5'-nucleotidase--adenylate cyclase relationships in mouse thymocytes. A re-evaluation of the effects of concanavalin A on cyclic AMP levels

Identification of cyclic AMP-phosphodiesterase variants from the PDE4D gene expressed in human peripheral mononuclear cells

To determine whether the expression of different PDE4D variants is unique to the rat or conserved through evolution, we have characterized the different PDE4D mRNAs expressed in human peripheral blood mononuclear cells. RT-PCR was performed using primers based on rat sequences and mRNAs from mononuclear cells. The specifically amplified fragments had a size identical to that predicted for rat PDE4D1, PDE4D2 and PDE4D3. Sequencing confirmed that these fragments are derived from the human PDE4D gene. Their sequence was highly homologous to that reported for the rat variants. cDNAs corresponding to the entire ORF of human PDE4D2 and PDE4D3 were expressed in mammalian cells, causing a large increase in PDE activity. Western blot analysis of human peripheral blood mononuclear cell extracts demonstrated the presence of proteins corresponding to the recombinant PDE4D1 and PDE4D2. The pattern of splicing and different promoter usage of the PDE4D gene is therefore conserved during evolution, which indicates an important physiological role.

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.

Enhancement of 5'-nucleotidase activity of human leukemic cells after fractionation: implications for cancer and aging

Previous studies reported that 5'-nucleotidase activity was undetectable or at much lower levels in the homogenate of human chronic lymphocytic leukemic (CCL) cells than in normal lymphocytes. In the present study, 5'-nucleotidase specific activity in acute myelocytic leukemia (AML), which varied in a range from undetectable to 1.4 (nmoles/min.mg protein), was enhanced by cell fractionation, from undetectable in the homogenate, up to 18.8 +/- 1.2, 6.4 +/- 0.7 and 0.68 +/- 0.12 in plasma membranes, microsomes, and cytosol fraction, respectively. In a further fractionation of the cytosol of various leukemic cells with ammonium sulfate, 5'-nucleotidase specific activity increased up to 14-fold in the 60% (NH4)2SO4 fraction, with a recovery of 1266 +/- 115%. These data suggest that 5'-nucleotidase activity in fractionated leukemic cells is higher than reported previously and that the sum of 5'-nucleotidase activity in subcellular compartments is higher than that detected in the homogenate. Furthermore, even when 5'-nucleotidase was undetectable in a homogenate, it became detectable in the plasma membranes, suggesting that its ecto-enzyme function is still active in leukemic cells. The undetectable or low 5'-nucleotidase in the homogenate is indicative of (1) the enzyme itself being in an inactive form but becoming active after the fractionations, or (2) the presence of a factor(s) that prevents the enzyme from being detected but that is separated from the enzyme by the fractionations. In both cases, the rate of nucleotide catabolism by inactive 5'-nucleotidase in rapidly proliferating leukemic cells should be slower than when the enzyme is active. The present finding is consistent with our previous findings that during normal cell aging the high 5'-nucleotidase activity is associated with senescent non-proliferating cells but low or undetectable activity with rapidly proliferating immortal cells. The implications of 5'-nucleotidase for DNA synthesis in aging and cancer are discussed.

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.

Calcium dependency of antigen-specific (T3-Ti) and alternative (T11) pathways of human T-cell activation

Human T lymphocytes are activated by two lineage-specific surface components: the antigen/major histocompatibility complex receptor (T3-Ti) and the unrelated T11 molecule. Interaction of either of these with their respective ligands leads to T-cell proliferation via an interleukin 2(IL-2) dependent autocrine mechanism. To begin to characterize the molecular details of the activation process, the role of Ca2+ was examined using human T-cell clones and monoclonal antibodies directed against their surface components. Here, we show that within minutes of triggering either the T3-Ti or T11 molecule, there is a large increase in intracellular Ca2+ concentration, as measured by quin-2 fluorescence. This is essential for induction of T-cell proliferation in inducer, suppressor, and cytotoxic clones and therefore presumably is required at an early step in the autocrine growth pathway. Thus, chelating exogenous Ca2+ with EGTA specifically inhibits proliferation triggered by anti-T3-Ti or anti-T11 monoclonal antibodies, but it does not affect triggering by exogenous IL-2. In addition, the Ca2+ ionophore A23187 can, by itself, initiate clonal proliferation.

Inherited disorders of purine metabolism--underlying molecular mechanisms

An overview of inherited disorders of purine metabolism, concentrating on well established enzyme defects is given. Included are HPRT and the LNS, APRT and 2,8-dihydroxyadenine lithiasis, hyperactivity of PRPP synthetase, ADA and PNP and immunodeficiencies. Emphasis is put on underlying molecular mechanisms on the gene-, enzyme-, or metabolite level for a better understanding of the events leading from the genotype to the clinical phenotype. Finally some aspects of extracellular purine nucleotide metabolism catalyzed by cell surface-bound ectoenzymes are discussed.

Thyrotropin increases 5'-nucleotidase activity in primary cultures of porcine thyroid cells

Thyrotropin given to support-anchored cultures of porcine thyroid cells, either in a serum-containing or in a serum-free system, produced an increase of about 50% in the total activity of 5'-nucleotidase. In the serum-free culture, in which TSH was administered to well-reformed follicles, this increase in 5'-nucleotidase activity concerns both the ecto-enzymic and intracellular forms of the enzyme and it coincides with the period of several days during which several glycosyltransferase activities are elevated and thyroglobulin production increased. Taken together, and in view of a recent in vitro study (Brandan and Fleisher, 1982) documenting the fate of uridine diphosphate in Golgi vesicles, these results suggest that there might be a functional correlation between the stimulation of 5'-nucleotidase and an increased production of nucleoside mono- and diphosphates when the activity of a number of glycosyltransferases is increased.

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.

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.

Enzyme analysis and subcellular fractionation of human peripheral blood lymphocytes with special reference to the localization of putative plasma membrane enzymes

Human lymphocytes were isolated from defibrinated blood by Ficoll-Hypaque centrifugation with erythrocyte hypotonic lysis. Homogenates of mixed lymphocytes were subjected to analytical subcellular fractionation by sucrose gradient centrifugation in a Beaufay automatic zonal rotor. The principal organelles were characterized by their marker enzymes: cytosol (lactate dehydrogenase), plasma membrane (5'-nucleotidase), endoplasmic reticulum (neutral alpha-glucosidase), mitochondria (malate dehydrogenase), lysosomes (N-acetyl-beta-glucosaminidase), peroxisomes (catalase). gamma-Glutamyl transferase was exclusively localized to the plasma membrane. Leucine amino-peptidase, especially when assayed in the presence of Co2+, was also partially localized to the plasma membrane. Experiments with diazotized sulphanilic acid, a non-permeant enzyme inhibitor, showed that these plasma membrane enzymes are present on the cell surface. No detectable alkaline phosphatase was found in the lymphocytes. Acid phosphatase and beta-glucuronidase were localized to lysosomes and there was some evidence for lysosomal heterogeneity. Leucine amino peptidase, optimal at pH 8.0, showed a partial localization to intracellular vesicles, possibly lysosomes, especially when assayed in the presence of EDTA. These studies provide a technique for determining the intracellular distribution of hitherto unassigned lymphocyte constituents and serve as a basis for investigating the cell pathology of lymphocytic disorders.

Association of 5'nucleotidase activity with immature granulocytes in the bone marrow of guinea pigs

Bone marrow leukocytes from adult strain 2 guinea pigs were found to have appreciable levels of 5' adenosine monophosphate hydrolytic activity (105 nmole/h/10(6) cells). On the basis of substrate specificity studies, enzyme inhibition studies, and thin-layer chromatographic analysis of the reaction product, the activity is related to 5' nucleotidase (5'N). The enzyme activity was associated with the membrane-enriched particulate fraction of lysed bone marrow cells. The bone marrow cell-associated 5'N activity was consistently very high in all five strains of guinea pigs examined (77--127 nmole/h/10(6) cells) and the range of activity was at least 10-fold greater than that observed for bone marrow cells obtained from mice, rabbits or rats. Furthermore, the bone marrow cell-associated 5'N activity in strain 2 guinea pigs was 5-fold greater than that observed for spleen and at least 13-fold greater than that of blood, mesenteric lymph nodes or thymus obtained from the same animal. Fractionation of strain 2 guinea pig bone marrow cells on Percoll density gradients showed that as the proportion of immature granulocytes increased in the various cell fractions, so did the 5'N activity. The cell fraction that sedimented at a density of 1.071 g/ml had the highest 5'N activity and the majority of the cells (94%) were immature granulocytes. The bone marrow compared to blood and spleen had the highest number of total granulocytes and the highest percentage of immature granulocytes. We conclude that the elevated bone marrow-derived 5'N activity in guinea pigs is associated with the resident population of immature granulocytes in that tissue.

Properties and sidedness of 5'-nucleotidase in rat red cell ghosts

The occurrence of a Mg++-activated 5'-AMPase activity in rat ghosts is demonstrated. This activity is inhibited by the alpha, beta-methyleneadenosine 5'-diphosphate, a specific inhibitor of 5'-nucleotidase. The enzyme has an apparent Km of approximately 90 microM, and is located on the exterior side of the plasma membrane.

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

Lymphocytes possess membrane adenosine receptors through which this nucleoside or analogs activate adenylate cyclase. Several adenosine analogs - 5'-N-ethylcarboxamide adenosine (NECA), N6-(L-2-phenyl-isopropyl) adenosine (PIA), 2-chloro adenosine (Cl-Ado) - were tested on the adenylate cyclase of mouse thymocyte homogenates; the order of potencies: NECA greater than Cl-Ado greater than PIA is consistent with the order which has been assumed to be characteristic of Ra type adenosine receptors. The stimulation of thymocyte adenylate cyclase by NECA was biphasic while the responses to Cl-Ado and PIA were not, which suggests the existence of Ra sub-types of adenosine receptors. Moreover, these adenosine receptors were found to be present in several lymphocyte subclasses: mature and immature mouse thymocytes, B and T mouse splenocytes, human peripheral blood lymphocytes, and also in lymphoblastoid cell lines. No crucial differences were observed between the adenylate cyclase stimulations of the various population by NECA.

Plasma membrane enzymes in BALB/c lymphomas with either T or B cell properties, I. 5'-Nucleotidase

Comparison of membrane bound 5'-nucleotidase activity has been made in crude extracts and plasma membrane fractions from Abelson virus transformed lymphomas, IgM, IgG and IgA producing plasmacytomas and from thymomas with different surface antigen markers. 5'Nucleotidase activity was characterised by the following criteria : 1) optimal pH for enzyme activity, 2) specificity of 5'AMP as a substrate at the optimal pH, 3) specific inhibition of the enzyme by alpha beta methylene ADP, 4) inhibition by EDTA. 5'-Nucleotidase was found to be present on the following B cells : Abelson virus transformed lymphomas, plasmacytomas and LPS-stimulated blasts from nu/nu spleens. The enzyme was also found in one thymoma Lut 13, but it was absent from all the other thymomas studied. A possible relationship between 5'-Nucleotidase and purine metabolism in lymphocyte subpopulations is suggested by the results.