Different responses of Fe transporters in Caco-2/HT29-MTX cocultures than in independent Caco-2 cell cultures

Interaction and competition for intestinal absorption by zinc, iron, copper, and manganese at the intestinal mucus layer

Trace elements such as zinc, manganese, copper, or iron are essential for a wide range of physiological functions. It is therefore crucial to ensure an adequate supply of these elements to the body. Many previous investigations have dealt with the role of transport proteins, in particular their selectivity for, and competition between, different ions. Another so far less well investigated major factor influencing the absorption of trace elements seems to be the intestinal mucus layer. This gel-like substance covers the entire gastrointestinal tract and its physiochemical properties can be mainly assigned to the glycoproteins it contains, so-called mucins. Interaction with mucins has already been demonstrated for some metals. However, knowledge about the impact on the respective bioavailability and competition between those metals is still sketchy. This review therefore aims to summarize the findings and knowledge gaps about potential effects regarding the interaction between gastrointestinal mucins and the trace elements iron, zinc, manganese, and copper. Mucins play an indispensable role in the absorption of these trace elements in the neutral to slightly alkaline environment of the intestine, by keeping them in a soluble form that can be absorbed by enterocytes. Furthermore, the studies so far indicate that the competition between these trace elements for uptake already starts at the intestinal mucus layer, yet further research is required to completely understand this interaction.

Iron delivery systems for controlled release of iron and enhancement of iron absorption and bioavailability

Iron deficiency is a global nutritional problem, and adding iron salts directly to food will have certain side effects on the human body. Therefore, there is growing interest in food-grade iron delivery systems. This review provides an overview of iron delivery systems, with emphasis on the controlled release of iron during gastrointestinal digestion, as well as the enhancement of iron absorption and bioavailability. Iron-bearing proteins are easily degraded by digestive enzymes and absorbed through receptor-mediated endocytosis. Instead, protein aggregates are slowly degraded in the stomach, which delays iron release and serves as a potential iron supplement. Amino acids, peptides and polysaccharides can bind iron through iron binding sites, but the formed compounds are prone to dissociation in the stomach. Moreover, peptides and polysaccharides can deliver iron by mediating the formation of ferric oxyhydroxide which is absorbed through endocytosis or bivalent transporter 1. In addition, liposomes are unstable during gastric digestion and iron is released in large quantities. Complexes formed by polysaccharides and proteins, and microcapsules formed by polysaccharides can delay the release of iron in the gastric environment and prolong iron release in the intestinal environment. This review is conducive to the development of iron functional ingredients and dietary supplements.

A Guide to Human Zinc Absorption: General Overview and Recent Advances of In Vitro Intestinal Models

Zinc absorption in the small intestine is one of the main mechanisms regulating the systemic homeostasis of this essential trace element. This review summarizes the key aspects of human zinc homeostasis and distribution. In particular, current knowledge on human intestinal zinc absorption and the influence of diet-derived factors on bioaccessibility and bioavailability as well as intrinsic luminal and basolateral factors with an impact on zinc uptake are discussed. Their investigation is increasingly performed using in vitro cellular intestinal models, which are continually being refined and keep gaining importance for studying zinc uptake and transport via the human intestinal epithelium. The vast majority of these models is based on the human intestinal cell line Caco-2 in combination with other relevant components of the intestinal epithelium, such as mucin-secreting goblet cells and in vitro digestion models, and applying improved compositions of apical and basolateral media to mimic the in vivo situation as closely as possible. Particular emphasis is placed on summarizing previous applications as well as key results of these models, comparing their results to data obtained in humans, and discussing their advantages and limitations.

Enzymatic treatment of pork protein for the enhancement of iron bioavailability

The typical intervention for iron-deficiency anaemia is through oral supplementation with iron salts, which have unpleasant side effects. Therefore, there is a need for the development of supplements which will be absorbed more effectively and may have fewer side effects. This study investigated the effects of partially hydrolysed pork proteins on the bioavailability of non-haem iron. The peptides were derived using either pepsin or a combination of bacterial and fungal proteases, and their ability to deliver iron was evaluated in a rat intestine epithelial tissue model. The greatest iron absorption was achieved with peptides hydrolysed by pepsin of low molecular weight (<6-8 kDa). The peptides hydrolysed with bacterial and fungal enzymes may have bound to the iron too strongly, affecting bioavailability. Finally, hydrolysing proteins using pepsin in the presence of iron produces a complex that resulted in more ferritin expression than mixing the peptides with iron after hydrolysis.

Intestinal expression of genes implicated in iron absorption and their regulation by hepcidin

BACKGROUND & AIMS

Through inhibition of iron absorption and iron mobilization from tissue stores, hepcidin exerts a negative control on iron homeostasis. Hepcidin, in fact, promotes the degradation of ferroportin (Fpn1), the iron exporter molecule expressed on the membrane of hepatocytes and macrophages, thus preventing iron release from cells to plasma. Hepcidin effects on enterocytes, however, are less clear. Aim of the present study was to further investigate the regulation of iron absorption by hepcidin.

METHODS

The transcriptional response of human duodenal mucosa to hepcidin was investigated using organ cultures of duodenal biopsies perendoscopically collected from healthy controls. Biopsies were cultured for 4 h with or without hepcidin-25 and were then assayed for the expression of iron-related genes.

RESULTS

In samples that had not been exposed to hepcidin, correlations were found between the expression of genes involved in iron absorption: DMT1, Fpn1, Dcytb and HCP1. In ex vivo experiments hepcidin down-regulated mRNA levels of the iron transporters Fpn1, and DMT1, of the ferric reductase Dcytb, of the ferroxidase hephaestin, and of the putative heme carrier protein HCP1.

CONCLUSIONS

Through the reported transcriptional changes hepcidin can modulate several steps of the iron absorption process, including the reduction of dietary iron by Dcytb, its uptake by enterocytes through DMT1, the mucosal uptake of heme iron by HCP1, and enterocyte iron release to plasma by Fpn1 in conjunction with hephaestin.

Cell Systems to Investigate the Impact of Polyphenols on Cardiovascular Health

Polyphenols are a diverse group of micronutrients from plant origin that may serve as antioxidants and that contribute to human health in general. More specifically, many research groups have investigated their protective effect against cardiovascular diseases in several animal studies and human trials. Yet, because of the excessive processing of the polyphenol structure by human cells and the residing intestinal microbial community, which results in a large variability between the test subjects, the exact mechanisms of their protective effects are still under investigation. To this end, simplified cell culture systems have been used to decrease the inter-individual variability in mechanistic studies. In this review, we will discuss the different cell culture models that have been used so far for polyphenol research in the context of cardiovascular diseases. We will also review the current trends in cell culture research, including co-culture methodologies. Finally, we will discuss the potential of these advanced models to screen for cardiovascular effects of the large pool of bioactive polyphenols present in foods and their metabolites.

A tunable Caco-2/HT29-MTX co-culture model mimicking variable permeabilities of the human intestine obtained by an original seeding procedure

Standard monoculture models utilizing Caco-2 monolayers were extensively used to mimic the permeability of the human intestinal barrier. However, they exhibit numerous limitations such as the lack of mucus layer, an overestimation of the P-gp-mediated efflux and a low paracellular permeability. Here, we suggest a new procedure to set up an in vitro model of intestinal barrier to adjust gradually the properties of the absorption barrier. Mucin-secreting HT29-MTX cells were added to Caco-2 absorptive cells in a Transwell® at different time intervals. Effects of seeding day of HT29-MTX on the paracellular permeability of lucifer yellow (LY) and on the P-gp-mediated efflux of rhodamine 123 were investigated. Apparent permeability of the rhodamine 123 in the secretory direction was highly dependent on the seeding day of goblet cells. Transepithelial electrical resistance values and LY transport across the co-cultures in the apical-to-basolateral direction were intermediary between single Caco-2 and HT29-MTX models. Early seeding days of HT29-MTX allowed increasing the fraction of goblet cells in the co-culture. Co-culture permeability was unchanged between 21 and 30 days after Caco-2 seeding, corresponding to the period of use for Caco-2-based cell models. Thus, the HT29-MTX seeding day was a key factor to set up an in vitro intestinal model with tailor-made barrier properties in terms of P-gp expression and paracellular permeability.

Salmonella adhesion, invasion and cellular immune responses are differentially affected by iron concentrations in a combined in vitro gut fermentation-cell model

In regions with a high infectious disease burden, concerns have been raised about the safety of iron supplementation because higher iron concentrations in the gut lumen may increase risk of enteropathogen infection. The aim of this study was to investigate interactions of the enteropathogen Salmonella enterica ssp. enterica Typhimurium with intestinal cells under different iron concentrations encountered in the gut lumen during iron deficiency and supplementation using an in vitro colonic fermentation system inoculated with immobilized child gut microbiota combined with Caco-2/HT29-MTX co-culture monolayers. Colonic fermentation effluents obtained during normal, low (chelation by 2,2'-dipyridyl) and high iron (26.5 mg iron/L) fermentation conditions containing Salmonella or pure Salmonella cultures with similar iron conditions were applied to cellular monolayers. Salmonella adhesion and invasion capacity, cellular integrity and immune response were assessed. Under high iron conditions in pure culture, Salmonella adhesion was 8-fold increased compared to normal iron conditions while invasion was not affected leading to decreased invasion efficiency (-86%). Moreover, cellular cytokines IL-1β, IL-6, IL-8 and TNF-α secretion as well as NF-κB activation in THP-1 cells were attenuated under high iron conditions. Low iron conditions in pure culture increased Salmonella invasion correlating with an increase in IL-8 release. In fermentation effluents, Salmonella adhesion was 12-fold and invasion was 428-fold reduced compared to pure culture. Salmonella in high iron fermentation effluents had decreased invasion efficiency (-77.1%) and cellular TNF-α release compared to normal iron effluent. The presence of commensal microbiota and bacterial metabolites in fermentation effluents reduced adhesion and invasion of Salmonella compared to pure culture highlighting the importance of the gut microbiota as a barrier during pathogen invasion. High iron concentrations as encountered in the gut lumen during iron supplementation attenuated Salmonella invasion efficiency and cellular immune response suggesting that high iron concentrations alone may not lead to an increased Salmonella invasion.

Modulation of mucin mRNA (MUC5AC and MUC5B) expression and protein production and secretion in Caco-2/HT29-MTX co-cultures following exposure to individual and combined Fusarium mycotoxins

Intestinal epithelial cells (IECs) are a critical component of the innate local immune response. In order to reduce the risk of pathogen infection or xenobiotic intoxication, different host defense mechanisms have been evolved. Evidence has shown that upon ingestion of food or feed contaminated with toxins (e.g., mycotoxins), IECs respond by regulating mucin secretions, which act as a physical barrier inhibiting bacterial attachment and subsequent infection-related processes. However, the effect of Fusarium mycotoxins on mucin production remains unclear. Consequently, the aim of this study was to evaluate individual and interactive effects of four common Fusarium mycotoxins, deoxynivalenol, nivalenol, zearalenone, and fumonisins B1 on mRNA expression and secretion of mucins, MUC5AC, and MUC5B, as well as total mucin-like glycoprotein secretion, using Caco-2 (absorptive-type) and HT29-MTX (secretive-type) cells and their co-cultures (initial seeding ratios Caco-2/HT29-MTX: 90/10 and 70/30). Our results showed that individual and mixtures of mycotoxins significantly modulated MUC5AC and MUC5B mRNA and protein, and total mucin-like glycoprotein secretion as measured by quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, and enzyme-linked lectin assay, respectively. Additive effects were not always observed for mixtures. Also, the present study showed that in co-cultures, lower MUC5AC and MUC5B mRNA, protein and total mucin production occurred following exposure, which might suggest higher intestinal permeability and susceptibility to toxin exposure. This study demonstrates the importance of selecting an appropriate cell model for the in vitro investigation of Fusarium mycotoxin effects either alone or in combinations on the immunological defense mechanisms of IECs, and will contribute to improved toxin risk assessments.

Potential of phytase-mediated iron release from cereal-based foods: a quantitative view

The major part of iron present in plant foods such as cereals is largely unavailable for direct absorption in humans due to complexation with the negatively charged phosphate groups of phytate (myo-inositol (1,2,3,4,5,6)-hexakisphosphate). Human biology has not evolved an efficient mechanism to naturally release iron from iron phytate complexes. This narrative review will evaluate the quantitative significance of phytase-catalysed iron release from cereal foods. In vivo studies have shown how addition of microbially derived phytases to cereal-based foods has produced increased iron absorption via enzyme-catalysed dephosphorylation of phytate, indicating the potential of this strategy for preventing and treating iron deficiency anaemia. Despite the immense promise of this strategy and the prevalence of iron deficiency worldwide, the number of human studies elucidating the significance of phytase-mediated improvements in iron absorption and ultimately in iron status in particularly vulnerable groups is still low. A more detailed understanding of (1) the uptake mechanism for iron released from partially dephosphorylated phytate chelates, (2) the affinity of microbially derived phytases towards insoluble iron phytate complexes, and (3) the extent of phytate dephosphorylation required for iron release from inositol phosphates is warranted. Phytase-mediated iron release can improve iron absorption from plant foods. There is a need for development of innovative strategies to obtain better effects.

Mucus as a barrier to lipophilic drugs

Mucus is a complex hydrogel, comprising glycoproteins, lipids, salts, DNA, enzymes and cellular debris, covering many epithelial surfaces in the human body. Once secreted, mucin forms a barrier to protect the underlying tissues against the extracellular environment. Mucus can therefore adversely affect the absorption or action of drugs administered by the oral, pulmonary, vaginal, nasal or other routes. Solubility and lipophilicity are key factors determining drug absorption, as a drug has to be soluble in the body fluids at the site of absorption and must also possess enough lipophilicity to permeate the biological membrane. Evidence has accumulated over the past 40 years indicating that poorly soluble drugs will interact with mucus glycoprotein. Studies of the permeability of native or purified mucous gels are important when it comes to understanding the relative importance of hindered diffusion versus drug binding in mucous layers. This review highlights the current understanding of the drug-mucin interaction and also examines briefly the interaction of polymers and particles with the mucus matrix. While the concept of mucoadhesion was thought to provide an intensified and prolonged contact to mucosal absorption sites, mucopenetrating properties are nowadays being discussed for (nano)particulate carriers to overcome the mucus as a barrier and enhance drug delivery through mucus.

Human iron transporters

Human iron transporters manage iron carefully because tissues need iron for critical functions, but too much iron increases the risk of reactive oxygen species. Iron acquisition occurs in the duodenum via divalent metal transporter (DMT1) and ferroportin. Iron trafficking depends largely on the transferrin cycle. Nevertheless, non-digestive tissues have a variety of other iron transporters that may render DMT1 modestly redundant, and DMT1 levels exceed those needed for the just-mentioned tasks. This review begins to consider why and also describes advances after 2008 that begin to address this challenge.