Alcohol Causes Alveolar Epithelial Oxidative Stress by Inhibiting the Nuclear Factor (Erythroid-Derived 2)–Like 2–Antioxidant Response Element Signaling Pathway

Abstract

Excessive alcohol use increases the risk of acute lung injury and pneumonia. Chronic alcohol ingestion causes oxidative stress within the alveolar space, including near depletion of glutathione (GSH), which impairs alveolar epithelial and macrophage function, in experimental animals and human subjects. However, the fundamental mechanism(s) by which alcohol induces such profound lung oxidative stress is unknown. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a redoxsensitive master transcription factor that regulates activation of the antioxidant response element (ARE). As the alveolar epithelium controls GSH levels within the alveolar space, we hypothesized that alcohol also decreases Nrf2 expression and/or activation within the alveolar epithelium. In this study, we determined that alcohol ingestion in vivo or direct alcohol exposure in vitro down-regulated the Nrf2-ARE pathway in lung epithelial cells, decreased the expression of antioxidant genes, and lowered intracellular GSH levels. RNA silencing of Nrf2 gene expression in alveolar epithelial cells in vitro decreased expression of these same antioxidant genes, and likewise lowered intracellular GSH levels, findings that mirrored the effects of alcohol. In contrast, treating alcoholexposed alveolar epithelial cells in vitro with the Nrf2 activator, sulforaphane, preserved Nrf2 expression, ARE activation, intracellular GSH levels, and epithelial barrier function. These new experimental findings implicate down-regulation of the Nrf2-ARE signaling pathway as a fundamental mechanism by which alcohol causes profound oxidative stress and alveolar epithelial dysfunction, and suggest that treatments, such as sulforaphane, that activate this pathway could mitigate the pathophysiological consequences of alcohol on the lung and other organs.Keywords: acute respiratory distress syndrome; glutathione; lung; redox signaling; oxidative stress Alcohol is the most commonly used and abused drug throughout the United States and the rest of the world. Although the overuse of alcohol is commonly associated with disorders of the liver, brain, and gastrointestinal tract, the role of alcohol in diseases of the respiratory system is becoming more widely recognized. Although a connection between alcohol abuse and pneumonia has been known for centuries, it was only in 1996 that an epidemiological association with the acute respiratory distress syndrome (ARDS) was recognized (1), an observation later validated in a prospective study (2). ARDS is a severe form of acute lung injury, characterized by alveolar epithelial barrier disruption and flooding of the alveolar space with proteinaceous fluid that interferes with gas exchange and causes profound respiratory failure. ARDS can result from a wide range of critical illnesses, including pneumonia, sepsis, massive gastric aspiration, severe burns, and trauma; recent estimates place its incidence at approximately 200,000 cases per year in the United States and, despite aggressive supportive care, is associated with a mortality rate o...

Chronic Alcohol Ingestion in Rats Alters Lung Metabolism, Promotes Lipid Accumulation, and Impairs Alveolar Macrophage Functions

“…These include altered oxidant-antioxidant balance (24,25) and direct suppressive effects on AMs (26,27). In this study, we describe a mechanism in which EtOH impairs lung lipid homeostasis, associated with altered activation of AMPK, with the net effects of enhanced lipid synthesis and impaired immune function.…”

Nrf2 regulates PU.1 expression and activity in the alveolar macrophage

“…However, this study suggests that SFP can also induce the PU.1-dependent immune defenses indirectly through its activation of Nrf2. Therefore, if these findings are borne out in human subjects, SFP and/or other similar phytochemicals could prove to be effective in promoting and/or restoring a healthier alveolar environment in individuals with underlying alcohol abuse or human immunodeficiency virus (HIV) infection, which at least experimentally are now recognized to inhibit both the antioxidant and immune defense systems in the airways (10,12).…”

“…A mechanism for the severe oxidative stress in the alcoholic lung remained mysterious until we more recently determined that it also inhibits the expression and activity of nuclear factor (erythroid-derived 2)-like 2 (Nrf2), the master transcription factor that activates a program of genes that protect the cell from oxidative stress via a consensusbinding sequence called the antioxidant response element (ARE). (10). Interestingly, alcohol-induced inhibition of Nrf2-ARE signaling is also mediated in a zinc-dependent manner, and dietary zinc supplementation likewise restores antioxidant defenses within the airway (17).…”

Donor Alcohol Abuse and Lung Injury Following Allograft Transplantation: A Cautionary Tale But Also a Call to Action

“…We then performed a prospective study in individuals with severe sepsis, using a more rigorous definition of alcohol abuse (the Short Michigan Alcohol Screening Test [SMAST]), and determined that the relative risk of ARDS was closer to 4-fold in those critically ill patients who met the definition of alcohol abuse (Moss et al, 2003). Over the past 2 decades, we have used experimental and clinical studies to determine the mechanisms by which alcohol abuse causes such profound oxidative stress within the airways and renders the lung susceptible to injury in response to a "second hit" such as sepsis, aspiration, or trauma Jensen et al, 2013;Mehta et al, 2011;Pelaez et al, 2004;Sueblinvong et al, 2014).…”

“…For example, glutathione depletion and the subsequent oxidative stress is a cardinal feature of the alcoholic lung and could be targeted in this setting as glutathione replacement mitigates the risk of acute lung injury in experimental models (Holguin et al, 1998;Velasquez et al, 2002). A multifactorial strategy of glutathione supplementation combined with treatments such as sulforaphane that activate the Nrf2-antioxidant response element and restore glutathione in the lungs of alcohol-fed animals (Jensen et al, 2013) could potentially improve the viability of lung allografts and make them safer for transplantation. Other complementary strategies that are efficacious in experimental models of alcohol-mediated lung dysfunction include delivering recombinant granulocyte-macrophage colony-stimulating factor down the airway (Pelaez et al, 2004) or restoring zinc levels in the alveolar space (Mehta et al, 2011).…”