1,25-Dihydroxycholecalciferol and glucocorticosteroid regulation of adenylate cyclase in an osteoblast-like cell line.

To study regulation of the parathyroid hormone (PTH)-responsive adenylate cyclase of osteoblast-like cells by 1,25-dihydroxyvitamin D (1,25(OH)2D), cAMP levels and adenylate cyclase activity were assayed in the hormone-responsive ROS 17/2.8 rat osteosarcoma cell line. Treatment of cells with 1,25(OH)2D3: alone markedly attenuated the cAMP response to subsequent PTH; decreased adenylate cyclase stimulated by PTH; and completely antagonized the positive regulatory effects of cell treatment with glucocorticosteroid (GC) on these responses to PTH. Sterol receptor mediation was indicated by specificity for the 1,25(OH)2D metabolite and high sensitivity (half-maximal attenuation at 7 X 10(-11) M). The effects of 1,25(OH)2D and GC were primarily on the maximal activity of adenylate cyclase and not on sensitivity to Mg2+, guanine nucleotide, or PTH. GC augmentation of ROS 17/2.8 cell cAMP accumulation was also seen with another receptor agonist (beta-adrenergic), cholera toxin or forskolin; 1,25(OH)2D antagonized all these GC effects. Opposing effects of GC and 1,25(OH)2D were seen as well on activation of the guanine nucleotide-binding regulatory protein (Ns) by guanyl-5'-yl imidodiphosphate and F- and on activation of the catalyst (C) by Mn2+. In contrast, with the activators other than PTH, cell treatment with 1,25(OH)2D in the absence of GC produced only minor attenuation of cAMP accumulation and no effect on adenylate cyclase activities. The data suggest that GC acts strongly on or near the PTH receptor-Ns complex in ROS 17/2.8 and to a lesser degree on the Ns-C interaction. Direct GC enhancement of C could not be concluded because of the influence of Ns on forskolin action and present data that Mn2+ does not uncouple Ns from C in this system. A GC effect on membrane structure or composition, as seen in other cell types, could explain these changes in adenylate cyclase function without the need to postulate multiple mechanisms. The data dissociate two 1,25(OH)2D effects, direct attenuation of activation of Ns via the PTH receptor and interference with the as yet undefined mechanism(s) of GC augmentation. These may represent dissimilar pathways of 1,25(OH)2D action on osteoblasts.