Subcellular location of heme oxygenase 1 and 2 and divalent metal transporter 1 in relation to endocytotic markers during heme iron absorption

Background and Aim: Heme is an important dietary micronutrient, although its absorptive mechanisms are poorly understood. One hypothesis suggests enterocytes take up heme by receptor‐mediated endocytosis (RME) which then undergoes catabolism by heme oxygenase (HO) inside internalized vesicles. This would require the translocation of HO‐1 or HO‐2 to endosomes and/or lysosomes and the presence of a transporter, possibly divalent metal transporter 1 (DMT1), to transfer released iron to the cytoplasm. Currently, the location of HO‐1 and HO‐2 in enterocytes is unknown. Methods: We studied the subcellular location of HO‐1, HO‐2, and DMT1 in the proximal small intestine of rats by confocal immunofluorescence microscopy up to 4 h after a dose of heme or ferrous iron. Double‐labeling was performed with endocytotic (EEA1, Lamp1) and structural markers (F‐actin). Results: HO‐1 was distributed evenly throughout the cytoplasm of enterocytes and did not colocalize with endocytotic markers in any condition. HO‐2 staining remained constant with dosing, presenting as a dense band in the apical cytoplasm that colocalized extensively with endosomes. DMT1 staining was markedly reduced by ferrous iron, but not heme and did not exhibit colocalization with endocytotic markers. Conclusion: The subcellular location of HO‐2 is consistent with the RME hypothesis for heme uptake and may suggest a possible role for this enzyme in heme degradation. The lack of translocation of DMT1 with heme dosing suggests another protein may be present to transport iron released from heme.