Antisense down-regulation of lipocalin-interacting membrane receptor expression inhibits cellular internalization of lipocalin-1 in human NT2 cells.

There is increasing experimental evidence demonstrating that many lipocalins bind to specific cell surface receptors. However, whereas the binding of lipocalins to their lipophilic ligands has now been characterized in much detail, there is a lack of knowledge about the nature of lipocalin receptors, the physiological role of receptor binding, and the molecular mechanism of ligand delivery. We previously identified a novel human membrane protein (lipocalin-1-interacting membrane receptor (LIMR)), which interacts with lipocalin-1 (Wojnar, P., Lechner, M., Merschak, P., and Redl, B. (2001) J. Biol. Chem. 276, 20206-20212). In the present study, we investigated the physiological role of LIMR and found this protein to be essential for mediating internalization of lipocalin-1 (Lcn-1) in NT2 cells, leading to its degradation. Whereas control NT2 cells rapidly internalized (125)I-Lcn-1 or fluorescein isothiocyanate-labeled Lcn-1, NT2 cells that were made LIMR deficient by cDNA antisense expression greatly accumulated Lcn-1 in the culture medium but did not internalize it. Because sequence and structure analysis indicated that proteins similar to LIMR are present in several organisms and at least two closely related orthologues are found in human and mouse, we suggest LIMR to be the prototype of a new family of endocytic receptors, which are topographically characterized by nine putative transmembrane domains and a characteristic large central cytoplasmic loop.

Human lipocalin-1, a physiological scavenger of lipophilic compounds, is produced by corticotrophs of the pituitary gland.

Lipocalin-1 (Lcn-1), a member of the lipocalin superfamily that binds a broad array of different chemical classes of lipophilic ligands, is believed to act as a physiological scavenger of potentially harmful lipophilic molecules. Thus far, it was thought to be produced exclusively by a number of exocrine glands and tissues, including lachrymal and lingual glands, prostate, secretory glands of the tracheobronchial tract, and sweat glands. Using Northern blotting analysis, we were able to demonstrate Lcn-1 expression by the human pituitary gland. Moreover, double immunolabeling with antibodies against Lcn-1 and pituitary gland hormones and detection with fluorophore-conjugated secondary antibodies revealed that Lcn-1 is specifically produced by corticotrophs, clearly indicating that its distribution is not restricted to exocrine tissues.