Role of PAK6 in cancer.

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PAKs are a family of serine threonine kinases characterized by a conserved N-terminal domain (CRIB domain) that binds the p21 GTPases, Cdc42 and Rac. The Group I PAKs (PAK1-3) have an autoinhibitory domain, which binds to and inhibits the kinase domain. The binding of Cdc42 and Rac proteins to the CRIB domain blocks this autoinhibitory domain, resulting in autophosphorylation and activation of kinase activity. Group II PAKs (PAK4-6) do not appear to have an autoinhibitory domain as their kinase activities are not activated by the binding of GTP-Cdc42. PAK6 was cloned by our lab, and independently by another lab, as an androgen receptor (AR) interacting protein. PAK6 also binds to the estrogen receptor (ERα) ligand binding domain and binding to both the AR and ERα are hormone independent. PAK6 RNA expression is highest in brain and testis, with lower levels in prostate, breast, and other tissues, but the specific cell types expressing PAK6 and the biological role of the AR and ERα interactions remain to be determined. Our experimental studies using affinity purified PAK6 antibody showed that PAK6 protein is expressed in prostate (LAPC4, LNCaP, CWR22Rv1, and PC3) and breast (HS578T, MDA-MB-231, MCF7, and HCC1937) cancer cell lines. By immunohistochemistry using the affinity purified antibody on tissue arrays, it was found that PAK6 is expressed in normal prostate epithelium (on the membrane) and its expression is increased and more cytoplasmic in many advanced prostate cancers. Our in vitro transfection data using CRIB domain mutants show that PAK6 is targeted to the plasma membrane by its CRIB domain, and that the CRIB domain is not involved in the .activation of PAK6 kinase activity. In contrast, we have recently reported that PAK6 is activated by MKK6 and p38 MAP kinase, suggesting that PAK6 plays a role in the cellular response to stress (Kaur et al., J. Biol. Chem. 2005;280(5):3323-30). We are currently using immunopurification of epitope tagged PAK6 in conjunction with mass spectroscopy to identify PAK6 associated proteins. One potential partner for PAK6 that we recently found by this approach is nucleolin, which is normally expressed in nucleolus, cytoplasm and as well as on the cell membrane. Nucleolin is an RNA binding protein whose activity is increased in response to stress by p38 MAP kinase, suggesting a link to PAK6. We are currently further assessing this interaction and identifying additional PAK6 associated proteins.