Biliary excretion of iron and ferritin in idiopathic hemochromatosis

Bile from the hemojuvelin-deficient mouse model of iron excess is enriched in iron and ferritin

Iron is an essential nutrient but is toxic in excess. Iron deficiency is the most prevalent nutritional deficiency and typically linked to inadequate intake. Iron excess is also common and usually due to genetic defects that perturb expression of hepcidin, a hormone that inhibits dietary iron absorption. Our understanding of iron absorption far exceeds that of iron excretion, which is believed to contribute minimally to iron homeostasis. Prior to the discovery of hepcidin, multiple studies showed that excess iron undergoes biliary excretion. We recently reported that wild-type mice raised on an iron-rich diet have increased bile levels of iron and ferritin, a multi-subunit iron storage protein. Given that genetic defects leading to excessive iron absorption are much more common causes of iron excess than dietary loading, we set out to determine if an inherited form of iron excess known as hereditary hemochromatosis also results in bile iron loading. We employed mice deficient in hemojuvelin, a protein essential for hepcidin expression. Mutant mice developed bile iron and ferritin excess. While lysosomal exocytosis has been implicated in ferritin export into bile, knockdown of Tfeb, a regulator of lysosomal biogenesis and function, did not impact bile iron or ferritin levels. Bile proteomes differed between female and male mice for wild-type and hemojuvelin-deficient mice, suggesting sex and iron excess impact bile protein content. Overall, our findings support the notion that excess iron undergoes biliary excretion in genetically determined iron excess.

Biliary excretion of excess iron in mice requires hepatocyte iron import by Slc39a14

Iron is essential for erythropoiesis and other biological processes, but is toxic in excess. Dietary absorption of iron is a highly regulated process and is a major determinant of body iron levels. Iron excretion, however, is considered a passive, unregulated process, and the underlying pathways are unknown. Here we investigated the role of metal transporters SLC39A14 and SLC30A10 in biliary iron excretion. While SLC39A14 imports manganese into the liver and other organs under physiological conditions, it imports iron under conditions of iron excess. SLC30A10 exports manganese from hepatocytes into the bile. We hypothesized that biliary excretion of excess iron would be impaired by SLC39A14 and SLC30A10 deficiency. We therefore analyzed biliary iron excretion in Slc39a14-and Slc30a10-deficient mice raised on iron-sufficient and -rich diets. Bile was collected surgically from the mice, then analyzed with nonheme iron assays, mass spectrometry, ELISAs, and an electrophoretic assay for iron-loaded ferritin. Our results support a model in which biliary excretion of excess iron requires iron import into hepatocytes by SLC39A14, followed by iron export into the bile predominantly as ferritin, with iron export occurring independently of SLC30A10. To our knowledge, this is the first report of a molecular determinant of mammalian iron excretion and can serve as basis for future investigations into mechanisms of iron excretion and relevance to iron homeostasis.

Transfusional iron overload and intravenous iron infusions modify the mouse gut microbiota similarly to dietary iron

Iron is essential for both microorganisms and their hosts. Although effects of dietary iron on gut microbiota have been described, the effect of systemic iron administration has yet to be explored. Here, we show that dietary iron, intravenous iron administration, and chronic transfusion in mice increase the availability of iron in the gut. These iron interventions have consistent and reproducible effects on the murine gut microbiota; specifically, relative abundance of the Parabacteroides and Lactobacillus genera negatively correlate with increased iron stores, whereas members of the Clostridia class positively correlate with iron stores regardless of the route of iron administration. Iron levels also affected microbial metabolites, in general, and indoles, in particular, circulating in host plasma and in stool pellets. Taken together, these results suggest that by shifting the balance of the microbiota, clinical interventions that affect iron status have the potential to alter biologically relevant microbial metabolites in the host.

Gastrointestinal iron excretion and reversal of iron excess in a mouse model of inherited iron excess

The current paradigm in the field of mammalian iron biology states that body iron levels are determined by dietary iron absorption, not by iron excretion. Iron absorption is a highly regulated process influenced by iron levels and other factors. Iron excretion is believed to occur at a basal rate irrespective of iron levels and is associated with processes such as turnover of intestinal epithelium, blood loss, and exfoliation of dead skin. Here we explore iron excretion in a mouse model of iron excess due to inherited transferrin deficiency. Iron excess in this model is attributed to impaired regulation of iron absorption leading to excessive dietary iron uptake. Pharmacological correction of transferrin deficiency not only normalized iron absorption rates and halted progression of iron excess but also reversed body iron excess. Transferrin treatment did not alter the half-life of ⁵⁹Fe in mutant mice. ⁵⁹Fe-based studies indicated that most iron was excreted via the gastrointestinal tract and suggested that iron-loaded mutant mice had increased rates of iron excretion. Direct measurement of urinary iron levels agreed with ⁵⁹Fe-based predictions that urinary iron levels were increased in untreated mutant mice. Fecal ferritin levels were also increased in mutant mice relative to wild-type mice. Overall, these data suggest that mice have a significant capacity for iron excretion. We propose that further investigation into iron excretion is warranted in this and other models of perturbed iron homeostasis, as pharmacological targeting of iron excretion may represent a novel means of treatment for diseases of iron excess.

Physiology and pathophysiology of iron in hemoglobin-associated diseases

Iron overload and iron toxicity, whether because of increased absorption or iron loading from repeated transfusions, can be major causes of morbidity and mortality in a number of chronic anemias. Significant advances have been made in our understanding of iron homeostasis over the past decade. At the same time, advances in magnetic resonance imaging have allowed clinicians to monitor and quantify iron concentrations noninvasively in specific organs. Furthermore, effective iron chelators are now available, including preparations that can be taken orally. This has resulted in substantial improvement in mortality and morbidity for patients with severe chronic iron overload. This paper reviews the key points of iron homeostasis and attempts to place clinical observations in patients with transfusional iron overload in context with the current understanding of iron homeostasis in humans.

Iron distribution in the liver and duodenum during seasonal iron overload in Svalbard reindeer

Seasonal iron overload in Svalbard reindeer was studied by light and electron microscopy and by X-ray microanalysis. The hepatic iron overload was of two types. The first type was characterized by massive siderosis of both parenchymal and non-parenchymal cells caused by a diet very rich in iron but low in energy and protein. Hepatocytes contained a moderate amount of free ferritin particles in the cytosol together with numerous siderosomes. This pattern is similar to that seen in primary haemochromatosis and thalassaemia. Kupffer cells contained large quantities of cytosolic ferritin, siderosomes and lysosomes with disintegrating red blood cells as seen in thalassaemia. The second type was characterized by massive non-parenchymal siderosis caused by an energy- and protein-poor diet with normal iron concentration. Hepatocytes contained little cytosolic ferritin and few siderosomes, but there were abundant electron-dense bodies without iron (i.e., autophagosomes). Kupffer cells were as described above. Ferritin was also present within the duodenal mucosa of these animals, located within enterocytes and lamina propria macrophages, as well as in the extracellular space and capillary and lacteal lumina. Ferritin was also present in the acinar cells of submucosal Brunner's glands. Changes consistent with exchange of ferritin particles between different cell types were observed. The role of ferritin as a possible iron transporter in this condition is discussed.

Iron excretion in iron-overloaded rats following the change from an iron-loaded to an iron-deficient diet

BACKGROUND

Iron stores in the body are thought to be regulated by a mechanism associated with the rate of iron absorption from the diet, with no significant role played by iron excretion. We report the existence of an iron excretory process that results in the loss of significant amounts of liver iron.

METHODS AND RESULTS

Rats were fed 3% carbonyl iron for 9 weeks, which resulted in a 20-fold increase in liver non-haem iron. When the rats on this iron-loaded diet were switched to a low iron diet for 2 and 7 days, liver non-haem iron levels fell 30% and 45%, respectively. A similar fall in transferrin-bound plasma iron was also seen. As the liver iron had not redistributed to other body compartments, it was concluded that the iron had been excreted and that the excreted iron represented a loss of 22% and 28% in total body non-haem iron over 2 and 7 days, respectively. Ligation of the common bile duct in iron loaded rats that had been switched to the iron-deficient diet was accompanied by a similar loss of liver iron and also hepatocellular damage. In addition, measurement of enterocyte iron levels showed that only approximately 5% of the total iron excreted was found in these cells.

CONCLUSION

Neither bile nor enterocytes play a significant role in iron excretion. The similarity in the degree of fall in transferrin-bound iron levels with a change in diet suggests that iron excretion involves the uptake and excretion of transferrin bound-iron, possibly by goblet cells. The observed hypertrophy of the intestinal mucosa associated with carbonyl iron feeding may facilitate hypersecretion of mucous and the excretion of this iron.

Effect of bile acids on lipid peroxidation: the role of iron

The toxic effect of hydrophobic bile acids is claimed to be in part mediated by lipid peroxidation. Conversely, antioxidant properties of tauroursodeoxycholic acid (TUDC), a hydrophilic bile acid, have been suggested as a possible mechanism by which TUDC confers its beneficial effect in a variety of diseases. We have investigated the effect of taurodeoxycholic acid (TDC), a hydrophobic bile acid and TUDC on lipid peroxidation using a pure lipid system both in the presence and absence of iron ions. Neither TDC nor TUDC showed any effect on spontaneous lipid peroxidation of phosphatidylcholine liposomes or sodium arachidonate solution. This lack of effect excludes the possibility of direct prooxidant or antioxidant properties for TDC and TUDC. Addition of ferrous ions (0.1 mM) to the lipid system brought about a linear increase in lipid peroxidation with time. The presence of TDC caused an increase in the rate and extent of iron-stimulated lipid peroxidation. The propensity of bile acids to increase iron-induced lipid peroxidation was related to hydrophobicity of the individual bile acids, with the highest effect observed with taurolithocholic acid, whereas TUDC did not have any influence. The TDC-induced increase in the iron-stimulated lipid peroxidation was concentration dependent. Addition of TUDC (10 mM) completely abolished the effect of TDC (2 mM) on iron-induced lipid peroxidation. This finding suggests that TUDC does not function as an antioxidant per se but may prevent lipid peroxidation caused by TDC. In conclusion, only in the presence of iron ions, hydrophobic bile acids may enhance lipid peroxidation. TUDC has no antioxidant activity per se but may counter the TDC-induced increase in iron-stimulated lipid peroxidation.

Ultrastructural sequences during liver iron overload in genetic hemochromatosis

BACKGROUND/AIMS

The pathway through which iron contributes to liver cell damage and cirrhosis in genetic hemochromatosis is not clear. The objective of the present study was to describe the ultrastructural changes in liver biopsies of patients in various stages of this condition and to correlate these with clinical, histopathological and biochemical data.

METHODS

Liver biopsies from 20 patients with genetic hemochromatosis were examined by transmission electron microscopy. The use of unstained thin (60 nm) sections facilitated the identification and localization of the electron-opaque compounds ferritin and hemosiderin in various liver cells. Stained thin sections permitted evaluation of the concomitant subcellular damage and collagen deposition. The ultrastructural observations were corroborated with the histopathological findings and biochemical data.

RESULTS

All patients had liver iron overload, which was classified as mild, moderate or severe. In the stage of mild overload (hepatic iron concentration HIC 93.5+/-23.3 micromol/g and hepatic iron index HII 2.3+/-0.7), cytosolic ferritin and scarce pericanalicular lysosomes (siderosomes) were seen in periportal hepatocytes (acinar zone 1), without evidence of organelle damage, and in the absence of sinusoidal cell siderosis. In moderate overload (HIC 190.8+/-41.5 micromol/g and HII 4.3+/-1.9), ferritin was identified in hepatocytes of all acinar zones, and the pericanalicular siderosomes were abundant, especially in acinar zone 1. Single hepatocytes showed organelle damage and occasional sinusoidal cells showed siderosis. In severe overload (HIC 308+/-49.0 micromol/g and HII 7.5+/-1.7), hepatocytes of all acinar zones were filled with large, hemosiderin-containing siderosomes, and changes in mitochondria, smooth and rough endoplasmic reticulum and nuclei were conspicuous. Marked sinusoidal cell siderosis and collagen deposition were observed predominantly in this stage.

CONCLUSIONS

Electron microscopy has shown that during the long, latent stage of "compensated" genetic hemochromatosis, hepatocytes display only minimal subcellular changes, other than iron overload. "Decompensated" overload, characterized by extensive subcellular pathology and focal necrosis, is reached when 1) the hepatocytic siderosis is generalized (i.e. beyond pericanalicular polarization of siderosomes in hepatocytes, and beyond zone 1 in the acinus); and 2) there is evidence of massive siderosis of sinusoidal cells. These findings support the concept of a critical level of hepatic iron concentration beyond which organelle damage is conspicuous and liver cell injury may become irreversible.

Abnormal bile acid metabolism and neonatal hemochromatosis: a subset with poor prognosis

BACKGROUND

Inborn errors of bile acid synthesis are newly recognized disorders that may cause the phenotypic appearance of neonatal hepatitis or neonatal cholestasis.

METHODS

This is a clinicopathologic study of two sets of siblings with cholestatic neonatal liver failure.

RESULTS

In 3 of the infants, diagnostic evaluation, including analysis of urinary bile salts, revealed a predominance of 7 alpha-hydroxy-3-oxo-4-cholenoic and 7 alpha, 12 alpha-dihydroxy-3-oxo-4-cholenoic acids, a pattern consistent with delta 4-3-oxosteroid 5 beta-reductase deficiency, which could be primary or secondary. The fourth infant died before such testing could be carried out. In addition, all 4 infants had histologically disseminated hemochromatosis and met diagnostic criteria for neonatal hemochromatosis. In the 3 infants studied, histologic examination of the liver disclosed giant cell hepatitis with extensive loss of hepatic parenchyma and rapid progression to cirrhosis. Early treatment with ursodeoxycholic acid and cholic acid, previously reported as effective therapy, was given to 2 siblings; it failed to reverse or halt the liver damage, and both infants died. One infant, with the original diagnosis of neonatal hemochromatosis, was treated with a variety of antioxidants and chelation therapy, as recently reported. No improvement was demonstrated, and he went on to liver transplantation.

CONCLUSIONS

The presentation of delta 4-3-oxosteroid 5 beta-reductase deficiency as neonatal hemochromatosis may represent a distinct subset of this disorder with an accelerated course, no response to therapy and poor prognosis.

Spontaneous iron overload in Sprague-Dawley rats

In a review of the toxicological studies performed in our laboratory during the period 1986-1995, we occasionally observed significant iron overloading in the liver. Liver tissue was examined by light and electron microscopy, and the results were analyzed by sex and age (7, 9, 11, 19, 31, 59, and 111 wk). The intensity of iron overload increased with age: the accumulation began in pericanalicular siderosomes of periportal hepatocytes and extended progressively to the entire lobule and also to nonhepatocytic cells (Kupffer cells in sinusoids and macrophages around bile ducts in portal tracts) and occasionally with distortion of sinusoids by sideroblastic nodules and moderate enlargement of portal tracts in the oldest animals. No significant inflammatory infiltrates, degeneration, necrosis or fibrosis were noted. Hepatocyte pigmentation alone was prominent at 9 and 11 wk. The frequency of pigmentation of parenchymal and nonhepatocytic cells increased from 9 wk for females; in males, this was seen only at 111 wk. The intensity of pigmentation of nonhepatocytic cells versus parenchymal cells increased with aging. The frequency of those different types of iron overloading was higher for females up to 111 wk. The pathology of spontaneous iron overloading in the Sprague-Dawley rat, described here in spite of differences, has some similarities to that of human hereditary hemochromatosis.

Distribution of iron during embryogenesis and early larval life in sea lampreys (Petromyzon marinus)

The distribution of ferric iron was examined in eggs, embryos, and early larvae (up to 77 d post fertilization (PF)) of the sea lamprey, Petromyzon marinus, using the Prussian blue staining technique. Iron was not detected in eggs and embryos but was detected in the lumen of the oesophagus at 17 – 33 days PF, when the newly hatched larvae burrow and commence exogenous feeding. The posterior intestine and hindgut became the primary location of iron absorption by 36 days PF, and subsequent development (36 – 63 days PF) showed an iron distribution suggesting elimination of excess metal through exfoliation of epithelial cells of the posterior intestine. The deposition of iron in macrophages of the intestine and liver (Kupffer cells) by 40 days PF may be related to both erythrophagocytosis and erythropoiesis at this time. Iron deposits in the macrophages of the pronephros and the atrium of the heart at 42 – 44 days PF were likely a consequence of endocytosis/filtration of the metal from circulating plasma. The commencement of iron deposition in specific tissues of larval lampreys seems to be correlated with the time they begin filter-feeding. Macrophages play an important role in iron metabolism in early larval life of lampreys.

The hypotransferrinaemic mouse: ultrastructural and laser microprobe analysis observations

Homozygote hypotransferrinaemic mice (hpx/hpx) have cytopathological features similar to those of human congenital atransferrinaemia, genetic haemochromatosis, and neonatal haemochromatosis. These conditions all have in common high levels of cytotoxic non-transferrin-bound serum iron. This study describes the ultrastructural features of iron overload in liver, pancreas, heart, and small intestine of 2- and 12-month-old hypotransferrinaemic mice. Electron microscopic studies of unstained sections showed early parenchymal cell siderosis, with accumulation of numerous ferritin particles and clusters in the cytosol, as well as ferritin and haemosiderin in lysosomes (siderosomes). In the 12-month-old animals, iron was also found in Kupffer cells and macrophages in other tissues. In addition, there were conspicuous iron-containing compounds in the bile canaliculi, and marked iron deposition in the pancreas and heart. Laser microprobe mass analysis (LAMMA) enabled localization and relative quantitation of iron deposition in subcellular compartments providing in situ documentation of iron accumulation in siderosomes and contributed in assessing total cytosolic iron in various cell types. Moreover, it demonstrated the importance and magnitude of the biliary route for iron excretion in these animals.

Delta 4-3-oxosteroid 5 beta-reductase deficiency causing neonatal liver failure and hemochromatosis

Neonatal liver failure was evaluated in two infants. Neither infant had evidence of congenital infection, galactosemia, alpha 1-antitrypsin deficiency, tyrosinemia, Zellweger syndrome, or hemophagocytic lymphohistiocytosis. Abnormal levels of iron were detected in the minor salivary glands of the first infant and in the explanted liver of the second. Analyses of urinary bile salts by fast-atom bombardment ionization mass spectrometry and gas chromatography-mass spectrometry revealed a paucity of primary bile acids and a predominance of 7 alpha-hydroxy-3-oxo-4-cholenoic and 7 alpha,12 alpha-dihydroxy-3-oxo-4-cholenoic acids. These findings are consistent with delta 4-3-oxosteroid 5 beta-reductase deficiency, a primary genetic defect in bile acid synthesis. Postmortem evaluation of the first infant revealed significant iron deposition in the liver, pancreas, thyroid, adrenal glands, myocardium, stomach, and submucosal glands of the respiratory tract. In both infants examination of the liver revealed extensive loss of hepatic parenchyma. These cases expand the clinical spectrum of bile acid metabolism defects to include neonatal liver failure with associated hemochromatosis.

Acute infusions of bile salts increase biliary excretion of iron in iron-loaded rats

The mechanisms of biliary excretion of iron are not well known. The aim of this study was to examine the effect of choleresis induced by several agents on biliary iron excretion in iron-loaded rats. Iron overload was obtained with a diet supplemented by 3% iron carbonyl during a 6-week period. Bile was collected with an external bile fistula. Biliary iron concentration was measured by atomic absorption spectrophotometry, and hepatic iron concentration was measured by a chemical method. Compared with controls, iron overload resulted in a 14-fold increase in hepatic iron concentration but only a 3.9-fold increase in biliary iron output. In iron-loaded rats, taurocholate infusion caused a 1.8-fold significant increase in biliary iron output. Dehydrocholate, given at the same dose, induced a significant but less pronounced (1.3-fold) increase in biliary iron output in spite of a higher bile flow. Taurochenodeoxycholate, tauroursodeoxycholate, and tauro-7-ketolithocholate induced an increase in biliary iron output similar to that observed with taurocholate. The canalicular bile salt-independent choleretic dihydroxydibutyl ether caused a significant but less pronounced increase in biliary iron output (1.4-fold). These results confirm that in iron-loaded rats biliary iron excretion is increased much less than hepatic iron concentration. They show that in iron loaded rats (a) bile salts can increase biliary iron secretion, and (b) this increase is related in part to choleresis and in part to bile salts themselves. This increase may be related to an interaction of iron with bile salt monomers and/or micelles.

Alterations in the structure, physicochemical properties, and pH of hepatocyte lysosomes in experimental iron overload

While hemochromatosis is characterized by sequestration of iron-protein complexes in hepatocyte lysosomes, little is known about the effects of excess iron on these organelles. Therefore, we studied the effects of experimental iron overload on hepatocyte lysosomal structure, physicochemical properties, and function in rats fed carbonyl iron. A sixfold increase (P less than 0.0001) in hepatic iron and a fivefold increase in lysosomal iron (P less than 0.01) was observed after iron loading; as a result, hepatocyte lysosomes became enlarged and misshapen. These lysosomes displayed increased (P less than 0.0001) fragility; moreover, the fluidity of lysosomal membranes isolated from livers of iron-loaded rats was decreased (P less than 0.0003) as measured by fluorescence polarization. Malondialdehyde, an end product of lipid peroxidation, was increased by 73% (P less than 0.008) in lysosomal membranes isolated from livers of iron-overloaded rats. While amounts of several individual fatty acids in isolated lysosomal membranes were altered after iron overload, cholesterol/phospholipid ratios, lipid/protein ratios, double-bond index, and total saturated and unsaturated fatty acids remained unchanged. The pH of lysosomes in hepatocytes isolated from livers of iron-loaded rats and measured by digitized video microscopy was increased (control, 4.70 +/- 0.05; iron overload, 5.21 +/- 0.10; P less than 0.01). Our results demonstrate that experimental iron overload causes marked alterations in hepatocyte lysosomal morphology, an increase in lysosomal membrane fragility, a decrease in lysosomal membrane fluidity, and an increase in intralysosomal pH. Iron-catalyzed lipid peroxidation is likely the mechanism of these structural, physicochemical, and functional disturbances.

Mechanisms of metal transport across liver cell plasma membranes

The liver's pivotal role in the homeostasis of essential trace metals and detoxification of exogenous metals is attributed to its ability to efficiently extract metals from plasma, metabolize, store, and redistribute them in various forms either into bile or back into the bloodstream. Bidirectional transport across the sinusoidal plasma membrane allows the liver to control plasma concentrations and therefore availability to other tissues. In contrast, transport across the canalicular membrane is largely, but not exclusively, unidirectional and is a major excretory pathway. Although each metal has relatively distinct hepatic transport characteristics, some generalizations can be made. First, movement of metals from plasma to bile follows primarily a transcellular route. The roles of the paracellular pathway and of ductular secretion appear minimal. Second, intracellular binding proteins and in particular metallothionein play only indirect roles in transmembrane flux. The amounts of metallothionein normally secreted into plasma and bile are quite small and cannot account for total metal efflux. Third, metals traverse liver cell plasma membranes largely by facilitated diffusion, and by fluid-phase, adsorptive, and receptor-mediated endocytosis/exocytosis. There is currently no evidence for primary active transport. Because of the high rate of hepatocellular membrane turnover, metal transport via endocytic vesicles probably makes a larger contribution than previously recognized. Finally, there is significant overlap in substrate specificity on the putative membrane carriers for the essential trace metals. For example, zinc and copper share many transport characteristics and apparently compete for at least one common transport pathway. Similarly, canalicular transport of five of the metals discussed in this overview (Cu, Zn, Cd, Hg, and Pb) is linked to biliary GSH excretion. These metals may be transported as GSH complexes by the canalicular glutathione transport system(s). Unfortunately, none of the putative membrane carrier proteins have been studied at the subcellular or molecular level. Our knowledge of their biochemical properties is rudimentary and rests almost entirely on indirect evidence obtained in vivo or in intact cell systems. The challenge for the future is to isolate and characterize these putative metal carriers, and to determine how they are functionally regulated.

Biliary excretion of iron in healthy man and in patients with alcoholic cirrhosis of the liver

We measured the biliary excretion of iron in 11 patients with alcoholic cirrhosis of the liver and in 10 healthy controls using an intestinal perfusion technique. In the patients with cirrhosis increased amounts of iron in liver tissue were present. The concentrations of iron in the bile samples were determined by atomic absorption spectrometry. The biliary excretion of iron in the healthy controls was 0.32 +/- 0.09 mumol/h and in the patients with cirrhosis it was 0.45 +/- 0.14 mumol/h. The biliary excretion of iron in the patients with cirrhosis was not reduced, indicating that other mechanisms than a reduced biliary excretion of iron must be responsible for the accumulation of iron in liver tissue in alcoholic cirrhosis.

Liver cell damage and lysosomal iron storage in patients with idiopathic hemochromatosis. A light and electron microscopic study

Eleven patients with idiopathic hemochromatosis were subjected to percutaneous liver biopsy, and seven were rebiopsied after repeated phlebotomies. The liver tissue was examined by light and electron microscopy. Ultrastructural morphometry was also performed and iron content was determined. Initially, all biopsies displayed large iron-laden lysosomes in the hepatocytes. Lysosomal volume density was increased and correlated well with liver iron content (r = 0.84, p less than 0.005). Neither mitochondria nor the endoplasmic reticulum showed any ultrastructural changes, except in the necrotic cells of biopsies with the highest liver iron content. In these cases, iron-laden lysosomes were also encountered in the Kupffer cells. Following treatment, liver iron content and lysosomal volume density were normalized. More specifically, iron content was 14.1 +/- 2.1 micrograms Fe/mg protein before and 1.3 +/- 0.3 micrograms Fe/mg protein after treatment (p less than 0.001). Lysosomal volume density was 6.1 +/- 0.8% before and 1.8 +/- 0.2% after treatment (p less than 0.001). Hence, in the precirrhotic stage of idiopathic hemochromatosis, the first evident ultrastructural changes are in the lysosomal compartment. These changes correlate well with the iron overload, also in advanced stages of the disease, and are reversed after iron removal.

Mechanisms of transport of nontransferrin-bound iron in basolateral and canalicular rat liver plasma membrane vesicles

Although most iron in plasma is bound to transferrin, recent evidence suggests that the nontransferrin-bound fraction contributes to hepatic iron loading and toxicity seen in iron-overload disorders. Our studies of isolated perfused rat liver previously demonstrated saturable uptake of nontransferrin-bound iron that continues despite hepatic iron overload. To further characterize the mechanism of transport of this form of iron, we measured binding of 55Fe-labeled ferrous ascorbate to rat liver plasma membrane vesicles under varying conditions. Binding of 5 mumol/L iron by both basolateral and canalicular membranes was time-dependent and linear for the first 5 sec. Initial rate of binding of ferrous ascorbate to basolateral membrane vesicles was temperature dependent and increased by calcium but, in contrast to the perfused rat liver, was not inhibited by other divalent cations. Binding velocities by basolateral membrane vesicles were saturable at increasing iron concentration (Km = 33 mumol/L, Vmax = 16 pmol/mg protein/sec). Ferrous iron binding by canalicular membrane vesicles was also temperature dependent, but initial association rates were not saturable over the concentration range studied (2 to 20 mumol/L). We conclude that nontransferrin-bound iron associates with basolateral liver plasma membrane vesicles by a saturable mechanism sensitive to temperature and calcium and consistent with a membrane carrier. Other divalent cations do not inhibit membrane association but may compete for a subsequent cytosolic binding site.

Transferrin receptor expression in rat liver: immunohistochemical and biochemical analysis of the effect of age and iron storage

Hepatic transferrin receptors were studied in normal male rats at 1 to 59 wk after weaning, using immunohistochemical and biochemical techniques. The number of transferrin receptors measured and the intensity of the staining in situ decreased rapidly during the first 10 wk of life and more slowly thereafter. Immunohistochemistry further demonstrated changes in the topographical and (sub)cellular localization of the transferrin receptor. In the young rat livers, staining was almost exclusively present on hepatocytes in acinar zone 2 + 3 in a honeycomb to sinusoidal pattern. With aging, a panacinar heterogeneous and mainly sinusoidal staining of hepatocytes was more frequent. Kupffer cell positivity was more obvious as compared with the young rat livers. The observed changes in transferrin receptor expression may partly be explained by age-dependent alterations in DNA synthesis and proliferative potential of the liver cells. A series of rats were iron loaded with carbonyl iron up to 39 wk and "unloaded" by administration of a normal diet during 20 wk. In these animals, serial histochemical studies showed predominantly parenchymal (7 to 14 wk), mixed parenchymal and reticuloendothelial (39 wk) and almost exclusive reticuloendothelial siderosis (59 wk). In the siderotic livers transferrin receptor numbers tended to be lower than in the controls with significant differences after 14 and 39 wk. Immunohistochemistry showed decreased parenchymal but increased reticuloendothelial transferrin receptor expression with iron load. After the period of unloading, parenchymal transferrin receptors were virtually absent despite the negligible siderosis of these cells. In contrast, siderotic reticuloendothelial cells were intensely positive. These findings support down-regulation of parenchymal transferrin receptor resulting from iron storage. However, the positivity of siderotic reticuloendothelial cells and the absence of re-emergence of parenchymal receptors in conditions of minimal parenchymal and prominent reticuloendothelial siderosis need further elucidation.

Aetiology and pathophysiology of chronic liver disorders

The aetiology of chronic liver disease covers a wide range of congenital or acquired abnormalities of the hepatocellular biochemical network. Although our knowledge has considerably increased in recent years, the aetiology of chronic liver disease often remains obscure. Acquired irreversible disturbances of normal liver function can be mediated by hepatotrophic viruses, chemicals, chronic oxygen depletion, or interference with the immune system. Considerable progress has been made in the detection and characterisation of hepatitis B, C, and D viruses as causative agents of chronic active hepatitis. Alcohol abuse remains the predominant cause of chronic liver disease in the Western world. The targets of autoantibodies used to diagnose autoimmune diseases of the liver and primary biliary cirrhosis continue to be biochemically defined. Their significance for the aetiology of the disease, however, remains to be established. Nonparenchymal cells play an important role in the sequence of events following hepatocellular injury and ultimately leading to liver cirrhosis. They release vasoactive compounds, cytokines, and other important mediators, and participate in the modulation of the extracellular matrix that is characteristic of liver fibrosis and cirrhosis. The biochemical basis of liver cell necrosis remains poorly defined. In spite of recent progress, and the detection of some new pathogenic principles that help in the understanding of the complications of chronic liver disease such as portal hypertension, oesophagogastric variceal bleeding, portosystemic encephalopathy, ascites, and other metabolic disturbances, many questions concerning the aetiology and pathophysiology of chronic liver disease and its complications remain to be answered.