Ceruloplasmin enhances smooth muscle cell- and endothelial cell-mediated low density lipoprotein oxidation by a superoxide-dependent mechanism

Ran-2, a glial lineage marker, is a GPI-anchored form of ceruloplasmin

Cell interactions in the nervous system are frequently mediated by surface proteins that are attached to the membrane by a glycosyl phosphatidylinositol (GPI) anchor. In this study, we have characterized the expression of such proteins on glial cells. We have detected a major GPI-anchored protein on astrocytes and Schwann cells, with a molecular weight of 140 kD. When Schwann cells were treated with forskolin to promote a myelinating phenotype, expression of this 140-kD protein dramatically decreased, whereas another GPI-anchored protein of 80 kD was strongly induced; expression of other integral membrane proteins were likewise dramatically altered. The size and pattern of expression of the 140-kD protein suggested that it might correspond to the Ran-2 antigen, a glial lineage marker. This notion was confirmed by immunoprecipitating this 140-kD protein with the Ran-2 monoclonal antibody. The Ran-2 antigen is expressed over the entire Schwann cell surface in a punctate fashion; it is removed by phosphatidylinositol phospholipase C treatment, thereby confirming that it is GPI-anchored. When Schwann cells are cocultured with neurons, the Ran-2 antigen initially concentrates at sites of Schwann cell contact with neurons, suggesting that it may play a role in early Schwann cell-neuron interactions; it is then downregulated. Protein sequencing of the Ran-2 antigen immunopurified from rat brain membranes showed complete identity over two extended segments with the copper binding protein ceruloplasmin. These findings indicate that astrocytes and Schwann cells express a novel GPI-anchored form of ceruloplasmin and suggest that this GPI form plays a role in axonal-glial interactions.

Ceruloplasmin copper induces oxidant damage by a redox process utilizing cell-derived superoxide as reductant

Oxidative damage by transition metals bound to proteins may be an important pathogenic mechanism. Ceruloplasmin (Cp) is a Cu-containing plasma protein thought to be involved in oxidative modification of lipoproteins. We have previously shown that Cp increased cell-mediated low-density lipoprotein (LDL) oxidation by a process requiring cell-derived superoxide, but the underlying chemical mechanism(s) is (are) unknown. We now show that superoxide reduction of Cp Cu is a critical reaction in cellular LDL oxidation. By bathocuproine disulfonate (BCS) binding and by superoxide utilization, we showed that exogenous superoxide reduces a single Cp Cu atom, the same Cu required for LDL oxidation. The Cu atom remained bound to Cp during the redox cycle. Three avenues of evidence showed that vascular cells reduce Cp Cu by a superoxide-dependent process. The 2-fold higher rate of Cp Cu reduction by smooth muscle cells (SMC) compared to endothelial cells (EC) was consistent with their relative rates of superoxide release. Furthermore, Cp Cu reduction by cells was blocked by Cu,Zn superoxide dismutase (SOD1). Finally, the level of superoxide produced by EC and SMC was sufficient to cause the amount of Cu reduction observed. An important role of Cp Cu reduction in LDL oxidation was suggested by results showing that SOD1 inhibited Cp Cu reduction and LDL oxidation by SMC with equal potency, while tumor necrosis factor-alpha stimulated both processes. In summary, these results show that superoxide is a critical cellular reductant of divalent transition metals involved in oxidation, and that protein-bound Cu is a substrate for this reaction. The role of these mechanisms in oxidative processes in vivo has yet to be defined.

Copper-dependent inflammation and nuclear factor-kappaB activation by particulate air pollution

Particulate air pollution causes increased cardiopulmonary morbidity and mortality, but the chemical determinants responsible for its biologic effects are not understood. We studied the effect of total suspended particulates collected in Provo, Utah, an area where an increase in respiratory symptoms in relation to levels of particulate pollution has been well documented. Provo particulates caused cytokine-induced neutrophil chemoattractant-dependent inflammation of rat lungs. Provo particulates stimulated interleukin-6 (IL-6) and IL-8 production, increased IL-8 messenger RNA (mRNA) and enhanced expression of intercellular adhesion molecule-1 (ICAM-1) in cultured BEAS-2B cells, and stimulated IL-8 secretion in primary cultures of human bronchial epithelium. Cytokine secretion was preceded by activation of the transcription factor nuclear factor-kappaB (NF-kappaB) and was reduced by treatment of cultures with superoxide dismutase, deferoxamine, or N-acetylcysteine. These biologic effects were replicated by culturing BEAS cells with quantities of Cu2+ found in Provo extract. IL-8 secretion by BEAS cells could be modified by addition of normal constituents of airway lining fluid to the culture medium. Mucin significantly reduced IL-8 secretion, and ceruloplasmin significantly increased IL-8 secretion and activation of NF-kappaB. These findings suggest that copper ions may cause some of the biologic effects of inhaled particulate air pollution in the Provo region of the United States, and may provide an explanation for the sensitivity of asthmatic individuals to Provo particulates that has been observed in epidemiologic studies.

Cellular thiol production and oxidation of low-density lipoprotein

Compelling evidence suggests that low-density lipoprotein (LDL) is oxidized by cells within the arterial intima and that, once oxidized, it is profoundly atherogenic. The precise mechanism(s) by which cells promote the oxidation of LDL in vivo are not known; in vitro, however, oxidation of LDL can be enhanced by a number of differing mechanisms, including reaction with free and protein-bound metal ions, thiols, reactive oxygen species, lipoxygenase, myeloperoxidase and peroxynitrite. This review is concerned with the mechanisms by which cells enhance the oxidation of LDL in the presence of transition metals; in particular, the regulation, pro- and anti-oxidant consequences, and mechanism of action of cellular thiol production are examined, and contrasted with thiol-independent oxidation of LDL in the presence of transition metals.

Stretch force on vascular smooth muscle cells enhances oxidation of LDL via superoxide production

Hemodynamic forces on vasculature profoundly influence atherogenesis. We examined the effect of stretch force on the oxidation of low-density lipoprotein (LDL) by rat aortic smooth muscle cells (RASM) and superoxide production. Stretch force was imposed on RASM cultured on deformable dishes by stretching the dishes. Incubation of native LDL with static RASM for 24 h resulted in LDL oxidation as indicated by increases in thiobarbituric acid-reacting substances from 9.5 +/- 2.3 to 24.5 +/- 2.3 nmol malondialdehyde/mg. Stretch force on RASM augmented cell-mediated LDL oxidation to 149.3 +/- 17.1% concomitantly with increase in superoxide production. LDL oxidation was inhibited by superoxide dismutase or depletion of the metal ion in the culture medium, indicating that it was a metal ion-dependent and superoxide-mediated process. The enhancement of LDL oxidation by stretch force was inhibited by diphenyliodonium, indicating the involvement of the NADH/NADPH oxidase system. Our findings suggest that the increased oxidant stress induced by stretch force is one of the potential mechanisms whereby hypertension facilitates atherosclerosis.

Copper stimulates proliferation of human endothelial cells under culture

Copper ions stimulate proliferation of human umbilical artery and vein endothelial cells but not human dermal fibroblasts or arterial smooth muscle cells. Incubation of human umbilical vein endothelial cells for 48 h with 500 microM CuSO4 in a serum-free medium in the absence of exogenous growth factors results in a twofold increase in cell number, similar to the cell number increase induced by 20 ng/ml of basic fibroblast growth factor under the same conditions. Copper-induced proliferation of endothelial cells is not inhibited by 10% fetal bovine serum or by the presence of antibodies against a variety of angiogenic, growth, and chemotactic factors including angiogenin, fibroblast growth factors, epidermal growth factor, platelet-derived growth factor, tumor necrosis factor-alpha, transforming growth factor-beta, macrophage/monocyte chemotactic and activating factor, and macrophage inflammatory protein-1alpha. Moreover, despite the previous observations that copper increased total specific binding of 125I-angiogenin to endothelial cells, binding to the 170 kDa receptor is not changed; hence, the mitogenic activity of angiogenin is not altered by copper. Copper-induced proliferation, along with early reports that copper induces migration of endothelial cells, may suggest a possible mechanism for the involvement of copper in the process of angiogenesis.

Identification of the prooxidant site of human ceruloplasmin: a model for oxidative damage by copper bound to protein surfaces

Free transition metal ions oxidize lipids and lipoproteins in vitro; however, recent evidence suggests that free metal ion-independent mechanisms are more likely in vivo. We have shown previously that human ceruloplasmin (Cp), a serum protein containing seven Cu atoms, induces low density lipoprotein oxidation in vitro and that the activity depends on the presence of a single, chelatable Cu atom. We here use biochemical and molecular approaches to determine the site responsible for Cp prooxidant activity. Experiments with the His-specific reagent diethylpyrocarbonate (DEPC) showed that one or more His residues was specifically required. Quantitative [14C]DEPC binding studies indicated the importance of a single His residue because only one was exposed upon removal of the prooxidant Cu. Plasmin digestion of [14C]DEPC-treated Cp (and N-terminal sequence analysis of the fragments) showed that the critical His was in a 17-kDa region containing four His residues in the second major sequence homology domain of Cp. A full length human Cp cDNA was modified by site-directed mutagenesis to give His-to-Ala substitutions at each of the four positions and was transfected into COS-7 cells, and low density lipoprotein oxidation was measured. The prooxidant site was localized to a region containing His426 because CpH426A almost completely lacked prooxidant activity whereas the other mutants expressed normal activity. These observations support the hypothesis that Cu bound at specific sites on protein surfaces can cause oxidative damage to macromolecules in their environment. Cp may serve as a model protein for understanding mechanisms of oxidant damage by copper-containing (or -binding) proteins such as Cu, Zn superoxide dismutase, and amyloid precursor protein.

High ceruloplasmin levels in rats without high lipoprotein oxidation rates

The serum copper protein ceruloplasmin, though sometimes considered an antioxidant, can promote lipoprotein oxidation in vitro under certain conditions. However, an inflammation-induced, two week elevation of rat serum ceruloplasmin levels did not render low plus very low density lipoproteins more prone to oxidation in vitro. Thus, high ceruloplasmin levels in vivo are not necessarily associated with high lipoprotein oxidation rates.