Induction of release of secretory nonpancreatic phospholipase A2 from human articular chondrocytes.

Objective. Secretory nonpancreatic phospholipase A 2 (sPLA 2 ) is a known inducer/promoter of the inflammatory process in the joints. It correlates with disease activity in adult and juvenile rheumatoid arthritis. Synovial fluids contain high concentrations of sPLA 2 . We discovered that human articular cartilage contains large quantities of sPLA 2 and that cultured chondrocytes constitutively synthesize and release sPLA 2 . To test the mechanisms controlling the release of sPLA 2 , we exposed cultured human articular chondrocytes to cytokines and other agents, known to induce sPLA 2 in other cells. Methods. Chondrocytes obtained from cartilage of normal appearance from rheumatoid and osteoarthritic joints, and from normal, neonatal joints were compared to rabbit articular chondrocytes. Radiolabeled Escherichia coli derived phospholipid assay and ELISA technique using monoclonal antibodies against recombinant human synovial type sPLA 2 were employed. The cells were grown as monolayers as well as in alginate beads. Results. Human articular chondrocytes from both arthritic and neonatal joints released sPLA 2 constitutively but could not be further stimulated with interleukin-1β (IL-1β), tumor necrosis factor α (TNF-α), IL-6, oncostatin M. lipopolysaccharide (LPS), or forskolin. Marked stimulation was observed when the cells were exposed to 8-bromo cyclic adenosine monophosphate (cAMP). Growing the cells as monolayers or in alginate beads did not change the results. In contrast to human cells, rabbit chondrocytes responded to IL-1β and IL-1/TNF, but not to TNF-α alone, with a very marked increase in extracellular sPLA 2 activity. Conclusion. Human articular chondrocytes synthesize and constitutively release sPLA 2 . Such continuous release is most probably responsible for the high concentration of sPLA 2 in articular cartilage and may be the source of synovial fluid sPLA 2 . To our knowledge, human articular chondrocytes are the only sPLA 2 producing cells tested to date that do not respond to cytokine stimulation with increased sPLA 2 activity ; yet enhancement was seen with 8-bromo cAMP. It seems therefore that, human articular chondrocytes possess signalling mechanisms for the release of sPLA 2 unlike those from other mammalian cells. The significance of this observation remains to be elucidated.