Constitutive Biosynthesis of Bone Gla Protein in a Human Osteosarcoma Cell Line*

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Abstract
A human osteosarcoma cell line derived from cells obtained from a patient with Paget''s disease is shown to synthesize and secrete bone Gla protein (BGP); (osteocalcin), a noncollagenous bone matrix protein. Using a human BGP-specific RIA [radioimmunoassay] it was shown that the human osteosarcoma cells synthesize significant amounts of BGP without any prior induction of BGP synthesis by 1,25-dihydroxyvitamin D. After specific immunoprecipitation of poly-A+ RNA in vitro translation products with antibodies to BGP, it was found that BGP is synthesized as a precursor with an apparent MW of 13.5 K, as demonstrated on 15% sodium dodecyl sulfate-polyacrylamide gels. Pulse labeling of the osteosarcoma cells with [3H]proline reveals that the cells synthesize mature BGP of 12,000 MW as well as a higher MW precursor (13,500) of the protein.