Alkaline phosphatase inhibition by parathyroid hormone and isoproterenol in a clonal rat osteosarcoma cell line. Possible mediation by cyclic AMP

The effect of parathyroid hormone (PTH 1-34 bovine) on alkaline phosphatase activity was investigated in an osteoblast-like clonal cell line derived from rat osteosarcoma (ROS 17/2). ROS 17/2 alkaline phosphatase resembled the bone enzyme in levamisole sensitivity and electrophoretic mobility but differed in heat stability. The specific activity of ROS 17/2 alkaline phosphatase increased with time in culture. This increase was inhibited by PTH (1-34) and (-)-isoproterenol in a dose-dependent manner starting at near-physiological hormone concentrations. The ID₅₀ values were 0.02 nM for PTH (1-34) and 1.7 nM for isoproterenol. The two hormones stimulated ROS 17/2 adenylate cyclase, albeit at higher concentrations: K_m values were 13 nM for PTH (1-34) and 16 nM for isoproterenol. The rise in alkaline phosphatase was also inhibited by dibutyryl cyclic AMP and 8-bromocyclic AMP (0.1 mM). These findings further document the osteoblastic properties of the ROS 17/2 osteosarcoma cell line, suggest that PTH inhibition of alkaline phosphatase represents a physiological response to the hormone in these cells, and implicate cyclic AMP as a mediator of this PTH effect.