Up-regulation and activation of lipocalin-2 causes endothelial dysfunction and hypertension in obese mice

Abstract

Lipocalin-2 is an adipokine with pro-inflammatory properties which is upregulated in obese humans and animals. Augmented lipocalin-2 expression is implicated in the pathogenesis of obesity-associated cardiovascular abnormalities. Lipocalin-2 deficiency (Lcn2-KO) protects mice against endothelial dysfunction and arterial blood pressure elevation induced by high fat diet. The administration of lipocalin-2 causes abnormal endothelium-dependent responses in mice on a high-fat diet.

In wild type mice fed standard chow, non-esterified free fatty acids (NEFAs) treatment attenuated insulin-evoked relaxations in aortae and enhances acetylcholine-stimulated contractions in carotid arteries. However, mice without lipocalin-2 given the same diet were resistant to NEFAs-induced endothelial dysfunction. Administration of lipocalin-2 and linoleic acid (18:2n-6) in combination, but not separately, caused vascular inflammation and endothelial dysfunction in both aortae and carotid arteries of Lcn2-KO mice. Linoleic acid treatment extended the circulating half-life of lipocalin-2 and facilitated the accumulation of this adipokine in arteries.

In the blood and in tissues, lipocalin-2 was present as both a high and a low molecular weight species, which correspond to polyaminated and deamidated forms, respectively. Polyaminated lipocalin-2 was rapidly cleared from the circulation. Adipose tissue was a major site for lipocalin-2 deamidation. The circulating levels and the arterial accumulation of deamidated lipocalin-2 were significantly enhanced by treatment with linoleic acid, which prevented its interactions with matrix metalloproteinase 9 (MMP9). A human lipocalin-2 mutant with cysteine 87 replaced by alanine (C87A) contained less polyamines and exhibited a reduced capacity to form heterodimeric complexes with MMP9. After injection, C87A remained in the circulation for a prolonged period of time and evoked endothelial dysfunction in the absence of linoleic acid.

Linoleic acid time-and concentration-dependently stimulated the release of high molecular weight lipocalin-2 species (polyaminated) from neutrophils, whereas the low molecular weight lipocalin-2 species (deamidated) was released from differentiated 3T3-L1 cells. Bone marrow transplantation experiment showed that the high molecular weightlipocalin-2 species was derived mainly from bone marrow cells, whereas the low molecular weight species was produced from peripheral tissues. Endothelial function was improved in wild type mice receiving bone marrow cells from Lcn2-KO mice.

Taken in conjunction, the findings support that the lipid (linoleic acid)-adipokine (lipocalin-2) cross-regulation and the adipose-vascular axis play an important role in the pathogenesis of vascular dysfunction and hypertension associated with obesity.