Expression of ferritin receptor in placental microvilli membrane in pregnant women with different iron status at mid-term gestation

The interplay between maternal-infant anemia and iron deficiency

Iron deficiency anemia in pregnancy is a major public health problem known to cause maternal morbidity and adverse birth outcomes, and it may also have lasting consequences on infant development. However, the impact of the maternal hematological environment on fetal and infant hemoglobin and iron stores in the first year of life remains unclear. This review of the epidemiological evidence found that severe maternal iron deficiency anemia in pregnancy is associated with lower ferritin, and to a lesser degree hemoglobin levels, in infants at birth. Emerging data also suggests that severe anemia in pregnancy increases the risk of iron deficiency and anemia in infants 6-12 months of age, although longitudinal studies are limited. Effective anemia prevention in pregnancy, such as iron supplementation, could reduce the risk of infant anemia and iron deficiency during the first year of life; however, more evidence is needed to determine the functional impact of iron supplementation in pregnancy on infant hematological indices.

The placenta: the forgotten essential organ of iron transport

Optimal iron nutrition in utero is essential for development of the fetus and helps establish birth iron stores adequate to sustain growth in early infancy. In species with hemochorial placentas, such as humans and rodents, iron in the maternal circulation is transferred to the fetus by directly contacting placental syncytiotrophoblasts. Early kinetic studies provided valuable data on the initial uptake of maternal transferrin, an iron-binding protein, by the placenta. However, the remaining steps of iron trafficking across syncytiotrophoblasts and through the fetal endothelium into the fetal blood remain poorly characterized. Over the last 20 years, identification of transmembrane iron transporters and the iron regulatory hormone hepcidin has greatly expanded the knowledge of cellular iron transport and its regulation by systemic iron status. In addition, emerging human and animal data demonstrating comprised fetal iron stores in severe maternal iron deficiency challenge the classic dogma of exclusive fetal control over the transfer process and indicate that maternal and local signals may play a role in regulating this process. This review compiles current data on the kinetic, molecular, and regulatory aspects of placental iron transport and considers new questions and knowledge gaps raised by these advances.

Oxidative stress and the homeodynamics of iron metabolism

Iron and oxygen share a delicate partnership since both are indispensable for survival, but if the partnership becomes inadequate, this may rapidly terminate life. Virtually all cell components are directly or indirectly affected by cellular iron metabolism, which represents a complex, redox-based machinery that is controlled by, and essential to, metabolic requirements. Under conditions of increased oxidative stress—i.e., enhanced formation of reactive oxygen species (ROS)—however, this machinery may turn into a potential threat, the continued requirement for iron promoting adverse reactions such as the iron/H2O2-based formation of hydroxyl radicals, which exacerbate the initial pro-oxidant condition. This review will discuss the multifaceted homeodynamics of cellular iron management under normal conditions as well as in the context of oxidative stress.

Maternal serum ferritin concentration is positively associated with newborn iron stores in women with low ferritin status in late pregnancy

Iron deficiency (ID) is common in pregnant women and infants, particularly in developing countries. The relation between maternal and neonatal iron status remains unclear. This study considered the issue in a large sample of mother-newborn pairs in rural southeastern China. Hemoglobin (Hb) and serum ferritin (SF) were measured in 3702 pregnant women at ≥37 wk gestation and in cord blood of their infants born at term (37-42 wk gestation). Maternal anemia (Hb <110 g/L) was present in 27.5% and associated with maternal SF <20 μg/L in 86.9%. Only 5.6% of neonates were anemic (Hb <130 g/L) and 9.5% had cord-blood SF <75 μg/L. There were low-order correlations between maternal and newborn iron measures (r = 0.07-0.10 for both Hb and SF; P ≤ 0.0001 due to the large number). We excluded 430 neonates with suggestion of inflammation [cord SF >370 μg/L, n = 208 and/or C-reactive protein (CRP) >5 mg/L, n = 233]. Piecewise linear regression analyses identified a threshold for maternal SF at which cord-blood SF was affected. For maternal SF below the threshold of 13.6 μg/L (β = 2.4; P = 0.001), cord SF was 0.17 SD lower than in neonates whose mothers had SF above the threshold (167 ± 75 vs. 179 ± 80 μg/L). The study confirmed that ID anemia remains common during pregnancy in rural southeastern China. Despite widespread maternal ID, however, iron nutrition seemed to meet fetal needs except when mothers were very iron deficient. The impact of somewhat lower cord SF on iron status later in infancy warrants further study.

Serum ferritin: Past, present and future

BACKGROUND

Serum ferritin was discovered in the 1930s, and was developed as a clinical test in the 1970s. Many diseases are associated with iron overload or iron deficiency. Serum ferritin is widely used in diagnosing and monitoring these diseases.

SCOPE OF REVIEW

In this chapter, we discuss the role of serum ferritin in physiological and pathological processes and its use as a clinical tool.

MAJOR CONCLUSIONS

Although many aspects of the fundamental biology of serum ferritin remain surprisingly unclear, a growing number of roles have been attributed to extracellular ferritin, including newly described roles in iron delivery, angiogenesis, inflammation, immunity, signaling and cancer.

GENERAL SIGNIFICANCE

Serum ferritin remains a clinically useful tool. Further studies on the biology of this protein may provide new biological insights.

Binding and uptake of H-ferritin are mediated by human transferrin receptor-1

Ferritin is a spherical molecule composed of 24 subunits of two types, ferritin H chain (FHC) and ferritin L chain (FLC). Ferritin stores iron within cells, but it also circulates and binds specifically and saturably to a variety of cell types. For most cell types, this binding can be mediated by ferritin composed only of FHC (HFt) but not by ferritin composed only of FLC (LFt), indicating that binding of ferritin to cells is mediated by FHC but not FLC. By using expression cloning, we identified human transferrin receptor-1 (TfR1) as an important receptor for HFt with little or no binding to LFt. In vitro, HFt can be precipitated by soluble TfR1, showing that this interaction is not dependent on other proteins. Binding of HFt to TfR1 is partially inhibited by diferric transferrin, but it is hindered little, if at all, by HFE. After binding of HFt to TfR1 on the cell surface, HFt enters both endosomes and lysosomes. TfR1 accounts for most, if not all, of the binding of HFt to mitogen-activated T and B cells, circulating reticulocytes, and all cell lines that we have studied. The demonstration that TfR1 can bind HFt as well as Tf raises the possibility that this dual receptor function may coordinate the processing and use of iron by these iron-binding molecules.

Soybean ferritin: implications for iron status of vegetarians

Meeting the requirement for absorbed iron is difficult for vegetarians, and their iron status often is lower than that of nonvegetarians. Beans contain ferritin in low concentrations, but it is possible to enhance this content by plant breeding or by inserting the gene for ferritin into plants, eg, soybeans. Because each ferritin molecule can bind to thousands of iron atoms, this may be a sustainable means to increase the iron contents of plants. Before such efforts are launched, it is important to determine whether iron in ferritin is bioavailable. This has been assessed in vitro by using human intestinal (Caco-2) cells and in vivo by using radiolabeled ferritin and whole-body counting in human subjects. Dietary factors affecting iron absorption, eg, ascorbic acid, phytate, and calcium, had limited effect on iron uptake from intact ferritin by Caco-2 cells, which suggests that ferritin-bound iron is absorbed via a mechanism different from that of nonheme iron. In an in vitro digestion system, ferritin was shown to be relatively resistant to proteolytic enzymes. Binding of ferritin to Caco-2 cells was shown to be saturable, and the kinetics for binding were characteristic of a receptor-mediated process. In human subjects, iron from purified soybean ferritin given in a meal was as well absorbed as iron from ferrous sulfate. In conclusion, iron is well absorbed from ferritin and may represent a means of biofortification of staple foods such as soybeans.

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.

Receptor-mediated uptake of ferritin-bound iron by human intestinal Caco-2 cells

Ferritin (Ft) is a large iron (Fe)-binding protein ( approximately 450 kDa) that is found in plant and animal cells and can sequester up to 4500 Fe atoms per Ft molecule. Our previous studies on intestinal Caco-2 cells have shown that dietary factors affect the uptake of Fe from Ft in a manner different from that of Fe from FeSO4, suggesting a different mechanism for cellular uptake. The objective of this study was to determine the mechanism for Ft-Fe uptake using Caco-2 cells. Binding of (59)Fe-labeled Ft at 4 degrees C showed saturable kinetics, and Scatchard analysis resulted in a K(d) of 1.6 muM, strongly indicating a receptor-mediated process. Competitive binding studies with excess unlabelled Ft significantly reduced binding, and uptake studies at 37 degrees C showed saturation after 4 h. Enhancing and blocking endocytosis using Mas-7 (a G-protein activator) and hypertonic medium (0.5 M sucrose), respectively, demonstrated that Ft-Fe uptake by Mas-7-treated cells was 140% of control cells, whereas sucrose treatment resulted in a statistically significant reduction in Ft-Fe uptake by 70% as compared to controls. Inhibition of macropinocytosis with 5-(N,N-dimethyl)-amiloride (Na+/H+ antiport blocker) resulted in a decrease (by approximately 20%) in Ft-Fe uptake at high concentrations of Ft, suggesting that enterocytes can use more than one Ft uptake mechanism in a concentration-dependent manner. These results suggest that Ft uptake by enterocytes is carried out via endocytosis when Ft levels are within a physiological range, whereas Ft at higher concentrations may be absorbed using the additional mechanism of macropinocytosis.

Intracellular iron transport and storage: from molecular mechanisms to health implications

Maintenance of proper "labile iron" levels is a critical component in preserving homeostasis. Iron is a vital element that is a constituent of a number of important macromolecules, including those involved in energy production, respiration, DNA synthesis, and metabolism; however, excess "labile iron" is potentially detrimental to the cell or organism or both because of its propensity to participate in oxidation-reduction reactions that generate harmful free radicals. Because of this dual nature, elaborate systems tightly control the concentration of available iron. Perturbation of normal physiologic iron concentrations may be both a cause and a consequence of cellular damage and disease states. This review highlights the molecular mechanisms responsible for regulation of iron absorption, transport, and storage through the roles of key regulatory proteins, including ferroportin, hepcidin, ferritin, and frataxin. In addition, we present an overview of the relation between iron regulation and oxidative stress and we discuss the role of functional iron overload in the pathogenesis of hemochromatosis, neurodegeneration, and inflammation.

Caco-2 intestinal epithelial cells absorb soybean ferritin by mu2 (AP2)-dependent endocytosis

Iron deficiency, a condition currently affecting approximately 3 billion people, persists in the 21st century despite half a millennium of medical treatment. Soybean ferritin (SBFn), a large, stable protein nanocage around a mineral with hundreds of iron and oxygen atoms, is a source of nutritional iron with an unknown mechanism for intestinal absorption. Iron absorption from SBFn is insensitive to phytate, suggesting an absorption mechanism different from for the ferrous transport. Here, we investigated the mechanism of iron absorption from mineralized SBFn using Caco-2 cells (polarized in bicameral inserts) as an intestinal cell mode and analyzed binding, internalization and degradation with labeled SBFn ((131)I or fluorescent labels), confocal microscopy, and immunoanalyses to show: 1) saturable binding to the apical cell surface; dissociation constant of 7.75 +/- 0.88 nmol/L; 2) internalization of SBFn that was dependent on temperature, concentration, and time; 3) entrance of SBFn iron into the labile iron pool (calcein quenching); 4) degradation of the SBFn protein cage; and 5) assembly peptide 2 (AP2)-/clathrin-dependent endocytosis (sensitivity of SBFn uptake to hyperosmolarity, acidity, and RNA interference to the mu(2) subunit of AP2), and resistance to filipin, a caveolar endocytosis inhibitor. The results support a model of SBFn endocytosis through the apical cell membrane, followed by protein cage degradation, mineral reduction/dissolution, and iron entry to the cytosolic iron pool. The large number of iron atoms in SBFn makes iron transport across the cell membrane a much more efficient event for SBFn than for single iron atoms as heme or ferrous ions.

Effects of dietary factors on iron uptake from ferritin by Caco-2 cells

Biofortification of staple foods with iron (Fe) in the form of ferritin (Ft) is now possible, both by conventional plant breeding methods and transgenic approaches. Ft-Fe from plants and animals is absorbed well (25-30%) by human subjects, but little is known about dietary factors affecting its absorption. We used human intestinal Caco-2 cells and compared Fe absorption from animal Ft and FeSO4 to determine the effects of inhibitors and enhancers, such as phytic acid, ascorbic acid, tannic acid, calcium and heme. When postconfluent cells were coincubated with 59Fe-labeled (1 microM) FeSO4 and dietary factors, at different molar ratios of dietary factor to Fe (phytic acid:Fe, 10:1; ascorbic acid:Fe, 50:1; tannic acid:Fe, 50:1; calcium:Fe, 10:1 and hemin:Fe, 10:1), all inhibited uptake from FeSO4, except ascorbate, confirming earlier studies. In contrast, these dietary factors had little or no effect on Fe uptake from undigested Ft or Ft digested in vitro at pH 4, except tannins. However, results after in vitro digestion of Ft at pH 2 were similar to those obtained for FeSO4. These results suggest that Fe uptake occurs from both undigested as well as digested Ft but, possibly, via different mechanisms. The Fe-Ft stability shown here could minimize Fe-induced oxidation of Fe-supplemented food products.

TIM-2 is expressed on B cells and in liver and kidney and is a receptor for H-ferritin endocytosis

T cell immunoglobulin-domain and mucin-domain (TIM) proteins constitute a receptor family that was identified first on kidney and liver cells; recently it was also shown to be expressed on T cells. TIM-1 and -3 receptors denote different subsets of T cells and have distinct regulatory effects on T cell function. Ferritin is a spherical protein complex that is formed by 24 subunits of H- and L-ferritin. Ferritin stores iron atoms intracellularly, but it also circulates. H-ferritin, but not L-ferritin, shows saturable binding to subsets of human T and B cells, and its expression is increased in response to inflammation. We demonstrate that mouse TIM-2 is expressed on all splenic B cells, with increased levels on germinal center B cells. TIM-2 also is expressed in the liver, especially in bile duct epithelial cells, and in renal tubule cells. We further demonstrate that TIM-2 is a receptor for H-ferritin, but not for L-ferritin, and expression of TIM-2 permits the cellular uptake of H-ferritin into endosomes. This is the first identification of a receptor for ferritin and reveals a new role for TIM-2.

Diet-induced iron deficiency anemia and pregnancy outcome in rhesus monkeys

BACKGROUND

Iron deficiency anemia (IDA) is relatively common in the third trimester of pregnancy, but causal associations with low birth weight and compromised neonatal iron status are difficult to establish in human populations.

OBJECTIVE

The objective was to determine the effects of diet-induced IDA on intrauterine growth and neonatal iron status in an appropriate animal model for third-trimester IDA in women.

DESIGN

Hematologic and iron-status measures, pregnancy outcomes, and fetal and neonatal evaluations were compared between pregnant rhesus monkeys (n = 14) fed a diet containing 10 microg Fe/g diet from the time of pregnancy detection (gestation days 28-30) and controls (n = 24) fed 100 microg Fe/g diet.

RESULTS

By the third trimester, 79% of the iron-deprived dams and 29% of the control monkeys had a hemoglobin concentration <11 g/dL. There were also significant group differences in hematocrit, mean corpuscular volume, transferrin saturation, serum ferritin, and serum iron. At birth, the newborns of monkeys iron-deprived during pregnancy had significantly lower hemoglobin, mean corpuscular volume, and mean corpuscular hemoglobin values and a lower ratio of erythroid to total colony-forming units in bone marrow than did the control newborns. Pregnancy weight gain did not differ significantly between the iron-deprived and control dams, and the fetuses and newborns of the iron-deprived dams were not growth retarded relative to the controls. Gestation length, the number of stillbirths, and neonatal neurobehavioral test scores did not differ significantly by diet group.

CONCLUSION

These data indicate that an inadequate intake of iron from the diet during pregnancy in rhesus monkeys can lead to compromised hematologic status of the neonate without indications of growth retardation or impaired neurologic function at birth.

Maternal iron status influences iron transfer to the fetus during the third trimester of pregnancy

BACKGROUND

The effect of maternal iron status on fetal iron deposition is uncertain.

OBJECTIVE

We used a unique stable-isotope technique to assess iron transfer to the fetus in relation to maternal iron status.

DESIGN

The study group comprised 41 Peruvian women. Of these women, 26 received daily prenatal supplements containing iron and folate (n = 11; Fe group) or iron, folate, and zinc (n = 15; Fe+Zn group) from week 10-24 of pregnancy to 1 mo postpartum. The remaining 15 women (control group) received iron supplementation only during the final month of pregnancy. During the third trimester of pregnancy (+/- SD: 32.9 +/- 1.4 wk gestation) oral 57Fe (10 mg) and intravenous 58Fe (0.6 mg) stable iron isotopes were administered to the women, and isotope enrichment and iron-status indicators were measured in cord blood at delivery.

RESULTS

The net amount of 57Fe in the neonates' circulation (from maternal oral dosing) was significantly related to maternal iron absorption (P < 0.005) and inversely related to maternal iron status during the third trimester of pregnancy: serum ferritin (P < 0.0001), serum folate (P < 0.005), and serum transferrin receptors (P < 0.02). Significantly more 57Fe was transferred to the neonates in non-iron-supplemented women: 0.112 +/- 0.031 compared with 0.078 +/- 0.042 mg in the control group (n = 15) and the Fe and Fe+Zn groups (n = 24), respectively (P < 0.01). In contrast, 58Fe tracer in the neonates' circulation was not significantly related to maternal iron status.

CONCLUSION

The transfer of dietary iron to the fetus is regulated in response to maternal iron status at the level of the gut.