Mononuclear cells are not involved in BGP synthesis and secretion

Osteocalcin or bone GLA protein (BGP) is found at high levels in only two tissues, the extracellular matrix of bone and dentine. Tissue culture experiments have demonstrated that BGP is synthesized by two osteoblastic osteosarcoma cell lines (ROS 2/3 and 17/2) and by normal osteoblastic cells in primary culture. BGP was not found in rat cartilage nor in liver, kidney, lung, spleen, brain, heart, thymus, skeletal muscle. In this study secretion of BGP was assayed by RIA in the supernatants of 48-hour cultures of peripheral blood lymphocytes or monocytes. Lymphocyte cultures were carried out using RPMI-1640 supplemented with L-glutamine and antibiotics at the concentration of 1 x 10(6) cells/ml and activated by PHA (10 ng/ml). Peripheral blood monocytes were purified by adherence to plastic Petri dishes and treated with cold PBS supplemented with EDTA. Monocytes were cultured as previously described and stimulated with LPS (50 micrograms/ml). Cell-free supernatants were obtained by centrifugation and stored at -20 degrees C, until the BGP assay was performed. The authors did not observe secretion of detectable amounts of BGP in the supernatants of short-term lymphocyte or monocyte cultures. These data indicate that circulating mononuclear cells are not involved in BGP synthesis and secretion.