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.

Automated summary

In most cells, cAMP acts as a short-lived second messenger, confined to the intracellular space and controlled by G protein actions. However, in human airway smooth muscle cells, activation of the cell-surface receptor β(2)AR leads to the release of cAMP into the extracellular space. This release is mediated by the ABC transporter ABCC1 and may have implications for the treatment of airflow obstruction in asthma.