In most living cells, the second-messenger roles for adenosine 3',5'-cyclic monophosphate (cAMP) are short-lived, confined to the intracellular space, and tightly controlled by the binary switch-like actions of Gαs (stimulatory G protein)-activated adenylyl cyclase (cAMP production) and cAMP-specific PDE (cAMP breakdown). Here, by using human airway smooth muscle (HASM) cells in culture as a model, we report that activation of the cell-surface β2AR (β2-adrenoceptor), a Gs-coupled GPCR (G protein-coupled receptor), evokes cAMP egress to the extracellular space. Increased extracellular cAMP levels ([cAMP]e) are long-lived in culture and are induced by receptor-dependent and receptor-independent mechanisms in such a way as to define a universal response class of increased intracellular cAMP levels ([cAMP]i). We find that HASM cells express multiple ATP-binding cassette (ABC) membrane transporters, with ABCC1 (ABC subfamily member C 1) being the most highly enriched transcript mapped to MRPs (multidrug resistance-associated proteins). We show that pharmacological inhibition or downregulation of ABCC1 with siRNA markedly reduces β2AR-evoked cAMP release from HASM cells. Furthermore, inhibition of ABCC1 activity or expression decreases basal tone and increases β-agonist-induced HASM cellular relaxation. These findings identify a previously unrecognized role for ABCC1 in the homeostatic regulation of [cAMP]i in HASM that may be conserved traits of the Gs-GPCRs (Gs-coupled family of GPCRs). Hence, the general features of this activation mechanism may uncover new disease-modifying targets in the treatment of airflow obstruction in asthma. Surprisingly, we find that serum cAMP levels are elevated in a small cohort of patients with asthma as compared with control subjects, which warrants further investigation.
Keywords: airway smooth muscle; asthma; cAMP; multidrug resistance–associated proteins; β2-adrenoceptor.