SPARC, A SECRETED PROTEIN ASSOCIATED WITH MORPHOGENESIS AND TISSUE REMODELING, INDUCES EXPRESSION OF METALLOPROTEINASES IN FIBROBLASTS THROUGH A NOVEL EXTRACELLULAR MATRIX-DEPENDENT PATHWAY

SPARC (osteonectin/BM40) is a secreted protein that modifies the inter action of cells with extracellular matrix (ECM). When we added SPARC t o cultured rabbit synovial fibroblasts and analyzed the secreted prote ins, we observed an increase in the expression of three metalloprotein ases-collagenase, stromelysin, and the 92-kD gelatinase-that together can degrade both interstitial and basement membrane matrices. We furth er characterized the regulation of one of these metalloproteinases, co llagenase, and showed that both collagenase mRNA and protein are upreg ulated in fibroblasts treated with SPARC. Experiments with synthetic S PARC peptides indicated that a region in the neutral alpha-helical dom ain III of the SPARC molecule, which previously had no described funct ion, was involved in the regulation of collagenase expression by SPARC . A sequence in the carboxyl-terminal Ca2+-binding domain IV exhibited similar activity, but to a lesser extent. SPARC induced collagenase e xpression in cells plated on collagen types I, II, III, and V, and on vitronectin, but not on collagen type IV. SPARC also increased collage nase expression in fibroblasts plated on ECM produced by smooth muscle cells, but not in fibroblasts plated on a basement membrane-like ECM from Engelbreth-Holm-Swarm sarcoma. Collagenase was induced within 4 h in cells treated with phorbol diesters or plated on fibronectin fragm ents, but was induced after 8 h in cells treated with SPARC. A number of proteins were transiently secreted by SPARC-treated cells within 6 h of treatment. Conditioned medium that was harvested from cultures 7 h after the addition of SPARC, and depleted of residual SPARC, induced collagenase expression in untreated fibroblasts; thus, part of the re gulation of collagenase expression by SPARC appears to be indirect and proceeds through a secreted intermediate. Because the interactions of cells with ECM play an important role in regulation of cell behavior and tissue morphogenesis, these results suggest that molecules like SP ARC are important in modulating tissue remodeling and cell-ECM interac tions.