Effects of Acute Hypoxia and Lipopolysaccharide on Nitric Oxide Synthase-2 Expression in Acute Lung Injury

Abstract

The potential role of nitric oxide synthase-2 (NOS2) in acute lung injury (ALI) has gained increasing attention. This study evaluates the effects of hypoxia, an important feature of ALI, on NOS2 expression in a rat model of ALI caused by exposure to hypoxia and LPS. Exposure to hypoxia alone had no effect on the expression of NOS2 in rat lungs. LPS treatment resulted in a significant increase in NOS2 in the lungs, which was further enhanced by concomitant exposure to hypoxia. Immunohistochemical analysis and in situ hybridization showed no changes in the expression of NOS2 in lung resident cells under any conditions. The increase in NOS2 levels is mainly due to the influx of NOS2-expressing inflammatory cells. By morphologic analysis, these inflammatory cells were identified as neutrophils, lymphocytes, and monocytes. In vitro experiments of lung epithelial and endothelial cell lines showed no detectable expression of NOS2 with any of the treatments. In a macrophage cell line, LPS-induced NOS2 expression was not affected by the concomitant exposure to hypoxia. In conclusion, LPS increases NOS2 expression in rat lungs through the recruitment of NOS2-producing leukocytes. Simultaneous exposure to LPS and hypoxia results in a greater influx of inflammatory cells that further enhances NOS2 expression.Keywords: acute respiratory distress syndrome; endotoxin; leukocytes; sepsis Nitric oxide (·NO) is a multifunctional, short-lived gas that may contribute to toxic free radical production and potentiate lung injury. ·NO is synthesized by a family of enzymes called nitric oxide synthases (NOS). There are three NOS isoforms, namely, NOS1 (neuronal NOS), NOS2 (inducible NOS), and NOS3 (endothelial NOS). These isoenzymes were first purified from rat brain (1), murine macrophages (2), and bovine aortic endothelial cells (3), respectively. NOS enzymes are classically classified as either constitutive (NOS1 and NOS3) or inducible (NOS2), although it is now known that the expression of NOS1 and NOS3 can be induced (4), and that NOS2 is constitutively expressed in numerous cell types (5). The so-called constitutive isoforms (NOS1 and NOS3) release low amounts of ·NO and their activity is dependent on calcium and calmodulin (6). On the other hand, NOS2 produces large amounts of ·NO after induction by stimuli such as endotoxin and inflammatory cytokines (7-9). All three isoenzymes have been found in the lung (10-12) and mediate a wide variety of biological events in the respiratory system, (Received in original form September 11, 2002; accepted in final form May 16, 2003) Supported by the Spanish Ministry of Health (FIS 00/0698 and RTIC C03/10) and the Fundació n Echébano.Correspondence and requests for reprints should be addressed to Javier J. Zulueta, M.D., Pulmonary Medicine Service, Clínica Universitaria de Navarra, University of Navarra, Avenida Pío XII, 36, 31008 Pamplona, Spain. E-mail: jzulueta@unav.es This article has an online supplement, which is accessible from this issue's table of contents online at www.atsjournals...

Histochemical Alterations in One Lung Ventilation

“…In addition, the low systemic NO strongly suggests that there was no overt inflammatory response to the surgery or OLV. This is consistent with previous results from in vivo models of hypoxia where it has been shown that hypoxia alone does not affect inducible NOS (NOS2) [18]. Increased NOS2 expression requires other inflammatory stimuli such as lipopolysaccharide or IFN-␥.…”

Lipopolysaccharide plus hypoxia and reoxygenation synergistically reduce electrical coupling between microvascular endothelial cells by dephosphorylating Connexin40

“…For example, low doses of LPS following ischemia/reperfusion of gut in the rat converted moderate lung dysfunction into advanced organ failure (Koike et al, 1992). Agorreta et al (2003) showed in rat macrophages that H/R, which itself does not upregulate expression of inducible nitric oxide synthase (iNOS), can enhance LPS-induced upregulation of iNOS. Regarding the priming the response to I/R by sepsis, Colletti and Green (2001) demonstrated that lung and liver injury in rats was significantly increased when LPS was injected prior to hepatic I/R, as compared to I/R alone.…”

Clinical and pathological features of fat embolism with acute respiratory distress syndrome

“…We have also identified the lung as the major site that produces NO, which is toxic to the lung [28]. Several experimental studies have indicated that the release of NO via iNOS is responsible for oxidative stress and lung injury following smoke inhalation [29], exposure to ozone [30], carrageen treatment [31], acute hypoxia [32], acid aspiration [33] and endotoxaemia [34][35][36][37]. In a murine model of sepsis, Razavi et al [38] proposed that up-regulation of iNOS in the lung caused neutrophil infiltration.…”