OSTEOCALCIN SECRETION BY THE HUMAN OSTEOSARCOMA CELL-LINE MG-63

The human osteosarcoma cell line MG‐63 has been used to study the production of the bone‐specific protein, osteocalcin. In the absence of any stimuli, MG‐63 cells secreted very low levels of osteocalcin. The secretion of osteocalcin started after a lag time of 10‐12 h upon 1,25‐(OH)2D3 treatment. Osteocalcin secretion was measured at doses as low as 0.03 nM (fourfold increase, p < 0.05), and this activity increased further with higher doses of 1,25‐(OH)2D3 to reach a plateau at 50 nM. The secretion increased transiently from very low levels in sparse cell cultures to peak values in subconfluent cultures (+ 40%), two‐ to threefold above values obtained for confluent cells. Values for confluent cells average 55.9 + 2.0 ng/ml protein per 48 h. A similar behavior is observed for 1,25‐(OH)2D3 receptor concentration under similar experimental conditions. Bmax increased transiently from sparse to subconfluent cell cultures (40 ‐60% confluent) and reached values 50% lower in confluent cells. However, the receptor affinity was not affected by cell density. MG‐63 cells also possessed an alkaline phosphatase isoenzyme of the bone‐liver‐kidney type that was stimulated by 1,25‐(OH)2D3 treatment (two‐ to threefold) and inhibited by parathyroid hormone (40 nM, ‐25%, p < 0.025). PTH and PGE2 increased cAMP production in a dose‐dependent manner, but the cells were irresponsive to salmon calcitonin. Basal and PTH‐responsive cyclic AMP production were also modulated by cell density. Dexamethasone pretreatment (100 nM, 48 h) stimulated the PTH‐dependent cAMP production but failed to influence the response to PGE2. Vitamin D3‐induced osteocalcin secretion was inhibited by 40 nM PTH (‐20%, p < 0.01) and 5 nM PGE2 (‐36%, p < 0.005), a situation that could be related to the ability of these hormones to stimulate cAMP in these cells. These results show that the MG‐63 cell line is a good human osteoblastlike cell model in which bone‐specific protein synthesis (osteocalcin and alkaline phosphatase) is modulated in response to 1,25‐(OH)2D3 and PTH. Copyright © 1990 ASBMR