Arthritis & Rheumatism Official Journal of the American College of Rheumatology EDITORIAL JOINT DESTRUCTION IN ARTHRITIS: METALLOPROTEINASES IN THE SPOTLIGHT CONSTANCE E . BRINCKERHOFF The erosion of connective tissue (cartilage, tendon, and bone) that accompanies both rheumatoid arthritis (RA) and osteoarthritis (OA) is, unfortunately, well known to physicians and patients alike. This destruction is mediated largely by collagenase and stromelysin, enzymes that are produced by the synovial fibroblasts (1). Collagenase is rate limiting in collagen degradation, while stromelysin degrades noncollagen proteins, e.g., laminin, fibronectin, and proteoglycans. Both enzymes belong to the gene family of metalloproteinases, which comprises at least 10 members (2). All metalloproteinases are active at neutral pH, contain zinc as an integral part of their structure, require Ca++ for activity, and are inhibited by tissue inhibitor of metalloproteinases (TIMP). Thus, TIMP, also a product of synovial fibroblasts, provides a potential mechanism for controlling degradation of the extracellular matrix, but its actual role in modulating disease has been debated. Until now, most of our information on the biochemical and biologic features of these enzymes has been derived from in vitro studies utilizing either From the Department of Medicine and the Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire. Supported by NIH grant AR-26599 and by grants from the Council for Tobacco Research and the RGK Foundation, Austin, Texas. Constance E. Brinckerhoff, PhD: Professor of Medicine and Biochemistry. Address reprint requests to Constance E. Brinckerhoff, PhD, Department of Medicine, Dartmouth Medical School, Hanover, NH 03756. primary cultures of human synovium taken from RA and OA tissue, or experimentally stimulated normal synovial cells (1,3-5). These studies showed that synovial fibroblasts can produce massive amounts of both collagenase and stromelysin, and that these enzymes, acting in concert, can degrade essentially all components of the extracellular matrix. Thus, the association of collagenase and stromelysin with arthritic synovium and with connective tissue destruction was obvious. However, knowledge of the subtle details of localization and quantitation of metalloproteinase production by various subpopulations of arthritic tissue was lacking, and this has prevented precise definition of pathologic mechanisms operating in each disease. Three papers in this issue of Arthritis and Rheumatism (6-8) go a long way toward remedying this situation. Using the technique of in situ hybridization to detect and quantify messenger RNA (mRNA) present in individual cells, the authors document the production of collagenase and stromelysin mRNA in synovial tissue from patients with RA and OA. All 3 studies demonstrated production of these enzymes by synovial lining cells of fibroblast or macrophage origin, but not by lymphocytes. Levels of mRNA were greater in tissue from patients with RA compared with OA, supporting the observation that RA is generally the more aggressive disease. The report by Firestein et al (6) carefully quantitates collagenase mRNA in synovial lining biopsy specimens before and after intraarticular steroid therapy. The authors found that the expression of collagenase and of several other genes (including TIMP) Arthritis and Rheumatism, Vol. 34, No. 9 (September 1991) 1073 1074 correlated with the degree of synovial inflammation. Of particular interest are the findings that 1) the intraarticular injection of corticosteroids resulted in marked clinical improvement which was accompanied by decreased expression of collagenase mRNA and, 2 ) this clinically meaningful analysis technique could be used with small amounts of material obtained by a minimally invasive biopsy. Gravallese and colleagues (7) extend these findings by demonstrating in vivo what has been observed in vitro: the coordinate expression of both collagenase and stromelysin by synovial cells. They go on to show a quantitative relationship in the activation of the two genes and suggest that this activation may be linked to the degree of hyperplasia. The studies by McCachren (8) concentrate on the relationship between metalloproteinase and TIMP mRNA levels. He found that the synovial lining cells produce all 3 mRNAs simultaneously, providing another mechanism by which cells may regulate the extent of matrix degradation. This is an important finding since it demonstrates that the inhibitor is in close physical proximity to the enzyme, and therefore, its presence may be physiologically relevant to disease outcome. Further, he found that TIMP levels were lower in RA patients than in OA patients, suggesting that this regulator of connective tissue destruction may be less effective in RA, where metalloproteinase levels are higher. Thus, these 3 papers, for the first time, 1) document the coordinate expression in vivo of collagenase and stromelysin mRNAs by synovial lining cells, 2) show that this expression is greater in RA tissue versus OA tissue, and 3) imply that modulation of these mRNA levels, either therapeutically by steroids or physiologically by TIMP, may influence disease outcome. The findings are significant because they validate in vivo the results of previous experiments in vitro. Moreover, they permit histologic and “geographic” mapping of the cells that are making these destructive enzymes and begin to delineate the specific pathophysiologic mechanisms involved in RA and OA. Equally important, they establish the power of quantitative in situ hybridization and prove the feasibility of using modern molecular biology techniques to evaluate disease severity and therapeutic efficacy. It is heartening to finally be able to address questions of why, how, and where these enzymes, long known to mediate the irrevocable erosion of BRINCKERHOFF connective tissue in arthritis, are produced. A better understanding of the precise molecular mechanisms regulating the expression of the genes for collagenase and stromelysin, and of how this knowledge can be applied to clinical medicine, is still needed. For example, we know that both inducers (9-12) and inhibitors (13,14) of collagenase production affect transcription of the collagenase gene by acting on a 9-basepair sequence of DNA (termed the AP-1 site) located in the promoter region. We also know that additional DNA sequences participate in these responses (11,12). Determining just where these regulatory sequences are and what transcription factors bind to them will require years of intensive research but may, in the long run, lead to new and better therapeutic approaches. In the immediate future, however, it is exciting to see that techniques of modern molecular biology are applicable to clinical rheumatology and that the crucial role of metalloproteinases in the pathology of arthritic disease is becoming fully recognized. REFERENCES 1. Werb Z: Proteinases and matrix degradation, Textbook of Rheumatology. Edited by WN Kelley, ED Harris Jr, S Ruddy, CB Sledge. Philadelphia, WB Saunders, 1989 2. Matrisian L: Metalloproteinases and their inhibitors in matrix remodeling. Trends Genet 6:121-125, 1990 3. Fini ME, Karmilowicz MJ, Ruby PL, Beeman AM, Borges KA, Brinckerhoff CE: Cloning of a complementary DNA for rabbit proactivator: a metalloproteinase that activates synovial cell collagenase, shares homology with stromelysin and transin, and is coordinately regulated with collagenase. Arthritis Rheum 30: 12541264, 1987 4. Krane SM, Conca W, Stephenson ML, Amento EP, Goldring MB: Mechanisms of matrix degradation in rheumatoid arthritis. Ann N Y Acad Sci 580:340-354, 1990 5 . MacNaul KL, Chartrain N, Lark M, Tocci MJ, Hutchinson NI: Discoordinate expression of stromelysin, collagenase and tissue inhibitor of metalloproteinases-1 in rheumatoid human synovial fibroblasts. J Biol Chem 265:1723&17245, 1990 6. Firestein GS, Paine MM, Littman BH: Gene expression (collagenase, tissue inhibitor of metalloproteinases, complement, and HLA-DR) in rheumatoid arthritis and osteoarthritis synovium: quantitative analysis and effect of intraarticular corticosteroids. Arthritis Rheum 34: 1094-1 105, 1991 7. Gravallese EM, Darling JM, Ladd AL, Katz JN, Glimcher LH: In situ hybridization studies of stromelysin and EDITORIAL collagenase messenger RNA expression in rheumatoid synovium. Arthritis Rheum 34:1076-1084, 1991 8. McCachren SS: Expression of metalloproteinases and metalloprotease inhibitor in human arthritic synovium. Arthritis Rheum 34:1085-1093, 1991 9. Brenner DA, O'Hara M, Angel P, Chojkier M, Karin M: Prolonged activation of jun and collagenase genes by tumor necrosis factor alpha. Nature 337:661-663, 1989 10. Conca M, Kaplan PB, Krane SM: Increases in levels of procollagenase mRNA in cultured fibroblasts induced by human interleukin lp or serum follow c-jun expression and are dependent on new protein synthesis. J Clin Invest 83:1753-1757, 1989 11. Angel P, Baumann IB, Stein B, Delium H, Rahmsdorf HJ, Herrlich P: 12-0-Tetradecanoyl-phorbol-13-acetate induction of the human collagenase gene is mediated by 1075 an inducible enhancer element located in the 5' flanking region. Mol Cell Biol 7:2256-2266, 1987 12. Auble DT, Brinckerhoff CE: The AP-I sequence is necessary but not sufficient for phorbol induction of collagenase in fibroblasts. Biochemistry 30:46294635, 1991 13. Jonat C, Ramsdorf HJ, Park K-K, Cat0 ACB, Gebel S, Ponta H , Herrlich P: Antitumor promotion and antiinflammation: down-modulation of AP-1 (Fos/Jun) activity by glucocorticoid hormone. Cell 62:1189-1204, 1990 14. Yang-Yen H-F, Chambard J-C, Sun Y-L, Smeal T, Schmidt TJ, Drouin J , Karin M: Transcriptional interference between c-Jun and the glucocorticoid receptor: mutual inhibition of DNA binding due to direct proteinprotein interaction. Cell 62:1205-1215, 1990
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