Dysregulation of cell adhesion and motility is known to be an important factor 
in the development of tumor malignancy. Actopaxin (α-parvin) is a paxillin, 
integrin-linked kinase, and F-actin binding focal adhesion protein with several 
serine phosphorylation sites in the amino terminus that contribute to the 
regulation of cell spreading and migration. Here, phosphorylation of actopaxin 
is shown to contribute to the regulation of matrix degradation and cell 
invasion. Osteosarcoma cells stably expressing wild type (WT), 
nonphosphorylatable (Quint), and phosphomimetic (S4D/S8D) actopaxin demonstrate 
that actopaxin phosphorylation is necessary for efficient Src and matrix 
metalloproteinase-driven degradation of extracellular matrix. Rac1 was found to 
be required for actopaxin-induced matrix degradation whereas inhibition of 
myosin contractility promoted degradation in the phosphomutant-expressing Quint 
cells, indicating that a balance of Rho GTPase signaling and regulation of 
cellular tension are important for the process. Furthermore, actopaxin forms a 
complex with the Rac1/Cdc42 GEF β-PIX and Rac1/Cdc42 effector PAK1, to regulate 
actopaxin-dependent matrix degradation. Actopaxin phosphorylation is elevated in 
the invasive breast cancer cell line MDA-MB-231 compared with normal breast 
epithelial MCF10A cells. Expression of the nonphosphorylatable Quint actopaxin 
in MDA-MB-231 cells inhibits cell invasion whereas overexpression of WT 
actopaxin promotes invasion in MCF10A cells. Taken together, this study 
demonstrates a new role for actopaxin phosphorylation in matrix degradation and 
cell invasion via regulation of Rho GTPase signaling.