Endogenous, hyperactive Rac3 controls proliferation of breast cancer cellsby a p21-activated kinase-dependent pathway

Uncontrolled cell proliferation is a major feature of cancer. Experimental cellular models have implicated some members of the Rho GTPase family in th is process. However, direct evidence for active Rho GTPases in tumors or ca ncer cell lines has never been provided. In this paper, we show that endoge nous, hyperactive Rac3 is present in highly proliferative human breast canc er-derived cell lines and tumor tissues. Rac3 activity results from both it s distinct subcellular localization at the membrane and altered regulatory factors affecting the guanine nucleotide state of Rac3. Associated with act ive Rac3 was deregulated, persistent kinase activity of two isoforms of the Rac effector p21-activated kinase (Pak) and of c-Jun N-terminal kinase (JN K). Introducing dominant-negative Rac3 and Pak1 fragments into a breast can cer cell line revealed that active Rac3 drives Pak and JNK kinase activitie s by two separate pathways. Only the Rac3-Pak pathway was critical for DNA synthesis, independently of JNK. These findings identify Rac3 as a consiste ntly active Rho GTPase in human cancer cells and suggest an important role for Rac3 and Pak in tumor growth.