Dexamethasone Increases 1,25-Dihydroxyvitamin D3Receptor Levels and Augments Bioresponses in Rat Osteoblast-Like Cells*

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Abstract
Glucocorticoids increase the level of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors in primary cultures of rat calvarial osteoblast-like (OB) cells. The present study investigated how this dexamethasone (DEX) up-regulation of 1,25-(OH)2D3 receptors modulates three 1,25-(OH)2D3 bioresponses: inhibition of collagen synthesis, stimulation of osteocalcin synthesis, and induction of 25-hydroxyvitamin D3-24-hydroxylase activity. Pretreatment of OB cells with 13 nM DEX for 24 h doubled the 1,25-(OH)2D3 receptor level without changing receptor affinity for 1,25-(OH)2D3. Cultures were then treated with varying concentrations of 1,25-(OH)2D3 to study bioresponses. After DEX treatment to increase the 1,25-(OH)2D3 receptor level, the magnitude and sensitivity of all three 1,25-(OH)2D3 bioresponses were enhanced. The maximal 1,25-(OH)2D3 inhibition of collagen synthesis was increased by DEX pretreatment compound to control values: 30 to 50% (1 day treatment) and 50 to 70% (2 day treatment). The sensitivity to 1,25-(OH)2D3, as measured by reduction of the half-maximal inhibitory dose (ED50), was increased 50%. This potentiation of 1,25-(OH)2D3 inhibitory action on collagen synthesis was still evident after correction for the inhibitory effect on collagen synthesis by DEX alone. The maximal stimulation of osteocalcin by 1,25-(OH)2D3 was also enhanced from 2- to 3-fold in controls to over 4- to 5-fold by DEX pretreatment. Similarly, the ED50 of the response was reduced 50%. For the induction of 25-hydroxyvitamin D3-24-hydroxylase activity, DEX doubled the enzyme activity over that seen with 1,25-(OH)2D3 alone, but only slightly affected the sensitivity of the enzyme induction. In conclusion, after DEX up-regulation of 1,25-(OH)2D3 receptor levels, there was a general potentiation of 1,25-(OH)2D3 bioresponses in rat OB cells. However, the detailed patterns of the augmented responses were different for each of the three biological functions we studied.