Effects of ghrelin on pulmonary NOD2 mRNA expression and NF-κB activation when protects against acute lung injury in rats challenged with cecal ligation and puncture

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Abstract

Background

Many studies have shown that ghrelin can down-regulate inflammatory cytokine expression via the inhibition of NF-κB activity and therefore, its administration to septic patients is considered beneficial. However, our knowledge of ghrelin's effects on the upstream activators of the NF-κB pathway, such as NOD2, is still limited. This study aimed to investigate the possible involvement of the NOD2 signaling pathway in the anti-inflammatory effects of ghrelin.

Methods

Twenty-four male SD rats received cecal ligation and puncture (CLP) or sham operation, followed by infusion of saline or ghrelin. The lungs were harvested 6 h after CLP or sham operation and analyzed for lung histopathology, neutrophil infiltration, inflammatory cytokines (TNF-α, and IL-6), NOD2 mRNA expression, and activation of NF-κB. Furthermore, survival was recorded for ten days in additional groups of rats.

Results

Compared with sham group, neutrophil infiltration, TNF-α and IL-6 levels, NOD2 mRNA expression, as well as NF-κB activation in lungs from rats undergoing CLP were significantly increased. After the administration of ghrelin, all inflammatory parameters analyzed were lower than those without ghrelin following CLP. In addition, ghrelin improved survival after CLP.

Conclusion

Our results indicate that in a CLP model of sepsis, the beneficial effects that ghrelin has on inflammatory outcomes are mediated at least in part through inhibition of NOD2 expression upstream of NF-κB.

Highlights

► Ghrelin profoundly attenuated CLP-induced acute lung injury. ► Ghrelin decreased NOD2 mRNA expression in lungs from CLP challenged rats. ► Ghrelin inhibited activation of NF-κB in lungs from CLP challenged rats. ► The effect of ghrelin may depend on inhibiting the NOD2/NF-κB signaling pathway.

Introduction

Sepsis, which is defined as a systemic inflammatory response syndrome (SIRS) with a suspected source of infection, continues to be one of the major causes of death in intensive care units (ICU) [1]. Previous studies have reported that more than 750,000 people develop sepsis in America each year, with a mortality of 28.6% [2]. Although much research has gone into understanding the process of sepsis, attempts to apply results in clinical settings have proved unsuccessful and the mortality rate of sepsis has not declined markedly. Thus, identifying a novel, effective therapy for sepsis is still necessary.

Sepsis results from a harmful host response to infection and therefore, components of the innate immune response have attracted considerable attention recently. Essential steps in the development of sepsis include the recognition of bacterial ligands by innate immune receptors, the activation of complex intracellular signaling pathways, and the induction of numerous inflammatory cytokines [1], [3]. Nucleotide-binding oligomerization domain 2 (NOD2) is an important intracellular pattern recognition receptor involved in a variety of host immune responses to pathogens [4]. During an infection, NOD2 in the host cells recognizes the peptidoglycan (PepG) component of bacterial cell walls. Upon binding PepG, NOD2 transmits signals to receptor-interacting protein 2 (Rip2) to activate the transcription factor nuclear factor-kappa B (NF-κB), which induces its translocation to the nucleus [5], [6]. Within the nucleus, NF-κB promotes the expression of inflammatory genes, such as those for TNF-α [7]. However, a persistent accumulation of NF-κB within the nucleus is associated with a higher mortality and degree of organ dysfunction, including acute lung injury (ALI) [8]. In addition, polymorphisms within the NOD2 gene correlate with sepsis outcomes, which are due to a differential ability of NOD2 to identify bacteria and activate NF-κB [9]. These lines of evidence suggest that the NOD2/NF-κB pathway may have an important role in sepsis.

Ghrelin, an endogenous ligand of growth hormone secretagogue receptor-1a (GHSR-1a), was originally purified from rat stomach but is also expressed in many other tissues [10]. In addition to a potent stimulatory effect on growth hormone release in humans and rodents, ghrelin also has other endocrine and non-endocrine activities [11]. Jacob et al. have shown that circulating levels of ghrelin decreased significantly in an experimental model of rodent sepsis [12], with the administration of ghrelin, pro-inflammatory cytokine production decreased, organ injury attenuated and survival rate improved [13], [14]. Recent studies have shown that ghrelin can attenuate severe sepsis-induced ALI and mortality through the inhibition of NF-κB [15], [16]. However, whether ghrelin's inhibitory effects on NF-κB come from the inhibition of upstream signaling molecules such as NOD2, has not yet been addressed [17]. To examine this question, we have used the well-characterized CLP model of sepsis and examined the effects of ghrelin on CLP-induced ALI, pulmonary NOD2 mRNA expression, and activation of the NF-κB.

Section snippets

Animals

Male Sprague–Dawley (SD) rats (220–250 g) were purchased from the Animal Center of the Chinese Academy of Science, Shanghai, China [certification No: SCXK (Shanghai) 2007–0005]. Rats were housed in a controlled temperature and humidity room with a 12-hour light/dark cycle with free access to water. The procedures were approved by the Institutional Animal Care Committee.

Experimental protocol

Twenty four male SD rats were randomly divided into four groups: sham operation group, sham operation plus ghrelin group, CLP

Systemic hemodynamics

Baselines of MAP and HR were comparable among the experimental groups. There were no significant differences in MAP and HR within groups or among groups during the experiment, and all values were within normal ranges for adult male rats. Except for the survival study, no animals died before the end of the experiment (data not shown).

Lung histopathological change

Rats in the CLP group exhibited extensive lung injury as showed by hematoxylin and eosin staining. Compared with sham group, lungs from CLP-treated rats showed

Discussion

In this study, we found that the infusion of ghrelin profoundly attenuated CLP-induced ALI. To shed light on the underlying mechanisms, we focused on the NOD2/NF-κB pathway. Our results show that ghrelin significantly decreases NOD2 mRNA expression and inhibits the activation of NF-κB in lungs from CLP challenged rats. These results indicated that the anti-inflammatory effects of ghrelin act at least in part through inhibition of NOD2 expression upstream of NF-κB.

It is generally accepted that

Conflict of interest

The authors have no conflict of interest to declare.

Acknowledgment

This work was supported by Grant 30901409 from the National Nature Science Foundation of China; Specialized Research Fund for Outstanding Young Teachers of Higher Education to Dr. Xiaomei Feng, Shanghai; Research Fund for Outstanding Young Teachers of Ruijin Hospital to Dr. Xiaomei Feng, Shanghai Jiaotong University, Shanghai; and also supported by Research Fund for Outstanding Young Teachers of Shanghai Jiaotong University to Dr. Xiaomei Feng, Shanghai.

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