The effect of serum iron concentration on iron secretion into mouse milk

CREB-H is a stress-regulator of hepcidin gene expression during early postnatal development

Hepcidin, the hepatic iron hormone, is the central regulator of iron homeostasis. Cyclic AMP-Responsive Element-Binding protein 3-like 3 (CREB3L3/CREB-H) is a liver homeostatic regulator of essential nutrients (i.e. glucose and lipids) and has been previously involved in hepcidin response to pathologic stress signals. Here, we asked whether CREB-H has also a physiologic role in iron homeostasis through hepcidin. To this end, we analyzed hepcidin gene expression and regulation in the liver of wild type and Creb3l3 knockout mice during early postnatal development, as a model of "physiologic" stressful condition. The effect of iron challenge in vivo and BMP6 stimulation in vitro have been also addressed. In addition, we investigated the BMP signaling pathway and hepcidin promoter activity following CREB3L3 silencing and hepcidin promoter mutation in HepG2 cells. Creb3l3 knockout suckling and young-adult mice showed a prominent serum and hepatic iron accumulation, respectively, due to impaired hepcidin mRNA expression which progressively returned to normal level in adult mice. Interestingly, upon iron challenge, while the upstream BMP/SMAD signaling pathway controlling hepcidin was equally responsive in both strains, hepcidin gene expression was impaired in knockout mice and more iron accumulated in the liver. Accordingly, hepcidin gene response to BMP6 was blunted in primary CREB-H knockout hepatocytes and in HepG2 cells transfected with CREB-H siRNA or carrying a hepcidin promoter mutated in the CREB-H binding site. In conclusion, CREB-H has a role in maintaining the homeostatic balance of iron traffic through hepcidin during the critical postnatal period and in response to iron challenge. KEY MESSAGES: CREB-H KO mice develop liver iron overload shortly after weaning that normalizes in adulthood. CHEB-H is involved in hepcidin gene response to oral iron in vivo. CREB-H loss hampers hepcidin promoter response to BMP6. CREB-H is a key stress-sensor controlling hepcidin gene transcription in physiologic and pathophysiologic states.

Effects of iron deficiency and iron supplementation at the host-microbiota interface: Could a piglet model unravel complexities of the underlying mechanisms?

Iron deficiency is the most prevalent human micronutrient deficiency, disrupting the physiological development of millions of infants and children. Oral iron supplementation is used to address iron-deficiency anemia and reduce associated stunting but can promote infection risk since restriction of iron availability serves as an innate immune mechanism against invading pathogens. Raised iron availability is associated with an increase in enteric pathogens, especially Enterobacteriaceae species, accompanied by reductions in beneficial bacteria such as Bifidobacteria and lactobacilli and may skew the pattern of gut microbiota development. Since the gut microbiota is the primary driver of immune development, deviations from normal patterns of bacterial succession in early life can have long-term implications for immune functionality. There is a paucity of knowledge regarding how both iron deficiency and luminal iron availability affect gut microbiota development, or the subsequent impact on immunity, which are likely to be contributors to the increased risk of infection. Piglets are naturally iron deficient. This is largely due to their low iron endowments at birth (primarily due to large litter sizes), and their rapid growth combined with the low iron levels in sow milk. Thus, piglets consistently become iron deficient within days of birth which rapidly progresses to anemia in the absence of iron supplementation. Moreover, like humans, pigs are omnivorous and share many characteristics of human gut physiology, microbiota and immunity. In addition, their precocial nature permits early maternal separation, individual housing, and tight control of nutritional intake. Here, we highlight the advantages of piglets as valuable and highly relevant models for human infants in promoting understanding of how early iron status impacts physiological development. We also indicate how piglets offer potential to unravel the complexities of microbiota-immune responses during iron deficiency and in response to iron supplementation, and the link between these and increased risk of infectious disease.

Iron mobilization during lactation reduces oxygen stores in a diving mammal

The profound impacts that maternal provisioning of finite energy resources has on offspring survival have been extensively studied across mammals. This study shows that in addition to calories, high hemoprotein concentrations in diving mammals necessitates exceptional female-to-pup iron transfer. Numerous indices of iron mobilization (ferritin, serum iron, total-iron-binding-capacity, transferrin saturation) were significantly elevated during lactation in adult female Weddell seals (Leptonychotes weddellii), but not in skip-breeders. Iron was mobilized from endogenous stores for incorporation into the Weddell seal’s milk at concentrations up to 100× higher than terrestrial mammals. Such high rates of iron offload to offspring drew from the female’s own heme stores and led to compromised physiologic dive capacities (hemoglobin, myoglobin, and total body oxygen stores) after weaning their pups, which was further reflected in shorter dive durations. We demonstrate that lactational iron transfer shapes physiologic dive thresholds, identifying a cost of reproduction to a marine mammal.

Here, the authors show that Weddell seal mothers mobilize endogenous iron stores during lactation to provide to pups, resulting in iron concentrations in milk 100x higher than terrestrial mammals. This was associated with reduced dive durations in the mother, a cost of reproduction.

Modeling the dynamics of mouse iron body distribution: hepcidin is necessary but not sufficient

BACKGROUND

Iron is an essential element of most living organisms but is a dangerous substance when poorly liganded in solution. The hormone hepcidin regulates the export of iron from tissues to the plasma contributing to iron homeostasis and also restricting its availability to infectious agents. Disruption of iron regulation in mammals leads to disorders such as anemia and hemochromatosis, and contributes to the etiology of several other diseases such as cancer and neurodegenerative diseases. Here we test the hypothesis that hepcidin alone is able to regulate iron distribution in different dietary regimes in the mouse using a computational model of iron distribution calibrated with radioiron tracer data.

RESULTS

A model was developed and calibrated to the data from adequate iron diet, which was able to simulate the iron distribution under a low iron diet. However simulation of high iron diet shows considerable deviations from the experimental data. Namely the model predicts more iron in red blood cells and less iron in the liver than what was observed in experiments.

CONCLUSIONS

These results suggest that hepcidin alone is not sufficient to regulate iron homeostasis in high iron conditions and that other factors are important. The model was able to simulate anemia when hepcidin was increased but was unable to simulate hemochromatosis when hepcidin was suppressed, suggesting that in high iron conditions additional regulatory interactions are important.

Nutrient transport in the mammary gland: calcium, trace minerals and water soluble vitamins

Milk nutrients are secreted by epithelial cells in the alveoli of the mammary gland by several complex and highly coordinated systems. Many of these nutrients are transported from the blood to the milk via transcellular pathways that involve the concerted activity of transport proteins on the apical and basolateral membranes of mammary epithelial cells. In this review, we focus on transport mechanisms that contribute to the secretion of calcium, trace minerals and water soluble vitamins into milk with particular focus on the role of transporters of the SLC series as well as calcium transport proteins (ion channels and pumps). Numerous members of the SLC family are involved in the regulation of essential nutrients in the milk, such as the divalent metal transporter-1 (SLC11A2), ferroportin-1 (SLC40A1) and the copper transporter CTR1 (SLC31A1). A deeper understanding of the physiology and pathophysiology of these transporters will be of great value for drug discovery and treatment of breast diseases.

Mammary gland secretion: hormonal coordination of endocytosis and exocytosis

The mammary epithelium coordinates the uptake of milk precursors and the transport of milk components in order to produce milk of relatively constant composition at a particular stage of lactation, as long as the mammary gland is healthy. The mammary epithelial cell controls the uptake of blood-borne molecules at its basal side and the release of products into milk at its apical side, through mechanisms of internalization (endocytosis) and mechanisms of release (exocytosis). These events are strictly dependent on the physiological stage of the mammary gland. This review addresses the mechanisms responsible for these processes and points out new questions that remain to be answered concerning possible interconnections between them, for an optimal milk secretion.

Iron administration reduces airway hyperreactivity and eosinophilia in a mouse model of allergic asthma

The prevalence of allergic diseases has increased dramatically during the last four decades and is paralleled by a striking increase in iron intake by infants in affluent societies. Several studies have suggested a link between increased iron intake and the marked increase in prevalence of allergic diseases. We hypothesized that the increased iron intake by infants offers an explanation for the increased prevalence of allergic disease in industrialized societies during the past four decades. A well-established mouse model of ovalbumin (OVA)-driven allergic asthma was used to test the effects of differences in iron intake and systemic iron levels on the manifestations of allergic asthma. Surprisingly, iron supplementation resulted in a significant decrease in airway eosinophilia, while systemic iron injections lead to a significant suppression of both allergen-induced airway eosinophilia and hyperreactivity compared to placebo. In contrast, mice fed on an iron-deprived diet did not show any difference in developing experimentally induced allergic asthma when compared to those fed on an iron-sufficient control diet. In contrast to our hypothesis, airway manifestations of allergic asthma are suppressed by both increased levels of iron intake and systemic iron administrations in the mouse model.

Increased murine neonatal iron intake results in Parkinson-like neurodegeneration with age

Iron elevation is well-documented in the Parkinsonian midbrain but its cause and contribution to subsequent neurodegeneration remain unknown. Mice administered iron at doses equivalent to those found in iron-fortified human infant formula during a developmental period equivalent to the first human year of life display progressive midbrain neurodegeneration and enhanced vulnerability to toxic injury. This may have major implications for the impact of neonatal iron intake as a potential risk factor for later development of Parkinson's disease (PD).

Lactoferrin (Lf): Retinoid interactions in the mammary glands of transgenic mice overexpressing human Lf

Induction of protein expression in a tissue-specific manner by gene transfer over-expression techniques has been one means to define the function of a protein in a biological paradigm. Studies with retinoid reporter constructs transfected in mammary cell lines suggests that lactoferrin (Lf) affects retinoid signaling pathways and alters apoptosis. We tested the effects and interactions of over-expressed mammary-specific human lactoferrin (hLf) and dietary retinol palmitate on lactation and mammary gland development in mice. Increased retinol palmitate in the diet increased daily retinol equivalents (RE) to 2.6-fold over the normal mouse control diet. Transgene (Tg) expression in the dam fed control diet depressed pup weight gain. Severe depression of pup weight gain was observed when homozygote TgTg dams were fed the RE diet. Normal weight gain was restored when pups were placed with a wild type dam fed the RE diet; conversely, normal growing pups from the wild type dams showed declining weight gains when fostered to the TgTg RE-fed dams. Northern analysis of mammary tissue extracts showed a reduction in WAP and an increase in IGFBP-3 mRNA that was associated with the presence of the transgene. Histological evaluation of 3 days lactating mammary tissue showed mammary epithelial cells from TgTg animals contained excessive secretory products, suggesting a block in cellular secretion mechanisms. In addition, the mammary cells displayed a cellular apical membrane puckering that extended into the alveoli lumens. These studies demonstrate an in vivo interaction of Tg-hLf expression and dietary retinoids in mouse mammary glands. While normal mammary gland physiology may not be representative by these experiments because high Lf concentrations during early lactation are abnormal, the demonstrated biological interaction suggests that typical periods of high Lf concentrations may have impact upon developing and involuting mammary glands.

Albumin transcytosis across the epithelium of the lactating mouse mammary gland

Murine milk contains 18 mg ml(-1) serum albumin, a concentration equal to that in the serum of the lactating mouse. We examined cellular transport using in vivo methods in the mouse. At steady state the specific activity of (125)I-albumin injected into the blood stream was equal in plasma and whey, confirming that milk albumin is extra-mammary in origin. Fluorescent albumin crossed the gland from basolateral surface to lumen via cytoplasmic vesicles, but was not transported in the apical to basal direction. Albumin was segregated from transferrin at the basal surface of the epithelial cells and did not colocalize with either caveolin-1 or -2. Vesicular transport was not disrupted by filipin providing additional evidence that, unlike the vascular endothelium, caveoli are not involved. Cytoplasmic albumin was localized to vesicles containing IgA and transport was disrupted by agents that interfere with clathrin-mediated endocytosis. Together, these findings provide evidence that albumin is transported across the mammary epithelium by the same pathway as immunoglobulin. The possibility that the massive transfer of albumin into mouse milk is mediated by fluid phase transport is considered.

Physiology of erythropoietin during mammalian development

Growth is a fundamental process of mammalian development. Several observations regarding regulation of erythropoiesis during growth are not easily explained by the hypoxia-erythropoietin (Epo) concept. This review focuses primarily on this aspect of the physiology of Epo. The question is raised of whether this regulation during growth is based on the hypoxia-Epo mechanism alone, or whether Epo acts in concert with general growth-promoting factors, particularly growth hormone (GH) and the insulin-like growth factors (IGF-I and -II). Supporting the latter hypothesis is the observation that the Epo and GH/IGF systems are activated by hypoxia and share similar receptors and pathways. Recent studies indicate that human fetal and infant growth is stimulated by GH, IGF-I and IGF-II. Epo, GH and IGFs are expressed early in fetal life. Although the rate of erythropoiesis in the fetus is high, serum Epo levels are low. The Epo response to hypoxia in the fetus and neonate is reduced compared with adults. Following delivery the Epo levels vary between species, probably related to the oxygen transport capacity of the hemoglobin (Hb) mass. IGF-I levels are low in the fetus and increase slowly following birth, except in preterm infants in whom the levels decline. In all mammals Hb declines following birth, giving rise to "early anemia". Except in the human, Epo levels increase proportionally with the fall in Hb, but there is a discrepancy between the curves for serum immunoreactive Epo (siEpo) and for erythropoiesis stimulating factors (ESF): the latter include other stimulatory factors in addition to Epo. Hypertransfusion of mice in the period of "early anemia" suppresses siEpo, but not ESF and erythropoiesis, as it does in adult mice. GH and IGF-I have direct effects on erythropoiesis in vitro and act particularly at the later stages of red cell differentiation. IGF-I acts synergistically with Epo, and its effects are most marked when Epo levels are low. Human recombinant (rhu) IGF-I stimulates erythropoiesis in neonatal rats, but not in newborn mice and lambs. In adult mice, in hypophysectomized rats and in mice with end-stage renal failure, however, a stimulatory effect of this growth factor was found on red cell production. RhuGH stimulates erythropoiesis in GH-deficient short children.

CONCLUSION

Fetal and early postnatal erythropoiesis are dependent on factors in addition to Epo. The likely candidates are GH and IGF-I. The in vitro stimulating effects of these factors on erythropoiesis are convincing, but more data are needed on the in vivo effects.

Does human lactoferrin in the milk of transgenic mice deliver iron to suckling neonates?

Lactoferrin is an iron-binding glycoprotein abundantly present in human milk, and has been postulated both to increase and to decrease intestinal iron absorption. To examine this problem, the interaction of milk iron with pup hemoglobin was studied in controls and in transgenic mice overexpressing human lactoferrin in their milk (2 lines expressing 12 mg/mL and 4 mg/mL, respectively). At day 14 of gestation, pregnant mice were switched from a diet of commercial chow containing iron at 300 mg/kg to diets containing 5, 15, or 50mg iron/kg; controls continued on chow. Nontransgenic pups were cross-fostered to transgenic dams to ensure that any results found in the pups were the effect of milk components. The hemoglobin level in the blood of 10-day-old suckling neonates was measured and calculated as total hemoglobin per pup. The total hemoglobin levels were lower in the pups receiving milk high in human lactoferrin, but the difference reached significance (P < 0.02) only at the highest level of dietary iron. Our findings do not support the hypothesis that lactoferrin functions as an intestinal iron scavenger, at least at high doses.