Background/Aims: To elucidate the influence and mode of action of HMR1726 (the active metabolite of leflunomide) on TNF-a and IL-17 activated metalloproteinases expression in synoviocytes. Methods: Synovial fibroblasts from RA and OA patients were stimulated with both cytokines and altered gene expression in the presence or absence of leflunomide was detected by microarray analyses and quantitative RT-PCR. Protein expression was detected by western blotting and commercial ELISAs. Results: Microarray analyses revealed that the addition of HMR1726 (50µM) to TNF-a and IL-17-stimulated synoviocytes induced gene expression of metallo-proteinases, especially MMP-1 and -3 in comparison to activated synoviocytes in the absence of leflunomide. To confirm these data, we examined the influence of different concentrations of HMR1726 in synoviocytes from further 5 OA and 7 RA patients by quantitative PCR. HMR1726 gradually induced MMP-1 and MMP-3 gene expression in a dose-dosedependent manner. Similar results were observed on protein levels. Examination of signal transduction pathways participating in the regulation of leflunomideinduced MMPs expression showed that the mechanism underlying activation of MMP-1 is in part p38- and activation of MMP-3 was MEK1/2-dependent. Conclusion: Leflunomide was not able to abolish expression of metallo-proteinases in synoviocytes activated with TNF-a and IL-17.

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