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

Abstract

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 H(2)O(2) or FeSO(4), which typically induce lipid peroxidation, significantly enhanced the expression of lcn-1. Binding studies revealed that arachidonic acid and several lipid peroxidation products including 7beta-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 F(2)-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.

Differences in neuroanatomical sites of apoD elevation discriminate between schizophrenia and bipolar disorder

“…Moreover, recent studies have demonstrated that another lipocalin, human tear lipocalin, acts as a scavenger for harmful lipophilic molecules and, hence, may work as a defense against the deleterious effects of oxidative stress. 51 A similar role may be provided by apoD in psychiatric disorders as it has been suggested that oxidative stress or damage contributes to the pathophysiology of schizophrenia. 52 Free radicals are reactive chemical species generated during normal metabolic processes and can damage lipids, proteins and DNA.…”

Molten globule state of tear lipocalin: ANS binding restores tertiary interactions

“…Numerous putative functions, most of which are linked to the binding of various classes of ligand, have been suggested for TL [15][16][17][18][19][20][21][22][23]. One postulate is that TL stabilizes and modulates lipid in the tear film through a molten globule state triggered by low pH at the aqueous-lipid interface [5].…”

Evidence for internal and external binding sites on human tear lipocalin

“…TL binds a host of other biologically important compounds including sapid molecules [8], retinol [9], potentially harmful lipid oxidation products [10], antioxidants [11], and fungal siderphores [12]. Several putative functions have been assigned to TL such as scavenging lipid from the corneal surface to prevent the formation of lipid induced dry spots [13], solubilization of lipid in tears (2), antimicrobial activity [14], cysteine proteinase inhibition [15], endonuclease activity [16] and scavenging of toxic molecules [10]. The solution structure of TL has been resolved by site directed tryptophan fluorescence and verified by crystallography of TL [17,18].…”