Structural and functional analysis of the 5'-flanking region of the rat ceruloplasmin gene

FOXO1 stimulates ceruloplasmin promoter activity in human hepatoma cells treated with IL-6

FOXO1, a member of the winged-helix family of transcription factors, is a ubiquitously expressed protein involved in regulating a variety of cellular processes including glucose homeostasis, apoptosis, cell cycle control, muscle differentiation, and angiogenesis. In addition to these biological functions, FOXO1 is a key player in the oxidative stress response by stimulating the expression of metal-containing anti-oxidant proteins such as manganese superoxide dismutase, selenoprotein P, and catalase. Evidence in the literature suggests that FOXO1 may also be capable of regulating the expression of the anti-oxidant protein Ceruloplasmin (Cp), a six-copper-containing protein synthesized and secreted mainly by the liver. In the present report, we demonstrate that FOXO1 stimulates Cp promoter activity in conjunction with the cytokine IL-6. Through deletional analysis and in vitro binding studies, we determine the DNA sequence responsible for the FOXO1-dependent regulation of the Cp proximal promoter. Finally, we demonstrate that FOXO1 is capable of enhancing the expression of endogenous Cp in human hepatic carcinoma cells treated with IL-6. These results allow us to identify FOXO1 as a regulator of Cp expression to promote the anti-oxidant pathway in response to IL-6 signaling.

Interleukin-1β induces ceruloplasmin and ferroportin-1 gene expression via MAP kinases and C/EBPβ, AP-1, and NF-κB activation

Previously, we demonstrated that IL-1β was able to increase iron efflux from glial cells through a coordinate induction of both ferroportin-1 (Fpn) and ceruloplasmin (Cp) synthesis. In this study, we have investigated the signaling pathways that are involved in the transcriptional activation of the Cp and Fpn. Our data show that the expression of Cp and Fpn in response to IL-1β requires the activation of MAP kinase pathways as a consequence of an IL-1β receptor stimulation. Moreover, we have observed that IL-1β regulates the expression of Cp and Fpn genes through (i) p38 MAPK-mediated activation of C/EBP transcription factor, (ii) ERK1/2-, JNK1- and partially p38 MAPK-dependent activation of AP-1, and through (iii) activation of NF-κB partially mediated by p38 MAPK.

Genistein up-regulates the iron efflux system in glial cells

Astrocytes accumulate iron under chronic oxidative stress conditions in ageing and neurological disorders. The soybean isoflavone genistein possesses antioxidant properties and selective estrogen-like activities. Here, a possible role of genistein in modulation of iron transport was explored in glial cells. Genistein significantly increased iron export through estrogen receptor-beta-dependent p38 MAPK activation. Evidence is presented that this effect is associated to a p38 MAPK-triggered up-regulation of the iron export system made by ceruloplasmin and ferroportin-1, a pathway requiring activation of the transcription factor C/EBP.

Transcriptional regulation of ceruloplasmin by an IL-6 response element pathway

Cp is an acute phase reactant protein that also acts as a ferroxidase, and thus indirectly decreases the production of the reactive oxygen species hydroxyl radical. Ceruloplasmin (Cp) expression is induced by a variety of central nervous system injuries, but the mechanism by which this occurs is unclear. Based on the fact that peripheral nerve injury induces interleukin-6 (IL-6) expression and that there are three IL-6 response elements in the upstream region of the Cp gene, we studied their role in transcriptional regulation of Cp in astrocytic C6 glioma cells, using transfection of a rat Cp-luciferase construct, followed by sequential and simultaneous mutation of the IL-6 response elements. We found that 0.8 kb of sequence upstream to the rat ceruloplasmin start site was sufficient to drive luciferase expression in C6 glioma cells. Cells transfected with Cp-luc and treated with 100 ng/ml rat IL-6 induced 216.8% +/- 4.6% of control activity. Mutagenesis of the IL-6 response elements decreased luciferase activity, with the maximal decline (9.7 +/- 0.7% of wild-type) after mutation of the second site. Mutagenesis of multiple sites decreased activity beyond mutagenesis of single sites with mutation of all three sites decreasing activity to 5.3 +/- 0.4% of wild-type. Gel shift and supershift assays indicated that activation of Cp in these cells was not via STAT-3. These results are consistent with a signaling process via IL-6 response elements for Cp upregulation.

Role of hypoxia-inducible factor-1 in transcriptional activation of ceruloplasmin by iron deficiency

A role of the copper protein ceruloplasmin (Cp) in iron metabolism is suggested by its ferroxidase activity and by the tissue iron overload in hereditary Cp deficiency patients. In addition, plasma Cp increases markedly in several conditions of anemia, e.g. iron deficiency, hemorrhage, renal failure, sickle cell disease, pregnancy, and inflammation. However, little is known about the cellular and molecular mechanism(s) involved. We have reported that iron chelators increase Cp mRNA expression and protein synthesis in human hepatocarcinoma HepG2 cells. Furthermore, we have shown that the increase in Cp mRNA is due to increased rate of transcription. We here report the results of new studies designed to elucidate the molecular mechanism underlying transcriptional activation of Cp by iron deficiency. The 5'-flanking region of the Cp gene was cloned from a human genomic library. A 4774-base pair segment of the Cp promoter/enhancer driving a luciferase reporter was transfected into HepG2 or Hep3B cells. Iron deficiency or hypoxia increased luciferase activity by 5-10-fold compared with untreated cells. Examination of the sequence showed three pairs of consensus hypoxia-responsive elements (HREs). Deletion and mutation analysis showed that a single HRE was necessary and sufficient for gene activation. The involvement of hypoxia-inducible factor-1 (HIF-1) was shown by gel-shift and supershift experiments that showed HIF-1alpha and HIF-1beta binding to a radiolabeled oligonucleotide containing the Cp promoter HRE. Furthermore, iron deficiency (and hypoxia) did not activate Cp gene expression in Hepa c4 hepatoma cells deficient in HIF-1beta, as shown functionally by the inactivity of a transfected Cp promoter-luciferase construct and by the failure of HIF-1 to bind the Cp HRE in nuclear extracts from these cells. These results are consistent with in vivo findings that iron deficiency increases plasma Cp and provides a molecular mechanism that may help to understand these observations.

Ceruloplasmin and cardiovascular disease

Transition metal ion-mediated oxidation is a commonly used model system for studies of the chemical, structural, and functional modifications of low-density lipoprotein (LDL). The physiological relevance of studies using free metal ions is unclear and has led to an exploration of free metal ion-independent mechanisms of oxidation. We and others have investigated the role of human ceruloplasmin (Cp) in oxidative processes because it the principal copper-containing protein in serum. There is an abundance of epidemiological data that suggests that serum Cp may be an important risk factor predicting myocardial infarction and cardiovascular disease. Biochemical studies have shown that Cp is a potent catalyst of LDL oxidation in vitro. The pro-oxidant activity of Cp requires an intact structure, and a single copper atom at the surface of the protein, near His(426), is required for LDL oxidation. Under conditions where inhibitory protein (such as albumin) is present, LDL oxidation by Cp is optimal in the presence of superoxide, which reduces the surface copper atom of Cp. Cultured vascular endothelial and smooth muscle cells also oxidize LDL in the presence of Cp. Superoxide release by these cells is a critical factor regulating the rate of oxidation. Cultured monocytic cells, when activated by zymosan, can oxidize LDL, but these cells are unique in their secretion of Cp. Inhibitor studies using Cp-specific antibodies and antisense oligonucleotides show that Cp is a major contributor to LDL oxidation by these cells. The role of Cp in lipoprotein oxidation and atherosclerotic lesion progression in vivo has not been directly assessed and is an important area for future studies.

Delayed translational silencing of ceruloplasmin transcript in gamma interferon-activated U937 monocytic cells: role of the 3' untranslated region

Ceruloplasmin (Cp) is an acute-phase protein with ferroxidase, amine oxidase, and pro- and antioxidant activities. The primary site of Cp synthesis in human adults is the liver, but it is also synthesized by cells of monocytic origin. We have shown that gamma interferon (IFN-gamma) induces the synthesis of Cp mRNA and protein in monocytic cells. We now report that the induced synthesis of Cp is terminated by a mechanism involving transcript-specific translational repression. Cp protein synthesis in U937 cells ceased after 16 h even in the presence of abundant Cp mRNA. RNA isolated from cells treated with IFN-gamma for 24 h exhibited a high in vitro translation rate, suggesting that the transcript was not defective. Ribosomal association of Cp mRNA was examined by sucrose centrifugation. When Cp synthesis was high, i.e., after 8 h of IFN-gamma treatment, Cp mRNA was primarily associated with polyribosomes. However, after 24 h, when Cp synthesis was low, Cp mRNA was primarily in the nonpolyribosomal fraction. Cytosolic extracts from cells treated with IFN-gamma for 24 h, but not for 8 h, contained a factor which blocked in vitro Cp translation. Inhibitor expression was cell type specific and present in extracts of human cells of myeloid origin, but not in several nonmyeloid cells. The inhibitory factor bound to the 3' untranslated region (3'-UTR) of Cp mRNA, as shown by restoration of in vitro translation by synthetic 3'-UTR added as a "decoy" and detection of a binding complex by RNA gel shift analysis. Deletion mapping of the Cp 3'-UTR indicated an internal 100-nucleotide region of the Cp 3'-UTR that was required for complex formation as well as for silencing of translation. Although transcript-specific translational control is common during development and differentiation and global translational control occurs during responses to cytokines and stress, to our knowledge, this is the first report of translational silencing of a specific transcript following cytokine activation.

Coordinated regulation of ceruloplasmin and metallothionein mRNA by interleukin-1 and copper in HepG2 cells

During the acute phase response, cytokines induce hepatic metallothionein and ceruloplasmin synthesis and the uptake of metals. We have investigated how copper and cytokines may interact in controlling ceruloplasmin (CP) and metallothionein mRNA in liver cells. We found that IL-1alpha, IL-1beta and IL-6 increased both metallothionein-1 (MT-1) and metallothionein-2 (MT-2) mRNA in HepG2 cells. The time and pattern of induction was different, both IL-1alpha and IL-1beta inducing two peaks of MT-1 and MT-2, with that of MT-2 being much larger. IL-6 induced only low levels of both MT-1 and MT-2 mRNA. CP mRNA was also increased after 16 h by IL-1beta, whereas IL-1alpha induced two CP peaks at 8 and 20 h, while IL-6 had little effect. Copper administration gave rise to substantially increased MT-1 mRNA, a slightly lower increase in MT-2 and also a significant increase in CP mRNA with similar kinetics. These parallel increases in MT and CP mRNA suggest that the coordinated expression of these proteins may be important for their synthesis during the acute phase response.

Direct and indirect approaches to neuroprotective therapy of glaucomatous optic neuropathy

Retinal ganglion cell death is the final common pathway of virtually all diseases of the optic nerve, including glaucomatous optic neuropathy. In recent years it has been shown that retinal ganglion cells die after axonal injury via a programmed cell death process called apoptosis. The dynamics of retinal ganglion cell death reflect the timing and degree of the axonal injury, rather than its nature. For example, whether mediated by ischemia (corresponding to abnormalities of peripapillary circulation) or compression (e.g., changes in retrograde transport caused by increased intraocular pressure), the end result is a series of changes at the level of the axon, which subsequently affect the retinal ganglion cell body. Our studies on neuroprotection of retinal ganglion cells have focused on general mechanisms applicable to axonal injuries. By dissecting the pathways by which retinal ganglion cells die in these situations, strategies for protection may become manifest. We and others have found that production of certain reactive oxygen species is a necessary step for neuronal death after neurotrophin deprivation. In response, cells invoke compensatory mechanisms to maintain survival in the face of this attack. We have studied the transcriptional regulation of one candidate compensatory gene and discuss it as a model for gene-based approaches to neuroprotective therapy for glaucomatous optic neuropathy. By approaching the problem of therapy from this point of view, it may become possible to prevent irreversible glaucomatous optic nerve changes by inducing endogenous cell-rescue mechanisms and, thus, with the retinal ganglion cells' own defense mechanisms, to prevent its death.

Identification and functional expression of HAH1, a novel human gene involved in copper homeostasis

To search for a mammalian homologue of ATX1, a human liver cDNA library was screened and a cDNA clone was isolated, which encodes a protein with 47% amino acid identity to Atx1p including conservation of the MTCXGC copper-binding domain. RNA blot analysis using this cDNA identified an abundant 0.5-kilobase mRNA in all human tissues and cell lines examined. Southern blot analysis using this same clone indicated that the corresponding gene exists as a single copy in the haploid genome, and chromosomal localization by fluorescence in situ hybridization detected this locus at the interface between bands 5q32 and 5q33. Yeast strains lacking copper/zinc superoxide dismutase (SOD1) are sensitive to redox cycling agents and dioxygen and are auxotrophic for lysine when grown in air, and expression of this human ATX1 homologue (HAH1) in these strains restored growth on lysine-deficient media. Yeast strains lacking ATX1 are deficient in high affinity iron uptake and expression of HAH1 in these strains permits growth on iron-depleted media and results in restoration of copper incorporation into newly synthesized Fet3p. These results identify HAH1 as a novel ubiquitously expressed protein, which may play an essential role in antioxidant defense and copper homeostasis in humans.

Excess copper and ceruloplasmin biosynthesis in long-term cultured hepatocytes from Long-Evans Cinnamon (LEC) rats, a model of Wilson disease

Immortalized hepatic cell lines obtained from laboratory animals or patients with defects in copper metabolism in the liver provide new approaches to examine related metabolism and toxicity. We established a series of hepatic cell lines from the liver of Long-Evans Cinnamon (LEC) rats, using recombinant adenovirus which expresses SV40 large T. Cells from the LEC rats were cultured and accumulated larger amounts of copper than did control cells, when the concentrations of copper in the culture medium exceeded 5 microM. The secretion of ceruloplasmin (CP) from the cultured cells was not reduced in hepatocytes from LEC cells, as compared with the control cells. As accumulation of copper did not affect CP secretion, CP production was not likely to be affected by the accumulation of copper in LEC rat hepatocytes. The production of holo-CP was further investigated by transfection of human CP cDNA and detection of human holo-CP by immunological procedures and use of a monoclonal antibody (mAb CP60) which recognizes human holo-CP but not human apo-CP and rat CP. Hepatocytes from the LEC rats processed and secreted holo-CP into the medium, even with excess copper present in the medium. These observations suggest that the genetic defect in LEC rats did not alter biosynthetic and secretory pathways of CP and that the intracellular copper concentration did not regulate the synthesis and processing of CP in the cultured hepatocytes. Low ceruloplasmin levels are observed in most, but not all, patients with Wilson disease, as well as in LEC rats. Our results do suggest that the copper transporting ATPase encoded in the Wilson disease gene is not a integral part of the biochemical mechanism of copper incorporation into apoprotein. The cell lines and immunological procedures we used are expected to add to information on biologically important process related to copper metabolism and to CP biosynthesis.

Ceruloplasmin gene defect associated with epilepsy in EL mice

Epilepsy is a dominant trait in EL mice, a model for human complex partial seizures. We recently mapped the major gene, El-1, to chromosome 9 near the predicted location for the ceruloplasmin (Cp) gene. We now present evidence for a partial duplication in the Cp gene in EL mice. This Cp duplication is coinherited with seizures in backcross generations and is associated with enhanced expression of Cp mRNA and increased Cp oxidase activity. Moreover, the duplication is associated with an enhanced frequency of double recombinants, simulating negative interference. The findings are relevant to the basic mechanisms of epilepsy and to theories of genetic recombination and gene mapping.