New Insights into Intestinal Heme and Non-heme Iron Absorption: Regulation by Dietary Factors
Open Access
- Author:
- Ma, Qianyi
- Graduate Program:
- Nutrition
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 04, 2012
- Committee Members:
- Okhee Han, Dissertation Advisor/Co-Advisor
Okhee Han, Committee Chair/Co-Chair
A Catharine Ross, Committee Member
Terryl Johnson Hartman, Committee Member
Margherita Teresa Anna Cantorna, Committee Member
Debashis Ghosh, Committee Member - Keywords:
- Intestinal iron absorption
Polyphenol
Heme
Green tea
Ferroportin-1 - Abstract:
- Iron is an essential mineral in all mammals required for many physiological processes, including oxygen transport and storage, electron transfer and DNA synthesis. Iron absorption is crucial in maintaining systemic iron homeostasis because there is no physiological mechanism for iron excretion, which distinguishes iron from other minerals. Dietary factors, such as phytate, ascorbic acid and animal tissues, have been proven consistently to influence intestinal iron absorption. The unique components of animal tissues that enhance intestinal iron absorption are coined as “meat factor”. However, the influence of polyphenols on iron absorption and the nature of the “meat factor” remain unresolved. As the major transporters for intestinal iron absorption have been identified in the past decade, the focus has shifted to the molecular mechanism of intestinal heme iron absorption. To address these identified gaps of knowledge, our research focused on defining the effect of polyphenols on iron absorption and elucidating the interactions between heme and non-heme iron during iron absorptive processes using the human intestine-like Caco-2 cell model. In the first study, our objective was to investigate the effect of several bioactive polyphenolic compounds, including (-) -epigallocatechin-3-gallate (EGCG), grape seed extract (GSE) and green tea extract (GT) on intestinal heme iron absorption. Dietary polyphenolic compounds has attracted increasing attentions recently because they have a wide range of health benefits. Previous studies have revealed the inhibiting effect of polyphenols from black tea on non-heme iron absorption. Our initial study showed the inhibitory effect of EGCG and GSE on non-heme iron absorption. We explored whether polyphenols from green tea and grape seed extract could inhibit intestinal absorption of heme iron in Caco-2 cells. Both EGCG and GSE decreased (p < 0.05) transepithelial transport of heme-derived 55Fe. Although apical heme-55Fe uptake was increased (p < 0.05) by GSE, the amount of total 55Fe released to basolateral chamber was negligible. In contrast, EGCG moderately decreased the apical uptake of heme-55Fe, whereas the basolateral iron transfer was extremely low. All three types of polyphenolic compounds tested, including EGCG, GSE and GT, significantly inhibited heme-55Fe absorption in a dose-dependent manner. The addition of ascorbic acid did not modulate the inhibitory effect of these polyphenols on heme iron absorption when the cells were treated with polyphenols at a concentration of 46 mg/L. However, ascorbic acid was able to offset or reverse the inhibitory effect of polyphenolic compounds when lower concentrations of polyphenols were added (≤ 4.6 mg/L). In summary, findings from the first study suggest that bioactive dietary polyphenols inhibit heme iron absorption mainly by reducing basolateral iron exit rather than decreasing apical heme iron uptake in intestinal cells. In addition, the results imply that regular consumption of dietary ascorbic acid can easily counteract the inhibitory effect of very low concentrations of dietary polyphenols on heme iron absorption but cannot counteract the inhibitory actions of higher concentrations of polyphenols. In the second study, our objective was to explore the mechanism of increased iron absorption promoted by meat factor of animal tissues and whether and how the absorption of heme and non-heme iron interfere with each other. We examined cellular uptake and transepithelial absorption of non-heme 55Fe in human intestine-like Caco-2 cells in response to treatment of different doses of heme or the heme precursor, protoporphyrin IX, for different time periods. We found that non-heme 55Fe absorption across Caco-2 cell monolayer was enhanced by heme via redistributing iron importer divalent metal-transporter-1 (DMT1) from cytosol to apical membrane and iron exporter ferroportin1 (FPN1) from cytosol to basolateral membrane. Interestingly, heme-55Fe absorption was also increased by treatment of non-heme iron. Although non-heme iron alone did not affect FPN1 localization, heme and non-heme iron together synergistically redistributed FPN1 to basolateral membrane. In addition, the heme precursor protoporphyrin IX itself was sufficient to increase non-heme iron absorption, while the competition between the iron released from the heme moiety and non-heme iron was negligible compared to compensatory increase of FPN1 on basolateral membrane. Finally, non-heme 55Fe absorption was also enhanced adaptively by pretreatment of heme for 2 – 3 days, possible as a consequence of increased FPN1 transcription induced by heme. In conclusion, our research supports that polyphenols from green tea and grape seed extracts drastically block intestinal absorption of both heme and non-heme iron and that intestinal absorption of heme and non-heme iron are reciprocal of each other via regulation of DMT1 and FPN1 using the Caco-2 cells model.