Regulation of human lymphoblast plasma membrane 5'-nucleotidase by zinc

Complex regulation of ecto-5'-nucleotidase/CD73 and A2AR-mediated adenosine signaling at neurovascular unit: A link between acute and chronic neuroinflammation

The review summarizes available data regarding the complex regulation of CD73 at the neurovascular unit (NVU) during neuroinflammation. Based on available data we propose the biphasic pattern of CD73 regulation at NVU, with an early attenuation and a postponed up-regulation of CD73 activity. Transient attenuation of CD73 activity on leukocyte/vascular endothelium and leukocyte/astrocyte surface, required for the initiation of a neuroinflammatory response, may be effectuated either by catalytic inhibition of CD73 and/or by shedding of the CD73 molecule from the cell surface, while postponed induction of CD73 is effectuated by transcriptional up-regulation of Nt5e and posttranslational modifications. Neuroinflammatory conditions are also associated with significant enhancement and gain-of-function of A_2AR-mediated adenosine signaling. However, in contrast to the temporary prevalence of A_2AR over A₁R signaling during an acute inflammatory response, prolonged induction of A_2AR and resulting perpetual CD73/A_2AR coupling may be a contributing factors in the transition between acute and chronic neuroinflammation. Thus, pharmacological targeting of the CD73/A_2AR axis may attenuate inflammatory response and ameliorate neurological deficits in chronic neuroinflammatory conditions.

Zinc deficiency reduces paclitaxel efficacy in LNCaP prostate cancer cells

Chemotherapeutics used to treat prostate cancer are often from a class of drugs that target microtubule networks, such as paclitaxel. A previous report indicated that supplemental zinc sensitized prostate cancer cells to paclitaxel-induced apoptosis, suggesting that increased zinc levels might enhance paclitaxel efficacy. The effect of zinc deficiency on paclitaxel activity is not known though, so we tested this in two prostate cancer cell lines maintained under moderately zinc-deficient conditions. LNCaP and PC3 cell lines were used as models of early and late-stage prostate cancer, respectively. Cells cultured in reduced zinc levels did not demonstrate altered cell viability, growth rates, or intracellular zinc content. Additionally, zinc deficiency alone had no apparent effect on cell cycle kinetics or apoptosis levels. However, the IC(50) for paclitaxel-induced cell cycle arrest increased in LNCaP cells from zinc-deficient compared to zinc-replete conditions. Consequently, paclitaxel-induced apoptosis was reduced in LNCaP cells from zinc-deficient compared to zinc-replete conditions. In PC3 cells, the effects of paclitaxel were independent of zinc status. Reduced extracellular zinc levels were shown to affect paclitaxel activity in a prostate cancer cell line. Given the prevalence of zinc deficiency, determining how chemotherapeutic action is modulated by zinc adequacy may have clinical importance.

Zinc-related metallothionein metabolism in bovine pulmonary artery endothelial cells

Bovine pulmonary artery endothelial cells (BPAEC) were cultured in vitro under a variety of conditions to investigate how metallothionein (MT) might participate in zinc homeostasis. Experimental conditions included 10% serum to ensure that the in vitro environment would be a better reflection of the in vivo situation than with protein-free medium. MT was increased by acutely high zinc concentrations (100-200 micromol/L) in the extracellular environment. MT was relatively insensitive to moderate changes in zinc concentration (2-50 micromol/L), even after prolonged exposure for 7 to 12 days. BPAEC had reduced MT content when grown in medium containing serum that had been dialyzed to remove components with a molecular mass of less than 1,000, including zinc. Because the principal source of the major minerals in the experimental medium was not the serum, their concentrations in the final medium were not significantly influenced by serum dialysis. Restoring the zinc concentration in the medium containing the dialyzed serum did not restore MT content in BPAEC, suggesting that some small molecular weight molecule other than zinc established their basal MT content. This study did not identify these putative factors in serum, but hormones are likely candidates. Forty-eight-hour incubations of BPAEC with interleukin (IL-6) or dexamethasone increased cellular MT; however, 17beta-estradiol decreased MT, and IL-1 and adenosine 3',5'-cyclic phosphate (cAMP) had no discernible effect. We conclude that extracellular zinc concentrations have relatively little impact on the cellular concentrations of MT and zinc of BPAEC in vitro. Zinc homeostasis by BPAEC is not maintained by changing the MT concentration in response to changes in the extracellular zinc environment. (J. Nutr. Biochem. 10:00-00, 1999).

Differences in the cellular zinc content and 5'-nucleotidase activity of normal and acrodermatitis enteropathica (AE) fibroblasts

The acrodermatitis enteropathica (AE) mutation affects zinc (Zn) metabolism in human fibroblasts. We hypothesize that the mutation affects the cell Zn content, which subsequently affects the activity of various zinc-dependent enzymes, such as 5'-nucleotidase. Therefore, normal and AE fibroblasts were grown in normal medium containing physiological levels of Zn (16 micromol/L) for approximately 24 h. The medium was replaced by normal medium (16 micromol/L Zn), Zn-depleted medium (1.5 micromol/L Zn), or Zn-supplemented medium (200 micromol/L Zn) for another 24 h. Regardless of the Zn concentration of the growth medium, the AE fibroblasts contained significantly less Zn than normal fibroblasts grown in comparable medium. Nevertheless, growth of the fibroblasts in 200 micromol/L Zn medium significantly increased the cell Zn content fourfold of both normal and AE fibroblasts. The activity of 5'-nucleotidase in the AE fibroblasts grown in 16 micromol/L Zn or 1.5 micromol/L Zn medium was also significantly lower than in normal fibroblasts. Changing the growth medium from 16 micromol/L Zn to 1.5 micromol/L Zn medium did not affect the activity of the enzyme in either genotype. Cells grown in 200 micromol/L Zn medium exhibited threefold greater 5'-nucleotidase activity in AE fibroblasts, but had no affect on enzyme activity in normal cells. In summary, altering the cell Zn content of normal fibroblasts did not result in a significant change in their 5'-nucleotidase activity. However, AE fibroblasts grown in 200 micromol/L Zn medium exhibited recovery of their 5'-nucleotidase activity to normal levels. These results support the hypothesis that the AE mutation affects the cellular Zn content. The lower cell Zn content subsequently affects the activity of 5'-nucleotidase.

Decreased expression of CD73 (ecto-5'-nucleotidase) in the CD8+ subset is associated with zinc deficiency in human patients

We used flow cytometry to observe the changes in T cell populations resulting from zinc deficiency in subjects with sickle cell anemia (SCA) and in healthy human volunteers without SCA. Zinc deficiency was associated with significant decreases in cellular zinc concentration, CD4+/CD8+ ratio, and percentage of CD73+ cells in the CD8+ population. The decrease in the percentage of CD73+ cells in the CD8+ subset was significantly correlated with lymphocyte zinc concentration and was accompanied by essentially no change in the percentage of CD11b+ cells in the CD8+ subset. Daily oral zinc supplementation in nine zinc-deficient human volunteers (25 mg elemental zinc) and in seven zinc-deficient SCA subjects (50 mg elemental zinc) resulted in increases in the absolute lymphocyte count and significant increases in the CD4+/CD8+ ratio and in the percentage of CD73+ cells in the CD8+ subset. In zinc-supplemented subjects, the increase in the percentage of CD73+ cells was accompanied by a significant decrease in the percentage of CD11b+ cells in the CD8+ subset. Changes in the CD4+/CD8+ and CD73+/CD11b- cell ratios in the CD8+ subset after treatment may provide a useful diagnostic criterion for zinc deficiency in humans.

Murine ecto-5'-nucleotidase (CD73): cDNA cloning and tissue distribution

The murine cDNA, encoding the purine catabolic enzyme, ecto-5'-nucleotidase (NT), was cloned and the tissue-specific distribution of both the mRNA and enzyme activity was examined. Starting with kidney RNA and primers based on the known rat sequence, reverse transcriptase-polymerase chain reaction (RT-PCR) was utilized to obtain the complete sequence for the translated portion of the murine cDNA. Murine NT is 94% identical to human NT at the amino acid (aa) level and 86% identical at the nucleotide (nt) level. NT enzyme assays revealed greater than tenfold more NT activity in mature vs. immature murine T- and B-lymphocytes. A similar increase in NT activity was also found when the pre-B-cell line, 70Z/3, was induced to produce surface kappa light chains with lipopolysaccharide (LPS) and gamma-interferon (gamma-IFN). Thus, culture systems in which murine lymphocytes mature may be useful for examining the mechanisms of NT gene regulation, as well as the function of NT in the immune system. In tissues, enzyme activity varied over 30-fold, from the lowest levels in skeletal muscle, thymus and spleen to highest in placenta, kidney and forestomach. Levels of mRNA, as determined by RNase protection assay, showed increased NT expression in the early gestation site, as compared to non-pregnant uterus, and in day-19.5 placenta, as compared to day-13 chorioallantoic placenta. Messenger RNA levels were in general proportional to enzyme activity, except in the lung and glandular stomach where mRNA levels were higher than expected, based on enzyme activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Zinc transport into endothelial cells is a facilitated process

The kinetics of zinc transport were examined by measuring the uptake of 65Zn into cultured endothelial cells. This served as a suitable model for characterizing the transport of zinc across a biological membrane (i.e., the plasma membrane). The transport process was saturable under physiological conditions, which indicates a facilitating transport mechanism. Within the physiological range of zinc concentrations, the maximum zinc transport rate was 27 pmoles zinc/(min x mg protein) and it was half maximal at 4.1 microM zinc. Cadmium competitively inhibited zinc transport (Ki = 6.5 microM), while equimolar concentrations of copper and manganese were ineffectual. The rate of zinc transport was substantially reduced at lower temperatures and in the presence of sulfhydryl blockers (sodium iodoacetate and N-ethylmaleimide). Inhibitors of energy metabolism (2,4-dinitrophenol and sodium azide) failed to disrupt zinc transport. These results demonstrate that zinc transport into endothelial cells is a facilitated process (i.e., it is carrier mediated and energy-independent).

5'-nucleotidase from bull seminal plasma, chicken gizzard and snake venom is a zinc metalloprotein

Using flame atomic absorption spectrometry the tight association of zinc to three different purified 5'-nucleotidases at a molar ratio of 2 could be proven. These 5'-nucleotidases purified from bull seminal plasma (BSP), chicken gizzard (CG) and snake venom (SV) are thus zinc metalloproteins. Removal of zinc results in the loss of their AMPase activity, which could be fully restored after readdition of zinc at a molar ratio of 2, for BSP and CG, and 1.5, for SV 5'-nucleotidase. Reactivation of their AMPase activity after the removal of zinc could also be obtained by addition of cobalt and copper ions, which were found to also bind with a molar ratio of 2 to the three 5'-nucleotidases tested.

T-lymphocyte dysfunction in the elderly associated with zinc deficiency and subnormal nucleoside phosphorylase activity: effect of zinc supplementation

The present study was conducted in an attempt to explore a possible mechanism for zinc-deficiency-induced T-lymphocyte dysfunction in the elderly. Eight elderly subjects aged 65-78 years served in this study. All subjects were anergic, zinc-deficient and had low activity of erythrocyte nucleoside phosphorylase. Baseline values for zinc status and immunological indices were established after which subject were treated with zinc acetate (60 mg elemental zinc per day given orally) for a period of 4 1/2 months. Zinc supplementation resulted in a significant increase in the concentrations of zinc in plasma (P less than 0.001), erythrocytes (P less than 0.05), lymphocytes (P less than 0.001) and neutrophils (P less than 0.005). The activity of nucleoside phosphorylase in erythrocytes increased significantly (P less than 0.001) as a result of zinc treatment. This was associated with significant improvement in the delayed cutaneous hypersensitivity reactions.

ESR study of 5'-nucleotidase from bull seminal plasma

5'-Nucleotidase of bull seminal plasma has been spin labeled with the sulfhydryl reagent 3-maleimidoproxyl. ESR analysis reveals the presence of two classes of labeled sites. The first is characterized by a long spin label rotational correlation time, from which a protein diameter of about 70 A can be estimated, under the assumption of a spherical shape. The second class is characterized by a shorter correlation time of the covalently bound spin labels and binding of the substrate sodium thymidine 5'-monophosphate to 5'-nucleotidase results in a reduction of their mobility. Low-temperature ESR analysis shows that no paramagnetic ion is bound to the native protein.

Physiology and metabolism of essential trace elements: an outline

Man depends on at least nine trace elements--iron, zinc, copper, manganese, iodine, chromium, selenium, molybdenum and cobalt--for optimum metabolic function. These elements serve a variety of functions including catalytic, structural and regulatory activities in which they interact with macromolecules such as enzymes, pro-hormones, pre-secretory granules and biological membranes. These micronutrients are involved, therefore, in all major metabolic pathways at levels which are so fundamental that the features of deficiency of many of them are protean and non-specific. In considering the metabolism of the elements themselves, they fall into two groups: those which exist normally as cations and those present as anions. The latter group are absorbed relatively easily and whole-body homeostasis is mediated mainly by renal excretion. The cations need specific pathways for absorption and their homeostasis is effected by gastrointestinal and biliary secretion. Some elements are absorbed more efficiently as organic complexes. The net achievement of the metabolic pathways for each element is to deliver it to its functional site(s) by exploiting its physicochemical characteristics to avoid interactions with other inorganic nutrients.

Dinucleosidetetraphosphatase inhibition by Zn(II)

Almost complete inhibition of partially purified dinucleoside-tetraphosphatase (EC 3. 6. 1. 17) was observed with 5 microM Zn(II). The inhibition was reversed by EDTA and was time dependent, reaching a maximum after 5 min of incubation at 37 degrees C. Zn(II) behaved as a non-competitive inhibitor of the reaction, leaving unaltered the Km value for the enzyme towards diadenosine tetraphosphate. The cellular level of this compound may be directly related to the Zn(II) content since, besides the inhibition here described, Zn(II) has been reported by others to be an activator of the synthesis of diadenosine tetraphosphate by sheep liver lysyl- and phenylalanyl-t RNA synthetases.