OSTEOPONTIN IS PRODUCED BY RAT CARDIAC FIBROBLASTS AND MEDIATES A(II)-INDUCED DNA-SYNTHESIS AND COLLAGEN GEL CONTRACTION

Angiotensin II (A(II)) is a critical factor in cardiac remodeling whic h involves hypertrophy, fibroblast proliferation, and extracellular ma trix production. However, little is known about the mechanism by which A(II) accelerates these responses. Osteopontin is an acidic phosphopr otein with RGD (arginine-glycine-aspartate) sequences that are involve d in the vascular smooth muscle cell remodeling process, We identified the presence of osteopontin mRNA and protein in cultured rat cardiac fibroblasts and its prominent regulation by A(II) (10(-11) M). Osteopo ntin message levels were increased fourfold (P < 0.01) and protein fiv efold (P < 0.05) at 24 h after addition of A(II) (10(-7) M), This resp onse was inhibited by the AT(1) receptor blocker, losartan, Osteoponti n mRNA levels were increased in hypertrophied ventricles from animals with renovascular hypertension (1.6-fold, P < 0.05) and aortic banding (2.9-fold, P < 0.05). To examine the function of osteopontin, we dete rmined its effects on (a) the ability of cardiac fibroblasts to contra ct three-dimensional collagen gels and (b) cardiac fibroblast growth. A monoclonal antibody against osteopontin partially blocked A(II)-indu ced three-dimensional collagen gel contraction by cardiac fibroblasts (64 +/- 4 vs, 86 +/- 5% in the presence of antibody, P < 0.05), while osteopontin itself promoted contraction of the gels by fibroblasts (71 +/- 5%, P < 0.05 compared with control), Either a monoclonal antibody against beta(3) integrin which is a ligand for osteopontin or the RGD peptide blocked both A(II) and osteopontin-induced collagen gel contr action, Thus, the osteopontin RGD sequence binds to beta(3) integrins on the fibroblast to promote fibroblast binding to collagen, A(II) ind uced a threefold increase in DNA synthesis of cardiac fibroblasts, whi ch was completely blocked by antibodies against osteopontin and beta(3 ) integrin, or by RGD peptide, but not by controls. Thus, A(II)-induce d growth of cardiac fibroblasts also requires osteopontin engagement o f the beta(3) integrin, Taken together, these results provide the firs t evidence that osteopontin is a potentially important mediator of A(I I) regulation of cardiac fibroblast behavior in the cardiac remodeling process.