Figure 2
From: Signaling and regulation of G protein-coupled receptors in airway smooth muscle
Gs-coupled receptor signaling in airway smooth muscle. Gs-coupled receptors on airway smooth muscle (ASM) are activated by endogenous agents such as circulating catecholamines, prostaglandins and iso-prostanes, adenosine and vasoactive intestinal peptide (VIP). Activated Gαs binds to and activates membrane bound adenylyl cyclase (AC). AC is comprised of eight membrane-spanning α-helices, and two cytosolic domains which are required for catalytic activity and integrate various regulatory signals. The cytosolic domains possess specific binding sites for the G-protein subunits Gαs, Gαi, and Gβγ. Of the nine know AC isoforms, AC V and VI appear be expressed and functionally important in human ASM. Adenylyl cyclase activation catalyzes the formation of cyclic AMP from cytoplasmic ATP. Cyclic AMP is a ubiquitous second messenger whose principal function is to activate protein kinase A (PKA). Inactive PKA exists as a complex comprising two regulatory and two catalytic subunits. The high affinity binding of cyclic AMP to domains in the regulatory region induces a conformational change forcing the release of the active catalytic subunits. PKA-mediated phosphorylation of various intracellular proteins has widespread effects in ASM. PKA can phosphorylate certain Gq-coupled receptors as well as phospholipase C (PLC) and thereby inhibit G protein-coupled receptor (GPCR) -PLC-mediated phosphoinositide (PI) generation, and thus calcium flux. PKA phosphorylates the inositol 1,4,5-trisphosphate (IP3) receptor to reduce its affinity for IP3 and further limit calcium mobilization. PKA phosphorylates myosin light chain kinase (MLCK) and decreases its affinity to calcium calmodulin, thus reducing activity and myosin light chain (MLC) phosphorylation. PKA also phosphorylates KCa++ channels in ASM, increasing their open-state probability (and therefore K+ efflux) and promoting hyperpolarization. Through its phosphorylation of the transcription factor CREB and its (typically inhibitory) effects on GPCR and receptor tyrosine kinase signaling, PKA regulates the transcription of numerous genes. Recent studies suggest that cAMP/PKA mediates regulation of the expression of numerous immunomodulatory proteins in ASM including IL-6, RANTES, eotaxin, and GM-CSF [53, 54, 274–276]. Although poorly characterized, the growth inhibitory effect of Gs-coupled receptor activation in ASM is consistent with the known effects of PKA on mitogenic signaling. These effects include inhibition of p42/p44 MAPK signaling via phosphorylation and inhibition of the upstream intermediate raf-1, and via inhibition of promitogenic transcriptional regulation mediated by phospho-CREB. Lastly, Gs-coupled receptor activation is also believed to promote PKA-independent effects, including gating of KCa++ channels directly by Gαs [56], and actin polymerization via an unestablished mechanism [55].