The p21-activated kinase 1 (Pak1) is a serine/threonine kinase whose activity is regulated by both Rho GTPases and AGC kinase family members. It plays a role in cytoskeletal remodeling and cell motility as well as cell proliferation, angiogenesis, tumorigenesis and metastasis. An involvement of Pak1 in renal cell carcinoma (RCC), which remains highly refractory to chemotherapy and radiotherapy, remains to be investigated. Pak1 expression, phosphorylation and kinase activity were examined in RCC cell lines and human tissue from normal and renal carcinoma. We report increased Pak1 expression and constitutive activity in the membrane and nucleus but not the cytoplasm of resected human RCC. To study a role for Pak1 in RCC, we developed 786-0 clones that expressed either a kinase-active Pak1(L83,L86) 2 different Pak1 dominant negative mutants, Pak1(R299) and Pak1(L83,L86,R299) or Pak1 siRNA. The expression of Pak1(L83,L86) increased 786-0 proliferation, motility and anchorage independent growth, while the dominant negative mutants and Pakl siRNA abrogated these effects. In addition, Pak1(L83,L86) conferred resistance to 5-fluorouracil with a 40%+/-10% increase in cell viability. Conversely, Pak1(L83,L86,R299) Pak1(R299) and Pak1 siRNA conferred sensitivity with a 65.2%+/-5.5%, 69.2%+/-3.3% and 73.0%+/-8.4% loss in viability, respectively. Finally, Pakl plays a role in renal tumor growth in vivo. Only 33% of mice developed tumors in the Pak1(L83,L86,R299) group and no tumors developed from Pak1(R299) cell challenge. Together these findings point to Pakl as an exciting target for therapy of renal cancer, which remains highly refractory to existing treatments. (c) 2007 Wiley-Liss, Inc.