RB Penn, SG Kelsen and JL Benovic
Department of Pharmacology, Jefferson Cancer Institute, Thomas Jefferson University, Philadelphia, PA 19107.

Despite the importance of beta 2-adrenergic receptor (beta 2AR) stimulation in mediating airway epithelial cell function, little is known regarding its regulation in airway epithelium. Perturbations of the airway environment associated with disease states, including the management of bronchomotor tone with beta-agonists, expose airways to putative regulators of beta 2AR signal transduction. In this communication, we describe the desensitization of beta 2AR signal transduction in the human airway epithelial cell line BEAS-2B. Examination of both beta-agonist- and prostaglandin E2 (PGE2)-mediated cAMP generation in BEAS-2B cells reveals both agonist-specific (homologous) and non-agonist-specific (heterologous) desensitization of these G protein-coupled receptor pathways. Short-term homologous desensitization of beta 2AR-mediated cAMP generation was characterized by an approximately 60% loss of maximal responsiveness to isoproterenol (ISO) when cells were pretreated 30 min with 10 microM ISO. A reduced sensitivity to ISO was also evidenced by an approximately 4-fold increase in the EC50 for ISO stimulation of adenylyl cyclase (AC). Short-term heterologous desensitization was characterized by an increase in EC50 (approximately 2- to 3-fold) with no change in maximal responsiveness to ISO in cells pretreated with either forskolin or PGE2. Qualitatively similar findings characterized short-term homologous and heterologous desensitization of PGE2-mediated AC activity. Short-term agonist-specific desensitization of the beta 2AR was associated with, but not dependent upon, rapid beta 2AR sequestration. Long-term pretreatment of cells with 10 nM ISO and 1 microM PGE2 eliminated AC responsiveness to subsequent ISO and PGE2 stimulation, respectively. Exposure of BEAS-2B cells to ISO for 24 h resulted in an approximately 70% loss of beta 2ARs, whereas chronic forskolin or PGE2 pretreatment had no effect on beta 2AR number. Long- term pretreatment of cells designed to elicit heterologous desensitization was associated with reductions in maximal responsiveness to ISO and PGE2 that appear to be related to a loss in inherent AC activity. These findings hold strong implications regarding the effect of beta 2AR desensitization on epithelial cell function and the role of beta-agonists in the management of airway disease.

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