Multiple pathways ensure retinoid delivery to milk: studies in genetically modified mice

Use of Theoretical Women and Model-Based Compartmental Analysis to Evaluate the Impact of Vitamin A Intake with or without a Daily Vitamin A Supplement on Vitamin A Total Body Stores and Balance During Lactation

BACKGROUND

Inadequate vitamin A (VA) intake is common among lactating women in many communities worldwide, but high-dose VA supplementation for postpartum women is not recommended by the World Health Organization as an effective intervention.

OBJECTIVES

To simulate the impact of VA intake via diet and daily VA supplements on VA total body stores (TBS) and balance in theoretical lactating women with low/moderate TBS.

METHODS

We studied 6 theoretical subjects with assigned values for TBS from 219-624 μmol. Using Simulation, Analysis, and Modeling software and a previously published compartmental model for whole-body VA metabolism, we simulated TBS over 6 mo of established lactation for each subject under 4 conditions: 1) prelactation VA intake was increased to maintain VA balance (LSS); 2) prelactation VA intake was maintained (NLSS); 3) VA intake was the same as 2) but a daily VA supplement (2.8 μmol/d) was added (NLSS+S); and 4) VA intake was as 1) and the daily VA supplement was included (LSS+S).

RESULTS

To compensate for the loss of VA via milk while VA balance was maintained (LSS) over 6 mo of lactation, VA intake had to increase by 0.8-1.87 μmol/d (n = 6) compared with NLSS. Over 6 mo of NLSS treatment, VA balance was negative (geometric mean, -0.77 μmol/d) compared with LSS, whereas balance was positive under NLSS+S and LSS+S conditions (0.75 and 1.5 μmol/d, respectively). For LSS, the proportion of total VA disposal was 37% via breastmilk, 32% from VA stores, and 32% from nonstorage tissues.

CONCLUSIONS

Adding a daily VA supplement (2.8 μmol/d) to the diet of lactating women with suboptimal VA intake may effectively counterbalance the negative VA balance resulting from the output of VA via breastmilk and thus benefit both mother and infant by maintaining or increasing VA stores and breastmilk VA concentration.

Development of a Compartmental Model for Studying Vitamin A Kinetics and Status in Theoretical Lactating Women

BACKGROUND

Low vitamin A status and suboptimal milk vitamin A concentrations are problems in many populations worldwide. However, limited research has been done on whole-body vitamin A kinetics in women of reproductive age, especially during lactation.

OBJECTIVES

Goals were to develop compartmental models describing retinol kinetics in theoretical nonlactating (NL) and lactating (L) women and to determine whether the retinol isotope dilution (RID) method accurately predicted vitamin A total body stores (TBS) in the groups and individuals.

METHODS

We adapted 12 previously-used theoretical females with assigned values for retinol kinetic parameters and TBS (225-1348 μmol); subjects were NL or L (nursing one 3- to 6-mo-old infant) during 49-d kinetic studies after isotope dosing. We used an established compartmental model, adding a compartment for chylomicrons and, for L, another for mammary gland milk with inputs from holo-retinol-binding protein and chylomicron retinyl esters and output to milk. Using compartmental analysis, we simulated tracer responses in compartments of interest and calculated TBS using the RID equation TBS =   FaS/SAp [Fa, fraction of dose in stores; S, retinol specific activity in plasma/specific activity in stores; SAp, specific activity of retinol in plasma].

RESULTS

Models for both groups were well identified. Simulated plasma tracer responses were similar for NL and L, with L always below NL; milk tracer paralleled plasma from 10 d postdosing. Geometric mean FaS ratios (L/NL) were ∼0.75 during days 2-30. Using appropriate group FaS, RID provided accurate TBS predictions for >80% of NL and L subjects after day 18 when CV% for FaS was ∼10%.

CONCLUSIONS

These new physiologically-based models for vitamin A kinetics may be useful for future research in women of reproductive age. Results indicate that, in groups like these, RID to assess an individual's vitamin A status should be done at 21-28 d after isotope dosing.

Systematic Review and Meta-Analysis of the Relative Dose-Response Tests to Assess Vitamin A Status

Vitamin A (VA) is an essential nutrient often lacking in the diets of people in developing countries. Accurate biomarkers of VA status are vital to inform public health policy and monitor interventions. The relative dose-response (RDR) and modified-RDR (MRDR) tests are semi-quantitative screening tests for VA deficiency that have been used in Demographic and Health Surveys and VA intervention studies. A systematic review and meta-analysis of sensitivity and specificity were conducted to summarize the physiological evidence to support the RDR tests as methods to assess VA status and investigate the impact of different pathological and physiological states on the tests. A total of 190 studies were screened for inclusion, with 21 studies comparing the RDR tests with the gold-standard biomarker, liver VA concentration (68% and 80% sensitivity and 85% and 69% specificity for the RDR and MRDR, respectively). Nearly all studies with VA interventions in VA-deficient populations demonstrated a response of the tests to VA intake that would be expected to improve VA status. The impacts of chronic liver disease, protein malnutrition, age, pregnancy and lactation, infection and inflammation, and various other conditions were examined in 51 studies. The RDR and MRDR tests were reported to have been used in 39 observational studies, and the MRDR has been used in at least 6 national micronutrient surveys. The RDR and MRDR are sensitive tests for determining population VA status and assessing VA interventions. Although they are robust to most physiological and pathological states, caution may be warranted when using the tests in neonates, individuals with chronic liver disease, and those with protein or iron malnutrition. Research on further improvements to the tests to increase accessibility, such as sampling breast milk instead of blood or using intramuscular doses in subjects with malabsorption, will allow wider adoption. This review was registered with PROSPERO as CRD42019124180.

Modified relative dose response values differ between lactating women in the United States and Indonesia

IMPACT STATEMENT

Vitamin A (VA) deficiency is a major health issue globally, and lactating women are particularly vulnerable due to increased needs for milk production. Accurate detection of VA deficiency is important; however, most population surveys measure VA status using serum retinol, which is affected by inflammation and lacks sensitivity. The modified relative dose response (MRDR) test qualitatively distinguishes between VA deficiency and sufficiency and could improve population surveys if completed in a randomly selected subsample of individuals in surveys. The original relative dose response test required two blood samples, while MRDR requires only one, a significant improvement in accessibility of the technique by decreasing burden on subjects and investigators. This work demonstrates significant deficiency in Indonesian women compared with US women. In combination with previous research using lactating sows, these human data support milk as a surrogate for blood in the MRDR, which may be less invasive, but requires further validation.

Maternal Circulating Vitamin Status and Colostrum Vitamin Composition in Healthy Lactating Women-A Systematic Approach

Colostrum is the first ingested sole nutritional source for the newborn infant. The vitamin profile of colostrum depends on the maternal vitamin status, which in turn is influenced by diet and lifestyle. Yet, the relationship between maternal vitamin status and colostrum vitamin composition has not been systematically reviewed. This review was conducted with the aim to generate a comprehensive overview on the relationship between maternal serum (plasma) vitamin concentration and corresponding colostrum composition. Three electronic databases, Embase (Ovid), Medline (Ovid), and Cochrane, were systematically searched based on predefined inclusion and exclusion criteria. Finally, a total of 11 eligible publications were included that examined the vitamins A, C, D, E, and K in both biological fluids. Maternal vitamin A, D, E, and K blood levels were unrelated to colostrum content of the respective vitamins, and serum vitamin A was inversely correlated with colostrum vitamin E. Colostrum versus maternal serum vitamins were higher for vitamins A, C, and K, lower for vitamin D, and divergent results were reported for vitamin E levels. Colostrum appears typically enriched in vitamin A, C, and K compared to maternal serum, possibly indicative of active mammary gland transport mechanisms. Inter-individual and inter-study high variability in colostrum's vitamin content endorses its sensitivity to external factors.

Retinol-to-Fat Ratio and Retinol Concentration in Human Milk Show Similar Time Trends and Associations with Maternal Factors at the Population Level: A Systematic Review and Meta-Analysis

Vitamin A in human milk is critical for meeting infant requirements and building liver stores needed after weaning. A number of studies have measured milk retinol, but only a subset have corrected for fat, which serves as the retinol carrier in breast milk. The purpose of the present work was to review and analyze studies in which human-milk retinol concentrations were reported in relation to milk fat and to compare these results with unadjusted breast-milk retinol concentrations in terms of time trends over the course of lactation, influences of maternal nutritional and constitutional factors, and effects of maternal vitamin A supplementation. A systematic approach was used to search the available literature by using the US National Library of Medicine's MEDLINE/PubMed bibliographic search engine. Observational and intervention studies were included if the research was original and the retinol-to-fat ratio (retinol:fat) in human milk was measured at ≥1 time point during the first 12 mo of lactation. Retinol:fat and retinol were highest in colostrum, declined rapidly in early lactation, and achieved statistical stability by 2 and 4 wk lactation, respectively. In mature milk, retinol concentration was positively correlated with milk fat (r = 0.61, P = 0.008). Breast-milk retinol:fat and retinol were positively associated with maternal vitamin A intake but were associated with plasma retinol only when dietary intake was inadequate. Postpartum supplementation with high-dose vitamin A (200,000-400,000 IU) resulted in significantly higher breast-milk retinol:fat for 3 mo and retinol for 6 mo (P < 0.05). In populations, the 2 indexes show similar trends and associations with maternal factors. Future studies should monitor how the uptake of retinol into the mammary gland affects maternal vitamin reserves, particularly in women who are at risk of vitamin A deficiency.

Vitamin A intake of Brazilian mothers and retinol concentrations in maternal blood, human milk, and the umbilical cord

Objectives To analyse intake of vitamin A (VA) and retinol concentrations in maternal blood, breast milk (BM), and the umbilical cord (UC) of newborns, and to determine the associations among these variables. Methods We performed a cross-sectional, epidemiological study of 180 mother-newborn dyads. Maternal and UC blood samples and BM were collected. VA intake by the mother over 30 days was assessed using a questionnaire. Results Mean retinol concentrations in maternal serum, the UC, and BM were 0.65 ± 0.27, 0.36 ± 0.18, and 2.95 ± 2.70 µmol/L, respectively. Retinol concentrations <0.70 µmol/L were found in 57.2% of maternal blood samples and in 94.9% of UC samples. A total of 27.9% of BM samples showed retinol concentrations <1.05 µmol/L. Mean VA intake by the mothers was 1041.33 ± 1187.86 µg retinol activity equivalents/day and was inadequate (<550 µg retinol activity equivalents/day) in 44.7%. Conclusions High proportions of insufficient retinol concentrations were observed in the UC, maternal blood, and BM. A high percentage of pregnant women had inadequate VA intake. Mothers with insufficient serum retinol concentrations had newborns with lower retinol concentrations in the UC. Higher retinol concentrations were observed in maternal blood and the UC with a higher VA intake.

Molecular Basis for Vitamin A Uptake and Storage in Vertebrates

The ability to store and distribute vitamin A inside the body is the main evolutionary adaptation that allows vertebrates to maintain retinoid functions during nutritional deficiencies and to acquire new metabolic pathways enabling light-independent production of 11-cis retinoids. These processes greatly depend on enzymes that esterify vitamin A as well as associated retinoid binding proteins. Although the significance of retinyl esters for vitamin A homeostasis is well established, until recently, the molecular basis for the retinol esterification enzymatic activity was unknown. In this review, we will look at retinoid absorption through the prism of current biochemical and structural studies on vitamin A esterifying enzymes. We describe molecular adaptations that enable retinoid storage and delineate mechanisms in which mutations found in selective proteins might influence vitamin A homeostasis in affected patients.

Vitamin A Absorption, Storage and Mobilization

It is well established that chylomicron remnant (dietary) vitamin A is taken up from the circulation by hepatocytes, but more than 80 % of the vitamin A in the liver is stored in hepatic stellate cells (HSC). It presently is not known how vitamin A is transferred from hepatocytes to HSCs for storage. Since retinol-binding protein 4 (RBP4), a protein that is required for mobilizing stored vitamin A, is synthesized solely by hepatocytes and not HSCs, it similarly is not known how vitamin A is transferred from HSCs to hepatocytes. Although it has long been thought that RBP4 is absolutely essential for delivering vitamin A to tissues, recent research has proven that this notion is incorrect since total RBP4-deficiency is not lethal. In addition to RBP4, vitamin A is also found in the circulation bound to lipoproteins and as retinoic acid bound to albumin. It is not known how these different circulating pools of vitamin A contribute to the vitamin A needs of different tissues. In our view, better insight into these three issues is required to better understand vitamin A absorption, storage and mobilization. Here, we provide an up to date synthesis of current knowledge regarding the intestinal uptake of dietary vitamin A, the storage of vitamin A within the liver, and the mobilization of hepatic vitamin A stores, and summarize areas where our understanding of these processes is incomplete.

Effect of maternal vitamin A supplementation on retinol concentration in colostrum

OBJECTIVE

To investigate the effect of vitamin A supplementation on the retinol concentration in colostrum under fasting and postprandial conditions.

METHODS

This was a quasi-experimental study, with before and after assessments, conducted with 33 patients treated at a public maternity hospital. Blood and colostrum samples were collected under fasting conditions in the immediate postpartum period. A second colostrum collection occurred two hours after the first meal of the day, at which time a mega dose of 200,000 IU of retinyl palmitate was administered. On the following day, the colostrum was collected again under fasting and postprandial conditions. Serum and colostrum retinol concentrations were determined by high performance liquid chromatography.

RESULTS

The serum retinol concentration was 37.3 (16.8-62.2) μg/dL, indicating adequate nutritional status. The colostrum retinol concentration before supplementation was 46.8 (29.7-158.9) μg/dL in fasting and 67.3 (31.1-148.7) μg/dL in postprandial condition (p < 0.05), showing an increase of 43.8%. After supplementation, the values were 89.5 (32.9-264.2) μg/dL and 102.7 (37.3-378.3) μg/dL in fasting and postprandial conditions, respectively (p < 0.05), representing an increase of 14.7%.

CONCLUSIONS

This study demonstrated that maternal supplementation with high doses of vitamin A in postpartum resulted in a significant increase of the retinol concentration in colostrum under fasting conditions, with an even greater increase after a meal.

Adipose-specific lipoprotein lipase deficiency more profoundly affects brown than white fat biology

Adipose fat storage is thought to require uptake of circulating triglyceride (TG)-derived fatty acids via lipoprotein lipase (LpL). To determine how LpL affects the biology of adipose tissue, we created adipose-specific LpL knock-out (ATLO) mice, and we compared them with whole body LpL knock-out mice rescued with muscle LpL expression (MCK/L0) and wild type (WT) mice. ATLO LpL mRNA and activity were reduced, respectively, 75 and 70% in gonadal adipose tissue (GAT), 90 and 80% in subcutaneous tissue, and 84 and 85% in brown adipose tissue (BAT). ATLO mice had increased plasma TG levels associated with reduced chylomicron TG uptake into BAT and lung. ATLO BAT, but not GAT, had altered TG composition. GAT from MCK/L0 was smaller and contained less polyunsaturated fatty acids in TG, although GAT from ATLO was normal unless LpL was overexpressed in muscle. High fat diet feeding led to less adipose in MCK/L0 mice but TG acyl composition in subcutaneous tissue and BAT reverted to that of WT. Therefore, adipocyte LpL in BAT modulates plasma lipoprotein clearance, and the greater metabolic activity of this depot makes its lipid composition more dependent on LpL-mediated uptake. Loss of adipose LpL reduces fat accumulation only if accompanied by greater LpL activity in muscle. These data support the role of LpL as the "gatekeeper" for tissue lipid distribution.

A microparticle/hydrogel combination drug-delivery system for sustained release of retinoids

PURPOSE

To design and develop a drug-delivery system containing a combination of poly(D,L-lactide-co-glycolide) (PLGA) microparticles and alginate hydrogel for sustained release of retinoids to treat retinal blinding diseases that result from an inadequate supply of retinol and generation of 11-cis-retinal.

METHODS

To study drug release in vivo, either the drug-loaded microparticle-hydrogel combination was injected subcutaneously or drug-loaded microparticles were injected intravitreally into Lrat(-/-) mice. Orally administered 9-cis-retinoids were used for comparison and drug concentrations in plasma were determined by HPLC. Electroretinography (ERG) and both chemical and histologic analyses were used to evaluate drug effects on visual function and morphology.

RESULTS

Lrat(-/-) mice demonstrated sustained drug release from the microparticle/hydrogel combination that lasted 4 weeks after subcutaneous injection. Drug concentrations in plasma of the control group treated with the same oral dose rose to higher levels for 6-7 hours but then dropped markedly by 24 hours. Significantly increased ERG responses and a markedly improved retinal pigmented epithelium (RPE)-rod outer segment (ROS) interface were observed after subcutaneous injection of the drug-loaded delivery combination. Intravitreal injection of just 2% of the systemic dose of drug-loaded microparticles provided comparable therapeutic efficacy.

CONCLUSIONS

Sustained release of therapeutic levels of 9-cis-retinoids was achieved in Lrat(-/-) mice by subcutaneous injection in a microparticle/hydrogel drug-delivery system. Both subcutaneous and intravitreal injections of drug-loaded microparticles into Lrat(-/-) mice improved visual function and retinal structure.

Uptake of dietary retinoids at the maternal-fetal barrier: in vivo evidence for the role of lipoprotein lipase and alternative pathways

Dietary retinoids (vitamin A and its derivatives) contribute to normal embryonic development. However, the mechanism(s) involved in the transfer of recently ingested vitamin A from mother to embryo is not fully understood. We investigated in vivo whether lipoprotein lipase (LPL) facilitates the placental uptake of dietary retinyl ester incorporated in chylomicrons and their remnants and its transfer to the embryo. We examined the effects of both genetic ablation (MCK-L0 mice) and pharmacological inhibition (P-407) of LPL by maintaining wild type and MCK-L0 mice on diets with different vitamin A content or administering them an oral gavage dose of [(3)H]retinol with or without P-407 treatment. We showed that LPL expressed in placenta facilitates uptake of retinoids by this organ and their transfer to the embryo, mainly through its catalytic activity. In addition, through its "bridging function," LPL can mediate the acquisition of nascent chylomicrons by the placenta, although less efficiently. Quantitative real-time PCR and Western blot analysis showed that placental LPL acts in concert with LDL receptor and LRP1. Finally, by knocking out the retinol-binding protein (RBP) gene in the MCK-L0 background (MCK-L0-RBP(-/-) mice) we demonstrated that the placenta acquires dietary retinoids also via the maternal circulating RBP-retinol complex. RBP expressed in the placenta facilitate the transfer of postprandial retinoids across the placental layers toward the embryo.

Retinyl ester hydrolases and their roles in vitamin A homeostasis

In mammals, dietary vitamin A intake is essential for the maintenance of adequate retinoid (vitamin A and metabolites) supply of tissues and organs. Retinoids are taken up from animal or plant sources and subsequently stored in form of hydrophobic, biologically inactive retinyl esters (REs). Accessibility of these REs in the intestine, the circulation, and their mobilization from intracellular lipid droplets depends on the hydrolytic action of RE hydrolases (REHs). In particular, the mobilization of hepatic RE stores requires REHs to maintain steady plasma retinol levels thereby assuring constant vitamin A supply in times of food deprivation or inadequate vitamin A intake. In this review, we focus on the roles of extracellular and intracellular REHs in vitamin A metabolism. Furthermore, we will discuss the tissue-specific function of REHs and highlight major gaps in the understanding of RE catabolism. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Vitamin A metabolism: an update

Retinoids are required for maintaining many essential physiological processes in the body, including normal growth and development, normal vision, a healthy immune system, normal reproduction, and healthy skin and barrier functions. In excess of 500 genes are thought to be regulated by retinoic acid. 11-cis-retinal serves as the visual chromophore in vision. The body must acquire retinoid from the diet in order to maintain these essential physiological processes. Retinoid metabolism is complex and involves many different retinoid forms, including retinyl esters, retinol, retinal, retinoic acid and oxidized and conjugated metabolites of both retinol and retinoic acid. In addition, retinoid metabolism involves many carrier proteins and enzymes that are specific to retinoid metabolism, as well as other proteins which may be involved in mediating also triglyceride and/or cholesterol metabolism. This review will focus on recent advances for understanding retinoid metabolism that have taken place in the last ten to fifteen years.

Vitamin A deficiency results in meiotic failure and accumulation of undifferentiated spermatogonia in prepubertal mouse testis

Vitamin A (retinol) is required for maintenance of adult mammalian spermatogenesis. In adult rodents, vitamin A withdrawal is followed by a loss of differentiated germ cells within the seminiferous epithelium and disrupted spermatogenesis that can be restored by vitamin A replacement. However, whether vitamin A plays a role in the differentiation and meiotic initiation of germ cells during the first round of mouse spermatogenesis is unknown. In the present study, we found that vitamin A depletion markedly decreased testicular expression of the all-trans retinoic acid-responsive gene, Stra8, and caused meiotic failure in prepubertal male mice lacking lecithin:retinol acyltransferase (Lrat), encoding for the major enzyme in liver responsible for the formation of retinyl esters. Rather than undergoing normal differentiation, germ cells accumulated in the testes of Lrat(-/-) mice maintained on a vitamin A-deficient diet. These results, together with our previous observations that germ cells fail to enter meiosis and remain undifferentiated in embryonic vitamin A-deficient ovaries, support the hypothesis that vitamin A regulates the initiation of meiosis I of both oogenesis and spermatogenesis in mammals.

Chylomicron- and VLDL-derived lipids enter the heart through different pathways: in vivo evidence for receptor- and non-receptor-mediated fatty acid uptake

Lipids circulate in the blood in association with plasma lipoproteins and enter the tissues either after hydrolysis or as non-hydrolyzable lipid esters. We studied cardiac lipids, lipoprotein lipid uptake, and gene expression in heart-specific lipoprotein lipase (LpL) knock-out (hLpL0), CD36 knock-out (Cd36(-/-)), and double knock-out (hLpL0/Cd36(-/-)-DKO) mice. Loss of either LpL or CD36 led to a significant reduction in heart total fatty acyl-CoA (control, 99.5 ± 3.8; hLpL0, 36.2 ± 3.5; Cd36(-/-), 57.7 ± 5.5 nmol/g, p < 0.05) and an additive effect was observed in the DKO (20.2 ± 1.4 nmol/g, p < 0.05). Myocardial VLDL-triglyceride (TG) uptake was reduced in the hLpL0 (31 ± 6%) and Cd36(-/-) (47 ± 4%) mice with an additive reduction in the DKO (64 ± 5%) compared with control. However, LpL but not CD36 deficiency decreased VLDL-cholesteryl ester uptake. Endogenously labeled mouse chylomicrons were produced by tamoxifen treatment of β-actin-MerCreMer/LpL(flox/flox) mice. Induced loss of LpL increased TG levels >10-fold and reduced HDL by >50%. After injection of these labeled chylomicrons in the different mice, chylomicron TG uptake was reduced by ∼70% and retinyl ester by ∼50% in hLpL0 hearts. Loss of CD36 did not alter either chylomicron TG or retinyl ester uptake. LpL loss did not affect uptake of remnant lipoproteins from ApoE knock-out mice. Our data are consistent with two pathways for fatty acid uptake; a CD36 process for VLDL-derived fatty acid and a non-CD36 process for chylomicron-derived fatty acid uptake. In addition, our data show that lipolysis is involved in uptake of core lipids from TG-rich lipoproteins.