Human tear lipocalin [lipocalin 1 (lcn-1); von Ebner's gland protein] is a member of the lipocalin superfamily that is known to bind an unusual variety of lipophilic ligands. Because of its properties and its tissue-specific expression it has been suggested that lcn-1 might act as a physiological protection factor of epithelia. Overexpression of lcn-1 under certain disease conditions supported such a function. However, experimental investigations into its exact biological role and its mode of expression were impeded because lcn-1 was previously found to be produced only in serous glands. To overcome this problem we therefore sought a cell line that produced lcn-1 endogenously. Using reverse-transcriptase-mediated PCR analysis we found expression of lcn-1 in the human teratocarcinoma-derived NT2 precursor cells. Under normal conditions the production of lcn-1 is low. However, treatment of the cells with H2O2 or FeSO4, which typically induce lipid peroxidation, significantly enhanced the expression of lcn-1. Binding studies revealed that arachidonic acid and several lipid peroxidation products including 7β-hydroxycholesterol, 8-isoprostane and 13-hydroxy-9,11-octadecadienoic acid specifically bind to lcn-1. To investigate the physiological consequence of this observation we purified holo-(lcn-1) from culture medium and extracted the bound ligands. The presence of F2-isoprostanes in the extracts obtained from the fractions containing lcn-1 indicates that these typical lipid peroxidation products are indeed ligands of the protein in vivo. These results support the idea that lcn-1 acts as a physiological scavenger of potentially harmful lipophilic molecules; lcn-1 might therefore be a novel member of the cellular defence against the deleterious effects of oxidative stress.
Abbreviations used: AA, arachidonic acid; DAUDA, 11-(dansylamino)undecanoic acid; EIA, enzyme immunoassay; FCS, fetal calf serum; F2-iP, F2-isoprostanes; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; HODE, 13-hydroxy-9,11-octadecadienoic acid; lcn-1, lipocalin 1 (human tear lipocalin, von Ebner's gland protein); LCN1, gene encoding lcn-1; PG, prostaglandin; ROS, reactive oxygen species; RT–PCR, reverse-transcriptase-mediated PCR.