Epidermal growth factor upregulates β-adrenergic receptor signaling in a human salivary cell line

Abstract

The effects of epidermal growth factor (EGF) on the β-adrenergic receptor-coupled adenylyl cyclase system were studied in a human salivary cell line (HSY). The β-adrenergic agonist isoproterenol (10−5 M) stimulated adenylyl cyclase activity by ∼2-fold, and the isoproterenol response was increased 1.8-fold after prolonged (48 h) exposure to EGF (5 × 10−10 M). In contrast, enzyme activation via stimulatory prostaglandin receptors and by agents acting on nonreceptor components of the adenylyl cyclase system was not enhanced by EGF. β-Adrenergic receptor density, assessed by binding of the β-adrenergic receptor antagonist (−)-[125I]iodopindolol, was increased threefold after EGF treatment. Competition binding studies with unlabeled antagonists selective for β1- and β2-adrenergic receptor subtypes indicated that the increase in (−)-[125I]iodopindolol binding sites induced by EGF reflected an increased number of β2-adrenergic receptors. Likewise, Northern blot analysis of RNA from EGF-treated cells revealed selective induction of β2-adrenergic receptor mRNA, which was blocked by the RNA synthesis inhibitor actinomycin D. The increase in β-adrenergic receptor density produced by EGF was unaltered after phorbol ester-induced downregulation of protein kinase C (PKC). Enhancement of isoproterenol-responsive adenylyl cyclase activity and phosphorylation of mitogen-activated protein kinase (MAPK) by EGF were both blocked by the MAPK pathway inhibitor PD-98059. The results suggest that in HSY cells EGF enhances β-adrenergic responsiveness by upregulating β2-adrenergic receptor expression at the transcriptional level. Moreover, the stimulatory effect of EGF on β2-adrenergic receptor signaling appears to be mediated by the MAPK pathway and independent of PKC activation.