Secretion and uptake of copper via a small copper carrier in blood fluid.

Studies with Wilson disease model mice that accumulate excessive copper, due to a dysfunctional ATP7B "copper pump" resulting in decreased biliary excretion, showed that the compensatory increase in urinary copper loss was due to a small copper carrier (∼1 kDa) (SCC). We show here that SCC is also present in the blood plasma of normal and Wilson disease model mice and dogs, as determined by ultrafiltration and size exclusion chromatography (SEC). It is secreted by cultured hepatic and enterocytic cells, as determined by pretreatment with 67Cu nitrilotriacetate (NTA) or nonradioactive 5-10 µM Cu-NTA, and collecting and examining 3 kDa ultrafiltrates of the conditioned media, where a single major copper peak is detected by SEC. Four different cultured cell types exposed to the radiolabeled SCC all took up the 67Cu at various rates. Rates differed somewhat when uptake was from Cu-NTA. Uptake of SCC-67Cu was inhibited by excess nonradioactive Cu(I) or Ag(I) ions, suggesting competition for uptake by copper transporter 1 (CTR1). Knockout of CTR1 in fibroblasts reduced uptake rates by 60%, confirming its participation, but also involvement of other transporters. Inhibitors of endocytosis, or an excess of metal ions taken up by divalent metal transporter 1, did not decrease SCC-67Cu uptake. The results imply that SCC may play a significant role in copper transport and homeostasis, transferring copper particularly from the liver (but also intestinal cells) to other cells within the mammalian organism, as well as spilling excess into the urine in copper overload-as an alternative means of copper excretion.

Mobilization of iron from ferritin: new steps and details.

Much evidence indicates that iron stored in ferritin is mobilized through protein degradation in lysosomes, but concerns about this process have lingered, and the mechanistic details of its aspects are lacking. In the studies presented here, 59Fe-labeled ferritin was induced by preloading hepatic (HepG2) cells with radiolabeled Fe. Placing these cells in a medium containing desferrioxamine resulted in the loss of ferritin-59Fe, but adding high concentrations of reducing agents or modulating the internal GSH concentration failed to alter the rates of ferritin-59Fe release. Confocal microscopy showed that Fe deprivation increased the movement of ferritin into lysosomes and hyperaccumulation was observed when lysosomal proteolysis was inhibited. It also resulted in the rapid movement of DMT1 to lysosomes, which was inhibited by bafilomycin. Ferrihydrite crystals isolated from purified rat liver/spleen ferritin were solubilized at pH 5 and 7 by GSH, ascorbate, citrate and lysosomal fluids obtained from livers and J774a.1 macrophages. The inhibition of DMT1/Nramp2 and siRNA knockdown of Nramp1 each reduced the transfer of 59Fe from lysosomes to the cytosol; and hepatocyte-specific knockout of DMT1 in mice prevented the release of Fe from the liver responding to EPO treatment, but did not inhibit lysosomal ferritin degradation. We conclude that ferritin-Fe mobilization does not occur through changes in cellular concentrations of reducing/chelating agents but by the coordinated movement of ferritin and DMT1 to lysosomes, where the ferrihydrite crystals exposed by ferritin degradation dissolve in the lysosomal fluid, and the reduced iron is transported back to the cytosol via DMT1 in hepatocytes, and by both DMT1 and Nramp1 in macrophages, prior to release into the blood or storage in ferritin.

Ceruloplasmin and other copper binding components of blood plasma and their functions: an update.

We know that blood plasma contains many proteins and also other components that bind copper. The largest contributor to copper in the plasma is ceruloplasmin, which accounts for 40-70 percent. Apart from ceruloplasmin and albumin, most of these components have not been studied extensively, and even for ceruloplasmin and albumin, much remains to be discovered. New components with new functions, and new functions of known components are emerging, some warranting reconsideration of earlier findings. The author's laboratory has been actively involved in research on this topic. This review summarizes and updates our knowledge of the nature and functions of ceruloplasmin and the other known and emerging copper-containing molecules (principally proteins) in this fluid, to better understand how they contribute to copper homeostasis and consider their potential significance to health and disease.

Iron prevents ferritin turnover in hepatic cells.

It has long been assumed that iron regulates the turnover of ferritin, but evidence for or against this idea has been lacking. This issue was addressed using rat hepatoma cells with characteristics of hepatocytes subjected to a continuous influx of iron. Iron-pretreated cells were pulsed with [(35)S]Met for 60 min or with (59)Fe overnight and harvested up to 30 h thereafter, during which they were/were not cultured with ferric ammonium citrate (FAC; 180 microm). Radioactivity in ferritin/ferritin subunits of cell heat supernatants was determined by autoradiography of rockets obtained by immunoelectrophoresis or after precipitation with ferritin antibody and SDS-PAGE. Both methods gave similar results. During the +FAC chase, the concentration of ferritin in the cells increased linearly with time. Without FAC, the half-life of (35)S-ferritin was 19-20 h; with FAC there was no turnover. Without FAC, the iron in ferritin had an apparent half-life of 20 h; in the presence of FAC there was no loss of (59)Fe. Without FAC, concentrations of ferritin iron and protein also decreased in parallel. We conclude that a continuous influx of excess iron can completely inhibit the degradation of ferritin protein and that the iron and protein portions of ferritin molecules may be coordinately degraded.

Copper and genomic stability in mammals.

As the free ion and in the form of some complexes, there is no doubt that copper can promote damage to cellular molecules and structures through radical formation. At the same time, and perhaps as a consequence, mammals have evolved means of minimizing levels of free copper ions and destructive copper complexes that enter the organism and its cells. These means include tight binding of copper ions to protein carriers and transporters; direct exchange of copper between protein carriers, transporters, and cuproenzymes; and mobilization of secretory mechanisms and excretory pathways, as needed. As a consequence, normally, and except under certain genetic conditions, copper is likely to be benign to most mammals and not responsible for genomic instability, including fragmentation of and/or alterations to DNA, induction of mutations or apoptosis, or other toxic events. Indeed, cuproenzymes are important members of the antioxidant system of the organism.

Transport of silver in virgin and lactating rats and relation to copper.

Virgin and lactating Sprague Dawley rats were used to determine whether the pathways of silver transport to tissues and milk resemble those for copper. Rats were injected i.p.with small amounts of 110AgNO3. Blood and tissues were examined at various times thereafter for total radioactivity and for incorporation into copper binding proteins in plasma and milk. As with 67Cu, much of the 110Ag was rapidly incorporated into the liver. Skeletal muscle, spleen, mammary gland, ovaries, uterus and adrenals also were significant initial accumulation sites, with or without lactation. Lactation enhanced uptake by the mammary gland, and radioactivity rapidly entered the milk and milk ceruloplasmin. In the plasma, most of the 110Ag bound to a single component of apparent molecular weight 800 k throughout the 52 h period examined. A small proportion was also incorporated into plasma ceruloplasmin, as determined by immunoprecipitation and native gel electrophoresis. There was little or no association of 110Ag with albumin or transcuprein. The binding of 110Ag to the 800 kDa protein was tight. Off rates during pH 7 dialysis were <2.5%/day even in the presence of 100 microM histidine or Cu(II), but were accelarated by mercaptoethanol. Subunits of 145 and 45 kDa in virtually pure peak fractions were those of alpha1-macroglobulin. We conclude that silver resembles copper in aspects of its tissue distribution, response to lactation, and incorporation into ceruloplasmin. However its main plasma carrier appears to be alpha1-macroglobulin, a different macroglobulin than that involved in copper transport.

Milk ceruloplasmin and its expression by mammary gland and liver in pigs.

Concentrations of ceruloplasmin and copper in milk and blood plasma, the nature of milk ceruloplasmin, and the effects of lactation and gestation on these parameters, as well as the expression of ceruloplasmin mRNA by the mammary gland, were examined in pigs. As seen previously in humans, ceruloplasmin and copper concentrations in sow milk were much higher a few days after birth than 1 month later, averaging 26.5 and 6.6 mg ceruloplasmin/L (by immunoassay) and 1.67 and 0.34 mg total Cu/L, on days 3 and 33 postpartum, respectively. Values for ceruloplasmin oxidase activity (measured with p-phenylene diamine) were 7.8 and 1.3 nmol/min/L, respectively. Daily milk ceruloplasmin production went from 61 to 22 mg/day and daily copper output from 38 to 12 mg/day. In contrast, there was little or no variation in serum ceruloplasmin concentration during lactation or gestation, although total plasma copper was high at the end of gestation. Milk ceruloplasmin was of the same apparent size as serum ceruloplasmin, as determined by SDS-PAGE and immunoblotting, and ceruloplasmin mRNAs of liver and mammary gland were indistinguishable by Northern analysis and RT-PCR of the various exons. Expression of total RNA and ceruloplasmin mRNA, as detected in biopsies of mammary gland, increased markedly upon onset of lactation and then declined during the next month in conjunction with a drop in milk ceruloplasmin production. The results indicate that milk ceruloplasmin, while being the same protein as in plasma, is not derived from the plasma but is produced by the mammary gland.

Measurement of acute phase proteins in the rat brain: contribution of vascular contents.

A localized acute phase response occurs in the brain in Alzheimer's disease. Acute phase proteins have previously been measured in brain homogenates to quantify this response. The extent to which measurements of these proteins reflect brain parenchymal contents, as opposed to vascular contents, is unknown. In this study, the acute phase proteins ceruloplasmin (CP), complement factor 3 (C3), haptoglobin (HP), and albumin were measured in regional brain homogenates from phosphate buffered saline-perfused and sham-perfused rats (n = 7-9/group). Interleukin 1-beta (IL1-beta) and copper were also measured. Mean CP, C3, HP, and albumin concentrations in perfused specimens decreased by 94%, 88%, 90%, and 81% vs. sham-perfused specimens (all p < 0.001), while IL1-beta and copper were unchanged. These results suggest that acute phase protein measurements in brain homogenates reflect primarily vascular contents. However, IL1-beta and copper concentrations in brain homogenates are minimally influenced by vascular contents.

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.

Copper transport.

In adult humans, the net absorption of dietary copper is approximately 1 mg/d. Dietary copper joins some 4-5 mg of endogenous copper flowing into the gastrointestinal tract through various digestive juices. Most of this copper returns to the circulation and to the tissues (including liver) that formed them. Much lower amounts of copper flow into and out of other major parts of the body (including heart, skeletal muscle, and brain). Newly absorbed copper is transported to body tissues in two phases, borne primarily by plasma protein carriers (albumin, transcuprein, and ceruloplasmin). In the first phase, copper goes from the intestine to the liver and kidney; in the second phase, copper usually goes from the liver (and perhaps also the kidney) to other organs. Ceruloplasmin plays a role in this second phase. Alternatively, liver copper can also exit via the bile, and in a form that is less easily reabsorbed. Copper is also present in and transported by other body fluids, including those bathing the brain and central nervous system and surrounding the fetus in the amniotic sac. Ceruloplasmin is present in these fluids and may also be involved in copper transport there. The concentrations of copper and ceruloplasmin in milk vary with lactational stage. Parallel changes occur in ceruloplasmin messenger RNA expression in the mammary gland (as determined in pigs). Copper in milk ceruloplasmin appears to be particularly available for absorption, at least in rats.

Secretion of ferritin by rat hepatoma cells and its regulation by inflammatory cytokines and iron.

The possibility that serum ferritin is a secreted protein and an acute phase reactant regulated by inflammatory hormones and iron was examined in a hepatic cell line that secretes plasma proteins. Differentiated rat hepatoma cells released albumin and ferritin into the medium, as determined by rocket immunoelectrophoresis and isolation of ferritin by standard procedures plus immunoaffinity chromatography, following labeling with radioactive amino acid. Administration of interleukin-1-beta (IL-1) or tumor necrosis factor-alpha (TNF) doubled the amounts of ferritin released into the medium over 24 and 48 hours. Together, the cytokines had more than an additive effect. Albumin secretion was diminished by IL-1, but not TNF. Iron, administered as an iron dextran complex or as a 1:1 chelate with nitrilotriacetate (Fe-NTA), also enhanced ferritin release, but had no effect on albumin. Intracellular ferritin concentrations did not change significantly with cytokine treatment, but increased in response to iron. With or without treatments, release of ferritin and albumin from cells into the medium was inhibited by brefeldin A, an inhibitor of Golgi function. The effect of each of the cytokines and of iron on ferritin and albumin was also blocked by dichlorofuranosylbenzimidazole (DRB), an inhibitor of transcription. The stimulatory effect of Fe-NTA on ferritin secretion was diminished by TNF, and this was partially counteracted by IL-1, indicating additional regulatory complexity. These results show for the first time that hepatic cells secrete ferritin, that this ferritin secretion is regulated by iron and inflammatory cytokines, and that the mechanisms of regulation differ from those for intracellular ferritin. The results would explain why serum ferritin increases in inflammation or when iron flux is enhanced.

Serum ferritin: does it differ from tissue ferritin?

Serum ferritin isolated from the horse was structurally compared with horse spleen ferritin and was found to differ markedly in molecular weight, iron content, carbohydrate, subunit size and amino acid sequence. The results are summarized and initial results obtained with candidate clones of pieces of two serum ferritin subunits are described.

Copper biochemistry and molecular biology.

In this review, our basic and most recent understanding of copper biochemistry and molecular biology for mammals (including humans) is described. Information is provided on the nutritional biochemistry of copper, including food sources, intestinal absorption, transport, tissue distribution, and excretion, along with descriptions of copper binding proteins and other factors involved and their roles in these processes. The metabolism of copper and its importance for the functions of a roster of vital enzymes is detailed. Its potential toxicology is also addressed. Alterations in copper metabolism associated with genetic and nongenetic diseases are summarized, including potential connections to inflammation, cancer, atherosclerosis, and anemia, and the effects of genetic copper deficiency (Menkes syndrome) and copper overload (Wilson disease). Understanding these diseases suggests new ways of viewing the normal functions of copper and provides new insights into the details of copper transport and distribution in mammals.

Copper transport in the Nagase analbuminemic rat.

The copper binding components of serum and potential importance of albumin to copper transport were investigated in adult Nagase analbuminemic and Sprague-Dawley rats of both sexes. There was a sex difference in total plasma copper concentrations, which were 60 and 130% higher than in the parent strain, in male and female Nagase rats, respectively. The higher levels of plasma copper were accounted for by two- and threefold increases in ceruloplasmin, as measured by p-phenylenediamine oxidase activity and copper by atomic absorption after gel chromatography. Other nonalbumin plasma proteins were also present in higher concentrations. Albumin concentrations were one-four-thousandth that of Sprague-Dawley rats, at 15 micrograms/ml (determined by rocket immunoelectrophoresis and comparative Western blotting). The tissue distribution and rate of uptake of intravenously injected 67 Cu(II) were unaffected by the lack of circulating albumin. 67Cu entered the liver of Nagase rats at least as rapidly as in the parent strain and reemerged in the blood on ceruloplasmin at an accelerated rate. The initial binding of 67Cu(II) to plasma components was primarily to transcuprein compared with albumin in the case of Sprague-Dawley rats. In the Nagase rats, the rest of the 67Cu bound primarily to nonalbumin proteins with about the same size as albumin; in Sprague-Dawley rats, it bound primarily to transcuprein. Limited analyses of tissue copper confirmed previous reports showing no striking differences from the parent strain. We conclude that albumin is not critical to the normal distribution and metabolism of copper and that it may serve more as a reservoir for excess plasma copper than as a specific conduit for the delivery of this element to hepatocytes or other cells.

Ceruloplasmin and copper transport during the latter part of gestation in the rat.

We have examined the tissue uptake of 67Cu from ceruloplasmin versus albumin and transcuprein, after its intravenous administration to pregnant rats, in the last 4 days of gestation. 67Cu infused as in vivo-labeled ceruloplasmin remained on ceruloplasmin in the maternal circulation over the 4- to 6-hr time period examined, as determined by gel chromatography and immunoreactivity. That infused as in vitro-labeled serum was initially on transcuprein and albumin but soon also with new ceruloplasmin. On the basis of percent dose as well as total actual Cu transferred (taking into account the sizes of the two plasma Cu pools), ceruloplasmin was the preferred source of Cu for most tissues. Total uptake of Cu from ceruloplasmin was seven times greater than that from albumin and transcuprein for the placenta, whole fetus, and fetal liver. It was 2- to 6-fold greater for other tissues (except liver and kidney). When synthesis of maternal 67Cu-ceruloplasmin (from 67Cu administered on albumin and transcuprein) was inhibited with cycloheximide, uptake by nonhepatic tissues was reduced markedly. In the fetal circulation, entering 67Cu was initially associated with transcuprein and alpha-fetoprotein (or albumin), but then also appeared with ceruloplasmin. Specific receptors for ceruloplasmin were detected on membranes from the placenta as well as fetal liver; mRNA for ceruloplasmin was detected on the endoplasmic reticulum-bound polyribosomes of placenta/yolk sac, and of fetal and maternal liver. We conclude that Cu destined for the fetus is delivered mainly or exclusively by ceruloplasmin. It may enter via placental receptors, arriving in fetal plasma in ionic form, for later incorporation into fetal ceruloplasmin. The importance of ceruloplasmin as a source of plasma Cu for nonhepatic organs is also confirmed.

Synthesis and turnover of ceruloplasmin in rats treated with 17 beta-estradiol.

The effects of estrogen on synthesis and turnover of ceruloplasmin were studied in adult female Fischer rats. Daily treatment with 140 microgram 17 beta-estradiol resulted in a slow rise of ceruloplasmin concentrations, as measured by p-phenylenediamine oxidase activity, leading to a 70% increase by 7 days and a tripling by Day 14. Ceruloplasmin protein concentrations increased to the same degree, based on yields of the protein obtained during purification. Effects of estrogen on rates of synthesis (incorporation of [3H]leucine) were followed, using immunoprecipitation of total ceruloplasmin or isolation of its two major isoforms (Rfs 0.4 and 0.6 in native gel electrophoresis). Synthesis was increased by 7 days and was 2.5 times that of controls by Day 14. Both forms of ceruloplasmin showed the same specific activities and degree of increase in rate of synthesis. Rates of ceruloplasmin turnover were unaffected, based on double labeling with 3H- and 14C-leucine, but were three- to fourfold faster than for total plasma protein. The enzymatically more active 0.6 Rf form of ceruloplasmin had a faster turnover rate than the 0.4 Rf form. Estrogen treatment doubled ceruloplasmin mRNA levels by 7 days and almost tripled them by Day 14. Most of the ceruloplasmin mRNA was associated with the endoplasmic reticulum-bound polyribosomes. We conclude that estrogen increases the rate of synthesis of two forms of ceruloplasmin by indirectly increasing liver concentrations of its mRNA but has no effect on ceruloplasmin turnover.

Differential effects of iron and inflammation on ferritin synthesis on free and membrane-bound polyribosomes of rat liver.

We have examined the distribution of ferritin mRNA to free and endoplasmic reticulum (ER)-bound liver polyribosomes during inflammation and iron treatment of rats. Postnuclear tissue supernatants were fractionated on a discontinuous sucrose gradient developed to separate free and bound polyribosomes. Total RNA recovered averaged 3.2 mg/g tissue, 40% of which was with ER and 30% with the free polyribosomes, about 25% being with the postribosomal/RNP fraction. Slot-blot hybridization of equal portions of RNA revealed that 12 h after injection of turpentine to induce inflammation, ferritin mRNA was concentrated on the ER-bound polyribosomes, while it was concentrated on the free polyribosomes 2 h after injection of ferric ammonium citrate. Differences were highly significant, based on multiple determinations and densitometry. Profiles of ferritin mRNA distribution on linear sucrose gradients corroborated the differential findings. Concentrations of total ferritin mRNA per gram liver doubled with iron treatment but were not significantly different 12 h after turpentine treatment. At the same time point after turpentine, ferritin protein synthesis was increased twofold, as measured by the 1 h incorporation of [14C]leucine. We conclude that a significant portion of ferritin mRNA always associates with the ER-bound polyribosomes, and that inflammation and iron differentially alter the polysomal distribution of ferritin mRNA, suggesting that two different kinds of mRNA may be involved.

Ferritin synthesis on polyribosomes attached to the endoplasmic reticulum.

The evidence that ferritin is synthesized both on free polyribosomes and on polyribosomes attached to the endoplasmic reticulum is reviewed. Evidence that some ferritin is secreted from cells after synthesis on bound polyribosomes was found to be inconclusive.

Heart tissue contains small and large aggregates of ferritin subunits.

Ferritin purified from horse heart and applied to nondenaturing polyacrylamide gel electrophoresis migrated as a single band that stained for both iron and protein. This ferritin contained almost equal amounts of fast- and slow-sedimenting components of 58 S and 3-7 S, which could be separated on sucrose density gradients. Iron removal reduced the sedimentation coefficient of the fast-sedimenting ferritin to 18 S, and sedimentation equilibrium gave a molecular weight 650,000, with some preparations containing ferritin of 500,000 molecular weight as well. Sedimentation rates of the 3 S and 7 S ferritins were not affected by iron removal, and sedimentation equilibrium data were consistent with Mr's 40,000 and 180,000, respectively. Preparations of ferritin extracted from horse spleen contained only 67 S (holo) or 16 S (apo) ferritin and no slow-sedimenting species. When examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, all of the ferritins contained the usual H and L subunits (23 and 20 kDa, respectively), but the slow-sedimenting (3 S and 7 S) heart apoferritins also contained appreciable quantities (ca 25%) of three larger subunits of 42, 55, and 65 kDa. All the subunits reacted positively in Western blots to polyclonal antibodies made against specially purified large heart or spleen ferritins containing only 20- and 23-kDa subunits. Similar results were obtained for ferritins from rat heart. The results indicate that mammalian heart tissue is peculiar not just in having an abnormally large iron-rich ferritin but also in having iron-poor ferritins of much lower molecular weight, partly composed of larger subunits.