SIGNAL TRANSDUCTION PATHWAYS INVOLVED IN VASCULAR SMOOTH MUSCLE CELL MIGRATION

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SUMMARY OF WORK Mechanical injury to blood vessels results in the relatively slow accumulation of vascular smothh muscle cells (VSMCs), which normally reside in the medial (M) cell layer, at the luminal interface to form a neointima (N), a process that mimics and is an experimental model for transplant atherosclerosis and vessel restenosis. We have previously shown that activation of calcium/calmodulin-dependent protein kinase II (CamKII) is a key intracellular signaling event regulating PDGF-directed vascular smooth muscle cell (VSMC) migration out of the media. CamKII integrates signals from integrins, such as alphav/beta3, integrated associated proteins (e.g., IAP),and extracellular matirx molecules, such as thrombospondin (TSP)to coordinate chemoattractant-induced cell movement into the neointima. More recently, we have investigated the role of apoptosis in neointima formation. Normal blood vessels do not express the muscle-specific repressor of apoptosis, known as ARC. However, ARC selectively accumulates in the developing neointima after vessel injury. N-VSMCs isolated from the site of injury show increased resistance to apoptosis compared to isolated M-VSMCs. Forced expression of ARC, but not its CARD-defective mutant, in M-VSMCs to levels comparable to that in N-VMSCs increases their resistance to apoptosis. Upregulation of ARC in respond to vessel injury, therefore, may confer on N-VSMCs a differntial resistant to apoptosis that contributes to neointimal cell accumulation. recognition. Accordingly, antibodies to TSP, IAP, or alphav/beta3 all block VSMC migration as well as CamKII activation. Forced expression of constitutively activated CamKII restores migration to antibody-treated cells as well as to cells isolated from IAP-/- mice. In other cell types, we have shown that outside-in signaling from the beta3 integrin complex to other integrins (integrin crosstalk) is also mediated by CamKII and its effects on myosin light chain kinase (MLCK), which in nonmuscle cells promotes stress fiber formation. In these cell types as well as in migrating VSMCs, pharmacological inhibitors of MLCK mimic the effect of CamKII activation, promote migration, and inhibit stress fiber formation while promoting cortical actin deposition. The inhibitory effect of CamKII on MLCK activity may be direct via phosphorylation of the enzyme or indirect through phosphorylation of intermediary molecules. One possible intermediary is TIAM, a rac-GEF that has recently been shown to be a substrate for CamKII phosphorylation. Phosphorylation of TIAM by CamKII results in increased rac activity, cortical actin formation, and MCLK inhibition through the rac-associated kinase, PAK. We have shown that a dominant negative mutant of TIAM inhibits PDGF-directed migration and this inhibition can be overriden by constitutuvely activated mutants of rac, but not of CamKII. These results demonstrate that the regulation of rac is a critical downstream effector of CamKII in the regulation of VSMC migration and the crosstalk between different integrins in the cell.