Adenosine receptors linked to adenylate cyclase activity in human neuroblastoma cells: modulation during cell differentiation

Prognostic significance of NT5E/CD73 in neuroblastoma and its function in CSC stemness maintenance

Cluster of differentiation 73 (CD73), a cell surface enzyme that catalyzes adenosine monophosphate (AMP) breakdown to adenosine, is differentially expressed in cancers and has prognostic significance. We investigated its expression profile in neuroblastoma (NB), its association with NB clinical outcomes, and its influence in the regulation of cancer stem cells' (CSCs) stemness maintenance. RNA-Seq data mining (22 independent study cohorts, total n = 3836) indicated that high CD73 can predict good NB prognosis. CD73 expression (immunohistochemistry) gauged in an NB patient cohort (n = 87) showed a positive correlation with longer overall survival (OS, P = 0.0239) and relapse-free survival (RFS, P = 0.0242). Similarly, high CD73 correlated with longer OS and RFS in advanced disease stages, MYCN non-amplified (MYCN-na), and Stage-4-MYCN-na subsets. Despite no definite association in children < 2 years old (2Y), high CD73 correlated with longer OS (P = 0.0294) and RFS (P = 0.0315) in children > 2Y. Consistently, high CD73 was associated with better OS in MYCN-na, high-risk, and stage-4 subsets of children > 2Y. Multivariate analysis identified CD73 as an independent (P = 0.001) prognostic factor for NB. Silencing CD73 in patient-derived (stage 4, progressive disease) CHLA-171 and CHLA-172 cells revealed cell-line-independent activation of 58 CSC stemness maintenance molecules (QPCR profiling). Overexpressing CD73 in CHLA-20 and CHLA-90 cells with low CD73 and silencing in CHLA-171 and CHLA-172 cells with high CD73 showed that CD73 regulates epithelial to mesenchymal transition (E-Cadherin, N-Cadherin, Vimentin), stemness maintenance (Sox2, Nanog, Oct3/4), self-renewal capacity (Notch), and differentiation inhibition (leukemia inhibitory factor, LIF) proteins (confocal-immunofluorescence). These results demonstrate that high CD73 can predict good prognosis in NB, and further suggest that CD73 regulates stemness maintenance in cells that defy clinical therapy.

Role of Ectonucleotidases in Synapse Formation During Brain Development: Physiological and Pathological Implications

BACKGROUND

Extracellular adenine nucleotides and nucleosides, such as ATP and adenosine, are among the most recently identified and least investigated diffusible signaling factors that contribute to the structural and functional remodeling of the brain, both during embryonic and postnatal development. Their levels in the extracellular milieu are tightly controlled by various ectonucleotidases: ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPP), alkaline phosphatases (AP), ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) and ecto-5'- nucleotidase (eN).

METHODS

Studies related to the expression patterns of ectonucleotidases and their known features during brain development are reviewed, highlighting involvement of these enzymes in synapse formation and maturation in physiological as well as in pathological states.

RESULTS

During brain development and in adulthood all ectonucleotidases have diverse expression pattern, cell specific localization and function. NPPs are expressed at early embryonic days, but the expression of NPP3 is reduced and restricted to ependymal area in adult brain. NTPDase2 is dominant ectonucleotidase existing in the progenitor cells as well as main astrocytic NTPDase in the adult brain, while NTPDase3 is fully expressed after third postnatal week, almost exclusively on varicose fibers. Specific brain AP is functionally associated with synapse formation and this enzyme is sufficient for adenosine production during neurite growth and peak of synaptogenesis. eN is transiently associated with synapses during synaptogenesis, however in adult brain it is more glial than neuronal enzyme.

CONCLUSION

Control of extracellular adenine nucleotide levels by ectonucleotidases are important for understanding the role of purinergic signaling in developing tissues and potential targets in developmental disorders such as autism.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Tissue-nonspecific alkaline phosphatase promotes axonal growth of hippocampal neurons

Axonal growth is essential for establishing neuronal circuits during brain development and for regenerative processes in the adult brain. Unfortunately, the extracellular signals controlling axonal growth are poorly understood. Here we report that a reduction in extracellular ATP levels by tissue-nonspecific alkaline phosphatase (TNAP) is essential for the development of neuritic processes by cultured hippocampal neurons. Selective blockade of TNAP activity with levamisole or specific TNAP knockdown with short hairpin RNA interference inhibited the growth and branching of principal axons, whereas addition of alkaline phosphatase (ALP) promoted axonal growth. Neither activation nor inhibition of adenosine receptors affected the axonal growth, excluding the contribution of extracellular adenosine as a potential hydrolysis product of extracellular ATP to the TNAP-mediated effects. TNAP was colocalized at axonal growth cones with ionotropic ATP receptors (P2X₇ receptor), whose activation inhibited axonal growth. Additional analyses suggested a close functional interrelation of TNAP and P2X₇ receptors whereby TNAP prevents P2X₇ receptor activation by hydrolyzing ATP in the immediate environment of the receptor. Furthermore inhibition of P2X₇ receptor reduced TNAP expression, whereas addition of ALP enhanced P2X₇ receptor expression. Our results demonstrate that TNAP, regulating both ligand availability and protein expression of P2X₇ receptor, is essential for axonal development.

The Hox genes and their roles in oncogenesis

Hox genes, a highly conserved subgroup of the homeobox superfamily, have crucial roles in development, regulating numerous processes including apoptosis, receptor signalling, differentiation, motility and angiogenesis. Aberrations in Hox gene expression have been reported in abnormal development and malignancy, indicating that altered expression of Hox genes could be important for both oncogenesis and tumour suppression, depending on context. Therefore, Hox gene expression could be important in diagnosis and therapy.

Activation of Src/kinase/phospholipase C/mitogen-activated protein kinase and induction of neurite expression by ATP, independent of nerve growth factor

Extracellular ATP has been reported to potentiate the neurite outgrowth induced by nerve growth factor. In the present study the neurotrophic effect of ATP and other nucleotides was examined in mouse neuroblastoma neuro2a cells which lack nerve growth factor receptor. Exposure of neuro2a cells to ATP resulted in a dramatic increase in neurite bearing cells as compared with untreated control cells. Experiments performed with purinergic receptor agonists and antagonists suggest that the ATP stimulates neurite outgrowth via P2 receptors. Neurite outgrowth was completely blocked by P2 receptor antagonist suramin whereas the P1 receptor antagonist CGS15943 was ineffective. P1 receptor agonist 5'-(N-ethylcarboxamido)adenosine failed to induce neurite outgrowth. The potency order of different P2 receptor agonists was ATP=ATPgammaS>ADP>>2Me-S-ATP. It was insensitive to UTP and antagonist pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid) suggesting the involvement of P2Y11 receptor in the observed neuritogenic effect. The signaling pathway leading to ATP-induced neuritogenesis was investigated. The neuritogenic effect of ATP is independent of rise in intracellular Ca(2+) as pharmacological profile of neuritogenic P2Y receptor does not match with that of P2Y2 receptor associated with [Ca(2+)](i) signaling cascade. Exposure of cells to ATP caused activation of Src kinase, phospholipase Cgamma and extracellular signal-regulated kinases ERK1/2. Mitogen-activated protein kinase (MAPK) inhibitor U0126 drastically reduced the number of neurite bearing cells in ATP-treated cultures implying that the neurotrophic effect of ATP is mediated by MAPK. Our results demonstrate that ATP can stimulate neurite outgrowth independent of other neurotrophic factors and can be an effective trophic agent.

Developmental profile of NTPDase activity in synaptic plasma membranes isolated from rat cerebral cortex

In the present study the developmental profile of ATP-hydrolyzing activity promoted by NTPDase 1, its kinetic properties and the enzyme protein abundance associated with synaptic plasma membrane from rat cerebral cortex were characterized. NTPDase 1 activity increased from birth to day 30; afterwards it decreased and remained unchanged from adulthood (90 days) to senescence (365 days). Kinetic analysis revealed that enzyme exhibited the highest specific activity at day 30 and highest apparent affinity for ATP at day 365; however, V(max)/K(m) values remained unchanged for each age studied. Immunoblot analysis demonstrated that relative abundance of NTPDase 1 is highest at day 15 during ontogeny. The discrepancy between maximum enzyme activity and maximum enzyme protein abundance indicates that NTPDase 1 may have an additional role during development.

Ecto-nucleotidase activities in spinal cord of rats changes as function of age

An increase in ADP hydrolysis was observed in spinal cord synaptosomal fractions of 2-month-old Wistar male rats, when compared to other ages (1, 4 and 6 months of age), while no change in ATPase activity was observed. Conversely, in female rats, whilst no change in ADPase activity was observed in the spinal cord synaptosomal fraction, ATPase activity diminished with age, in 1-6-month-old animals. 5'-Nucleotidase activity was higher in the 4-month-old male and female rats in relation to 1 and 2-month-old animals. In the female rats, this activity continued to increase at least until 6 months of age. In conclusion, adenine nucleotides hydrolysis in synaptosomes from rat spinal cord is influenced by age and by gender. Since both ATP and adenosine may act as neuromodulators in the spinal cord, influencing several processes such as nociception, the regulation of ATP-metabolizing enzymes in spinal cord is probably important for the normal function of this tissue at different ages.

Involvement of astrocytes in purine‐mediated reparative processes in the brain

Astrocytes are involved in multiple brain functions in physiological conditions, participating in neuronal development, synaptic activity and homeostatic control of the extracellular environment. They also actively participate in the processes triggered by brain injuries, aimed at limiting and repairing brain damages. Purines may play a significant role in the pathophysiology of numerous acute and chronic disorders of the central nervous system (CNS). Astrocytes are the main source of cerebral purines. They release either adenine-based purines, e.g. adenosine and adenosine triphosphate, or guanine-based purines, e.g. guanosine and guanosine triphosphate, in physiological conditions and release even more of these purines in pathological conditions. Astrocytes express several receptor subtypes of P1 and P2 types for adenine-based purines. Receptors for guanine-based purines are being characterised. Specific ecto-enzymes such as nucleotidases, adenosine deaminase and, likely, purine nucleoside phosphorylase, metabolise both adenine- and guanine-based purines after release from astrocytes. This regulates the effects of nucleotides and nucleosides by reducing their interaction with specific membrane binding sites. Adenine-based nucleotides stimulate astrocyte proliferation by a P2-mediated increase in intracellular [Ca2+] and isoprenylated proteins. Adenosine also, via A2 receptors, may stimulate astrocyte proliferation, but mostly, via A1 and/or A3 receptors, inhibits astrocyte proliferation, thus controlling the excessive reactive astrogliosis triggered by P2 receptors. The activation of A1 receptors also stimulates astrocytes to produce trophic factors, such as nerve growth factor, S100beta protein and transforming growth factor beta, which contribute to protect neurons against injuries. Guanosine stimulates the output of adenine-based purines from astrocytes and in addition it directly triggers these cells to proliferate and to produce large amount of neuroprotective factors. These data indicate that adenine- and guanine-based purines released in large amounts from injured or dying cells of CNS may act as signals to initiate brain repair mechanisms widely involving astrocytes.

Extracellular adenosine deprivation induces epithelial differentiation of HT29 cells: evidence for a concomitant adenosine A(1)/A(2) receptor balance regulation

HT29 cells display an undifferentiated phenotype in culture. However, numerous treatments are able to induce both epithelial differentiation and cell growth inhibition. We have previously demonstrated that adenosine and its analogues act through specific adenosine receptors to modulate cell proliferation in HT29 and other human colon adenocarcinoma cell lines. Among the treatments tested, the most potent inhibition of HT29 cell growth was induced by deprivation of extracellular adenosine using adenosine deaminase. Here, we investigated the capacity of adenosine deaminase to initiate epithelial differentiation. After 1 month of daily addition of 10 U/ml adenosine deaminase to the culture medium, HT29 cells were cloned by limited dilution. Among the clones obtained, we focused our attention on clone 13. Microscopic visualization and proliferation studies indicated that cells from this clone grew very slowly and in a pseudo-monolayer, in marked contrast with the situation observed in the mother HT29 cell line. In addition, clone 13 cells displayed epithelial features that mimic the enterocytic differentiation of Caco-2 cells. These modifications were accompanied by dramatic changes in the activity of adenosine receptors, as demonstrated by pharmacological studies. In contrast to the original HT29 cells, clone 13 as well as Caco-2 cells displayed (i) a very low number of adenosine A(1) receptors, and (ii) increases in intracellular cAMP levels when challenged with adenosine analogues. It is hypothesized that a loss of adenosine A(1) receptors, with no change or a concomitant increase in adenosine A(2) receptors, results in the emergence of adenosine A(2) receptor-mediated differentiation and inhibition of proliferation, through a cAMP-dependent pathway.

Trophic effects of purines in neurons and glial cells

In addition to their well known roles within cells, purine nucleotides such as adenosine 5' triphosphate (ATP) and guanosine 5' triphosphate (GTP), nucleosides such as adenosine and guanosine and bases, such as adenine and guanine and their metabolic products xanthine and hypoxanthine are released into the extracellular space where they act as intercellular signaling molecules. In the nervous system they mediate both immediate effects, such as neurotransmission, and trophic effects which induce changes in cell metabolism, structure and function and therefore have a longer time course. Some trophic effects of purines are mediated via purinergic cell surface receptors, whereas others require uptake of purines by the target cells. Purine nucleosides and nucleotides, especially guanosine, ATP and GTP stimulate incorporation of [3H]thymidine into DNA of astrocytes and microglia and concomitant mitosis in vitro. High concentrations of adenosine also induce apoptosis, through both activation of cell-surface A3 receptors and through a mechanism requiring uptake into the cells. Extracellular purines also stimulate the synthesis and release of protein trophic factors by astrocytes, including bFGF (basic fibroblast growth factor), nerve growth factor (NGF), neurotrophin-3, ciliary neurotrophic factor and S-100beta protein. In vivo infusion into brain of adenosine analogs stimulates reactive gliosis. Purine nucleosides and nucleotides also stimulate the differentiation and process outgrowth from various neurons including primary cultures of hippocampal neurons and pheochromocytoma cells. A tonic release of ATP from neurons, its hydrolysis by ecto-nucleotidases and subsequent re-uptake by axons appears crucial for normal axonal growth. Guanosine and GTP, through apparently different mechanisms, are also potent stimulators of axonal growth in vitro. In vivo the extracellular concentration of purines depends on a balance between the release of purines from cells and their re-uptake and extracellular metabolism. Purine nucleosides and nucleotides are released from neurons by exocytosis and from both neurons and glia by non-exocytotic mechanisms. Nucleosides are principally released through the equilibratory nucleoside transmembrane transporters whereas nucleotides may be transported through the ATP binding cassette family of proteins, including the multidrug resistance protein. The extracellular purine nucleotides are rapidly metabolized by ectonucleotidases. Adenosine is deaminated by adenosine deaminase (ADA) and guanosine is converted to guanine and deaminated by guanase. Nucleosides are also removed from the extracellular space into neurons and glia by transporter systems. Large quantities of purines, particularly guanosine and, to a lesser extent adenosine, are released extracellularly following ischemia or trauma. Thus purines are likely to exert trophic effects in vivo following trauma. The extracellular purine nucleotide GTP enhances the tonic release of adenine nucleotides, whereas the nucleoside guanosine stimulates tonic release of adenosine and its metabolic products. The trophic effects of guanosine and GTP may depend on this process. Guanosine is likely to be an important trophic effector in vivo because high concentrations remain extracellularly for up to a week after focal brain injury. Purine derivatives are now in clinical trials in humans as memory-enhancing agents in Alzheimer's disease. Two of these, propentofylline and AIT-082, are trophic effectors in animals, increasing production of neurotrophic factors in brain and spinal cord. Likely more clinical uses for purine derivatives will be found; purines interact at the level of signal-transduction pathways with other transmitters, for example, glutamate. They can beneficially modify the actions of these other transmitters.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Expression of purine metabolism-related enzymes by microglial cells in the developing rat brain

The nucleoside triphosphatase (NTPase), nucleoside diphosphatase (NDPase), 5'-nucleotidase (5'-Nase), and purine nucleoside phosphorylase (PNPase) activity has been examined in the cerebral cortex, subcortical white matter, and hippocampus from embryonic day (E)16 to postnatal day (P)18. Microglia display all four purine-related enzymatic activities, but the expression of these enzymatic activities differed depending on the distinct microglial typologies observed during brain development. We have identified three main morphologic typologies during the process of microglial differentiation: ameboid microglia (parenchymatic precursors), primitive ramified microglia (intermediate forms), and resting microglia (differentiated cells). Ameboid microglia, which were encountered from E16 to P12, displayed the four enzymatic activities. However, some ameboid microglial cells lacked 5'-Nase activity in gray matter, and some were PNPase-negative in both gray and white matter. Primitive ramified microglia were already observed in the embryonic period but mostly distributed during the first 2 postnatal weeks. These cells expressed NTPase, NDPase, 5'-Nase, and PNPase. Similar to ameboid microglia, we found primitive ramified microglia lacking the 5'-Nase and PNPase activities. Resting microglia, which were mostly distinguishable from the third postnatal week, expressed NTPase and NDPase, but they lacked or displayed very low levels of 5'-Nase activity, and only a subpopulation of resting microglia was PNPase-positive. Apart from cells of the microglial lineage, GFAP-positive astrocytes and radial glia cells were also labeled by the PNPase histochemistry. As shown by our results, the differentiation process from cell precursors into mature microglia is accompanied by changes in the expression of purine-related enzymes. We suggest that the enzymatic profile and levels of the different purine-related enzymes may depend not only on the differentiation stage but also on the nature of the cells. The use of purine-related histoenzymatic techniques as a microglial markers and the possible involvement of microglia in the control of extracellular purine levels during development are also discussed.

Adenosine modulates cell proliferation in human colonic carcinoma. II. Differential behavior of HT29, DLD-1, Caco-2 and SW403 cell lines

In a previous study, we provided evidence that extracellular adenosine modulates growth of the poorly differentiated colonic adenocarcinoma cells HT29 and proposed that adenosine A1 receptors might mediate proliferative effects. We now extend our investigations to a group of colonic adenocarcinoma cells at different stages of enterocytic differentiation. In HT29, DLD-1, Caco-2 and SW403, proliferation was decreased in the presence of adenosine deaminase (5 or 10 U/ml), 5'-N-ethylcarboxamido-adenosine (NECA; 1 microM), xanthine amine congener and 8-phenyltheophylline (both at 1 nM or 1 microM). NECA stimulated cAMP accumulation in all cell lines except for HT29. In the presence of forskolin (adenyl cyclase activator) cAMP accumulation was inhibited at sub-nanomolar concentrations of NECA and stimulated at micromolar concentrations in all four cell lines. The inhibitory response disappeared in the presence of 50 nM cyclopentyladenosine (CPA). The binding of [3H]cyclopentyl-1,3-dipropylxanthine and [3H]NECA was also investigated in the four cell lines. Results of displacement experiments were consistent with the idea that poorly differentiated cells with high proliferation rates (e.g. HT29) express mainly adenosine A receptors. In contrast, displacement curves with more differentiated cells exhibiting low proliferation rates (e.g. Caco-2, DLD-1, SW403) displayed two components. The high-affinity component was no longer seen in competition experiments performed in the presence of [3H]NECA and 50 nM CPA. Together, our results further support the idea that extracellular adenosine stimulates cell proliferation in colonic adenocarcinoma cells. The effects might involve cAMP-coupled adenosine receptors.

Protein kinase C regulates adenosine A2a receptor mRNA expression in SH-SY5Y cells

Protein kinase C regulates mRNAs encoding several G protein-linked receptors but its role in adenosine A2a receptor expression is not known. We tested the hypothesis that protein kinase C activated by tetradecanoyl phorbol acetate (TPA) regulates adenosine A2a receptor mRNA levels. SH-SY5Y human neuroblastoma cells express adenosine receptors which positively couple to adenylyl cyclase with a pharmacologic profile expected of the A2a subtype. Northern blotting demonstrated an adenosine A2a receptor mRNA species of similar molecular size in SH-SY5Y cells and in human brain. TPA increased adenosine A2a receptor mRNA in a dose- and time-dependent fashion. Transcription or translation inhibition prevented increases in adenosine A2a receptor mRNA. Bisindolylmaleimide blocked TPA effects. Adenosine A2a receptor mRNA stability was unchanged by TPA. This study identifies a human neuroblastoma cell line expressing functional adenosine A2a receptors. Protein kinase C activation appears to enhance transcription of the adenosine A2a receptor gene.

Down-regulation of adenosine A2A receptors upon NGF-induced differentiation of PC12 cells

PC12 cell differentiation was induced by one week of nerve growth factor (NGF) treatment and adenosine A2A receptor expression and activity were analysed. Undifferentiated PC12 cells expressed very high levels of adenosine A2A receptors (approximately equal to 2 pmol/mg) and exhibited strong cyclic AMP (cAMP) responses when stimulated with the selective adenosine A2A receptor agonist 2-[p-(2-carbonylethyl) phenylethylamino-5'-N-ethylcarboxamidoadenosine]. NGF-induced differentiation was accompanied by a down-regulation of adenosine A2A receptors: receptor binding decreased to 500 fmol/mg, immunoreactive A2A receptor protein was decreased by about half and cAMP production was reduced by 60%. In situ hybridization experiments demonstrated a heterogenous distribution of A2A receptor mRNA and a decreased number of strongly labelled cells after NGF treatment. Stimulation of the cells with the non-selective adenosine receptor agonist N-ethylcarboxamidoadenosine (NECA) inhibited NGF-induced mitogen-activated protein kinase activation. These results thus show that NGF-induced differentiation of PC12 cells is accompanied by a decrease in A2A receptor-mediated cAMP accumulation. This might be a way for PC12 cells to counteract an inhibitory effect of A2A receptor activation on some aspects of neurotrophin signalling.

GTP and guanosine synergistically enhance NGF-induced neurite outgrowth from PC12 cells

Six per cent of rat pheochromocytoma (PC12) cells extended neurites (processes greater than one cell diameter in length) in the presence of 300 microM extracellular GTP or 300 microM guanosine for 48 hr, compared to only 2.5% of cells in control cultures. In the presence of 40 ng/ml of 2.5S NGF, about 20-35% of PC12 cells had neurites after 48 hr, and the addition of 300 microM guanosine or GTP together with NGF synergistically increased the proportion of cells with neurites to 40-65%. GTP and guanosine also increased the average number of branches per neurite, from 0.6 in NGF-treated cultures to 1.2 (guanosine) or 1.5 (GTP). Neurites formed after exposure to NGF alone had axonal characteristics as determined by immunocytochemistry with antibody, SMI-31, against axonal-specific polyphosphorylated neurofilament epitopes. Neurites generated with the addition of both guanosine or GTP had the same characteristics. GTP probably did not exert its effects via the P2X or P2Y purinoceptors because the adenine nucleotides ATP, ATP gamma S, ADP beta S, and ADP, which are all agonists of these receptors, inhibited rather than enhanced, NGF-induced neurite outgrowth. UTP also enhanced the proportion of cells with neurites, although not to the same degree as did GTP. This may indicate activity through a P2U-like nucleotide receptor. However, the response profile obtained, GTP > UTP >> ATP, does not fit the profile of any known P2Y, P2X or P2U receptor. The poorly hydrolyzable GTP analogues, GTP gamma S and GDP beta s were also unable to enhance the proportion of cells with neurites. This implied that GTP may produce its effects through a GTP-specific ectoenzyme or kinase. This idea was supported by results showing that another poorly hydrolyzable analogue, GMP-PCP, competitively inhibited the effects of GTP on neurite outgrowth. GTP did not exert its effects after hydrolysis to guanosine since the metabolic intermediates GDP and GMP were also ineffective in enhancing the proportion of cells with neurites. Moreover, the effects of GTP and guanosine were mutually additive, implying that these two purines utilized different signal transduction mechanisms. The effects of guanosine were not affected by the nucleoside uptake inhibitors nitrobenzylthioinosine (NBTI) and dipyridamole, indicating that a transport mechanism was not involved. Guanosine also did not activate the purinergic P1 receptors, because the A2 receptor antagonists, 1,3-dipropyl-7-methylxanthine (DPMX) or CGS15943, and the A1 receptor antagonist, 1,3-dipropyl-8-(2-amino-4-chloro)xanthine (PACPX) did not inhibit its reaction. Therefore guanosine enhanced neurite outgrowth by a signal transduction mechanism that does not include the activation of the P1 purinoceptors. The enhancement of the neuritogenic effects of NGF by GTP and guanosine may have physiological implications in sprouting and functional recovery after neuronal injury in the CNS, due to the high levels of nucleosides and nucleotides released from dead or injured cells.

Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells

In addition to their well-established roles as neurotransmitters and neuromodulators, growing evidence suggests that nucleotides and nucleosides might also act as trophic factors in both the central and peripheral nervous systems. Specific extracellular receptor subtypes for these compounds are expressed on neurons, glial and endothelial cells, where they mediate strikingly different effects. These range from induction of cell differentiation and apoptosis, mitogenesis and morphogenetic changes, to stimulation of synthesis or release, or both, of cytokines and neurotrophic factors, both under physiological and pathological conditions. Nucleotides and nucleosides might be involved in the regulation of development and plasticity of the nervous system, and in the pathophysiology of neurodegenerative disorders. Receptors for nucleotides and nucleosides could represent a novel target for the development of therapeutic strategies to treat incurable diseases of the nervous system, including trauma- and ischemia-associated neurodegeneration, demyelinating and aging-associated cognitive disorders.

Adenosine-induced apoptosis in chick embryonic sympathetic neurons: a new physiological role for adenosine

1. A newly found action of adenosine in neurons, which may have an important physiological function in the growth and development of the sympathetic nervous system, is described. Adenosine (1-100 microM) inhibited neurite outgrowth within the first 24 h and killed about 80% of sympathetic neurons supported by nerve growth factor over the next 2 days in culture. Neurons supported by excess KCl, forskolin or phorbol 12,13-dibutyrate were equally susceptible to the toxic actions of adenosine. Inosine, guanosine or hypoxanthine (all 100-300 microM) were without effect on neuronal growth and survival. 2. Specific agonists of adenosine A1 and A2 receptors were not neurotoxic, and toxic effects of adenosine were not antagonized by aminophylline. These results rule out involvement of adenosine receptors and the adenylyl cyclase-cAMP signalling system in neurotoxic actions of adenosine. 3. Adenosine toxicity was prevented by inhibitors of the adenosine membrane transporter, suggesting an intracellular site of action of adenosine. 4. Inhibitors of adenosine deaminase dramatically facilitated the toxic action so that physiologically relevant concentrations of adenosine were neurotoxic. 5. Adenosine kinase activity of sympathetic neurons was dose-dependently inhibited by 5'-iodotubercidin (3-100 nM). 5'-Iodotubercidin (100 nM) completely protected neurons against toxicity of adenosine plus adenosine deaminase inhibitors. These results provide convincing evidence that phosphorylation of the nucleoside is an essential requirement for initiation of adenosine toxicity. 6. Sympathetic neurons were successfully rescued from the lethal effects of adenosine deaminase inhibitor plus adenosine by uridine or 2-deoxycytidine, but not by nicotinamide or 2-deoxyguanosine, suggesting that depletion of pyrimidine nucleotides by phosphorylated adenosine compounds and consequent inhibition of DNA synthesis produces neuronal death. 7. DNA fragmentation, assessed by the fluorescent dye bisbenzimide and by the TUNEL (terminal deoxynucleotidyl transferase-mediated nick end labelling) method, indicated that neuronal death induced by adenosine was apoptotic. 8. We conclude that adenosine deaminase and adenosine kinase play an important role in the metabolism of intracellular concentrations of adenosine and thereby regulate the growth and development of sympathetic neurons. Our study highlights, for the first time, the importance of adenosine as a mediator of programmed cell death of neurons supported by nerve growth factor.

Modulation of astroglial cell proliferation by analogues of adenosine and ATP in primary cultures of rat striatum

We have studied the possible purinoceptor-mediated modulation of astroglial cell proliferation in neuron-glia primary cultures obtained from rat corpus striatum. Cultures were grown for three days in the presence of either 2-chloro-adenosine or alpha beta-methylene-ATP (which behave as agonists of adenosine/P1 and ATP/P2 purinoceptors, respectively), and then immunostained with an antibody to glial fibrillary acidic protein. 2-Chloro-adenosine decreased and alpha beta-methylene-ATP increased the number of astroglial cells in culture. For both derivatives, the effect was dose-dependent. The effect of alpha beta-methylene-ATP was antagonized by the trypanoside suramin, suggesting the involvement of a suramin-sensitive P2 purinoceptor, whereas the effect of 2-chloro-adenosine was not reversed by the P1 purinoceptor antagonist p-sulphonyl-phenyl-theophylline, implying the activation of a xanthine-insensitive adenosine purinoceptor subtype. In order to evaluate the extent of astrocyte proliferation in the presence of these two analogues, some cultures were incubated with bromodeoxyuridine for 24 h before fixing, and then double-immunostained for glial fibrillary acidic protein and bromodeoxyuridine. The percentage of bromodeoxyuridine positive astrocytes was significantly increased after exposure to both agents. It is therefore concluded that purines can modulate astroglial cells in opposite ways, inducing decreases or increases of cell number by activation of P1 and P2 purinoceptors, respectively. For the P2 purinoceptor-mediated effect, there was a quantitative correlation between the percentage of bromodeoxyuridine positive astrocytes and the cell number. For the P1 purinoceptor-mediated effect, no apparent correlation between these two parameters was found. This suggests the activation of independent effects, which involve other mechanisms besides the stimulation of DNA synthesis, and which eventually result in a reduction of cell number. The possible relevance of these findings to in vivo regulation of astrocyte cell function as well as in trauma- and ischaemia-associated hypergliosis is discussed.

Effects of adenosine analog PIA (n-phenylisopropyladenosine) on FSH-stimulated cyclic AMP (cAMP) production in the rat seminiferous epithelium

In rat seminiferous epithelium, FSH-stimulated cAMP production is cyclically modulated by spermatogenic cells and is highest in stages XIV-V and lowest in stages VII-VIII of the epithelial cycle. Adenosine has been proposed to be an inhibitory paracrine molecule in Sertoli cells. In this paper the effect of adenosine analog n-phenylisopropyladenosine (PIA) on FSH-stimulated cAMP production was studied in staged rat seminiferous tubules. In low responsive stages VII-VIII of the cycle, 100 nM and 10 microM PIA inhibited FSH-stimulated cAMP production by 24% and 28%, respectively. To study whether PIA effect is mediated through Gi-protein, pertussis toxin (PT) pretreatment was used to block the Gi-protein. PT pretreatments of 3 or 18 h caused 42% or 16% elevation in FSH-stimulated cAMP production, respectively. PIA blocked the stimulation caused by PT pretreatment. At 38 days post irradiation, when spermatocytes and round spermatids were decreased in number, in stages VII-VIII of the cycle the inhibitory effect of PIA was abolished. In high responsive stages XIV-V of the cycle, 100 nM PIA stimulated cAMP production by 27%, while 10 microM PIA had no effect. At 38 days post irradiation FSH response was decreased by 19% when compared to non-irradiated level, and PIA stimulated FSH-stimulated cAMP production by 22%. The results suggest that there are stage-specific mechanisms for adenosine-dependent regulation of FSH-stimulated cAMP production in the rat seminiferous epithelium. Advanced spermatogenic cells seem to maintain the mechanisms that include PIA-mediated inhibition of FSH response. Other mechanisms than PT-sensitive Gi-protein seem to be involved in the inhibition.

Effect of an adenosine A1 agonist injected into substantia nigra on kindling of epileptic seizures and convulsion duration

The substantia nigra pars reticulata (SNr) has been reported to be critically involved in the development and propagation of epileptic seizures, while extracellular adenosine appears to be important for making seizures stop. In the present study, an adenosine A1 receptor agonist [N6-cyclohexyladenosine (CHA); 2.0 nmol/side, or vehicle] was injected bilaterally into the SNr shortly before each of the first five of a series of daily kindling stimuli delivered to the rat amygdala. Injections did not affect the acquisition of kindled afterdischarges or the rate at which seizures developed over subsequent kindling sessions, but convulsions occurring 48-72 h after treatment were significantly shortened. Thus, purinergic mechanisms in the SNr do not appear to be specifically involved in the acquisition of kindled seizures but may contribute to a postictal inhibitory process that shortens the convulsive component.

Prolonged in vitro exposure of rat brain slices to adenosine analogues: selective desensitization of adenosine A1 but not A2 receptors

Agonist-induced desensitization of adenosine A1 and A2 receptors was studied in rat striatum slices maintained in carbo-oxygenated Krebs buffer. Slices were exposed to adenosine analogues (either cyclo-pentyl-adenosine or N-ethyl-carboxamido-adenosine) for selected time periods (15-60 min) and repeatedly washed at the end of agonist exposure. Agonist-induced changes of adenosine receptors were then evaluated in P2 fractions prepared from slices by measuring A1 and A2 receptor-regulated adenylate cyclase. A1 receptors were rapidly desensitized by agonist exposure, as shown by a gradual loss of A1 receptor-mediated inhibition of basal cyclase activity and cAMP formation, which was evident within 15-30 min after addition of the adenosine analogue. Agonist-induced desensitization of A1 receptors was dose- and time-dependent, and seemed quicker in onset with cyclo-pentyl-adenosine, according to the higher A1 selectivity of this receptor agonist, with respect to N-ethyl-carboxamido-adenosine. Binding of the A1-selective agonist [3H]cyclo-hexyl-adenosine was unaffected by the desensitization procedure at any of the exposure periods utilized, suggesting that an uncoupling of A1 receptors from their transduction system is indeed responsible for the loss of functional activity. Loss of A1 receptor function was accompanied by a time-dependent amplification of A2 receptor-mediated stimulation of adenylate cyclase activity, likely due to an 'unmasking' of A2 stimulatory receptor function as a consequence of the desensitization of A1 inhibitory receptors. All these effects could be completely counteracted by the concomitant exposure to an adenosine receptor antagonist, and specifically involved the coupling mechanisms of adenosine receptors with their effector system.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal-type nicotinic receptors in human neuroblastoma and small-cell lung carcinoma cell lines

A beta subunit of the neuronal nicotinic receptor, sharing 88% homology with the rat beta 4 subunit, has been cloned from a human neuroblastoma cell line. The gene encoding the human beta 4 subunit is expressed in association with the alpha 3 gene in neuroblastoma and small-cell lung carcinoma cell lines. Patch-clamp experiments and radioligand binding assays confirm that these neuroendocrine tumor cell lines express functional neuronal nicotinic receptors. We suggest that these receptors might play a crucial role in the control of neurotransmitter and hormone secretion from neurosecretory human tumors.

Adenosine receptors in rat basophilic leukaemia cells: transductional mechanisms and effects on 5-hydroxytryptamine release

1. The presence of adenosine receptors linked to adenylate cyclase activity and their functional role in calcium-evoked 5-hydroxytryptamine (5-HT) release was investigated in rat basophilic leukaemia (RBL) cells, a widely used model for studying the molecular mechanisms responsible for stimulus-secretion coupling. 2. In [3H]-5-HT-loaded cells triggered to release by the calcium ionophore A23187, a biphasic modulation of 5-HT secretion was induced by adenosine analogues, with inhibition of stimulated release at nM and potentiation at microM concentrations, suggesting the presence of adenosine receptor subtypes mediating opposite effects on calcium-dependent release. This was also confirmed by results obtained with other agents interfering with adenosine pharmacology, such as adenosine deaminase and the non-selective A1/A2 antagonist 8-phenyl-theophylline. 3. Similar biphasic dose-response curves were obtained with a variety of adenosine analogues on basal adenylate cyclase activity in RBL cells, with inhibition and stimulation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) production at nM and microM concentrations, respectively. The rank order of potency of adenosine analogues for inhibition and stimulation of adenylate cyclase activity and the involvement of G-proteins in modulation of cyclic AMP levels suggested the presence of cyclase-linked A1 high-affinity and A2-like low-affinity adenosine receptor subtypes. However, the atypical antagonism profile displayed by adenosine receptor xanthine antagonists on cyclase stimulation suggested that the A2-like receptor expressed by RBL cells might represent a novel cyclase-coupled A2 receptor subtype.4. Micromolar concentrations of adenosine analogues could also increase inositol phospholipid hydrolysis and inositol tris-phosphate formation in both unstimulated cells and in cells triggered to release by the calcium ionophore. The stimulation was constant, small and additive to that exerted by the calcium ionophore.5. It is concluded that RBL cells express both A1 and A2-like adenosine receptors which exert opposite effects on 5-HT release and intracellular cyclic AMP levels. However, besides modulation of cyclic AMP levels, additional transduction pathways, such as modulation of phospholipase C activity, may contribute to the release response evoked by adenosine analogues in this cell-line.

Characterization of the effect of the adenosine agonist cyclohexyladenosine on platelet activating factor-induced increases in [Ca2+]i in human platelets in vitro

Previous studies have shown that adenosine agonists acting at A-2 receptors inhibit platelet aggregation. Since an increase in cytosolic Ca2+ concentration (delta [Ca2+]i) is closely associated with the time frame of platelet aggregation, we have examined the effect of adenosine receptor function on induced increases of [Ca2+]i by a potent platelet activator, platelet activating factor (PAF). We loaded washed platelets with Fura-2, then induced increases in [Ca2+]i with various concentrations of PAF, and then determined EC50 values (PAF concentration at half-maximal response) and values for maximal response of delta[Ca2+]i (max-delta[Ca2+]i). The EC50 for PAF-delta[Ca2+]i was 112 +/- 37 (SD) pM and the max-delta[Ca2+]i was 284 +/- 138 (SD) nM. Our results show that PAF-delta[Ca2+]i was inhibited in a non-competitive manner by the adenosine receptor agonist cyclohexyladenosine (CHA) with an IC50 of 14.9 microM. This inhibition was partially reversed by theophylline, an adenosine receptor antagonist, with an IC50 of 19 microM. Based on the results of these studies together with evidence from other research groups that platelets do not possess A-1 receptors, our results suggest that CHA inhibited PAF-delta[Ca2+]i in platelets through an activation of A-2 receptors.