Sustained Adenosine Exposure Causes Lung Endothelial Barrier Dysfunction via Nucleoside Transporter–Mediated Signaling

Abstract

Previous studies by our group as well as others have shown that acute adenosine exposure enhances lung vascular endothelial barrier integrity and protects against increased permeability lung edema. In contrast, there is growing evidence that sustained adenosine exposure has detrimental effects on the lungs, including lung edema. It is well established that adenosine modulates lung inflammation. However, little is known concerning the effect of sustained adenosine exposure on lung endothelial cells (ECs), which are critical to the maintenance of the alveolar-capillary barrier. We show that exogenous adenosine plus adenosine deaminase inhibitor caused sustained elevation of adenosine in lung ECs. This sustained adenosine exposure decreased EC barrier function, elevated cellular reactive oxygen species levels, and activated p38, JNK, and RhoA. Inhibition of equilibrative nucleoside transporters (ENTs) prevented sustained adenosine-induced p38 and JNK activation and EC barrier dysfunction. Inhibition of p38, JNK, or RhoA also partially attenuated sustained adenosine-induced EC barrier dysfunction. These data indicate that sustained adenosine exposure causes lung EC barrier dysfunction via ENT-dependent intracellular adenosine uptake and subsequent activation of p38, JNK, and RhoA. The antioxidant N-acetylcysteine and the NADPH inhibitor partially blunted sustained adenosine-induced JNK activation but were ineffective in attenuation of p38 activation or barrier dysfunction. p38 was activated exclusively in mitochondria, whereas JNK was activated in mitochondria and cytoplasm by sustained adenosine exposure. Our data further suggest that sustained adenosine exposure may cause mitochondrial oxidative stress, leading to activation of p38, JNK, and RhoA in mitochondria and resulting in EC barrier dysfunction.Keywords: adenosine deaminase; equilibrative nucleoside transporters; oxidative stress; MAP kinases; RhoA Increased permeability pulmonary edema is a life-threatening complication characteristic of acute lung injury (ALI) and acute respiratory distress syndrome. The purine nucleoside adenosine has been shown to protect (1-4) and cause (5) pulmonary edema and inflammation in animal models of ALI. The mechanisms governing these paradoxical effects of adenosine are poorly understood.Adenosine is a potent signaling molecule. Under homeostatic conditions, extracellular adenosine concentrations are low (40-600 nM) (6). However, extracellular adenosine levels are increased in response to tissue injury. High plasma adenosine (10-190 mM) has been reported in patients with sepsis-induced ALI (7, 8) and tissue ischemia (9). Adenosine is also significantly elevated in the lungs of animals with ALI caused by acute hypoxia (10-13), high tidal volume ventilation (14), endotoxin (15), and bleomycin (16). Acutely elevated adenosine has been shown to have a protective, antiinflammatory effect in various animal models of ALI (1-4). However, nonsurviving patients with sepsis have much higher plasma adenosine than survivors (7), suggesti...

Pulmonary Endothelial Cell Apoptosis in Emphysema and Acute Lung Injury

“…Activation of p38 has also been implicated in homocysteine-induced apoptosis of endothelial progenitor cells (Bao et al 2010) and cardiomyocytes (Wang et al 2011). We have shown that sustained exposure to exogenous adenosine causes mitochondrial defects and endothelial apoptosis via mitochondrial oxidative stress-induced activation of p38 (Lu et al 2012, 2013). Active p38 causes apoptosis by direct phosphorylation, and thus inhibition of Bcl-2 (De Chiara et al 2006; Farley et al 2006) and by increasing mitochondrial translocation of Bax (Capano and Crompton 2006).…”

Interrelated role of cigarette smoking, oxidative stress, and immune response in COPD and corresponding treatments

“…However, most importantly, through the stimulation of CD40, an important TNF receptor, IFN-␥ has been thought to induce the Fas-apoptotic pathway in epithelial cells and stimulate the enlargement of emphysematous airways (110). In addition, extracellular adenosine is often increased during tissue injury and inflammatory insults by the stimulation of ectonucleotidases, CD39 and CD73, or by variations in the activity and expression of adenosine deaminase (76). Recent research has shown that sustained, elevated adenosine exposure may contribute to CS-induced lung endothelial cell apoptosis and emphysema through the activation of p38 and JNK in mitochondria and eventually mitochondrial defects that also lead to cell apoptosis (77).…”

Blockade of equilibrative nucleoside transporter 1/2 protects against Pseudomonas aeruginosa–induced acute lung injury and NLRP3 inflammasome activation

“…21,22 In this study, we show that pharmacological inhibition of ENT1/2 significantly increased lung adenosine levels and attenuated P. aeruginosa-induced acute lung injury. 21,22 In this study, we show that pharmacological inhibition of ENT1/2 significantly increased lung adenosine levels and attenuated P. aeruginosa-induced acute lung injury.…”

“…[18][19][20] We have previously shown that sustained elevated adenosine causes lung endothelial cell barrier dysfunction and apoptosis via ENT1/2dependent mechanisms, 21,22 suggesting that intracellular adenosine uptake is detrimental to lung endothelial cells. ENT1 and ENT2 are major adenosine transporters expressed in lung microvascular endothelial cells and alveolar epithelial cells.…”

“…19 Whether inhibition of ENT1/2 protects against P. aeruginosainduced acute lung injury is unknown. 21, 22 We therefore hypothesized that blocking intracellular adenosine uptake by inhibition of ENT1/2 and simultaneous increase in extracellular adenosine signaling would protect against P. aeruginosa-induced acute lung injury. 19 Intracellular adenosine uptake adversely affects endothelial barrier function and causes endothelial apoptosis.…”