Mice lacking lipocalin-2 are protected from developing insulin resistance associated with aging and obesity

Abstract

Background and aims: Obesity is characterized as a low-grade inflammation of the adipose tissue and increased circulating concentrations of pro-inflammatory cytokines. The “inflamed” adipose tissue can secrete a large amount of pro-inflammatory adipokines / cytokines that act as mediators for obesity-related metabolic syndrome. Previous studies have shown that the expression and production of lipocalin-2, a pro-inflammatory adipokine, are elevated in obese animal and human subjects. In this study, we aim to evaluate whether lipocalin-2 deficiency can protect the mice from developing systemic insulin resistance. Methods and results: C57 wildtype and lipocalin-2 deficient mice (Lcn2-KO) were given free access to high fat diet from the age of 4 weeks for the induction of dietary obesity. In addition, genetic obesity was induced by cross-breeding male C57BL/6J db/+ mice with female Lcn2-KO mice to generate the leptin receptor (Lepr)-/-/lipocalin-2-/- double knockout mice (DKO). Deficiency of lipocalin-2 in mice prevents age-associated decline in insulin sensitivity and the development of insulin resistance induced by high fat diet and genetically obesity. In diet-induced obesity, lower fasting blood glucose was consistently observed in Lcn2-KO mice (5.6±1.18 mmol/L vs that in C57 wildtype mice, 8.4±1.51 mmol/L, p<0.05). HOMA-IR index was increased by 77.93% in C57 wildtype after 18-weeks high fat diet feeding. On the other hand, in mice under normal diet, at the age of 21-week, the fasting glucose levels of Lcn2-KO mice were significantly lower than those of C57 mice (3.62±0.35 mmol/L in Lcn2-KO mice vs 5.05±0.3 mmol/L in C57 wildtype). Both fasting serum insulin levels and HOMO-IR indexes were largely reduced in 24-week old Lcn2-KO mice, by 44.6% and 79.35% respectively comparing to those of the control mice. Similar phenomena were observed in DKO mice, which did not develop hyperinsulinemia as in the leptin receptor (Lepr)-/- (db/db) counterparts (516.778.421±73.225 µU/ml in wildtype db/db mice, 97.67±35.63 µU/ml in DKO mice, p<0.05). Lipocalin-2 deficiency attenuated peripheral insulin resistance and macrophage infiltration in adipose tissue. Further analysis revealed that obesity- and lipopolysaccharide-induced expressions of TNFα, a pro-inflammatory cytokine, were completed blocked by lipocalin-2 deficiency in epidydimal adipose tissue as revealed by quantitative PCR analysis On the other hand, increased expression of IκB found in Lcn2-KO mice while incubation of lipocalin-2 in differentiated 3T3-L1 adipocytes lowered IκB expression and increased NFκB activity. Conclusion: Lipocalin-2 plays critical roles in causing obesity-induced insulin resistance through regulating TNFα expressions, which might account for its systematic metabolic and pro-inflammatory activities.