PAK1 induces podosome formation in A7r5 vascular smooth muscle cells in a PAK-interacting exchange factor-dependent manner.

Remodeling of the vascular smooth muscle cytoskeleton is essential for cell motility involved in the development of diseases such as arteriosclerosis and restenosis. The p21-activated kinase (PAK), which is an effector of the Rho GTPases Rac and Cdc42, has been shown to be involved in cytoskeletal remodeling and cell motility. We show herein that expression of cytoskeletally active constructs of PAK1 is able to induce the formation of dynamic, podosome-like F-actin columns in the A7r5 vascular smooth muscle cell line. Most of these actin columns appear at the junctions between stress fibers and focal adhesions and contain several known podosomal protein markers, such as cortactin, Arp2/3, alpha-actinin, and vinculin. The kinase activity of PAK plays a role in the regulation of the turnover rates of these actin columns but is not essential for their formation. The ability of PAK to interact with the PAK-interacting exchange factor (PIX) but not with Rac or Cdc42, however, is required for the formation of the actin columns as well as for the translocation of PIX and G protein-coupled receptor kinase-interacting protein (GIT) to focal adhesions adjacent to the actin columns. These findings suggest that interaction between PAK and PIX, as well as the recruitment of PIX and GIT to focal adhesions, plays an important role in the formation of actin columns that resemble podosomes induced by phorbol ester in vascular smooth muscle cells.