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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.

Background: Lipocalin 2 (LCN2), also known as neutrophil gelatinase-associated lipocalin, is a 25 kDa protein involved in immune defense, inflammation, and metabolism. Results: LCN2 is widely expressed across various tissues, including immune cells, bone, adipose tissue, liver, kidneys, lung, spleen, and epithelial cells, and exhibits sex- and fat depot-specific expression patterns. Structurally, LCN2 contains a hydrophobic lipid-binding pocket and glycosylation sites, enabling it to interact with diverse ligands and form dimers. In innate immunity, LCN2 plays a critical role by sequestering iron-laden siderophores, thereby restricting bacterial growth. Beyond its role in infection control, LCN2 is implicated in metabolic inflammation and diseases such as obesity and diabetes. Recent research has highlighted a pivotal role for LCN2 in mitochondrial phospholipid metabolism and mitochondrial function. In metabolic diseases and mitochondrial metabolism, LCN2 appears to display paradoxical effects. While some studies link it to improved insulin sensitivity, glucose regulation, and mitochondrial function, others associate it with insulin resistance, obesity, and mitochondrial dysfunction. These inconsistencies may arise from differences in experimental conditions and study populations. Conclusions: This review provides an up-to-date summary of LCN2’s multifaceted roles in obesity, diabetes, energy balance, and mitochondrial function, emphasizing its context-dependent effects. LCN2 appears to have dual roles, exerting both protective and detrimental outcomes depending on the physiological or pathological context, sex, cell types, and experimental conditions. Further research is necessary to unravel its complex functions and resolve conflicting findings, particularly in metabolic disorders.

Apart from a well-known role in the innate immune system, lipocalin 2 (Lcn2) has been recently characterized as a critical regulator of thermogenesis and lipid metabolism. However, the physiological mechanism through which Lcn2 regulates cellular metabolism and thermogenesis in adipocytes remains unknown. We found that Lcn2 expression and secretion are significantly upregulated by arachidonic acid (AA) and mTORC1 inhibition in differentiated inguinal adipocytes. AA-induced Lcn2 expression and secretion correlate with the inflammatory NFkB activation. Lcn2 deficiency leads to the upregulation of cyclooxygenase-2 (COX2) expression, as well as increased biosynthesis and secretion of prostaglandins (PGs), particularly PGE2 and PGD2, induced by AA in adipocytes. Furthermore, Lcn2 deficiency affects the mTOR signaling regulation of thermogenic gene expression, lipogenesis, and lipolysis. The loss of Lcn2 dismisses the effect of mTORC1 inhibition by rapamycin on COX2, thermogenesis genes, lipogenesis, and lipolysis, but has no impact on p70 S6Kinase-ULK1 activation in Lcn2-deficient adipocytes. We conclude that Lcn2 converges the COX2-PGE2 and mTOR signaling pathways in the regulation of thermogenesis and lipid metabolism in adipocytes.

Background/objectivesPentraxin 3 (PTX3) has been characterized as a soluble and multifunctional pattern recognition protein in the regulation of innate immune response. However, little is known about its role in adipose tissue inflammation and obesity. Herein, we investigated the role of PTX3 in the regulation of lipopolysaccharide (LPS)-induced inflammation in adipocytes and adipose tissue, as well as high-fat diet (HFD)-induced metabolic inflammation in obesity.MethodsPtx3 knockdown 3T3-L1 Cells were generated using shRNA for Ptx3 gene and treated with different inflammatory stimuli. For the in vivo studies, Ptx3 knockout mice were treated with 0.3 mg/kg of LPS for 6 h. Adipose tissues were collected for gene and protein expression by qPCR and western blotting, respectively. Ptx3 knockout mice were fed with HFD for 12 week since 6 week of age.ResultsWe observed that the expression of PTX3 in adipose tissue and serum PTX3 were markedly increased in response to LPS administration. Knocking down Ptx3 in 3T3-L1 cells reduced adipogenesis and caused a more profound and sustained upregulation of proinflammatory gene expression and signaling pathway activation during LPS-stimulated inflammation in 3T3-L1 adipocytes. In vivo studies showed that PTX3 deficiency significantly exacerbated the LPS-induced upregulation of inflammatory genes and downregulation of adipogeneic genes in visceral and subcutaneous adipose tissue of mice. Accordingly, LPS stimulation elicited increased activation of nuclear factor-κB (NF-κB) and p44/42 MAPK (Erk1/2) signaling pathways in visceral and subcutaneous adipose tissue. The expression of PTX3 in adipose tissue was also induced by HFD, and PTX3 deficiency led to the upregulation of proinflammatory genes in visceral adipose tissue of HFD-induced obese mice.ConclusionsOur results suggest a protective role of PTX3 in LPS- and HFD-induced sustained inflammation in adipose tissue.

Adipose tissue dysfunction in response to obesity or aging is a major underlying factor in the development of metabolic diseases. Therefore, identifying mechanisms that promote adipose tissue health in the face of metabolic challenges is essential for preventing further deterioration of whole-body metabolic homeostasis. Lipocalin 2 (Lcn2) is an adipose tissue-derived secreted protein that has previously been shown to play a role in diet-induced obesity, metabolic disease, and brown adipose tissue (BAT) thermogenesis. This thesis project addresses two main questions: 1) whether Lcn2 regulates thermogenic activation, or beiging, of inguinal white adipose tissue (iWAT) and 2) whether overexpression of Lcn2 promotes whole-body metabolic homeostasis and healthspan. In the first project, we found Lcn2 deficient inguinal adipocytes have decreased expression of thermogenic genes and reduced mitochondrial capacity under basal conditions. Further, Lcn2 is necessary for retinoic acid (RA) induction of beiging markers in inguinal adipocytes, particularly in the presence of insulin. Lcn2 is required for insulin-stimulated localization of the retinoic acid receptor-alpha (RAR-) to the plasma membrane. Together, this suggests Lcn2 promotes RA-induced thermogenesis, possibly through regulation of RAR- at the plasma membrane. In the second project, we further investigated the role of Lcn2 in WAT beiging and energy metabolism in vivo. Utilizing an ap2-promoter-driven Lcn2 transgenic (Tg) mouse model, we determined whether overexpression of Lcn2 in adipose tissue is sufficient to promote beiging/thermogenesis. We found overexpression of Lcn2 in adipose tissue leads to improved cold adaptation and an increase in beiging markers in iWAT, including uncoupling protein 1 (Ucp1) gene expression. Lcn2 Tg mice had a trend towards increased fat utilization, alongside increased oxidative gene expression and decreased adipocyte size in iWAT. In the third project, we investigated the role of Lcn2 in adipose tissue function and metabolic healthspan. We determined whether overexpression of Lcn2 in adipose tissue can prevent the age-associated decline in adipogenesis, serum metabolic parameters, and liver function. Following aging, Lcn2 Tg mice maintained higher levels of adipogenic markers and reduced adipocyte size in iWAT relative to WT mice, suggesting improved adipose tissue health. Aged Lcn2 Tg mice had decreased serum triglycerides, significantly better maintenance of glucose tolerance, and protection from liver lipid accumulation resulting in decreased markers of liver inflammation and steatosis. In conclusion, Lcn2 has a novel role in promoting WAT beiging and adipose tissue health, protecting against age-related metabolic deterioration, and improving metabolic healthspan.

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 TNF alpha , IL-1 beta , and IL-6, IL-1 beta 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 NF Kappa B 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.