A New Function of The Lubricin and It's Role in the Pathogenesis of Arthritis.

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We previously found that the lubricin mRNA deleting the exon 6 (sMSF) is present in synovial fluids from rheumatoid arthritis (RA). To detect the sMSF protein, rabbit anti-human lubricin polyclonal antibody was produced immunizing the purified lubricin, and that reactivity with lubricin and sMSF recombinant protein were confirmed. In the detection of sMSF using that antibody and western blotting, a lot of the sMSF protein was present in synovial fluids from RA patients in comparison with that from OA patients.The presence of the complex of lubricin and hyaluronic acid (HA) has not been clarified in vivo. Therefore, we tried to examine that complex in synovial fluids from RA and OA patients. The molecular weight (M.W.) of lubricin was ranged from 300 kDa to 5,000 kDa in RA and OA synovial fluids in the fractionation using Sepharose-S 500. There was much of the lower M.W. lubricin in synovial fluids from RA patients than OA patients. To determine whether the high M.W. lubricin is the complex with HA, HA was trapped with HA binding protein (HABP) that is specifically bound to HA, and then, lubricin in the HABP binding proteins was detected using anti-lubricin antibody and western blotting. The results showed that lubricin in RA and OA synovial fluids was present in the complex with HA. In addition, we found that lubricin is bound to joint cartilage via HA.Recently, the high M.W. (3,000 kDa) or low M.W. (1,000 kDa) HA are available as a drug for treatment of arthritis. Thus, it is an interesting subject to know how these HA are contributed to the boundary lubrication by forming the complex with lubricin.