Effects of leflunomide on hyaluronan synthases (HAS): NF-kappa B-independent suppression of IL-1-induced HAS1 transcription by leflunomide.

Despite evidence that points to unfettered hyaluronic acid (HA) production as a culprit in the progression of rheumatic disorders, little is known about differences in regulation and biological functions of the three hyaluronan synthase (HAS) genes. Testing the effects of drugs with proven anti-inflammatory effects could help to clarify biological functions of these genes. In this study, we demonstrate that leflunomide suppresses HA release in fibroblast-like synoviocytes (FLS) in a dose-dependent manner. We further demonstrate that leflunomide suppresses HA synthase activity, as determined by (14)C-glucuronic acid incorporation assays. Additional experiments revealed that in FLS, leflunomide specifically blocked the induction of HAS1. HAS2 and HAS3, genes that are, in contrast to HAS1, constitutively expressed in FLS, are not significantly affected. Leflunomide can function as a NF-kappaB inhibitor. However, EMSA experiments demonstrate that at the concentrations used, leflunomide neither interferes with IL-1beta- nor with PMA-induced NF-kappaB translocation. Furthermore, reconstituting the pyrimidine synthase pathway did not lead to the restoration of IL-1beta-induced HAS1 activation. More importantly, two tyrosine kinase inhibitors mimicked the effect of leflunomide in that both blocked IL-1beta-induced HAS1 activation without affecting HAS2 or HAS3. These data point at HAS1 activation as the possible cause for unfettered HA production in rheumatoid arthritis and might explain, at least in part, the beneficial effects of leflunomide treatment. These findings also support the concept that IL-1beta-induced HAS1 activation depends on the activation of tyrosine kinases, and indicate that leflunomide blocks HA release by suppressing tyrosine kinases rather than through inhibition of NF-kappaB translocation.