Adenosine-induced IL-6 expression in pituitary folliculostellate cells is mediated via A2b adenosine receptors coupled to PKC and p38 MAPK

A2B adenosine receptor signaling and regulation

The A2B adenosine receptor (A2BR) is one of the four adenosine-activated G protein-coupled receptors. In addition to adenosine, protein kinase C (PKC) was recently found to activate the A2BR. The A2BR is coupled to both Gs and Gi, as well as Gq proteins in some cell types. Many primary cells and cell lines, such as bladder and breast cancer, bronchial smooth muscle, skeletal muscle, and fat cells, express the A2BR endogenously at high levels, suggesting its potentially important role in asthma, cancer, diabetes, and other conditions. The A2BR has been characterized as both pro- and anti-inflammatory, inducing cell type-dependent secretion of IL-6, IL-8, and IL-10. Theophylline and enprofylline have long been used for asthma treatment, although it is still not entirely clear if their A2BR antagonism contributes to their therapeutic effects or side effects. The A2BR is required in ischemic cardiac preconditioning by adenosine. Both A2BR and protein kinase C (PKC) contribute to cardioprotection, and both modes of A2BR signaling can be blocked by A2BR antagonists. Inhibitors of PKC and A2BR are in clinical cancer trials. Sulforaphane and other isothiocyanates from cruciferous vegetables such as broccoli and cauliflower have been reported to inhibit A2BR signaling via reaction with an intracellular A2BR cysteine residue (C210). A full, A2BR-selective agonist, critical to elucidate many controversial roles of the A2BR, is still not available, although agonist-bound A2BR structures have recently been reported.

Activation of adenosine A2B receptor alleviates myocardial ischemia-reperfusion injury by inhibiting endoplasmic reticulum stress and restoring autophagy flux

Myocardial ischemia-reperfusion injury (MIRI) poses a significant threat to patients with coronary heart disease. Adenosine A2A receptors have been known as a protective role in MIRI by regulating autophagy, so we assumed that activation of adenosine A2B receptor (A2BAR) might exert a similar effect during MIRI and underlying mechanism be related to proteostasis maintenance as well. In situ hearts were subjected to 30 min of ischemia and 120 min of reperfusion (IR), while invitro cardiomyocytes from neonatal rats experienced 6 h of oxygen-glucose deprivation followed by 12 h of reoxygenation (OGDR). Initially, we observed that post-ischemia-reperfusion induced autophagy flux blockade and ERS both in vivo and in vitro, evident through the increased expression of p62, LC3II, and BIP, which indicated the deteriorated proteostasis. We used a selective A2BAR agonist, Bay 60-6583, to explore the positive effects of A2BAR on cardiomyocytes and found that A2BAR activation rescued damaged cardiac function and morphological changes in the IR group and improved frail cell viability in the OGDR group. The A2BAR agonist also alleviated the blockage of autophagic flux, coupled with augmented ERS in the IR/OGDR group, which was reassured by using an autophagy inhibitor chloroquine (CQ) and ERS inhibitor (4-PBA) in vitro. Additionally, considering cAMP/PKA as a well-known downstream effector of A2BAR, we utilized H89, a selective PKA inhibitor. We observed that the positive efficacy of Bay 60-6583 was inhibited by H89. Collectively, our findings demonstrate that the A2BAR/cAMP/PKA signaling pathway exerts a protective role in MIRI by mitigating impaired autophagic flux and excessive ERS.

MTAP loss correlates with an immunosuppressive profile in GBM and its substrate MTA stimulates alternative macrophage polarization

Glioblastoma (GBM) is a lethal brain cancer known for its potent immunosuppressive effects. Loss of Methylthioadenosine Phosphorylase (MTAP) expression, via gene deletion or epigenetic silencing, is one of the most common alterations in GBM. Here we show that MTAP loss in GBM cells is correlated with differential expression of immune regulatory genes. In silico analysis of gene expression profiles in GBM samples revealed that low MTAP expression is correlated with an increased proportion of M2 macrophages. Using in vitro macrophage models, we found that methylthioadenosine (MTA), the metabolite that accumulates as a result of MTAP loss in GBM cells, promotes the immunosuppressive alternative activation (M2) of macrophages. We show that this effect of MTA on macrophages is independent of IL4/IL3 signaling, is mediated by the adenosine A_2B receptor, and can be pharmacologically reversed. This study suggests that MTAP loss in GBM cells may contribute to the immunosuppressive tumor microenvironment, and that MTAP status should be considered for characterizing GBM immune states and devising immunotherapy-based approaches for treating MTAP-null GBM.

Mesenchymal stem/stromal cells-derived IL-6 promotes nasopharyngeal carcinoma growth and resistance to cisplatin via upregulating CD73 expression

Previous studies have implicated the important role of mesenchymal stem/stromal cells (MSCs) within tumor microenvironment (TME) in the pathogenesis and progression of nasopharyngeal carcinoma (NPC), but the potential mechanisms are still unclear. Herein, we showed that an elevated IL-6 level was positively correlated with elevated expression of CD73 in TME of NPC. NPC specimens with an IL-6^highCD73^high phenotype showed higher expression levels of gp80, gp130, p-STAT3, MMP-9 and α-SMA, and clinically, a poorer prognosis than those with an IL-6^lowCD73^low phenotype. We found that stimulation with conditioned media derived from IL-6 gene knocked out MSC (MSC^IL6KO-CM) down-regulated the expression of CD73, IL-6, gp80, p-STAT3, and proliferative cell nuclear antigen (PCNA) in CNE-2 NPC cells. Meanwhile, NPC cells co-cultured with MSC^IL6KO-CM were more sensitive to cisplatin than those co-cultured with MSC-CM. Additionally, MSC-derived IL-6 transcriptionally upregulated CD73 expression via activating STAT3 signaling pathway in NPC cells. In summary, our findings suggest that MSCs promote NPC progression and chemoresistance by upregulation of CD73 expression via activating STAT3 signaling pathway.

Shockwaves Inhibit Chondrogenic Differentiation of Human Mesenchymal Stem Cells in Association with Adenosine and A2B Receptors

Extracorporeal shockwave therapy (ESWT) has emerged as the important choice for the treatment of many orthopedic disorders. Our previous mechanistic studies suggest that ESWT promoted osteogenesis of human mesenchymal stem cells (hMSCs) through mechanisms that involve adenosine 5'-triphosphate (ATP) release. In this study, we investigated the effect of ESWT on chondrogenesis of hMSCs. We demonstrate that ESWT treatment caused a significant release of adenosine from hMSCs; ESWT treatment increased the levels of A2B receptor (A2BR) in hMSCs under 3-D culture conditions. ESWT, exogenous adenosine and specialized A2BR agonist suppressed hMSC chondrogenic differentiation through downregulating the expressions of aggrecan (ACAN), Collagen Type I alpha 2(COL1A2), Collagen Type II alpha 1(COL2A1), Sex-Determining Region YBox 9 (SOX9) and Sex-Determining Region YBox 6 (SOX6). Selective A2BR antagonists induced chondrogenic differentiation of hMSCs. This study indicated that shockwave therapy inhibits hMSC chondrogenic differentiation through or partially through regulation of adenosine release and activation of A2B receptor under 3-D culture conditions.

Adenosine A2B Receptors - Mediated Induction of Interleukin-6 in Skeletal Muscle Cells

Objectives

Inflammatory response and cytokine activation are markedly stimulated in skeletal muscle during various conditions. Interleukin-6 (IL-6), a pro-inflammatory cytokine, has pleiotropic effects on skeletal muscle. Adenosine, released by all cell types, binds to a class of G protein-coupled receptors to induce various skeletal muscle effects. The aim of this work was to investigate whether activation of adenosine receptors, particularly adenosine A2B receptors, could stimulate IL-6 gene expression in rat L6 skeletal muscle cells.

Materials and Methods

The rat L6 skeletal muscle cells were cultured in 25 cm² flasks. These differentiated cells were treated and then quantitative reverse transcription-polymerase chain reaction (Probe-based) was used to analyze IL-6 gene expression level among different treatment conditions.

Results

Adenosine-5'-N-ethyluronamide (NECA), a stable adenosine analogue, concentration- and time-dependently stimulates IL-6 gene expression in skeletal muscle cells. The effect of NECA is inhibited by a selective adenosine A2B receptor antagonist, PSB 603. By using cyclic adenosine monophosphate (cAMP)-arising reagent forskolin, cAMP is found to be involved in the up-regulation of IL-6 induction.

Conclusion

Here, a novel relationship between adenosine and IL-6 up-regulation has been demonstrated for the first time; IL-6 up-regulation induced by NECA is mediated by adenosine A2B receptor activation in skeletal muscle and is dependent on mainly a cAMP pathway. Adenosine A2B receptors are, thus, potentially important pharmacological targets in treating inflammation and related diseases in skeletal muscle tissues.

A2B adenosine receptors stimulate IL-6 production in primary murine microglia through p38 MAPK kinase pathway

The hallmark of neuroinflammation is the activation of microglia, the immunocompetent cells of the CNS, releasing a number of proinflammatory mediators implicated in the pathogenesis of neuronal diseases. Adenosine is an ubiquitous autacoid regulating several microglia functions through four receptor subtypes named A₁, A_2A, A_2B and A₃ (ARs), that represent good targets to suppress inflammation occurring in CNS. Here we investigated the potential role of ARs in the modulation of IL-6 secretion and cell proliferation in primary microglial cells. The A_2BAR agonist 2-[[6-Amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]-2-pyridinyl]thio]-acetamide (BAY60-6583) stimulated IL-6 increase under normoxia and hypoxia, in a dose- and time-dependent way. In cells incubated with the blockers of phospholipase C (PLC), protein kinase C epsilon (PKC-ε) and PKC delta (PKC-δ) the IL-6 increase due to A_2BAR activation was strongly reduced, whilst it was not affected by the inhibitor of adenylyl cyclase (AC). Investigation of cellular signalling involved in the A_2BAR effect revealed that only the inhibitor of p38 mitogen activated protein kinase (MAPK) was able to block the agonist's effect on IL-6 secretion, whilst inhibitors of pERK1/2, JNK1/2 MAPKs and Akt were not. Stimulation of p38 by BAY60-6583 was A_2BAR-dependent, through a pathway affecting PLC, PKC-ε and PKC-δ but not AC, in both normoxia and hypoxia. Finally, BAY60-6583 increased microglial cell proliferation involving A_2BAR, PLC, PKC-ε, PKC-δ and p38 signalling. In conclusion, A_2BARs activation increased IL-6 secretion and cell proliferation in murine primary microglial cells, through PLC, PKC-ε, PKC-δ and p38 pathways, thus suggesting their involvement in microglial activation and neuroinflammation.

Adenosine receptor neurobiology: overview

Adenosine is a naturally occurring nucleoside that is distributed ubiquitously throughout the body as a metabolic intermediary. In the brain, adenosine functions as an important upstream neuromodulator of a broad spectrum of neurotransmitters, receptors, and signaling pathways. By acting through four G-protein-coupled receptors, adenosine contributes critically to homeostasis and neuromodulatory control of a variety of normal and abnormal brain functions, ranging from synaptic plasticity, to cognition, to sleep, to motor activity to neuroinflammation, and cell death. This review begun with an overview of the gene and genome structure and the expression pattern of adenosine receptors (ARs). We feature several new developments over the past decade in our understanding of AR functions in the brain, with special focus on the identification and characterization of canonical and noncanonical signaling pathways of ARs. We provide an update on functional insights from complementary genetic-knockout and pharmacological studies on the AR control of various brain functions. We also highlight several novel and recent developments of AR neurobiology, including (i) recent breakthrough in high resolution of three-dimension structure of adenosine A2A receptors (A2ARs) in several functional status, (ii) receptor-receptor heterodimerization, (iii) AR function in glial cells, and (iv) the druggability of AR. We concluded the review with the contention that these new developments extend and strengthen the support for A1 and A2ARs in brain as therapeutic targets for neurologic and psychiatric diseases.

Osteoblast differentiation and survival: A role for A2B adenosine receptor allosteric modulators

The A2B adenosine receptor (A2B AR), activated in response to high levels of endogenous adenosine, is the major AR subtype involved in mesenchymal stem cell (MSC) differentiation to osteoblasts and bone formation. For this reason, targeting of A2B AR with selective allosteric modulators may represent a promising pharmacological approach to the treatment of bone diseases. Herein, we report the characterization of a 3-keto-indole derivative, 2-(1-benzyl-1H-indol-3-yl)-2-oxo-N-phenylacetamide (KI-7), as A2B AR positive allosteric modulator in MSCs, demonstrating that this compound is able to potentiate the effects of either adenosine and synthetic orthosteric A2B AR agonists in mediating osteoblast differentiation in vitro. In detail, we observed that MSC treatment with KI-7 determined an increase in the expression of osteoblast-related genes (Runx2 and osterix) and osteoblast marker proteins (phosphatase alkaline and osteocalcin), associated with a stimulation of osteoblast mineralization. In the early phase of differentiation programme, KI-7 significantly potentiated physiological and A2B AR agonist-mediated down-regulation of IL-6 release. Conversely, during the late stage of differentiation, when most of the cells have an osteoblast phenotype, KI-7 caused a sustained raise in IL-6 levels and an improvement in osteoblast viability. These data suggest that a positive allosteric modulation of A2B AR not only favours MSC commitment to osteoblasts, but also ensures a greater survival of mature osteoblasts. Our study paves the way for a therapeutic use of selective positive allosteric modulators of A2B AR in the control of osteoblast differentiation, bone formation and fracture repair.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

Vascular endothelial growth factor secretion from pituitary folliculostellate cells: role of KATP channels

Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen responsible for physiological and pathological angiogenesis. Abnormal regulation of VEGF expression in anterior pituitary folliculostellate (FS) cells has been implicated in pituitary tumour progression. FS and endocrine cells express VEGF, which is considered to be secreted by the constitutive pathway. The present study investigated the mechanism of VEGF secretion in TtT/GF cells, a mouse FS cell line. TtT/GF cells were shown to express VEGF(164), the most potent and bioavailable isoform of VEGF. Immunofluorescence and immunogold electron microscopy localised VEGF to the cytoplasm and small electron-lucent vesicles. Pituitary adenylate cyclase-activating polypeptide (PACAP), a well-documented stimulant of VEGF secretion, caused a robust increase in VEGF secretion over 24 h. Glyburide, an ABCA1 and K(ATP) channel blocker, also caused an increase in VEGF secretion when applied alone, and amplified the response to PACAP. Other ABCA1 transport blockers did not affect VEGF secretion. Exposure of TtT/GF cells to cycloheximide with PACAP or glyburide inhibited the increased secretion of VEGF, consistent with control of secretion at the transcription level. The SUR2B/Kir6.1 form of K(ATP) channels was shown to be expressed by TtT/GF cells. Diazoxide, a K(ATP) activator, inhibited PACAP- and PACAP + glyburide-stimulated VEGF secretion but not that of glyburide alone. These data suggest that K(ATP) channels are expressed by FS cells and play a significant role in the control of VEGF secretion, and also that activation of K(ATP) channels inhibits the secretion of VEGF at the level of transcription.

Adenosine A2B receptor-mediated leukemia inhibitory factor release from astrocytes protects cortical neurons against excitotoxicity

Background

Neuroprotective and neurotrophic properties of leukemia inhibitory factor (LIF) have been widely reported. In the central nervous system (CNS), astrocytes are the major source for LIF, expression of which is enhanced following disturbances leading to neuronal damage. How astrocytic LIF expression is regulated, however, has remained an unanswered question. Since neuronal stress is associated with production of extracellular adenosine, we investigated whether LIF expression in astrocytes was mediated through adenosine receptor signaling.

Methods

Mouse cortical neuronal and astrocyte cultures from wild-type and adenosine A_2B receptor knock-out animals, as well as adenosine receptor agonists/antagonists and various enzymatic inhibitors, were used to study LIF expression and release in astrocytes. When needed, a one-way analysis of variance (ANOVA) followed by Bonferroni post-hoc test was used for statistical analysis.

Results

We show here that glutamate-stressed cortical neurons induce LIF expression through activation of adenosine A_2B receptor subtype in cultured astrocytes and require signaling of protein kinase C (PKC), mitogen-activated protein kinases (MAPKs: p38 and ERK1/2), and the nuclear transcription factor (NF)-κB. Moreover, LIF concentration in the supernatant in response to 5′-N-ethylcarboxamide (NECA) stimulation was directly correlated to de novo protein synthesis, suggesting that LIF release did not occur through a regulated release pathway. Immunocytochemistry experiments show that LIF-containing vesicles co-localize with clathrin and Rab11, but not with pHogrin, Chromogranin (Cg)A and CgB, suggesting that LIF might be secreted through recycling endosomes. We further show that pre-treatment with supernatants from NECA-treated astrocytes increased survival of cultured cortical neurons against glutamate, which was absent when the supernatants were pre-treated with an anti-LIF neutralizing antibody.

Conclusions

Adenosine from glutamate-stressed neurons induces rapid LIF release in astrocytes. This rapid release of LIF promotes the survival of cortical neurons against excitotoxicity.

A novel mechanism of control of NFκB activation and inflammation involving A2B adenosine receptors

The nuclear factor kappa B (NFκB) pathway controls a variety of processes, including inflammation, and thus, the regulation of NFκB has been a continued focus of study. Here, we report a newly identified regulation of this pathway, involving direct binding of the transcription factor NFκB1 (the p105 subunit of NFκB) to the C-terminus of the A(2B) adenosine receptor (A(2B)AR), independent of ligand activation. Intriguingly, binding of A(2B)AR to specific sites on p105 prevents polyubiquitylation and degradation of p105 protein. Ectopic expression of the A(2B)AR increases p105 levels and inhibits NFκB activation, whereas p105 protein levels are reduced in cells from A(2B)AR-knockout mice. In accordance with the known regulation of expression of anti- and pro-inflammatory cytokines by p105, A(2B)AR-null mice generate less interleukin (IL)-10, and more IL-12 and tumor necrosis factor (TNF-α). Taken together, our results show that the A(2B)AR inhibits NFκB activation by physically interacting with p105, thereby blocking its polyubiquitylation and degradation. Our findings unveil a surprising function for the A(2B)AR, and provide a novel mechanistic insight into the control of the NFκB pathway and inflammation.

Regulation of the p38 mitogen-activated protein kinase and dual-specificity phosphatase 1 feedback loop modulates the induction of interleukin 6 and 8 in cells infected with coronavirus infectious bronchitis virus

Induction of pro-inflammatory response is a crucial cellular process that detects and controls the invading viruses at early stages of the infection. Along with other innate immunity, this nonspecific response would either clear the invading viruses or allow the adaptive immune system to establish an effective antiviral response at late stages of the infection. The objective of this study was to characterize cellular mechanisms exploited by coronavirus infectious bronchitis virus (IBV) to regulate the induction of two pro-inflammatory cytokines, interleukin (IL)-6 and IL-8, at the transcriptional level. The results showed that IBV infection of cultured human and animal cells activated the p38 mitogen-activated protein kinase (MAPK) pathway and induced the expression of IL-6 and IL-8. Meanwhile, IBV has developed a strategy to counteract the induction of IL-6 and IL-8 by inducing the expression of dual-specificity phosphatase 1 (DUSP1), a negative regulator of the p38 MAPK, in order to limit the production of an excessive amount of IL-6 and IL-8 in the infected cells. As activation of the p38 MAPK pathway and induction of IL-6 and IL-8 may have multiple pathogenic effects on the whole host as well as on individual infected cells, regulation of the p38 MAPK and DUSP1 feedback loop by IBV may modulate the pathogenesis of the virus.

Methylmercury-induced IL-6 release requires phospholipase C activities

Methylmercury (MeHg) is a neurotoxin capable of causing severe damage to the CNS, especially in the developing fetus. Glia in the CNS release a number of cytokines that are important for proper CNS development and function. We reported earlier that MeHg could induce interleukin-6 (IL-6) release in primary mouse glia. This finding is significant considering previous reports indicating that sustained IL-6 exposure could be detrimental to cerebellar granule neurons, one of the major cellular targets of MeHg cytotoxicity. By using pharmacological antagonists against phophatidycholine- and phosphoinositol-specific phospholipase C, the current study indicated that phospholipase C activity was necessary for MeHg-induced IL-6 release. Results from pharmacological antagonists further suggested that the calcium signaling initiated by phospholipase C appeared essential for this event. In contrast, protein kinase C activity did not appear to be important. Even though mitogen-activated protein kinases were important for IL-6 release in some experimental systems, these enzymes did not appear to be required for MeHg-induced IL-6 release in glia. Based on these data and those reported by us and others, there is a possibility that MeHg-induced phospholipase C activation initiates a calcium signaling that causes phospholipase A(2) activation. This, in turn, leads to arachidonic acid and lysophosphatidyl choline generation, both of which are potent inducers for IL-6 release.

Adenosine regulates thrombomodulin and endothelial protein C receptor expression in folliculostellate cells of the pituitary gland

Adenosine stimulates the release of interleukin 6 (IL-6) and vascular endothelial growth factor from folliculostellate cells of the anterior pituitary gland indicating that such cells are also involved in the communication between the immune and endocrine systems during stress and inflammation. In order to understand the precise actions of adenosine on folliculostellate cells, DNA microarray analysis was used to determine global changes in gene expression. Hierarchical clusters revealed, of the genes that had altered expression, the majority were suppressed and many, such as B cell translocation gene 2 and cyclin-dependent kinase inhibitor 2b were related to cell cycle arrest or inhibition of proliferation. Several of the up-regulated genes were associated with cytokine signalling or membrane receptor activity. The most notable of these being IL-6, sulfiredoxin 1, endothelial protein C receptor (EPCR) and thrombomodulin (THBD) which can all play a role in controlling inflammation. The EPCR and THBD pathway is well known in anti-coagulation but also has anti-inflammatory and anti-apoptotic properties. Up-regulation of EPCR and THBD in folliculostellate cells was confirmed by qRT-PCR and western blotting analysis and their expression were also demonstrated in many of the hormone-secreting cells of the anterior pituitary gland. Our findings suggest that adenosine can stimulate expression of stress and inflammation related genes from folliculostellate cells of the anterior pituitary gland. These genes include EPCR and THBD, neither of which has been previously identified in the pituitary gland.

Signaling by purinergic receptors and channels in the pituitary gland

Adenosine 5'-triphosphate is frequently released by cells and acts as an agonist for G protein-coupled P2Y receptors and ligand-gated P2X cationic channels in numerous tissues. The breakdown of ATP by ectonucleotidases not only terminates its extracellular messenger functions, but also provides a pathway for the generation of two additional agonists: adenosine 5'-diphosphate, acting via some P2Y receptors, and adenosine, a native agonist for G protein-coupled adenosine receptors. In the pituitary gland, adenosine 5'-triphosphate is released from the endings of magnocellular hypothalamic neurons and by anterior pituitary cells through pathway(s) that are still not well characterized. This gland also expresses several members of each family of purinergic receptors. P2X and adenosine receptors are co-expressed in the somata and nerve terminals of vasopressin-releasing neurons as well as in some secretory pituitary cells. P2X receptors stimulate electrical activity and modulate InsP(3)-dependent calcium release from intracellular stores, whereas adenosine receptors terminate electrical activity. Calcium-mobilizing P2Y receptors are expressed in pituicytes, folliculo-stellate cells and some secretory cells of the anterior pituitary.

Discovery of N-(5,6-diarylpyridin-2-yl)amide derivatives as potent and selective A(2B) adenosine receptor antagonists

The synthesis and SAR of a series of N-(5,6-diarylpyridin-2-yl)amide derivatives as potent A(2B) adenosine receptor antagonists is described. Several compounds showed good selectivity versus other adenosine receptors. The potent and selective analogue 9 was shown to have good oral bioavailability in the rat.

Discovery of potent and selective bicyclic A(2B) adenosine receptor antagonists via bioisosteric amide replacement

Several new potent and selective A(2B) adenosine receptor antagonists have been prepared in which the aryl-amide moiety of the lead series, exemplified by 1a, has been replaced by bioisosteric bicyclic moieties. Although the majority of compounds had generally improved microsomal stability as compared to 1a, this was not translated into overall improvements in the pharmacokinetic profiles of a representative set of compounds.

Stimulation of adenosine A(2B) receptors induces interleukin-6 secretion in cardiac fibroblasts via the PKC-delta-P38 signalling pathway

BACKGROUND AND PURPOSE

Inflammatory response and cytokine activation are markedly stimulated after myocardial infarction, and contribute to cardiac remodelling. Interleukin-6 (IL-6), a pro-inflammatory cytokine, has pleiotropic effects on cardiac remodelling. Adenosine, released by all cell types, binds to a class of G protein-coupled receptors to induce various cardiovascular effects. The aim of this work was to investigate whether activation of adenosine receptors, particularly A(2B) adenosine receptors, could stimulate IL-6 secretion in cardiac fibroblasts (CFs).

EXPERIMENTAL APPROACH

elisa was used to assess IL-6 concentration in supernatant, and immunostaining was used to analyse IL-6 protein level in CFs. The levels of phosphorylated and total p38, extracellular signal-regulated kinase, c-Jun N-terminal kinase and protein kinase C-delta (PKC-delta) were determined by Western blot analysis.

KEY RESULTS

Adenosine-5'-N-ethyluronamide (NECA), a stable adenosine analogue, dose- and time-dependently stimulated IL-6 secretion in CFs. The effect of NECA was dose-dependently inhibited by an A(2B) antagonist, and silencing of the A(2B) receptor also inhibited IL-6 secretion. By using PKC isoform-selective inhibitors and translocation peptide inhibitors, the PKC-delta isoform was found to be involved in the up-regulation of IL-6 production. Inhibition of p38 by SB203580, and adenoviral transfer of dominant-negative p38 inhibited NECA-induced IL-6 production. Furthermore, PKC-delta functioned as an upstream regulator of p38 MAPK in this process.

CONCLUSIONS AND IMPLICATIONS

We demonstrated a novel relationship between adenosine and IL-6 secretion, in that IL-6 secretion induced by NECA was mediated by adenosine A(2B) receptor activation in CFs and was dependent on a PKCdelta-P38 pathway.

Purinergic regulation of hypothalamopituitary functions

The hypothalamus controls the release of hormones by the pituitary and is involved in control of food and water intake, sexual behavior, reproduction and daily cycles in physiological state and behavior, temperature regulation and emotional responses. Adenosine-5'-triphosphate (ATP) and its metabolic products contribute to these functions, acting as agonists for adenosine and P2Y receptors and two-transmembrane domain P2X receptor channels. This review summarizes the recent findings on purinergic receptor expression and their roles in signaling and cellular function in secretory and supporting cells of the hypothalamopituitary system. ATP secretion by these tissues, the enzymes involved in ATP hydrolysis, and the relevance of this pathway for sequential activation of receptors and termination of signaling are also discussed.

5'-N-ethylcarboxamide induces IL-6 expression via MAPKs and NF-kappaB activation through Akt, Ca(2+)/PKC, cAMP signaling pathways in mouse embryonic stem cells

Many studies suggest that adenosine modulates cell responses in a wide array of tissues through potent and selective regulation of cytokine production. This study examined the effects of adenosine on interleukin (IL)-6 expression and its related signal pathways in mouse embryonic stem (ES) cells. In this study, the adenosine analogue 5'-N-ethylcarboxamide (NECA) increased IL-6 protein expression level. Mouse ES cells expressed the A(1), A(2A), A(2B), and A(3) adenosine receptors (ARs), whose expression levels were increased by NECA and NECA-induced increase of IL-6 mRNA expression or secretion level was inhibited by the non-specific AR inhibitor, caffeine. NECA increased Akt and protein kinase C (PKC) phosphorylation, intracellular Ca(2+) and cyclic adenosine monophosphate (cAMP) levels, which were blocked by caffeine. On the other hand, NECA-induced IL-6 secretion was partially inhibited by Akt inhibitor, bisindolylmaleimide I (PKC inhibitor), SQ 22536 (adenylate cyclate inhibitor) and completely blocked by the 3 inhibitor combination treatment. In addition, NECA increased mitogen activated protein kinase' (MAPK) phosphorylation, which were partially inhibited by the Akt inhibitor, bisindolylmaleimide I, and SQ 22536 and completely blocked by the 3 inhibitor combination treatment. NECA-induced increases of IL-6 protein expression and secretion levels were inhibited by MAPK inhibition. NECA-induced increase of nuclear factor (NF)-kappaB phosphorylation was inhibited by MAPK inhibitors. NECA also increased cAMP response element-binding protein (CREB) phosphorylation, which was blocked by MAPK or NF-kappaB inhibitors. Indeed, NECA-induced increase of IL-6 protein expression and secretion was blocked by NF-kappaB inhibitors. In conclusion, NECA stimulated IL-6 expression via MAPK and NF-kappaB activation through Akt, Ca(2+)/PKC, and cAMP signaling pathways in mouse ES cells. J. Cell. Physiol. 219: 752-759, 2009. (c) 2009 Wiley-Liss, Inc.

Differential role of the carboxy-terminus of the A(2B) adenosine receptor in stimulation of adenylate cyclase, phospholipase Cbeta, and interleukin-8

In human mast cells and microvascular endothelial cells, the A(2B) adenosine receptor controls at least three independent signaling pathways, i.e., Gs-mediated stimulation of adenylate cyclase, Gq-mediated stimulation of phospholipase Cbeta, and Gs/Gq-independent upregulation of IL-8. Functional analysis of cells transfected with full-length and truncated receptor constructs revealed that the A(2B) receptor C-terminus is important for coupling to Gs and Gq proteins. Removal of the entire cytoplasmic portion in the A(2B) receptor C-terminus rendered it incapable of stimulating adenylate cyclase and phospholipase Cbeta. Conversely, removal of the distal 16 amino acids facilitated signal transduction from the receptor to the downstream Gs but not Gq proteins. However, the A(2B) receptor C-terminus is not essential for upregulation of IL-8. Analysis of chimeric A(2A)/A(2B) receptors demonstrated that only chimeras containing the third intracellular loop of the A(2B) receptor mediated agonist-dependent IL-8 reporter stimulation, suggesting that this domain is important for upregulation of IL-8.

Effect of A2B adenosine receptor gene ablation on proinflammatory adenosine signaling in mast cells

Pharmacological studies suggest that A(2B) adenosine receptors mediate proinflammatory effects of adenosine in human mast cells in part by up-regulating production of Th2 cytokines and angiogenic factors. This concept has been recently challenged by the finding that mast cells cultured from bone marrow-derived mast cells (BMMCs) of A(2B) knockout mice display an enhanced degranulation in response to FcepsilonRI stimulation. This finding was interpreted as evidence of anti-inflammatory functions of A(2B) receptors and it was suggested that antagonists with inverse agonist activity could promote activation of mast cells. In this report, we demonstrate that genetic ablation of the A(2B) receptor protein has two distinct effects on BMMCs, one is the previously reported enhancement of Ag-induced degranulation, which is unrelated to adenosine signaling; the other is the loss of adenosine signaling via this receptor subtype that up-regulates IL-13 and vascular endothelial growth factor secretion. Genetic ablation of A(2B) receptors had no effect on A(3) adenosine receptor-dependent potentiation of Ag-induced degranulation in mouse BMMCs, but abrogated A(2B) adenosine receptor-dependent stimulation of IL-13 and vascular endothelial growth factor secretion. Adenosine receptor antagonists MRS1706 and DPCPX with known inverse agonist activity at the A(2B) subtype inhibited IL-13 secretion induced by the adenosine analog NECA, but did not mimic the enhanced Ag-induced degranulation observed in A(2B) knockout BMMCs. Thus, our study confirmed the proinflammatory role of adenosine signaling via A(2B) receptors and the anti-inflammatory actions of A(2B) antagonists in mouse BMMCs.

Adenosine promotes IL-6 release in airway epithelia

In the airway epithelia, extracellular adenosine modulates a number of biological processes. However, little is known about adenosine's role in the inflammatory responses of airway epithelial cells. Recent studies suggest that the chronic elevation of extracellular adenosine in mice leads to pulmonary inflammation and fibrosis. Yet, the underlying molecular mechanism has not been well understood and little attention has been paid to the role of airway epithelia in adenosine-triggered inflammation. In the present work, we examined the role of adenosine in releasing IL-6 from airway epithelia. In Calu-3 human airway epithelial cells, apical but not basolateral adenosine elicited robust, apically polarized release of IL-6, along with proinflammatory IL-8. Both protein kinase A and protein kinase C mediated the adenosine-induced IL-6 release, at least partly via phosphorylation of CREB. Protein kinase C appeared to phosphorylate CREB through activating ERK. In addition, A2A but not A2B adenosine receptors were specifically required for the adenosine-induced IL-6 release. Furthermore, in rat bronchoalveolar lavage fluid, adenosine triggered the release of IL-6 as well as proinflammatory IL-1beta. Adenosine also mediated the release of a considerable portion of the LPS-induced IL-6 in rat bronchoalveolar lavage fluid. Our findings provide a possible molecular link between extracellular adenosine elevation and lung inflammation and fibrosis.

Netrin-1-independent adenosine A2b receptor activation regulates the response of axons to netrin-1 by controlling cell surface levels of UNC5A receptors

Growth cone response to the bifunctional guidance cue netrin-1 is regulated by the activity of intracellular signaling intermediates such as protein kinase C-alpha (PKCalpha) and adenylyl cyclase. Among the diverse cellular events these enzymes regulate is receptor trafficking. Netrin-1, itself, may govern the activity of these signaling intermediates, thereby regulating axonal responses to itself. Alternatively, other ligands, such as activators of G protein-coupled receptors, may regulate responses to netrin-1 by governing these signaling intermediates. Here, we investigate the mechanisms controlling activation of PKCalpha and the subsequent downstream regulation of cell surface UNC5A receptors. We report that activation of adenosine receptors by adenosine analogs, or activation of the putative netrin-1 receptor, the G protein-coupled receptor adenosine A2b receptor (A2bR) results in PKCalpha-dependent removal of UNC5A from the cell surface. This decrease in cell surface UNC5A reduces the number of growth cones that collapse in response to netrin-1 and converts repulsion to attraction. We show these A2bR-mediated alterations in axonal response are not because of netrin-1 because netrin-1 neither binds A2bR, as assayed by protein overlay, nor stimulates PKCalpha-dependent UNC5A surface loss. Our results demonstrate that netrin-1-independent A2bR signaling governs the responsiveness of a neuron to netrin-1 by regulating the levels of cell surface UNC5A receptor.

Effect of A2B adenosine receptor gene ablation on adenosine-dependent regulation of proinflammatory cytokines

Pharmacological studies suggest that A(2B) adenosine receptors mediate proinflammatory effects of adenosine. This concept was recently challenged by the finding that A(2B) adenosine receptor knockout (A(2B)KO) mice had moderate inflammation due to elevated basal plasma tumor necrosis factor (TNF)-alpha and an exaggerated response to lipopolysaccharide (LPS) challenge. However, it is unclear whether this phenomenon actually reflects the loss of putative taming of proinflammatory cytokine production via activation of A(2B) receptors by endogenous adenosine. In this report, we examined adenosine receptor-dependent regulation of interleukin (IL)-6 and TNF-alpha blood plasma levels in A(2B)KO and wild-type mice in vivo and their release from peritoneal macrophages ex vivo. Stimulation of adenosine receptors with 5'-N-ethylcarboxamidoadenosine (NECA) up-regulated IL-6 and suppressed LPS-induced TNF-alpha in wild-type mice. The selective A(2B) antagonists 3-isobutyl-8-pyrrolidinoxanthine and 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl)xanthine (MRS 1754) inhibited NECA-induced IL-6 release but not the suppression of LPS-induced TNF-alpha secretion from macrophages. Genetic ablation of A(2B) receptors abrogated NECA-induced increases in IL-6 release from mouse peritoneal macrophages and dramatically reduced the ability of NECA to raise IL-6 plasma levels in vivo. In contrast, the absence of A(2B) adenosine receptors did not affect NECA-induced suppression of LPS-activated TNF-alpha release in macrophages, nor did it reduce the ability of NECA to suppress LPS-induced increase in TNF-alpha plasma levels in vivo. Thus, our results indicate that stimulation of A(2B) receptors up-regulates the proinflammatory cytokine IL-6 and argue against the recently suggested anti-inflammatory role of A(2B) receptors in suppression of LPS-stimulated TNF-alpha production by adenosine.

Adenosine stimulates connexin 43 expression and gap junctional communication in pituitary folliculostellate cells

Adenosine is known to stimulate interleukin (IL)-6 and vascular endothelial growth factor (VEGF) secretion from pituitary TtT/GF folliculostellate [corrected] (FS) cells indicating that it is an important paracrine regulator of anterior pituitary function. This study demonstrates that rodent anterior pituitary cell lines produce extracellular adenosine that is able to increase intercellular gap junction communication in FS cells. Ecto-5'-nucleotidase (CD73), the enzyme that generates adenosine from AMP, was demonstrated by immunocytochemistry in approximately 20% of anterior pituitary cells, and some of these cells colocalized with prolactin and growth hormone. CD73 mRNA and protein were detected in GH3 and MMQ (somatotroph-lactotroph lineages) and TtT/GF cells, and enzyme activity was demonstrated by the conversion of exogenously added fluorescent ethenoAMP to ethenoadenosine. Adenosine production, as measured by HPLC, was detected in GH3 (1 microM/h) and MMQ (3 microM/h) but not in TtT/GF cells. Adenosine (EC50: 0.5 microM) and NECA (universal adenosine receptor agonist; EC50 0.1 microM) stimulated connexin 43 (Cx43) mRNA and protein expression within 1-2 h in TtT/GF cells. Adenosine and NECA also stimulated gap junctional intercellular communication (as assessed by transmission of Alexa Fluor 488) by 6- to 8-fold in comparison with untreated TtT/GF cells. In cocultures of MMQ and TtT/GF cells, Cx43 expression in TtT/GF cells increased in proportion to the number of MMQ cells plated out. These data suggest that adenosine, formed locally in the anterior pituitary gland can stimulate gap junction communication in FS cells.

New insights into the regulation of inflammation by adenosine

Adenosine, long known as a regulator of cardiovascular function, has recently been identified as a significant paracrine inhibitor of inflammation that acts primarily by activation of A2A adenosine receptors (A2AARs) on lymphoid or myeloid cells. In this issue of the JCI, Yang et al. describe a proinflammatory phenotype resulting from deletion of the gene encoding the A2B adenosine receptor (A2BAR) in the mouse, suggesting that activation of the A2BAR can also have antiinflammatory effects (see the related article beginning on page 1913). Nevertheless, the role of the A2BAR remains enigmatic since its activation can either stimulate or inhibit the release of proinflammatory cytokines in different cells and tissues.

Mechanism of 2-chloroadenosine toxicity to PC3 cell line

BACKGROUND

2-CADO inhibits the growth of several types of cells and causes apoptosis by a mechanism which involves adenosine receptors or cellular uptake or both.

METHODS

Androgen-independent (PC3) prostate cancer cells were used in the study and proliferation, cell-cycle progression, and apoptosis analyzed. Deoxy-and ribonucleoside triphosphate pools were determined by HPLC. The molecular mechanism was examined by assessing the involvement of DNA synthesizing enzymes in the cellular response.

RESULTS

2-CADO treatment dramatically reduced the number of prostate cancer cells and permanently stopped cell-cycle progression in the S-phase. The role of 2-CADO in prostate cancer cells is uptake-mediated and followed by sequential phosphorylations to 2-Cl-ATP that irreversibly inhibits several key-enzymes for DNA biosynthesis.

CONCLUSIONS

Arrest of DNA synthesis responsible for toxicity of 2-CADO to PC3 cells is due to the lack of substrates for DNA polymerization caused by irreversible inhibition of purine/pyrimidine ribo-and 2-deoxyribonucleotides salvage enzymes.

Human osteoblast precursors produce extracellular adenosine, which modulates their secretion of IL-6 and osteoprotegerin

We showed that human osteoprogenitor cells produced adenosine and expressed ecto-5'-nucleotidase and all four adenosine receptor subtypes. Adenosine stimulated IL-6 but inhibited osteoprotegerin secretion, suggesting that adenosine is a newly described regulator of progenitor cell function.

INTRODUCTION

Maintaining skeletal homeostasis relies on there being a balance between bone formation and resorption; an imbalance between these processes can lead to diseases such as osteoporosis and rheumatoid arthritis. Recent reports showed that locally produced ATP, acting through P2 receptors, has pronounced effects on bone formation. However, ATP can be enzymatically cleaved to adenosine that has little or no activity at P2 receptors but mediates its action through the P1 family of receptors. We studied whether adenosine may also have an important role in controlling bone cell differentiation and function.

MATERIALS AND METHODS

Extracellular adenosine levels were analyzed by high-performance liquid chromatography in HCC1 and bone marrow stromal (BMS) cells. Ecto-5'-nucleotidase (CD73) expression and activity was determined by RT-PCR, immunocytochemistry, and the cleavage of etheno-AMP to ethenoadenosine. Adenosine receptor expression and activity were determined by RT-PCR and cAMP measurements. The effects of adenosine receptor agonists on IL-6, osteoprotegerin (OPG), and RANKL expression were determined by ELISA and QRT-PCR.

RESULTS

HCC1 and BMS cells produce adenosine and express CD73 and all four adenosine receptor subtypes. The A2b receptor was shown to be functionally dominant in HCC1 cells, as determined by cAMP production and in its stimulation of IL-6 secretion. Adenosine receptor agonism also inhibited OPG secretion and OPG but not RANKL mRNA expression.

CONCLUSIONS

Our findings show that HCC1 and primary BMS cells produce adenosine, express CD73 and all four adenosine receptor subtypes. In HCC1 cells, adenosine has a potent stimulatory action on IL-6 secretion but an inhibitory action on OPG expression. These data show for the first time that adenosine may be an important regulator of progenitor cell differentiation and hence an important local contributor to the regulation of bone formation and resorption.

AICAR stimulates IL-6 production via p38 MAPK in cardiac fibroblasts in adult mice: a possible role for AMPK

Though known as a sensor of energy balance, AMP-activated protein kinase (AMPK) was recently shown to limit damage and apoptotic activity and contribute to the late preconditioning in heart. Interleukin-6 was also reported to involve in anti-apoptosis and cardio-protection in myocardium. Interestingly, both AMPK activity and IL-6 level were increased in response to ischemia, hypertrophy and oxidative stress. To determine whether AMPK activation will promote IL-6 production, cardiac fibroblasts (CFs) from mice were incubated with AMPK activator, 5-aminoimidazole-4-carboxamide-1-4-ribofuranoside (AICAR). The results demonstrated that AICAR time and dose-dependently stimulated IL-6 production by ELISA and immunofluorescence. Pretreatment with p38 mitogen-activated protein kinase (MAPK) inhibitor blocked AICAR-induced IL-6 production; furthermore, AICAR-activated p38 MAPK phosphorylation by Western blot. To confirm that the increase in IL-6 production is ascribed to AMPK activation, we used another known AMPK activator, metformin. It also dose-dependently potentiated IL-6 production in CFs, and this potentiation could be reversed by p38 MAPK inhibitor. In conclusion, AMPK activation promoted IL-6 production in CFs via p38 MAPK-dependent pathway.

IL-6 expression induced by adenosine A2b receptor stimulation in U373 MG cells depends on p38 mitogen activated kinase and protein kinase C

Adenosine binds to a class of G-protein coupled receptors, which are further distinguished as A(1), A(2a), A(2b) and A(3) adenosine receptors. As we have shown earlier, the stable adenosine analogue NECA (N6-(R)-phenylisopropyladenosine) stimulates IL-6 expression in the human astrocytoma cell line U373 MG via the A(2b) receptor. The mechanism by which NECA promotes astrocytic IL-6 expression has not been identified. By using various inhibitors of signal transduction, we found that p38 mitogen-activated protein kinases (MAPK) activation (inhibitor SB202190), but not extracellular signal-regulated kinase (ERK) (PD98059) and c-jun N-terminal kinase (JNK)(SP600125), is essential in the NECA-induced signalling cascade that leads to the increase in IL-6 synthesis in U373 MG cells. Results obtained with protein kinase C (PKC) inhibitors that have different substrate specificities, indicated that the PKC delta and epsilon isoforms are also involved in adenosine receptor A(2b) dependent upregulation of IL-6 expression. This is supported by the fact that NECA induced the activation of PKC delta and epsilon in U373 MG cells.

Expression, Pharmacological Profile, and Functional Coupling of A2BReceptors in a Recombinant System and in Peripheral Blood Cells Using a Novel Selective Antagonist Radioligand, [3H]MRE 2029-F20

In this study, we compared the pharmacological and biochemical characteristics of A(2B) adenosine receptors in recombinant (hA(2B)HEK293 cells) and native cells (neutrophils, lymphocytes) by using a new potent 8-pyrazole xanthine derivative, [(3)H]N-benzo[1,3]dioxol-5-yl-2-[5-(1,3-dipropyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yl-oxy]-acetamide] ([(3)H]MRE 2029-F20), that has high affinity and selectivity for hA(2B) versus hA(1),hA(2A), and hA(3) subtypes. [(3)H]MRE 2029-F20 bound specifically to the hA(2B) receptor stably transfected in human embryonic kidney (HEK) 293 cells with K(D) of 2.8 +/- 0.2 nM and B(max) of 450 +/- 42 fmol/mg of protein. Saturation experiments of [(3)H]MRE 2029-F20 binding in human neutrophils and lymphocytes detected a single high-affinity binding site with K(D) values of 2.4 +/- 0.5 and 2.7 +/- 0.7 nM, respectively, and B(max) values of 79 +/- 10 and 54 +/- 8 fmol/mg of protein, respectively, in agreement with real-time reverse transcription polymerase chain reaction studies showing the presence of A(2B) mRNA. The rank order of potency of typical adenosine ligands with recombinant hA(2B) receptors was consistent with that typically found for interactions with the A(2B) subtype and was also similar in peripheral blood cells. 5'-N-Ethyl-carboxamidoadenosine stimulated cAMP accumulation in both hA(2B)HEK293 and native cells, whereas phospholipase C activation was observed in recombinant receptors and endogenous subtypes expressed in neutrophils but not in lymphocytes. MRE 2029-F20 was revealed to be a potent antagonist in counteracting the agonist effect in both signal transduction pathways. In conclusion, [(3)H]MRE 2029-F20 is a selective and high-affinity radioligand for the hA(2B) adenosine subtype and may be used to quantify A(2B) endogenous receptors. In this work, we demonstrated their presence and functional coupling in neutrophils and lymphocytes that play a role in inflammatory processes in which A(2B) receptors may be involved.

Pituitary cell lines and their endocrine applications

The pituitary gland is an important component of the endocrine system, and together with the hypothalamus, exerts considerable influence over the functions of other endocrine glands. The hypothalamus either positively or negatively regulates hormonal productions in the pituitary through its release of various trophic hormones which act on specific cell types in the pituitary to secrete a variety of pituitary hormones that are important for growth and development, metabolism, reproductive and nervous system functions. The pituitary is divided into three sections-the anterior lobe which constitute the majority of the pituitary mass and is composed primarily of five hormone-producing cell types (thyrotropes, lactotropes, corticotropes, somatotropes and gonadotropes) each secreting thyrotropin, prolactin, ACTH, growth hormone and gonadotropins (FSH and LH) respectively. There is also a sixth cell type in the anterior lobe-the non-endocrine, agranular, folliculostellate cells. The intermediate lobe produces melanocyte-stimulating hormone and endorphins, whereas the posterior lobe secretes anti-diuretic hormone (vasopressin) and oxytocin. Representative cell lines of all the six cell types of the anterior pituitary have been established and have provided valuable information on genealogy of the various cell lineages, endocrine feedback control of hormone synthesis and secretions, intrapituitary interactions between the various cell types, as well as the role of specific transcription factors that determine each differentiated cell phenotype. In this review, we will discuss the morphology and function of the cell types that make up the anterior pituitary, and the characteristics of the various functional anterior pituitary cell systems that have been established to be representative of each anterior pituitary cell lineage.