p21-activated kinase 5 inhibits camptothecin-induced apoptosis in colorectal carcinoma cells

p21-activated kinase 5 (PAK5) is a recently identified member of the group B PAK family. The PAK proteins are effectors of the small GTPase Cdc42 and Rac1 and are known to regulate cell motility and activate cell-survival signaling pathways. Especially, the mitochondrial localization of PAK5 is vital to its effects on apoptosis and cell survival. Previously, we demonstrated that PAK5 expression increased significantly during the malignant progression of colorectal carcinoma (CRC) and that PAK5 promoted CRC metastasis by regulating CRC cell adhesion and migration. In the present study, we aim to investigate the role of PAK5 in camptothecin-induced apoptosis and its potential mechanism of action. Our results showed that overexpression of PAK5 inhibited camptothecin-induced apoptosis by inhibiting the activity of caspase-8 in CRC cells. Accordingly, knockdown of PAK5 in LoVo cells resulted in increased apoptosis. Mechanistically, we found that PAK5 directly phosphorylated Bad on serine 112 and indirectly led to phosphorylation of serine 136 via the Akt pathway. In conclusion, our study revealed previously unappreciated inhibitory role of PAK5 in camptothecin-induced apoptosis, thus suggesting PAK5 as a novel therapeutic target in CRC.