes phosphorylate pRb to give ppRb, 5) which then activates E2F-driven transcriptional activation.6) E2F promotes expression of the thymidine kinase, thymidylate synthetase (TS), dihydrofolate reductase, cyclin E, and DNA polymerase-α genes, among others.Recent studies have revealed that CDK activity during G1 is negatively regulated by two families of CDK inhibitor proteins; the INK4 family including p16INK4a and the CIP/KIP family which includes p21 WAF1/ CIP1 and p27 Kip1