INHIBITION OF TERMINAL DIFFERENTIATION AND MATRIX CALCIFICATION IN CULTURED AVIAN GROWTH-PLATE CHONDROCYTES BY ROUS-SARCOMA VIRUS TRANSFORMATION

Endochondral bone formation involves the progression of epiphyseal gro wth plate chondrocytes through a sequence of developmental stages whic h include proliferation, differentiation, hypertrophy, and matrix calc ification. To study this highly coordinated process, we infected growt h plate chondrocytes with Rous sarcoma virus (RSV) and studied the eff ects of RSV transformation on cell proliferation, differentiation, mat rix synthesis, and mineralization. The RSV-transformed chondrocytes ex hibited a distinct bipolar, fibroblast-like morphology, while the mock -infected chondrocytes had a typical polygonal morphology. The RSV-tra nsformed chondrocytes actively synthesized extracellular matrix protei ns consisting mainly of type I collagen and fibronectin. RSV-transform ed cells produced much less type X collagen than was produced by mock- transformed cells. There also was a significant reduction of proteogly can levels secreted in both the cell-matrix layer and culture media fr om RSV-transformed chondrocytes. RSV-transformed chondrocytes expresse d two-to-threefold more matrix metalloproteinase, while expressing onl y one-half to one-third of the alkaline phosphatase activity of mock i nfected cells. Finally, RSV-transformed chondrocytes failed to calcify the extracellular matrix, while mock-transformed cells deposited high levels of calcium and phosphate into their extracellular matrix. Thes e results collectively indicate that RSV transformation disrupts the p reprogrammed differentiation pattern of growth plate chondrocytes and inhibit chondrocyte terminal differentiation and mineralization. They also suggest that the expression of extracellular matrix proteins, typ e II and type X collagens, and the cartilage proteoglycans are importa nt for chondrocyte terminal differentiation and matrix calcification. (C) 1998 Wiley-Liss, Inc.