AbstractBACKGROUNDThe neurofibromatosis 2 (NF2) tumor suppressor gene product merlin is an important regulator of contact‐dependent cell proliferation. Phosphorylation of merlin at serine 518 (Ser518) by the Rac effector p21‐activated kinase (PAK) inactivates merlin's growth suppressing function, and is regulated by cell‐culture conditions, including cell density, cell/substrate attachment, and growth factor availability. We examined the regulation of merlin expression and merlin phosphorylation in prostate cancer cells.METHODSPhosphorylation of merlin in five prostate cancer cell lines (LNCaP, DU145, PC3, 22RV1, and LAPC‐4) was examined by Western blotting using anti‐phospho‐merlin (Ser518) antibody. The activity of PAK, an upstream regulator of merlin phosphorylation, was measured by Western blotting using phospho‐PAK (Ser141) antibody. The effects of various cell‐culture conditions on the phosphorylation levels of merlin and PAK were analyzed.RESULTSBoth merlin expression and phosphorylation were low in LNCaP, PC3, 22RV1, and LAPC‐4 prostate cancer cells. In DU145 cells, total and phosphorylated merlin were abundant, but phosphorylation was not inhibited by high cell density, serum withdrawal, the addition of hyaluronic acid or inhibition of CD44 expression, all of which are reported to inhibit merlin phosphorylation in non‐neoplastic cells. PAK activation was elevated in DU145 cells and the addition of a PAK‐specific inhibitor peptide but not the Rac1‐specific inhibitor NSC23766 inhibited both PAK and merlin phosphorylation.CONCLUSIONSMerlin is inactivated in DU145 prostate cancer cells by PAK‐mediated constitutive phosphorylation, identifying a novel mechanism of merlin inactivation in neoplastic cells. Prostate 68:975–984, 2008. © 2008 Wiley‐Liss, Inc.