The role of monoamine oxidase (MAO) and translocator protein (TSPO) on mechanisms linking cigarette smoke to airway injury : in vitro and in vivo

Abstract

Chronic obstructive pulmonary disease (COPD), characterized by progressively irreversible airflow limitation and airway inflammation, is one of the leading causes of mortality worldwide. Cigarette smoking is the primary risk factor in the development of COPD. Currently, there is no effective treatment to cure COPD. As a result, a variety of studies have been carried out to determine novel potential therapeutic targets based on the pathogenesis, including oxidative stress, chronic inflammation and proteases/anti-proteases. There is increasing evidence demonstrating that mitochondrial dysfunction may play an important role in developing COPD, thus indicating that mitochondria are potential therapeutic targets. Mitochondrial dysfunction can be driven by oxidative stress. Monoamine oxidases (MAOs, type A and B) are mitochondrial bound proteins that are responsible for catalyzing the oxidative deamination of a variety of primary amines, neurotransmitters and hormones. The catalytic reaction generates hydrogen peroxide (H2O2) as a byproduct, providing a novel source of ROS in the cells. Upon exposure to cigarette smoke medium (CSM), only MAO-B showed an elevation of protein expression and activity, in line with increased ROS levels in airway epithelial cells (AECs). Besides, CSM was found to disrupt the antioxidant systems, including superoxide dismutase (SOD), catalase (CAT), heme oxygenase-1 (HO-1), NAD(P)H dehydrogenase (quinone) 1 (NQO1) and glutathione (GSH), resulting in the activation of nuclear factor-erythroid 2 related factor 2 (Nrf2) signaling pathway. At the same time, CSM caused elevation of pro-inflammatory cytokine interleukin-8 (IL-8) release via activation of nuclear factor nuclear factor-κB (NF-κB) signaling pathway. Furthermore, CSM was found to cause mitochondrial damage, which was characterized by interruption of mitochondrial network, loss of mitochondrial membrane potential (ΔΨm) and ATP production. Pretreatment of a selective MAO-B inhibitor selegiline could prevent CSM-induced AECs injury via inhibiting ROS generation, leading to a significant restoration of antioxidant systems, reduction of IL-8 release, reversal of mitochondrial dysfunction and apoptosis. In a 7-day cigarette smoke (CS)-exposed rat model, intraperitoneal injection of selegiline alleviated CS-induced oxidative stress and mitochondrial dysfunction in the lungs. In addition, selegiline demonstrated an anti-inflammatory effect in attenuating CS-induced inflammatory responses in bronchoalveolar lavage (BAL). On the other hand, the opening of mitochondrial permeability transition pore (mPTP) is believed as an inducer of mitochondrial dysfunction. There is a variety of evidence demonstrating that the 18 kDa translocator protein (TSPO) participates in the formation of mPTP and plays a role in the regulation of mPTP opening. In this study, I determined the role of TSPO using its antagonist PK 11195 and agonist AC-5216 for comparison in the CSM-exposed AECs in vitro. Blockade of TSPO by PK 11195 reversed CSM-induced mitochondrial dysfunction, oxidative stress, inflammation and apoptosis; in contrast, activation of TSPO by AC-5216 enhanced CSM-induced alterations. In summary, I demonstrated two mitochondrial targets linked to mitochondrial dysfunction: MAO-B via ROS generation and TSPO via mPTP opening. Inhibition of MAO-B or TSPO could alleviate CS-induced airway injury. To sum up, this project highlights the importance of mitochondria as a promising target for the development of disease-modifying pharmacotherapy against cigarette smoking-related diseases, such as COPD.