Proteasome inhibitors potentiate etoposide-induced cell death in human astrocytoma cells bearing a mutated p53 isoform

Resistance to anticancer agents is often due to defects of intracellular pathways of cell death. Thus, the identification of the apoptotic pathways that can still be recruited by chemotherapeutic agents in cancerous cells can disclose new opportunities to treat malignancies. Here we show that human astrocytoma ADF cells (which are resistant to "mitochondriotropic" agents as well as to the antineoplastic drug etoposide and to proteasome inhibitors when used alone) undergo dramatic apoptotic death when exposed to a combination protocol based on the use of etoposide in the presence of proteasome inhibitors. Sensitization to cell death involved an autoamplifying loop of caspase activation, where the "executioner" phase of apoptosis was sustained by cooperation of caspase-2, -9, -8, and -3. We also show that sensitization of cells to the combination protocol involved the nuclear relocalization of p53, despite the presence of a polymorphism in its DNA-binding domain, suggesting the likely induction of p53-dependent proapoptotic genes. Conversely, p53 phosphorylation on Ser-15 did not play any role in apoptosis. In conclusion, use of etoposide in combination with proteasome inhibitors may represent an effective strategy to restore sensitivity to apoptosis in human astrocytoma cells bearing multiple defects of intracellular apoptotic pathways.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Early and transient alteration of adenosine A2A receptor signaling in a mouse model of Huntington disease

Huntington Disease (HD) is characterized by choreic involuntary movements and striatal vulnerability. A2A receptors expressed on GABAergic striatal neurons have been suggested to play a pathogenetic role. Previous data demonstrated the presence of an aberrant alteration of A2A receptor-dependent adenylyl cyclase in an in vitro model of the disease (striatal cells expressing mutant huntingtin) and in peripheral circulating cells of HD patients. Here, we investigated whether this dysfunction is present in the R6/2 HD transgenic mouse model, by analyzing striatal A2A receptor-binding and adenylyl cyclase activity at different developmental stages in comparison with age-matched wild type animals. A transient increase in A2A receptor density (Bmax) and A2A receptor-dependent cAMP production at early presymptomatic ages (7-14 postnatal days) was found. Both alterations normalized to control values starting from postnatal day 21. In contrast, A2A receptor mRNA, as detected by real time PCR, dramatically decreased starting from PND21 until late symptomatic stages (12 weeks of age). The discrepancy between A2A receptor expression and density suggests compensatory mechanisms. These data, reproducing ex vivo the previous observations in vitro, support the hypothesis that an alteration of A2A receptor signaling is present in HD and might represent an interesting target for neuroprotective therapies.

Roles of P2 receptors in glial cells: focus on astrocytes

Central nervous system glial cells release and respond to nucleotides under both physiological and pathological conditions, suggesting that these molecules play key roles in both normal brain function and in repair after damage. In particular, ATP released from astrocytes activates P2 receptors on astrocytes and other brain cells, allowing a form of homotypic and heterotypic signalling, which also involves microglia, neurons and oligodendrocytes. Multiple P2X and P2Y receptors are expressed by both astrocytes and microglia; however, these receptors are differentially recruited by nucleotides, depending upon specific pathophysiological conditions, and also mediate the long-term trophic changes of these cells during inflammatory gliosis. In astrocytes, P2-receptor-induced gliosis occurs via activation of the extracellular-regulated kinases (ERK) and protein kinase B/Akt pathways and involves induction of inflammatory and anti-inflammatory genes, cyclins, adhesion and antiapoptotic molecules. While astrocytic P2Y₁ and P2Y_2,4 are primarily involved in short-term calcium-dependent signalling, multiple P2 receptor subtypes seem to cooperate to astrocytic long-term changes. Conversely, in microglia, exposure to inflammatory and immunological stimuli results in differential functional changes of distinct P2 receptors, suggesting highly specific roles in acquisition of the activated phenotype. We believe that nucleotide-induced activation of astrocytes and microglia may originally start as a defence mechanism to protect neurons from cytotoxic and ischaemic insults; dysregulation of this process in chronic inflammatory diseases eventually results in neuronal cell damage and loss. On this basis, full elucidation of the specific roles of P2 receptors in these cells may help exploit the beneficial neuroprotective features of activated glia while attenuating their harmful properties and thus provide the basis for novel neuroprotective strategies that specifically target the purinergic system.

CysLT1 receptor is a target for extracellular nucleotide-induced heterologous desensitization: a possible feedback mechanism in inflammation

Both cysteinyl-leukotrienes and extracellular nucleotides mediate inflammatory responses via specific G-protein-coupled receptors, the CysLT and the P2Y receptors, respectively. Since these mediators accumulate at sites of inflammation, and inflammatory cells express both classes of receptors, their responses are likely to be crossregulated. We investigated the molecular basis of desensitization and trafficking of the CysLT1 receptor constitutively and transiently expressed in the human monocyte/macrophage-like U937 or COS-7 cells in response to LTD4 or nucleotides. Exposure to agonist induced a rapid homologous desensitization of the CysLT1 receptor [as measured by the reduction in the maximal agonist-induced intracellular cytosolic Ca2+ ([Ca2+]i) transient], followed by receptor internalization (as assessed by equilibrium binding and confocal microscopy). Activation of P2Y receptors with ATP or UDP induced heterologous desensitization of the CysLT1 receptor. Conversely, LTD4-induced CysLT1 receptor activation had no effect on P2Y receptor responses, which suggests that the latter have a hierarchy in producing desensitizing signals. Furthermore, ATP/UDP-induced CysLT1 receptor desensitization was unable to cause receptor internalization, induced a faster recovery of CysLT1 functionality and was dependent upon protein kinase C. By contrast, homologous desensitization, which is probably dependent upon G-protein-receptor kinase 2 activation, induced a fast receptor downregulation and, accordingly, a slower recovery of CysLT1 functionality. Hence, CysLT1 receptor desensitization and trafficking are differentially regulated by the CysLT1 cognate ligand or by extracellular nucleotides. This crosstalk may have a profound physiological implication in the regulation of responses at sites of inflammation, and may represent just an example of a feedback mechanism used by cells to fine-tune their responses.

Pathophysiological roles of P2 receptors in glial cells

Extracellular nucleotides act through specific receptors on target cells: the seven ionotropic P2X and the eight G protein-coupled P2Y receptors. All these receptors are expressed by brain astroglia and microglia. In astrocytes, P2 receptors have been implicated in short-term calcium-dependent cell-cell communication. Upon mechanical stimulation or activation by other transmitters, astrocytes release ATP and respond to ATP with a propagating wave of intracellular calcium increases, allowing a homotypic astrocyte-astrocyte communication, as well as an heterotypic signalling which also involves neurons, oligodendrocytes and microglia. Astrocytic P2 receptors also mediate reactive astrogliosis, a reaction contributing to neuronal death in neurodegenerative diseases. Signalling leading to inflammatory astrogliosis involves induction of cyclo-oxygenase 2 through stimulation of ERK1,2 and of the transcriptional factors AP-1 and NF-kappaB. Microglia also express several P2 receptors linked to intracellular calcium increases. P2 receptor subtypes are differentially regulated by typical proinflammatory signals for these cells (e.g. lipopolysaccharide), suggesting specific roles in brain immune responses. Globally, these findings highlight the roles of P2 receptors in glial cell pathophysiology suggesting a contribution to neurodegenerative diseases characterized by excessive gliosis and neuro-inflammation. They also open up the possibility of modulating brain damage by ligands selectively targeting the specific P2 receptor subtypes involved in the gliotic response.

CysLT1 leukotriene receptor antagonists inhibit the effects of nucleotides acting at P2Y receptors

Montelukast and pranlukast are orally active leukotriene receptor antagonists selective for the CysLT1 receptor. Conversely, the hP2Y(1,2,4,6,11,12,13,14) receptors represent a large family of GPCRs responding to either adenine or uracil nucleotides, or to sugar-nucleotides. Montelukast and pranlukast were found to inhibit nucleotide-induced calcium mobilization in a human monocyte-macrophage like cell line, DMSO-differentiated U937 (dU937). Montelukast and pranlukast inhibited the effects of UTP with IC50 values of 7.7 and 4.3 microM, respectively, and inhibited the effects of UDP with IC50 values of 4.5 and 1.6 microM, respectively, in an insurmountable manner. Furthermore, ligand binding studies using [3H]LTD4 excluded the possibility of orthosteric nucleotide binding to the CysLT1 receptor. dU937 cells were shown to express P2Y2, P2Y4, P2Y6, P2Y11, P2Y13 and P2Y14 receptors. Therefore, these antagonists were studied functionally in a heterologous expression system for the human P2Y receptors. In 1321N1 astrocytoma cells stably expressing human P2Y(1,2,4,6) receptors, CysLT1 antagonists inhibited both the P2Y agonist-induced activation of phospholipase C and intracellular Ca2+ mobilization. IC50 values at P2Y1 and P2Y6 receptors were <1 microM. In control astrocytoma cells expressing an endogenous M3 muscarinic receptor, 10 microM montelukast had no effect on the carbachol-induced rise in intracellular Ca2+. These data demonstrated that CysLT1 receptor antagonists interact functionally with signaling pathways of P2Y receptors, and this should foster the study of possible implications for the clinical use of these compounds in asthma or in other inflammatory conditions.

P2 receptors in human heart: upregulation of P2X6 in patients undergoing heart transplantation, interaction with TNFalpha and potential role in myocardial cell death

ATP acts as a neurotransmitter via seven P2X receptor-channels for Na(+) and Ca(2+), and eight G-protein-coupled P2Y receptors. Despite evidence suggesting roles in human heart, the map of myocardial P2 receptors is incomplete, and their involvement in chronic heart failure (CHF) has never received adequate attention. In left myocardia from five to nine control and 5-12 CHF subjects undergoing heart transplantation, we analyzed the full repertoire of P2 receptors and of 10 "orphan" P2Y-like receptors. All known P2Y receptors (i.e. P2Y(1,2,4,6,11,12,13,14)) and two P2Y-like receptors (GPR91 and GPR17) were detected in all subjects. All known P2X(1-7) receptors were also detected; of these, only P2X(6) was upregulated in CHF, as confirmed by quantitative real time-PCR. The potential significance of this change was studied in primary cardiac fibroblasts freshly isolated from young pigs. Exposure of cardiac fibroblasts to ATP or its hydrolysis-resistant-analog benzoylATP induced apoptosis. TNFalpha (a cytokine implicated in CHF progression) exacerbated cell death. Similar effects were induced by ATP and TNFalpha in a murine cardiomyocytic cell line. In cardiac fibroblasts, TNFalpha inhibited the downregulation of P2X(6) mRNA associated to prolonged agonist exposure, suggesting that, by preventing ATP-induced P2X(6) desensitization, TNFalpha may abolish a defense mechanism meant at avoiding Ca(2+) overload and, ultimately, Ca(2+)-dependent cell death. This may provide a basis for P2X(6) upregulation in CHF. In conclusion, we provide the first characterization of P2 receptors in the human heart and suggest that the interaction between TNFalpha and the upregulated P2X(6) receptor may represent a novel pathogenic mechanism in CHF.

To be or not to be (inflamed) – is that the question in anti-inflammatory drug therapy of neurodegenerative disorders?

A sustained inflammatory reaction is present in acute (e.g. stroke) and chronic (e.g. Alzheimer's disease, Parkinson's disease and multiple sclerosis) neurodegenerative disorders. Inflammation, which is fostered by both residential glial cells and blood-circulating cells that infiltrate the diseased brain, probably starts as a time- and site-specific defense mechanism that could later evolve into a destructive and uncontrolled reaction. In this article, we review the crucial dichotomy of brain inflammation, where failure to resolve an acute beneficial response could lead to a vicious and anarchic state of chronic activation. The possible use of non-steroidal anti-inflammatory drugs for the management of neurodegenerative diseases is discussed in light of recent data demonstrating a neuroprotective role of local innate and adaptive immune responses. Novel therapeutic approaches must rely on potentiation of endogenous anti-inflammatory pathways, identification of early markers of neuronal deterioration and a combination treatment involving immune modulation and anti-inflammatory therapies.

Upregulation of A2A adenosine receptor expression by TNF-alpha in PBMC of patients with CHF: a regulatory mechanism of inflammation

BACKGROUND

Tumor necrosis factor (TNF)-alpha plays a role in congestive heart failure (CHF). A2A adenosine receptor (A(2A)R) activation on immune cells putatively reduces the release of cytokines contributing to CHF progression. The study is aimed at determining the role of the A(2A)R in the modulation of TNF-alpha production, and the ex vivo effect of TNF-alpha on A(2A)R in peripheral blood mononuclear cells (PBMC) from CHF patients.

METHODS AND RESULTS

Plasma levels of TNF-alpha and TNF-alpha production from lipopolysaccharide (LPS)-stimulated PBMC were evaluated in 26 CHF patients in comparison to controls. The effects of the A(2A)R agonist CGS-21680 and antagonist ZM-241385 on TNF-alpha production from PBMC were also evaluated. Finally, reverse transcriptase-polymerase chain reaction and Western blot analyses of A(2A)R in PBMC were performed in TNF-alpha-treated and untreated cells. TNF-alpha production from LPS-stimulated PBMC was enhanced in CHF patients with respect to controls. CGS-21680 blunted TNF-alpha production in both groups; ZM-241385 reverted this effect. A(2A)R expression in PBMC was higher in CHF patients than in controls. TNF-alpha addition produced an increase in A(2A)R in PBMC from controls but not in PBMC from CHF patients.

CONCLUSIONS

PBMC from CHF patients show an upregulation of A(2A)R-mediated inhibition of TNF-alpha, which may represents a mechanism of protection against inappropriate cytokine production.

Resistance of human astrocytoma cells to apoptosis induced by mitochondria-damaging agents: possible implications for anticancer therapy

The success of anticancer chemotherapy is often hampered by resistance to apoptosis, which may depend on defects in intracellular cell death pathways. Characterizing the alterations of these pathways is a prerequisite for developing alternative and effective antitumoral strategies. Here, we investigated the susceptibility of a human astrocytoma cell line, ADF, to apoptotic cell death induced by mitochondria-damaging agents. Neither the anticancer agent betulinic acid nor the "mitochondriotropic" poisons 2-deoxy-d-ribose and potassium cyanide induced apoptosis of these cells, despite induction of highly significant mitochondrial depolarization, eventually resulting in necrotic death. Resistance to apoptosis was not due to presence of the multidrug resistance pump or to impaired expression of caspase-8, caspase-9, or "executioner" caspase-3. Cloning of caspase-9 revealed the presence of full-length caspase-9alpha and a short variant (caspase-9beta), which, in other tumors, acts as a dominant negative of the long isoform. All analyzed clones showed a point mutation in the prodomain region that is known to interact with mitochondria-released factors. Thus, in these human astrocytoma cells, mitochondria-damaging agents induce a regulated form of mitochondrial-dependent necrotic cell death (oncosis). Resistance to apoptosis is due to an intrinsic defect of caspase-9, leading to inhibition of enzyme activation and/or impaired interaction with proteins released from depolarized mitochondria. These results may have implications for developing strategies aimed at overcoming tumor resistance to chemotherapy.

Regulation of A2B adenosine receptor functioning by tumour necrosis factor a in human astroglial cells

Low-affinity A2B adenosine receptors (A2B ARs), which are expressed in astrocytes, are mainly activated during brain hypoxia and ischaemia, when large amounts of adenosine are released. Cytokines, which are also produced at high levels under these conditions, may regulate receptor responsiveness. In the present study, we detected A2B AR in human astrocytoma cells (ADF) by both immunoblotting and real-time PCR. Functional studies showed that the receptor stimulated adenylyl cyclase through Gs proteins. Moreover, A2B ARs were phosphorylated and desensitized following stimulation of the receptors with high agonist concentration. Tumour necrosis factor alpha (TNF-alpha) treatment (24- h) increased A2B AR functional response and receptor G protein coupling, without any changes in receptor protein and mRNA levels. TNF-alpha markedly reduced agonist-dependent receptor phosphorylation on threonine residues and attenuated agonist-mediated A2B ARs desensitization. In the presence of TNF-alpha, A2B AR stimulation in vitro induced the elongation of astrocytic processes, a typical morphological hallmark of in vivo reactive astrogliosis. This event was completely prevented by the selective A2B AR antagonist MRS 1706 and required the presence of TNF-alpha. These results suggest that, in ADF cells, TNF-alpha selectively modulates A2B AR coupling to G proteins and receptor functional response, providing new insights to clarify the pathophysiological role of A2B AR in response to brain damage.

Pathophysiological roles of extracellular nucleotides in glial cells: differential expression of purinergic receptors in resting and activated microglia

Microglial cells are the major cellular elements with immune function inside the CNS and play important roles in orchestrating inflammatory brain response to hypoxia and trauma. Although a complete knowledge of the endogenous factors leading to a prompt activation of microglia is not yet available, activation of P2 purinoreceptors by extracellular ATP has been indicated as a primary factor in microglial response. A still unresolved question, however, is which subtype(s) of P2 receptors mediate(s) the response to ATP. By a combination of RT-PCR, Western blotting, and single-cell calcium imaging, we assessed the presence and the activity of P2 receptor subtypes in the mouse microglial cell line N9. All members of the P2 receptor family, including the recently reported receptor for sugar nucleotides (P2Y(14)), were found to be present in these cells at mRNA and/or protein level. The functionality of the receptors was assessed by analysis of the calcium responses evoked by specific agonists both in N9 cells and in primary microglia from rat brain. Interestingly, a different functional profile of P2 receptors was observed in resting or in LPS-activated N9 cells. Overnight exposure to LPS increased P2Y(6) and P2Y(14), decreased P2X(7), and left unchanged P2Y(1) and P2Y(2,4) receptor activity. The change in the P2 receptor profile in activated cells suggests selective roles for specific P2 receptor subtypes in microglial activation triggered by LPS. We speculate that modulation of microglial cell function via subtype-selective P2 receptor ligands may open up new strategies in the therapeutic management of inflammatory neurological diseases characterized by abnormal microglia response.

Cloning, pharmacological characterisation and distribution of the rat G-protein-coupled P2Y(13) receptor

The human P2Y(13) receptor is a new receptor characterized by coupling to Gi, responsiveness to adenine di-phospho-nucleotides and blockade by the P2Y antagonist AR-C69931MX. The mouse P2Y(13) ortholog has also been reported. Here we report, for the first time, the cloning of rat P2Y(13) receptor, its pharmacological analysis and tissue distribution. Rat P2Y(13) is 79% and 87% identical to human and mouse P2Y(13) receptors, respectively. Expression of rP2Y(13) receptor in 1321N1 cells induced the appearance of responses to the typical P2Y(13) receptor agonists ADP and 2MeSADP, as detected by stimulation of [(35)S]GTPgammaS binding. Agonist activities were higher in cells transfected with rP2Y(13) receptor in the presence of the Galpha(16) subunit; in all cases agonist effects were abolished by pertussis toxin pre-treatment. At variance from both human and mouse receptors, ADP was more potent than 2MeSADP. Other nucleotides and sugar-nucleotides were ineffective. Both in the absence and presence of Galpha(16), activation of rP2Y(13) receptor by ADP and 2MeSADP was completely inhibited by nM concentrations of AR-C69931MX. In contrast, no inhibition of rP2Y(13) receptor was induced by the selective P2Y(1) receptor antagonist MRS2179. rP2Y(13) receptor showed highest expression levels in spleen, followed by liver and brain (with particularly high levels in cortex and striatum as reported in man), suggesting important roles in the nervous and immune systems. Expression levels comparable to those of the other cloned P2Y receptors were found in primary rat astrocytes, indicating a possible role in reactive astrogliosis. Hence, rat P2Y(13) receptor displays several similarities but also interesting differences with its human and mouse orthologs, that will have to be taken into account when characterizing the pathophysiological roles of this receptor in the rat animal models.

Peripheral benzodiazepine receptor ligands: mitochondrial transmembrane potential depolarization and apoptosis induction in rat C6 glioma cells

The peripheral benzodiazepine receptor (PBR) is a component of a multiprotein complex, located at the contact site between the inner and outer mitochondrial membranes, which constitutes the mitochondrial permeability transition (MPT)-pore. The opening of the MPT-pore, leading to the transmembrane mitochondrial potential (DeltaPsi(m)) dissipation, is a critical event in the mechanism of apoptosis. In the present work, we investigated the ability of the specific PBR ligands, PK 11195 or Ro5-4864, to affect mitochondrial potential and to induce apoptotic cell death in rat C6 glioma cells. Both specific ligands inhibited cell survival in a dose- and time-dependent manner, as assessed by MTS conversion assay, whereas the non-site selective ligand Diazepam or the low-affinity benzodiazepine Clonazepam showed no significant effects. After cell exposure to PK 11195 or Ro5-4864 we evidenced typical alterations of apoptotic cell death such as DNA fragmentation and chromatin condensation assessed by flow cytometric and transmission electron microscopy (TEM) analysis, respectively. Activation of the "effector" caspase-3 confirmed the ability of specific PBR ligands to induce apoptosis. Moreover, PK 11195 and Ro5-4864 induced a decrease of DeltaPsi(m), as evidenced by JC-1 flow cytometry analysis. Our data demonstrate the pro-apoptotic effects of specific PBR ligands on rat C6 glioma cells.

Adenosine-induced cell death: evidence for receptor-mediated signalling

Adenosine modulates the proliferation, survival and apoptosis of many different cell types, ranging from epithelial, endothelial and smooth muscle cells, to cells of the immune and neural lineages. In this review, we critically discuss the available in vitro and in vivo data which support a role for adenosine in both development-associated apoptosis, and in diseases characterized by either pathologically increased cell death (e.g., ischemia, trauma and aging-associated neurodegeneration) or abnormally reduced spontaneous apoptosis (e.g., cancer). Particular emphasis is given to the possible role of extracellular adenosine receptors, since these may represent novel and attractive molecular targets for the pharmacological modulation of apoptosis. In some instances, adenosine-induced cell death has been demonstrated to require entry of the nucleoside inside cells; however, in many other cases, activation of specific adenosine extracellular receptors has been demonstrated. Of the four G protein-coupled adenosine receptors so far identified, the A2A and the A3 receptors have been specifically implicated in modulation of cell death. For the A3 receptor, results obtained by exposing both cardiomyocytes and brain astrocytes to graded concentrations of selective agonists suggest induction of both cell protection and cell death. Such opposite effects, which likely depend on the degree of receptor activation, may have important therapeutic implications in the pharmacological modulation of cardiac and brain ischemia. For the A2A receptor, recent intriguing data suggest a specific role in immune cell death and immunosuppression, which may be relevant to both adenosine-deaminase-immunodeficiency syndrome (a pathology characterized by accumulation of adenosine to toxic levels) and in tumors where induction of apoptosis via activation of specific extracellular receptors may be desirable. Finally, preliminary data suggest that, in a similar way to the adenosine-deaminase-immunodeficiency syndrome, the abnormal accumulation of adenosine in degenerative muscular diseases may contribute to muscle cell death. Although the role of adenosine receptors in this effect still remains to be determined, these data suggest that adenosine-induced apoptosis may also represent a novel pathogenic pathway in muscular dystrophies.

Antitumor Effects of Cannabidiol, a Nonpsychoactive Cannabinoid, on Human Glioma Cell Lines

Recently, cannabinoids (CBs) have been shown to possess antitumor properties. Because the psychoactivity of cannabinoid compounds limits their medicinal usage, we undertook the present study to evaluate the in vitro antiproliferative ability of cannabidiol (CBD), a nonpsychoactive cannabinoid compound, on U87 and U373 human glioma cell lines. The addition of CBD to the culture medium led to a dramatic drop of mitochondrial oxidative metabolism [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide test] and viability in glioma cells, in a concentration-dependent manner that was already evident 24 h after CBD exposure, with an apparent IC(50) of 25 microM. The antiproliferative effect of CBD was partially prevented by the CB2 receptor antagonist N-[(1S)-endo-1,3,3-trimethylbicyclo[2,2,1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528; SR2) and alpha-tocopherol. By contrast, the CB1 cannabinoid receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride (SR141716; SR1), capsazepine (vanilloid receptor antagonist), the inhibitors of ceramide generation, or pertussis toxin did not counteract CBD effects. We also show, for the first time, that the antiproliferative effect of CBD was correlated to induction of apoptosis, as determined by cytofluorimetric analysis and single-strand DNA staining, which was not reverted by cannabinoid antagonists. Finally, CBD, administered s.c. to nude mice at the dose of 0.5 mg/mouse, significantly inhibited the growth of subcutaneously implanted U87 human glioma cells. In conclusion, the nonpsychoactive CBD was able to produce a significant antitumor activity both in vitro and in vivo, thus suggesting a possible application of CBD as an antineoplastic agent.

Aberrant A2A receptor function in peripheral blood cells in Huntington's disease

A2A adenosine receptors specifically found on striatal medium spiny neurons play a major role in sensory motor function and may also be involved in neuropsychiatric and neurodegenerative disorders. One hypothesis concerning Huntington's disease (HD) proposes that an imbalance of the cortico-striatal pathway, due to the mutation in the HD gene, leads to striatal vulnerability. An A2A receptor dysfunction has been previously demonstrated in striatal cells engineered to express mutant huntingtin. Here we tested whether a similar dysfunction (i.e., the binding and functional parameters of A2A adenosine receptors) is present in peripheral blood cells (platelets, lymphocytes, and neutrophils) of subjects carrying the mutant gene. This study involved 48 heterozygous and three homozygous patients compared with 58 healthy subjects. Moreover, we selected seven at-risk mutation carriers. A2A receptor density and function are substantially increased in peripheral blood cells from both patients and subjects at the presymptomatic stage. In the neutrophils of the three homozygous HD subjects receptor dysfunction was higher than in heterozygotes. These data indicate the existence of an aberrant A2A receptor phenotype in the peripheral blood cells of subjects carrying the HD mutation. Future studies will assess whether this parameter can be exploited as a peripheral biomarker of Huntington's disease.

Nucleotide-mediated calcium signaling in rat cortical astrocytes: Role of P2X and P2Y receptors

ATP is the dominant messenger for astrocyte-to-astrocyte calcium-mediated communication. Definition of the exact ATP/P2 receptors in astrocytes and of their coupling to intracellular calcium ([Ca(2+)](i)) has important implications for brain physiology and pathology. We show that, with the only exception of the P2X(6) receptor, primary rat cortical astrocytes express all cloned ligand-gated P2X (i.e., P2X(1-5) and P2X(7)) and G-protein-coupled P2Y receptors (i.e., P2Y(1), P2Y(2), P2Y(4), P2Y(6), and P2Y(12)). These cells also express the P2Y-like UDP-glucose receptor, which has been recently recognized as the P2Y(14) receptor. Single-cell image analysis showed that only some of these receptors are coupled to [Ca(2+)](i). While ATP induced rapid and transient [Ca(2+)](i) increases (counteracted by the P2 antagonists suramin, pyridoxal-phosphate-6-azophenyl-2'-4'-disulfonic acid and oxidized ATP), the P2X(1)/P2X(3) agonist alphabetameATP produced no changes. Conversely, the P2X(7) agonist BzATP markedly increased [Ca(2+)](i); the presence and function of the P2X(7) receptor was also confirmed by the formation of the P2X(7) pore. ADP and 2meSADP also produced [Ca(2+)](i) increases antagonized by the P2Y(1) antagonist MRS2179. Some cells also responded to UTP but not to UDP. Significant responses to sugar-nucleotides were also detected, which represents the first functional response reported for the putative P2Y(14) receptor in a native system. Based on agonist preference of known P2 receptors, we conclude that, in rat astrocytes, ATP-induced calcium rises are at least mediated by P2X(7) and P2Y(1) receptors; additional receptors (i.e., P2X(2), P2X(4), P2X(5), P2Y(2), P2Y(4), and P2Y(14)) may also contribute.

Blockade of A2A adenosine receptors prevents basic fibroblast growth factor-induced reactive astrogliosis in rat striatal primary astrocytes

Previous literature data show that blockade of A(2A) adenosine receptors via selective antagonists induces protection in various models of neurodegenerative diseases. The mechanisms underlying this effect are still largely unknown. Since it is known that excessive reactive astrogliosis is a factor contributing to cell death in diseases characterized by neurodegenerative events, the present study has been aimed at determining whether selective A(2A) receptor antagonists can counteract the formation of reactive astrocytes induced in vitro by basic fibroblast growth factor (bFGF), a typical trigger of this reaction. Exposure of primary rat striatal astrocytes to the selective A(2A) antagonist SCH58261 resulted in concentration-dependent abolition of bFGF induction of astrogliosis in vitro. This effect could also be reproduced with the chemically unrelated A(2A) antagonist KW-6002. The direct activation of A(2A) adenosine receptors by selective receptor agonists was not sufficient per se to induce astrogliosis, suggesting that the A(2A) receptor needs to act in concert with other bFGF-induced genes to trigger the formation of reactive astrocytes. These results provide a mechanism at the basis of the neuroprotection induced by A(2A) receptor antagonists in models of brain damage and highlight this adenosine receptor subtype as a novel target for the pharmacological modulation of the gliotic reaction.

A key role for caspase-2 and caspase-3 in the apoptosis induced by 2-chloro-2'-deoxy-adenosine (cladribine) and 2-chloro-adenosine in human astrocytoma cells

Both the anticancer agent 2-chloro-2'-deoxy-adenosine (Cladribine) and its derivative 2-chloro-adenosine induce apoptosis of human astrocytoma cells (J Neurosci Res 60:388-400, 2000). In this study, we have analyzed the involvement of caspases in these effects. Both compounds produced a gradual and time-dependent activation of "effector" caspase-3, which preceded the appearance of the nuclear signs of apoptosis, suggesting a temporal correlation between these two events. Moreover, the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-dl-Asp-fluoromethylketone (fmk) suppressed both caspase-3 activation and apoptosis induction. "Initiator" caspase-9 and caspase-8 were only marginally activated at later times in the apoptotic process. Accordingly, at concentrations that selectively inhibit these caspases, neither N-benzyloxycarbonyl-Leu-Glu-His-Asp-fmk nor N-benzyloxycarbonyl-Ile-Glu-Thr-Asp-fmk could prevent adenosine analog-induced cell death. To definitively rule out a role for the caspase-9/cytochrome c-dependent mitochondrial pathway of cell death, neither adenosine analog had any effect on mitochondrial membrane potential, which was instead markedly reduced by other apoptotic stimuli (e.g., deoxyribose, NaCN, and betulinic acid). Consistently, although the latter triggered translocation of mitochondrial cytochrome c to the cytoplasm, no cytosolic accumulation of cytochrome c was detected with adenosine analogs. Conversely, 1 to 7 h after addition of either adenosine analog (i.e., before the appearance of caspase-3 activation), caspase-2 activity was surprisingly and markedly increased. The selective caspase-2 inhibitor N-benzyloxy carbonyl-Val-Asp-Val-Ala-Asp-fmk significantly reduced both adenosine analogs-induced caspase-2 activation and the associated cell death. We conclude that adenosine analogs induce the apoptosis of human astrocytoma cells by activating an atypical apoptotic cascade involving caspase-2 as an initiator caspase, and effector caspase-3. Therefore, these compounds could be effectively used in the pharmacological manipulation of tumors characterized by resistance to cell death via either the mitochondrial or caspase-8/death receptor pathways.

Changes of peripheral A2A adenosine receptors in chronic heart failure and cardiac transplantation

Peripheral blood mononuclear cells of chronic heart failure (CHF) patients produce great amounts of pro-inflammatory cytokines, indicating that circulating cells are activated and could mirror changes occurring in inflammatory cells infiltrating the failing heart. Adenosine is a regulatory metabolite acting through four membrane receptors that are linked to adenylyl cyclase: activation of the A2A receptor subtype has been reported to inhibit cytokine release. Changes of the adenosinergic system may play a role in CHF development. Here we report an increase of A2A receptor expression, density, and coupling to adenylyl cyclase in blood circulating cells of CHF patients. A2A receptor up-regulation was also found in the explanted hearts of these patients, suggesting that changes of peripheral adenosine receptors mirror changes occurring in the disease target organ. In a cohort of patients followed longitudinally after heart transplantation, alterations of peripheral A2A adenosine receptor progressively normalized to control values within 6 months, suggesting that improvement of cardiac performance is accompanied by progressive restoration of a normal adenosinergic system. These results validate the importance of the A2A receptor in human diseases characterized by a marked inflammatory/immune component and suggest that the evaluation of this receptor in peripheral blood cells may be useful for monitoring hemodynamic changes and the efficacy of pharmacological and non-pharmacological treatments in CHF patients.