Osteopontin - An intrinsic inhibitor of inflammation in cartilage

Objective. To identify extracellular and intraarticular matrix components t hat are differentially expressed in normal and osteoarthritis (OA)-affected cartilage and to investigate their functions with respect to regulation of mediators of inflammation. Methods. Differential-display reverse transcriptase-polymerase chain reacti on (RT-PCR) analysis of a pool of messenger RNA (mRNA) from 10 human OA car tilage samples and 5 normal cartilage samples was performed using arbitrary primers. Confirmatory analysis of the up-regulated transcripts of fibronec tin (FN) and osteopontin (OPN) was performed by RT-PCR of individual RNA sa mples from a separate set of donors. The effect of recombinant OPN (or anti -OPN antiserum) on chondrocyte function was examined by analyzing the spont aneous or interleukin-1 (IL-1)-induced release of nitric oxide (NO) and pro staglandin E-2 (PGE(2)) from human OA-affected cartilage under ex vivo cond itions. Results. Up-regulation (300-700%) of FN and OPN mRNA was observed in human OA-affected cartilage as compared with normal cartilage. Functional analysi s of the role of OPN in OA cartilage showed that 1) Addition of 1 mu /ml (2 0 nM) of recombinant OPN to human OA-affected cartilage under ex vivo condi tions inhibited spontaneous and IL-1 beta -induced NO and PGE(2) production , and 2) neutralization of intraarticular OPN with anti-OPN antiserum augme nted NO production. Conclusion. The data indicate that one of the functions of intraarticular O PN, which is overexpressed in OA cartilage, is to act as an innate inhibito r of IL-1, NO, and PGE(2) production. These findings suggest that the produ ction of pleiotropic mediators of inflammation that influence cartilage hom eostasis, such as NO and PGE(2), is regulated by the interaction of chondro cytes with differentially expressed proteins within the extracellular matri x.