Objective. In rheumatoid arthritis (RA), it is well known that rheumatoid synovial fibroblasts (RSF) produce matrix metalloproteinases (MMPs) when stimulated with proinflammatory cytokines such as interleukin-1β (IL-1β), which causes joint destruction. We have previously shown that hyaluronan (HA) inhibits IL-1β actions in RSF via CD44, the principal HA receptor. However, CD44 mediates HA effects only partially, and intracellular events after the HA binding to its receptors remain unclear. We investigated the role of intercellular adhesion molecule-1 (ICAM-1), another cell surface receptor for HA, and the intracellular signalling pathways in the actions of HA.

Methods. RSF were isolated from rheumatoid synovial tissues by enzymatic digestion and cultured in monolayers. The confluent cells were incubated for 48 h with IL-1β, IL-1β in the presence of HA, or IL-1β in the presence of HA with pretreatment with anti-ICAM-1 antibody. Secretion of MMP-1 and MMP-3 was analysed by immunoblotting and immunofluorescence cytochemistry. Immunofluorescence cytochemistry was also performed to evaluate binding of HA to ICAM-1. The phosphorylation of nuclear factor (NF)-κB and mitogen-activated protein kinases (MAPKs) was analysed by immunoblotting.

Results. Production of MMP-1 and MMP-3 by RSF was stimulated by IL-1β. HA at ≥2 mg/ml significantly inhibited MMP production induced by IL-1β in a dose-dependent manner. Moreover, pretreatment with anti-ICAM-1 antibody at 50 μg/ml significantly blocked the effects of HA on the actions of IL-1β on RSF, as shown by immunoblotting and immunofluorescence cytochemistry. Another immunofluorescence cytochemistry study demonstrated that HA bound RSF via ICAM-1. Inhibition studies revealed the requirement of NF-κB, p38 and c-jun NH₂-terminal kinase (JNK) for IL-1β-induced MMP production. IL-1β activated all three pathways, whereas HA down-regulated their phosphorylation. Pretreatment with anti-ICAM-1 antibody reversed the inhibitory effects of HA on the activation of NF-κB and p38 without affecting JNK.

Conclusion. HA suppresses IL-1β-enhanced MMP-1 and MMP-3 synthesis in RSF via ICAM-1 through down-regulation of NF-κB and p38. Intra-articular injection of HA of high molecular weight may work through such a mechanism in RA joints.