RECOMBINANT HUMAN BONE MORPHOGENETIC PROTEIN-2 MAINTAINS THE ARTICULAR CHONDROCYTE PHENOTYPE IN LONG-TERM CULTURE

Bone morphogenetic proteins have been shown to increase matrix synthes is by articular ghondrocytes in short-term cultures. Members of this f amily of proteins have also been shown to induce endochondral ossifica tion in vivo. The present study was performed to determine if the addi tion of human recombinant bone morphogenetic protein-2 to a long-term monolayer articular chondrocyte cell culture system affected the abili ty of the chondrocytes to divide in vitro, whether the cytokine altere d expression of the articular chondrocyte phenotype and synthesis of m atrix proteoglycans, and whether the cytokine was capable of inducing differentiation to a hypertrophic chondrocyte. Human recombinant bone morphogenetic protein-2 did not alter cell proliferation. It caused 3. 5-6.2 times more proteoglycan synthesis by articular chondrocytes duri ng each of the time points tested after 4 days in culture. Total prote oglycan accumulation in the extracellular matrix after 28 days in cult ure was 6.7 times as great in the treated cultures as in the control. Treatment with human recombinant bone morphogenetic protein-2 maintain ed the articular chondrocyte phenotype of cells in culture as demonstr ated by Northern blot analysis: the expression of type-I collagen gene s was increased and that of type-II collagen and aggrecan mRNA was los t in untreated chondrocyte cultures after 14-21 days in culture. In co ntrast, exposure to 100 ng/ml human recombinant bone morphogenetic pro tein-2 maintained expression of type-II collagen and increased express ion of aggrecan compared with controls during the 28-day culture perio d. Northern blot analysis of the expression of type-X collagen and ost eocalcin by chondrocytes treated with human recombinant bone morphogen etic protein-2 showed a lack of expression of these genes, indicating no alteration in phenotype. These experiments demonstrated the ability of human recombinant bone morphogenetic protein-2 to promote the arti cular chondrocyte phenotype and matrix synthesis in long-term culture. Characteristics of cell growth were not affected, and the cytokine di d not induce differentiation to a hypertrophic chondrocyte.