Background: Lipocalin-2 is a proinflammatory adipokine upregulated in obese humans and animals. A pathogenic role of lipocalin-2 in hypertension has been suggested. Mice lacking lipocalin-2 are protected from dietary obesity-induced cardiovascular dysfunctions. Administration of lipocalin-2 causes abnormal vasodilator responses in mice on a high-fat diet (HFD).
Methods and results: Wild-type and lipocalin-2 knockout mice were fed with standard chow or HFD. Immunoassays were performed for evaluating the circulating and tissue contents of lipocalin-2. The relaxation and contraction of arteries were studied using a wire myograph. Blood pressure was monitored with implantable radio telemetry. Dietary obesity promoted the accumulation of lipocalin-2 protein in blood and arteries. Deficiency of this adipokine protected mice from dietary obesity-induced elevation of blood pressure. Mass spectrometry analysis revealed that human and murine lipocalin-2 were modified by polyamination. 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 (18:2n-6), which bound to lipocalin-2 with high affinity and prevented its interactions with matrix metalloproteinase 9 (MMP9). Combined administration of linoleic acid with lipocalin-2 caused vascular inflammation and endothelial dysfunction and raised the blood pressure of mice receiving standard chow. 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 treatment, C87A remained in the circulation for a prolonged period of time and evoked endothelial dysfunction in the absence of linoleic acid.
Conclusions: Polyamination facilitates the clearance of lipocalin-2, whereas the accumulation of deamidated lipocalin-2 in arteries causes vascular inflammation, endothelial dysfunction, and hypertension.
Keywords: adipokine; endothelial dysfunction; inflammation; lipotoxicity; obesity.