MATRIX METALLOPROTEINASE-8 IS EXPRESSED IN RHEUMATOID SYNOVIAL FIBROBLASTS AND ENDOTHELIAL-CELLS - REGULATION BY TUMOR-NECROSIS-FACTOR-ALPHA AND DOXYCYCLINE

Neutrophil collagenase (matrix metalloproteinase-8 or MMP-8) is regard ed as being synthesized exclusively by polymorphonuclear neutrophils ( PMN). However, in vivo MMP-8 expression was observed in mononuclear fi broblast-like cells in the rheumatoid synovial membrane. In addition, we detected MMP-8 mRNA expression in cultured rheumatoid synovial fibr oblasts and human endothelial cells. Up-regulation of MMP-8 was observ ed after treatment of the cells with either tumor necrosis factor-alph a (10 ng/ml) or phorbol 12-myristate 13-acetate (10 nM). Western analy sis showed a similar regulation at the protein level. The size of secr eted MMP-8 was 50 kDa, which is about 30 kDa smaller than MMP-8 from P MN. Conditioned media from rheumatoid synovial fibroblasts contained b oth type I and II collagen degrading activity. However, degradation of type II collagen, but not that of type I collagen, was completely inh ibited by 50 mu M doxycycline, suggesting specific MMP-8 activity. In addition, doxycycline down-regulated MMP-8 induction, at both the mRNA and protein levels. Thus MMP-8 exerts markedly wider expression in hu man cells than had been thought previously, implying that PMN are not the only source of cartilage degrading activity at arthritic sites. Th e inhibition of both MMP-8 activity and synthesis by doxycycline provi des an incentive for further studies on the clinical effects of doxycy cline in the treatment of rheumatoid arthritis.