Inhibition Of Human Airway Smooth Muscle Cell Proliferation By Beta-2 Adrenergic Receptor Activation And Cyclic Amp-Elevating Agents: Evidence For Epac Involvement

Mechanisms by which beta-adrenergic receptor (βAR) agonists inhibit proliferation of human airway smooth muscle (HASM) cells were investigated in relation to smooth muscle hyperplasia in asthma. Cells were treated for 24 hrs and then proliferation was assessed by [3H]thymidine uptake. Epidermal growth factor (EGF) stimulated proliferation by about 10-fold. The βAR agonist isoproterenol (Iso) and the β2AR-selective agonists albuterol (Alb) and salmeterol (Sal) inhibited EGF stimulation by more than 50%, with EC50 values of 7, 100, and 21 nM, respectively. Sal was a weak partial agonist for cAMP elevation but fully effective for inhibiting proliferation. ICI 118551 (β2AR antagonist) inhibited the Iso effect with 100x greater potency than ICI 89406 (β1AR antagonist). The cAMP-elevating agents PGE2, forskolin (Fsk), db-cAMP, and 8-Br-cAMP all decreased EGF-induced proliferation. Inhibition by db-cAMP and 8Br-cAMP occurred only at 1 mM, whereas the EPAC-selective analog 8CPT-2Me-cAMP inhibited with an EC50 of 1.5 μM. Iso, Fsk, db-cAMP, and 8CPT-2Me-cAMP also inhibited proliferation stimulated by lysophosphatidic acid (LPA) and LPA synergism with EGF. These data suggest that β2AR activation decreases HASM proliferation by cAMP regulation of EPAC. Supported by an AHA predoctoral fellowship to KMK and by GlaxoSmithKline.