Agonist- and hydrogen peroxide-mediated oxidation of the β2 adrenergic receptor: evidence of receptor s-sulfenation as detected by a modified biotin-switch assay

ROS-mediated regulation of β2AR function: Does oxidation play a meaningful role towards β2-agonist tachyphylaxis in airway obstructive diseases?

β2-adrenergic receptor (β2AR) agonists are the clinical gold standard for treatment and prophylaxis of airway constriction in pulmonary obstructive diseases such as asthma and COPD. Inhaled β2-agonists elicit rapid bronchorelaxation of the airway smooth muscle, yet, clinical tachyphylaxis to this response can occur over repeated and chronic use, which reduces the bronchodilatory effectiveness. Several mechanisms have been proposed to impart β2-agonist tachyphylaxis, most notably β2AR desensitization. However, airway tissue is known to be highly oxidative, particularly in obstructive disease states where reactive oxygen species (ROS) generation is upregulated and ROS degradation is suboptimal yielding a large oxidative burden. Recent evidence demonstrates that β2AR can regulate ROS generation and that ROS can post-translationally alter β2AR cysteine residues via oxidation, leading to distinct functional receptor outcomes. Herein, we discuss the growing evidence for β2AR mediated ROS generation in airway cells and the role of ROS in regulating β2AR via cysteine-oxidation of the receptor. Given the functional consequence of the β2AR-ROS signaling axis in the airways, we also discuss the potential role of ROS in mediating β2-agonist tachyphylaxis.

Agonists and hydrogen peroxide mediate hyperoxidation of β2-adrenergic receptor in airway epithelial cells: Implications for tachyphylaxis to β2-agonists in constrictive airway disorders

Asthma and other airway obstructive disorders are characterized by heightened inflammation and excessive airway epithelial cell reactive oxygen species (ROS), which give rise to a highly oxidative environment. After decades of use, β2-adrenergic receptor (β2AR) agonists remain at the forefront of treatment options for asthma, however, chronic use of β2-agonists leads to tachyphylaxis to the bronchorelaxant effects, a phenomenon that remains mechanistically unexplained. We have previously demonstrated that β2AR agonism increases ROS generation in airway epithelial cells, which upholds proper receptor function via feedback oxidation of β2AR cysteine thiolates to Cys-S-sulfenic acids (Cys-SOH). Our previous results also demonstrate that prevention of normal redox cycling of this post-translational oxi-modification back to the thiol prevents proper receptor function. Given that Cys-S-sulfenic acids can be irreversibly overoxidized to Cys-S-sulfinic (Cys-SO2H) or S-sulfonic (Cys-SO3H) acids, which are incapable of further participation in redox reactions, we hypothesized that β2-agonist tachyphylaxis may be explained by hyperoxidation of β2AR to S-sulfinic acids. Here, using airway epithelial cell lines and primary small airway epithelial cells from healthy and asthma-diseased donors, we show that β2AR agonism generates H2O2 in a receptor and NAPDH oxidase-dependent manner. We also demonstrate that acute and chronic receptor agonism can facilitate β2AR S-sulfination, and that millimolar H2O2 concentrations are deleterious to β2AR-mediated cAMP formation, an effect that can be rescued to a degree in the presence of the cysteine-donating antioxidant N-acetyl-L-cysteine. Our results reveal that the oxidative state of β2AR may contribute to receptor functionality and may, at least in part, explain β2-agonist tachyphylaxis.

H2S regulates redox signaling downstream of cardiac β-adrenergic receptors in a G6PD-dependent manner

Stimulating β-adrenergic receptors (β-AR) culminates in pathological hypertrophy - a condition underlying multiple cardiovascular diseases (CVDs). The ensuing signal transduction network appears to involve mutually communicating phosphorylation-cascades and redox signaling modules, although the regulators of redox signaling processes remain largely unknown. We previously showed that H₂S-induced Glucose-6-phosphate dehydrogenase (G6PD) activity is critical for suppressing cardiac hypertrophy in response to adrenergic stimulation. Here, we extended our findings and identified novel H₂S-dependent pathways constraining β-AR-induced pathological hypertrophy. We demonstrated that H₂S regulated early redox signal transduction processes - including suppression of cue-dependent production of reactive oxygen species (ROS) and oxidation of cysteine thiols (R-SOH) on critical signaling intermediates (including AKT1/2/3 & ERK1/2). Consistently, the maintenance of intracellular levels of H₂S dampened the transcriptional signature associated with pathological hypertrophy upon β-AR-stimulation, as demonstrated by RNA-seq analysis. We further prove that H₂S remodels cell metabolism by promoting G6PD activity to enforce changes in the redox state that favor physiological cardiomyocyte growth over pathological hypertrophy. Thus, our data suggest that G6PD is an effector of H₂S-mediated suppression of pathological hypertrophy and that the accumulation of ROS in the G6PD-deficient background can drive maladaptive remodeling. Our study reveals an adaptive role for H₂S relevant to basic and translational studies. Identifying adaptive signaling mediators of the β-AR-induced hypertrophy may reveal new therapeutic targets and routes for CVD therapy optimization.

The skeletal muscle relaxer cyclobenzaprine is a potent non-competitive antagonist of histamine H1 receptors

Cyclobenzaprine is a tricyclic dimethylpropanamine skeletal muscle relaxant, which is used clinically to decrease muscle spasm and hypercontractility, as well as acute musculoskeletal pain. Although the absolute mechanism of action of cyclobenzaprine remains elusive, it is known to mediate its effects centrally, via inhibition of tonic somatic motor function, likely through modulation of noradrenergic and serotonergic systems. While cyclobenzaprine is effective as a muscle relaxant, greater than 30% of patients experience drowsiness and sedative/hypnotic effects, yet, the mechanisms that cause this adverse effect is also undescribed. Based on this common adverse effect profile and the structural similarity of cyclobenzaprine to tricyclic antidepressants, as well as ethanolamine first-generation antihistamines, we hypothesized that cyclobenzaprine facilitates sedative effects via off-target antagonism of central histamine H1 receptors (H1R). Here, for the first time, we present data that demonstrate that cyclobenzaprine exhibits low nanomolar affinity for the cloned human H1R, as well as that expressed in both rat and mouse brain. Using saturation radioligand binding, we also demonstrate that cyclobenzaprine binds to the H1R in a non-competitive manner. Similarly, functional assays measuring both Ca⁺² influx and novel TRUPATH G-protein subunit BRET biosensors reveal that cyclobenzaprine also blocks histamine-mediated H1R functional activity in a non-competitive manner, whereas the classical H1R antagonist diphenhydramine does so competitively. Given that cyclobenzaprine readily crosses the blood-brain barrier and its muscle relaxant effects occur centrally, our data suggest that off-target central antagonism of H1R by cyclobenzaprine facilitates the significant sedative effect of this agent seen in patients. Significance Statement Cyclobenzaprine, a clinically used muscle relaxant that is strongly linked to sedation, demonstrates high affinity non-competitive antagonism at the histamine H1 receptor. This effect likely modulates the high degree of sedation patients experience.

In cardiac muscle cells, both adrenergic agonists and antagonists induce reactive oxygen species from NOX2 but mutually attenuate each other's effects

In cardiac muscle cells adrenergic agonists stimulate the generation of reactive oxygen species, followed by redox signaling. We postulated that the antagonists would attenuate such reactive oxygen species generation by the agonists. H9c2 cardiac myoblasts, neonatal rat cardiac myocytes, and HEK293 cells expressing β₁/β₂ adrenoceptors were stimulated with several agonists and antagonists. All the agonists and antagonists independently generated reactive oxygen species; but its generation was minimum whenever an agonists was added together with an antagonist. We monitored the Ca++ signaling in the treated cells and obtained similar results. In all treatment sets, superoxide and H₂O₂ were generated in the mitochondria and the cytosol respectively. NOX2 inhibitor gp91ds-tat blocked reactive oxygen species generation by both the agonists and the antagonists. The level of p47phox subunit of NOX2 rapidly increased upon treatment, and it translocated to the plasma membrane, confirming NOX2 activation. Inhibitor studies showed that the activation of NOX2 involves ERK, PI3K, and tyrosine kinases. Recombinant promoter-reporter assays showed that reactive oxygen species generated by both the agonists and antagonists modulated downstream gene expression. Mice injected with the β-adrenergic agonist isoproterenol and fed with the antagonist metoprolol showed a robust induction of p47phox in the heart. We conclude that both the agonism and antagonism of adrenoceptors initiate redox signaling but when added together, they mutually counteract each other's effects. Our study thus highlights the importance of reactive oxygen species in adrenoceptor agonism and antagonism with relevance to the therapeutic use of the β blockers.

Redox Regulation of Lipid Mobilization in Adipose Tissues

Lipid mobilization in adipose tissues, which includes lipogenesis and lipolysis, is a paramount process in regulating systemic energy metabolism. Reactive oxygen and nitrogen species (ROS and RNS) are byproducts of cellular metabolism that exert signaling functions in several cellular processes, including lipolysis and lipogenesis. During lipolysis, the adipose tissue generates ROS and RNS and thus requires a robust antioxidant response to maintain tight regulation of redox signaling. This review will discuss the production of ROS and RNS within the adipose tissue, their role in regulating lipolysis and lipogenesis, and the implications of antioxidants on lipid mobilization.

Aquaporin 8 is involved in H2 O2 -mediated differential regulation of metabolic signaling by α1 - and β-adrenoceptors in hepatocytes

Reactive oxygen species participate in regulating intracellular signaling pathways. Herein, we investigated the reported opposite effects of hydrogen peroxide (H₂ O₂ ) on metabolic signaling mediated by activated α₁ - and β-adrenoceptors (ARs) in hepatocytes. In isolated rat hepatocytes, stimulation of α₁ -AR increases H₂ O₂ production via NADPH oxidase 2 (NOX2) activation. We find that the H₂ O₂ thus produced is essential for α₁ -AR-mediated activation of the classical hepatic glycogenolytic, gluconeogenic, and ureagenic responses. However, H₂ O₂ inhibits β-AR-mediated activation of these metabolic responses. We show that H₂ O₂ mediates its effects on α₁ -AR and β-AR by permeating cells through aquaporin 8 (AQP8) channels and promoting Ca²⁺ mobilization. Thus, our findings reveal a novel NOX2-H₂ O₂ -AQP8-Ca²⁺ signaling cascade acting downstream of α₁ -AR in hepatocytes, which, by negatively regulating β-AR signaling, establishes negative crosstalk between the two pathways.

Cysteine redox state regulates human β2-adrenergic receptor binding and function

Bronchoconstrictive airway disorders such as asthma are characterized by inflammation and increases in reactive oxygen species (ROS), which produce a highly oxidative environment. β2-adrenergic receptor (β2AR) agonists are a mainstay of clinical therapy for asthma and provide bronchorelaxation upon inhalation. We have previously shown that β2AR agonism generates intracellular ROS, an effect that is required for receptor function, and which post-translationally oxidizes β2AR cysteine thiols to Cys-S-sulfenic acids (Cys-S-OH). Furthermore, highly oxidative environments can irreversibly oxidize Cys-S-OH to Cys-S-sulfinic (Cys-SO₂H) or S-sulfonic (Cys-SO₃H) acids, which are incapable of further participating in homeostatic redox reactions (i.e., redox-deficient). The aim of this study was to examine the vitality of β2AR-ROS interplay and the resultant functional consequences of β2AR Cys-redox in the receptors native, oxidized, and redox-deficient states. Here, we show for the first time that β2AR can be oxidized to Cys-S-OH in situ, moreover, using both clonal cells and a human airway epithelial cell line endogenously expressing β2AR, we show that receptor redox state profoundly influences β2AR orthosteric ligand binding and downstream function. Specifically, homeostatic β2AR redox states are vital toward agonist-induced cAMP formation and subsequent CREB and G-protein-dependent ERK1/2 phosphorylation, in addition to β-arrestin-2 recruitment and downstream arrestin-dependent ERK1/2 phosphorylation and internalization. On the contrary, redox-deficient β2AR states exhibit decreased ability to signal via either Gαs or β-arrestin. Together, our results demonstrate a β2AR-ROS redox axis, which if disturbed, interferes with proper receptor function.

The β2-adrenergic receptor-ROS signaling axis: An overlooked component of β2AR function?

β2-Adrenergic receptor (β2AR) agonists are clinically used to elicit rapid bronchodilation for the treatment of bronchospasms in pulmonary diseases such as asthma and COPD, both of which exhibit characteristically high levels of reactive oxygen species (ROS); likely secondary to over-expression of ROS generating enzymes and chronically heightened inflammation. Interestingly, β2AR has long-been linked to ROS, yet the involvement of ROS in β2AR function has not been as vigorously studied as other aspects of β2AR signaling. Herein, we discuss the existing body of evidence linking β2AR activation to intracellular ROS generation and importantly, the role of ROS in regulating β2AR function. The reciprocal interplay of the β2AR and ROS appear to endow this receptor with the ability to self-regulate signaling efficacy and ligand binding, hereby unveiling a redox-axis that may be unfavorably altered in pathological states contributing to both disease progression and therapeutic drug responses.

Oxidation inhibits PTH receptor signaling and trafficking

Reactive Oxygen Species (ROS) increase during aging, potentially affecting many tissues including brain, heart, and bone. ROS alter signaling pathways and constitute potential therapeutic targets to limit oxidative damaging effects in aging-associated diseases. Parathyroid hormone receptors (PTHR) are widely expressed and PTH is the only anabolic therapy for osteoporosis. The effects of oxidative stress on PTHR signaling and trafficking have not been elucidated. Here, we used Fluorescence Resonance Energy Transfer (FRET)-based cAMP, ERK, and calcium fluorescent biosensors to analyze the effects of ROS on PTHR signaling and trafficking by live-cell imaging. PTHR internalization and recycling were measured in HEK-293 cells stably transfected with HA-PTHR. PTH increased cAMP production, ERK phosphorylation, and elevated intracellular calcium. Pre-incubation with H₂O₂ reduced all PTH-dependent signaling pathways. These inhibitory effects were not a result of PTH oxidation since PTH incubated with H₂O₂ triggered similar responses. PTH promoted internalization and recycling of the PTHR. Both events were significantly reduced by H₂O₂ pre-incubation. These findings highlight the role of oxidation on PTHR signaling and trafficking, and suggest the relevance of ROS as a putative target in diseases associated with oxidative stress such as age-related osteoporosis.

Effect of penehyclidine hydrochloride on β-arrestin-1 expression in lipopolysaccharide-induced human pulmonary microvascular endothelial cells

β-arrestins are expressed proteins that were first described, and are well-known, as negative regulators of G protein-coupled receptor signaling. Penehyclidine hydrochloride (PHC) is a new anti-cholinergic drug that can inhibit biomembrane lipid peroxidation, and decrease cytokines and oxyradicals. However, to date, no reports on the effects of PHC on β-arrestin-1 in cells have been published. The aim of this study was to investigate the effect of PHC on β-arrestin-1 expression in lipopolysaccharide (LPS)-induced human pulmonary microvascular endothelial cells (HPMEC). Cultured HPMEC were pretreated with PHC, followed by LPS treatment. Muscarinic receptor mRNAs were assayed by real-time quantitative PCR. Cell viability was assayed by the methyl thiazolyl tetrazolium (MTT) conversion test. The dose and time effects of PHC on β-arrestin-1 expression in LPS-induced HPMEC were determined by Western blot analysis. Cell malondialdehyde (MDA) level and superoxide dismutase (SOD) activity were measured. It was found that the M3 receptor was the one most highly expressed, and was activated 5 min after LPS challenge. Furthermore, 2 μg/mL PHC significantly upregulated expression of β-arrestin-1 within 10 to 15 min. Compared with the control group, MDA levels in cells were remarkably increased and SOD activities were significantly decreased in LPS pretreated cells, while PHC markedly decreased MDA levels and increased SOD activities. We conclude that PHC attenuated ROS injury by upregulating β-arrestin-1 expression, thereby implicating a mechanism by which PHC may exert its protective effects against LPS-induced pulmonary microvascular endothelial cell injury.

Reactive oxygen species are required for β2 adrenergic receptor-β-arrestin interactions and signaling to ERK1/2

The β2-adrenergic receptor (β2AR) is the prototypical member of the heptahelical G protein-coupled receptor (GPCR) superfamily and is well-known to elicit biological effects through both G protein-dependent and G protein-independent signaling cascades. Agonism of β2AR has been described to promote phosphorylation and activation of extracellular signal-regulated kinases (ERK1/2) via a G-protein/PKA pathway that transpires rapidly upon receptor agonism, as well as by a distinct β-arrestin-mediated pathway that occurs at later time points. We have previously shown that β2AR agonism promotes generation of intracellular reactive oxygen species (ROS) and that β2AR-associated G protein signaling is dependent on ROS formation. It has also been suggested that β2AR-mediated ROS generation occurs via recruitment of β-arrestins. In this study, we confirm the effects of β-arrestin on β2AR-induced ROS generation, and investigate the ROS-dependency of β-arrestin-linked β2AR signaling. In HEK293 cells, both coimmunoprecipitation and BRET studies reveal that ROS are vital for the physical interaction of β2AR with β-arrestin partner proteins. Using phosphorylation of ERK1/2 as a functional endpoint to assess the role of ROS in β2AR-β-arrestin signaling, our results show that inhibition of intracellular ROS abrogates both the β-arrestin and G protein-mediated phosphorylation of ERK1/2 upon agonism of β2AR. Importantly, both the G protein and β-arrestin components were reversed upon exogenous administration of ROS, suggesting a critical role for oxidants in stabilization of β2AR. Taken together, our data signify that ROS serve purposeful roles in stabilizing both G protein- and β-arrestin-mediated β2AR signaling in HEK293 cells.