Regulation of hepatic metabolism by extracellular nucleotides and eicosanoids. The role of cell heterogeneity

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Extracellular adenosine production by ecto-5'-nucleotidase protects during murine hepatic ischemic preconditioning

BACKGROUND & AIMS

The liver tolerates ischemia/reperfusion (IR) poorly. The discovery of ischemic preconditioning (IP) has raised hopes that natural pathways could be activated to increase hepatic resistance to ischemia. However, mechanisms of hepatic IP remain largely unknown. Extracellular adenosine has been implicated as an innate anti-inflammatory metabolite, particularly during ischemia. We investigated whether ecto-5'-nucleotidase (CD73), the "pacemaker" enzyme of extracellular adenosine production, is critical for hepatic protection by IP.

METHODS

Mice were subjected to 4 cycles of portal triad occlusion and reperfusion (3 minutes of ischemia/3 minutes of reperfusion) prior to IR or IR alone.

RESULTS

Hepatic IP was associated with a significant induction of CD73 transcript and protein. Targeted gene deletion or pharmacologic inhibition of CD73 abolished hepatic protection by IP as measured by lactate dehydrogenase, aspartate aminotransferase, and alanine aminotransferase serum levels or histologic injury. Increases in extracellular adenosine with IP were significantly attenuated in cd73-deficient (cd73(-/-)) mice. Reconstitution of cd73(-/-) mice with soluble 5'-nucleotidase resulted in complete restoration of hepatoprotection by IP, and hepatic injury following ischemia was attenuated by treatment of WT mice with soluble 5'-nucleotidase. Mice deficient in CD73 did not demonstrate the same degree of IP-dependent inhibition of acute phase complement gene expression/activation as did wild-type mice suggesting that extracellular adenosine attenuates hepatic IR via complement regulation.

CONCLUSIONS

Extracellular adenosine production by CD73 mediates protection during murine hepatic IP. Use of soluble 5'-nucleotidase may be a potential therapeutic for hepatic ischemia.

Regulation of cytosolic free calcium concentration by extracellular nucleotides in human hepatocytes

The effects of extracellular ATP and other nucleotides on the cytosolic free Ca2+ concentration ([Ca2+]i) have been studied in single primary human hepatocytes and in human Hep G2 and HuH-7 hepatoma cells. ATP, adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), and UTP caused a concentration-dependent biphasic increase in [Ca2+]i with an initial peak followed by a small sustained plateau in most cells. In some cells, however, repetitive Ca2+ transients were observed. The rank order of potency was ATP >/= UTP > ATPgammaS, and complete cross-desensitization of the Ca2+ responses occurred between ATP and UTP. The initial transient peak in [Ca2+]i was resistant to extracellular Ca2+ depletion, which demonstrates mobilization of internal Ca2+ by inositol 1,4,5-trisphosphate whose formation was enhanced by ATP and UTP. In contrast, the sustained plateau phase required influx of external Ca2+. Ca2+ influx occurs most likely through a capacitative Ca2+ entry mechanism, which was shown to exist in these cells by experiments performed with thapsigargin. On the molecular level, specific mRNA coding for the human P2Y1, P2Y2, P2Y4, and P2Y6 receptors could be detected by RT-PCR in Hep G2 and HuH-7 cells. However, ADP and UDP, which are agonists for P2Y1 and P2Y6 receptors, respectively, caused no changes in [Ca2+]i, demonstrating that these receptors are not expressed at a functional level. Likewise, alpha,beta-methylene-ATP, beta,gamma-methylene-ATP, AMP, and adenosine were inactive in elevating [Ca2+]i, suggesting that the ATP-induced increase in [Ca2+]i was not caused by activation of P2X or P1 receptors. Thus, on the basis of the pharmacological profile of the nucleotide-induced Ca2+-responses, extracellular ATP and UTP increase [Ca2+]i by activating P2Y2 and possibly P2Y4 receptors coupled to the Ca2+-phosphatidylinositol signaling cascade in human hepatocytes. This suggests that extracellular nucleotides from various sources may contribute to the regulation of human liver cell functions.

Steady-state binding of [3H]ATP to rat liver plasma membranes and competition by various purinergic agonists and antagonists

Steady-state analysis of nucleotide-binding sites on rat liver plasma membranes was carried out using 3H-labelled ATP as radioligand under complete inhibition of ecto-ATPase activity by excess EDTA. Binding of [3H]ATP to the membranes is saturable, reversible and apparently involves one population of specific binding sites with Kd of about 90 nM and binding capacity (Bmax) of 15 pmol/mg protein. A broad spectrum of purinergic agonists and antagonists was examined as potential inhibitors of the measured binding. The displacement studies showed the following rank order of inhibitory potency for [3H]ATP-binding sites (pIC50 values in parentheses): ATPgammaS (7.49)>2-MeSATP (7.18)>ATP (6.91)>ADPbetaS (6.64)>/=ADP (6.56)>>RB2 (6.14)>>suramin (5.40)>>Ap4A (4. 57)>alpha,beta-MeATP (4.19)>/=beta,gamma-MeATP (3.97). AMP, adenosine, Ap5A, PPADS, beta-glycerophosphate as well as non-adenine nucleoside triphosphates GTP, UTP and CTP did not exert any effect on the measured binding at concentration ranges of 10-6-10-4 M. In order to ascertain whether ATP and its analogues are capable of interacting with the same binding domain, 2-MeSATP and ADP were treated as alternative ligands that could compete with unlabelled ATP for its binding sites. A 2-fold increase of Kd value for ATP-receptor interaction was observed in the presence of 2-MeSATP (60 nM) or ADP (250 nM) without any modulation of Bmax value, confirming that inhibitory effects of these compounds are competitive in nature. These studies demonstrate that ATP and its analogues are able to interact with a single binding domain on liver plasma membranes, which may be identified as ligand-binding component of P2 purinoceptors of the P2Y1 subtype.

Reduction of the concentrations of prostaglandins E2 and F2alpha, and thromboxane B2 in cultured rat hepatocytes treated with the peroxisome proliferator ciprofibrate

Several hypolipidemic drugs, plasticizers and other chemicals induce peroxisome proliferation and hepatic tumors in rodents, but the mechanism by which they induce tumors is not fully understood. Their carcinogenic activity may be related to alterations in gene expression, such as induction of peroxisomal beta-oxidation enzymes or of the cytochrome P450 4A family. These enzymes metabolize lipids, including eicosanoids and their precursor fatty acids. Because eicosanoids likely play a role in the carcinogenic process, alterations in their concentration by xenobiotics may be important in their carcinogenic or promoting activities. In this study we used isolated hepatocytes to study if peroxisome proliferators alter the metabolism of prostaglandins (PG) and thromboxanes (Tx). Isolated rate hepatocytes were cultured for 4 days with 2 concentrations of ciprofibrate (CIP): 100 and 400 microM. Fatty acyl CoA oxidase activities of the 100 and 400 microM CIP treatment groups at the end of the experiment were increased 5.3 and 9.6 times, respectively. TxB2 and PGF2alpha concentrations in cultures treated with CIP were significantly lower than the control at days 3 and 4, whereas a lower concentration of PGE2 was seen at day 4 only. These studies show that PG and Tx concentrations in cultured hepatocytes are lowered by the peroxisome proliferator CIP.

Hepatic purine and pyrimidine metabolism: implications for antiviral chemotherapy of viral hepatitis

The use of nucleoside analogues as antiviral agents is expanding. For most nucleoside analogues, intracellular phosphorylation is the major prerequisite for activity. Antiviral activity may be limited by poor uptake, absence of appropriate activating enzymes, catabolism, and competition from endogenous nucleotides. Appreciation of these factors, which are species-, tissue- and cell-specific is important in the understanding of the pharmacology and toxicology of nucleoside analogues. The use of nucleoside analogues against the agents of viral hepatitis is inherently problematic for many reasons including active hepatic nucleoside catabolism, probable absence of virus-specific activating enzymes, competition from endogenous nucleotides synthesised de novo or derived from RNA turnover, and factors related to mitochondrial toxicity. Despite these drawbacks, some nucleoside analogues have been found efficacious against hepatitis B virus and it is likely that as knowledge of their mechanism of action accumulates, their efficacy can be improved both by rational drug design and by use in combination with other drugs, including interferon.

Peptidoleukotrienes increase the efflux of glutathione from perfused rat liver

The effect of peptidoleukotrienes (LT) on the efflux of glutathione (GSH) from the perfused rat liver was investigated. LTD4, C4 and E4 were infused at a final concentration of 20 nM into the portal vein of rat livers perfused with Krebs-Henseleit buffer. Perfusion pressure, efflux of glucose and release of LDH increased during the infusion of LTC4 and D4 and returned to baseline upon cessation of the infusion of LT. In contrast, the efflux of GSH did not change during the infusion of LT, but increased from 15 +/- 2 to 26 +/- 4 nmol/min.g liver 20 min after cessation of the infusion of LTC4. LTE4 did not increase the efflux of LDH, glucose, lactate, or GSH. During the LTC4- and LTD4-induced rise in perfusion pressure bile-flow decreased transiently by one third. The biliary excretion of GSH, however, decreased by an average of 75% and recovered more slowly than the cholestasis. In the presence of the selective LTD4 receptor antagonist LY171883 the effects of LTC4 and LTD4 were largely abolished. The delayed effects of LT on GSH efflux suggest that LT shift the efflux of GSH from the canalicular towards the sinusoidal side of the hepatocyte independent of other effects of LT on hepatic function. The sustained increase in efflux of GSH resulting from LT will raise the extracellular concentration of this antioxidant, such that more GSH is available at sites of inflammation to detoxify reactive oxygen species released by activated inflammatory cells.

Eicosanoids released following inhibition of the endoplasmic reticulum Ca2+ pump stimulate Ca2+ efflux in the perfused rat liver

In the isolated perfused rat liver 2,5-di(tert-butyl)hydroquinone (tBuHQ), a selective inhibitor of the endoplasmic reticulum Ca2+ pump, induces a prolonged glucose output and stimulates Ca2+ efflux. The present study shows that tBuHQ depleted the hormone-sensitive Ca2+ pool in the perfused liver, abolishing the vasopressin- or phenylephrine-induced Ca2+ efflux. The effects of tBuHQ were reversible, since the response to these agonists gradually returned within 1 hr of perfusion, and protein synthesis was not required for this recovery. Since tBuHQ does not cause Ca2+ efflux from isolated hepatocytes, we examined the mechanism responsible for the tBuHQ-induced Ca2+ efflux observed in the intact liver. The cyclooxygenase inhibitor indomethacin prevented the Ca2+ extrusion stimulated by tBuHQ, but not that induced by vasopressin. During infusion of tBuHQ there was a 9-fold increase in the concentration of thromboxane B2 in the perfusate. The Ca2+ efflux response to tBuHQ was inhibited by the thromboxane/prostaglandin endoperoxide receptor antagonist, L-655,240 (3-[1-(4-chlorobenzyl)-5-fluoro-3-methyl-indol-2-yl]2,2-dimethylpropa noic acid) in the absence of any effect on thromboxane B2 release. Thus, the inhibition of the endoplasmic reticulum Ca2+ pump by tBuHQ results in a rise in the cytosolic Ca2+ concentration in non-parenchymal cells, leading to the formation of cyclooxygenase products. The released eicosanoids, in turn, stimulate Ca2+ efflux from hepatocytes.

The complex interaction of ATP and UTP with isolated hepatocytes. How many receptors?

1. ATP exerts multiple receptor-mediated effects on isolated hepatocytes: glycogenolysis through the activation of glycogen phosphorylase (cAMP-independent, IP3/calcium-mediated), inactivation of glycogen synthase, inhibition of the glucagon effect on cAMP, activation of phospholipase D. The fact that some of these effects can be selectively altered and that they are not, or differently, reproduced by some other analogues of ATP, suggests the presence of more than one receptor. (i) Pertussis toxin abolishes the anti-glucagon effect of ATP without affecting its glycogenolytic effect. (ii) Single cell calcium measurements reveal major differences between ATP and ADP, (iii) 2MeSATP and ADP beta S, in clear contrast to ATP, barely increase the levels of IP3 and their glycogenolytic effects is completely blocked by phorbol ester treatment of hepatocytes. (iv) 2MeSATP differs from ADP beta S since it has no anti-glucagon effect. 2. Effects of UTP on isolated hepatocytes so far do not show any difference with effects of ATP, suggesting interaction with the same receptor(s). 3. It is proposed that liver plasma membranes contain (at least) three different receptors mediating (a) the activation of phospholipase C, (b) the activation of phospholipase D and (c) the inhibition of adenylate cyclase.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Survey of the occurrence of adenosine polyphosphatase in extracellular matrix of rat tissues. A cytochemical investigation

The extracellular presence of adenosine polyphosphatase was investigated in a number of rat tissues. The enzyme was demonstrated in basement membranes of epithelial cells of duodenum, urinary bladder, tongue, choroid plexus, submandibular salivary gland, lung and kidney, as well as in basement membranes of capillaries in these tissues. Furthermore adenosine polyphosphatase was demonstrated on collagen fibrils and in the cytoplasm of fibroblasts of all investigated tissues. It appears that the presence of adenosine polyphosphatase in basement membranes is a widespread phenomenon. Since extracellular ADP and ATP are known to promote respectively platelet aggregation and inflammation, the presence of extracellular ADP and ATP-hydrolyzing activity might contribute to inhibit these processes.

Prostanoid release of cultured liver sinusoidal endothelial cells in response to endotoxin and tumor necrosis factor. Comparison with umbilical vein endothelial cells

Vascular endothelial cells release prostanoids, especially prostacyclin, when properly stimulated. In addition to short acting stimuli like thrombin and histamine an increased prostanoid release occurs in the presence of endotoxin, interleukin 1 or tumor necrosis factor (TNF). The response of sinusoidal endothelial liver cells to such stimuli - probably important in hepatic inflammatory disease - is unknown. Sinusoidal endothelial liver cells from the guinea pig were isolated by centrifugal elutriation and investigated as confluent monolayers. Their prostanoid release in response to endotoxin and human recombinant TNF was determined by radioimmunoassays and compared to that obtained with cultured human umbilical vein endothelial cells. A pronounced time- and dose-dependent release of prostanoids was found with both cell types in response to endotoxin. In contrast to umbilical vein cells, liver endothelial cells produced not only large amounts of 6-keto-PGF1 alpha and some PGE2 but also thromboxane B2. Only umbilical vein endothelial cells responded to TNF with an increased 6-keto-PGF1 alpha release, emphasising the metabolic differences between both cell types.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.