Study On The Role Of Adenosine A₁Receptor In The Pathogenesis Of Liver Disease

Adenosine is a natural metabolite in organisms, which plays an important role in many physiological and pathological processes. Extracellular adenosine concentrations are dynamically regulated by a variety of pathophysiological conditions. Adenosine once released can activate cell-surface adenosine receptors which in turn regulate cellular function. Adenosine receptors are coupled to G proteins, though altering the activity of a second messenger system, can modulate protein kinase signaling pathways and regulate downstream target gene expression. Because adenosine receptors mediate extracellular and intracellular signal transduction, they attract more and more attentions as potential targets for disease. There is growing evidence that adenosine receptors could be promising therapeutic targets in a wide range of diseases, including cerebral and cardiac ischemic diseases, sleep disorders, immune and inflammatory disorders, and cancer. However, adenosine receptor’s role and value in liver disease have not yet been fully revealed. In this study, by using adenosine receptor knockout animals, we examined the contribution of adenosine-adenosine receptor signaling pathway to liver metabolic disease, and demonstrated its regulation mechanism at the molecular level. We also evaluate the potential role of adenosine receptors as gene or drug targets in the prevention and treatment of liver disease.In this study, we found that adenosine A1receptor (A1AR) involves in the development of liver fibrosis. A1AR plays a contradictory role in carbon tetrachloride-(CCl4) and bile duct ligation-(BDL) induced liver fibrosis. Our results demonstrated that, lack of A1AR attenuates hepatic fibrosis resulting from chronic CCl4exposure, with markedly decreased collagen deposition and reduced hepatic stellate cell activation in A1AR-/-mice. Whereas, hepatic fibrosis caused by BDL ligation was aggravated in A1AR-/-mice, with significantly increased collagen synthesis and induced fibroblasts activation in portal tracts. We further investigated the different molecular mechanisms through which A1AR gene regulates CCl4-and BDL-induced hepatic fibrosis. A1AR deficiency reduces acute reactivity to liver injury induced by a single CCl4injection. In A1AR-/-mice, serum transaminase levels were lower and the extent of hepatocyte damage was reduced, which was probably achieved by down-regulating hepatic CYP2E1and UCP2gene expression. Besides, the levels of "profibrotic mediators", including TGFβ2, TNFα, IL-1, and IL-6were decreased in A/AR-/-mice during chronic CCl4treatment. Whereas in the BDL model, enhanced biliary infarcts and cholangiocyte proliferation due to elevation of bile acid levels should be the primary causes leading to increased fibrosis in A1AR-/-mice. These results indicate that A/AR participates in the pathogenesis of hepatic fibrosis with a complex mechanism, and the effect of targeting adenosine and its receptors in the prevention of hepatic fibrosis should be cautiously evaluated.We found that A1AR involves in the pathogenesis of intrahepatic cholestatic liver injury, and is a potential gene and drug target for the treatment of intrahepatic cholestatsis. In the mouse model of a-naphthylisothiocyanate-(ANIT) induced intrahepatic cholestasis, hepatic adenosine levels and A1AR expression were markedly increased, which suggested that adenosine-A1AR signal may participate in the development of ANIT-induced liver injury. Our study revealed that mice lacking A1AR are protected from ANIT induced liver injury as evidenced by lower serum liver enzyme levels and reduced extent of histological necrosis. Bile acid accumulation in liver and serum was markedly diminished in A1AR-/-mice compared with wild-type (WT) mice, however, biliary and urinary outputs of bile acids were significantly enhanced in A1AR-/-mice. In the liver, mRNA expression of genes related to bile acid transport Bsep and Mdr2and hydroxylation Cyp3all was increased in A1AR-/-mice. In the kidney, A1AR deficiency prevented the decrease of glomerular filtration rate caused by ANIT. In addition, treatment of mice with A1AR antagonist also protected against ANIT hepatotoxicity. Our results indicated that lack of A1AR gene protects mice from ANIT-induced cholestasis by enhancing toxic biliary constituents efflux through biliary excretory route and renal elimination system and suggested a potential role of A1AR as therapeutic target for the treatment of intrahepatic cholestasis.We also found that endogenous A1AR activation protects against acute ethanol-induced liver injury. Mice lacking A1AR were more susceptible to acute ethanol-induced liver damage than WT mice, which was evidenced by elevated serum transaminase levels and increased extent of histopathological changes. Ethanol induced triglycerides accumulation in the serum and liver, and this accumulation was augmented in A1AR-/-mice. Analysis of gene expression in the liver revealed up-regulated mRNA levels of lipogenic genes (including:FAS, SCD1, ACC1, DGAT2, and PPARy) in A1AR-/-mice after ethanol treatment. In addition, lack of A1AR aggravated lipid peroxidation and worsened antioxidants depletion which was caused by ethanol exposure. A subsequent study revealed that pharmacological block of A1AR also exacerbated liver injury and steatosis induced by acute ethanol administration. In conclusion, these results indicated that endogenous A1AR protects mice against acute ethanol-induced liver injury by reducing oxidative stress and decreasing lipid accumulation though down-regulation of lipogenesis-related genes. In addition, we found that lack of A1AR exaggerates the development of nonalcoholic fatty liver and insulin resistance. Compared with WT mice,"western diet" induced a series of metabolic abnormalities in A1AR-/-mice, such as increased body weight and fat, adipose tissue hypertrophy, high blood lipids, deteriorated hepatic steatosis and insulin resistance. Further studies showed that increased lipogenesis due to up-regulated mRNA expression of lipogenic genes (including:FAS, SCD1, ACC1, DGAT2, and PPARy) in A1AR-/-mice after high-fat diet feeding. Besides, lack of A1AR exaggerated inflammation and macrophage infiltration into white adipose tissue, and increased mRNA expression of F4/80, MCP-1, and TNFa. Inflammation and macrophage infiltration into adipose tissue appear to participate in the pathogenesis of obesity-induced insulin resistance. Moreover, A1AR deficiency enhanced expression of C/EBPa and PPARy, the major transcriptional regulators of adipocyte differentiation, which should contribute to increased adipogenesis and obesity in A1AR-/-mice. Taken together, our results indicated that endogenous A1AR activation protects mice from high fat diet-induced metabolic disorders, though inhibiting hepatic lipogenesis, decreasing adipocyte differentiation, and alleviating inflammation in adipose tissue.