Cyclic nucleotide phosphodiesterase in human bronchial epithelial cells: characterization of isoenzymes and functional effects of PDE inhibitors

Cyclic AMP (adenosine 3':5'-cyclic monophosphate, cAMP) is an intracellular second messenger that mediates the actions of endogenous hormones and neurotransmitters and also of drugs such as beta-adrenoceptor agonists. The presence of functional beta-adrenoceptors on human airway epithelial cells has been demonstrated but the expression of the cAMP-metabolizing enzyme, cyclic nucleotide phosphodiesterase (PDE) in these cells has not been studied. We investigated the profile of activity of the different PDE isoenzymes in lysates of a pulmonary epithelial cell line, A549, and of human bronchial epithelial (HBE) cells grown in primary culture. The effects of non-selective and isoenzyme-selective PDE inhibitors on beta-agonist-induced elevations in intracellular cAMP concentrations and the production of interleukin (IL) 8 and prostaglandin (PG) E2 was also investigated. A549 cells expressed a high level of PDE4, lower levels of PDE1 and PDE3, and minor PDE5 activity. Primary HBE cultures expressed PDE4 and PDE1 activity at approximately equal levels with small additional PDE3 and PDE5 activities. The total PDE activity of the HBE cells was approximately nine-fold lower than that of A549 cells. The beta-adrenoceptor agonist salbutamol, caused a slow, concentration-dependent increase in intracellular cAMP levels in HBE cells which was not affected by a non-selective PDE inhibitor, IBMX (100 microM), or by a selective PDE4 inhibitor, rolipram (100 microM). Zardaverine, a dual-selective PDE3/PDE4 inhibitor, had no effect on cAMP levels at 10 microM but did cause a significant enhancement of salbutamol-induced elevations at 100 microM (150+/-36 pmol/10(5) cells at 10 microM salbutamol vs. 64+/-25 pmol/10(5) cells in the absence of zardaverine; n=3,P<0.01). Neither basal nor tumour necrosis factor alpha (10 ng/ml)-induced IL8 secretion was affected by salbutamol (10 microM) in the absence or presence of IBMX (100 microM). Salbutamol (10 microM), alone or in the presence of IBMX (100 microM) or rolipram (100 microM), also failed to affect basal or bradykinin (1 microM)-induced PGE2 release. Zardaverine (100 microM) caused a significant increase in basal PGE2 release but this was not enhanced in the presence of salbutamol (10 microM) and was not related to changes in cAMP levels. We conclude that HBE cells express a low total PDE activity, made up predominantly of PDE1 and PDE4 isoenzymes, and that intracellular cAMP levels in HBE cells are not related to the production of IL8 or PGE2.