The induction of lipocalin-2 protein expression in vivo and in vitro.

Lipocalin-2 (LCN2) is secreted from adipocytes, and its expression is up-regulated in obese and diabetic mice and humans. LCN2 expression and secretion have been shown to be induced by two proinflammatory cytokines, IFNγ and TNFα, in cultured murine and human adipocytes. In these studies, we demonstrated that IFNγ and TNFα induced LCN2 expression and secretion in vivo. Although we observed a strong induction of LCN2 expression and secretion from white adipose tissue (WAT) depots, the induction of LCN2 varied among different insulin-sensitive tissues. Knockdown experiments also demonstrated that STAT1 is required for IFNγ-induced lipocalin-2 expression in murine adipocytes. Similarly, knockdown of p65 in adipocytes demonstrated the necessity of the NF-κB signaling pathway for TNFα-mediated effects on LCN2. Activation of ERKs by IFNγ and TNFα also affected STAT1 and NF-κB signaling through modulation of serine phosphorylation. ERK activation-induced serine phosphorylation of both STAT1 and p65 mediated the additive effects of IFNγ and TNFα on LCN2 expression. Our results suggest that these same mechanisms occur in humans as we observed STAT1 and NF-κB binding to the human LCN2 promoter in chromatin immunoprecipitation assays performed in human fat cells. These studies substantially increase our knowledge regarding the requirements and mechanisms used by proinflammatory cytokines to induce LCN2 expression.