Biosynthesis of the vitamin K-dependent matrix Gla protein (MGP) in chondrocytes: a fetuin–MGP protein complex is assembled in vesicles shed from normal but not from osteoarthritic chondrocytes

SummaryObjectiveMineralization has been observed in osteoarthritic cartilage but the mechanisms are incompletely understood. Vitamin K is an essential cofactor in post-translational modification of proteins where specific Glu residues become modified to Ca++ binding γ-carboxyglutamic acid residues (Gla). One such protein, matrix Gla protein (MGP), is a known mineralization inhibitor. This study determined if synthesis of MGP and formation of a fetuin–MGP protein complex was altered in chondrocytes and vesicles from osteoarthritis (OA) cartilage.MethodsChondrocytes and vesicles were isolated from normal and OA human articular cartilage and lysates prepared. Specific antibodies were used in immunoblotting to detect the mature fully γ-carboxylated form of MGP (cMGP) and non-γ-carboxylated MGP (ucMGP) as well as fetuin and MGP–fetuin complexes. γ-carboxylase activity was measured by 14CO2 incorporation into the carboxylase peptide substrate FLEEL. Immunocytochemistry was used to examine fetuin in cartilage sections and uptake of biotin-labeled fetuin by isolated chondrocytes.ResultsChondrocytes and vesicles from osteoarthritic tissue produced significantly less cMGP compared to those from normal cartilage. This correlated with significantly less vitamin K-dependent γ-carboxylase enzyme activity in OA chondrocytes. Fetuin was found to be present in articular cartilage and cultured chondrocytes were capable of fetuin uptake. A fetuin–MGP complex was identified in normal chondrocytes and in vesicles shed from these cells but not in OA cells or vesicles.ConclusionsThe absence of cMGP and of the cMGP–fetuin complex in OA cells and OA vesicles may be an important mechanism for increased mineralization of osteoarthritic cartilage.