A self-restricted CD38-connexin 43 cross-talk affects NAD+ and cyclic ADP-ribose metabolism and regulates intracellular calcium in 3T3 fibroblasts

Connexin 43 (Cx43) hexameric hemichannels, recently demonstrated to mediate NAD(+) transport, functionally interact in the plasma membrane of several cells with the ectoenzyme CD38 that converts NAD(+) to the universal calcium mobilizer cyclic ADP-ribose (cADPR). Here we demonstrate that functional uncoupling between CD38 and Cx43 in CD38-transfected 3T3 murine fibroblasts is paralleled by decreased [Ca(2+)](i) levels as a result of reduced intracellular conversion of NAD(+) to cADPR. A sharp inverse correlation emerged between [Ca(2+)](i) levels and NAD(+) transport (measured as influx into cells and as efflux therefrom), both in the CD38(+) cells (high [Ca(2+)](i), low transport) and in the CD38(-) fibroblasts (low [Ca(2+)](i), high transport). These differences were correlated with distinctive extents of Cx43 phosphorylation in the two cell populations, a lower phosphorylation with high NAD(+) transport (CD38(-) cells) and vice versa (CD38(+) cells). Conversion of NAD(+)-permeable Cx43 to the phosphorylated, NAD(+)-impermeable form occurs via Ca(2+)-stimulated protein kinase C (PKC). Thus, a self-regulatory loop emerged in CD38(+) fibroblasts whereby high [Ca(2+)](i) restricts further Ca(2+) mobilization by cADPR via PKC-mediated disruption of the Cx43-CD38 cross-talk. This mechanism may avoid: (i) leakage of NAD(+) from cells; (ii) depletion of intracellular NAD(+) by CD38; (iii) overproduction of intracellular cADPR resulting in potentially cytotoxic [Ca(2+)](i).

The hypothalamic-pituitary-adrenocortical and gonadal axis function in rheumatoid arthritis

The altered cortisol and adrenal androgen (i.e., dehydroepiandrosterone sulfate = DHEAS) secretion, observed during testing in rheumatoid arthritis (RA) patients not treated with corticosteroids, should be clearly regarded as a "relative adrenal insufficiency" in the setting of a sustained inflammatory process, as shown by high serum IL-6 levels. Androgens seem implicated in the pathophysiology of autoimmune disorders, including RA, as natural immunosuppressors. Low plasma and synovial fluid testosterone concentrations are observed in male RA patients; low plasma DHEAS levels are mainly observed in female RA patients. The menopausal peak of RA suggests that estrogens and/or progesterone deficiency also play a role in the disease, and many data indicate that estrogens suppress cellular immunity, but stimulate humoral immunity (i.e., deficiency promotes cellular Th1-type immunity). Gene polymorphisms for enzymes involved in the steroidogenesis seem to further complicate the role of sex hormones in the susceptibility to autoimmunity. Acquired changes of sex steroid metabolism seem to also play a role in the peripheral sex hormone levels. In conclusion, a complex interaction between the hypothalamus-pituitary-adrenocortical and gonadal axis functions is evident in RA.

Presence of estrogen receptors in human myeloid monocytic cells (THP-1 cell line)

OBJECTIVE

To test THP-1 cells for the presence of estrogen receptors (ER) since studies have demonstrated in vivo and in vitro, the influence of estrogens on cells involved in immune response (i.e. macrophages), and since it has been demonstrated that human myeloid monocytic THP-1 cells acquire phenotypic and functional macrophage-like features after incubation with several cytokines or pharmacological agents.

DESIGN

Stimulation of THP-1 cells with phorbol myristate acetate (PMA) to prompt their differentiation into macrophage-like cells and evaluation of the possible induction of ER.

METHODS

The expression of ER was analyzed by immunocytochemical assay, reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot analysis.

RESULTS

After stimulation by PMA, the human myeloid monocytic THP-1 cells showed the presence of ER, together with markers of monocytic cell differentiation such as CD68, CD54 and HLA-DR.

CONCLUSION

Estrogen effects may be exerted directly through ER on monocytes/macrophages. PMA-treated THP-1 cells may constitute a useful in vitro model to determine the effects of estrogens on macrophage-like cells and their implications in the inflammatory and immune processes.

Antiproliferative and antiinflammatory effects of methotrexate on cultured differentiating myeloid monocytic cells (THP-1) but not on synovial macrophages from patients with rheumatoid arthritis

OBJECTIVE

We investigated the antiproliferative and antiinflammatory effects of methotrexate (MTX) on differentiating/differentiated cells, namely cultured human monocytic myeloid cells (THP-1), and primary cultures of synovial macrophages from patients with rheumatoid arthritis (RA).

METHODS

We evaluated early and late apoptosis as well as natural cytokine inhibitor production, such as the interleukin 1 (IL-1) receptor antagonist (IL-1ra) and the soluble tumor necrosis factor receptor (sTNFr).

RESULTS

Within THP-1 cells we observed a significant (p < 0.001) dose-dependent inhibition of proliferation (at 24-48 and 72-96 h) and a significant presence of apoptosis (at 24-48 h) with MTX concentrations of 500, 100, and 75 microg/ml compared with untreated controls. No significant changes were observed with 5 microg/ml or 500 to 50, and 5 ng/ml. A significant increase of IL-1ra (p < 0.001) was observed with MTX concentrations of 5 microg (51.43 +/- 2.53 vs 16.22 +/- 5.19 pg/ml control) and 500 ng (36.43 +/- 3.3 vs 16.22 +/- 5.19 pg/ml control) at all the tested times. No significant changes were observed for the sTNFr p75. Evaluating the RA synovial macrophages, we obtained no significant effects on cell proliferation and apoptosis with MTX treatment at 24 h and at the concentration of 50 microg/ml (achievable in the serum with low dose MTX treatment in RA). No significant changes were observed for the IL-1ra and no detectable levels for the sTNFr p75 were detected after treatment with MTX.

CONCLUSION

This study shows that the antiproliferative and antiinflammatory effects of MTX on human cultured monocytes are dose-dependent. The antiproliferative activity seems to be mediated by cell apoptosis and the antiinflammatory activity seems to be related to cytokine inhibitor release.

Probing the catalytic mechanism of GDP-4-keto-6-deoxy-d-mannose Epimerase/Reductase by kinetic and crystallographic characterization of site-specific mutants

GDP-4-keto-6-deoxy-d-mannose epimerase/reductase is a bifunctional enzyme responsible for the last step in the biosynthesis of GDP-l-fucose, the substrate of fucosyl transferases. Several cell-surface antigens, including the leukocyte Lewis system and cell-surface antigens in pathogenic bacteria, depend on the availability of GDP-l-fucose for their expression. Therefore, the enzyme is a potential target for therapy in pathological states depending on selectin-mediated cell-to-cell interactions. Previous crystallographic investigations have shown that GDP-4-keto-6-deoxy-d-mannose epimerase/reductase belongs to the short-chain dehydrogenase/reductase protein homology family. The enzyme active-site region is at the interface of an N-terminal NADPH-binding domain and a C-terminal domain, held to bind the substrate. The design, expression and functional characterization of seven site-specific mutant forms of GDP-4-keto-6-deoxy-d-mannose epimerase/reductase are reported here. In parallel, the crystal structures of the native holoenzyme and of three mutants (Ser107Ala, Tyr136Glu and Lys140Arg) have been investigated and refined at 1. 45-1.60 A resolution, based on synchrotron data (R-factors range between 12.6 % and 13.9 %). The refined protein models show that besides the active-site residues Ser107, Tyr136 and Lys140, whose mutations impair the overall enzymatic activity and may affect the coenzyme binding mode, side-chains capable of proton exchange, located around the expected substrate (GDP-4-keto-6-deoxy-d-mannose) binding pocket, are selectively required during the epimerization and reduction steps. Among these, Cys109 and His179 may play a primary role in proton exchange between the enzyme and the epimerization catalytic intermediates. Finally, the additional role of mutated active-site residues involved in substrate recognition and in enzyme stability has been analyzed.

Properties of the evoked spatio-temporal electrical activity in neuronal assemblies

Properties of neural computation were studied in two types of neuronal networks: isolated leech ganglia and neuronal cultures of dissociated cortical neurons from neonatal rats. With appropriate experimental set-ups it was possible to obtain a precise description of the spread of excitation induced by specific inputs. The evoked spatio-temporal electrical activity was characterized by large variability and the electrical activity of neurons activated by the same stimulation was found to be statistically independent to a high degree. The variability presumably originates from basic properties of synaptic transmission, which is stochastic in nature. As a consequence, the large variability of the evoked spatio-temporal electrical activity appears to be a general property of neural computation and a typical feature of neuronal assemblies. It is shown, however, that the observed statistical independence of co-activated neurons may be used to reduce the effects of variability by appropriately averaging or pooling the electrical activity.

Structural and enzymatic characterization of human recombinant GDP-D-mannose-4,6-dehydratase

GDP-D-mannose-4,6-dehydratase (GMD) is the key enzyme in the 'de novo' pathway of GDP-L-fucose biosynthesis. The reported cDNA sequences for human GMD predict three forms of different length, whose 'in vivo' occurrence and molecular properties are completely undefined. Here, we report the expression in Escherichia coli and the properties of each native recombinant GMD form. Only the 42 kDa long GMD (L-GMD) and the 40.2 kDa (M-GMD) forms were recovered as soluble functional proteins, while the 38.7 kDa form, short GMD (S-GMD), lacking an N-terminal domain critical for dinucleotide binding, was inactive and formed a precipitate. Both L-GMD and M-GMD are homodimers and contain 1 mol of tightly bound NADP+. Their kinetic properties (Km, Kcat) are apparently identical and both forms are non-competitively feedback-inhibited by GDP-L-fucose to a similar extent. M-GMD is the predominant enzyme form expressed in several human cell lines. These data seem to suggest that modulation of the 'de novo' pathway of GDP-L-fucose biosynthesis involves mechanisms other than differential 'in vivo' expression of GMD forms.

GDP-4-keto-6-deoxy-D-mannose epimerase/reductase from Escherichia coli, a key enzyme in the biosynthesis of GDP-L-fucose, displays the structural characteristics of the RED protein homology superfamily

BACKGROUND

The process of guanosine 5'-diphosphate L-fucose (GDP-L-fucose) biosynthesis is conserved throughout evolution from prokaryotes to man. In animals, GDP-L-fucose is the substrate of fucosyltransferases that participate in the biosynthesis and remodeling of glycoconjugates, including ABH blood group and Lewis-system antigens. The 'de novo' pathway of GDP-L-fucose biosynthesis from GDP-D-mannose involves a GDP-D-mannose 4,6 dehydratase (GMD) and a GDP-4-keto-6-deoxy-D-mannose epimerase/reductase (GMER). Neither of the catalytic mechanisms nor the three-dimensional structures of the two enzymes has been elucidated yet. The severe leukocyte adhesion deficiency (LAD) type II genetic syndrome is known to result from deficiencies in this de novo pathway.

RESULTS

The crystal structures of apo- and holo-GMER have been determined at 2.1 A and 2.2 A resolution, respectively. Each subunit of the homodimeric (2 x 34 kDa) enzyme is composed of two domains. The N-terminal domain, a six-stranded Rossmann fold, binds NADP+; the C-terminal domain (about 100 residues) displays an alpha/beta topology. NADP+ interacts with residues Arg12 and Arg36 at the adenylic ribose phosphate; moreover, a protein loop based on the Gly-X-X-Gly-X-X-Gly motif (where X is any amino acid) stabilizes binding of the coenzyme diphosphate bridge. The nicotinamide and the connected ribose ring are located close to residues Ser107, Tyr136 and Lys140, the putative GMER active-site center.

CONCLUSIONS

The GMER fold is reminiscent of that observed for UDP-galactose epimerase (UGE) from Escherichia coli. Consideration of the enzyme fold and of its main structural features allows assignment of GMER to the reductase-epimerase-dehydrogenase (RED) enzyme homology superfamily, to which short-chain dehydrogenase/reductases (SDRs) also belong. The location of the NADP+ nicotinamide ring at an interdomain cleft is compatible with substrate binding in the C-terminal domain.

The metabolism of 6-deoxyhexoses in bacterial and animal cells

L-fucose and L-rhamnose are two 6-deoxyhexoses naturally occurring in several complex carbohydrates. In prokaryotes both of them are found in polysaccharides of the cell wall, while in animals only L-fucose has been described, which mainly participates to the structure of glycoconjugates, either in the cell membrane or secreted in biological fluids, such as ABH blood groups and Lewis system antigens. L-fucose and L-rhamnose are synthesized by two de novo biosynthetic pathways starting from GDP-D-mannose and dTDP-D-glucose, respectively, which share several common features. The first step for both pathways is a dehydration reaction catalyzed by specific nucleotide-sugar dehydratases. This leads to the formation of unstable 4-keto-6-deoxy intermediates, which undergo a subsequent epimerization reaction responsible for the change from D- to L-conformation, and then a NADPH-dependent reduction of the 4-keto group, with the consequent formation of either GDP-L-fucose or dTDP-L-rhamnose. These compounds are then the substrates of specific glycosyltransferases which are responsible for insertion of either L-fucose or L-rhamnose in the corresponding glycoconjugates. The enzyme involved in the first step of GDP-L-fucose biosynthesis in E. coli, i.e., GDP-D-mannose 4,6 dehydratase, has been recently expressed as recombinant protein and characterized in our laboratory. We have also cloned and fully characterized a human protein, formerly named FX, and an E. coli protein, WcaG, which display both the epimerase and the reductase activities, thus indicating that only two enzymes are required for GDP-L-fucose production. Fucosylated complex glycoconjugates at the cell surface can then be recognized by specific counter-receptors in interacting cells, these mechanisms initiating important processes including inflammation and metastasis. The second pathway starting from dTDP-D-glucose leads to the synthesis of antibiotic glycosides or, alternatively, to the production of dTDP-L-rhamnose. While several sets of data are available on the first enzyme of the pathway, i.e., dTDP-D-glucose dehydratase, the enzymes involved in the following steps still need to be identified and characterized.

Preliminary crystallographic investigations of recombinant GDP-4-keto-6-deoxy-D-mannose epimerase/reductase from E. coli

The GDP-4-keto-6-deoxy-D-mannose epimerase/reductase (GM_ER) isolated from E. coli has been overexpressed as a GST-fusion protein and purified to homogeneity. The enzyme, an NADP+(H)-binding homodimer of 70 kDa, is responsible for the production of GDP-L-fucose. GM_ER shows significant structural homology to the human erythrocyte protein FX, which is involved in blood-group glycoconjugate biosynthesis, displaying 3,5 epimerase/reductase activity on GDP-4-keto-6-deoxy-D-mannose. GM_ER has been crystallized in a trigonal crystalline form, containing one molecule per asymmetric unit, suitable for high-resolution crystallographic investigations.

Defective intracellular activity of GDP-D-mannose-4,6-dehydratase in leukocyte adhesion deficiency type II syndrome

Leukocyte adhesion deficiency type II (LAD II) is a rare genetic disease characterized by severe immunodeficiency which is related to defective expression in leukocytes of sialyl-Lewis X (SLeX), a fucosylated ligand for endothelial selectins. The molecular basis of LAD II is still unknown, but has been tentatively localized in the de novo pathway of GDP-L-fucose biosynthesis from GDP-D-mannose. Here, we demonstrate that in cell lysates from a LAD II patient, GDP-D-mannose-4,6-dehydratase (GMD), the first of the two enzymes of the pathway has a defective activity compared to control subjects. GMD in cell lysates from both parents showed intermediate activity levels. Cloning of GMD from patient and control lymphocytes ruled out any mutation affecting the amino acid GMD sequence and the purified recombinant proteins from both controls and the patient showed identical specific activities. Since the levels of immunoreactive GMD in cell lysates were comparable in the patient and in controls, the biochemical deficiency of intracellular GMD activity in LAD II seems to be due to mutation(s) affecting some still unidentified GMD-regulating protein.

Expression, purification and characterization of GDP-D-mannose 4,6-dehydratase from Escherichia coli

GDP-D-mannose dehydratase (GMD) catalyzes the first step of the pathway that converts GDP-D-mannose to GDP-L-fucose in bacteria, plants and mammals. Recently, the gene coding for GMD has been identified and sequenced in E. coli. Based on this sequence, we have expressed and purified GMD in E. coli as a glutathione transferase (GST) fusion protein. The fused GST-GMD protein and the thrombin-cleaved GMD were then characterized. The catalytically active form of both enzyme species seems to be a hexamer of 410 and 250 kDa, respectively. The GST-GMD fusion protein has a Km of 0.22 +/- 0.04 mM and a specific activity of 2.3 +/- 0.2 micromol/h/mg. Ca2+ and Mg2+ activate GMD, while GDP-L-beta-fucose, the end-product of the pathway, inhibits it specifically. The GST-GMD fusion protein contains one mole of tightly bound NADP+ per mole of hexamer. Apparently, this NADP+ is involved in the catalytic mechanism of GMD.

Effects of the murine L929 and L1210 cell lines on nitric oxide and TNF-alpha production by RAW 264.7 murine macrophages

Co-cultures of the murine macrophagic cell line RAW 264.7 with the L929 fibrosarcoma cell line, but not with the leukemia L1210 cell line, showed enhanced NO production over control RAW 264.7 cells. This potentiating effect, which was observed in detectably mycoplasma-free conditions and required low concentrations of recombinant murine IFN-gamma, was due to soluble factors released from L929 cells and not to physical contact between the two cell types. The soluble factors were able to induce TNF-alpha in the macrophages and to potentiate the TNF-alpha release induced by IFN-gamma. Increased generation of NO in RAW 264.7 cells co-cultured with L929 cells was prevented by a neutralizing anti-TNF-alpha antibody, suggesting that TNF-alpha is an autocrine factor for iNOS expression in these conditions. Also the L929 cell line showed a 4- to 5-fold enhanced NO production following co-culture with RAW 264.7 cells, thus indicating that exposure of tumor cells to macrophages can lead to an increased iNOS expression in tumor cells themselves.

Synthesis of GDP-L-fucose by the human FX protein

FX is a homodimeric NADP(H)-binding protein of 68 kDa, first identified in human erythrocytes, from which it was purified to homogeneity. Its function has been unrecognized despite partial structural and genetic characterization. Recently, on the basis of partial amino acid sequence, it proved to be the human homolog of the murine protein P35B, a tumor rejection antigen. In order to address the biochemical role of FX, its primary structure was completed by cDNA sequencing. This sequence revealed a significant homology with many proteins from different organisms. Specifically, FX showed a remarkable similarity with a putative Escherichia coli protein, named Yefb, whose gene maps in a region of E. coli chromosome coding for enzymes involved in synthesis and utilization of GDP-D-mannose. Accordingly, a possible role of FX in this metabolism was investigated. The data obtained indicate FX as the enzyme responsible for the last step of the major metabolic pathway resulting in GDP-L-fucose synthesis from GDP-D-mannose in procaryotic and eucaryotic cells. Specifically, purified FX apparently catalyzes a combined epimerase and NADPH-dependent reductase reaction, converting GDP-4-keto-6-D-deoxymannose to GDP-L-fucose. This is the substrate of several fucosyltranferases involved in the correct expression of many glyconjugates, including blood groups and developmental antigens.