The effects of 1,25-dihydroxyvitamin D3 and dexamethasone on rat osteoblast-like primary cell cultures: receptor occupancy and functional expression patterns for three different bioresponses.
The effects of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and dexamethasone to regulate collagen and osteocalcin synthesis and induction of 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) activity were studied in rat osteoblast-like cell primary cultures. In this culture system, the basal levels of collagen and osteocalcin synthesis increased with rising cell density in culture. At maximal doses, both 1,25-(OH)2D3 (8.1 nM) and dexamethasone (130 nM) reduced collagen synthesis to about 50% of the control levels, 1,25-(OH)2D3 affected osteocalcin synthesis in a biphasic manner: stimulatory at low doses, which peaked near 0.33 nM to reach 3- to 5-fold the basal level, followed by a gradual return to the basal level at higher concentrations. Dexamethasone had only a slight stimulatory effect on osteocalcin. 1,25-(OH)2D3 also induced 24-hydroxylase activity in rat osteoblast-like cells, while dexamethasone had no effect on the enzyme. Induction of enzyme activity achieved a 4- to 6-fold rise, but required higher concentrations of 1,25-(OH)2D3 to achieve maximal levels (16 nM). The half-maximal doses (ED50) of 1,25-(OH)2D3 required for each bioresponse were different. The approximate ED50 for the inhibition of collagen synthesis was near the Kin (0.4 nM; apparent dissociation constant of receptor nuclear internalization), while the ED50 for osteocalcin synthesis (0.08 nM) was below the Kin, and the ED50 for 24-hydroxylase induction (20 nM) was greater than the Kin. The ED50 for dexamethasone on collagen synthesis (20 nM) was about 5-fold higher than the Kin (4 nM) of dexamethasone receptor binding. The potencies of various vitamin D3 metabolites in all three functional responses followed their abilities to compete for the 1,25-(OH)2D3 receptor, indicating that these actions were 1,25-(OH)2D3 receptor mediated. In summary, these studies explored bone cell bioresponses to 1,25-(OH)2D3 and dexamethasone and examined the relationship between receptor occupancy and functional expression. Each action exhibited a different dose-response pattern, implying that different levels of control are required for each individual response.