Increased PAK6 expression in prostate cancer and identification of PAK6 associated proteins.

PAK6 is a member of the p21-activated kinase (PAK) family of serine/threonine kinases that was originally cloned from prostate cancer (PCa) cells as an androgen receptor interacting protein, but its cellular distribution and functions have not been established. An affinity purified rabbit anti-PAK6 antiserum was generated to assess PAK6 protein expression. PAK6 associated proteins were identified by immunopurification of 3xFlag-tagged PAK6 followed by LC/MS/MS. We confirmed that PAK6 protein is expressed in prostate and breast cancer cell lines. PAK6 expression in LNCaP PCa cells was not directly androgen regulated, but was markedly increased when the cells were cultured for 6-8 weeks in steroid hormone depleted medium. By immunohistochemistry, PAK6 was weakly expressed in normal prostate epithelium. Its expression was increased in primary and metastatic PCa, and was further increased in tumors that relapsed after androgen deprivation therapy. LC/MS/MS identified IQ motif containing GTPase activating protein 1 (IQGAP1) and protein phosphatase 1B (PP1B) as candidate PAK6 interacting proteins, and these findings were confirmed by coimmunoprecipitation. These results indicate that PAK6 contributes to PCa development and progression after androgen deprivation therapy, and that it may play roles in the regulation of motility and in stress responses.