Lactoferrin receptor mediates apo- but not holo-lactoferrin internalization via clathrin-mediated endocytosis in trophoblasts

摘要

LfR [Lf (lactoferrin) receptor] is expressed in most mammalian tissues, including placental trophoblasts, and is presumed to mediate the internalization of Lf. However, the physiological significance of trophoblast LfR is not understood. Using the CT (cytotrophoblast) cell model BeWo, we demonstrated that transfection with LfR siRNA (small interfering RNA) significantly decreased apo- but not holo-Lf uptake compared with mock-transfected controls and that apo- but not holo-Lf significantly increased MMP (matrix metalloproteinase)-2 activity. As Lf functionality is related to the presence (holo-Lf) or absence (apo-Lf) of iron within the Lf molecule, our results suggest that apo-Lf may play a role in cellular invasion. Moreover, we detected LfR (similar to 105 kDa) in association with the plasma membrane, and ligand blotting confirmed that Lf binds to a LfR of similar to 105 kDa. Apo-Lf treatment significantly increased LfR abundance at the plasma membrane and internalization probably occurs via clathrin-mediated endocytosis through early and recycling endosomes, as LfR was co-localized with EEA1 (early endosome antigen 1) and TfR (transferrin receptor) using confocal microscopy, and hypertonic medium (0.4 M sucrose) significantly inhibited apo-Lf internalization. In summary, our data demonstrate that apo- but not holo-Lf is internalized by LfR and suggest that, following internalization via LfR, apo-Lf plays a role in CT invasiveness by inducing MMP-2 activity. Moreover, LfR facilitates apo-Lf uptake specifically through clathrin-mediated endocytosis into early endosomes and potentially into a recycling pathway. Taken together, our data provide a new dimension in understanding ligand-dependant function that may be directly related to the ability of LfR to selectively internalize apo- but not holo-Lf.