Secretion of the vitamin K-dependent protein of bone by rat osteosarcoma cells. Evidence for an intracellular precursor.

Four clonal cell lines derived from a rat osteosarcoma were tested for the ability to secrete the gamma-carboxyglutamic acid-containing protein of bone (BGP) using a specific radioimmunoassay for this protein. Two cell lines secreted BGP into culture media while the other two did not. Other investigators have shown that these two cell lines are also the only ones with the high parathyroid hormone responsiveness and alkaline phosphatase activity expected for osteoblast cells in culture. Both cell lines also form a mineralized sarcoma when implanted in rats. The BGP in culture media is identical in molecular weight and in electrophoretic mobility with the 5800-dalton BGP purified from rat bone. Thus, BGP is probably secreted by osteosarcoma cells directly and not derived from an extracellular precursor by proteolytic cleavage. There are two immunoreactive components within osteosarcoma cells which secrete BGP. One component is identical in molecular weight and electrophoretic mobility with BGP from rat bone. The other component has a higher molecular mass (approximately 9000 daltons) and about half the electrophoretic mobility of BGP from bone. The presence of both components within these cells raises the possibility that the larger component may be an intracellular precursor which is processed to BGP prior to secretion.