Constitutive biosynthesis of bone Gla protein in a human osteosarcoma cell line

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, we show 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, we found that BGP is synthesized as a precursor with an apparent mol wt of 13.5K, as demonstrated on 15% sodium dodecyl sulfate-polyacrylamide gels. Finally, pulse labeling of the osteosarcoma cells with [3H]proline reveals that the cells synthesize mature BGP of 12,000 mol wt as well as a higher mol wt precursor (13,500) of the protein.