Caspase-9 mediates Puma activation in UCN-01-induced apoptosis

The protein kinase inhibitor 7-hydroxystaurosporine (UCN-01) is one of the most potent and frequently used proapoptotic stimuli. The BH3-only molecule of Bcl-2 family proteins has been reported to contribute to UCN-01-induced apoptosis. Here we have found that UCN-01 triggers Puma-induced mitochondrial apoptosis pathway. Our data confirmed that Akt-FoxO3a pathway mediated Puma activation. Importantly, we elucidate the detailed mechanisms of Puma-induced apoptosis. Our data have also demonstrated that caspase-9 is a decisive molecule of Puma induction after UCN-01 treatment. Caspase-9 mediates apoptosis through two kinds of feedback loops. On the one hand, caspase-9 enhances Puma activation by cleaving Bcl-2 and Bcl-xL independent of caspase-3. On the other hand, caspase-9 directly activated caspase-3 in the presence of caspase-3. Caspase-3 could cleave XIAP in an another positive feedback loop to further sensitize cancer cells to UCN-01-induced apoptosis. Therefore, caspase-9 mediates Puma activation to determine the threshold for overcoming chemoresistance in cancer cells.The apoptosis pathway is closely related to the Bcl-2 family proteins in which antiapoptotic members sequester multidomain proapoptotic proteins, thereby inhibiting their active role in apoptosis. In contrast, BH3-only proteins that are considered stress sensors can dissociate Bax-like proteins from their antiapoptotic sequestrators, and thus leading to apoptosis.¹The expression of Bcl-2 family proteins is regulated during carcinogenesis,¹ and the expression of both the Bcl-2 and Bcl-xL antiapoptotic proteins is associated with resistance to antitumor agents such as cisplatin (CP).² The inhibition of the protective function of antiapoptotic Bcl-2 members can either restore the normal apoptotic process in cancer cells or circumvent resistance to chemotherapy.^3,4 In this regard, enhanced expression of BH3-only proteins can effectively bind the antiapoptotic members and prevent the function of these proteins.Some reports suggest that the BH3-only protein Puma has important roles in p53-dependent and -independent apoptosis in human cancer cells and mediates cell death through the Bcl-2 family proteins Bax/Bak and the mitochondrial pathway.^5,6 Our studies also reveal that Puma upregulation induces cell apoptosis in chemoresistant ovarian cancer cells,^7,8 confirming the requisite role of Puma in chemosensitivity.7-Hydroxystaurosporine (UCN-01) is a protein kinase C-selective inhibitor that is successfully used in phase I and II clinical trials.^9,10 As a modulator, UCN-01 enhances the cytotoxicity of other anticancer drugs such as DNA-damaging agents and antimetabolite drugs by putative abrogation of G2- and/or S-phase accumulation induced by these anticancer agents.¹¹ As a single agent, UCN-01 exhibits two key biochemical effects, namely accumulation of cells in the G1 phase of the cell cycle and induction of apoptosis.¹² Both these effects may be important for its anticancer activity. Previous studies have demonstrated that UCN-01 potently decreased the levels of activated the phosphorylation level of Akt (p-Akt) in in vitro or in in vivo systems.^{12, 13, 14} Some researchers have also approved that UCN-01 can modulate Bcl-2 family members to potentiate apoptosis in cancer cells.^15,16 These reports suggest that Akt and Bcl-2 family proteins may be the potent targets of UCN-01 to trigger cancer cell apoptosis.In this study, we also investigate the role of Puma in UCN-01-induced apoptosis and confirm that p53-independent Puma induction is pivotal for the anticancer effects of UCN-01. Moreover, we first elucidate the detailed mechanism of Puma-induced apoptosis after UCN-01 treatment. We found that Puma expression mediated caspase-9 and caspase-3 activation. Among the caspase proteins, caspase-9 has a key role in Puma-induced apoptosis. Our data demonstrated that caspase-9 could mediate Puma-induced apoptosis through two feedback pathways. On the one hand, activated caspase-9 was initiated followed by caspase-3 activity, and activated caspase-3 cleaved XIAP in a positive feedback loop to strengthen Puma expression. On the other hand, caspase-9 itself cleaved antiapoptotic Bcl-2 and Bcl-xL to positively enhance Puma induction. These results provide the detailed mechanistic insight into therapeutic response to UCN-01 and the theoretical basis for its applications.