Netrin-1 Promotes Epithelial Sodium Channel-Mediated Alveolar Fluid Clearance via Activation of the Adenosine 2B Receptor in Lipopolysaccharide-Induced Acute Lung Injury

Abstract

Background: The epithelial sodium channel (ENaC) is the driving force for pulmonary edema absorption in acute lung injury (ALI). Netrin-1 is a newly found anti-inflammatory factor that works by activating the adenosine 2B receptor (A2BAR). Meanwhile, activated A2BAR has the potential to enhance ENaC-dependent alveolar fluid clearance (AFC). However, whether netrin-1 can increase ENaC-mediated AFC by activating A2BAR remains unclear. Objectives: To investigate the effect of netrin-1 on AFC in ALI and clarify the pathway via which netrin-1 regulates the expression of ENaC in vivo and in vitro. Methods: An ALI model was established by intratracheal instillation of lipopolysaccharide (LPS; 5 mg/kg) in C57BL/J mice, followed by netrin-1 with or without pretreatment with PSB1115, via the caudal vein. Twenty-four hours later, the lungs were isolated for determination of the bronchoalveolar lavage fluid, the lung wet/dry weight (W/D) ratio, AFC, the expressions of α-, β-, and γ-ENaC, and cyclic adenosine monophosphate (cAMP) levels. LPS-stimulated MLE-12 cells were incubated with netrin-1 with or without preincubation with PSB1115. Twenty-four hours later, the expressions of α-, β-, and γ-ENaC were detected. Results: In vivo, netrin-1 expression was significantly decreased during ALI. Substituted netrin-1 significantly dampened the lung injury, decreased the W/D ratio, and enhanced AFC, the expressions of α-, β-, and γ-ENaC, and cAMP levels in ALI, which were abolished by specific A2BAR inhibitor PSB1115. In vitro, netrin-1 increased the expressions of α-, β-, and γ-ENaC, which were prevented by PSB1115. Conclusion: These results indicate that netrin-1 dampens pulmonary inflammation and increases ENaC-mediated AFC to alleviate pulmonary edema in LPS-induced ALI by enhancing cAMP levels through the activation of A2BAR.

4-Hydroxyphenylacetic Acid Attenuated Inflammation and Edema via Suppressing HIF-1α in Seawater Aspiration-Induced Lung Injury in Rats

“…The acute lung injury (ALI) induced by seawater aspiration had an acuter and rapider course than that induced by LPS [4,6,7,8]. The majority death induced by seawater aspiration happened within the first 6 h (Figure 2), which is greatly shorter than 40 h in LPS-induced ALI [29].…”

“…Hypoxia increases HIF-1α protein by elevating its protein stability but not by prompting its mRNA expression [4,6,7,8]. However, instillation of seawater increased not only HIF-1α protein but also its mRNA expression (Figure 4).…”

“…A recent study showed that ALI induced by seawater aspiration is severer than that by freshwater, the reason of which could concern osmotic pressure [5]. Hypoxia and hypertonicity are the two impact factors in seawater aspiration-induced ALI which had an acuter and rapider course than that induced by lipopolysaccharide (LPS) [4,6,7,8]. Besides mechanical ventilation [9,10,11], therapies which could attenuate the injuries in lungs at the early stage of seawater aspiration-induced ALI are required to prevent the occurrence of ARDS.…”

Adenosine A2B Receptor: From Cell Biology to Human Diseases

“…The major signaling pathway of A 2B AR is suggested to be the pathway involving adenylyl cyclase (AC) that leads to an increase in intracellular cAMP levels and results in the subsequent activation of PKA and other cAMP effectors such as Epac (Peakman and Hill, 1994; Murakami et al, 2000; Sitaraman et al, 2001; Lynge et al, 2003; Fang and Olah, 2007; Darashchonak et al, 2014; He et al, 2014). However, the A 2B AR-Gq-PLC pathway also mediates several crucial functions of A 2B AR (Gao et al, 1999; Linden et al, 1999; Panjehpour et al, 2005), and A 2B AR further couples to the MAPK and arachidonic acid signaling pathways and regulates membrane ion channels probably through G-protein βγ subunits (Feoktistov et al, 1999; Jimenez et al, 1999; Schulte and Fredholm, 2003a,b; Donoso et al, 2005).…”

“…Thus, DCC was proposed to mediate netrin signaling in axon growth and guidance independently of A 2B AR activation (Stein et al, 2001). Intriguingly, more recent studies have reported that netrin-1 attenuates neutrophil transmigration and hypoxia-induced inflammation (Rosenberger et al, 2009), alveolar fluid clearance (He et al, 2014), and diabetic nephropathy (Tak et al, 2013) and induces cancer-cell invasion (Rodrigues et al, 2007) in an A 2B AR-dependent manner. These results appear to support the general notion that A 2B AR mediates the function of netrin-1 at least in certain tissues.…”