Inhibition of p21‐activated kinase 6 (PAK6) increases radiosensitivity of prostate cancer cells

Abstract

p21-activated kinase 6 (PAK6) is a serine/threonine kinase belonging to the p21-activated kinase (PAK) family. We investigated the role of PAK6 in radiation-induced cell death in human prostate cancer cells. We used a short hairpin RNA (shRNA) strategy to stably knock down PAK6 in PC3 and DU145 cells. Radiation sensitivities were compared in PAK6 stably knockdown cells versus the scrambled shRNA-expressing control cells. PAK6 mRNA and protein levels in PC3 and DU145 cells were upregulated upon exposure to 6 Gy of radiation. After irradiation, an increased percentage of apoptotic cells and cleaved caspase-3 levels were demonstrated in combination with a decrease in cell viability and a reduction in clonogenic survival in PAK6-knockdown cells. In addition, transfection with PAK6 shRNA blocked cells in a more radiosensitive G2-M phase and increased levels of DNA double-strand breaks. We further explored the potential mechanisms by which PAK6 mediates resistance to radiation-induced apoptosis. Inhibition of PAK6 caused a decrease in Ser(112) phosphorylation of BAD, a proapoptotic member of the Bcl-2 family, which led to enhanced binding of BAD to Bcl-2 and Bcl-X(L) and release of cytochrome c culminating into caspase activation and cell apoptosis. The combination of PAK6 inhibition and irradiation resulted in significantly decreased survival of prostate cancer cells. The underlying mechanisms by which targeting PAK6 may improve radiation response seem to be multifaceted, and involve alterations in cell cycle distribution and impaired DNA double-strand break repair as well as relieved BAD phosphorylation.