In vitro proteoglycan synthesis in response to extracts of demineralized bone

Two extracts of bovine bone, bone morphogenetic protein (BMP) supplied by the UCLA Bone Research Laboratory, and osteogenic factor extract (OFE) supplied by the industrial group Celtrix Pharmaceuticals, were tested for the ability to transform embryonic skeletal muscle into cartilage. Skeletal muscle was placed into organ cultures on substrata of Type I collagen and fed with concentrations of the extracts that their originators reported to be effective; however, only BMP was capable of eliciting the morphologic differentiation of cartilage. In contrast, both extracts supported patterns of glycosaminoglycan synthesis that mimicked the biochemical differentiation of cartilage-type extracellular matrix. Bone morphogenetic protein differed from OFE in its ability to elicit high levels of hyaluronic acid synthesis, although BMP and OFE upregulated synthesis of hyaluronic acid that was of sufficient chain length to support proteoglycan aggregate formation. Proteoglycan extracts of the cell layer and medium demonstrated that most of the proteoglycan synthesized in response to BMP was an aggrecan-like material, which was lost to the medium. That which synthesized in response to OFE was a proteoglycan with short glycosaminoglycan chains that had only a limited ability to aggregate. These results demonstrate that BMP is effective in promoting chondrogenesis by virtue of its ability to promote the synthesis of hyaluronic acid, and aggrecan, but suggests that other accessory matrix components must also be synthesized to anchor aggrecan in the cell layer. The ability to stimulate the synthesis of these other components may be lost on purification of BMP. Consequently, BMP may initiate several activities that collectively upregulate chondrogenesis and the production of cartilage extracellular matrix.