Adenosine inhibits cytosolic calcium signals and chemotaxis in hepatic stellate cells

ORAI2 is Important for the Development of Early-Stage Postirradiation Fibrosis in Salivary Glands

PURPOSE

Although postirradiation hyposalivation significantly impairs patient quality of life, the underlying mechanisms driving radiation-induced salivary gland fibrosis and hyposalivation remain poorly understood. This study aims to explore the role of calcium-mediated signaling pathways in radiation-induced salivary gland fibrosis.

METHODS AND MATERIALS

Primary human submandibular gland (SG) cells and C57BL/6J female mouse SGs were exposed to irradiation to model fibrosis development. Following 15 Gy irradiation exposure, RNA sequencing and bioinformatic analysis were conducted on mouse SGs. The effects of store-operated calcium entry (SOCE) inhibition using SKF96365 and YM58483 on fibrosis markers were assessed in vitro and in vivo. Additionally, the involvement of ORAI2 protein and the newly identified JNK/NFAT1/transforming growth factor β1 (TGF-β1) signaling axis in SG fibrosis was explored.

RESULTS

We identified that the calcium release-activated calcium modulator ORAI2 was important in promoting early-stage postirradiation fibrosis in SGs. Calcium channel signaling was activated in both human patients and irradiated C57BL/6J female mice SGs. Inhibition of SOCE signaling effectively blocked fibrosis in an ORAI2-dependent manner 30 days after irradiation. Our mechanistic studies revealed a novel ORAI2/JNK/NFAT1 axis within the SOCE pathway critical in driving TGF-β1-mediated fibrogenesis. Encouragingly, pharmacologic inhibition of NFAT1 significantly mitigated radiation-induced SG fibrosis and restored saliva flow to 84.61% of normal levels in treated mice 30 days after irradiation, without detectable side effects.

CONCLUSIONS

Our findings highlight the significance of the ORAI2-mediated calcium signaling pathway, specifically via the ORAI2/JNK/NFAT1 axis, in promoting TGF-β1 expression and contributing to the development of early-stage salivary gland fibrosis following irradiation exposure. Targeting the ORAI2/JNK/NFAT1 axis emerges as a promising therapeutic strategy to alleviate radiation-induced hyposalivation and fibrosis, potentially improving the quality of life for patients undergoing radiation therapy.

Nimodipine ameliorates liver fibrosis via reshaping liver immune microenvironment in TAA-induced in mice

Nimodipine, a calcium antagonist, exert beneficial neurovascular protective effects in clinic. Recently, Calcium channel blockers (CCBs) was reported to protect against liver fibrosis in mice, while the exact effects of Nimodipine on liver injury and hepatic fibrosis remain unclear. In this study, we assessed the effect of nimodipine in Thioacetamide (TAA)-induced liver fibrosis mouse model. Then, the collagen deposition and liver inflammation were assessed by HE straining. Also, the frequency and phenotype of NK cells, CD4+T and CD8+T cells and MDSC in liver and spleen were analyzed using flow cytometry. Furthermore, activation and apoptosis of primary Hepatic stellate cells (HSCs) and HSC line LX2 were detected using α-SMA staining and TUNEL assay, respectively. We found that nimodipine administration significantly attenuated liver inflammation and fibrosis. And the increase of the numbers of hepatic NK and NKT cells, a reversed CD4+/CD8+T ratio, and reduced the numbers of MDSC were observed after nimodipine treatment. Furthermore, nimodipine administration significantly decreased α-SMA expression in liver tissues, and increased TUNEL staining adjacent to hepatic stellate cells. Nimodipine also reduced the proliferation of LX2, and significantly promoted high level of apoptosis in vitro. Moreover, nimodipine downregulated Bcl-2 and Bcl-xl, simultaneously increased expression of JNK, p-JNK, and Caspase-3. Together, nimodipine mediated suppression of growth and fibrogenesis of HSCs may warrant its potential use in the treatment of liver fibrosis.

Purinergic Signaling in Non-Parenchymal Liver Cells

Purinergic signaling has emerged as an important paracrine-autocrine intercellular system that regulates physiological and pathological processes in practically all organs of the body. Although this system has been thoroughly defined since the nineties, recent research has made substantial advances regarding its role in aspects of liver physiology. However, most studies have mainly targeted the entire organ, 70% of which is made up of parenchymal cells or hepatocytes. Because of its physiological role, the liver is exposed to toxic metabolites, such as xenobiotics, drugs, and fatty acids, as well as to pathogens such as viruses and bacteria. Under injury conditions, all cell types within the liver undergo adaptive changes. In this context, the concentration of extracellular ATP has the potential to increase dramatically. Indeed, this purinergic response has not been studied in sufficient detail in non-parenchymal liver cells. In the present review, we systematize the physiopathological adaptations related to the purinergic system in chronic liver diseases of non-parenchymal liver cells, such as hepatic stellate cells, Kupffer cells, sinusoidal endothelial cells, and cholangiocytes. The role played by non-parenchymal liver cells in these circumstances will undoubtedly be strategic in understanding the regenerative activities that support the viability of this organ under stressful conditions.

Adenosine receptors are the on-and-off switch of astrocytic cannabinoid type 1 (CB1) receptor effect upon synaptic plasticity in the medial prefrontal cortex

The medial prefrontal cortex (mPFC) is involved in cognitive functions such as working memory. Astrocytic cannabinoid type 1 receptor (CB1R) induces cytosolic calcium (Ca2+) concentration changes with an impact on neuronal function. mPFC astrocytes also express adenosine A1 and A2A receptors (A1R, A2AR), being unknown the crosstalk between CB1R and adenosine receptors in these cells. We show here that a further level of regulation of astrocyte Ca2+ signaling occurs through CB1R-A2AR or CB1R-A1R heteromers that ultimately impact mPFC synaptic plasticity. CB1R-mediated Ca2+ transients increased and decreased when A1R and A2AR were activated, respectively, unveiling adenosine receptors as modulators of astrocytic CB1R. CB1R activation leads to an enhancement of long-term potentiation (LTP) in the mPFC, under the control of A1R but not of A2AR. Notably, in IP3R2KO mice, that do not show astrocytic Ca2+ level elevations, CB1R activation decreases LTP, which is not modified by A1R or A2AR. The present work suggests that CB1R has a homeostatic role on mPFC LTP, under the control of A1R, probably due to physical crosstalk between these receptors in astrocytes that ultimately alters CB1R Ca2+ signaling.

Coffee, adenosine, and the liver

A variety of observational studies have demonstrated that coffee, likely acting through caffeine, improves health outcomes in patients with chronic liver disease. The primary pharmacologic role of caffeine is to act as an inhibitor of adenosine receptors. Because key liver cells express adenosine receptors linked to liver injury, regeneration, and fibrosis, it is plausible that the biological effects of coffee are explained by effects of caffeine on adenosinergic signaling in the liver. This review is designed to help the reader make sense of that hypothesis, highlighting key observations in the literature that support or dispute it.

Adenosine A2A Receptor Regulates microRNA-181b Expression in Aorta: Therapeutic Implications for Large-Artery Stiffness

Background The identification of large-artery stiffness as a major, independent risk factor for cardiovascular disease-associated morbidity and death has focused attention on identifying therapeutic strategies to combat this disorder. Genetic manipulations that delete or inactivate the translin/trax microRNA-degrading enzyme confer protection against aortic stiffness induced by chronic ingestion of high-salt water (4%NaCl in drinking water for 3 weeks) or associated with aging. Therefore, there is heightened interest in identifying interventions capable of inhibiting translin/trax RNase activity, as these may have therapeutic efficacy in large-artery stiffness. Methods and Results Activation of neuronal adenosine A2A receptors (A2ARs) triggers dissociation of trax from its C-terminus. As A2ARs are expressed by vascular smooth muscle cells (VSMCs), we investigated whether stimulation of A2AR on vascular smooth muscle cells promotes the association of translin with trax and, thereby increases translin/trax complex activity. We found that treatment of A7r5 cells with the A2AR agonist CGS21680 leads to increased association of trax with translin. Furthermore, this treatment decreases levels of pre-microRNA-181b, a target of translin/trax, and those of its downstream product, mature microRNA-181b. To check whether A2AR activation might contribute to high-salt water-induced aortic stiffening, we assessed the impact of daily treatment with the selective A2AR antagonist SCH58261 in this paradigm. We found that this treatment blocked aortic stiffening induced by high-salt water. Further, we confirmed that the age-associated decline in aortic pre-microRNA-181b/microRNA-181b levels observed in mice also occurs in humans. Conclusions These findings suggest that further studies are warranted to evaluate whether blockade of A2ARs may have therapeutic potential in treating large-artery stiffness.

Pathophysiology roles for adenosine 2A receptor in obesity and related diseases

Obesity, a burgeoning worldwide health system challenge, is associated with several comorbidities, including non-alcoholic fatty liver disease (NAFLD), diabetes, atherosclerosis, and osteoarthritis, leading to serious problems to people's health. Adenosine is a critical extracellular signaling molecule that has essential functions in regulating most organ systems by binding to four G-protein-coupled adenosine receptors, denoted A₁ , A_2A , A_2B , and A₃ . Among the receptors, a growing body evidence highlights the key roles of the adenosine 2A receptor (A_2A R) in obesity and related diseases. In the current review, we summarize the effects of A_2A R in obesity and obesity-associated non-alcoholic fatty liver disease, diabetes, atherosclerosis, and osteoarthritis, to clarify the complicated impacts of A_2A R on obesity and related diseases.

Pathophysiological mechanisms of hepatic stellate cells activation in liver fibrosis

Liver fibrosis is a complex pathological process controlled by a variety of cells, mediators and signaling pathways. Hepatic stellate cells play a central role in the development of liver fibrosis. In chronic liver disease, hepatic stellate cells undergo dramatic phenotypic activation and acquire fibrogenic properties. This review focuses on the pathophysiological mechanisms of hepatic stellate cells activation in liver fibrosis. They enter the cell cycle under the influence of various triggers. The “Initiation” phase of hepatic stellate cells activation overlaps and continues with the “Perpetuation” phase, which is characterized by a pronounced inflammatory and fibrogenic reaction. This is followed by a resolution phase if the injury subsides. Knowledge of these pathophysiological mechanisms paved the way for drugs aimed at preventing the development and progression of liver fibrosis. In this respect, impairments in intracellular signaling, epigenetic changes and cellular stress response can be the targets of therapy where the goal is to deactivate hepatic stellate cells. Potential antifibrotic therapy may focus on inducing hepatic stellate cells to return to an inactive state through cellular aging, apoptosis, and/or clearance by immune cells, and serve as potential antifibrotic therapy. It is especially important to prevent the formation of liver cirrhosis since the only radical approach to its treatment is liver transplantation which can be performed in only a limited number of countries.

A Review of Liver Fibrosis and Emerging Therapies

With the increasing burden of liver cirrhosis, the most advanced stage of hepatic fibrosis, there is a need to better understand the pathological processes and mechanisms to target specific treatments to reverse or cease fibrosis progression. Antiviral therapy for hepatitis B and C has effectively treated underlying causes of chronic liver disease and has induced fibrosis reversal in some; however, this has not been targeted for the majority of aetiologies for cirrhosis including alcohol or nonalcoholic steatohepatitis. Fibrosis, characterised by the accumulation of extracellular matrix proteins, is caused by chronic injury from toxic, infectious, or metabolic causes. The primary event of fibrogenesis is increased matrix production and scar formation mediated by the hepatic stellate cell, which is the principal cell type involved. Experimental models using rodent and human cell lines of liver injury have assisted in better understanding of fibrogenesis, especially in recognising the role of procoagulant factors. This has led to interventional studies using anticoagulants in animal models with reversal of fibrosis as the primary endpoint. Though these trials have been encouraging, no antifibrotic therapies are currently licenced for human use. This literature review discusses current knowledge in the pathophysiology of hepatic fibrosis, including characteristics of the extracellular matrix, signalling pathways, and hepatic stellate cells. Current types of experimental models used to induce fibrosis, as well as up-to-date anticoagulant therapies and agents targeting the hepatic stellate cell that have been trialled in animal and human studies with antifibrotic properties, are also reviewed.

Prophylactic versus therapeutic role of the transplanted CD34+ Umbilical Cord Blood Stem Cells and Wharton Jelly Mesenchymal Stem Cells in early / acute hepatic S. mansoni granulomas reversal in mice; a novel approach

PURPOSE

Praziquantel (PZQ) is the conventional antibilharzial agent. Nevertheless, no antibilharzial prophylactic agents or 100% curable therapy approved and no reported data about use of human CD34⁺ Umbilical Cord Blood Stem Cells (CD34⁺UCBSCs) or Wharton Jelly Mesenchymal Stem Cells (WJMSCs) in prevention and/or complete eradication of acute S.mansoni granulomas in liver. We aimed to study possible prophylactic vs therapeutic role of human CD34⁺UCBSCs and WJMSCs in acute hepatic bilharzial granulomas in pre vs post-infected mice.

METHODS

Seventy mice were divided into 7 groups (10 mice each): Normal, S.mansoni-infected, post-infected PZQ-treated, CD34⁺UCBSCs pre and post-infected, WJMSCs pre and post-infected. Serological, parasitological, histopathological evaluation using OCT4 & TGFB immunohistochemistry and quantitative image analysis assessment of TGFB-stained fibrogenesis in liver granulomas performed.

RESULTS

Histopathologically, surprisingly and significantly, the prophylactic pre-infection stem cells (CD34⁺UCBSCs and WJMSCs) & similarly the post-infection CD34⁺UCBSCs treatment revealed eradication/reversal of the entire granulomas and no fibrosis. Moreover, post-infection PZQ treatment showed fewer and significantly smaller granulomas than post-infection WJMSCs treatment. Nevertheless, post-infection WJMSCs exhibited non-significant less TGFB-stained fibrogenesis.

CONCLUSION

CD34⁺UCBSCs exerted the best prophylactic and therapeutic roles in prevention and complete cure of acute hepatic S.mansoni granulomas over WJMSCs and PZQ. In contrast, only pre-infection WJMSCs exhibited similar preventive (prophylactic) effect. On the contrary, post-infection WJMSCs were the worst (incompletely reversed granulomas). Post-infection Praziquantel was overall better therapeutically than WJMSCs in this regard. Accordingly, when it comes to WJMSCs application, WJMSCs are better used as a pre-infection prophylactic and preventive tool rather than a post-infection therapy. Further studies are needed.

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.

Extracellular adenosine: a critical signal in liver fibrosis

Extracellular adenosine nucleoside is a potent, endogenous mediator that signals through specific G protein-coupled receptors, and exerts pleiotropic effects on liver physiology, in health and disease. Particularly, adenosinergic or adenosine-mediated signaling pathways impact the progression of hepatic fibrosis, a common feature of chronic liver diseases, through regulation of matrix deposition by liver myofibroblasts. This review examines the current lines of evidence on adenosinergic regulation of liver fibrosis and myofibroblasts, identifies unanswered research questions, and proposes important future areas of investigation.

Platelet-Derived Growth Factor Receptor α Contributes to Human Hepatic Stellate Cell Proliferation and Migration

Platelet-derived growth factor receptor α (PDGFRα), a tyrosine kinase receptor, is up-regulated in hepatic stellate cells (HSCs) during chronic liver injury. HSCs mediate hepatic fibrosis through their activation from a quiescent state partially in response to profibrotic growth factors. HSC activation entails enhanced expression of profibrotic genes, increase in proliferation, and increase in motility, which facilitates migration within the hepatic lobule. We show colocalization of PDGFRα in murine carbon tetrachloride, bile duct ligation, and 0.1% 3,5-diethoxycarbonyl-1,4-dihydrocollidine models of chronic liver injury, and investigate the role of PDGFRα on proliferation, profibrotic gene expression, and migration in primary human HSCs (HHSteCs) using the PDGFRα-specific inhibitory monoclonal antibody olaratumab. Although lacking any effects on HHSteC transdifferentiation assessed by gene expression of ACTA2, TGFB1, COL1A1, SYP1, and FN1, olaratumab specifically reduced HHSteC proliferation (AlamarBlue assay) and cell migration (transwell migration assays). Using phospho-specific antibodies, we show that olaratumab attenuates PDGFRα activation in response to PDGF-BB, and reduced phosphorylation of extracellular signal-regulated kinase 1 and 2, Elk-1, p38, Akt, focal adhesion kinase, mechanistic target of rapamycin, C10 regulator of kinase II, and C10 regulator of kinase-like, suggesting that PDGFRα contributes to mitogenesis and actin reorganization through diverse downstream effectors. Our findings support a distinct contribution of PDGFRα signaling to HSC proliferation and migration and provide evidence that inhibition of PDGFRα signaling could alter the pathogenesis of hepatic fibrosis.

Signaling pathways involving adenosine A2A and A2B receptors in wound healing and fibrosis

Collagen and matrix deposition by fibroblasts is an essential part of wound healing but also contributes to pathologic remodeling of organs leading to substantial morbidity and mortality. Adenosine, a small molecule generated extracellularly from adenine nucleotides as a result of direct stimulation, hypoxia, or injury, acts via a family of classical seven-pass G protein-coupled protein receptors, A2A and A2B, leading to generation of cAMP and activation of downstream targets such as PKA and Epac. These effectors, in turn, lead to fibroblast activation and collagen synthesis. The regulatory actions of these receptors likely involve multiple interconnected pathways, and one of the more interesting aspects of this regulation is opposing effects at different levels of cAMP generated. Additionally, adenosine signaling contributes to fibrosis in organ-specific ways and may have opposite effects in different organs. The development of drugs that selectively target these receptors and their signaling pathways will disrupt the pathogenesis of fibrosis and slow or arrest the progression of the important diseases they underlie.

NLRC5 regulates TGF-β1-induced proliferation and activation of hepatic stellate cells during hepatic fibrosis

Therapeutic management of liver fibrosis remains an unsolved clinical problem. Hepatic accumulation of extracellular matrix, mainly collagen, is mediated by the production of transforming growth factor-β1 (TGF-β1) in hepatic stellate cells (HSCs). NLRC5, the largest member of the NLR protein family, has recently been identified as a critical regulator of immune responses. Novel evidence shows that NLRC5 is an important negative modulator of inflammatory pathways. Herein, we determined the regulation of NLRC5 in liver fibrogenesis and its underlying mechanisms. We have shown that NLRC5 was upregulated in human liver fibrotic tissues. Overexpression of NLRC5 resulted in an upregulation of collagen 1 and α-smooth muscle actin expression in HSC LX-2 cells, which was inhibited by NLRC5 knockdown with its siRNA. Furthermore, NLRC5 deficiency significantly suppressed TGF-β1-induced proliferation but increased apoptosis (i.e., increased caspases-3, DR4 and DR5) in LX-2 cells. In addition, knockdown of NLRC5 promoted the activation of NF-κB signaling pathways but abrogated phosphorylation of Smad2 and Smad3 proteins in response to TGF-β1. These results indicate that NLRC5 is a potent pro-fibrogenic molecule for HSC activation through TGF-β1/Smad and NF-κB signaling pathways. NLRC5 inhibition would be a promising therapeutic avenue for treating hepatic fibrosis.

Inhibition of the CXCL12/CXCR4 chemokine axis with AMD3100, a CXCR4 small molecule inhibitor, worsens murine hepatic injury

AIM

Activation of hepatic stellate cells and development of chronic inflammation are two key features in the progression of hepatic fibrosis. We have shown that in vitro activated stellate cells increase their expression of CXCL12 as well as the receptor CXCR4 and that receptor engagement promotes a profibrogenic phenotype. Furthermore, injury promotes increased hepatic expression of CXCL12 and a massive infiltration of CXCR4-expressing leukocytes, granulocytes and myeloid cells. The primary site of inflammatory cell accumulation is around the CXCL12-rich portal tracts and within fibrotic septae, indicating a role for CXCR4 during injury. In order to characterize the relevance of the CXCR4/CXCL12 chemokine axis during hepatic injury we inhibited the axis using AMD3100, a CXCR4 small molecule inhibitor, in models of chronic and acute liver injury.

METHODS

Mice were subjected to acute and chronic CCl4 liver injury with and without AMD3100 administration. The degree of liver injury, fibrosis and the composition of the intrahepatic inflammatory response were characterized.

RESULTS

Treatment of mice with AMD3100 in the chronic CCl4 model of liver injury led to an increase in hepatic inflammation and fibrosis with a specific increase in intrahepatic neutrophils. Furthermore, in an acute model of CCl4 -induced liver injury, AMD3100 led to an increase in the number of intrahepatic neutrophils and a trend towards worse necrosis.

CONCLUSION

Together, this data suggests that inhibition of the CXCR4/CXCL12 chemokine axis is injurious through modulation of the hepatic inflammatory response and that this axis may serve a protective role in liver injury.

I drink for my liver, Doc: emerging evidence that coffee prevents cirrhosis

Evidence demonstrating that regular ingestion of coffee has salutary effects on patients with chronic liver disease is accumulating rapidly. Specifically, it appears that coffee ingestion can slow the progression of liver fibrosis, preventing cirrhosis and hepatocellular carcinoma (HCC). This should excite clinicians and scientists alike, since these observations, if true, would create effective, testable hypotheses that should lead to improved understanding on fibrosis pathogenesis and thus may generate novel pharmacologic treatments of patients with chronic liver disease.

This review is designed to examine the relevant clinical and epidemiological data in critical fashion and to examine the putative pharmacological effects of coffee relevant to the pathogenesis of liver fibrosis and cirrhosis. We hope that this will inspire relevant critical analyses, especially among “coffee skeptics”. Of note, one major assumption made by this review is that the bulk of the effects of coffee consumption are mediated by caffeine, rather than by other chemical constituents of coffee. Our rationales for this assumption are threefold: first, caffeine’s effects on adenosinergic signaling provide testable hypotheses; second, although there are  myriad chemical constituents of coffee, they are present in very low concentrations, and perhaps more importantly, vary greatly between coffee products and production methods (it is important to note that we do not dismiss the “botanical” hypothesis here; rather, we do not emphasize it at present due to the limitations of the studies examined); lastly, some (but not all) observational studies have examined both coffee and non-coffee caffeine consumption and found consistent effects, and when examined, no benefit to decaffeinated coffee has been observed. Further, in the interval since we examined this phenomenon last, further evidence has accumulated supporting caffeine as the effector molecule for coffee’s salutary effects.

Expression of mediators of purinergic signaling in human liver cell lines

Purinergic signaling regulates a diverse and biologically relevant group of processes in the liver. However, progress of research into functions regulated by purinergic signals in the liver has been hampered by the complexity of systems probed. Specifically, there are multiple liver cell subpopulations relevant to hepatic functions, and many of these have been effectively modeled in human cell lines. Furthermore, there are more than 20 genes relevant to purinergic signaling, each of which has distinct functions. Hence, we felt the need to categorize genes relevant to purinergic signaling in the best characterized human cell line models of liver cell subpopulations. Therefore, we investigated the expression of adenosine receptor, P2X receptor, P2Y receptor, and ecto-nucleotidase genes via RT-PCR in the following cell lines: LX-2, hTERT, FH11, HepG2, Huh7, H69, and MzChA-1. We believe that our findings will provide an excellent resource to investigators seeking to define functions of purinergic signals in liver physiology and liver disease pathogenesis.

Endoplasmic reticulum stress-induced hepatic stellate cell apoptosis through calcium-mediated JNK/P38 MAPK and Calpain/Caspase-12 pathways

Recent reports considered that it was the disturbance of calcium homeostasis and the accumulation of misfolded proteins in the endoplasmic reticulum (ER) that activated hepatic stellate cells (HSCs) apoptosis and promoted fibrosis resolution. However, the signal-transducing events that are activated by ER stress after HSCs activation were incompletely understood. In this study, we induced ER stress with thapsigargin (TG), and determined the activation of calpain and the cleavage of caspase by analyzing the protein levels and the correspondingly increased intracellular calcium levels and the induction of the proapoptotic transcription factor CHOP. Moreover, the phosphorylation of JNK and p38 MAPK were followed by the activation of the executioner caspases, caspase-3. As expected, preventing an increase in intracellular calcium levels using intracellular calcium chelators, EGTA, and BAPTA/AM, could substantially inhibit the phosphorylation of JNK and p38 MAPK, abolish the activation of calpains, namely caspase-12, caspase-9, and caspase-3, and provide significant protection for TG-treated activated HSCs. Interestingly, pretreatment with p38 MAPK inhibitor SB202190, JNK inhibitor SP600125, the pan-caspase inhibitor z-VAD-FMK, or calpain inhibitors calpeptin, significantly reduced the cell apoptosis and the cleavage of caspase-12 and caspase-3. However, pretreatment with z-VAD-FMK failed to reduce the activation of calpain. Additionally, pretreatment with SB202190 and SP600125 also decreased the expression of CHOP. Importantly, PDGF-induced collagen Col1α1 and α-smooth muscle actin (α-SMA), markers for the perpetuation phase of HSCs activation, were inhibited in TG-treated activated HSCs. These findings showed that the Calpain/Caspase-12 activation induced by ER stress and the JNK/p38 MAPK phosphorylation induced by the increase of intracellular calcium concentration releasing from ER are the novel signaling pathway underlying the molecular mechanism of fibrosis recovery.

Cellular and molecular mechanisms in the pathogenesis of liver fibrosis: An update

There have been considerable recent advances towards a better understanding of the complex cellular and molecular network underlying liver fibrogenesis. Recent data indicate that the termination of fibrogenic processes and the restoration of deficient fibrolytic pathways may allow the reversal of advanced fibrosis and even cirrhosis. Therefore, efforts have been made to better clarify the cellular and molecular mechanisms that are involved in liver fibrosis. Activation of hepatic stellate cells (HSCs) remains a central event in fibrosis, complemented by other sources of matrix-producing cells, including portal fibroblasts, fibrocytes and bone marrow-derived myofibroblasts. These cells converge in a complex interaction with neighboring cells to provoke scarring in response to persistent injury. Defining the interaction of different cell types, revealing the effects of cytokines on these cells and characterizing the regulatory mechanisms that control gene expression in activated HSCs will enable the discovery of new therapeutic targets. Moreover, the characterization of different pathways associated with different etiologies aid in the development of disease-specific therapies. This article outlines recent advances regarding the cellular and molecular mechanisms involved in liver fibrosis that may be translated into future therapies. The pathogenesis of liver fibrosis associated with alcoholic liver disease, non-alcoholic fatty liver disease and viral hepatitis are also discussed to emphasize the various mechanisms involved in liver fibrosis.

Inhibition of acid-sensing ion channel 1a in hepatic stellate cells attenuates PDGF-induced activation of HSCs through MAPK pathway

Acid-sensing ion channels (ASICs), a group of Na(+)-selective and Ca(2+)-permeant ligand-gated cation channels, can be transiently activated by extracellular acid. Among seven subunits of ASICs, acid-sensing ion channel 1a (ASIC1a), which is responsible for Ca(2+) transportation, is elevated in response to inflammation, tumor, and ischemic injury in central nervous system and non-neuronal tissues. In this study, we demonstrated for the first time the presence of ASIC1a in rat liver and hepatic stellate cells (HSCs). Furthermore, the expression of ASIC1a was increased in primary HSCs and liver tissues of CCl4-treated rats, suggesting that ASIC1a may play certain role in liver fibrosis. Interestingly, we identified that the level of ASIC1a was significantly elevated in response to platelet-derived growth factor (PDGF) induction in a time- and dose-dependent manner. It was also established that Ca(2+)-transporting ASIC1a was involved in acid-induced injury of different cell types. Moreover, inhibition or silencing of ASIC1a was able to inhibit PDGF-induced pro-fibrogenic effects of activated rat HSCs, including cell activation, de novo synthesis of extracellular matrix components through mitogen-activated protein kinase signaling pathway. Collectively, our studies identified that ASIC1a was expressed in rat liver and HSCs and provided a strong evidence for the involvement of the ASIC1a in the progression of hepatic fibrosis.

Finding ATF4/p75NTR/IL-8 signal pathway in endothelial-mesenchymal transition by safrole oxide

Targeting the endothelial-to-mesenchymal transition (EndoMT) may be a novel therapeutic strategy for cancer and various diseases induced by fibrosis. We aimed to identify a small chemical molecule as an inducer of EndoMT and find a new signal pathway by using the inducer. Safrole oxide (SFO), 50 µg/ml, could most effectively induce EndoMT within 12 h. To understand the underlying molecular mechanism, we performed microarray, quantitative real-time PCR and western blot analysis to find key factors involved in SFO-induced EndoMT and demonstrated the involvement of the factors by RNAi. The expression of activating transcription factor 4 (ATF4), p75 neurotrophin receptor (p75NTR), and interleukin 8 (IL-8) was greatly increased in SFO-induced EndoMT. Knockdown of ATF4 inhibited the SFO-induced EndoMT completely, and knockdown of p75NTR or IL-8 partially inhibited the EndoMT, which suggests that all three factors were involved in the process. Furthermore, knockdown of p75NTR inhibited the SFO-increased IL-8 expression and secretion, and knockdown of ATF4 inhibited SFO-increased p75NTR level significantly. The ATF4/p75NTR/IL-8 signal pathway may have an important role in EndoMT induced by SFO. Our findings support potential novel targets for the therapeutics of cancer and fibrosis disease.

Activation of adenosine receptor A2A increases HSC proliferation and inhibits death and senescence by down-regulation of p53 and Rb

Background and Aims: During fibrosis hepatic stellate cells (HSC) undergo activation, proliferation, and senescence but the regulation of these important processes is poorly understood. The adenosine A_2A receptor (A_2A) is known to be present on HSC, and its activation results in liver fibrosis. In this study, we tested if A_2A has a role in the regulation of HSC proliferation, apoptosis, senescence, and the relevant molecular mechanism.

Methods: The ability of adenosine to regulate p53 and Rb protein levels, proliferation, apoptosis and senescence was tested in the human HSC cell line LX-2 and rat primary HSC.

Results: Adenosine receptor activation down-regulates p53 and Rb protein levels, increases BrdU incorporation and increases cell survival in LX-2 cells and in primary rat HSC. These effects of NECA were reproduced by an adenosine A_2A receptor specific agonist (CGS21680) and blocked by a specific antagonist (ZM241385). By day twenty-one of culture primary rat HSC entered senescence and expressed β-gal which was significantly inhibited by NECA. Furthermore, NECA induced down regulation of p53 and Rb and Rac1, and decreased phosphorylation of p44-42 MAP Kinase in LX-2 cells and primary rat HSC. These effects were reproduced by the cAMP analog 8-Bromo-cAMP, and the adenylyl cyclase activator forskolin, and were blocked by PKA inhibitors.

Conclusions: These results demonstrate that A_2A receptor regulates a number of HSC fate decisions and induces greater HSC proliferation, reduces apoptosis and senescence by decreasing p53 and Rb through cAMP-PKA/Rac1/p38 MAPK pathway. This provides a mechanism for adenosine induced HSC regulation and liver fibrosis.

Caffeine inhibits the activation of hepatic stellate cells induced by acetaldehyde via adenosine A2A receptor mediated by the cAMP/PKA/SRC/ERK1/2/P38 MAPK signal pathway

Hepatic stellate cell (HSC) activation is an essential event during alcoholic liver fibrosis. Evidence suggests that adenosine aggravates liver fibrosis via the adenosine A2A receptor (A2AR). Caffeine, which is being widely consumed during daily life, inhibits the action of adenosine. In this study, we attempted to validate the hypothesis that caffeine influences acetaldehyde-induced HSC activation by acting on A2AR. Acetaldehyde at 50, 100, 200, and 400 μM significantly increased HSC-T6 cells proliferation, and cell proliferation reached a maximum at 48 h after exposure to 200 μM acetaldehyde. Caffeine and the A2AR antagonist ZM241385 decreased the cell viability and inhibited the expression of procollagen type I and type III in acetaldehyde-induced HSC-T6 cells. In addition, the inhibitory effect of caffeine on the expression of procollagen type I was regulated by A2AR-mediated signal pathway involving cAMP, PKA, SRC, and ERK1/2. Interestingly, caffeine’s inhibitory effect on the expression of procollagen type III may depend upon the A2AR-mediated P38 MAPK-dependent pathway. Conclusions: Caffeine significantly inhibited acetaldehyde-induced HSC-T6 cells activation by distinct A2AR mediated signal pathway via inhibition of cAMP-PKA-SRC-ERK1/2 for procollagen type I and via P38 MAPK for procollagen type III.

Coffee has hepatoprotective benefits in Brazilian patients with chronic hepatitis C even in lower daily consumption than in American and European populations

The potential role of coffee as a hepatoprotective substance for chronic liver diseases has been widely discussed. Our main aim was to evaluate the effect of coffee intake regarding clinical, biochemical tests and liver biopsy data in treatment naïve patients with chronic hepatitis C. One hundred and thirty-six patients with chronic hepatitis C, diagnosed through liver biopsy, or by means of clinical, ultrasound or endoscopic signs of cirrhosis, were assessed by determination of biochemical tests, metabolic and morphological alterations. Food frequency was scrutinized by using a structured questionnaire. Coffee intake represented more than 90% of the total daily caffeine, and the 75th percentile was 4-Brazilian coffee-cup/day (≥255 mL/day or ≥123 mg caffeine/day). According to caffeine intake, patients were divided into two groups (< or ≥123 mg caffeine/day). Patients with higher ingestion of caffeine had lower serum levels of aspartate aminotransferase (× upper limit of normal) (1.8 ± 1.5 vs 2.3 ± 1.5, p = 0.04), lower frequencies of advanced (F3, F4) fibrosis (23.5% vs 54.5%, p < 0.001) and of histological activity grade (A3, A4) observed in liver biopsies (13.8% vs 36.9%, p < 0.001). By multivariate logistic regression, fibrosis was independently associated with caffeine intake (OR– 0.16; 95%CI – 0.03–0.80; p = 0.026), γ-glutamil transferase serum levels and morphological activity. But only fibrosis was associated with histological activity. In conclusion caffeine consumption greater than 123 mg/day was associated with reduced hepatic fibrosis. In addition, this study supports the assumption that coffee intake has hepatoprotective benefits for Brazilian patients with chronic hepatitis C, even in lower doses than that of American and European population intake.

Adenosine 2A receptor antagonist prevented and reversed liver fibrosis in a mouse model of ethanol-exacerbated liver fibrosis

The effect of moderate alcohol consumption on liver fibrosis is not well understood, but evidence suggests that adenosine may play a role in mediating the effects of moderate ethanol on tissue injury. Ethanol increases the concentration of adenosine in the liver. Adenosine 2A receptor (A2AR) activation is known to enhance hepatic stellate cell (HSC) activation and A2AR deficient mice are protected from fibrosis in mice. Making use of a novel mouse model of moderate ethanol consumption in which female C57BL/6J mice were allowed continued access to 2% (vol/vol) ethanol (11% calories) or pair-fed control diets for 2 days, 2 weeks or 5 weeks and superimposed with exposure to CCl4, we tested the hypothesis that moderate ethanol consumption increases fibrosis in response to carbon tetrachloride (CCl4) and that treatment of mice with an A2AR antagonist prevents and/or reverses this ethanol-induced increase in liver fibrosis. Neither the expression or activity of CYP2E1, required for bio-activation of CCl4, nor AST and ALT activity in the plasma were affected by ethanol, indicating that moderate ethanol did not increase the direct hepatotoxicity of CCl4. However, ethanol feeding enhanced HSC activation and exacerbated liver fibrosis upon exposure to CCl4. This was associated with an increased sinusoidal angiogenic response in the liver. Treatment with A2AR antagonist both prevented and reversed the ability of ethanol to exacerbate liver fibrosis.

CONCLUSION

Moderate ethanol consumption exacerbates hepatic fibrosis upon exposure to CCl4. A2AR antagonism may be a potential pharmaceutical intervention to decrease hepatic fibrosis in response to ethanol.

Nilotinib induces apoptosis and autophagic cell death of activated hepatic stellate cells via inhibition of histone deacetylases

Increasing hepatic stellate cell (HSC) death is a very attractive approach for limiting liver fibrosis. Tyrosine kinase inhibitors have been shown to have anti-fibrotic properties, but the mechanisms are poorly understood. Here, we identified the mechanism of action of the second-generation tyrosine kinase inhibitor nilotinib in inducing HSC death. Human HSC line (LX-2) and rat HSCs were treated with nilotinib and its predecessor, imatinib, in the absence or presence of various blockers, known to interfere with death signaling pathways. Nilotinib, but not imatinib, induced progressive cell death of activated, but not quiescent, HSCs in a dose-dependent manner. Activated HSCs died through apoptosis, as denoted by increased DNA fragmentation and caspase activation, and through autophagy, as indicated by the accumulation of autophagic markers, light chain (LC)3A-II and LC3B-II. Although inhibition of caspases with Z-VAD-FMK suppressed nilotinib-induced HSCs' apoptosis, there was no increase in HSCs' survival, because autophagy was exacerbated. However, blocking the mitochondrial permeability transition pore (mPTP) opening with cyclosporin A completely abolished both apoptosis and autophagy due to nilotinib. Moreover, nilotinib treatment decreased the protein expression of histone deacetylases 1, 2 and 4. Interestingly, pretreament with C646, a selective p300/CBP histone acetyl transferase inhibitor, resulted in diverting nilotinib-induced apoptosis and autophagy towards necrosis. In conclusion, the identification of mPTP as a target of nilotinib in activated HSCs suggests coordination with histone deacetylases inhibition to induce apoptosis and autophagy. Thus, our study provides novel insights into the anti-fibrotic effects of nilotinib.

Activated hepatic stellate cells upregulate transcription of ecto-5'-nucleotidase/CD73 via specific SP1 and SMAD promoter elements

Adenosine is a potent modulator of liver fibrosis and inflammation. Adenosine has been shown to regulate such diverse activities as chemotaxis, contraction, and matrix production in hepatic stellate cells (HSC). Ecto-5'-nucleotidase/CD73 [EC 3.1.3.5] is the rate-limiting enzyme in adenosine production. Cd73-deficient mice are resistant to experimental liver fibrosis and have impaired adenosine generation. However, cell-specific expression and regulation of CD73 within the fibrotic liver have not been defined. In particular, prior evidence demonstrating that liver myofibroblasts, the cells believed to be responsible for matrix formation in the liver, express CD73 is lacking. Thus we tested the hypothesis that HSC and portal fibroblasts (PF), cells that undergo differentiation into liver myofibroblasts, express CD73 in a regulated fashion. We found that CD73 is weakly expressed in quiescent HSC and PF but is markedly upregulated at the transcriptional level in myofibroblastic HSC and PF. We furthermore found that CD73 protein and its functional activity are strongly increased in fibrous septa in rats subjected to experimental fibrosis. To determine the mechanism for the upregulation of Cd73 gene, we cloned the rat Cd73 promoter and then used serial truncation and site-directed mutagenesis to identify key regulatory elements. We identified two consensus SP1 motifs and one SMAD binding site, each of which was necessary for Cd73 gene upregulation. In conclusion, activated HSC upregulate Cd73 gene expression, via specific SP1 and SMAD promoter elements, after myofibroblastic differentiation. The ecto-5'-nucleotidase/CD73 enzyme is a novel cellular marker of activated liver myofibroblasts in vivo and in vitro and thus represents a promising molecular target for antifibrotic therapies in liver diseases.

Adenine induces differentiation of rat hepatic stellate cells

BACKGROUND AND AIMS

Adenine is a uric acid pathway metabolite of no known function, and has recently been identified as a ligand for a rat G protein-coupled receptor. Due to the known role of other uric acid pathway metabolites in HSC biology, we tested the ability of adenine to induce HSC differentiation.

METHODS

RT-PCR was performed for adenine receptor expression in T-6 and primary rat HSC. T-6 and primary rats HSC were cultured with and without adenine, and stellation examined. Next, we examined inhibition of calcium signaling using caged IP(3). To test if adenine inhibits HSC chemotaxis T-6 cells and rat HSCs were cultured with or without adenine for 24 h in a transwell assay with PDGF as the chemoattractant. cDNA was prepared from T-6 and primary HSC for quantification of collagen 1 mRNA using real-time PCR.

RESULTS

We found that mRNA for the adenine receptor is expressed in T-6 cells and primary rat HSC. Also, adenine induces HSC stellation and adenine inhibits IP(3) mediated increase in cytosolic [Ca(2+)](i) and inhibits chemotaxis in T-6 cells and primary rat HSC. Adenine was also shown to up-regulate α-SMA and collagen 1, and this effect is lost by using specific si-RNA for the adenine receptor. Finally, adenine inhibits endothelin-1-induced gel contraction.

CONCLUSIONS

The adenine receptor is present in T-6 cells and primary rats HSC. Adenine, via the adenine receptor, induces morphological change, and cytosolic calcium signaling, inhibits chemotaxis, and up-regulates collagen 1 mRNA in HSCs.

New insights regarding the regulation of chemotaxis by nucleotides, adenosine, and their receptors

The directional movement of cells can be regulated by ATP, certain other nucleotides (e.g., ADP, UTP), and adenosine. Such regulation occurs for cells that are "professional phagocytes" (e.g., neutrophils, macrophages, certain lymphocytes, and microglia) and that undergo directional migration and subsequent phagocytosis. Numerous other cell types (e.g., fibroblasts, endothelial cells, neurons, and keratinocytes) also change motility and migration in response to ATP, other nucleotides, and adenosine. In this article, we review how nucleotides and adenosine modulate chemotaxis and motility and highlight the importance of nucleotide- and adenosine-regulated cell migration in several cell types: neutrophils, microglia, endothelial cells, and cancer cells. We also discuss difficulties in conducting experiments and drawing conclusions regarding the ability of nucleotides and adenosine to modulate the migration of professional and non-professional phagocytes.

Ca²⁺/calmodulin-dependent protein kinase II mediates platelet-derived growth factor-induced human hepatic stellate cell proliferation

BACKGROUND AND AIM

Proliferation and activation of myofibroblastic hepatic stellate cells (HSCs) in response to growth factors is essential for the development of liver fibrosis. As one of the most potent factors, platelet-derived growth factor (PDGF) activates intracellular signals and contributes to sustained HSCs activation. Growing evidence has suggested that the Ca(2+) signal is involved in PDGF pathways. We showed previously for the first time that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is essential for human HSC proliferation. The inhibition of CaMKII by its specific inhibitor, KN-93, significantly decreased the HSC growth and increased expression of cell cycle suppressive regulators P53 and P21.

METHODS

In the present study, we investigated the role of CaMKII in PDGF-induced HSC proliferation and underlying mechanisms.

RESULTS

We confirmed that in human HSCs, PDGF significantly increased CaMKII mRNA levels, protein expression, and phosphorylation. The interruption of CaMKII by KN-93, specific inhibitory peptide (AIP), or specific CaMKII knockdown by its siRNA not only attenuated PDGF-induced HSC proliferation but also ERK1/2 phosphorylation. However, CaMKII had no effect on JNK phosphorylation. In addition, inhibitors of ERK1/2 (PD98059) and JNK (SP600125) did not affect CaMKII expression. Interruption of CaMKII-ERK cascade, not JNK signal, inhibited PDGF-induced HSC proliferation.

CONCLUSION

We confirmed that CaMKII mediated PDGF-induced human HSC proliferation through ERK1/2 but not the JNK mechanism. Our study shed light on CaMKII as a crucial signal in PDGF-activated HSCs and a potential therapeutic point in hepatic fibrosis.

New role of the human equilibrative nucleoside transporter 1 (hENT1) in epithelial-to-mesenchymal transition in renal tubular cells

Epithelial-to-mesenchymal transition (EMT) is an important pro-fibrotic event in which tubular epithelial cells are transformed into myofibroblasts. Nucleoside transporters (NT) are regulated by many factors and processes, some of which are involved in fibrosis, such as cytokines, inflammation, and proliferation. Equilibrative nucleoside transporter 1 (ENT1) has been proved to be the most widely expressed adenosine transporter. In that sense, ENT1 may be a key player in cell damage signaling. Here we analyze the role of human ENT1 (hENT1) in the EMT process in proximal tubular cells. Addition of the main inducer of EMT, the transforming growth factor-β1, to HK-2 cells increased hENT1 mRNA and protein level expression. ENT1-mediated adenosine uptake was also enhanced. When cells were incubated with dipyridamole to evaluate the potential contribution of ENT1 to EMT by blocking its transport activity, EMT was induced. Moreover, the knock down of hENT1 with siRNA induced EMT and collagen production in HK-2 cells. Kidneys isolated from ENT1 knockout mice showed higher levels of interstitial collagen and α-SMA positive cells than wild-type mice. Our results point to a new potential role of hENT1 as a modulator of EMT in proximal tubular cells. In this sense, hENT1 could be involved in renal protection processes, and the loss or reduced expression of hENT1 would lead to an increased vulnerability of cells to the onset and/or progression of renal fibrosis.

Adenosine receptors and fibrosis: a translational review

Adenosine—a purine nucleoside generated extracellularly from adenine nucleotides released by cells as a result of direct stimulation, hypoxia, trauma, or metabolic stress—is a well-known physiologic and pharmacologic agent. Recent studies demonstrate that adenosine, acting at its receptors, promotes wound healing by stimulating both angiogenesis and matrix production. Subsequently, adenosine and its receptors have also been found to promote fibrosis (excess matrix production) in the skin, lungs, and liver, but to diminish cardiac fibrosis. A commonly ingested adenosine receptor antagonist, caffeine, blocks the development of hepatic fibrosis, an effect that likely explains the epidemiologic finding that coffee drinking, in a dose-dependent fashion, reduces the likelihood of death from liver disease. Accordingly, adenosine may be a good target for therapies that prevent fibrosis of the lungs, liver, and skin.

Hepatic stellate cells and astrocytes: Stars of scar formation and tissue repair

Scar formation inhibits tissue repair and regeneration in the liver and central nervous system. Activation of hepatic stellate cells (HSCs) after liver injury or of astrocytes after nervous system damage is considered to drive scar formation. HSCs are the fibrotic cells of the liver, as they undergo activation and acquire fibrogenic properties after liver injury. HSC activation has been compared to reactive gliosis of astrocytes, which acquire a reactive phenotype and contribute to scar formation after nervous system injury, much like HSCs after liver injury. It is intriguing that a wide range of neuroglia-related molecules are expressed by HSCs. We identified an unexpected role for the p75 neurotrophin receptor in regulating HSC activation and liver repair. Here we discuss the molecular mechanisms that regulate HSC activation and reactive gliosis and their contributions to scar formation and tissue repair. Juxtaposing key mechanistic and functional similarities in HSC and astrocyte activation might provide novel insight into liver regeneration and nervous system repair.

Pathogenesis of liver fibrosis

Liver fibrosis is a major cause of morbidity and mortality worldwide due to chronic viral hepatitis and, more recently, from fatty liver disease associated with obesity. Hepatic stellate cell activation represents a critical event in fibrosis because these cells become the primary source of extracellular matrix in liver upon injury. Use of cell-culture and animal models has expanded our understanding of the mechanisms underlying stellate cell activation and has shed new light on genetic regulation, the contribution of immune signaling, and the potential reversibility of the disease. As pathways of fibrogenesis are increasingly clarified, the key challenge will be translating new advances into the development of antifibrotic therapies for patients with chronic liver disease.

Ecto-5'-nucleotidase/CD73 knockdown increases cell migration and mRNA level of collagen I in a hepatic stellate cell line

Ecto-5'-nucleotidase (eNT/CD73, E.C.3.1.3.5) is a glycosyl phosphatidylinositol (GPI)-linked cell-surface protein with several functions, including the local generation of adenosine from AMP, with the consequent activation of adenosine receptors and the salvaging of extracellular nucleotides. It also apparently functions independently of this activity, e.g., in the mediation of cell-cell adhesion. Liver fibrosis can be considered as a dynamic and integrated cellular response to chronic liver injury and the activation of hepatic stellate cells (HSCs) plays a role in the fibrogenic process. eNT/CD73 and adenosine are reported to play an important role in hepatic fibrosis in murine models. Knockdown of eNT/CD73 leads to an increase in mRNA expression of tissue non-specific alkaline phosphatase (TNALP), another AMP-degrading enzyme and thus no alteration is seen in the total ecto-AMPase activity of the cell. eNT/CD73 knockdown also leads to changes in the expression of collagen I and a clear alteration of cell migration. We suggest that eNT/CD73 protein expression controls cell migration and collagen expression in a mechanism independent of changes in nucleotide metabolism.

Extracellular nucleosides and nucleotides regulate liver functions via a complex system of membrane proteins

Nucleosides and nucleotides are now considered as extracellular signalling molecules, like neurotransmitters and hormones. Hepatic cells, amongst other cells, ubiquitously express specific transmembrane receptors that transduce the physiological signals induced by extracellular nucleosides and nucleotides, as well as various cell surface enzymes that regulate the levels of these mediators in the extracellular medium. Here, we cover various aspects of the signalling pathways initiated by extracellular nucleosides and nucleotides in the liver, and discuss their overall impact on hepatic physiology.

Prevention of in vitro hepatic stellate cells activation by the adenosine derivative compound IFC305

We have previously shown that adenosine and the aspartate salt of adenosine (IFC305) reverse pre-established CCl(4)-induced cirrhosis in rats. However, their molecular mechanism of action is not clearly understood. Hepatic stellate cells (HSC) play a pivotal role in liver fibrogenesis leading to cirrhosis, mainly through their activation, changing from a quiescent adipogenic state to a proliferative myofibrogenic condition. Therefore, we decided to investigate the effect of IFC305 on primary cultured rat HSC. Our results reveal that this compound suppressed the activation of HSC, as demonstrated by the maintenance of a quiescent cell morphology, including lipid droplets content, inhibition of α-smooth muscle actin (α-SMA) and collagen α1(I) expression, and up-regulation of MMP-13, Smad7, and PPARγ expression, three key antifibrogenic genes. Furthermore, IFC305 was able to repress the platelet-derived growth factor (PDGF)-induced proliferation of HSC. This inhibition was independent of adenosine receptors stimulation; instead, IFC305 was incorporated into cells by adenosine transporters and converted to AMP by adenosine kinase. On the other hand, addition of pyrimidine ribonucleoside as uridine reversed the suppressive effect of IFC305 on the proliferation and activation of HSC, suggesting that intracellular pyrimidine starvation would be involved in the molecular mechanism of action of IFC305. In conclusion, IFC305 inhibits HSC activation and maintains their quiescence in vitro; these results could explain in part the antifibrotic liver beneficial effect previously described for this compound on the animal model.

Cell death and fibrogenesis

Fibrosis is a common feature of chronic liver injury and is initiated by cell death inside the liver. Hepatocyte death results in apoptotic bodies and other cellular debris, which are phagocytosed by hepatic stellate cells (HSCs), resulting in their activation, proliferation, differentiation, and matrix deposition. This profibrotic effect of cellular death is balanced by an antifibrotic effect of HSC death. Many HSC survival signals are obtained from the extracellular matrix, and active proapoptotic signals are provided by immune cells, particularly natural killer (NK) cells. Quiescent HSCs are relatively resistant to apoptotic signals but become sensitive after activation. The important role of NK cells in inducing HSC apoptosis may explain the increased fibrosis associated with immune suppression (e.g., in the transplant recipient) and HIV infection. HSCs also undergo senescence, which limits their function and sensitizes them to apoptosis.

Caffeine, a drug for all seasons

Increased caffeine consumption is associated with reduced hepatic fibrosis. Apurva A. Modi, Jordan J. Feld, Yoon Park,David E. Kleiner, James E. Everhart, T. Jake Liang, Jay H. Hoofnagle. Hepatology, 2010 Jan;51(1):201-9.

Adenosine: tipping the balance towards hepatic steatosis and fibrosis

Fatty liver is commonly associated with alcohol ingestion and abuse. While the molecular pathogenesis of these fatty changes is well understood, the histochemical and pharmacological mechanisms by which ethanol stimulates these molecular changes remain unknown. During ethanol metabolism, adenosine is generated by the enzyme ecto-5'-nucleotidase, and adenosine production and adenosine receptor activation are known to play critical roles in the development of hepatic fibrosis. We therefore investigated whether adenosine and its receptors play a role in the development of alcohol-induced fatty liver. WT mice fed ethanol on the Lieber-DeCarli diet developed hepatic steatosis, including increased hepatic triglyceride content, while mice lacking ecto-5-nucleotidase or adenosine A1 or A2B receptors were protected from developing fatty liver. Similar protection was also seen in WT mice treated with either an adenosine A1 or A2B receptor antagonist. Steatotic livers demonstrated increased expression of genes involved in fatty acid synthesis, which was prevented by blockade of adenosine A1 receptors, and decreased expression of genes involved in fatty acid metabolism, which was prevented by blockade of adenosine A2B receptors. In vitro studies supported roles for adenosine A1 receptors in promoting fatty acid synthesis and for A2B receptors in decreasing fatty acid metabolism. These results indicate that adenosine generated by ethanol metabolism plays an important role in ethanol-induced hepatic steatosis via both A1 and A2B receptors and suggest that targeting adenosine receptors may be effective in the prevention of alcohol-induced fatty liver.

Changes in E-NTPDase 3 expression and extracellular nucleotide hydrolysis during the myofibroblast/lipocyte differentiation

Hepatic stellate cells (HSC) play a critical role in the development and maintenance of liver fibrosis. HSC are lipocytes that displayed the capacity to develop into myofibroblast-like cells. Ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) regulate the concentration of extracellular nucleotides, signaling molecules that play a role in the pathogenesis of hepatic fibrosis. In the present study, we identified and compared the expressions of E-NTPDase family members in two different phenotypes of the mouse hepatic stellate cell line (GRX) and evaluated the nucleotide hydrolysis by these cells. We show that both phenotypes of GRX cell line expressed NTPDase 3 and 5. However, only activated cells expressed NTPDase 6. In quiescent-like cells, the hydrolysis of triphosphonucleosides was significantly higher, and was related to an increase in Entpd3 mRNA expression. The diphosphonucleosides were hydrolyzed at a similar rate by two phenotypes of GRX cells. We suggest that up-regulation of Entpd3 mRNA expression modulates the extracellular concentration of nucleotides/nucleosides and affect P2-receptor signaling differently in quiescent-like cells and may play a role in the regulation of HSC functions.

A contradictory role of A1 adenosine receptor in carbon tetrachloride- and bile duct ligation-induced liver fibrosis in mice

Mice lacking A(1) adenosine receptors (A(1)AR) were thought to be protected from developing fatty liver; however, the contribution of A(1)AR to hepatic fibrosis has not been explored. Here we found that the expression of A(1)AR was decreased in fibrotic liver induced by chronic carbon tetrachloride (CCl(4)) but increased in that induced by bile duct ligation (BDL). Therefore, we examined whether A(1)AR contributes to hepatic fibrosis in CCl(4) and BDL animal models using A(1)AR knockout mice. Compared with wild-type (WT) mice, hepatic fibrosis resulting from chronic CCl(4) exposure was attenuated in A(1)AR(-/-) mice with markedly decreased collagen deposition and reduced hepatic stellate cell activation, whereas bile duct-ligated A(1)AR(-/-) mice displayed a significant increase in hepatic fibrosis. Hepatocyte damage was reduced in A(1)AR(-/-) mice after a single injection of CCl(4), with down-regulation of CYP2E1 and UCP2 gene expression in livers, which resulted in impaired liver sensitivity to CCl(4). However, BDL caused severe bile infarcts in livers of A(1)AR(-/-) mice, with significantly elevated levels of bile acid compared with those in WT mice. CCl(4) and BDL resulted in different expression patterns of genes involved in fibrogenesis in A(1)AR(-/-) mice. These results indicate that A(1)AR participates in the pathogenesis of hepatic fibrosis with a complex mechanism, and the effect of targeting adenosine and its receptors in the prevention of hepatic fibrosis should be cautiously evaluated.

Adenosine in fibrosis

Adenosine is an endogenous autocoid that regulates a multitude of bodily functions. Its anti-inflammatory actions are well known to rheumatologists since it mediates many of the anti-inflammatory effects of a number of antirheumatic drugs such as methotrexate. However, inflammatory and tissue regenerative responses are intricately linked, with wound healing being a prime example. It has only recently been appreciated that adenosine has a key role in tissue regenerative and fibrotic processes. An understanding of these processes may shed new light on potential therapeutic options in diseases such as scleroderma where tissue fibrosis features prominently.

Adenosine A(2A) receptor mediates microglial process retraction

Cell motility drives many biological processes, including immune responses and embryonic development. In the brain, microglia are immune cells that survey and scavenge brain tissue using elaborate and motile cell processes. The motility of these processes is guided by the local release of chemoattractants. However, most microglial processes retract during prolonged brain injury or disease. This hallmark of brain inflammation remains unexplained. We identified a molecular pathway in mouse and human microglia that converted ATP-driven process extension into process retraction during inflammation. This chemotactic reversal was driven by upregulation of the A(2A) adenosine receptor coincident with P2Y(12) downregulation. Thus, A(2A) receptor stimulation by adenosine, a breakdown product of extracellular ATP, caused activated microglia to assume their characteristic amoeboid morphology during brain inflammation. Our results indicate that purine nucleotides provide an opportunity for context-dependent shifts in receptor signaling. Thus, we reveal an unexpected chemotactic switch that generates a hallmark feature of CNS inflammation.

Inflammasome-mediated regulation of hepatic stellate cells

The inflammasome is a cytoplasmic multiprotein complex that has recently been identified in immune cells as an important sensor of signals released by cellular injury and death. Analogous to immune cells, hepatic stellate cells (HSC) also respond to cellular injury and death. Our aim was to establish whether inflammasome components were present in HSC and could regulate HSC functionality. Monosodium urate (MSU) crystals (100 microg/ml) were used to experimentally induce inflammasome activation in LX-2 and primary mouse HSC. Twenty-four hours later primary mouse HSC were stained with alpha-smooth muscle actin and visualized by confocal microscopy, and TGF-beta and collagen1 mRNA expression was quantified. LX-2 cells were further cultured with or without MSU crystals for 24 h in a transwell chemotaxis assay with PDGF as the chemoattractant. We also examined inhibition of calcium (Ca(2+)) signaling in LX-2 cells treated with or without MSU crystals using caged inositol 1,4,5-triphosphate (IP(3)). Finally, we confirmed an important role of the inflammasome in experimental liver fibrosis by the injection of carbon tetrachloride (CCl(4)) or thioacetamide (TAA) in wild-type mice and mice lacking components of the inflammasome. Components of the inflammasome are expressed in LX-2 cells and primary HSC. MSU crystals induced upregulation of TGF-beta and collagen1 mRNA and actin reorganization in HSCs from wild-type mice but not mice lacking inflammasome components. MSU crystals inhibited the release of Ca(2+) via IP(3) in LX-2 cells and also inhibited PDGF-induced chemotaxis. Mice lacking the inflammasome-sensing and adaptor molecules, NLRP3 and apoptosis-associated speck-like protein containing CARD, had reduced CCl(4) and TAA-induced liver fibrosis. We concluded that inflammasome components are present in HSC, can regulate a variety of HSC functions, and are required for the development of liver fibrosis.