Molecular Cloning and Functional Expression of a Human Intestinal Lactoferrin Receptor

摘要

Lactoferrin (Lf), a major iron-binding protein in human milk, has been suuggested to have multiple biological roles such as facilitating iron absroption, modulating the immune system, embryonic development, and cell proliferation. Our previous binidng studies suggested the presence of a specific receptor for Lf (LfR) in the small intestine of newborn infants, which may facilitate iron absorption. We here report the cloning and the functional expression of the human intestinal LfR and the evidence of its involvement in iron metabolism. The entire codingn region of the lFR cDNA was cloned by PCR based on amino acid sequences of the purified native LfR (nLfR). The recombinant LfR (rLfR) was then expressed in a baculovirus-insect cell system and purified by immobilized human Lf (hLf) affinity chromatography where binding of hLf tot he rLfR was partially Ca~(2+) dependent. The apparent molecular mass was 136 kDa under nonreducing conditions and 34 kDa under reducing conditions. ~(125)I-hLf bound to the rLfR with an apparent K_d of approx360 nM. these biochmeical properties of the rLfR are similar to those of the nLfR. RT-PCR revealed that the gene was expressed at high levels in fetal small intestine and in adult heart and at lower levels in Caco-2 cells. PI-PLC treatment of Caco-2 cells indicated that the LfR is GPI anchored. In Caco-2 cells transfected wiith the LfR gene. ~(125)I-hLf binding and ~(59)Fe-hLf uptake were increased by 1.7 and 3.4 times, respectively, compared to those in mock-transfected cells. Our findings demonstrate the presence of a unique receptor-mediated mechanism for nutrient uptake by the newborn.