Adipose tissue-derived cytokines (adipokines) are associated with the development of inflammation and insulin resistance. However, which adipokine(s) mediate this linkage and the mechanisms involved during obesity is poorly understood. Through proteomics and microarray screening, we recently identified lipocalin 2 (LCN 2) as an adipokine that potentially connects obesity and its related adipose inflammation. Herein we show that the levels of LCN2 mRNA are dramatically increased in adipose tissue and liver of ob/ob mice and primary adipose cells isolated from Zucker obese rats, and thiazolidinedione administration reduces LCN2 expression. Interestingly, addition of LCN2 induces mRNA levels of peroxisome proliferator-activated receptor-γ (PPARγ) and adiponectin. Reducing LCN2 gene expression causes decreased expression of PPARγ and adiponectin, slightly reducing insulin-stimulated Akt2 phosphorylation at Serine 473 in 3T3-L1 adipocytes. LCN2 administration to 3T3-L1 cells attenuated TNFα-effect on glucose uptake, expression of PPARγ, insulin receptor substrate-1, and glucose transporter 4, and secretion of adiponectin and leptin. When added to macrophages, LCN2 suppressed lipopolysaccharide-induced cytokine production. Our data suggest that LCN2, as a novel autocrine and paracrine adipokine, acts as an antagonist to the effect of inflammatory molecules on inflammation and secretion of adipokines.

OBJECTIVE Lipocalin (LCN) 2 belongs to the lipocalin subfamily of low–molecular mass–secreted proteins that bind small hydrophobic molecules. LCN2 has been recently characterized as an adipose-derived cytokine, and its expression is upregulated in adipose tissue in genetically obese rodents. The objective of this study was to investigate the role of LCN2 in diet-induced insulin resistance and metabolic homeostasis in vivo. RESEARCH DESIGN AND METHODS Systemic insulin sensitivity, adaptive thermogenesis, and serum metabolic and lipid profile were assessed in LCN2-deficient mice fed a high-fat diet (HFD) or regular chow diet. RESULTS The molecular disruption of LCN2 in mice resulted in significantly potentiated diet-induced obesity, dyslipidemia, fatty liver disease, and insulin resistance. LCN2^−/− mice exhibit impaired adaptive thermogenesis and cold intolerance. Gene expression patterns in white and brown adipose tissue, liver, and muscle indicate that LCN2^−/− mice have increased hepatic gluconeogenesis, decreased mitochondrial oxidative capacity, impaired lipid metabolism, and increased inflammatory state under the HFD condition. CONCLUSIONS LCN2 has a novel role in adaptive thermoregulation and diet-induced insulin resistance.

Lipocalin 2 (Lcn2) has been previously characterized as an adipokine/cytokine and implicated in obesity and inflammation. Herein, we investigated the role and potential mechanism of Lcn2 in the regulation of macrophage polarization in obesity-associated inflammation. We observed that Lcn2−/− mice displayed an up-regulation of expression of M1 macrophage marker Cd11c but a down-regulation of M2 marker arginase 1 in adipose tissue and liver of mice upon a high-fat diet feeding. Lcn2-deficient bone marrow–derived macrophages (BMDMs) were more sensitive to lipopolysaccharide (LPS) stimulation, leading to a more profound up-regulation of expression of pro-inflammatory markers than wild-type (WT) BMDMs. Accordingly, LPS stimulation elicited an increase in the activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), c-Jun, and STAT3 signaling pathways as well as an up-regualtion of expression of NF-κB and STAT3 target genes such as IL-1β, IL-6, iNOS, and MCP-1 in Lcn2−/− BMDMs compared with WT controls. Pre-treatment of recombinant Lcn2 attenuated LPS-stimulated degradation of IκBα and STAT3 phosphorylation as well as LPS-induced gene expression of IL-6 and iNOS in Lcn2−/− BMDMs. Moreover, the NFκB inhibitor markedly blocked LPS-stimulated STAT3 phosphorylation in Lcn2−/− BMDMs. These results together with the time course of Lcn2 secretion, NFκB and STAT3 phosphorylation in response to LPS stimulation, suggest that Lcn2 plays a role as an anti-inflammatory regulator in macrophage activation via modulating a feed-forward activation of NFκB-STAT3 loop.

Lipocalin 2 (Lcn2) has been recently characterized as a new adipokine having a role in innate immunity and energy metabolism. Nonetheless, the metabolic regulation of Lcn2 production in adipocytes has not been comprehensively studied. To better understand the Lcn2 biology, we investigated the regulation of Lcn2 expression in adipose tissue in response to metabolic stress in mice as well as the control of Lcn2 expression and secretion by cytokines and nutrients in 3T3-L1 adipocytes. Our results showed that the mRNA expression of Lcn2 was upregulated in white and brown adipose tissues as well as liver during fasting and cold stress in mice. Among pro-inflammatory cytokines TNFα, IL-1β, and IL-6, IL-1β showed most profound effect on Lcn2 expression and secretion in 3T3-L1 adipocytes. Insulin stimulated Lcn2 expression and secretion in a dose-dependent manner; this insulin effect was significantly abolished in the presence of low concentration of glucose. Moreover, insulin-stimulated Lcn2 expression and secretion was also attenuated when glucose was replaced by 3-O-methyl-d-glucose or by blocking NFκB pathway activation. Additionally, we showed that palmitate and oleate induced Lcn2 expression and secretion more significantly than EPA, while phytanic acid reduced Lcn2 production. Our results demonstrated that Lcn2 production in adipocytes is highly responsive to metabolic stress, cytokines, and nutrient signals, suggesting an important role of Lcn2 in adipocyte metabolism and inflammation.

We have previously identified lipocalin 2 (Lcn2) as a cytokine playing a critical role in the regulation of body fat mass, lipid metabolism, and insulin resistance. Lcn2 deficiency reduces PPARγ gene expression in adipocytes. In this study, we investigated the role of Lcn2 in PPARγ activation and function via assessing the insulin sensitization and fatty acid (FA) homeostasis of PPARγ agonist in high‐fat diet (HFD)‐induced obesity in Lcn2 ^−/− mice. We found that rosiglitazone (Rosi) significantly improved insulin sensitivity in Lcn2 ^−/− mice as effectively as in wild‐type (WT) mice; unfed‐state levels of blood glucose, free FAs, and triglycerides (TGs) were significantly reduced after a 25‐d treatment of Rosi in Lcn2 ^−/− mice. However, Rosi action on fat deposition and FA homeostasis was altered; Rosi‐induced body weight and subcutaneous fat gain and liver lipid accumulation were markedly lessened in Lcn2 ^−/− mice. The results of in vivo metabolic labeling showed that Rosi markedly reduced de novo lipogenesis in adipose tissue of Lcn2 ^−/− mice. In brown adipose tissue (BAT), the expression of the genes functioning in TG hydrolysis and mitochondrial oxidation was up‐regulated more in Lcn2 ^−/− than in WT mice. Most strikingly, Rosi stimulated significantly higher levels of uncoupling protein‐1 expression in BAT, and completely rescued cold intolerance in Lcn2 ^−/− mice. We demonstrate that Lcn2 is a critical selective modulator of PPARγ activation and function in lipid homeostasis and energy expenditure.—Jin, D., Guo, H., Bu, Y. S., Zhang, Y., Hannaford, J., Mashek, D. G., Chen, X. Lipocalin 2 is a selective modulator of peroxisome proliferator‐activated receptor‐γ activation and function in lipid homeostasis and energy expenditure. FASEB J. 25, 754–764 (2011). www.fasebj.org