Alterations in endogenous osteogenic protein-1 with degeneration of human articular cartilage

A synchronized balance between synthesis and breakdown of extracellular matrix (ECM) molecules in normal articular cartilage is disturbed in osteoarthritis (OA). The focus of our study is the anabolic factor, osteogenic protein-1 (OP-1) that is expressed in articular cartilage and is able to induce the synthesis of ECM components. The major aim was to investigate both qualitatively and quantitatively endogenous OP-1 in normal, degenerative, and OA cartilage. Normal and degenerative cartilage was obtained at autopsies from femoral condyles of human organ donors with no documented history of joint disease; OA cartilage was obtained from patients undergoing joint arthroplasty. Appearance of donor cartilage was evaluated by Collins scale, where normal cartilage is assigned grades 0-1, and degenerated cartilage is assigned grades 2-4. OP-1 mRNA expression was assessed by RT-PCR; OP-1 protein (pro- and active forms) was qualitatively analyzed by Western blotting and quantified by OP-1 ELISA. The highest levels of OP-1 expression (mRNA and protein) were detected in normal cartilage of grade 0. The concentration of OP-1 protein was about 50 ng per gram cartilage dry weight. With the progression of cartilage degeneration (increased Collins grades and OA) OP-1 protein was down-regulated up to 9-fold. These changes affected primarily the active form of OP-1. OP-1 message also declined in cartilages with the increase of degenerative changes. In conclusion, an overall decrease in endogenous OP-1 in degenerated and OA tissue suggests that OP-1 could be one of the factors responsible for normal homeostasis and matrix integrity in cartilage.