DEXAMETHASONE-TREATED ROS-17/2.8 RAT OSTEOSARCOMA CELLS ARE RESPONSIVE TO HUMAN CARBOXYLTERMINAL PARATHYROID-HORMONE PEPTIDE HPTH (53-84) - STIMULATION OF ALKALINE-PHOSPHATASE

We tested the effects of various parathyroid hormone (PTH) peptides on alkaline phosphatase (ALP) activity in the osteoblastic cell line ROS 17/2.8. In dexamethasone-treated ROS 17/2.8 cells there was a dose-related increase in ALP activity due to treatment with hPTH (53-84). ALP activity was stimulated by 10 nM hPTH (53-84) by a mean of 1.51 +/- 0.07-fold (P < 0.001) in nine experiments, whereas the same dose of bPTH (1-34) and bPTH (1-84) inhibited enzyme activity to 0.36 +/- 0.02-fold (P < 0.001) and 0.37 +/- 0.03-fold (P < 0.001), respectively. Significant stimulation of ALP activity occurred with doses of hPTH (53-84) as low as 0.01 nM. There was no stimulation of enzyme activity by hPTH (53-84) in the absence of dexamethasone; the maximum ALP response to hPTH (53-84) occurred between 96 and 144 hours, and no significant effect was seen at time periods less than 96 hours. The optimum dose of dexamethasone required to enable the response to hPTH (53-84) was 10 nM. Carboxylterminal PTH fragments had a specific stimulatory effect on ALP activity in dexamethasone-treated ROS 17/2.8 cells, but the aminoterminal PTH effect appeared to be dominant, as the equimolar combination of bPTH (1-34) and hPTH (53-84) resulted in inhibition of ALP activity. Thus, in order for the effects of carboxylterminal fragments to be manifest, the cells would have to be stimulated under conditions in which the aminoterminal receptor is unoccupied; this could occur under some in vivo conditions. The physiological significance of these observations is unknown, but the enabling effect of dexamethasone on hPTH (53-84) stimulation of ALP suggests that osteoblastic cells are responsive to this hormonal peptide at a certain stage of differentiation.