Down-regulation of liver iron-regulatory protein 1 in haemochromatosis

Cellular iron homoeostasis is maintained by iron sensor proteins known as iron-regulatory proteins (IRPs), which act post-transcriptionally by binding RNA stem-loop structures, termed iron-responsive elements (IREs), present on the mRNAs of proteins involved in iron storage, utilization and transport. IRP1 is a bifunctional protein that can act either as a cytoplasmic aconitase or as an IRE-binding protein. The RNA-binding activity of IRP1 is regulated post-translationally by the insertion or extrusion of a 4Fe-4S cluster, without changes in the levels of protein. In hereditary haemochromatosis (HH) accumulation of iron in parenchymal tissues, including the liver, occurs, possibly through dysfunctional IRP1. Investigation of IRP1 expression in liver biopsies from HH patients showed that the protein is completely absent or markedly reduced in heavily iron-loaded HH patients. Real-time PCR was then conducted in an attempt to investigate the mRNA levels and establish the underlying mechanism behind the disappearing act of IRP1. The two possibilities are: transcriptional regulation (through the inhibition of transcription) or post-transcriptional regulation (either through increased turnover of protein or inhibition of translation) of IRP1. Preliminary data suggest that transcription of IRP1 is not affected by chronic iron overload, and down-regulation may be attributable instead to degradation of the protein.

An iron-regulated ferric reductase associated with the absorption of dietary iron

The ability of intestinal mucosa to absorb dietary ferric iron is attributed to the presence of a brush-border membrane reductase activity that displays adaptive responses to iron status. We have isolated a complementary DNA, Dcytb (for duodenal cytochrome b), which encoded a putative plasma membrane di-heme protein in mouse duodenal mucosa. Dcytb shared between 45 and 50% similarity to the cytochrome b561 family of plasma membrane reductases, was highly expressed in the brush-border membrane of duodenal enterocytes, and induced ferric reductase activity when expressed in Xenopus oocytes and cultured cells. Duodenal expression levels of Dcytb messenger RNA and protein were regulated by changes in physiological modulators of iron absorption. Thus, Dcytb provides an important element in the iron absorption pathway.

Hepatic expression of hepatocyte growth factor gene mRNA in acute liver failure

Hepatocyte growth factor plays a key role in liver regeneration but the role of liver in its synthesis in acute liver failure is unclear. We therefore measured hepatic expression of hepatocyte growth factor mRNA in this condition in comparison to H3 histone mRNA, a marker of cellular proliferation. Hepatocyte growth factor mRNA levels were quantified by specific RNase protection assay in nine patients with acute liver failure and found to be similar to those in six normal controls. Hepatocyte proliferation, as assessed by H3 histone mRNA expression, was not detected in normal liver but was present in six of nine patients with acute liver failure (P < 0.05) and was not correlated with expression of hepatocyte growth factor mRNA (rs = -0.28). Liver is unlikely to be the source of the high serum hepatocyte growth factor levels observed in acute liver failure.

A novel binding site for the native hepatic acute-phase protein alpha1-antitrypsin expressed on the human hepatoma cell line HepG 2 and intestinal cell line Caco 2

AIMS/BACKGROUND

alpha1-antitrypsin (alpha1-AT) is a hepatic acute phase protein which predominantly inhibits neutrophil elastase. Besides this major function, we have also previously shown that alpha1-AT markedly increased H-ferritin mRNA expression and ferritin synthesis in the human hepatoma cell line HepG 2. These actions suggest that alpha1-AT might interact with HepG 2 cells via a specific cell surface binding site.

METHODS AND RESULTS

Using radio-labelled native alpha1-AT, we observed saturable binding to HepG 2 cells with a dissociation constant (Kd) of 63.3+/-6.9 nM and a maximal density of binding sites (Bmax) of 0.34+/-0.05 pmol/10(6) cells equivalent to 195,800+/-29,200 sites/cell. The binding of [125I]alpha1-AT was time dependent with a calculated association rate constant of 9.22+/-1.84x10(4)xM(-1)xmin(-1). Binding was highly specific since other acute phase proteins or protease inhibitors failed to block binding. Although alpha1-AT-trypsin, alpha1-AT-elastase and the pentapeptide FVYLI, the minimal binding sequence for the SEC receptor, increased [125I]alpha1-AT binding, in long term experiments these complexes failed to influence the number of alpha1-AT binding sites. Specific, saturable binding of [125I]alpha1-AT was also found on the human intestinal epithelial Caco 2 cells, but not on fibroblast or leukaemic cell lines.

CONCLUSION

These experiments demonstrate a specific, high affinity binding site for native alpha1-AT on HepG 2 and Caco 2 cells, cell lines derived from tissues involved in the acute phase response.

Mechanism of regulation of HGF/SF gene expression in fibroblasts by TGF-beta1

The effect of transforming growth factor beta 1 (TGF-beta1) on levels of hepatocyte growth factor/scatter factor (HGF/SF) gene transcripts was investigated in the human lung embryonic fibroblast cell line, MRC-5. TGF-beta1 markedly reduced the expression of the 6. 0-kb and 3.0-kb HGF/SF mRNA, which encode full-length HGF/SF, but it had little effect on the expression of the alternatively spliced 1. 5-kb mRNA, which encodes NK2, a competitive HGF/SF antagonist. Using actinomycin D to block RNA synthesis, it was observed that TGF-beta1 had little effect on the stability of the 1.5-kb NK2 mRNA but increased the rate of degradation of the 6.0- and 3.0-kb HGF/SF mRNA transcripts by a mechanism that was dependent on new protein synthesis. TGF-beta1 minimally increased rather than reduced HGF/SF promoter activity in cells transiently transfected with chloramphenicol acetyltransferase (CAT) reporter genes driven by HGF/SF gene 5'-flanking sequences. In MRC-5 cells, TGF-beta1 modulates HGF/SF gene transcripts at the posttranscriptional level in order to favour expression of the 1.5-kb mRNA that encodes the truncated protein NK2.

A novel duodenal iron-regulated transporter, IREG1, implicated in the basolateral transfer of iron to the circulation

Iron absorption by the duodenal mucosa is initiated by uptake of ferrous Fe(II) iron across the brush border membrane and culminates in transfer of the metal across the basolateral membrane to the portal vein circulation by an unknown mechanism. We describe here the isolation and characterization of a novel cDNA (Ireg1) encoding a duodenal protein that is localized to the basolateral membrane of polarized epithelial cells. Ireg1 mRNA and protein expression are increased under conditions of increased iron absorption, and the 5' UTR of the Ireg1 mRNA contains a functional iron-responsive element (IRE). IREG1 stimulates iron efflux following expression in Xenopus oocytes. We conclude that IREG1 represents the long-sought duodenal iron export protein and is upregulated in the iron overload disease, hereditary hemochromatosis.

The human and mouse GATA-6 genes utilize two promoters and two initiation codons

GATA-6 has been implicated in the regulation of myocardial differentiation during cardiogenesis. To determine how its expression is controlled, we have characterized the human and mouse genes. We have mapped their transcriptional start sites and demonstrate that two alternative promoters and 5' noncoding exons are utilized. Both transcript isoforms are expressed in the same tissue-specific and developmental stage-specific pattern, and their ratio appears similar wherever examined. The more upstream noncoding exon showed a substantial degree of homology between the two mammalian species, suggesting a conserved regulatory function. Moreover, in transfection assays we show that elements within this exon act to promote its transcription. Positive regulatory elements that effect transcription from the more downstream exon were not apparent in this assay, revealing a regulatory distinction between the two promoters. We also demonstrate alternative initiator codon usage in both the human and mouse GATA-6 genes. Both isoforms of the protein are synthesized in vitro regardless of which 5' noncoding exon is present in the RNA, although the larger protein has greater transcriptional activation potential in transfection assays. Thus, GATA-6 function in the cell is controlled by a complex interplay of transcriptional and translational regulation.

Hepatocellular carcinoma arising in the absence of cirrhosis in genetic haemochromatosis: three case reports and review of literature

Genetic haemochromatosis constitutes a high risk factor for the development of hepatocellular carcinoma. It is widely accepted that venesection prevents the evolution of cirrhosis in haemochromatosis and indirectly protects against the development of hepatocellular carcinoma. Clinical, pathological and radiological data are presented on three patients who did not conform to the 'siderosis-cirrhosis-carcinoma' sequence and in whom prompt and adequate iron depletion did not prevent the development of cancer. This is the first report of hepatocellular carcinoma intervening in non-cirrhotic liver in two siblings with genetic haemochromatosis. The current literature on the subject is reviewed. The direct oncogenic role of iron remains to be elucidated. Hepatocellular carcinoma should be considered as a differential diagnosis in patients with non-cirrhotic genetic haemochromatosis who present with clinical deterioration during the course of an otherwise uneventful venesection programme.

Predominance of the HLA-H Cys282Tyr mutation in Austrian patients with genetic haemochromatosis

BACKGROUND/AIMS

Genetic haemochromatosis is the most common autosomal recessive disorder in Northern European populations. A major histocompatibility complex class I-like gene, HLA-H, has been proposed to be responsible for genetic haemochromatosis. The prevalence of HLA-H gene mutations 282(TGC; Cys/TAC; Tyr) and 63(CAT; His/GAT; Asp) was determined in patients of Austrian origin.

METHODS

DNA extracted from the blood of 40 Austrian patients and 271 controls was used to amplify HLA-H gene fragments by the polymerase chain reaction method. The base changes responsible for mutations Cys282Tyr and His63Asp alter recognition sites for restriction enzymes SnaB I and Bcl I, respectively. Digestion products were separated by agarose gel electrophoresis and visualised by ethidium bromide staining.

RESULTS

Thirty-one (77.5%) genetic haemochromatosis patients were homozygous for mutation Cys282Tyr and three compound heterozygous for mutations Cys282Tyr and His63Asp. One patient was homozygous for mutation His63Asp but normal for mutation Cys282Tyr. Four patients were normal at both genetic loci and one patient was heterozygous for mutation His63Asp. One control subject homozygous for mutation Cys282Tyr was found on investigation to fulfill diagnostic criteria for haemochromatosis. Eight control subjects homozygous for mutation His63Asp showed no biochemical or clinical evidence of haemochromatosis indicating that this variant is not directly responsible for haemochromatosis. Absence of the Cys282Tyr mutation in six genetic haemochromatosis patients with distinct haplotypes indicates mutations within the HLA-H gene or at alternative genetic loci are the cause of genetic haemochromatosis in these patients.

CONCLUSIONS

The HLA-H Cys282Tyr defect is likely to play a key role in the pathogenesis of haemochromatosis in most patients. Predominance of a single HLA-H gene mutation in haemochromatosis allows presymptomatic screening by genotypic analysis.

Haemochromatosis: a gene at last?

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Over-expression of GATA-6 in Xenopus embryos blocks differentiation of heart precursors

Xenopus GATA-6 transcripts are first detected at the beginning of gastrulation in the mesoderm, and subsequent domains of expression include the field of cells shown to have heart-forming potential. In this region, GATA-6 expression continues only in those cells that go on to form the heart; however, a decrease occurs prior to terminal differentiation. Artificial elevation of GATA-6, but not GATA-1, prevents expression of both cardiac actin and heart-specific myosin light chain. This effect is heart-specific because cardiac actin expression is unaffected in somites. Expression of the earlier marker XNkx-2.5 was unaffected and morphological development of the heart was initiated independently of the establishment of the contractile machinery. We conclude that a reduction in the level of GATA-6 is important for the progression of the cardiomyogenic differentiation programme and that GATA-6 may act to maintain heart cells in the precursor state. At later stages, when the elevated GATA-6 levels had decayed, differentiation ensued but the number of cells contributing to the myocardium had increased, suggesting either that the blocked cells had proliferated or that additional cells had been recruited.

A duodenal mucosal abnormality in the reduction of Fe(III) in patients with genetic haemochromatosis

BACKGROUND

Previous in vitro studies have shown that the uptake of Fe(III) by freshly isolated duodenal mucosal biopsy specimens is increased in patients with genetic haemochromatosis. Moreover, in the mouse it has recently been found that reduction of Fe(III) to Fe(II) is a prerequisite for iron uptake by the proximal intestine.

AIMS/METHODS

This study used the in vitro technique to investigate the rates of reduction and uptake of 59Fe(III) by duodenal mucosal biopsy specimens obtained at endoscopy from treated and untreated patients with genetic haemochromatosis.

RESULTS

The rate of reduction of iron in the medium was proportional to the incubation time and was not caused by the release of reducing factors from the tissue fragments. Ferrozine, a specific Fe(II) chelator and ferricyanide, a non-permeable oxidising agent, inhibited uptake of 59Fe showing that reduction of Fe(III) precedes uptake. The rates (all values given as pmol/mg/min) of reduction (152 (49) v 92 (23)) and uptake (8.3 (4.0) v 3.6 (1.3), mean (SD)), were significantly increased in biopsy specimens from the untreated group (n = 6) compared with those from 10 control subjects (p < 0.04). Furthermore, the reduction and uptake rates were still increased in five patients in whom iron stores were normal after venesection treatment.

CONCLUSIONS

These results show that there is a persistent abnormality in the reduction and uptake of iron by the intestine in genetic haemochromatosis.

Conformational analysis of pentapeptide sequences matching a proposed recognition motif for lysosomal degradation

A selective pathway for the degradation of specific long-lived cytosolic proteins is activated in response to starvation in vivo or to serum withdrawal from cultured cells. It involves recognition of a targeting motif by a member of the hsp70 family. A 5-residue targeting motif has been proposed on the basis of sequence comparisons. We investigate whether there is any structural basis for this motif being the true recognition signal. We examine the conformations of four motif peptides in proteins that are either known to be serum regulated or are from related vertebrate species, and two equivalent peptides in bacterial proteins that closely resemble other regulated proteins. Our studies show that all the motif sequences are located near the ends of surface helices with one or more of the residues buried in the structure, yet it is known that members of the hsp70 family tend to interact with extended peptide chains. Furthermore, recognition by these proteins generally requires a specific ordering of key residues, yet the motif implies a largely order-independent sequence characterized by residue type only. We conclude that the proposed motif is unlikely to be the true targeting signal for lysosomal degradation unless additional factors apply.

Relationship between duodenal cytosolic aconitase activity and iron status in the mouse

Cytosolic aconitase activity was assayed in duodenal mucosa from mice subjected to a variety of manipulations known to modulate duodenal iron status and duodenal iron absorption. No changes in cytosolic aconitase activity were observed 1 h after oral FeSO4 dosing or intramuscular desferrioxamine treatment. Three days of hypoxic exposure and two weeks treatment with intramuscular iron dextran also had no effect on cytosolic aconitase. Three weeks growth on an iron deficient diet significantly reduced cytosolic aconitase activity. In no situation was there any evidence for significant amounts of inactive aconitase which could be activated in vitro with FeSO4/cysteine. These data suggest that duodenal cytosolic aconitase is not sensitive to acute changes in mucosal iron levels and is generally much less sensitive to body iron status than is duodenal iron absorption. There is evidence that chronic iron depletion reduces cytosolic aconitase to a relatively small degree but generally activity is maintained, consistent with an important metabolic role for the enzyme.

New polymorphic microsatellite markers place the haemochromatosis gene telomeric to D6S105

The haemochromatosis gene (HFE) is linked to both HLA-A and D6S105 on the short arm of chromosome 6 but these markers are separated by approximately 2 Mb of DNA. Most chromosomes carrying HFE have a common haplotype which extends from HLA-A to D6S105 and includes HLA-F. To localise the gene more precisely we have examined 10 microsatellite markers extending over a genetic distance of approximately 5 cM from D6S265 (within 100 kb of HLA-A on the centromeric side) to D6S299 (telomeric). The order of markers is D6S265, HLA-F, D6S258, D6S306, CS3, D6S105, D6S464, CS5, D6S461 and D6S299. We confirm that haemochromatosis appears to originate from a founder mutation which has multiplied in the population through successive generations. This mutation is associated with the haplotype D6S306-5, CS3-3, D6S105-8, D6S464-9 and CS5-4 which is found on approximately 70% of HFE chromosomes. We have applied a new and powerful, likelihood analysis for linkage disequilibrium. The maximum value of lambda (proportion of total possible association between a marker and disease) is 0.74 for marker CS5 (allele 4). A multipoint analysis also gives a maximum likelihood near marker CS5. We conclude that the HFE gene is likely to be located telomeric of D6S105 and close to CS5.

Antibodies to atypical mycobacteria in primary biliary cirrhosis

Recent work has shown that there is antigenic similarity between common bacterial proteins and epitopes on biliary epithelial cells. Following evidence that the sera of all of a series of patients with primary biliary cirrhosis contained antibodies to an extract of Mycobacterium gordonae (a ubiquitous environmental organism) and that these antibodies cross-reacted with the major mitochondrial M2 epitopes, the study below has been performed to investigate this phenomenon further. Using Western blotting of membrane extracts of Mycobacterium gordonae and seven other atypical mycobacterial species, no such antibodies were detected in the serum of patients with primary biliary cirrhosis, but antibodies to a 65 kDa mycobacterial protein were found in most patients with primary biliary cirrhosis as well as in normal controls and patients with other chronic liver diseases.

Metal ions catalytic for free radical reactions in the plasma of patients with fulminant hepatic failure

We propose that the frequency and severity of multi-organ failure (MOF) in fulminant hepatic failure (FHF) involves free radical damage caused by the presence of circulating iron and copper ions, catalytic for free radical reactions. The presence of such metal ions is demonstrated by using the sensitive bleomycin and phenanthroline assays. Antioxidant therapy, e.g., using chelating agents that prevent metal ions from stimulating free radical reactions, may have benefit in the treatment of FHF and its consequences.

Iterative endocytosis of transferrin by K562 cells

The effect of iron on the exocytosis of transferrin by K562 cells was studied by first allowing the cells to endocytose apotransferrin or diferric transferrin. Subsequent release of the apotransferrin was very rapid with a t 1/2 of 3.01 min, compared with 5.5 min for diferric transferrin. Release of apotransferrin was slowed by the weak base methylamine, t 1/2 8.0 min, but the effect of this agent was substantially greater when iron-transferrin was used, t 1/2 18.65 min, suggesting that methylamine affects both iron removal and receptor recycling. Release of iron-transferrin could be accelerated to a rate comparable with that of apotransferrin by addition of the permeant iron-chelator desferrioxamine. The difference in the rates of release of different forms of the protein could be explained by the re-endocytosis of the iron-rich protein, a process detected by the accelerated release of transferrin when the cells were washed in medium at pH 5.5 containing an iron-chelator or treated with a protease-containing medium to digest transferrin accessible at the cell surface. It appears that in cells incubated under control conditions, re-endocytosis of transferrin, which is incompletely depleted of iron, occurs and that a transferrin molecule may make two passes through the cell before all the iron is removed. This mechanism helps to explain why very little iron-transferrin is released from cells and why the efficiency of the iron uptake process is so high.

Reversibility of hypogonadotrophic hypogonadism associated with genetic haemochromatosis

OBJECTIVE

Recent reports have suggested that hypogonadotrophic hypogonadism in men with genetic haemochromatosis can be reversed by aggressive venesection therapy. We have studied prospectively men with this complication of haemochromatosis in order to document the frequency and completeness of recovery from hypogonadism.

PATIENTS AND DESIGN

Six men with symptomatic hypogonadotrophic hypogonadism and genetic haemochromatosis were studied before beginning venesection therapy and again after depletion of body iron stores.

MEASUREMENT

Symptoms of hypogonadism, serum gonadotrophins and serum total and free testosterone concentrations.

RESULTS

Five men aged 47-66 showed no symptomatic improvement and no change in serum gonadotrophin or testosterone concentrations. The symptoms of one man, aged 33, resolved completely after venesection and this was accompanied by increases in serum LH and FSH. Serum total and free testosterone concentrations increased fourfold, but remained subnormal.

CONCLUSION

Only one partial recovery from hypogonadotrophic hypogonadism was seen and this was in the youngest subject. In reviewing the other documented cases of reversal it would appear that the age at diagnosis is critical and there are no proven cases of reversal of hypogonadotrophic hypogonadism in men over the age of 40 at the start of venesection therapy.

Hip arthropathy in genetic hemochromatosis. Radiographic and histologic features

Genetic hemochromatosis, a disorder of iron metabolism, results in the deposition of massive amounts of iron in the tissues. Arthropathy is one of a number of clinical features associated with the disease. Characteristic radiographic features in the wrist and hand have been reported, and an increased incidence of severe hip disease has been observed. In this study, hip radiographs of 112 patients with genetic hemochromatosis and arthritis were reviewed, and histologic examination of 2 femoral heads was performed. Twenty-eight of the 112 patients (25%) had evidence of arthritis of the hip joint. In 23 (82%) of the 28 patients, this feature was thought to be associated with osteoarthritis; 2 of these patients had an atypical arthropathy associated with radiolucency of the femoral head and histologic features of atypical stripping of the cartilage from the subchondral bone. These atypical features were not thought to be due to avascular necrosis, pyrophosphate-associated arthropathy, apatite-associated deposition arthritis, or osteoarthritis, but may be typical of genetic hemochromatosis and possibly the result of increased susceptibility to shearing forces at the bone-cartilage interface. In 5 of the 28 patients (18%), chondrocalcinosis was the sole abnormal finding on radiography. Ten of the 28 patients eventually required hip surgery, which confirms the severity of the hip disease associated with genetic hemochromatosis.

Molecular analysis of the human MHC class I region in hereditary haemochromatosis. A study by pulsed-field gel electrophoresis

The unknown allele that predisposes to the development of haemochromatosis in man has been localized to the HLA class I region on the short arm of chromosome 6. We have utilized pulsed-field gel electrophoresis in conjunction with probes that map within, or in the vicinity of, this region to search for structural lesions that may further define the disease locus. Using the enzyme Mlu I, fragments that associated specifically with the HLA-A23, A31 and B8 alleles were identified. However, in members of three pedigrees affected by haemochromatosis, and in six unrelated patients with the disorder, no disease-specific differences were detected in the DNA fragments with four restriction enzymes and eight probes when compared with healthy individuals. These data suggest that the lesion responsible for hereditary haemochromatosis lies beyond the resolution of this technique and does not involve large structural deletions or extensive re-arrangements in this highly polymorphic region of the genome.

Mechanisms of inhibition of mononuclear cell activation by the iron-chelating agent desferrioxamine

Iron-withholding by the chelating agent desferrioxamine abrogates the proliferative response of human peripheral blood mononuclear cells (PBMC) to phytohaemagglutinin (PHA). The present study investigated whether desferrioxamine operates late in the activation process or, as recently suggested, at an early stage, by inhibiting the appearance of the interleukin-2 (IL-2) receptor. Human PBMC were stimulated with PHA (10 micrograms/ml) and [3H]thymidine ([3H]TdR) incorporation determined after 66 hr of culture. Greater than 90% inhibition was achieved by concentrations of desferrioxamine as low as 5 mumol/l present throughout culture, while IL-2 receptor expression (anti-Tac), analysed by FACS, was maintained at up to 75% of control levels. 300 mumol/l desferrioxamine present throughout culture abrogated [3H]TdR incorporation and additionally suppressed IL-2 receptor to 10-15% of control levels. In contrast, the same high dose of desferrioxamine when added for 2 hr to cells previously cultured for 66 hr produced 80% inhibition of [3H]TdR incorporation but failed to inhibit expression of the IL-2 receptor. Desferrioxamine rapidly achieved equilibrium across the cell membrane (within 60 min) and chelated 59Fe delivered to activated cells by the transferrin endocytic cycle. These results indicate that desferrioxamine can inhibit T-cell activation either early or late in the process by chelating iron and independently of an effect on the IL-2 receptor. In support of a dual effect of the drug is the finding that at 50 mumol/l, desferrioxamine-enhanced expression of the transferrin receptor occurred, an adaptive response made to intracellular iron depletion, while IL-2 receptor expression was inhibited.

Determinants of the interaction between the iron-responsive element-binding protein and its binding site in rat L-ferritin mRNA

Ferritin messenger RNA has been shown to be translationally inactivated by the binding of a cytosolic protein to a 28-nucleotide iron-responsive element (IRE) located in the 5'-untranslated region of the mRNA. This interaction has been studied using quantitative receptor-ligand binding methods with gel retardation and nitrocellulose filter binding assays for the separation of bound complex from free RNA. In competition assays the entire 5'-untranslated region and the isolated IRE bound identically. The specificity of the RNA binding was studied using IRE variants. Two IREs from transferrin receptor mRNA and several variants with single base substitutions in the stem or loop had similar affinities. RNAs which could not form a stem-loop structure bound 1000-fold less well. These studies demonstrate the importance of the RNA conformation and the relative insensitivity of binding to much of the primary sequence. Saturation assays with increasing concentrations of 32P-IRE resulted in a binding hyperbola characteristic of mass action binding to a single class of sites with a KD = 0.09 nM. At 37 degrees C the dissociation rate is 0.04 min-1 (t 1/2 = 17 min). This rate is fast enough to account for the shift of ferritin RNA from the ribonucleoprotein pool to polysomes after rats are injected with iron. Determination of the concentration of the repressor requires accounting for three interconverting pools: free active repressor, mRNA-bound protein, and inactive (low affinity) repressor. Rat liver cytosol has a concentration of free active repressor of about 1 pmol/mg protein. Protein bound to endogenous mRNA can be measured by pretreatment with micrococcal nuclease or by separation with DEAE-Sepharose chromatography; it is present at a level similar to that of the free active protein. Inclusion of high levels of thiol reductants in the binding incubations reduces the inactive or low affinity repressor, forming unstably activated protein which has the same KD as the endogenous active protein; this inactive or low affinity protein is 2-4 times more abundant. A mechanism for iron regulation is proposed which accounts for the kinetics, the multiple protein pools, and the characteristics of the protein in these pools.

Biochemical studies of the iron cores and polypeptide shells of haemosiderin isolated from patients with primary or secondary haemochromatosis

Haemosiderin isolated from different iron-loading syndromes, primary haemochromatosis (PHC) and secondary haemochromatosis (SHC) biochemically exhibited differences in both their iron core and peptide composition. The rate of release of iron from PHC haemosiderin to oxalate was 3-fold greater than that from SHC haemosiderin. The major peptides separated by SDS-PAGE showed a major band at Mr 20,000 for PHC haemosiderin and at Mr 15,000 for SHC haemosiderin.

Hypogonadism and sexual dysfunction in hemochromatosis: the effects of cirrhosis and diabetes

The contribution of diabetes and cirrhosis to sexual dysfunction and hypogonadism was evaluated by two-way analysis of variance in a group of 30 men with idiopathic hemochromatosis. The prevalence of severe sexual dysfunction was significantly higher in men with hemochromatosis than in a control group matched for prevalence of diabetes and age (P less than 0.001). In both controls and hemochromatosis patients the presence of diabetes was significantly associated with sexual dysfunction (P less than 0.005), but the more severe symptoms in the hemochromatosis patients were related to the additive effects of hypoandrogenism (P less than 0.01). Sexual dysfunction was a common early complaint in hemochromatosis patients, but these symptoms were frequently overlooked, leading to diagnostic delay. Mean testicular volume was a useful measure of gonadal status, being significantly correlated with indices of serum free testosterone (rs = 0.83; P less than 0.01) and LH (rs = 0.71; P less than 0.001). The presence of cirrhosis did not contribute significantly to symptomatology, but had an effect independent of and additive to hypogonadotropic hypogonadism in reducing serum free testosterone (P less than 0.02) and estradiol (P less than 0.002), an effect apparently mediated through central rather than testicular mechanisms. Hypoandrogenism was associated with an increase in serum sex hormone-binding globulin (SHBG) concentrations (P less than 0.005), but cirrhosis also had an independent effect in raising SHBG (P less than 0.005), which could not be accounted for by changes in circulating sex hormone concentrations. Thus, the evaluation of sexual dysfunction or hypogonadism in men with hemochromatosis requires consideration of the effects of both diabetes and cirrhosis. Because of the greater variance in SHBG some estimate of free testosterone rather than total testosterone is preferable.

Occupancy of the iron-binding sites of human transferrin in sera obtained from different anatomical sites

Transferrin is the major iron-transport protein in serum. Four molecular species (apo-, two mono-, and diferric transferrins) can be distinguished on the basis of their occupancy with iron. These species differ physicochemically and in the affinity with which they bind to the transferrin receptor. To elucidate the possible role of the four molecular species in directing the flow of iron between the major anatomical sites of iron release and utilization we have analyzed sera from eight stable multiorgan donors. The samples were obtained from veins draining the spleen, gut, and liver, and from the periphery. Employing polyacrylamide-gel electrophoresis in combination with crossed immunoelectrophoresis we were able to identify the four molecular species in all samples. Apo-transferrin was the predominant molecular species while diferric transferrin was the least abundant (P less than 0.01). The monoferric species were dominated by the acid-stable form (iron loading on the C-terminal end of the molecule) with ratios C/N from 1.2 (splenic and posthepatic serum) to 1.5-1.6 in mesenteric or peripheral samples respectively. We conclude from our study that it is unlikely that the monoferric transferrin species play a role in directing internal iron exchange and that in accordance with most of the literature the acid-stable form is preferentially loaded under physiological conditions of iron metabolism.

Optimizing the immunohistochemical signal from the transferrin receptor in liver tissue

Cellular expression of the transferrin receptor is determined by the proliferative state and iron requirements of the cell. Previous immunohistochemical studies using a number of anti-(transferrin receptor) monoclonal antibodies confirmed the biochemical evidence that hepatocytes express the receptor, although the distribution shown was patchy with only a small number of cells showing positive staining. In the present study, a number of techniques have been compared to optimize detection of the immunohistochemical signal from the transferrin receptor in human liver tissue. Using an alkaline phosphatase detection system, widespread expression of this receptor with both cytoplasmic and membrane staining was found in all parenchymal and non-parenchymal cells.

Non-transferrin-bound iron in plasma or serum from patients with idiopathic hemochromatosis. Characterization by high performance liquid chromatography and nuclear magnetic resonance spectroscopy

The nature of non-transferrin-bound iron in the plasma or serum of iron-overloaded hemochromatosis patients was studied by high performance liquid chromatography (HPLC) and high resolution nuclear magnetic resonance (NMR). 500-MHz proton Hahn spin-echo NMR spectra of plasma or serum, combined with the use of the iron chelator desferrioxamine, suggests complexation of iron ions with citrate and a possible involvement of acetate. Addition of FeCl3 to hemochromatosis samples broadened the NMR signals from citrate. HPLC analysis rigorously confirmed the presence of an iron-citrate complex in ultrafiltrates of plasma or serum studies with added FeCl3 or desferrioxamine supported this conclusion. It is proposed that non-transferrin-bound iron in the plasma of iron-overloaded patients exists largely as complexes with citrate and possibly also as ternary iron-citrate-acetate complexes. The presence of such complexes would account for the ability of non-transferrin-bound iron to be measurable by the bleomycin assay and for its rapid clearance from the circulation by the liver.

Non-caeruloplasmin copper and ferroxidase activity in mammalian serum. Ferroxidase activity and phenanthroline-detectable copper in human serum in Wilson's disease

It is thought that 5-10% of human serum copper consists of copper ions complexed with histidine, amino acids or albumin. The phenanthroline assay developed by Gutteridge (Biochem. J. 218, 983-985; 1984) is shown to measure all these forms of copper down to a sensitivity of 0.1 mumol/dm3, yet it does not detect any copper ions in freshly-prepared serum or plasma from rats, mice, rabbits or guinea-pigs, or freshly-prepared serum from humans. It is concluded that the "non-caeruloplasmin copper pool" is much smaller than has previously been supposed. No phenanthroline-detectable copper could be measured in serum freshly prepared from four patients with uncomplicated Wilson's disease, but it could be measured in serum from a patient with fulminant hepatic failure. After liver transplantation, concentrations of phenanthroline-detectable copper in this patient fell to zero within two days. Studies on the ferroxidase activity of freshly-prepared serum or plasma samples shows that little ferroxidase II activity is present in samples from healthy adults or from the patient with Wilson's disease and fulminant hepatic failure. In all the patients, ferroxidase I activities are sub-normal. Freshly-prepared plasma or serum samples from several animal species generally show lower ferroxidase I and greater ferroxidase II activities than do human samples, but only in rabbits does ferroxidase II account for a high proportion of total plasma ferroxidase activity. Storage of biological fluids can cause release of copper from caeruloplasmin and a rise in ferroxidase II activity; these events may have confused some earlier studies.

Regulation of the hepatic transferrin receptor in hereditary hemochromatosis

The liver is the main site of iron accumulation and pathologic sequelae in hereditary hemochromatosis. Whether this is a result solely of inappropriately increased absorption of iron by the gastrointestinal tract or a more generalized regulatory failure of iron balance is unknown. Using immunohistochemical techniques, we have examined the effects of therapeutic changes in liver iron stores on the expression of the hepatic transferrin receptor in hereditary hemochromatosis. Ten patients with untreated hereditary hemochromatosis had no detectable staining for transferrin receptor in their liver biopsies. All had increased hepatic ferritin (mean = 19.9 micrograms per mg protein, range = 1 to 31.7 micrograms per mg protein) and hepatic iron levels (mean = 36.2 micrograms per mg protein, range = 3.6 to 69.9 micrograms per mg protein). In contrast, hepatocyte transferrin receptor was detected in seven patients in whom hepatic iron stores were markedly depleted by venesection (hepatic ferritin mean = 0.32 microgram per mg protein, range = 0.16 to 0.53 microgram per mg protein; hepatic iron mean = 0.98 microgram per mg protein, range = 0.3 to 2.1 micrograms per mg protein). Sequential data from one patient confirmed the reexpression of receptor in response to therapeutic iron depletion, whereas data from another patient studied during treatment illustrated a reciprocal relationship between liver tissue distribution of iron and expression of transferrin receptor. The finding that appropriate physiologic regulation of the hepatic transferrin receptor operates in hereditary hemochromatosis does not support the concept of a generalized defect in receptor-mediated uptake of transferrin-bound iron.

Ferritin iron kinetics and protein turnover in K562 cells

The binding, incorporation, and release of iron by ferritin were investigated in K562 cells using both pulse-chase and long term decay studies with 59Fe-transferrin as the labeled iron source. After a 20-min pulse of labeled transferrin, 60% of the 59Fe was bound by ferritin with the proportion increasing to 70% by 4 h. This initial binding was reduced to 35% when the cells were exposed to the chelator desferrioxamine (5 mM) for an additional 30 min. By 4 h the association of 59Fe with ferritin was unaffected by the presence of the chelator, and levels of 59Fe-ferritin were identical to those in control cells (70%). Between 4-10h there was a parallel decline in 59Fe-ferritin in both control and desferrioxamine-treated cells. When incoming iron was bound by ferritin it was, therefore, initially chelatable but with time progressed to a further, nonchelatable compartment. In turnover studies where ferritin was preloaded with 59Fe by overnight incubation, 50% of the label was released from the protein by 18 h, contrasting with a t 1/2 for cellular iron release of approximately 70 h. The half-time of 59Fe release from ferritin was accelerated to 11 h by the presence of desferrioxamine. The half-time for ferritin protein turnover determined by [35S]methionine labeling was approximately 12 h in the presence or absence of the chelator. Thus, when the reassociation of iron with ferritin was prevented by the exogenous chelator there was a concordant decay of both protein and iron moieties. The direct involvement of lysosomes in this turnover was demonstrated by the use of the inhibitors leupeptin and methylamine which stabilized both 59Fe (t 1/2 = 24 h) and 35S (t 1/2 = 25.6 h) labels. We conclude that in this cell type the predominant mechanism by which iron is released from ferritin is through the constitutive degradation of the protein by lysosomes.

Bleomycin-detectable iron in serum from leukaemic patients before and after chemotherapy. Therapeutic implications for treatment with oxidant-generating drugs

Patients with acute myeloid leukaemia show elevated plasma iron and, in 2/6 cases studied, low-molecular-mass iron complexes capable of stimulating radical reactions were present in the plasma. Shortly after the onset of chemotherapy, there is a sharp rise in transferrin saturation and all patients studied showed low-molecular-mass iron in their plasma. It is proposed that such iron could interact with oxidants generated by certain drugs (e.g. adriamycin or daunorubicin) to facilitate tissue damage, and that some of the side-effects of chemotherapy might be ameliorated by careful co-administration of small doses of desferrioxamine.

Nontransferrin-bound iron in plasma from hemochromatosis patients: effect of phlebotomy therapy

Plasma from patients with iron overload resulting from idiopathic hemochromatosis contains nontransferrin-bound iron, measurable by the bleomycin, assay. During venesection therapy, the concentration of bleomycin iron declines in a way highly correlated with plasma ferritin concentrations. Even when patients had been venesected to give very low total plasma iron concentrations and high transferrin iron-binding capacity, bleomycin-detectable iron was still present at low concentrations. Bleomycin-detectable iron can stimulate damaging free radical reactions, and its persistence in plasma even after prolonged venesection might contribute to the tissue damage that results from iron overload.

Chelation of transferrin iron by desferrioxamine in K562 cells. The partition of iron between ferrioxamine and ferritin

In this study we have determined whether desferrioxamine can chelate iron delivered to human leukaemic cells by the transferrin endocytic cycle. The cellular uptake of desferrioxamine was investigated by an indirect method in which the conversion of repeated pulses of [59Fe]transferrin to [59Fe]ferrioxamine was determined at two concentrations of the drug. Maximum generation of [59Fe]ferrioxamine occurred in cells exposed to either 100 microM- or 500 microM-desferrioxamine after 40-60 min. Thereafter (up to 180 min) [59Fe]ferrioxamine levels remained steady with 20% of a 59Fe pulse partitioning to chelator at 100 microM and 50% at 500 microM. Of the cellular [59Fe]ferrioxamine loss 50% occurred within 90-120 min. In cells preloaded with desferrioxamine for 1 or 4 h the partitioning of iron during a 3 h incubation with [59Fe]transferrin was dependent upon the extracellular concentration of the chelator. Above 1 mM more than 80% of entering iron was converted to ferrioxamine and less than 5% partitioned to ferritin. Below this concentration (50-500 microM) a proportion of the iron became ferritin associated (7-41%). There was a linear increase in the total amount of intracellular [59Fe]ferrioxamine in accordance with cellular iron uptake showing that transferrin continued to cycle in the presence of high concentrations of desferrioxamine. The uptake of iron and generation of ferrioxamine were markedly reduced by 5 mM-methylamine, which prevented endosome acidification and uncoupling of iron from endocytosed transferrin.

Phytohaemagglutinin induced proliferation of lymphocytes from patients with rheumatoid arthritis and iron deficiency

The response of peripheral blood lymphocytes to stimulation by mitogens such as phytohaemagglutinin (PHA) is commonly depressed in both rheumatoid arthritis and iron deficiency, and as many rheumatoid patients are anaemic with evidence of abnormal iron metabolism it is possible that the same mechanism underlies the observed suppression in both conditions. In the present study the mitogenic response to PHA of lymphocytes from three rheumatoid patients, who were also iron deficient, and two healthy controls has been shown to be significantly less in iron deficient than iron containing media (p less than 0.001). In addition, iron deficient sera from these patients reduced the PHA induced proliferation of lymphocytes from a normal subject (p less than 0.01), an effect which was prevented by prior addition of iron to these serum samples. In iron containing media lymphocytes from five patients and two controls showed no difference in their response to PHA for both the minimum mitogen concentration which enhanced transformation and the peak [3H]thymidine uptake; but patients' lymphocytes showed significantly less response to PHA concentrations of 5 and 10 mg/l (p less than 0.02), resulting in a reduction in the area under the dose response curves up to 20 mg/l (p less than 0.05). These findings show both that iron deficient sera can impair PHA induced lymphocyte transformation and that lymphocytes from iron deficient rheumatoid patients have impaired responsiveness to PHA. Iron is known to be required intracellularly for the enzyme ribonucleotide reductase, which is important for DNA synthesis, and reduced activity of this enzyme could explain these observed effects.

Measurement of ferritin in serum by an indirect competitive enzyme-linked immunosorbant assay

This indirect competitive enzyme-linked immunosorbant assay (ELISA) for ferritin, unlike other currently available ELISAS, does not require use of an anti-ferritin antibody-enzyme conjugate. Designed for use on microtiter plates, the method has a precision and sensitivity similar to those of other immunoassays. The detection limit is 20 pg of ferritin per test (corresponding to 2.0 micrograms/L in serum samples). Comparison of results obtained on serum from 57 patients by this method with those from a conventional radioimmunoassay gave a correlation coefficient of 0.92.

Measurement of ferritin in serum by an indirect competitive enzyme-linked immunosorbant assay

This indirect competitive enzyme-linked immunosorbant assay (ELISA) for ferritin, unlike other currently available ELISAS, does not require use of an anti-ferritin antibody-enzyme conjugate. Designed for use on microtiter plates, the method has a precision and sensitivity similar to those of other immunoassays. The detection limit is 20 pg of ferritin per test (corresponding to 2.0 micrograms/L in serum samples). Comparison of results obtained on serum from 57 patients by this method with those from a conventional radioimmunoassay gave a correlation coefficient of 0.92.

Heterogeneous distribution of transferrin receptors on parenchymal and nonparenchymal liver cells: biochemical and morphological evidence

To investigate which cells of the liver express the receptor for transferrin, isolated rat liver cells produced by collagenase perfusion were fractionated by repeated differential centrifugation to produce hepatocytes (95% + 1%, mean +/- SD, n = 4) and nonparenchymal cells (97% + 1%, n = 3). Saturable, high-affinity binding of 125I-transferrin was demonstrated on intact cells at 4 degrees C, with average receptor numbers 20,900 +/- 3,160 (mean + SD, n = 4) for hepatocytes and 5,500 + 1,520 (n = 3) for nonparenchymal cells. Total cellular receptors measured in detergent permeabilized hepatocytes were 42,000 +/- 18,330 (mean +/- SD, n = 3) per cell and 14,760 +/- 7,120 (n = 3) per cell in the nonparenchymal fraction. Immunocytochemical demonstration of transferrin using antitransferrin, peroxidase antiperoxidase complex confirmed that both cell types bound transferrin. There was heterogeneity of the staining reaction since there was no detectable staining on 40% of hepatocytes and 60% of nonparenchymal cells. Microdensitometric analysis of the staining product corroborated the biochemical evidence that hepatocytes have, on average, more than three times more transferrin receptors than do nonparenchymal cells. These findings support the concept that the hepatocyte has a central role in the uptake and storage of transferrin iron.

Treatment of rheumatoid arthritis with desferrioxamine

Desferrioxamine was used to treat six patients who had rheumatoid arthritis refractory to conventional treatment, on the basis that levels of intra-articular iron would be reduced and inflammation lessened. After a period of initial intensive treatment which was limited by side effects, five patients continued on once-weekly maintenance doses. Two patients had temporary improvement in their symptoms, but relapsed in spite of continuing treatment. The remaining three patients completed six months of treatment with no improvement in their rheumatoid disease. There were no significant changes in rheumatological parameters, immunological markers of disease activity nor radiological evidence of improvement. Treatment did lead to significant falls in haemoglobin concentration (p less than 0.01), mean corpuscular volume (p less than 0.05) and serum ferritin levels (p less than 0.02). Therefore, in spite of a reduction in iron available for haem synthesis and a fall in tissue storage iron the rheumatoid inflammatory process persisted.

The effect of desferrioxamine on transferrin receptors, the cell cycle and growth rates of human leukaemic cells

The effect of the iron chelator, desferrioxamine, on transferrin binding, growth rates and the cell cycle was investigated in the human leukaemic cell line, K562. At all concentrations of the chelator (2-50 microM) binding of 125I-transferrin was increased by 24 h and reached a maximum at 72-96 h. Maximum binding (6-8-fold increased) occurred in cells treated with 20 microM-desferrioxamine, in contrast with control cells which, at 96 h, showed a 50% decrease over initial binding. Scatchard analysis at 4 degrees C showed that this increased binding was due to an increase in the number of receptors, as the Kd was similar in induced (1.8 nM) and control (1.5 nM) cells. After 96 h cells, cultured with 20 and 50 microM-desferrioxamine accumulated 59Fe from bovine transferrin at over twice the rate found with control cells, reflecting the increase in transferrin receptors. Although iron uptake was unimpaired by the chelator there was a dose-dependent inhibition of cell growth, with control cells completing three divisions in 96 h and those in 10 microM-desferrioxamine only two divisions. At the highest concentration (50 microM), cell division was abrogated although cell viability was maintained (85%). In contrast, DNA synthesis was not markedly affected, except at 50 microM-desferrioxamine when incorporation of [3H]thymidine was 52% of that in control cells. Flow cytometry revealed that there was a progressive accumulation of the cells in the active phases of their cycle (S, G2 + M). Desferrioxamine may increase transferrin receptors in two ways: by chelating a regulatory pool of iron within the cell, and by arresting cells in S phase when receptors are maximally expressed.

Wash off liver cytology: a complementary diagnostic tool to liver biopsy

Parenchymal and non-parenchymal cells were harvested by washing the liver tissue core and needle after percutaneous biopsy. The cytological material obtained was suitable for morphological analysis, including showing the presence of surface and cytoplasmic antigens using labelled antibody techniques. This technique provides a combined cytological and histological approach to the diagnosis of liver disease.

Intracellular forms of iron during transferrin iron uptake by mitogen-stimulated human lymphocytes

Transferrin receptors expressed by mitogen stimulated human lymphocytes mediate the uptake of transferrin iron into haem, ferritin and a non-haem, non-ferritin component. In spite of different rates of iron uptake by cells from different individuals, the proportional incorporation of 59Fe into these components was similar, suggesting that there was an obligatory relationship between the different forms of iron in the cells. By 3 h over 60% of the iron taken up was incorporated into ferritin while less than 10% was found in haem. Initially (10 min) non-haem, non-ferritin iron comprised 70% of total iron and this diminished to 30% by 3 h. At 10 min 80% of iron in the non-haem, non-ferritin component was retained by anti-transferrin affinity columns indicating it was transferrin-bound. The proportion retained fell to reach a steady state level of 50% by 60 min. These results indicate that 10-20% of the iron in the cells was not recognized as transferrin, ferritin or haem iron. The finding that iron incorporation into this unidentified pool reached equilibrium while that into haem and ferritin increased suggests the iron may act as a precursor for functional and storage compounds.

Intracellular processing of transferrin and iron by isolated rat hepatocytes

Transferrin bound by isolated rat hepatocytes is rapidly endocytosed and enters a compartment of low density. Little was found associated with the lysosomes, even though the protein was subsequently lost from the cells. Iron entering the cells on transferrin was subsequently found in a number of intracellular components: transferrin, haem, ferritin and a residual fraction. After 2 h incubation with 59Fe-transferrin almost 70% of the iron was in ferritin, and this proportion increased to 80% during a 'chase' experiment. Residual iron, because of its rapid increase at the start of the incubation and its decline during the 'chase', probably represents an intracellular transit pool, which at steady state was present at 23 pg/10(6) cells.