Aggrecan degradation is considered to play a key role in the progression of osteoarthritis (OA). Aggrecanases are members of a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family, and degrade aggrecan in OA cartilage. The aim of this study was to clarify the mechanisms of expression of ADAMTS4 induced by IL-1 beta in human fibroblast-like synoviocyte (HFLS) cells by high molecular weight hyaluronan (HMW-HA), a therapeutic agent used for OA. Monolayer cultures of HFLS cells were incubated with IL-1 beta and HMW-HA. In some experiments, cells were pretreated with the CD44 function-blocking monoclonal antibody or inhibitors of signaling pathways prior to addition of IL-1 beta and HMW-HA. The expressions of ADAMTS4 mRNA and protein were monitored using real-time RT-PCR, Western blotting, and immunofluorescence microscopy. To further determine the role of HMW-HA in IL-1 beta-induced ADAMTS4 expression, activation of p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), c-jun NH2-terminal kinase JNK), Akt, and NF-KB were analyzed by Western blotting. HMW-HA suppressed ADAMTS4 mRNA and protein expressions induced by IL-1 beta. Pretreatment with the anti-CD44 monoclonal antibody recovered the inhibitory effect of HMW-HA on expression of ADAMTS4 mRNA induced by IL-1 beta. Western blotting analysis revealed that 1-1 beta-induced phosphorylation of p38 MAPK and JNK protein were diminished by HMW-HA. Furthermore, inhibition of the p38 MAPK and JNK pathways by chemical inhibitors suppressed ADAMTS4 mRNA expression stimulated by IL-1 beta. These results suggest that HMW-HA plays an important role as a regulatory factor in synovial tissue inflammation. (C) 2013 Elsevier Inc. All rights reserved.