Lymphocyte membrane adenosine receptors coupled to adenylate cyclase: properties and occurrence in various lymphocyte subclasses

PhysiologicalControl ofImmuneResponse andInflammatoryTissueDamage byHypoxia-InducibleFactors andAdenosineA2AReceptors

Immune cell-mediated destruction of pathogens may result in excessive collateral damage to normal tissues, and the failure to control activated immune cells may cause immunopathologies. The search for physiological mechanisms that downregulate activated immune cells has revealed a critical role for extracellular adenosine and for immunosuppressive A2A adenosine receptors in protecting tissue from inflammatory damage. Tissue damage-associated deep hypoxia, hypoxia-inducible factors, and hypoxia-induced accumulation of adenosine may represent one of the most fundamental and immediate tissue-protecting mechanisms, with adenosine A2A receptors triggering "OFF" signals in activated immune cells. In these regulatory mechanisms, oxygen deprivation and extracellular adenosine accumulation serve as "reporters," while A2A adenosine receptors serve as "sensors" of excessive tissue damage. The A2A receptor-triggered generation of intracellular cAMP then inhibits activated immune cells in a delayed negative feedback manner to prevent additional tissue damage. Targeting A2A adenosine receptors may have important clinical applications.

The critical role of adenosine A2A receptors in downregulation of inflammation and immunity in the pathogenesis of infectious diseases

Adenosine can be described as a retaliatory metabolite, the production and release of which is usually enhanced under adverse environmental conditions. Binding via specific receptors, adenosine activates endogenous protective mechanisms aiming at the restoration of tissue homeostasis. While adenosinergic downregulation of tissue damage is beneficial in acute inflammation, chronic suppression of the immune system by adenosine may account for immunoparalysis in long-term septic patients.

Characterization of A2A adenosine receptors in human lymphocyte membranes by [3H]-SCH 58261 binding

1. The present study describes for the first time the characterization of the adenosine A2A receptor in human lymphocyte membranes with the new potent and selective antagonist radioligand, [3H]-5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo [4,3-e]-1,2,4 triazolo [1,5-c] pyrimidine, ([3H]-SCH 58261). In addition, both receptor affinity and potency of reference adenosine receptor agonists and antagonists were determined in binding and adenylyl cyclase studies. 2. Saturation experiments revealed a single class of binding sites with Kd and Bmax values of 0.85 nM and 35 fmol mg-1 protein, respectively. A series of adenosine receptor ligands were found to compete for the binding of 0.8 nM [3H]-SCH 58261 to human lymphocyte membranes with a rank order of potency consistent with that typically found for interactions with the A2A-adenosine receptor. In the adenylyl cyclase assay the same compounds exhibited a rank order of potency similar to that observed in binding experiments. 3. Thermodynamic data indicate that [3H]-SCH 58261 binding to human lymphocytes is entropy and enthalpy-driven, a finding in agreement with the thermodynamic behaviour of antagonists for rat striatal A2A-adenosine receptors. 4. It is concluded that in human lymphocyte membranes [3H]-SCH 58261 directly labels binding sites showing the characteristic properties of the adenosine A2A-receptor. The presence of A2A-receptors in peripheral tissue such as human lymphocytes strongly suggests an important role for adenosine in modulating immune and inflammatory responses.

Electrophysiological behavior of microglia

The present knowledge of voltage- and ligand-activated ion channels of cultured microglial cells is described and its relevance is discussed. All microglial cells cultured from rat or mouse brain express an inward rectifying K+ channel but no outward currents. This expression is not changed by the length of the cultivation period, nor is it different in freshly isolated cells. It makes the microglial cells distinct from peritoneal macrophages, which possess an outward rectifying K+ channel. In bone marrow, 2 populations of cells could be distinguished electrophysiologically, one with the channel pattern of macrophages and one with that of microglial cells. This finding is interesting in light of the fact that it is presently hypothesized that the differentiation of monocytes into microglia takes place exclusively during embryonic development but not in the adult. The available data thus support the hypothesis that within the bone marrow a population of macrophage precursor cells exists with a possible lineage relationship to brain macrophages. The lack of outward currents in the microglial cells has the functional consequence that even a small inward current leads to a large membrane depolarization, since K+ outward currents are not activated with the depolarization. The microglial cell is thus very sensitive to depolarizing events. We found that ATP induced an inward current and an increase in the conductance, whereas ADP, AMP, and adenosine did not. These relative potencies indicate that microglia possess a P2 purinoceptor linked to an ion channel. The amplitude of the inward current elicited by ATP is about 80 pA and is sufficient to depolarize microglial cells close to 0 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Methotrexate inhibits neutrophil function by stimulating adenosine release from connective tissue cells

Although commonly used to control a variety of inflammatory diseases, the mechanism of action of a low dose of methotrexate remains a mystery. Methotrexate accumulates intracellularly where it may interfere with purine metabolism. Therefore, we determined whether a 48-hr pretreatment with methotrexate affected adenosine release from [14C]adenine-labeled human fibroblasts and umbilical vein endothelial cells. Methotrexate significantly increased adenosine release by fibroblasts from 4 +/- 1% to 31 +/- 6% of total purine released (EC50, 1 nM) and by endothelial cells from 24 +/- 4% to 42 +/- 7%. Methotrexate-enhanced adenosine release from fibroblasts was further increased to 51 +/- 4% (EC50, 6 nM) and from endothelial cells was increased to 58 +/- 5% of total purine released by exposure to stimulated (fMet-Leu-Phe at 0.1 microM) neutrophils. The effect of methotrexate on adenosine release was not due to cytotoxicity since cells treated with maximal concentrations of methotrexate took up [14C]adenine and released 14C-labeled purine (a measure of cell injury) in a manner identical to control cells. Methotrexate treatment of fibroblasts dramatically inhibited adherence to fibroblasts by both unstimulated neutrophils (IC50, 9 nM) and stimulated neutrophils (IC50, 13 nM). Methotrexate treatment inhibited neutrophil adherence by enhancing adenosine release from fibroblasts since digestion of extracellular adenosine by added adenosine deaminase completely abrogated the effect of methotrexate on neutrophil adherence without, itself, affecting adherence. One hypothesis that explains the effect of methotrexate on adenosine release is that, by inhibition of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) transformylase, methotrexate induces the accumulation of AICAR, the nucleoside precursor of which (5-aminoimidazole-4-carboxamide ribonucleoside referred to hereafter as acadesine) has previously been shown to cause adenosine release from ischemic cardiac tissue. We found that acadesine also promotes adenosine release from and inhibits neutrophil adherence to connective tissue cells. The observation that the antiinflammatory actions of methotrexate are due to the capacity of methotrexate to induce adenosine release may form the basis for the development of an additional class of antiinflammatory drugs.

Overview. Purinergic mechanisms

An overview of the history of studies of the biological actions of extracellular ATP is presented. The basis of the subdivision of receptors for purines into P1-purinoceptors for adenosine and P2-purinoceptors for ATP and ADP are considered, as well as the recent proposal for subdivision of the ATP receptors into P2X-, P2Y-, P2Z-, and P2T- purinoceptor subtypes. These purinoceptor subtypes are discussed with respect to their transduction mechanisms, their distribution, and their physiological roles, including their roles in cotransmission and neuromodulation.

Extracellular ATP4- permeabilizes thymocytes not only to cations but also to low-molecular-weight solutes

Extracellular ATP increases the free cytosolic calcium concentration ([Ca2+]i) of thymocytes by changing the membrane permeability to divalent cations: (1) no [Ca2+]i elevation is observed in the absence of extracellular calcium and (2) ATP also promotes Mn2+ entry into thymocytes, as shown by a stimulated quench of the fluorescence of Quin2-loaded cells. Using 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5- hexatriene, a dye which usually does not pass through membranes, we showed that ATP permeabilizes thymocyte membranes not only to cations but also to low-molecular-weight solutes. The ATP effect is dose-dependent, specific and reversible; ATP4- appears to be the active species and probably acts through specific receptor(s). Both medullary and cortical thymocytes respond to ATP whereas splenocytes do not; this shows that the ATP effect is restricted to certain populations of lymphocytes. The thymoma-derived cell line, EL-4, the permeability of which is also sensitive to ATP, could provide a useful model for studying the putative ATP4- receptor(s) of thymocytes.

Adenosine, a cytoprotective autocoid: effects of adenosine on neutrophil plasma membrane viscosity and chemoattractant receptor display

At inflammatory sites neutrophils are stimulated to produce a variety of toxic agents, yet rarely harm the endothelium across which they migrate. We have recently found that endothelium releases adenosine which, acting via receptors on the surface of human neutrophils, inhibits generation of toxic metabolites by stimulated neutrophils but, paradoxically, promotes chemotaxis. Agents which diminish plasma membrane viscosity affect neutrophil function similarly, possibly by modulating chemoattractant receptor number or affinity. We therefore determined whether adenosine receptor agonists modulate neutrophil function by decreasing membrane viscosity and/or changing the affinity of chemoattractant (N-fMet-Leu-Phe, FMLP) receptors. Surprisingly, 5'-(N-ethylcarboxamido)adenosine (NECA, 10 microM), the most potent agonist at neutrophil adenosine receptors, increased plasma membrane viscosity, as measured by fluorescence anisotropy of the plasma membrane specific probe 1-(4-trimethylaminophenyl)-6-diphenyl-1,3,5-hexatriene (TMA-DPH), in unstimulated neutrophils from a mean microviscosity of 1.67 +/- 0.02 (S.E.) to 1.80 +/- 0.02 (p less than 0.001) while inosine (10 microM), a poor adenosine receptor agonist, had no effect (1.73 +/- 0.04, p = n.s. vs. control, p less than 0.01 vs. NECA). Adenosine receptor agonists increased plasma membrane viscosity in neutrophils with the same order of potency previously seen for inhibition of superoxide anion generation and enhancement of chemotaxis (NECA greater than adenosine = N6-phenylisopropyladenosine). The adenosine receptor antagonist 8-(p-sulfophenyl)theophylline reversed the effect of NECA on plasma membrane viscosity. Unlike other agents which modulate plasma membrane viscosity, NECA (10 microM) did not significantly change the number or affinity of [3H]FMLP binding sites on neutrophils. In contrast to the hypothesis of Yuli et al. these results indicate that occupancy of adenosine receptors on neutrophils increases plasma membrane viscosity without affecting chemoattractant receptor display.

Disparate functions of I-A and I-E molecules on B cells as evidenced by the inhibition with anti-I-A and anti-I-E antibodies of polyclonal B cell activation

Polyclonal differentiation of unprimed B cells into IgM-producing cells induced by lipopolysaccharide (LPS) or T cell-derived lymphokine B151-TRF2 has been shown to contain a process of I-A-restricted B-B cell interaction, so that the B cell responses are inhibited by monoclonal antibodies (mAb) specific for I-A molecules. On the other hand, the B cell responses are also inhibited by anti-I-E mAb, although I-E molecules are not involved in such B-B cell interaction. In this study, we examined the mechanism underlying the anti-I-E-mediated inhibition of the B cell responses. The B cell responses induced by LPS or B151-TRF2 were inhibited by either anti-I-A or anti-I-E mAb added on day 0 over a 5-day culture period, whereas when added on day 3 the responses were inhibited only by anti-I-E mAb and not by anti-I-A mAb. To gain insight into the mechanism underlying the anti-I-E-mediated inhibition, we prepared monovalent Fab and divalent F(ab')2 fragments of anti-I-A and anti-I-E mAb and examined their effects on the B cell responses. We found that the B cell responses were inhibited by the F(ab')2 but not Fab fragment of anti-I-E mAb, whereas the Fab fragment of anti-I-A mAb still gave effective inhibition. The F(ab')2 but not Fab fragment of anti-I-E mAb induced increases in cyclic AMP (cAMP) levels in B cells, whereas the undigested anti-I-A mAb did not induce such increases. Furthermore, adenylate cyclase inhibitors, which inhibit cellular cAMP accumulation, circumvented the B cell responses inhibited by anti-I-E but not anti-I-A mAb. Thus, these results indicate that the anti-I-E-mediated inhibition of the B cell responses requires increases in intracellular cAMP levels induced by cross-linking of I-E molecules. In contrast, anti-I-A mAb inhibits the B cell responses without cross-linking of I-A molecules and cAMP accumulation. These results reinforce a unique function of I-A molecules as restriction elements in the Ia-restricted B-B cell interaction.

The [Ca2+]i increase induced in murine thymocytes by extracellular ATP does not involve ATP hydrolysis and is not related to phosphoinositide metabolism

We have previously demonstrated that exogenous ATP can give medullary thymocytes the calcium message required for the induction of their blastogenesis. In the present study, using the highly sensitive calcium indicator Indo-1, we have measured the effect of exogenous nucleotides on the cytosolic-free calcium concentration [Ca2+]i of thymocytes, and determined inositol phosphate (IP) formation in the same cells, in parallel assays. The results were compared to those obtained with the mitogenic lectin concanavalin A (ConA) in similar experiments. They show that ATP does not mobilize calcium from its internal stores but stimulates its influx from the extracellular medium. Nevertheless, these data do not rule out the possibility that the nucleotide acts through specific P2 purinergic sites.

Qualitative and quantitative differences between Burkitt lymphoma and lymphoblastoid cell lines in the expression of membrane ecto-nucleotidases and in the response to agonists of the A2-type adenosine receptor

The activity of ecto-nucleotidases has been determined on intact cells from Burkitt lymphoma (BL) and lymphoblastoid cell lines established in vitro (LCLs). BL cells never express ecto-5'-nucleotidase (5'-N) and exhibit only low levels of ecto-ATPase activity, whereas LCL-cells usually express both enzymes to varying degrees. There is a certain correlation (r = 0.75) between 5'-N and ecto-ATPase. When cAMP formation in response to agonists of the A2-type (stimulating) adenosine receptor is measured in the same cell lines there is no correlation with the expression of ecto-nucleotidases. Within pairs of BL and LCL cell lines derived from the same donor, an inverse relationship between ecto-nucleotidases and the response to the adenosine receptor agonist N-ethyl-carboxamido-adenosine (NECA) is observed. BL cells show a good response to NECA, whereas this is low or absent in LCL cells. Treatment of cells which exhibit both 5'-N and the adenosine receptor with specific polyclonal or monoclonal antibodies against the enzyme does not impair the function of the receptor. Antisera against peptides of the membrane antigen BNLF I-MA, coded by the EBV-genome, do not co-precipitate 5'-N out of detergent extracts of LCL-cells. In both cases 5'-N cannot be closely associated with other membrane components. The differences between BL and LCL cells in ectonucleotidases and the adenosine receptor appear to be fairly stable phenotypical markers, independent of other parameters and suitable for use in distinguishing these cells in all populations.

Basal and adenosine receptor-stimulated levels of cAMP are reduced in lymphocytes from alcoholic patients

Alcoholism causes serious neurologic disease that may be due, in part, to the ability of ethanol to interact with neural cell membranes and change neuronal function. Adenosine receptors are membrane-bound proteins that appear to mediate some of the effects of ethanol in the brain. Human lymphocytes also have adenosine receptors, and their activation causes increases in cAMP levels. To test the hypothesis that basal and adenosine receptor-stimulated cAMP levels in lymphocytes might be abnormal in alcoholism, we studied lymphocytes from 10 alcoholic subjects, 10 age- and sex-matched normal individuals, and 10 patients with nonalcoholic liver disease. Basal and adenosine receptor-stimulated cAMP levels were reduced 75% in lymphocytes from alcoholic subjects. Also, there was a 76% reduction in ethanol stimulation of cAMP accumulation in lymphocytes from alcoholics. Similar results were demonstrable in isolated T cells. Unlike other laboratory tests examined, these measurements appeared to distinguish alcoholics from normal subjects and from patients with nonalcoholic liver disease. Reduced basal and adenosine receptor-stimulated levels of cAMP in lymphocytes from alcoholics may reflect a change in cell membranes due either to chronic alcohol abuse or to a genetic predisposition unique to alcoholic subjects.

Adenosine induced immunosuppression: the role of the adenosine receptor--adenylate cyclase interaction in the alteration of T-lymphocyte surface phenotype and immunoregulatory function

Adenosine (1 microM at 37 degrees C) rapidly modulates the expression of T-lymphocyte surface antigens (OKT4 and OKT8) and Fc gamma receptors, and increases T-suppressor activity for pokeweed mitogen driven in vitro immunoglobulin synthesis. The adenosine induced loss of OKT4 expressed was maximal within 5 min, while increased OKT8 expression developed more slowly; increased Fc gamma expression was maximal at 30 min after initiation of incubation with adenosine. Adenosine, 2-chloroadenosine and adenosine in the presence of nitrobenzylthioinosine (NBTI) induced the decrease in OKT4 expression. In contrast only adenosine induced the enhancement of OKT8 and Fc gamma receptor expression. Neither 2-chloroadenosine, a poorly transported analog, nor adenosine in the presence of the adenosine transport inhibitor NBTI were capable of enhancing OKT8 or Fc gamma receptor expression. Adenosine was shown to cause a rapid biphasic increase in cAMP, while 2-chloroadenosine and adenosine with NBTI induces a prolonged elevation in cAMP. Similarly isoproteranol which induces a sustained elevation in cAMP suppressed the adenosine induced increases in OKT8 and Fc gamma receptor expression. Incubation of T-helper/inducer lymphocytes with 1 microM adenosine for 30 min at 37 degrees C caused the loss of T-helper function; this loss of T-helper activity has previously been shown to result from the activation of T-suppressor cells. The loss of T-helper function was blocked by the simultaneous addition of isobutylmethylxanthine, an R site adenosine receptor antagonist, or NBTI, an adenosine transport inhibitor. Moreover, 2-chloroadenosine could not induce loss of T-helper function.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against functional adenosine receptors on murine lymphocytes

The effects of adenosine, 2-chloroadenosine (2Cl Ado) and N6-phenylisopropyladenosine (PIA) were examined on peripheral blood and splenic lymphocytes from mice. Lectin-stimulated DNA synthesis was antagonized by adenosine and 2Cl Ado at high concentrations. Lower concentrations of all three nucleosides produced an enhancement of lectin-stimulated thymidine uptake in splenic lymphocytes. Peripheral blood lymphocytes were found to exhibit only inhibitions of mitogenic stimulation suggesting a difference in response to nucleoside exposure between spleen and peripherally circulating cells. The synthesis of antibody to sheep red blood cells was inhibited in a non-cytotoxic manner by 2Cl Ado and PIA while adenosine was without effects. The receptor antagonist 8-phenyltheophylline was found to block nucleoside increases in thymidine uptake and low concentrations of 2Cl Ado with regard to antibody production. The effects of high concentrations of 2Cl Ado or PIA on humoral responses were not antagonized by receptor blockade. The data suggest that functional alterations of lymphocyte responses to nucleoside exposure are not a consequence of surface receptors for adenosine nucleosides in murine cells.

The proposed 5'-nucleotidase inhibitor in human leukemic cells is an artefact

It is now well established that human lymphoblastoid cell lines showing immaturity characters display ecto-5'-nucleotidase activities lower than normal levels. A recent paper (Sun, A.S., Holland, J.F. and Ohnuma, T. (1983) Biochim. Biophys. Acta 762, 577-584) mentioned that this phenomenon resulted from the presence of a 5'-nucleotidase inhibitor in these cell lines. We demonstrate here that the use of 5'-[3H]AMP as a substrate, and inadequate analysis of the products formed, led them to a misinterpretation. [3H]Adenosine derived from 5'-[3H]AMP hydrolysis was further transformed into [3H]inosine by the adenosine deaminase activity of the leukemic cell lines tested; [3H]inosine was precipitated with the excess substrate and was not taken into account in the ecto-5'-nucleotidase determination, which led the authors to confuse this adenosine deaminase activity with a 5'-nucleotidase inhibitor. We did not observe 5'-nucleotidase inhibition by leukemic cell cytosol when convenient assay methods were used and showed that the presence of such an inhibitor remains to be established.

Theophylline-like properties of xanthine analogs

Theophylline and other methylxanthines display a large number of biological effects, some of which are clinically important. The effects of these compounds are commonly ascribed to an inhibition of cyclic AMP breakdown. However, it becomes actually evident that another mechanism, namely adenosine receptor antagonism, could be responsible for certain methylxanthine effects. It could be of interest to find new compounds displaying only one of these mechanisms, either phosphodiesterase inhibition or adenosine receptor antagonism. We have studied several synthetic imidazol[1,2a]pyrazines, some of which display theophylline-like pharmacological properties at lower doses than theophylline. We showed that some of these compounds inhibited mitogen-induced [3H]-thymidine uptake by human lymphocytes, which is consistent with increases in cyclic AMP levels: the most efficient compounds were those which were better phosphodiesterase inhibitors than theophylline and poorer adenosine receptor antagonists.

Immunological aspects of psoriasis. VI. Impairment of isoprenaline and theophylline-induced inhibition of mitogen responsiveness

Pharmacological abnormalities occur in the psoriatic epidermis, and if similar abnormalities occur in the peripheral blood mononuclear cells they could impair the immune responses in psoriasis. In a paired control study, we have tested the capacity of histamine, isoprenaline and theophylline (10(-5) and 10(-7) M) to inhibit the mitogen responsiveness of blood mononuclear cells from normal and psoriatic subjects, using phytohaemagglutinin and concanavalin A. In the normal controls, mitogen responsiveness was inhibited by all three pharmacological agents by about 30 to 50%. In cells from psoriatic patients, the response in the presence of histamine was inhibited (as in the controls), but isoprenaline caused no inhibition, and theophylline paradoxically increased the mitogenic responses. These results suggest there is a defect in the pharmacological response of the blood mononuclear cells in psoriasis.

Inhibition by xanthine derivatives of adenosine receptor-stimulated cyclic adenosine 3',5'-monophosphate accumulation in rat and guinea-pig thymocytes

The effect of stable adenosine analogues, including adenosine 5'-N-ethylcarboxamide (NECA) and N6-L-phenylisopropyl-adenosine (L-PIA), were studied on cyclic adenosine 3', 5'-monophosphate (cyclic AMP) accumulation in rat and guinea-pig thymocytes. NECA was approximately 10 times more potent than L-PIA, in thymocytes from both species. D-PIA was more potent in guinea-pig than in rat thymocytes. The effect of a number of adenosine analogues followed the order: NECA greater than 2-chloro-adenosine greater than L-PIA greater than N6-cyclohexyl-adenosine (CHA), an order of potency characteristic for adenosine receptors of the A2-subtype. Thymocytes may be used as a model system to study the pharmacology of such receptors. Several xanthines were studied as antagonists of the NECA (1 microM)-induced cyclic AMP accumulation. The order of potency was: 1,3-diethyl-8-phenylxanthine greater than 8-phenyl-theophylline greater than IBMX = 8-p-sulphophenyltheophylline = verrophylline greater than theophylline greater than caffeine greater than enprofylline greater than theobromine greater than pentoxiphylline. The pA2 value for 8-phenyltheophylline was 0.35 microM, and the antagonism was shown to be competitive. The order of potency of the xanthine is virtually identical to that found earlier in several other systems in which the receptors are of the A1-subtype. None of the xanthine derivatives tested thus seem to discriminate between A1 and A2-receptor-mediated adenosine actions.