Invasion of the subendothelial space by vascular smooth muscle cells (VSMCs) 
contributes to the development and progression of diverse cardiovascular 
diseases. In this report we show that the expression of activated versions of 
Src, Cdc42 and Rac1, or a kinase-dead but open form of the p21-activated kinase 
(PAK1), induces primary rat aorta VSMCs to form extracellular matrix-degrading 
actin-rich protrusions that are morphologically similar to the invadopodia 
formed by highly invasive tumor cells. The matrix-degrading structures are 
enriched in known markers for invadopodia, including cortactin and 
tyrosine-phosphorylated cortactin and contain the matrix metalloproteinases 
MMP-9 and MT1-MMP and the urokinase plasminogen activator receptor (uPAR). In 
contrast to other cell types, invadopodia formation in VSMCs is only weakly 
supported by the phorbol ester PBDu. Invadopodia formation by Src was dependent 
on Cdc42, Rac, and ERK, but not on p38 MAPK. Invadopodia formation induced by 
kinase-dead PAK1 required Src and ERK activity and a direct interaction with the 
exchange factor PIX. VSMCs embedded in a three-dimensional collagen matrix 
formed actin- and cortactin-rich extensions that penetrated through holes in the 
matrix, suggesting that invadopodia-like structures are formed in a 
three-dimensional environment.