Effect of adrenaline and glucocorticoids on monocyte cAMP-specific phosphodiesterase (PDE4) in a monocytic cell line

Abstract

An increase in phosphodiesterase 4 (PDE4) in blood mononuclear white cells of patients with atopic dermatitis (AD) has been described. This and other skin disorders worsen during stress, during which there are increased circulating levels of adrenaline and glucocorticoids. The aim of this study was to analyse the effect of both these hormones on PDE4 in inflammatory cells. The human monocyte cell line U-937 was used as a model of blood mononuclear leucocytes. For this purpose, (1) the effect of adrenaline on PDE4 activity was determined, (2) the receptor mediating adrenaline actions was characterized, (3) the role of intracellular cAMP in PDE4 activation was investigated and (4) the effect of glucocorticoids on PDE4 activity was also ascertained. Adrenaline at a concentration of 1 µM produced a two- to threefold selective increase in PDE4 activity in U-937 cells after 4 h of incubation with the hormone. This stimulation was reversible, as well as concentration- and time-dependent. Cycloheximide (10 µM) induced a blockade of adrenaline-induced stimulation of PDE4. The stimulatory effect was inhibited by propranolol, but not by atenolol or phentolamine, in a concentration-dependent manner and was mimicked by salbutamol. The stimulation of PDE4 was paralleled by an increase in intracellular levels of cAMP. 8-Br-cAMP and forskolin also increased PDE4 activity. After 4 h of incubation in the presence of adrenaline, inhibition of cAMP degradation by rolipram further increased cAMP levels by about 300% and also enhanced PDE4 activity. These results suggest that adrenaline-induced stimulation of PDE4 is mediated by the β₂-adrenoceptor and is related to intracellular levels of cAMP, which might trigger expression and synthesis of the enzyme. The synthetic glucocorticoid dexamethasone (in the concentration range 10^–8 to 10^–6 M) showed no effect on PDE4 activity or on the cAMP accumulation induced by adrenaline, even after prolonged (24 h) incubation with the cells. Of the two stress hormones assayed, PDE4 activity was shown to be sensitive to adrenaline elevation but resistant to changes in glucocorticoid levels in the U-937 monocytic cell line.