Human tear lipocalin acts as an oxidative-stress-induced scavenger of potentially harmful lipid peroxidation products in a cell culture system

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 variet y of lipophilic ligands. Because of its properties and its tissue-specific expression it has been suggested that lcn-1 might act as a physiological pr otection 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 bec ause lcn-1 was previously found to be produced only in serous glands. To ov ercome this problem we therefore sought a cell line that produced lcn-1 end ogenously. Using reverse-transcriptase-mediated PCR analysis we found expre ssion of lcn-1 in the human teratocarcinoma-derived NT2 precursor cells. Un der normal conditions the production of lcn-1 is low. However, treatment of the cells with H2O2 or FeSO4, which typically induce lipid peroxidation, s ignificantly enhanced the expression of lcn-1. Binding studies revealed tha t arachidonic acid and several lipid peroxidation products including 7 beta -hydroxycholesterol, 8-isoprostane and 13-hydroxy-9,11-octadecadienoic aci d specifically bind to lcn-1. To investigate the physiological consequence of this observation we purified holo-(lcn-1) from culture medium and extrac ted the bound ligands. The presence of F-2-isoprostanes in the extracts obt ained from the fractions containing lcn-1 indicates that these typical lipi d peroxidation products are indeed ligands of the protein in vivo. These re sults support the idea that lcn-1 acts as a physiological scavenger of pote ntially harmful lipophilic molecules : lcn-1 might therefore be a novel mem ber of the cellular defence against the deleterious effects of oxidative st ress.