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

Abstract

Background 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. Methods 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. Results 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 Ser112 phosphorylation of BAD, a proapoptotic member of the Bcl-2 family, which led to enhanced binding of BAD to Bcl-2 and Bcl-XL and release of cytochrome c culminating into caspase activation and cell apoptosis. Conclusions 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.

PAK signalling during the development and progression of cancer

“…Interestingly, in these cells, PAK1 appeared to be the major Pak isoform required for invasiveness, despite the prominent expression of group II PAK4 and PAK6 34, 67, 69 .…”

Structure, biochemistry, and biology of PAK kinases

“…However, PAK6 downregulation in clear renal cell carcinoma is associated with unfavorable survival of patients (Liu et al, 2014). In human therapeutic treatments, PAK6 upregulation promotes resistance to 5- fluorouracil treatment in colon cancer patients (Chen et al, 2015), and its inhibition enhances the sensitivity of prostate cancer cells to radiation (Zhang et al, 2010) and docetaxel (Wen, et al, 2009). …”

Substrate and Inhibitor Specificity of the Type II p21-Activated Kinase, PAK6

“…In addition, mice with combined deletion of Pak5 and Pak6 show deficits in locomotion, learning and memory not associated with single deletions of either gene, suggesting functional redundancy between the two PAKs [12], [13]. While neuronal substrates specific to PAK6 have not been identified, PACSIN1 (Syndapin 1), an F-BAR protein involved in synaptic vesicle recycling, is phosphorylated redundantly by PAK4, PAK5 and PAK6 in vivo [14] PAK6 is overexpressed in prostate cancer [15], and its targeted inhibition could potentially decrease growth of prostate tumors [11] or sensitize prostate cancer cells to radiotherapy [16]. PAK6 has also been found to acquire somatic mutations in other solid tumors, including mutation of residue Pro52 to leucine in two independent melanomas [17], [18].…”