1 Alpha,25-dihydroxyvitamin D3 Receptors in Cultured Rat Osteoblast-like Cells. Glucocorticoid Treatment Increases Receptor Content

The direct actions of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) in target cells are initiated by the binding of hormone to specific receptor sites. The aims of this study were to characterize the receptor for 1,25(OH)2D3 in cultured rat osteoblast-like (OB) cells and to determine whether these receptors are regulated by either the culture cycle itself or glucocorticoid treatment. The 1,25(OH)2D3 receptor in rat OB cells exhibited the same apparent binding affinity (Kd = 0.1 nM) and sedimentation coefficient (3.2 S) as mouse OB cells and receptors in other target organs. However, the receptor concentration in rat OB cells was substantially lower than mouse OB cells (approximately 20%). The concentration of receptors in rat OB cells did not show a correlation with the rate of DNA synthesis and therefore did not exhibit an endogenous rhythm in receptor concentration as was previously seen in mouse OB cells. Also in contrast to mouse cells, where glucocorticoids caused a decrease in receptor level, dexamethasone induced a marked increase in receptor binding throughout the culture cycle. This increase was due to an increase in receptor number with no change in receptor affinity. The change was glucocorticoid-specific, dose-dependent with half-maximal stimulation occurring between 1.3 and 13 nM and exhibited a latent period of at least 4 h. The independence of receptor concentration from DNA synthesis rate was established by assessing receptors after stimulating cell proliferation with epidermal growth factor and inhibiting it with hydroxyurea. Neither treatment altered basal 1,25(OH)2D3 receptor concentration or prevented the marked increase in receptor levels elicited by dexamethasone. We conclude that although the biochemical characteristics of 1,25(OH)2D3 receptors are indistinguishable in rat and mouse OB cells, there are genuine species differences in the regulation of the receptor number as it relates to DNA synthesis rate and response to glucocorticoids.