Glucocorticoid receptors and actions in subpopulations of cultured rat bone cells. Mechanism of dexamethasone potentiation of parathyroid hormone-stimulated cyclic AMP production

Abstract

We have previously shown that bone cells possess glucocorticoid receptors and that, in addition to being inhibitory to cell growth, glucocorticoid treatment potentiates the ability of parathyroid hormone (PTH) to stimulate cyclic AMP (cAMP) formation. This study extends those observations to specific subpopulations of bone cells and explores the mechanism of the cAMP augmentation. Subpopulations of cultured bone cells derived from 20-d-old fetal rat calvaria were enriched for "osteoblast-like" (OB) and "osteoclast-like" (OC) cells by sequential collagenase digestion. OC cells released during the first 30 min of collagenase digestion were characterized by low alkaline phosphatase activity, a cAMP response to salmon calcitonin (CT), but only a small cAMP response to bovine PTH. In contrast, OB cells released between 30 and 120 min of collagenase digestion, possessed high alkaline phosphatase activity, responded with a large cAMP rise to PTH, but exhibited no response to CT. Glucocorticoid receptors, with similar properties, were demonstrated in both populations (K(d) congruent with 5 nM, N(maximum) congruent with 400 fmol/mg cytosol protein). Dexamethasone equivalently inhibited cell growth and alkaline phosphatase activity in both populations. Dexamethasone potentiation of cAMP generation occurred after PTH but not CT stimulation. A greater enhancement of cAMP generation observed in OB cells appears to result from two glucocorticoid actions: (a) stimulation of adenylate cyclase and (b) inhibition of phosphodiesterase. Only the latter mechanism was found in OC cells. Dexamethasone-treated cells showed an increase in both sensitivity and maximal response of cAMP to PTH. The possible relationship of these actions to the mechanism of glucocorticoid-induced osteopenia is discussed.