ADAMs are a gene family of multifunctional proteins. We undertook this study to determine which ADAM species is up-regulated in osteoarthritic (OA) cartilage and to examine its pathobiologic function.
Expression of the 13 different metalloproteinase-type ADAMs was screened by reverse transcription-polymerase chain reaction (PCR), and expression levels of prototype membrane-anchored ADAM-12 (ADAM-12m) were determined by real-time PCR. ADAM-12m expression in articular cartilage was examined by in situ hybridization, immunohistochemistry, and immunoblotting. Chondrocytes were used for functional analyses of ADAM-12m.
ADAM-12m was selectively expressed in 87% of OA cartilage, and the expression level was significantly higher in OA cartilage than in normal cartilage. In situ hybridization showed that OA chondrocytes were responsible for the expression. ADAM-12m was immunolocalized on the membranes of OA chondrocytes, and its immunoreactivity correlated directly with the Mankin score and with degrees of chondrocyte cloning and proliferation. Immunoblotting analysis of OA chondrocytes demonstrated an active form of ADAM-12m. ADAM-12m expression in OA chondrocytes was selectively enhanced by transforming growth factor beta (TGFbeta), which also induced chondrocyte proliferation and degradation of insulin-like growth factor binding protein 5 (IGFBP-5). TGFbeta-induced chondrocyte proliferation was inhibited by suppression of IGF-1 signaling. In addition, TGFbeta-induced chondrocyte proliferation, chondrocyte cloning in agarose gel culture, and digestion of IGFBP-5 were inhibited with ADAM inhibitor, anti-ADAM-12 antibody, and small interfering RNA for ADAM-12.
These data suggest a novel function of ADAM-12m in chondrocyte proliferation and cloning in OA cartilage through enhanced bioavailability of IGF-1 from the IGF-1-IGFBP-5 complex by selective IGFBP-5 digestion.