MIGRATION OF CULTURED VASCULAR SMOOTH-MUSCLE CELLS THROUGH A BASEMENT-MEMBRANE BARRIER REQUIRES TYPE-IV COLLAGENASE ACTIVITY AND IS INHIBITED BY CELLULAR-DIFFERENTIATION

The migration of vascular smooth muscle cells (VSMCs) from the tunica media to the neointima is a key event in the development and progressi on of many vascular diseases and a highly predictable consequence of m echanical injury to the blood vessel. In vivo, VSMCs are surrounded by and embedded in a variety of extracellular matrices (ECMs) that must be traversed during migration. One of the principal barriers to cell m ovement in the intact vessel is the basement membrane (BM) that surrou nds each VSMC and separates the VSMC-containing medial cell layer from the endothelium. We have used a Boyden chamber to monitor the ability of VSMCs to degrade a BM barrier as they migrate toward a chemoattrac tant and to define the role of extracellular proteases in this process . We show that cultured VSMCs can migrate across a BM barrier and that this ability was dependent on the phenotypic state of the cell. VSMCs maintained in a proliferating or ''synthetic'' state readily migrated across a BM toward a chemoattractant, whereas the migration of serum- starved/differentiated VSMCs was suppressed by >80% (P<.001). By use o f a number of peptides that inhibit matrix metalloproteinase (MMP) act ivity, the migration of proliferating VSMCs across the BM barrier was inhibited by >80% (P<.0001), whereas migration that occurred in the ab sence of the barrier was unaffected. Northern blotting and zymographic analyses indicated that 72-kD type IV collagenase (MMP2) was the prin cipal MMP expressed and secreted by these cells. Accordingly, antisera capable of selectively neutralizing MMP2 activity also inhibited VSMC migration across the barrier without significantly affecting the migr ation of VSMCs in the absence of the barrier. Finally, MMP2 activity w as also regulated by the phenotypic state of the cells in that MMP2 ac tivity expressed by serum-starved/differentiated VSMCs was 15% of that measured in proliferating VSMCs. Extrapolating to the in vivo situati on in which VSMCs reside in an ECM composed of various BM barriers, th ese results suggest that VSMC migration in vivo may be dependent on MM P2 activity. That activity, in turn, could be regulated by the phenoty pic state of VSMCs and increase as these cells undergo the transition from a quiescent and differentiated state to that of a dedifferentiate d, proliferating, and motile phenotype after injury to the vessel.