Nicotine Diminishes Alpha2-Adrenergic Receptor-Dependent Protection Against Oxidative Stress in H9c2 Cardiomyocytes

Abstract

Introduction: Nicotine is a major component of cigarette smoke with various detrimental cardiovascular effects, including increased oxidative stress in the heart. Agonism of α 2 -adrenergic receptors (ARs), such as with dexmedetomidine, has been documented to exert cardioprotective effects against oxidative stress and related apoptosis and necroptosis. α 2 -ARs are membrane-residing G protein-coupled receptors (GPCRs) that primarily activate Gi/o proteins. They are also subjected to GPCR-kinase (GRK)-2-dependent desensitization, which entails phosphorylation of the agonist-activated receptor by GRK2 to induce its decoupling from G proteins, thus terminating α 2 AR-mediated G protein signaling. Objective: In the present study, we sought to examine the effects of nicotine on α 2 AR signaling and effects in H9c2 cardiomyocytes exposed to H 2 O 2 to induce oxidative cellular damage. Methods and Results: As expected, treatment of H9c2 cardiomyocytes with H 2 O 2 significantly decreased cell viability and increased oxidative stress, as assessed by reactive oxygen species (ROS)-associated fluorescence levels (DCF assay) and superoxide dismutase activity. Both H 2 O 2 effects were partly rescued by α 2 AR activation with brimonidine in control cardiomyocytes but not in cells pretreated with nicotine for 24 hours, in which brimonidine was unable to reduce H 2 O 2 -induced cell death and oxidative stress. This was due to severe α 2 AR desensitization, manifested as very low Gi protein activation by brimonidine, and accompanied by GRK2 upregulation in nicotine-treated cardiomyocytes. Finally, pharmacological inhibition of adenylyl cyclase (AC) blocked H 2 O 2 -dependent oxidative damage in nicotine-pretreated H9c2 cardiomyocytes, indicating that α 2 AR activation protects against oxidative injury via its classic coupling to Gai-mediated AC inhibition. Discussion/Conclusions: Nicotine can negate the cardioprotective effects of α 2 AR agonists against oxidative injury, which may have important implications for patients treated with this class of drugs that are chronic tobacco smokers. Keywords: α 2 -adrenergic receptor, oxidative stress, cardiac myocyte, nicotine, GRK2, signal transduction, desensitization, α 2 -adrenergic agonist, adenylyl cyclase, superoxide dismutase

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