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

Nicotine is a major component of cigarette smoke with various detrimental cardiovascular effects, including increased oxidative stress in the heart. Agonism of α₂-adrenergic receptors (ARs), such as with dexmedetomidine, has been documented to exert cardioprotective effects against oxidative stress and related apoptosis and necroptosis. α₂-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 α₂AR-mediated G protein signaling.

In the present study, we sought to examine the effects of nicotine on α₂AR signaling and effects in H9c2 cardiomyocytes exposed to H₂O₂ to induce oxidative cellular damage.

As expected, treatment of H9c2 cardiomyocytes with H₂O₂ 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 H₂O₂ effects were partly rescued by α₂AR activation with brimonidine in control cardiomyocytes but not in cells pretreated with nicotine for 24 hours, in which brimonidine was unable to reduce H₂O₂-induced cell death and oxidative stress. This was due to severe α₂AR 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 H₂O₂-dependent oxidative damage in nicotine-pretreated H9c2 cardiomyocytes, indicating that α₂AR activation protects against oxidative injury via its classic coupling to Gai-mediated AC inhibition.

Nicotine can negate the cardioprotective effects of α₂AR agonists against oxidative injury, which may have important implications for patients treated with this class of drugs that are chronic tobacco smokers.