Conditioned medium taken from cultures of resting rabbit synovial fibroblasts contained a protein that prevented the synthesis of the neutral proteinase collagenase. Conditioned medium was concentrated 10-fold and placed on cultures of rabbit synovial fibroblasts along with an inducer of collagenase (phorbol myristate acetate or latex particles) and[3H]leucine. Collagenase production was measured by immunoprecipitation of culture medium with monospecific antibody. Gel filtration showed that the inhibitory factor had Mrs of 12,500, 25,000-50,000, and 150,000, suggesting that the protein may exist as aggregates. Activity was destroyed by boiling, by trypsin, and by dithiothreitol. Production of the inhibitory protein was prevented by cycloheximide. Isoelectric focusing purified the protein 100-to 150-fold and revealed pls in the range of 3.2-3.7. Glycosylation was demonstrated by binding to Con A-Sepharose. Our data indicate that rabbit synovial fibroblasts autoregulate collagenase production and suggest that the low levels of collagenase seen in resting cultures result from an active suppression of collagenase synthesis.The interstitial collagens are the body's most abundant structural proteins, and collagen remodeling occurs in a number of normal processes such as uterine resorption and wound healing (1-3) and in diseases such as rheumatoid arthritis and tumor invasion (4, 5) (for reviews, see refs. 6 and 7). The fact that collagen breakdown can be initiated only by the metalloproteinase collagenase (EC 3.4.24.7) makes this enzyme rate-limiting in collagenolysis and assures the importance of cellular mechanisms regulating its production. Although collagenase is produced by a variety of mammalian cell types (6, 7), the mechanisms governing collagenase synthesis and secretion differ. In polymorphonuclear leukocytes (8, 9) and monocytes/macrophages (10-12), collagenase is synthesized and stored within the cell in granules; secretion is regulated by factors controlling granule release rather than by those controlling enzyme synthesis (7). In contrast, fibroblast (e.g., skin and synovial cell) collagenase is not stored. Addition of an inducer stimulates transcription of collagenase mRNA followed by rapid synthesis and secretion of the enzyme (13-17).An abundant source of enzyme is that produced by fibroblasts derived from the synovial tissue lining the joints (4, 6, 7). Normally, these cells are quiescent, but proliferating synovial tissue in rheumatoid arthritis produces large quantities of collagenase, and the role of this enzyme in the destruction of connective tissues is well documented (4, 6, 7). The fact that these high levels of collagenase can be induced by treating cultures of resting synovial fibroblasts with a variety of stimuli makes these cells an attractive experimental model for studies on mechanisms controlling collagenase synthesis (14,(17)(18)(19).Over the past few years, considerable information has been obtained on the biosynthesis of synovial cell collagenase. Upon addition of a stimulus ...

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