Sensitization by dexamethasone of lymphocyte cyclic AMP formation: evidence for increased function of the adenylyl cyclase catalyst

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Abstract
Glucocorticoids and elevations of intracellular adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) may affect lymphocyte activation, proliferation and effector functions in similar ways. Therefore, we have investigated the effects of the glucorticoid, dexamethasone, on human lymphocyte cyclic AMP formation. Treatment of resting human lymphocytes with the glucocorticoid, dexamethasone, sensitized prostaglandin E2‐stimulated cyclic AMP accumulation in a time‐ and concentration‐dependent manner. In membranes of lymphocytes treated for 24 h with 100 nm dexamethasone, maximal adenylyl cyclase activity stimulated by prostaglandin E2, isoprenaline, guanosine 5′‐triphosphate (GTP), forskolin and MnCl2 was significantly enhanced; the EC50 for these agents was not significantly altered. β2‐Adrenoceptor density, immunodetectable α‐subunits of the G‐proteins Gs and Gi, and pertussis toxin‐substrates were not significantly altered by dexamethasone treatment. In dexamethasone‐treated lymphocytes, prostaglandin E2‐mediated inhibition of concanavalin A‐induced Ca2+ elevations was doubled compared to control cells. Based on these data and the observation that enhancement of forskolin‐ and MnCl2‐stimulated adenylyl cyclase activity could quantitatively account for the enhancement of prostaglandin E2‐, isoprenaline‐ or GTP‐stimulated adenylyl cyclase activity, we conclude that dexamethasone treatment sensitizes cyclic AMP formation in resting human lymphocytes by altering the adenylyl cyclase catalyst rather than G‐proteins or hormone receptors. This results in an enhanced capability of cyclic AMP generating agonists to inhibit early steps of lymphocyte activation.