Aggravation of chemically-induced injury in perfused rat liver by extracellular ATP

The Role of Extracellular Adenosine Triphosphate in Ischemic Organ Injury

OBJECTIVES

Ischemic tissue injury contributes to significant morbidity and mortality and is implicated in a range of pathologic conditions, including but not limited to myocardial infarction, ischemic stroke, and acute kidney injury. The associated reperfusion phase is responsible for the activation of the innate and adaptive immune system, further accentuating inflammation. Adenosine triphosphate molecule has been implicated in various ischemic conditions, including stroke and myocardial infarction.

STUDY SELECTION

Adenosine triphosphate is a well-defined intracellular energy transfer and is commonly referred to as the body's "energy currency." However, Laboratory studies have demonstrated that extracellular adenosine triphosphate has the ability to initiate inflammation and is therefore referred to as a damage-associated molecular pattern. Purinergic receptors-dependent signaling, proinflammatory cytokine release, increased Ca influx into cells, and subsequent apoptosis have been shown to form a common underlying extracellular adenosine triphosphate molecular mechanism in ischemic organ injury.

CONCLUSIONS

In this review, we aim to discuss the molecular mechanisms behind adenosine triphosphate-mediated ischemic tissue injury and evaluate the role of extracellular adenosine triphosphate in ischemic injury in specific organs, in order to provide a greater understanding of the pathophysiology of this complex process. We also appraise potential future therapeutic strategies to limit damage in various organs, including the heart, brain, kidneys, and lungs.

Exogenous adenosine 5'-triphosphate does not improve survival in rats with acute liver failure

BACKGROUND

Acute liver failure is associated with a marked depletion of intrahepatic adenosine 5'-triphosphate (ATP), a compound required for the maintenance of hepatic function and enhanced hepatic regeneration.

AIM

The aim of this study was to test the safety and efficacy of exogenous ATP at various doses in a rat model of acute liver failure.

METHODS

Adult male Sprague-Dawley rates (n = 56) received an intraperitoneal dose (1.0 g/kg) of the potent hepatotoxin D: -galactosamine (D: -galN). Thereafter, rats were divided into groups that received saline (n = 18), low (n = 8), moderate (n = 18) or high (n = 12) doses of ATP for 7 days.

RESULTS

There was an inverse correlation between ATP dose and survival such that rats treated with low dose ATP had the highest survival rate (50%) compared to moderate (39%) and high (17%) dose treated groups. However, survival in all treated groups was similar (P = 0.085) to that of controls (45%). Liver biochemistry, regenerative activity and ATP levels were similar in the highest survival group (low dose ATP) versus controls.

CONCLUSION

These findings suggest that exogenous ATP does not improve and indeed at high doses may impair survival in rats with acute liver failure. Further studies involving a wider range of ATP doses and different routes and frequency of ATP administration are required to determine whether exogenous ATP has therapeutic value in the treatment of acute liver failure.

Purinergic receptors in gastrointestinal inflammation

Purinergic receptors comprise a family of transmembrane receptors that are activated by extracellular nucleosides and nucleotides. The two major classes of purinergic receptors, P1 and P2, are expressed widely in the gastrointestinal tract as well as immune cells. The purinergic receptors serve a variety of functions from acting as neurotransmitters, to autocoid and paracrine signaling, to cell activation and immune response. Nucleosides and nucleotide agonist of purinergic receptors are released by many cell types in response to specific physiological signals, and their levels are increased during inflammation. In the past decade, the advent of genetic knockout mice and the development of highly potent and selective agonists and antagonists for the purinergic receptors have significantly advanced the understanding of purinergic receptor signaling in health and inflammation. In fact, agonist/antagonists of purinergic receptors are emerging as therapeutic modalities to treat intestinal inflammation. In this article, the distribution of the purinergic receptors in the gastrointestinal tract and their physiological and pathophysiological role in intestinal inflammation will be reviewed.

The role of purinergic signaling in the liver and in transplantation: effects of extracellular nucleotides on hepatic graft vascular injury, rejection and metabolism

Extracellular nucleotides (e.g. ATP, UTP, ADP) are released by activated endothelium, leukocytes and platelets within the injured vasculature and bind specific cell-surface type-2 purinergic (P2) receptors. This process drives vascular inflammation and thrombosis within grafted organs. Importantly, there are also vascular ectonucleotidases i.e. ectoenzymes that hydrolyze extracellular nucleotides in the blood to generate nucleosides (viz. adenosine). Endothelial cell NTPDase1/CD39 has been shown to critically modulate levels of circulating nucleotides. This process tends to limit the activation of platelet and leukocyte expressed P2 receptors and also generates adenosine to reverse inflammatory events. This vascular protective CD39 activity is rapidly inhibited by oxidative reactions, such as is observed with liver ischemia reperfusion injury. In this review, we chiefly address the impact of these signaling cascades following liver transplantation. Interestingly, the hepatic vasculature, hepatocytes and all non-parenchymal cell types express several components co-ordinating the purinergic signaling response. With hepatic and vascular dysfunction, we note heightened P2- expression and alterations in ectonucleotidase expression and function that may predispose to progression of disease. In addition to documented impacts upon the vasculature during engraftment, extracellular nucleotides also have direct influences upon liver function and bile flow (both under physiological and pathological states). We have recently shown that alterations in purinergic signaling mediated by altered CD39 expression have major impacts upon hepatic metabolism, repair mechanisms, regeneration and associated immune responses. Future clinical applications in transplantation might involve new therapeutic modalities using soluble recombinant forms of CD39, altering expression of this ectonucleotidase by drugs and/or using small molecules to inhibit deleterious P2-mediated signaling while augmenting beneficial adenosine-mediated effects within the transplanted liver.

Rat hepatocytes express functional P2X receptors

Extracellular ATP regulates many hepatic functions by stimulating purinergic receptors. Only the G protein-coupled P2Y receptors have been studied in hepatocytes. We investigated the functional expression of P2X receptors, the ATP-gated channels in rat hepatocytes. P2X4 and P2X7 transcripts and proteins were detected by RT-PCR and by both Western blotting and immunocytochemistry. High concentrations of ATP, and 2'-and 3'-O-(4-benzoylbenzoyl)-ATP the preferring agonist of P2X7, induced membrane blebbing and significant uptake of 4-[(3-methyl-2(3H)-benzoxazolylidene)methyl]-1-[3-(triethylammonio)propyl]diiodide, both of which were inhibited by oxidised ATP, a blocker of P2X receptors. These results provide evidence that P2X4 and P2X7 receptors are expressed and functional on rat hepatocytes, possibly playing an important role in the purinergic signaling complex in these cells.