Immune-Induced Expression of Lipocalin-2 in Brain Endothelial Cells: Relationship with Interleukin-6, Cyclooxygenase-2 and the Febrile Response

Interleukin (IL)-6 is critical for the febrile response to peripheral immune challenge. However, the mechanism by which IL-6 enables fever is still unknown. To characterise the IL-6-dependent fever generating pathway, we used microarray analysis to identify differentially expressed genes in the brain of lipopolysaccharide (LPS)-treated IL-6 wild-type and knockout mice. Mice lacking IL-6 displayed a two-fold lower expression of the lipocalin-2 gene (lcn2), and this difference was confirmed by real-time reverse transcriptase-polymerase chain reaction. Conversely, the induction of lipocalin-2 protein was observed in brain vascular cells following i.p. administration of recombinant IL-6, suggesting a direct relationship between IL-6 and lipocalin-2. Immunohistochemical analysis also revealed that LPS-induced lipocalin-2 is expressed by brain endothelial cells and is partly co-localised with cyclooxygenase-2 (Cox-2), the rate-limiting enzyme for the production of inflammatory induced prostaglandin E2 (PGE2), which is the key mediator of fever. The direct role of lipocalin-2 in fever was examined in LPS-challenged lipocalin-2 knockout mice. In both male and female mice, normal fever responses were observed at near-thermoneutral conditions (2930 degrees C) but when recorded at normal room temperature (1920 degrees C), the body temperature of lipocalin-2 knockout female mice displayed an attenuated fever response compared to their wild-type littermates. This difference was reflected in significantly attenuated mRNA expression of Cox-2 in the brain of lipocalin-2 knockout female mice, but not of male mice, following challenge with peripheral LPS. Our findings suggest that IL-6 influences the expression of lipocalin-2, which in turn may be involved in the control of the formation of Cox-2, and hence central PGE2-production. We have thus identified lipocalin-2 as a new factor in the pathway of inflammatory IL-6 signalling. However, the effect of lipocalin-2 on fever is small, being sex-dependent and ambient temperature-specific, and thus lipocalin-2 cannot be considered as a major mediator of the IL-6-dependent fever generating pathway.