INTRODUCTION: The metastatic progression of cancer is a direct result of the 
disregulation of numerous cellular signaling pathways, including those 
associated with adhesion, migration, and invasion. Members of the Rac family of 
small GTPases are known to act as regulators of actin cytoskeletal structures 
and strongly influence the cellular processes of integrin-mediated adhesion and 
migration. Even though hyperactivated Rac proteins have been shown to influence 
metastatic processes, these proteins have never been directly linked to 
metastatic progression.
METHODS: To investigate a role for Rac and Cdc42 in metastatic breast cancer 
cell invasion and migration, relative endogenous Rac or Cdc42 activity was 
determined in a panel of metastatic variants of the MDA-MB-435 metastatic human 
breast cancer cell line using a p21-binding domain-PAK pull down assay. To 
investigate the migratory and invasive potential of the Rac isoforms in human 
breast cancer, namely Rac1 and the subsequently cloned Rac3, we stably expressed 
either dominant active Rac1 or dominant active Rac3 into the least metastatic 
cell variant. Dominant negative Rac1 or dominant negative Rac3 were stably 
expressed in the most metastatic cell variant. Cell lines expressing mutant Rac1 
or Rac3 were analyzed using in vitro adhesion, migration and invasion assays.
RESULTS: We show that increased activation of Rac proteins directly correlates 
with increasing metastatic potential in a panel of cell variants derived from a 
single metastatic breast cancer cell line (MDA-MB-435). The same correlation 
could not be found with activated Cdc42. Expression of a dominant active Rac1 or 
a dominant active Rac3 resulted in a more invasive and motile phenotype. 
Moreover, expression of either dominant negative Rac1 or dominant negative Rac3 
into the most metastatic cell variant resulted in decreased invasive and motile 
properties.
CONCLUSION: This study correlates endogenous Rac activity with high metastatic 
potential and implicates Rac in the regulation of cell migration and invasion in 
metastatic breast cancer cells. Taken together, these results suggest a role for 
both the Rac1 and Rac3 GTPases in human breast cancer progression.