Lipocalin-2 Deficiency Impairs Thermogenesis and Potentiates Diet-Induced Insulin Resistance in Mice

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.Obesity is a major risk for developing insulin resistance, a hallmark of type 2 diabetes and other metabolic complications such as fatty liver, dyslipidemia, and atherosclerosis. Adipose tissue plays a central role in body weight homeostasis, inflammation, and insulin resistance via regulating lipid metabolism/storage and releasing a range of adipokines/cytokines (1–4). Adipose tissue in a variety of insulin-resistant states has been characterized by dysregulated lipid metabolism and altered production of adipokines/cytokines that, in sum, are important contributors to systemic inflammation and related metabolic disorders.Lipocalin (LCN) 2 (also known as neutrophil gelatinase–associated lipocalin [NGAL]), a lipocalin subfamily member, has been recently identified by our group and others (5,6) as an adipose-derived cytokine. LCN2 is a 25-kDa secreted protein initially identified from human neutrophils (7,8) and other immune cells and tissues that are exposed to microorganisms in the respiratory and gastrointestinal tract and is present abundantly in the circulation (9). Interestingly, lipocalins have structural similarity with fatty acid binding proteins (FABPs), and both are members of the multigene family of up and down β-barrel proteins (10). Both the intracellular FABPs and the extracellular lipocalins have a clearly defined β-barrel motif that forms either an interior cavity (FABP) or a deep pit (lipocalins) that constitutes the lipid binding domain (10). The extracellular lipocalins such as LCN2, retinol binding protein (RBP) 4, and α₂-microglobulin use a series of β-strands to form a globular domain with a deep depression resembling the calyx of a flower. Because of the unique structure, the lipocalins function as efficient transporters for a number of different hydrophobic ligands in extracellular milieus, including a variety of retinoids, fatty acids, biliverdin, pheromones, porphyrins, odorants, steroids, and iron. RBP4, one of the extracellular lipocalins, affects glucose metabolism and insulin sensitivity (11).Previous studies have demonstrated that LCN2 gene expression is upregulated in adipose tissue and liver of genetically obese animals (6). Rosiglitazone administration significantly reduces LCN2 expression in adipose tissue in obese animals (6), suggesting that the protein may function as a proinflammatory factor. Unexpectedly, the addition of LCN2 protein to the culture media of adipocytes and macrophages leads to the suppression of tumor necrosis factor (TNF)α- and lipopolysaccharide-induced cytokine/chemokine production, indicating an anti-inflammatory function (6). Most strikingly, LCN2 appears to protect against TNFα-induced insulin resistance in adipocytes. Unlike RBP4, increased production of LCN2 in obesity may be a protective mechanism against inflammation and insulin resistance.To evaluate this hypothesis, we assessed the metabolic and regulatory consequences of LCN2 deficiency. Herein, we show that the ablation of LCN2 profoundly impairs adaptive thermogenesis and exacerbates high-fat diet (HFD)- or age-induced insulin resistance and glucose homeostasis. LCN2-deficient mice have increased hepatic gluconeogenesis and inflammatory state and exhibit a cold sensitive phenotype.