Crosstalk between Gi and Gq/Gs pathways in airway smooth muscle regulates bronchial contractility and relaxation

Abstract

Receptor-mediated airway smooth muscle (ASM) contraction via G αq , and relaxation via G αs , underlie the bronchospastic features of asthma and its treatment. Asthma models show increased ASM G αi expression, considered the basis for the proasthmatic phenotypes of enhanced bronchial hyperreactivity to contraction mediated by M 3 -muscarinic receptors and diminished relaxation mediated by β 2 -adrenergic receptors (β 2 ARs). A causal effect between G i expression and phenotype has not been established, nor have mechanisms whereby G i modulates G q /G s signaling. To delineate isolated effects of altered G i , transgenic mice were generated overexpressing G αi2 or a G αi2 peptide inhibitor in ASM. Unexpectedly, G αi2 overexpression decreased contractility to methacholine, while G αi2 inhibition enhanced contraction. These opposite phenotypes resulted from different crosstalk loci within the G q signaling network: decreased phospholipase C and increased PKCα, respectively. G αi2 overexpression decreased β 2 AR-mediated airway relaxation, while G αi2 inhibition increased this response, consistent with physiologically relevant coupling of this receptor to both G s and G i . IL-13 transgenic mice (a model of asthma), which developed increased ASM G αi , displayed marked increases in airway hyperresponsiveness when G αi function was inhibited. Increased G αi in asthma is therefore a double-edged sword: a compensatory event mitigating against bronchial hyperreactivity, but a mechanism that evokes β-agonist resistance. By selective intervention within these multipronged signaling modules, advantageous G s /G q activities could provide new asthma therapies. IntroductionAirway smooth muscle (ASM) contraction and relaxation are primarily regulated by G protein-coupled receptors, the former mediated by receptors signaling to G q and the latter by those that couple to G s (1, 2). Many inflammatory cascades in asthma evoke bronchoconstriction by promoting local increases of G q receptor agonists such as acetylcholine, cysteinyl leukotrienes, prostaglandins, and histamine, which activate their cognate receptors expressed on ASM. There appear to be fewer G s -coupled receptors that act via endogenous agonists to counteract bronchoconstriction, but the β 2 -adrenergic receptor (β 2 AR) of ASM is the target of pharmacologically administered β-agonists and is typically highly effective in relaxing constricted airways. The molecular events and critical transduction elements for these 2 classes of receptors are well recognized. Agonist binding to receptors such as the M 3 -muscarinic receptor promote disassociation of heterotrimeric G q into G α and G βγ subunits, with the α subunit activating phospholipase C (PLC; which promotes inositol-3 phosphate and diacylglycerol production) and the latter activating PKC. Receptors such as the β 2 AR act via G αs to stimulate the effector adenylyl cyclase, resulting in cAMP production and activation of PKA. Substantial interest has

Prolonged heterologous β2-adrenoceptor desensitization promotes proasthmatic airway smooth muscle function via PKA/ERK1/2-mediated phosphodiesterase-4 induction

“…More recently, Liggett and co-workers (38) provided evidence supporting a direct causal relationship between changes in G i expression and altered airway responsiveness by demonstrating that transgenic mice that overexpress the G␣i2-subunit in ASM exhibit decreased ␤2AR-mediated airway relaxation, whereas ␤2AR function is increased in mice expressing a G␣i2 inhibitory peptide in ASM. Unexpectedly, these authors also reported that wild-type mice pretreated with PTX exhibited increased airway constrictor responses to methacholine, as did mice expressing the G␣i2 inhibitory peptide, whereas the bronchoconstrictor responses were decreased in mice overexpressing G␣i2 in ASM (38). The latter findings appear contradictory to those of the present study, as we found that pretreatment with PTX inhibited the heightened ASM constrictor responses to ACh accompanying heterologous ␤2AR desensitization while having no effect on ASM contractility under control conditions (Fig.…”

“…Moreover, in this context, recent studies have demonstrated that the acute cAMP responses detected in subcellular microdomains of HEK293 cells exposed to a ␤2AR or prostanoid receptor (EP2 and EP4) agonist are rapidly attenuated by PKA-dependent PDE4 activity (notably involving PDE4D5), which also mediates acute homologous desensitization of these receptors to subsequent agonist administration, as well as acute heterologous desensitization of the ␤2AR following exposure to an EP2/EP4 agonist (48,49). Of significance, this rapid PKA-dependent/PDE4D-mediated attenuation of cAMP signaling and desensitization to ␤2AR and EP2/EP4 agonists was not detected in the presence of receptor-independent activation of adenylyl cyclase in forskolin-exposed HEK293 cells (38). Recent evidence supports the notion that PDE4D5 represents the key regulator of acute ␤2AR-stimulated cAMP signaling in human ASM cells (5).…”

“…Although these discordant observations are not readily explained, apart from potential species differences, consideration should be given to other possibilities, including differences in the relative contributions of the regulatory actions of the ␣-vs. ␤␥-subunits of G i implicated under the different study conditions. In this respect, whereas Liggett and coworkers attributed the attenuating effect of G␣i2 on airway constrictor responsiveness to an observed downregulated expression of phospholipase C (PLC)␤3 in the G␣i2-overexpressing ASM cells (38), it is well established that the ␤␥-subunit of G i can activate PLC␤ isoforms, including PLC␤3 (7), thereby conferring PTX sensitivity to increases in agonistinduced inositol phospholipid signaling (20). Accordingly, the latter mechanism may account for the reported PTX-sensitive increase in ASM contractility associated with heightened PLCmediated inositol phospholipid signaling in ASM isolated from allergic rabbits (1), as well as in rabbit ASM passively sensitized with atopic asthmatic serum (20,22).…”

Repeated Bouts of Moderate-Intensity Aerobic Exercise Reduce Airway Reactivity in a Murine Asthma Model

“…Increases in PGE 2 levels within the lung have been implicated in the initiation and/or exacerbation of AHR (19,20). To determine the effect of moderate-intensity aerobic exercise on PGE 2 levels within the lungs of OVA-treated mice, mice were subjected to bronchoalveolar lavage and the resulting BALF was analyzed for changes in PGE 2 content.…”

Pilot study: The effect of reducing treatment on exercise induced bronchoconstriction

“…There appears to be a differentiation between downregulation of the β2AR (causing less effective bronchodilatation) and enhanced contractile signaling (causing enhanced AHR) due to chronic activation of the β2AR. 37 Both occur with regular use of beta2-agonists.…”