Regulation of YKL-40 production by human articular chondrocytes

Objective. YKL-40 (human cartilage glycoprotein 39) is one of the most abun dant proteins secreted by cultured chondrocytes. The objectives of the pres ent study were to identify regulators of YKL-40 production in cartilage and chondrocytes and to map the localization of YKL-40 in chondrocytes. Methods. Human articular chondrocytes and cartilage explants (obtained from subjects at autopsy, from a tissue bank, and from osteoarthritis [OA] pati ents undergoing total joint replacement surgery) were stimulated with cytok ines, growth factors, and other agents. YKL-40 expression was analyzed by N orthern blot and polymerase chain reaction. YKL-40 secretion into the media was determined by enzyme-linked immunosorbent assay. Results. YKL-40 production increased to very high levels during the early p hase of chondrocyte monolayer culture and in normal cartilage explant cultu res as a response to tissue injury. Spontaneous YKL-40 release was higher i n OA than in normal cartilage explant cultures. In chondrocyte monolayer cu ltures, interleukin-1 beta (IL-1 beta) and transforming growth factor beta (TGF beta) decreased the levels of secreted YKL-40, and this was associated with a reduction in YKL-40 messenger RNA levels. IL-1 beta, but not TGF be ta, reduced YKL-40 production in cartilage explant cultures. Media from exp lants treated with cycloheximide had no detectable YKL-40, suggesting that the released protein was newly synthesized. Immunofluorescence microscopy s howed YKL-40 staining in the Golgi system of the chondrocytes, but YKL-40 c ould not be detected in the extracellular matrix. Conclusion. The spontaneous increase in the production of YKL-40 in the ear ly phase of culture appears to represent a cellular response to changes in the extracellular matrix environment. This, coupled with the profound suppr essive effects of IL-1 beta and TGF beta on YKL-40 production, identifies a novel regulatory pattern for this major chondrocyte-derived protein.