Inhibition of ABCC1 Decreases cAMP Egress and Promotes Human Airway Smooth Muscle Cell Relaxation

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 G alpha(s) (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 beta(2)AR (beta(2)-adrenoceptor), a G(s)-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 beta(2)AR-evoked cAMP release from HASM cells. Furthermore, inhibition of ABCC1 activity or expression decreases basal tone and increases beta-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 G(s)-GPCRs (G(s)-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.