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

Ferritin (Ft) is a large iron (Fe)-binding protein (∼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 59Fe-labeled Ft at 4°C showed saturable kinetics, and Scatchard analysis resulted in a Kd of 1.6 μM, strongly indicating a receptor-mediated process. Competitive binding studies with excess unlabelled Ft significantly reduced binding, and uptake studies at 37°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 ∼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.