Purinergic signalling in the liver in health and disease

PANX1-mediated ATP release confers FAM3A's suppression effects on hepatic gluconeogenesis and lipogenesis

BACKGROUND

Extracellular adenosine triphosphate (ATP) is an important signal molecule. In previous studies, intensive research had revealed the crucial roles of family with sequence similarity 3 member A (FAM3A) in controlling hepatic glucolipid metabolism, islet β cell function, adipocyte differentiation, blood pressure, and other biological and pathophysiological processes. Although mitochondrial protein FAM3A plays crucial roles in the regulation of glucolipid metabolism via stimulating ATP release to activate P2 receptor pathways, its mechanism in promoting ATP release in hepatocytes remains unrevealed.

METHODS

db/db, high-fat diet (HFD)-fed, and global pannexin 1 (PANX1) knockout mice, as well as liver sections of individuals, were used in this study. Adenoviruses and adeno-associated viruses were utilized for in vivo gene overexpression or inhibition. To evaluate the metabolic status in mice, oral glucose tolerance test (OGTT), pyruvate tolerance test (PTT), insulin tolerance test (ITT), and magnetic resonance imaging (MRI) were conducted. Protein-protein interactions were determined by coimmunoprecipitation with mass spectrometry (MS) assays.

RESULTS

In livers of individuals and mice with steatosis, the expression of ATP-permeable channel PANX1 was increased (P < 0.01). Hepatic PANX1 overexpression ameliorated the dysregulated glucolipid metabolism in obese mice. Mice with hepatic PANX1 knockdown or global PANX1 knockout exhibited disturbed glucolipid metabolism. Restoration of hepatic PANX1 rescued the metabolic disorders of PANX1-deficient mice (P < 0.05). Mechanistically, ATP release is mediated by the PANX1-activated protein kinase B-forkhead box protein O1 (Akt-FOXO1) pathway to inhibit gluconeogenesis via P2Y receptors in hepatocytes. PANX1-mediated ATP release also activated calmodulin (CaM) (P < 0.01), which interacted with c-Jun N-terminal kinase (JNK) to inhibit its activity, thereby deactivating the transcription factor activator protein-1 (AP1) and repressing fatty acid synthase (FAS) expression and lipid synthesis (P < 0.05). FAM3A stimulated the expression of PANX1 via heat shock factor 1 (HSF1) in hepatocytes (P < 0.05). Notably, FAM3A overexpression failed to promote ATP release, inhibit the expression of gluconeogenic and lipogenic genes, and suppress gluconeogenesis and lipid deposition in PANX1-deficient hepatocytes and livers.

CONCLUSIONS

PANX1-mediated release of ATP plays a crucial role in maintaining hepatic glucolipid homeostasis, and it confers FAM3A's suppressive effects on hepatic gluconeogenesis and lipogenesis.

Purinergic ligands induce extracellular acidification and increased ATP turnover in HepG2 cells

Nucleosides and nucleotides at μM concentrations stimulated a 300% increase in acid secretion in HepG2 cells, which was quantitatively accounted for as increased export of lactate generated by glycogenolysis. Agonist selectivity encompassed nucleosides and nucleotides for all 5 natural nucleobases and, along with antagonist profiles, was inconsistent with a role for purinergic receptors in mediating this activity. Agonist catabolism did not contribute significantly to either low selectivity or lactate production. Lactate production was driven by an increase in ATP turnover of as much as 56%. For some agonists, especially adenosine, ATP turnover decreased precipitously at mM concentrations, correlating with known adenosine-stimulated apoptosis. We propose that nucleoside/nucleotide agonists induce a futile energy cycle via a novel mechanism, which results in increased ATP turnover and initiates a continuum of events that for some agonists culminates in apoptosis.

The Purinergic Nature of Pseudoxanthoma Elasticum

Pseudoxanthoma Elasticum (PXE) is an inherited disease characterized by elastic fiber calcification in the eyes, the skin and the cardiovascular system. PXE results from mutations in ABCC6 that encodes an ABC transporter primarily expressed in the liver and kidneys. It took nearly 15 years after identifying the gene to better understand the etiology of PXE. ABCC6 function facilitates the efflux of ATP, which is sequentially hydrolyzed by the ectonucleotidases ENPP1 and CD73 into pyrophosphate (PPi) and adenosine, both inhibitors of calcification. PXE, together with General Arterial Calcification of Infancy (GACI caused by ENPP1 mutations) as well as Calcification of Joints and Arteries (CALJA caused by NT5E/CD73 mutations), forms a disease continuum with overlapping phenotypes and shares steps of the same molecular pathway. The explanation of these phenotypes place ABCC6 as an upstream regulator of a purinergic pathway (ABCC6 → ENPP1 → CD73 → TNAP) that notably inhibits mineralization by maintaining a physiological Pi/PPi ratio in connective tissues. Based on a review of the literature and our recent experimental data, we suggest that PXE (and GACI/CALJA) be considered as an authentic "purinergic disease". In this article, we recapitulate the pathobiology of PXE and review molecular and physiological data showing that, beyond PPi deficiency and ectopic calcification, PXE is associated with wide and complex alterations of purinergic systems. Finally, we speculate on the future prospects regarding purinergic signaling and other aspects of this disease.

Intracellular metabolic profiling of drug resistant cells by surface enhanced Raman scattering

BACKGROUND

Intracellular metabolic profiling reveals real-time metabolic information useful for the study of underlying mechanisms of cells in particular conditions such as drug resistance. However, mass spectrometry (MS), one of the leading metabolomics technologies, usually requires a large number of cells and complex pretreatments. Surface enhanced Raman scattering (SERS) has an ultrahigh detection sensitivity and specificity, favorable for metabolomics analysis. However, some targeted SERS methods focus on very limited metabolite without global bioprofiling, and some label-free approaches try to fingerprint the metabolic response based on whole SERS spectral classification, but comprehensive interpretation of biological mechanisms was lacking. (95) RESULTS: We proposed a label-free SERS technique for intracellular metabolic profiling in complex cellular lysates within 3 min. We first compared three kinds of cellular lysis methods and sonication lysis shows the highest extraction efficiency of metabolites. To obtain comprehensive metabolic information, we collected a spectral set for each sample and further qualified them by the Pearson correlation coefficient (PCC) to calculate how many spectra should be acquired at least to gain the adequate information from a statistical and global view. In addition, according to our measurements with 10 pure metabolites, we can understand the spectra acquired from complex cellular lysates of different cell lines more precisely. Finally, we further disclosed the variations of 22 SERS bands in enzalutamide-resistant prostate cancer cells and some are associated with the androgen receptor signaling activity and the methionine salvage pathway in the drug resistance process, which shows the same metabolic trends as MS. (149) SIGNIFICANCE: Our technique has the capability to capture the intracellular metabolic fingerprinting with the optimized lysis approach and spectral set collection, showing high potential in rapid, sensitive and global metabolic profiling in complex biosamples and clinical liquid biopsy. This gives a new perspective to the study of SERS in insightful understanding of relevant biological mechanisms. (54).

Calcium signalling in hepatic metabolism: Health and diseases

The flexibility between the wide array of hepatic functions relies on calcium (Ca2+) signalling. Indeed, Ca2+ is implicated in the control of many intracellular functions as well as intercellular communication. Thus, hepatocytes adapt their Ca2+ signalling depending on their nutritional and hormonal environment, leading to opposite cellular functions, such as glucose storage or synthesis. Interestingly, hepatic metabolic diseases, such as obesity, type 2 diabetes and non-alcoholic fatty liver diseases, are associated with impaired Ca2+ signalling. Here, we present the hepatocytes' toolkit for Ca2+ signalling, complete with regulation systems and signalling pathways activated by nutrients and hormones. We further discuss the current knowledge on the molecular mechanisms leading to alterations of Ca2+ signalling in hepatic metabolic diseases, and review the literature on the clinical impact of Ca2+-targeting therapeutics.

CD73-generated extracellular adenosine promotes resolution of neutrophil-mediated tissue injury and restrains metaplasia in pancreatitis

Pancreatitis is currently the leading cause of gastrointestinal hospitalizations in the US. This condition occurs in response to abdominal injury, gallstones, chronic alcohol consumption or, less frequently, the cause remains idiopathic. CD73 is a cell surface ecto-5'-nucleotidase that generates extracellular adenosine, which can contribute to resolution of inflammation by binding adenosine receptors on infiltrating immune cells. We hypothesized genetic deletion of CD73 would result in more severe pancreatitis due to decreased generation of extracellular adenosine. CD73 knockout (CD73-/- ) and C57BL/6 (wild type, WT) mice were used to evaluate the progression and response of caerulein-induced acute and chronic pancreatitis. In response to caerulein-mediated chronic or acute pancreatitis, WT mice display resolution of pancreatitis at earlier timepoints than CD73-/- mice. Using immunohistochemistry and analysis of single-cell RNA-seq (scRNA-seq) data, we determined CD73 localization in chronic pancreatitis is primarily observed in mucin/ductal cell populations and immune cells. In murine pancreata challenged with caerulein to induce acute pancreatitis, we compared CD73-/- to WT mice and observed a significant infiltration of Ly6G+, MPO+, and Granzyme B+ cells in CD73-/- compared to WT pancreata and we quantified a significant increase in acinar-to-ductal metaplasia demonstrating sustained metaplasia and inflammation in CD73-/- mice. Using neutrophil depletion in CD73-/- mice, we show neutrophil depletion significantly reduces metaplasia defined by CK19+ cells per field and significantly reduces acute pancreatitis. These data identify CD73 enhancers as a potential therapeutic strategy for patients with acute and chronic pancreatitis as adenosine generation and activation of adenosine receptors is critical to resolve persistent inflammation in the pancreas.

Mitochondria Drive Immune Responses in Critical Disease

Mitochondria engage in multiple cellular and extracellular signaling pathways ranging from metabolic control, antiviral and antibacterial host defense to the modulation of inflammatory responses following cellular damage and stress. The remarkable contributions of these organelles to innate and adaptive immunity, shape cell phenotype and modulate their functions during infection, after trauma and in the setting of inflammatory disease. We review the latest knowledge of mitochondrial biology and then discuss how these organelles may impact immune cells to drive aberrant immune responses in critical disease.

Potential Implications of Exercise Training on Pannexin Expression and Function

Pannexins (PANX1, 2, 3) are channel-forming glycoproteins that are expressed throughout the cardiovascular and musculoskeletal system. The canonical function of these proteins is to release nucleotides that act as purinergic signalling at the cell membrane or Ca2+ channels at the endoplasmic reticulum membrane. These two forms of signalling are essential for autocrine and paracrine signalling in health, and alterations in this signalling have been implicated in the pathogenesis of many diseases. Many musculoskeletal and cardiovascular diseases are largely the result of a lack of physical activity which causes altered gene expression. Considering exercise training has been shown to alter a wide array of gene expression in musculoskeletal tissues, understanding the interaction between exercise training, gene function and expression in relevant diseases is warranted. With regards to pannexins, multiple publications have shown that exercise training can influence pannexin expression and may influence the significance of its function in certain diseases. This review further discusses the potential interaction between exercise training and pannexin biology in relevant tissues and disease models. We propose that exercise training in relevant animal and human models will provide a more comprehensive understanding of the implications of pannexin biology in disease.

Therapeutic potential for P2Y2 receptor antagonism

G protein-coupled receptors are the target of more than 30% of all FDA-approved drug therapies. Though the purinergic P2 receptors have been an attractive target for therapeutic intervention with successes such as the P2Y₁₂ receptor antagonist, clopidogrel, P2Y₂ receptor (P2Y₂R) antagonism remains relatively unexplored as a therapeutic strategy. Due to a lack of selective antagonists to modify P2Y₂R activity, studies using primarily genetic manipulation have revealed roles for P2Y₂R in a multitude of diseases. These include inflammatory and autoimmune diseases, fibrotic diseases, renal diseases, cancer, and pathogenic infections. With the advent of AR-C118925, a selective and potent P2Y₂R antagonist that became commercially available only a few years ago, new opportunities exist to gain a more robust understanding of P2Y₂R function and assess therapeutic effects of P2Y₂R antagonism. This review discusses the characteristics of P2Y₂R that make it unique among P2 receptors, namely its involvement in five distinct signaling pathways including canonical Gα_q protein signaling. We also discuss the effects of other P2Y₂R antagonists and the pivotal development of AR-C118925. The remainder of this review concerns the mounting evidence implicating P2Y₂Rs in disease pathogenesis, focusing on those studies that have evaluated AR-C118925 in pre-clinical disease models.

Type 2 Diabetes Mellitus (T2DM) and Carbohydrate Metabolism in Relation to T2DM from Endocrinology, Neurophysiology, Molecular Biology, and Biochemistry Perspectives

Type 2 diabetes mellitus (T2DM) is a severe disease caused by metabolic disorders, particularly carbohydrate metabolism disorders. The disease is a fatal global trouble characterised by high prevalence rates, causing death, blindness, kidney failure, myocardial infarction, amputation of lower limps, and stroke. Biochemical metabolic pathways like glycolysis, gluconeogenesis, glycogenesis, and glycogenolysis are critical pathways that regulate blood glucose levels with the glucokinase (GK) enzyme playing a central role in glucose homeostasis. Any factor that perturbs the aforementioned biochemical pathways is detrimental. Endocrinological, neurophysiological, and molecular biological pathways that are linked to carbohydrate metabolism should be studied, grasped, and manipulated in order to alleviate T2DM global chaos. The challenge, howbeit, is that, since the body is an integration of systems that complement one another, studying one “isolated” system is not very useful. This paper serves to discuss endocrinology, neurophysiology, and molecular biology pathways that are involved in carbohydrate metabolism in relation to T2DM.

Purinergic and Adenosinergic Signaling in Pancreatobiliary Diseases

Adenosine 5'-triphosphate (ATP), other nucleotides, and the nucleoside analogue, adenosine, all have the capacity to modulate cellular signaling pathways. The cellular processes linked to extracellular purinergic signaling are crucial in the initiation, evolution, and resolution of inflammation. Injured or dying cells in the pancreatobiliary tract secrete or release ATP, which results in sustained purinergic signaling mediated through ATP type-2 purinergic receptors (P2R). This process can result in chronic inflammation, fibrosis, and tumor development. In contrast, signaling via the extracellular nucleoside derivative adenosine via type-1 purinergic receptors (P1R) is largely anti-inflammatory, promoting healing. Failure to resolve inflammation, as in the context of primary sclerosing cholangitis or chronic pancreatitis, is a risk factor for parenchymal and end-organ scarring with the associated risk of pancreatobiliary malignancies. Emerging immunotherapeutic strategies suggest that targeting purinergic and adenosinergic signaling can impact the growth and invasive properties of cancer cells, potentiate anti-tumor immunity, and also block angiogenesis. In this review, we dissect out implications of disordered purinergic responses in scar formation, end-organ injury, and in tumor development. We conclude by addressing promising opportunities for modulation of purinergic/adenosinergic signaling in the prevention and treatment of pancreatobiliary diseases, inclusive of cancer.

A hormone complex of FABP4 and nucleoside kinases regulates islet function

The liberation of energy stores from adipocytes is critical to support survival in times of energy deficit; however, uncontrolled or chronic lipolysis associated with insulin resistance and/or insulin insufficiency disrupts metabolic homeostasis^1,2. Coupled to lipolysis is the release of a recently identified hormone, fatty-acid-binding protein 4 (FABP4)³. Although circulating FABP4 levels have been strongly associated with cardiometabolic diseases in both preclinical models and humans^4-7, no mechanism of action has yet been described^8-10. Here we show that hormonal FABP4 forms a functional hormone complex with adenosine kinase (ADK) and nucleoside diphosphate kinase (NDPK) to regulate extracellular ATP and ADP levels. We identify a substantial effect of this hormone on beta cells and given the central role of beta-cell function in both the control of lipolysis and development of diabetes, postulate that hormonal FABP4 is a key regulator of an adipose-beta-cell endocrine axis. Antibody-mediated targeting of this hormone complex improves metabolic outcomes, enhances beta-cell function and preserves beta-cell integrity to prevent both type 1 and type 2 diabetes. Thus, the FABP4-ADK-NDPK complex, Fabkin, represents a previously unknown hormone and mechanism of action that integrates energy status with the function of metabolic organs, and represents a promising target against metabolic disease.

Receptor-specific Ca2+ oscillation patterns mediated by differential regulation of P2Y purinergic receptors in rat hepatocytes

Extracellular agonists linked to inositol-1,4,5-trisphosphate (IP₃) formation elicit cytosolic Ca²⁺ oscillations in many cell types, but despite a common signaling pathway, distinct agonist-specific Ca²⁺ spike patterns are observed. Using qPCR, we show that rat hepatocytes express multiple purinergic P2Y and P2X receptors (R). ADP acting through P2Y1R elicits narrow Ca²⁺ oscillations, whereas UTP acting through P2Y2R elicits broad Ca²⁺ oscillations, with composite patterns observed for ATP. P2XRs do not play a role at physiological agonist levels. The discrete Ca²⁺ signatures reflect differential effects of protein kinase C (PKC), which selectively modifies the falling phase of the Ca²⁺ spikes. Negative feedback by PKC limits the duration of P2Y1R-induced Ca²⁺ spikes in a manner that requires extracellular Ca²⁺. By contrast, P2Y2R is resistant to PKC negative feedback. Thus, the PKC leg of the bifurcated IP₃ signaling pathway shapes unique Ca²⁺ oscillation patterns that allows for distinct cellular responses to different agonists.

•

Distinct stereotypic Ca²⁺ oscillations are elicited by P2Y1 and P2Y2 receptors

•

P2X receptors do not contribute to the generation of Ca²⁺ oscillations

•

Agonist-specific Ca²⁺ spike shapes reflect discrete modes of PKC negative feedback

•

Bifurcation of IP₃/PKC signaling yields unique Ca²⁺ oscillation signatures

Synapomorphic features of hepatic and pulmonary vasculatures include comparable purinergic signaling responses in host defense and modulation of inflammation

Hepatosplanchnic and pulmonary vasculatures constitute synapomorphic, highly comparable networks integrated with the external environment. Given functionality related to obligatory requirements of "feeding and breathing," these organs are subject to constant environmental challenges entailing infectious risk, antigenic and xenobiotic exposures. Host responses to these stimuli need to be both protective and tightly regulated. These functions are facilitated by dualistic, high-low pressure blood supply of the liver and lungs, as well as tolerogenic characteristics of resident immune cells and signaling pathways. Dysregulation in hepatosplanchnic and pulmonary blood flow, immune responses, and microbiome implicate common pathogenic mechanisms across these vascular networks. Hepatosplanchnic diseases, such as cirrhosis and portal hypertension, often impact lungs and perturb pulmonary circulation and oxygenation. The reverse situation is also noted with lung disease resulting in hepatic dysfunction. Others, and we, have described common features of dysregulated cell signaling during liver and lung inflammation involving extracellular purines (e.g., ATP, ADP), either generated exogenously or endogenously. These metabokines serve as danger signals, when released by bacteria or during cellular stress and cause proinflammatory and prothrombotic signals in the gut/liver-lung vasculature. Dampening of these danger signals and organ protection largely depends upon activities of vascular and immune cell-expressed ectonucleotidases (CD39 and CD73), which convert ATP and ADP into anti-inflammatory adenosine. However, in many inflammatory disorders involving gut, liver, and lung, these protective mechanisms are compromised, causing perpetuation of tissue injury. We propose that interventions that specifically target aberrant purinergic signaling might prevent and/or ameliorate inflammatory disorders of the gut/liver and lung axis.

P2Y2R Deficiency Ameliorates Hepatic Steatosis by Reducing Lipogenesis and Enhancing Fatty Acid β-Oxidation through AMPK and PGC-1α Induction in High-Fat Diet-Fed Mice

Non-alcoholic fatty liver disease (NAFLD) is a chronic metabolic liver disease associated with obesity and insulin resistance. Activation of the purinergic receptor P2Y2R has been reported to promote adipogenesis, inflammation and dyslipidemia in adipose tissues in obese mice. However, the role of P2Y2R and its mechanisms in NAFLD remain unknown. We hypothesized that P2Y2R deficiency may play a protective role in NAFLD by modulating lipid metabolism in the liver. In this study, we fed wild type and P2Y2R knockout mice with a high-fat diet (HFD) for 12 weeks and analyzed metabolic phenotypes. First, P2Y2R deficiency effectively improved insulin resistance with a reduction in body weight and plasma insulin. Second, P2Y2R deficiency attenuated hepatic lipid accumulation and injury with reduced alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels. Third, P2Y2R deficiency decreased the expression of fatty acid synthesis mediators (cluster of differentiation (CD36), fatty acid synthase (FAS), and stearoyl-CoA desaturase 1 (SCD1)); and increased the expression of adipose triglyceride lipase (ATGL), a lipolytic enzyme. Mechanistically, P2Y2R deficiency increased the AMP-activated protein kinase (AMPK) activity to improve mitochondrial fatty acid β-oxidation (FAO) by regulating acetyl-CoA carboxylase (ACC) and carnitine palmitoyltransferase 1A (CPT1A)-mediated FAO pathway. In addition, P2Y2R deficiency increased peroxisome proliferator-activated gamma co-activator-1α (PGC-1α)-mediated mitochondrial biogenesis. Conclusively, P2Y2R deficiency ameliorated HFD-induced hepatic steatosis by enhancing FAO through AMPK signaling and PGC-1α pathway, suggesting P2Y2R as a promising therapeutic target for NAFLD.

Adenosine A3 receptor activated in H2O2 oxidative stress of primary open-angle glaucoma

Background

Primary open-angle glaucoma (POAG), as one of the leading reasons for blindness, is mainly due to trabecular meshwork (TM) dysfunction. Bioinformatics analysis was used to find related genes involved in TM oxidative stress, which is a major cause of TM fibrosis.

Methods

A total of three datasets from the Gene Expression Omnibus (GEO) database were used to identify differentially expressed genes (DEGs). Gene expression relationships were enriched by the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and Gene Ontology (GO) pathways. The interaction network was listed by the protein-protein interaction (PPI) network. The expression of adenosine A3 receptor (ADORA3) was validated in POAG tissue and human trabecular meshwork cells (HTMCs) by western blot (WB) and reverse transcription polymerase chain reaction (RT-PCR). Additionally, WB and RT-PCR were used to measure oxidative stress injury relative protein and gene expression, respectively, such as fibronectin (FN), collagen-I (Col-I), and α-smooth muscle actin (α-SMA). Cell migration function and vitality were tested via transwell migration assay and Cell Counting Kit-8 (CCK-8). The cell vitality was measured using CCK-8.

Results

A total of 61 significant DEGs among the three data sources were analyzed. Among all three different datasets, two significant DEGs [ADORA3 and DNA damage-inducible transcript 4 protein (DDIT4)] were identified. The dataset ADORA3 was selected for further analysis. In the POAG TM tissue, ADORA3 was overexpressed at transcriptional and post-transcriptional levels. Overexpression of ADORA3 reduced TMC viability and migration but upregulated the extracellular matrix (ECM) proteins (FN, Col-I, and α-SMA) expression. It was found that ADORA3 can exacerbate oxidative stress injury in normal TMCs. These results indicated that ADORA3 might play an essential role in the occurrence and progression of POAG.

Conclusions

A total of 61 novel common DEGs identified are related to the development and prognosis of POAG. In the POAG, ADORA3 was verified as overexpressed; therefore, it may be associated with an oxidative stress injury in TMCs.

Hypoxia-inducible factor-1α-dependent induction of miR122 enhances hepatic ischemia tolerance

Hepatic ischemia and reperfusion (IR) injury contributes to the morbidity and mortality associated with liver transplantation. microRNAs (miRNAs) constitute a family of noncoding RNAs that regulate gene expression at the posttranslational level through the repression of specific target genes. Here, we hypothesized that miRNAs could be targeted to enhance hepatic ischemia tolerance. A miRNA screen in a murine model of hepatic IR injury pointed us toward the liver-specific miRNA miR122. Subsequent studies in mice with hepatocyte-specific deletion of miR122 (miR122loxP/loxP Alb-Cre+ mice) during hepatic ischemia and reperfusion revealed exacerbated liver injury. Transcriptional studies implicated hypoxia-inducible factor-1α (HIF1α) in the induction of miR122 and identified the oxygen-sensing prolyl hydroxylase domain 1 (PHD1) as a miR122 target. Further studies indicated that HIF1α-dependent induction of miR122 participated in a feed-forward pathway for liver protection via the enhancement of hepatic HIF responses through PHD1 repression. Moreover, pharmacologic studies utilizing nanoparticle-mediated miR122 overexpression demonstrated attenuated liver injury. Finally, proof-of-principle studies in patients undergoing orthotopic liver transplantation showed elevated miR122 levels in conjunction with the repression of PHD1 in post-ischemic liver biopsies. Taken together, the present findings provide molecular insight into the functional role of miR122 in enhancing hepatic ischemia tolerance and suggest the potential utility of pharmacologic interventions targeting miR122 to dampen hepatic injury during liver transplantation.

Structural and Functional Features of the P2X4 Receptor: An Immunological Perspective

Extracellular nucleotides are important mediators of activation, triggering various responses through plasma membrane P2 and P1 receptors. P2 receptors are further subdivided into ionotropic P2X receptors and G protein-coupled P2Y receptors. P2X4 is an ATP-gated cation channel broadly expressed in most tissues of the body. Within the P2X family, P2X4 has a unique subcellular distribution, being preferentially localized in lysosomes. In these organelles, high ATP concentrations do not trigger P2X4 because of the low pH. However, when the pH increases to 7.4, P2X4 can be stimulated by intra-lysosomal ATP, which is in its active, tetra-anionic form. Elucidation of P2X4, P2X3 and P2X7 structures has shed some light on the functional differences between these purinergic receptors. The potential interaction between P2X4 and P2X7 has been extensively studied. Despite intensive effort, it has not been possible yet to determine whether P2X4 and P2X7 interact as heterotrimers or homotrimers at the plasma membrane. However, several publications have shown that functional interactions between P2X4 and P2X7 do occur. Importantly, these studies indicate that P2X4 potentiates P2X7-dependent activation of inflammasomes, leading to increased release of IL-1β and IL-18. The role of P2X4 in various diseases could be beneficial or deleterious even though the pathophysiological mechanisms involved are still poorly defined. However, in diseases whose physiopathology involves activation of the NLRP3 inflammasome, P2X4 was found to exacerbate severity of disease. The recent production of monoclonal antibodies specific for the human and mouse P2X4, some of which are endowed with agonist or antagonist properties, raises the possibility that they could be used therapeutically. Analysis of single nucleotide polymorphisms of the human P2RX4 gene has uncovered the association of P2RX4 gene variants with susceptibility to several human diseases.

Clodronate, an inhibitor of the vesicular nucleotide transporter, ameliorates steatohepatitis and acute liver injury

The vesicular nucleotide transporter (VNUT) is responsible for the vesicular storage and release of ATP from various ATP-secreting cells, and it plays an essential role in purinergic signaling. Although extracellular ATP and its degradation products are known to mediate various inflammatory responses via purinoceptors, whether vesicular ATP release affects steatohepatitis and acute liver injury is far less understood. In the present study, we investigated the effects of clodronate, a potent and selective VNUT inhibitor, on acute and chronic liver inflammation in mice. In a model of methionine/choline-deficient diet-induced non-alcoholic steatohepatitis (NASH), the administration of clodronate reduced hepatic inflammation, fibrosis, and triglyceride accumulation. Clodronate also protected mice against high-fat/high-cholesterol diet-induced steatohepatitis. Moreover, prophylactic administration of clodronate prevented d-galactosamine and lipopolysaccharide-induced acute liver injury by reducing inflammatory cytokines and hepatocellular apoptosis. In vitro, clodronate inhibited glucose-induced vesicular ATP release mediated by VNUT and reduced the intracellular level and secretion of triglycerides in isolated hepatocytes. These results suggest that VNUT-dependent vesicular ATP release plays a crucial role in the recruitment of immune cells, cytokine production, and the aggravation of steatosis in the liver. Pharmacological inhibition of VNUT may provide therapeutic benefits in liver inflammatory disorders, including NASH and acute toxin-induced injury.

Vesicular ATP release from hepatocytes plays a role in the progression of nonalcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH) is becoming a growing public health problem along with the increase of metabolic syndrome worldwide. Extracellular nucleotides are known to serve as a danger signal by initiating purinergic signaling in many inflammatory disorders, although the role of purinergic signaling in the progression of NASH remains to be clarified. Vesicular nucleotide transporter (VNUT) is a key molecule responsible for vesicular ATP release to initiate purinergic signaling. Here, we studied the role of VNUT in the progression of nonalcoholic steatohepatitis. VNUT was expressed in mouse hepatocytes and associated, at least in part, with apolipoprotein B (apoB)-containing vesicles. High glucose stimulation evoked release of appreciable amount of ATP from hepatocytes, which disappeared in hepatocytes of Vnut knockout (Vnut-/-) mice. Glucose treatment also stimulated triglyceride secretion from hepatocytes, which was inhibited by PPADS and MRS211, antagonists of P2Y receptors, and clodronate, a VNUT inhibitor, and was significantly reduced in Vnut-/- mice. In vivo, postprandial secretion of triglyceride from hepatocytes was observed, while the serum triglyceride level was significantly reduced in Vnut-/- mice. On a high-fat diet, the liver of wild type mice exhibited severe inflammation, fibrosis, and macrophage infiltration, which is similar to NASH in humans, while this NASH pathology was not observed in Vnut-/- mice. These results suggest that VNUT-mediated vesicular ATP release regulates triglyceride secretion and involves in chronic inflammation in hepatocytes. Since blockade of vesicular ATP release protects against progression of steatohepatitis, VNUT may be a pharmacological target for NASH.

The Purinergic P2X7 Receptor-NLRP3 Inflammasome Pathway: A New Target in Alcoholic Liver Disease?

The World Health Organization has estimated that approximately 3 million deaths are attributable to alcohol consumption each year. Alcohol consumption is notably associated with the development and/or progression of many non-communicable inflammatory diseases-particularly in the liver. Although these alcoholic liver diseases were initially thought to be caused by the toxicity of ethanol on hepatocytes, the latest research indicates Kupffer cells (the liver macrophages) are at the heart of this "inflammatory shift". Purinergic signaling (notably through P2X7 receptors and the NLRP3 inflammasome) by Kupffer cells appears to be a decisive factor in the pathophysiology of alcoholic liver disease. Hence, the modulation of purinergic signaling might represent a new means of treating alcoholic liver disease. Here, we review current knowledge on the pathophysiology of alcoholic liver diseases and therapeutic perspectives for targeting these inflammatory pathways.

CD73 Maintains Hepatocyte Metabolic Integrity and Mouse Liver Homeostasis in a Sex-Dependent Manner

BACKGROUND & AIMS

Metabolic imbalance and inflammation are common features of chronic liver diseases. Molecular factors controlling these mechanisms represent potential therapeutic targets. CD73 is the major enzyme that dephosphorylates extracellular adenosine monophosphate (AMP) to form the anti-inflammatory adenosine. CD73 is expressed on pericentral hepatocytes, which are important for long-term liver homeostasis. We aimed to determine if CD73 has nonredundant hepatoprotective functions.

METHODS

Liver-specific CD73 knockout (CD73-LKO) mice were generated by targeting the Nt5e gene in hepatocytes. The CD73-LKO mice and hepatocytes were characterized using multiple approaches.

RESULTS

Deletion of hepatocyte Nt5e resulted in an approximately 70% reduction in total liver CD73 protein (P < .0001). Male and female CD73-LKO mice developed normally during the first 21 weeks without significant liver phenotypes. Between 21 and 42 weeks, the CD73-LKO mice developed spontaneous-onset liver disease, with significant severity in male mice. Middle-aged male CD73-LKO mice showed hepatocyte swelling and ballooning (P < .05), inflammation (P < .01), and variable steatosis. Female CD73-LKO mice had lower serum albumin levels (P < .05) and increased inflammatory genes (P < .01), but did not show the spectrum of histopathologic changes in male mice, potentially owing to compensatory induction of adenosine receptors. Serum analysis and proteomic profiling of hepatocytes from male CD73-LKO mice showed significant metabolic imbalance, with increased blood urea nitrogen (P < .0001) and impairments in major metabolic pathways, including oxidative phosphorylation and AMP-activated protein kinase (AMPK) signaling. There was significant hypophosphorylation of AMPK substrates in CD73-LKO livers (P < .0001), while in isolated hepatocytes treated with AMP, soluble CD73 induced AMPK activation (P < .001).

CONCLUSIONS

Hepatocyte CD73 supports long-term metabolic liver homeostasis through AMPK in a sex-dependent manner. These findings have implications for human liver diseases marked by CD73 dysregulation.

Targeting ectonucleotidases to treat inflammation and halt cancer development in the gut

CD39 and CD73 control cell immunity by hydrolyzing proinflammatory ATP and ADP (CD39) into AMP, subsequently converted into anti-inflammatory adenosine (CD73). By regulating the balance between effector and regulatory cells, these ectonucleotidases promote immune homeostasis in acute and chronic inflammation; while also appearing to limit antitumor effector immunity in gut cancer. This manuscript focuses on the pivotal role of CD39 and CD73 ectonucleotidase function in shaping immune responses in the gut. We focus on those mechanisms deployed by these critical and pivotal ectoenzymes and the regulation in the setting of gastrointestinal tract infections, inflammatory bowel disease and tumors of the gastrointestinal tract. We will highlight translational and clinical implications of the latest and most innovative basic research discoveries of these important players of the purinergic signaling. Immunotherapeutic strategies that have been developed to either boost or control ectonucleotidase expression and activity in important disease settings are also reviewed and the in vivo effects discussed.

[VNUT Is a Therapeutic Target for Type 2 Diabetes and NASH]

ATP, used in cells as an energy currency, also acts as an extracellular signaling molecule. Studies of purinergic receptor subtypes have revealed that purinergic chemical transmission plays important roles in various cell types. The vesicular nucleotide transporter (VNUT), the ninth transporter in the SLC17 organic anion transporter family, is essential for vesicular ATP storage and its subsequent release. The VNUT is localized on the membrane of secretory vesicles and actively transports ATP into vesicles using an electrochemical gradient of protons supplied by vacuolar proton ATPase (V-ATPase) as a driving force. ATP acts as a damage-associated molecular pattern (DAMPs), contributing to the persistence of chronic inflammation. Chronic inflammation induces systemic insulin resistance, which is the underlying pathology of type 2 diabetes and non-alcoholic fatty liver disease (NAFLD), ranging from simple steatosis to non-alcoholic steatohepatitis (NASH). We previously demonstrated that ATP transported in insulin granules via the VNUT negatively regulates insulin secretion. We also found that hepatocytes release ATP in a VNUT-dependent manner, which elicits hepatic insulin resistance and inflammation. VNUT-knockout mice exhibited improved glucose tolerance and were resistant to the development of high fat diet-induced NAFLD. In this article, we summarize recent advances in our understanding of the mechanism of the VNUT, the development of inhibitors, and its pathological involvement in type 2 diabetes and NAFLD. The pharmacological inhibition of the VNUT may represent a potential therapeutic approach for the treatment of metabolic diseases.

Purinergic Signaling in Liver Pathophysiology

Extracellular nucleosides and nucleotides activate a group of G protein-coupled receptors (GPCRs) known as purinergic receptors, comprising adenosine and P2Y receptors. Furthermore, purinergic P2X ion channels are activated by ATP. These receptors are expressed in liver resident cells and play a critical role in maintaining liver function. In the normal physiology, these receptors regulate hepatic metabolic processes such as insulin responsiveness, glycogen and lipid metabolism, and bile secretion. In disease states, ATP and other nucleotides serve as danger signals and modulate purinergic responses in the cells. Recent studies have demonstrated that purinergic receptors play a significant role in the development of metabolic syndrome associated non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), fibrosis, hepatocellular carcinoma (HCC) and liver inflammation. In this concise review, we dissect the role of purinergic signaling in different liver resident cells involved in maintaining healthy liver function and in the development of the above-mentioned liver pathologies. Moreover, we discuss potential therapeutic strategies for liver diseases by targeting adenosine, P2Y and P2X receptors.

Purinergic signaling in diabetes and metabolism

Purinergic signaling, a concept originally formulated by the late Geoffrey Burnstock (1929-2020), was found to modulate pathways in every physiological system. In metabolic disorders there is a role for both adenosine receptors and P2 (nucleotide) receptors, of which there are two classes, i.e. P2Y metabotropic and P2X ionotropic receptors. The individual roles of the 19 receptors encompassed by this family have been dissected - and in many cases the effects associated with specific cell types, including adipocytes, skeletal muscle, liver cells and immune cells. It is suggested that ligands selective for each of the four adenosine receptors (A₁, A_2A, A_2B and A₃), and several of the P2 subtypes (e.g. P2Y₆ or P2X7 antagonists) might have therapeutic potential for treating diabetes and obesity. This is a developing story with some conflicting conclusions relevant to drug discovery, which we summarize here.

P2X7 receptor deletion attenuates oxidative stress and liver damage in sepsis

Sepsis is a severe disease characterized by an uncontrolled systemic inflammation and consequent organ dysfunction generated in response to an infection. Extracellular ATP acting through the P2X7 receptor induces the maturation and release of pro-inflammatory cytokines (i.e., IL-1β) and the production of reactive nitrogen and oxygen species that lead to oxidative tissue damage. Here, we investigated the role of the P2X7 receptor in inflammation, oxidative stress, and liver injury in sepsis. Sepsis was induced by cecal ligation and puncture (CLP) in wild-type (WT) and P2X7 knockout (P2X7-/-) mice. The oxidative stress in the liver of septic mice was assessed by 2',7'-dichlorofluorescein oxidation reaction (DCF), thiobarbituric acid-reactive substances (TBARS), and nitrite levels dosage. The status of the endogenous defense system was evaluated through catalase (CAT) and superoxide dismutase (SOD) activities. The inflammation was assessed histologically and by determining the expression of inflammatory cytokines and chemokines by RT-qPCR. We observed an increase in the reactive species and lipid peroxidation in the liver of septic WT mice, but not in the liver from P2X7-/- animals. We found an imbalance SOD/CAT ratio, also only WT septic animals. The number of inflammatory cells and the gene expression of IL-1 β, IL-6, TNF-α, IL-10, CXCL1, and CXCL2 were higher in the liver of WT septic mice in comparison to P2X7-/- septic animals. In summary, our results suggest that the P2X7 receptor might be a therapeutic target to limit oxidative stress damage and liver injury during sepsis.

CD73 regulates hepatic stellate cells activation and proliferation through Wnt/β-catenin signaling pathway

Alcoholic liver fibrosis (ALF) is commonly associated with long-term alcohol consumption and the activation of hepatic stellate cells (HSCs). Inhibiting the activation and proliferation of HSCs is a critical step to alleviate liver fibrosis. Increasing evidence indicates that ecto-5'-nucleotidase (CD73) plays a vital role in liver disease as a critical component of extracellular adenosine pathway. However, the regulatory role of CD73 in ALF has not been elucidated. In this study, both ethanol plus CCl₄-induced liver fibrosis mice model and acetaldehyde-activated HSC-T6 cell model were employed and the expression of CD73 was consistently elevated in vivo and in vitro. C57BL/6 J mice were intraperitoneally injected with CD73 inhibitor Adenosine 5'-(α, β-methylene) diphosphate sodium salt (APCP) from 5th week to the 8th week in the development of ALF. The results showed APCP could inhibit the activation of HSCs, reduce fibrogenesis marker expression and thus alleviate ALF. Silencing of CD73 inhibited the activation of HSC-T6 cells and promoted apoptosis of activated HSC-T6 cells. What's more, the proliferation of HSC-T6 cells was inhibited, which was characterized by decreased cell viability and cycle arrest. Mechanistically, Wnt/β-catenin pathway was activated in acetaldehyde-activated HSC-T6 cells and CD73 silencing or overexpression could regulate Wnt/β-catenin signaling pathway. Collectively, our study unveils the role of CD73 in HSCs activation, and Wnt/β-catenin signaling pathway might be involved in this progression.

P2Y12 shRNA normalizes inflammatory dysfunctional hepatic glucokinase activity in type 2 diabetic rats

The celiac ganglion projects its postganglionic (including purinergic) fibers to the liver. P2Y12 receptor is one of the P2Y family members. We found that the expression levels of P2Y12 receptor in both celiac ganglia and liver were increased in type 2 diabetes mellitus (T2DM) rats which also displayed an enhanced activity of celiac sympathetic nerve discharge (SND). In addition, a marked decrease of hepatic glucokinase (GK) expression was accompanied by reduced hepatic glycogen synthesis in T2DM rats, whereas meanwhile the levels of NLRP3, active caspase-1, NF-κB, and interleukin-1β were elevated. All these abnormal alterations could be largely reversed after treatment of short hairpin RNA (shRNA) targeting P2Y12. Our results indicate that the silence of P2Y12 by shRNA may effectively correct the anomalous activity of celiac SND and improve the dysfunctional hepatic glucokinase by counteracting hepatocyte inflammation and likely pyroptosis due to activated NLRP3 inflammasome and caspase-1 signaling, thereby attenuating hyperglycemia in T2DM rats.

Hepatotoxicity of nutmeg: A pilot study based on metabolomics

Incidences of abuse and poisoning have been reported for nutmeg, a household spice made from grinding the seed of Myristica fragrans, owing to its hallucinogenic properties. However, there have been no reports on nutmeg hepatotoxicity in relation to dose and duration of exposure. To investigate the hepatotoxicity of different nutmeg exposure durations and doses, male mice were administered daily with normal saline, 1.0 g/kg nutmeg, or 4.0 g/kg nutmeg by intragastrical gavage for either 7 or 14 days (for a total of six treatment groups, n = 6). Body weight of each mouse was monitored daily. Histological analysis of liver tissues was performed using hematoxylin and eosin (H&E) staining to investigate the morphological changes in hepatocytes. Serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels were determined using enzyme-linked immunosorbent assay (ELISA) to investigate liver function. Metabolomics and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed between treatment groups for identifying differential metabolites. Mice in the nutmeg exposure groups exhibited slow growth trends, hepatocyte damage, and significantly elevated serum AST and ALT levels associated with nutmeg dose and exposure duration. Metabolomics and KEGG enrichment pathway analyses also revealed differential levels of some metabolites related to liver function upon nutmeg exposure. Therefore, the present study reasonably speculates that nutmeg exposure may cause liver damage and affect liver function depending on the dose and duration.

The effects of polydeoxyribonucleotide on wound healing and tissue regeneration: a systematic review of the literature

Aim: The present study evaluated the effects of polydeoxyribonucleotide (PDRN) on tissue regeneration, paying special attention to the molecular mechanisms that underlie its tissue remodeling actions to better identify its effective therapeutic potential in wound healing. Materials & methods: Strategic searches were conducted through MEDLINE/PubMed, Google Scholar, Scopus, Web of Science and the Cochrane Central Register of Controlled Trials, from their earliest available dates to March 2020. The studies were included with the following eligibility criteria: studies evaluating tissue regeneration, and being an in vitro, in vivo and clinical study. Results: Out of more than 90 articles, 34 fulfilled the eligibility criteria. All data obtained proved the ability of PDRN in promoting a physiological tissue repair through salvage pathway and adenosine A2A receptor activation. Conclusion: Up to date PDRN has proved promising results in term of wound regeneration, healing time and absence of side effects.

Purinergic signalling in liver diseases: Pathological functions and therapeutic opportunities

Extracellular nucleotides, including ATP, are essential regulators of liver function and serve as danger signals that trigger inflammation upon injury. Ectonucleotidases, which are expressed by liver-resident cells and recruited immune cells sequentially hydrolyse nucleotides to adenosine. The nucleotide/nucleoside balance orchestrates liver homeostasis, tissue repair, and functional restoration by regulating the crosstalk between liver-resident cells and recruited immune cells. In this review, we discuss our current knowledge on the role of purinergic signals in liver homeostasis, restriction of inflammation, stimulation of liver regeneration, modulation of fibrogenesis, and regulation of carcinogenesis. Moreover, we discuss potential targeted therapeutic strategies for liver diseases based on purinergic signals involving blockade of nucleotide receptors, enhancement of ectonucleoside triphosphate diphosphohydrolase activity, and activation of adenosine receptors.

Kupffer cell activation by different microbial lysates: Toll-like receptor-2 plays pivotal role on thromboxane A2 production in mice and humans

Thromboxane (TX) A₂ has been identified as an important intrahepatic vasoconstrictor upon Kupffer cell (KC) activation during infections such as spontaneous bacterial peritonitis (SBP). The study aimed to investigate the role of TLRs in the TXA₂ increase in liver nonparenchymal cells and their related mechanisms. Here, we identified TLR-2 as a common pathway for different microbials: microbial lysates including Gram-positive bacteria, Gram-negative bacteria, and fungi all increased TXA₂ secretion via activation of TLR-2 in human KCs, accompanied by increased expression and phosphorylation of Myd88-related pathway. Of all TLR agonists, only TLR-1, -2, and -4 agonists increased TXA₂ in human KCs. These results were further confirmed by mouse liver nonparenchymal cells. Comparing the effects of TLR-1, -2, and -4 antagonists, only TLR-2 antagonist showed inhibitory effects with all tested microbial lysates. Pretreatment with TLR-2 antagonist in human KCs blocked the secretion of IL-10, CXCL-10, TNF-α, and IL-6 induced by Gram-positive and Gram-negative bacterial stimulation. IL-23 and IL-1β were only induced by Gram-negative bacteria. Thus, TLR-2 might be a potential marker and an attractive target for future treatment of patients with SBP. In addition, IL-23 and IL-1β might distinguish early between Gram-positive and Gram-negative SBP.

Characterization of the N6-etheno-bridge method to assess extracellular metabolism of adenine nucleotides: detection of a possible role for purine nucleoside phosphorylase in adenosine metabolism

The goal of this study was to determine the validity of using N6-etheno-bridged adenine nucleotides to evaluate ecto-nucleotidase activity. We observed that the metabolism of N6-etheno-ATP versus ATP was quantitatively similar when incubated with recombinant CD39, ENTPD2, ENTPD3, or ENPP-1, and the quantitative metabolism of N6-etheno-AMP versus AMP was similar when incubated with recombinant CD73. This suggests that ecto-nucleotidases process N6-etheno-bridged adenine nucleotides similarly to endogenous adenine nucleotides. Four cell types rapidly (t1/2, 0.21 to 0.66 h) metabolized N6-etheno-ATP. Applied N6-etheno-ATP was recovered in the medium as N6-etheno-ADP, N6-etheno-AMP, N6-etheno-adenosine, and surprisingly N6-etheno-adenine; intracellular N6-etheno compounds were undetectable. This suggests minimal cellular uptake, intracellular metabolism, or deamination of these compounds. N6-etheno-ATP, N6-etheno-ADP, N6-etheno-AMP, N6-etheno-adenosine, and N6-etheno-adenine had little affinity for recombinant A1, A2A, or A2B receptors, for a subset of P2X receptors (3H-α,β-methylene-ATP binding to rat bladder membranes), or for a subset of P2Y receptors (35S-ATP-αS binding to rat brain membranes), suggesting minimal pharmacological activity. N6-etheno-adenosine was partially converted to N6-etheno-adenine in four different cell types; this was blocked by purine nucleoside phosphorylase (PNPase) inhibition. Intravenous N6-etheno-ATP was quickly metabolized, with N6-etheno-adenine being the main product in naïve rats, but not in rats pretreated with a PNPase inhibitor. PNPase inhibition reduced the urinary excretion of endogenous adenine and attenuated the conversion of exogenous adenosine to adenine in the renal cortex. The N6-etheno-bridge method is a valid technique to assess extracellular metabolism of adenine nucleotides by ecto-nucleotidases. Also, rats express an enzyme with PNPase-like activity that metabolizes N6-etheno-adenosine to N6-etheno-adenine.

Ectonucleoside triphosphate diphosphohydrolase 1 and 5'-nucleotidase ecto gene polymorphisms and acute cellular rejection after liver transplantation

The single nucleotide polymorphisms (SNPs) rs11188513, rs7071836, rs10748643, rs9450279, rs4458647, and rs6922 map in the genes of ectonucleoside triphosphate diphosphohydrolase 1 (ENTPD1) and 5'-nucleotidase ecto. We investigated whether these SNPs and haplotypes of these SNPs are associated with an acute cellular rejection after liver transplantation. A total of 69 recipients with an acute cellular rejection and 138 recipients without an acute cellular rejection were analyzed. Analyzed individually, no SNP demonstrates an association, but the haplotype rs11188513T-rs7071836G-rs10748643A of the ENTPD1 gene appeared more frequently in recipients without rejection and conversely, the haplotype rs11188513T-rs7071836G-rs10748643G of the ENTPD1 gene was more often represented in recipients with rejection. These two haplotypes seem to be important for the susceptibility of an acute cellular rejection after liver transplantation.

Disruption of hepatic one-carbon metabolism impairs mitochondrial function and enhances macrophage activity in methionine-choline-deficient mice

One-carbon metabolism is a collection of metabolic cycles that supports methylation and provides one-carbon bound folates for the de novo synthesis of purine and thymidine nucleotides. The methylation of phosphatidylethanolamine to form choline has been extensively studied in the context of fatty liver disease. However, the role of one-carbon metabolism in supporting nucleotide synthesis during liver damage has not been addressed. The objective of this study is to determine how the disruption of one-carbon metabolism influences nucleotide metabolism in the liver after dietary methionine and choline restriction. Mice (n=8) were fed a methionine-choline-deficient or control diet for 3 weeks. We treated mice with the compound alloxazine (0.5 mg/kg), a known adenosine receptor antagonist, every second day during the final week of feeding to probe the function of adenosine signaling during liver damage. We found that concentrations of several hepatic nucleotides were significantly lower in methionine- and choline-deficient mice vs. controls (adenine: 13.9±0.7 vs. 10.1±0.6, guanine: 1.8±0.1 vs. 1.4±0.1, thymidine: 0.0122±0.0027 vs. 0.0059±0.0027 nmol/mg dry tissue). Treatment of alloxazine caused a specific decrease in thymidine nucleotides, decrease in mitochondrial content in the liver and exacerbation of steatohepatitis as shown by the increased hepatic lipid content and altered macrophage morphology. This study demonstrates a role for one-carbon metabolism in supporting de novo nucleotide synthesis and mitochondrial function during liver damage.

Extracellular ATP and Purinergic P2Y2 Receptor Signaling Promote Liver Tumorigenesis in Mice by Exacerbating DNA Damage

Release of ATP to the extracellular compartment and subsequent activation of purinergic receptors is a conserved mechanism mediating inflammatory responses and cell fate decisions in various organs including the liver. Previous findings suggest that extracellular ATP may promote liver tumorigenesis, however, the underlying mechanisms are poorly understood. Therefore, our aim was to dissect the functions of extracellular ATP and P2Y₂ receptors (P2Y₂R) during hepatocarcinogenesis. Liver tumors were induced in wild-type and P2y₂r ^-/- knockout mice by intraperitoneal diethylnitrosamine (DEN) injection. Tumorigenesis was analyzed after 8 to 10 months and molecular analyses were performed at different stages of tumorigenesis in vivo, as well as in primary mouse hepatocytes in vitro. Liver tumor incidence and tumor numbers were strongly reduced in P2y₂r ^-/- mice, whereas tumor size and morphology were comparable to wild-type controls, suggesting that P2Y₂R contributes to tumor initiation. Mechanistically, hepatocyte proliferation in DEN-treated P2y₂r ^-/- mice was reduced, which correlated with reduced c-JUN and CCND1 but increased p21 expression. Moreover, DNA damage as determined by hepatocellular expression of γH2A.X and of genes related to genotoxic stress, as well as STAT3 phosphorylation, was reduced in the absence of P2y₂r. Administration of genotoxic agents to primary hepatocytes in vitro confirmed that DNA damage was indeed exacerbated by extracellular ATP, subsequent P2Y₂R activation, and downstream intracellular calcium-dependent signal transduction. In conclusion, our data reveal that extracellular ATP and subsequent P2Y₂R function stimulate DNA damage responses and hepatocyte proliferation, thereby promoting hepatocarcinogenesis. Targeting this pathway may be an attractive approach for chemoprevention of hepatocellular carcinoma. SIGNIFICANCE: Extracellular ATP and subsequent P2Y₂ receptor function stimulate DNA damage responses and hepatocyte proliferation, thereby promoting hepatocarcinogenesis in mice. Targeting this pathway may be an attractive approach for chemoprevention of hepatocellular carcinoma. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/4/699/F1.large.jpg.

Regulation of CD39 expression in ATP-P2Y2R-mediated alcoholic liver steatosis and inflammation

Inflammation plays a central role in the progression of alcoholic liver disease. ATP-P2Y2R signaling and CD39 play an important role in various diseases, but little is known about their role in alcoholic liver steatosis and inflammation. As a transmembrane hydrolase, CD39 hydrolyzes ATP, while the mutual regulation of CD39 and ATP-P2Y2R in alcoholic steatohepatitis is poorly understood. Here, we found that the expression of ATP, P2Y2R, and CD39 is increased significantly both in the liver of alcohol-fed mice and alcohol-induced RAW264.7 cell lines. In this study, C57BL/6 mice were intrapretationally injected with P2Y2R inhibitor suramin from day 4 until day 10 during the induction of a chronic/binge drinking model. Pharmacological blockade of P2Y2R largely prevents liver damage, lipid accumulation, and inflammation, with concomitant down-expression of CD39 in liver. We found that the inhibition of P2Y2R in vitro reduces inflammation via down-expression of interleukin 6 (IL-6), interleukin-1β (IL-1β), and tumor necrosis factor-alpha (TNF-α), and the expression of CD39 was reduced, whereas the activation of P2Y2R showed an opposite effect. Silencing of CD39 promoted the expression of ATP and P2Y2R. These results indicate that CD39 attenuates alcohol-induced steatohepatitis by scavenging extracellular ATP to indirectly regulate the expression of P2Y2R. Interestingly, P2Y2R paradoxically boosts CD39 activity. Thus, blockade of the extracellular ATP-P2Y2R signalling represents a potential therapeutic approach against alcoholic liver disease, and CD39 is a potential therapeutic target.

Purinergic signaling in hepatic disease

Extracellular purines (ATP and adenosine) are ubiquitous intercellular messengers. During tissular damage, they function as damage-associated molecular patterns (DAMPs). In this context, purines announce tissue alterations to initiate a reparative response that involve the formation of the inflammasome complex and the recruitment of specialized cells of the immune system. The present review focuses on the role of the purinergic system in liver damage, mainly during the onset and development of fibrosis. After hepatocellular injury, extracellular ATP promotes a signaling cascade that ameliorates tissue alterations to restore the hepatic function. However, if cellular damage becomes chronic, ATP orchestrates an aberrant reparative process that results in severe liver diseases such as fibrosis and cirrhosis. ATP and adenosine, their receptors, and extracellular ectonucleotidases are mediators of unique processes that will be reviewed in detail.

The Inhibition by Guanfu Base A of Neuropathic Pain Mediated by P2Y12 Receptor in Dorsal Root Ganglia

Activation of satellite glial cells (SGCs) in the dorsal root ganglia (DRG) is involved in mechanical and thermal hyperalgesia. The upregulated P2Y₁₂ receptor expressed in SGCs of the DRG participates in the nociceptive transmission of neuropathic pain. Guanfu base A (GFA) has been reported to exhibit antiarrhythmic and anti-inflammatory effects. In this study, we explored the effects of GFA on P2Y₁₂ receptor-mediated mechanical and thermal hyperalgesia in chronic constriction injury (CCI) rats. Sprague-Dawley rats were randomly divided into sham operation group (Sham), CCI operation group (CCI), CCI rats treated with guanfu base A group (CCI + GFA) and control rats treated with GFA group (Ctrl + GFA). Mechanical withdrawal threshold and thermal withdrawal latency were measured. P2Y₁₂ expression in L4-L6 dorsal root ganglion (DRG) was detected by quantitative real-time PCR and Western blot. After CCI treatment, mechanical and thermal hyperalgesia and the expression values of P2Y₁₂ receptor mRNA and protein in DRG were increased. Dual-labeling immunofluorescence showed that the coexpression of P2Y₁₂ receptor and glial fibrillary acidic protein (GFAP) in the DRG of CCI rats was increased compared to sham rats. GFA relieved mechanical and thermal hyperalgesia in the CCI rats, decreased the expression of P2Y₁₂ mRNA and protein and phosphorylation of p38 MAPK in the DRG, and increased the ADP-downregulated cAMP concentrations in HEK293 cells transfected with P2Y₁₂ plasmid. After CCI rats were treated with GFA, the coexpression of P2Y₁₂ receptor and GFAP in the DRG was significantly decreased compared to the untreated CCI group. Thus, downregulating the P2Y₁₂ receptor relieved mechanical and thermal hyperalgesia in the CCI rats.

Uridine adenosine tetraphosphate and purinergic signaling in cardiovascular system: An update

Uridine adenosine tetraphosphate (Up4A), biosynthesized by activation of vascular endothelial growth factor receptor (VEGFR) 2, was initially identified as a potent endothelium-derived vasoconstrictor in perfused rat kidney. Subsequently, the effect of Up4A on vascular tone regulation was intensively investigated in arteries isolated from different vascular beds in rodents including rat pulmonary arteries, aortas, mesenteric and renal arteries as well as mouse aortas, in which Up4A produces vascular contraction. In contrast, Up4A produces vascular relaxation in porcine coronary small arteries and rat aortas. Intravenous infusion of Up4A into conscious rats or mice decreases blood pressure, and intravenous bolus injection of Up4A into anesthetized mice increases coronary blood flow, indicating an overall vasodilator influence in vivo. Although Up4A is the first dinucleotide described that contains both purine and pyrimidine moieties, its cardiovascular effects are exerted mainly through activation of purinergic receptors. These effects not only encompass regulation of vascular tone, but also endothelial angiogenesis, smooth muscle cell proliferation and migration, and vascular calcification. Furthermore, this review discusses a potential role for Up4A in cardiovascular pathophysiology, as plasma levels of Up4A are elevated in juvenile hypertensive patients and Up4A-mediated vascular purinergic signaling changes in cardiovascular disease such as hypertension, diabetes, atherosclerosis and myocardial infarction. Better understanding the vascular effect of the novel dinucleotide Up4A and the purinergic signaling mechanisms mediating its effects will enhance its potential as target for treatment of cardiovascular disease.

The P2X4 purinergic receptor regulates hepatic myofibroblast activation during liver fibrogenesis

BACKGROUND & AIMS

Liver fibrosis is characterized by the accumulation of extracellular matrix produced by hepatic myofibroblasts (hMF), the activation of which is critical to the fibrogenic process. Extracellular ATP, released by dying or stressed cells, and its purinergic receptors, constitute a powerful signaling network after injury. Although the purinergic receptor P2X4 (P2RX4) is highly expressed in the liver, its functions in hMF had never been investigated during liver fibrogenesis.

METHODS

In vivo, bile duct ligation was performed and methionine- and choline-deficient diet administered in wild-type and P2x4 knock-out (P2x4-KO) mice. In vitro, hMF were isolated from mouse (wild-type and P2x4-KO) and human liver. P2X4 pharmacological inhibition (in vitro and in vivo) and P2X4 siRNAs (in vitro) were used. Histological, biochemical and cell culture analysis allowed us to study P2X4 expression and its involvement in the regulation of fibrogenic and fibrolytic factors, as well as of hMF activation markers and properties.

RESULTS

P2X4 genetic invalidation or pharmacological inhibition protected mice from liver fibrosis and hMF accumulation after bile duct ligation or methionine- and choline-deficient diet. Human and mouse hMFs expressed P2X4, mainly in lysosomes. Invalidation of P2X4 in human and mouse hMFs blunted their activation marker expression and their fibrogenic properties. Finally, we showed that P2X4 regulates calcium entry and lysosomal exocytosis in hMF, impacting on ATP release, profibrogenic secretory profile, and transcription factor activation.

CONCLUSION

P2X4 expression and activation is critical for hMF to sustain their activated and fibrogenic phenotype. Therefore, the inactivation of P2X4 may be of therapeutic interest during liver fibrotic diseases.

LAY SUMMARY

During chronic injury, the liver often repairs with fibrotic tissue, which impairs liver function, and for which there is currently no treatment. We found that a previously unexplored pathway involving the purinergic receptor P2X4, can modulate fibrotic liver repair. Therefore, this receptor could be of interest in the development of novel therapies for fibrotic liver diseases.

Extracellular ATP signaling and clinical relevance

Since purinergic signaling was discovered in the early 1970s, it has been shown that extracellular nucleotides, and their derivative nucleosides, are released in a regulated or unregulated manner by cells in various challenging settings and then bind defined purinergic receptors to activate intricate signaling networks. Extracellular ATP plays a role based on different P2 receptor subtypes expressed on specific cell types. Sequential hydrolysis of extracellular ATP catalyzed by ectonucleotidases (e.g. CD39, CD73) is the main pathway for the generation of adenosine, which in turn activates P1 receptors. Many studies have demonstrated that extracellular ATP signaling functions as an important dynamic regulatory pathway to coordinate appropriate immune responses in various pathological processes, including intracellular infection, host-tumor interaction, pro-inflammation vascular injury, and transplant immunity. ATP receptors and CD39 also participate in related clinical settings. Here, we review the latest research in to the development of promising clinical treatment strategies.

Deficiency of the Purinergic Receptor 2X7 Attenuates Nonalcoholic Steatohepatitis Induced by High-Fat Diet: Possible Role of the NLRP3 Inflammasome

Molecular mechanisms driving transition from simple steatosis to nonalcoholic steatohepatitis (NASH), a critical step in the progression of nonalcoholic fatty liver disease (NAFLD) to cirrhosis, are poorly defined. This study aimed at investigating the role of the purinergic receptor 2X₇ (PR2X₇), through the NLRP3 inflammasome, in the development of NASH. To this end, mice knockout for the Pr2x₇ gene (Pr2x₇^−/−) and coeval wild-type (WT) mice were fed a high-fat diet (HFD) or normal-fat diet for 16 weeks. NAFLD grade and stage were lower in Pr2x₇^−/− than WT mice, and only 1/7 Pr2x₇^−/− animals showed evidence of NASH, as compared with 4/7 WT mice. Molecular markers of inflammation, oxidative stress, and fibrosis were markedly increased in WT-HFD mice, whereas no or significantly reduced increments were detected in Pr2x₇^−/− animals, which showed also decreased modulation of genes of lipid metabolism. Deletion of Pr2x₇ gene was associated with blunted or abolished activation of NLRP3 inflammasome and expression of its components, which were induced in liver sinusoidal endothelial cells challenged with appropriate stimuli. These data show that Pr2x₇ gene deletion protects mice from HFD-induced NASH, possibly through blunted activation of NLRP3 inflammasome, suggesting that PR2X₇ and NLRP3 may represent novel therapeutic targets.

Distinct roles of ecto-nucleoside triphosphate diphosphohydrolase-2 (NTPDase2) in liver regeneration and fibrosis

Ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) are cell surface-located transmembrane ecto-enzymes of the CD39 superfamily which regulate inflammation and tissue repair by catalyzing the phosphohydrolysis of extracellular nucleotides and modulating purinergic signaling. In the liver, NTPDase2 is reportedly expressed on portal fibroblasts, but its functional role in regulating tissue regeneration and fibrosis is incompletely understood. Here, we studied the role of NTPDase2 in several models of liver injury using global knockout mice. Liver regeneration and severity of fibrosis were analyzed at different time points after exposure to carbon tetrachloride (CCl₄) or 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) or partial hepatectomy in C57BL/6 wild-type and globally NTPDase2-deficient (Entpd2 null) mice. After chronic CCl₄ intoxication, Entpd2 null mice exhibit significantly more severe liver fibrosis, as assessed by collagen content and histology. In contrast, deletion of NTPDase2 does not have a substantial effect on biliary-type fibrosis in the setting of DDC feeding. In injured livers, NTPDase2 expression extends from the portal areas to fibrotic septae in pan-lobular (CCl₄-induced) liver fibrosis; the same pattern was observed, albeit to a lesser extent in biliary-type (DDC-induced) fibrosis. Liver regeneration after partial hepatectomy is not substantively impaired in global Entpd2 null mice. NTPDase2 protects from liver fibrosis resulting from hepatocellular injury induced by CCl₄. In contrast, Entpd2 deletion does not significantly impact fibrosis secondary to DDC injury or liver regeneration after partial hepatectomy. Our observations highlight mechanisms relating to purinergic signaling in the liver and indicate possible therapeutic avenues and new cellular targets to test in the management of hepatic fibrosis.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

Combination treatment with 6-mercaptopurine and allopurinol in HepG2 and HEK293 cells - Effects on gene expression levels and thiopurine metabolism

Combination treatment with low-dose thiopurine and allopurinol (AP) has successfully been used in patients with inflammatory bowel disease with a so called skewed thiopurine metabolite profile. In red blood cells in vivo, it reduces the concentration of methylated metabolites and increases the concentration of the phosphorylated ones, which is associated with improved therapeutic efficacy. This study aimed to investigate the largely unknown mechanism of AP on thiopurine metabolism in cells with an active thiopurine metabolic pathway using HepG2 and HEK293 cells. Cells were treated with 6-mercaptopurine (6MP) and AP or its metabolite oxypurinol. The expression of genes known to be associated with thiopurine metabolism, and the concentration of thiopurine metabolites were analyzed. Gene expression levels were only affected by AP in the presence of 6MP. The addition of AP to 6MP affected the expression of in total 19 genes in the two cell lines. In both cell lines the expression of the transporter SLC29A2 was reduced by the combined treatment. Six regulated genes in HepG2 cells and 8 regulated genes in HEK293 cells were connected to networks with 18 and 35 genes, respectively, present at known susceptibility loci for inflammatory bowel disease, when analyzed using a protein-protein interaction database. The genes identified as regulated as well as the disease associated interacting genes represent new candidates for further investigation in the context of combination therapy with thiopurines and AP. However, no differences in absolute metabolite concentrations were observed between 6MP+AP or 6MP+oxypurinol vs. 6MP alone in either of the two cell lines. In conclusion; the effect of AP on gene expression levels requires the presence of 6MP, at least in vitro. Previously described AP-effects on metabolite concentrations observed in red blood cells in vivo could not be reproduced in our cell lines in vitro. AP’s effects in relation to thiopurine metabolism are complex. The network-identified susceptibility genes represented biological processes mainly associated with purine nucleotide biosynthetic processes, lymphocyte proliferation, NF-KB activation, JAK-STAT signaling, and apoptotic signaling at oxidative stress.