Published ahead of print on November 11, 2005, doi:10.1165/rcmb.2005-0320OC
© 2006 American Thoracic Society DOI: 10.1165/rcmb.2005-0320OC Chronic Ethanol Ingestion Increases Superoxide Production and NADPH Oxidase Expression in the LungDepartment of Medicine, Atlanta Veterans Affairs Medical Center and Emory University, and Department of Pediatrics and Biochemistry, Emory University, Atlanta, Georgia; and St. Vincent's Institute of Medical Research, and Department of Medicine, University of Melbourne, St. Vincent's Hospital, Fitzroy, Victoria, Australia Correspondence and requests for reprints should be addressed to John A. Polikandriotis, Ph.D., Department of Medicine, Atlanta VA and Emory University Medical Center, Atlanta VAMC (151-P), 1670 Clairmont Rd., Decatur, GA 30033. E-mail: jpolika{at}emory.edu
Alcohol abuse increases the incidence of acute respiratory distress syndrome and causes oxidative stress and cellular dysfunction in the lung. The mechanisms of ethanol (EtOH)-induced oxidative stress in the lung remain to be defined. Chronic alcohol ingestion has been associated with increased renin-angiotensin system (RAS) activity. Therefore, the current study investigated the ability of lisinopril, an angiotensin-converting enzyme (ACE) inhibitor, to modulate oxidative stress in the lung after chronic EtOH ingestion in a well-established rat model. Male Sprague-Dawley rats were fed liquid diets containing EtOH (36% of calories) or maltose-dextrin as an isocaloric substitution for EtOH (Control) for 6 wk. Selected animals were also treated with lisinopril (3 mg/liter) for 6 wk. Chronic EtOH ingestion increased bronchoalveolar lavage fluid glutathione disulfide levels and superoxide formation in lung parenchyma. These effects of EtOH were attenuated by lisinopril treatment. Chronic EtOH ingestion failed to increase ACE expression or angiotensin II levels in lung homogenates, but increased angiotensinogen, angiotensin II type 1 and type 2 receptor levels, and ACE activity. Chronic EtOH ingestion also increased the levels of the NADPH oxidase subunit, gp91phox, an effect that was attenuated by lisinopril, but had no effect on lung p22phox or p47phox levels. These findings suggest that EtOH-mediated RAS activation plays an important role in pulmonary oxidative stress and provide new insights into mechanisms by which EtOH causes oxidative stress in the lung and potential strategies of lung protection through ACE inhibition.
Key Words: ARDS chronic ethanol ingestion lung NADPH oxidase superoxide
The acute respiratory distress syndrome (ARDS) is a severe form of lung injury characterized by noncardiogenic pulmonary edema in response to diverse clinical scenarios such as sepsis, trauma, or aspiration that cause systemic activation of proinflammatory cascades (1). Despite extensive research, ARDS is associated with a 50% mortality rate, due in part to the lack of effective therapies (2). Therefore, identifying mechanisms that predispose the lung to acute injury has the potential to further our understanding of ARDS pathophysiology and the design of treatment strategies capable of limiting the development and/or severity of ARDS. Chronic alcohol abuse is a comorbid variable that increases the incidence of ARDS (3). While alcohol undoubtedly predisposes to ARDS through many mechanisms, oxidative stress is a key factor (47). Chronic alcohol ingestion reduces the level of the antioxidant glutathione (GSH) in bronchoalveolar lavage (BAL) fluid (8) and increases renin-angiotensin system (RAS) activity (911). Activation of the RAS involves renin-mediated cleavage of angiotensinogen into the decapeptide angiotensin I (Ang I). Angiotensinogen is generated mainly in the liver, but recent studies have confirmed mRNA expression in a wide range of tissues including the kidney, brain, vascular tissue, adrenal gland, and placenta (12). Renin is produced not only in the kidney in response to hypotension or volume depletion but also in vascular endothelial cells in response to more diverse stimuli (13). Ang I is converted to the octapeptide, angiotensin II (Ang II), by angiotensin-converting enzyme (ACE). Although ACE concentration is highest in the lung, it can also be found on the lumenal surface of vascular endothelial cells in other locations (14, 15). Once formed, Ang II exerts its cellular effects by binding to receptors on the cell surface, predominately Ang II Type 1 (AT1) and Type 2 (AT2) receptors (16). Ang II is a potent inducer of oxidative stress. Ang II stimulates NADPH oxidase expression and activity as well as superoxide production in cells of the vascular wall in vitro (1719) and in vivo (20), and increases NADPH oxidase-dependent superoxide production in coronary microvascular endothelial cells (21). Originally described in neutrophils, NADPH oxidase is a multicomponent, membrane-associated, enzyme that catalyzes the one electron reduction of oxygen to superoxide using NADH or NADPH as the electron donor (22). NADPH oxidase components include gp91phox, p22phox, p40phox, p47phox, and p67phox. The subunits gp91phox and p22phox reside in the plasma membrane and bind the components of the electron transport chain heme and FAD, forming cytochrome b558 (23, 24). The NADPH oxidase subunits p40phox, p47phox, and p67phox are cytosolic and are involved in activation of the enzyme complex (24, 25). Recently, we determined that in a rat model of chronic EtOH ingestion, the ACE inhibitor, lisinopril, prevented EtOH-induced lung glutathione depletion and protected against endotoxin-mediated epithelial dysfunction, suggesting the RAS as a key mediator of EtOH-induced lung dysfunction (26). To our knowledge, no previous studies have examined the effects of EtOH administration on lung angiotensin peptide levels. In this study, we hypothesized that RAS-mediated activation of NADPH oxidase is one mechanism by which chronic alcohol abuse increases susceptibility of the lung to oxidative stress and ARDS. The current report demonstrates that chronic EtOH ingestion stimulates RAS-mediated increases in superoxide production and gp91phox expression in the lung, leading to increased oxidative stress.
Animal Treatment Male Sprague-Dawley rats were fed the liquid Lieber-DeCarli diet containing EtOH (36% of calories) for 6 wk or an isocaloric control diet without EtOH (substitution of maltose-dextrin for EtOH) (27) as previously reported (26). During the first 2 wk of the dietary regimen the EtOH-fed rats were gradually acclimated to EtOH, receiving 12% of their total calories as EtOH for 1 wk, then 24% of their total calories as EtOH for 1 wk, and then 36% of their total calories as EtOH for 4 wk. Selected animals were also treated with lisinopril (3 mg/liter) in the Control or EtOH diets. The diets are otherwise identical in protein, lipid, and essential nutrient composition.
Dihydroethidium Staining
ACE Activity Assay
Analysis of Angiotensin Peptide Levels
Immunoprecipitation and Western Blotting
Real-Time PCR
Blood Pressure Measurements
Determination of Lung Lavage Glutathione Disulfide Concentration
Statistical Analysis
The Impact of Chronic EtOH Ingestion on the RAS Pathway in the Lung Rats were fed Control or EtOH diets for 6 wk. Homogenates prepared from lung tissue were subjected to Western blotting for specific components of the RAS pathway. As shown in Figure 1, compared with Control, chronic EtOH ingestion increased the levels of angiotensinogen and the Ang II receptors, AT1 and AT2, while having no effect on levels of ACE or renin. All blots were probed with antibodies to actin to ensure equal loading among lanes. Compared with Control, chronic EtOH ingestion produced roughly a 2-fold increase in ACE activity (Figure 1C), but did not increase lung Ang I or Ang II levels in the lung (Table 1).
Chronic EtOH Ingestion Had No Significant Effect on Blood Pressure Because chronic EtOH ingestion altered the levels and activity of RAS components that participate in hemodynamic regulation, blood pressure was measured after EtOH and lisinopril treatments. The results in Figure 2 illustrate that acute blood pressure measurements are comparable in the Control and EtOH-treated groups in anesthetized rats. Only the combination of the EtOH diet and lisinopril treatment (EtOH + lis) caused a small, but significant reduction in mean arterial pressure (MAP), systolic blood pressure (SBP), and diastolic blood pressure (DBP) compared with EtOH alone. As expected, lisinopril treatment was also associated with reduced lung Ang II levels (Table 1).
Chronic EtOH Ingestion Caused RAS-Dependent Oxidative Stress in the Lung We previously reported that chronic EtOH ingestion in the rat leads to oxidative stress in the lung characterized by reductions in glutathione levels in parenchymal lung cells and in BAL fluid (46). In addition, treating EtOH-fed rats with lisinopril (in a protocol identical to that employed in the current study) increased levels of glutathione in BAL fluid compared with untreated, EtOH-fed rats (26). To further characterize the oxidative stress mediated by chronic EtOH ingestion, BAL fluid of Control or EtOH-fed rats treated with or without the ACE inhibitor, lisinopril, was collected, and GSSG levels were measured. As shown in Figure 3, chronic EtOH ingestion increased GSSG levels of BAL fluid in the rat, an effect attenuated by treatment with lisinopril.
Chronic EtOH Ingestion Increased RAS-Dependent Lung Superoxide Generation To determine if chronic EtOH ingestion amplified the production of ROS in the lung, in situ, 30-µm-thick frozen sections from OCT-embedded lungs of Control or EtOH-fed rats treated with or without lisinopril were prepared in parallel. The sections were treated with dihydroethidium (DHE) and subjected to fluorescence confocal microscopy as described in MATERIALS AND METHODS. DHE fluorescence is specific for superoxide generation within the cell (28). In brief, superoxide oxidizes DHE to ethidium bromide, which binds to the DNA in the nucleus and fluoresces red. All images were taken with identical acquisition parameters. Compared with Control (Figure 4A), chronic EtOH ingestion (Figure 4B) substantially increased superoxide generation in rat lung. Inhibition of ACE with lisinopril had no effect on basal superoxide production (Figure 4C), but reduced EtOH-mediated increases in superoxide (Figure 4D).
EtOH-Induced Increases in Lung gp91phox Levels In Vivo Were Attenuated by Lisinopril Treatment NADPH oxidase constitutes a major source of superoxide formation in the vasculature (22). To examine changes in NADPH oxidase expression in the lung as a potential source of EtOH-mediated superoxide production, RNA was isolated from lung tissue of rats fed Control or EtOH diets with or without lisinopril for 6 wk, and real-time PCR for NADPH oxidase components was performed. As shown in Figure 5A, chronic EtOH ingestion increased gp91phox message levels, an effect that was inhibited by lisinopril. This EtOH-induced alteration in NADPH oxidase subunit expression was specific for gp91phox expression in that alterations in p22phox or p47phox expression were not observed (Figures 5B and 5C). In separate experiments, NADPH oxidase components were immunoprecipitated from lung homogenates. Consistent with examination of mRNA expression, chronic EtOH ingestion caused a similar ACE-dependent increase in gp91phox protein levels (Figure 6A) without altering protein levels for p22phox or p47phox (Figures 6B and 6C). Taken together, these results demonstrate that EtOH increased the expression of gp91phox in the lung, an effect that was prevented by in vivo treatment with the ACE inhibitor, lisinopril. These findings support the hypothesis that RAS signaling plays an important role in EtOH-induced NADPH oxidase expression in the lung.
Previous studies from our laboratories determined that chronic EtOH ingestion in rats renders the lung intrinsically susceptible to endotoxin-dependent edematous injury, supporting the observation that alcohol abuse predisposes the lung to injury (7). In addition, we have previously reported that EtOH ingestion markedly decreased the levels of the antioxidant glutathione in lung tissue and in BAL fluid (47). Recently, we found that lisinopril-mediated ACE inhibition prevented EtOH-induced glutathione depletion (26). The current study extends these observations to further clarify the mechanisms by which RAS mediates oxidative stress via NADPH oxidase activation in the lung during chronic EtOH ingestion. Consistent with EtOH-mediated reductions in BAL fluid glutathione, chronic EtOH ingestion increased GSSG levels of BAL fluid and enhanced superoxide generation in lung tissue, effects attenuated by lisinopril. Since the EtOH diet alone had no significant effect on blood pressure, these findings suggest that the effects of Ang II on oxidative stress in the lung relates more to its ability to modulate gene expression and are independent of its effects on blood pressure. The role of the RAS in the pathogenesis of lung injury and ARDS has been suggested by studies demonstrating increased circulating levels of Ang II in patients with ARDS (31, 32). In the current study, chronic EtOH ingestion increased components of the RAS pathway in the lung, specifically angiotensinogen, AT1, and AT2 expression, as well as ACE activity. Cleavage of angiotensinogen, the only known substrate for renin, is the rate-limiting factor in the cascade of Ang II production (33), suggesting that activation of renin following inflammatory stimuli could lead to robust increases in RAS activity in the lung after chronic EtOH ingestion. Although chronic EtOH ingestion alone did not increase lung Ang I or Ang II levels, we postulate that the enhanced AT1 and AT2 receptor expression in the lung caused by chronic EtOH ingestion results in an increased sensitivity to Ang II and therefore enhanced downstream RAS effector mechanisms in the lung. As expected, lisinopril reduced lung Ang II levels and tended to increase Ang I levels in Control and ETOH-fed animals. Lisinopril-mediated reductions in Ang II may therefore provide a plausible mechanism for attenuation of downstream RAS signaling in EtOH-fed animals. Among its many downstream targets, Ang II stimulates NADPH oxidase expression and activity, an important source of oxidative stress and ROS production in many tissues (34, 35). Chronic EtOH ingestion increased both the message and protein levels of the NADPH oxidase subunit, gp91phox, an effect attenuated by the ACE inhibitor, lisinopril. The pathophysiologic significance of gp91phox has been demonstrated in vivo where gp91phox/ mice, compared to wild-type mice, displayed less lung microvascular injury when challenged with intraperitoneal Escherichia coli (36). A 2-fold increase in superoxide formation caused by Ang II infusion has also been reported in the aortas of wild-type animals but not in gp91phox/ mice (37). In addition, in vitro evidence suggests that gp91phox expression provides dose-dependent regulation of superoxide generation in response to Ang II in human endothelial cells (38). Taken together, these findings suggest that EtOH-stimulated increases in RAS activity in the lung increased NADPH oxidase-dependent superoxide production by increasing gp91phox expression. The current study does not address the mechanism by which EtOH modulates RAS activity. Regardless of the precise mechanism, our study provides novel evidence that EtOH stimulates RAS activity in the lung and stimulates lung oxidative stress through an ACE-dependent mechanism and implicates gp91phox as a downstream effector. These findings extend previous work linking EtOH ingestion to oxidative stress in the lung and susceptibility to lung injury. These findings raise the possibility that alcohol abuse increases a patient's risk for acute lung injury in part by altering the RAS and specific NADPH oxidase component expression and thereby rendering the tissue susceptible to oxidative stress. Furthermore, this study raises the possibility that lung dysfunction could be modulated by treatment with ACE inhibitors. This may be particularly beneficial in patients with a history of alcohol abuse who are at more than twice the risk of developing ARDS in response to a given insult (3).
This work is supported by grants from the Veterans Affairs Research Service (D.M.G. and C.M.H.), the National Institute of Alcohol Abuse and Alcoholism (J.A.P., L.A.S.B., D.M.G., and C.M.H.), and by a Career Development Fellowship (Award CR 02M 0829) from the National Heart Foundation of Australia (D.J.C.). Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. Originally Published in Press as DOI: 10.1165/rcmb.2005-0320OC on November 11, 2005 Received in original form August 22, 2005 Accepted in final form October 27, 2005
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