Glomerular basement membrane-derived perlecan inhibits mesangial cell adhesion to fibronectin

To explore whether the heparan sulfate proteoglycan, periecan, may act as a regulator of glomerular cell behavior, we examined the effects of perlecan on adhesive properties of rat and human mesangial cells in culture. We observed that mesangial cells did not adhere to purified perlecan. Mesangial cell adhesion to fibronectin, but not to laminin, was inhibited when perlecan was present in the substratum, reaching complete inhibition at 20 micrograms/ml perlecan. Similarly, perlecan reduced mesangial cell adhesion to the 120 kDa fragment of fibronectin and to bovine serum albumin-coupled RGD peptides both lacking the fibronectin heparin-binding domains, indicating that the inhibitory effect of perlecan does not require interaction of its heparan sulfate chains with fibronectin. The core protein of perlecan seems to be relevant, since perlecan retained most of its anti-adhesive potency after total deglycosylation. It is unlikely that perlecan causes steric inhibition of the cell binding sites of fibronectin by direct binding, since the 120 kDa fragment of fibronectin which includes the cell binding site, did not bind to intact perlecan. In the presence of submaximal inhibitory concentrations of perlecan, soluble RGD peptides (100 micrograms/ml), by themselves without effect, completely inhibited cell adhesion to fibronectin. Using immunocytochemistry, we examined the effects of perlecan on the organization of fibronectin-specific, RGD-dependent beta 1 integrins in focal contacts. When perlecan was added to the substratum, human mesangial cells adhered to fibronectin only in the presence of collagen I. Under these conditions, perlecan did not significantly alter the organization of alpha 5 beta 1 integrin into focal contacts of mesangial cells which adhered to a substratum composed of collagen I plus fibronectin. These data suggest that perlecan exerts its anti-adhesive effects on mesangial cells via a core protein-dependent mechanism which reduces the avidity of the fibronectin receptor. The resulting avidity level is too low for mediating cell binding but sufficient to induce integrin organization into focal contacts.