Measuring non-polyaminated lipocalin-2 for cardiometabolic risk assessment

Abstract

Lipocalin-2 is a proinflammatory protein with diversified expression patterns and different structure-functional relationships. Both human and murine lipocalin-2 are post-translationally modified by polyamination, which regulates its circulating half-life. Non-polyaminated lipocalin-2 tends to be accumulated in the circulation and tissues, in turn contributing to the development of obesity-associated medical complications. The present study investigated expression and distribution of both polyaminated (pLcn2) and non-polyaminated (npLcn2) lipocalin-2 in biofluids and tissues collected from healthy human subjects and patients with heart failure, by using specific immunoassays. In healthy volunteers, the circulating and urinary levels of pLcn2 and npLcn2 exhibited gender differences, with females having lower npLcn2 in serum but higher npLcn2 in urine. While obese subjects had higher levels of pLcn2 and npLcn2, the latter showed a stronger correlation with increased components of metabolic syndrome. Compared with pLcn2, npLcn2 levels displayed positive correlations with heart rate, triglyceride, high-sensitive C-reaction protein and creatinine independently of age, gender, smoking and BMI. Both pLcn2 and npLcn2 were positively correlated with aldosterone levels and kidney injury molecule-1 in urine. In heart failure patients, increased npLcn2 levels were detected in the plasma when compared to those of healthy volunteers. Patients with more components of metabolic syndrome had elevated levels of npLcn2 in both plasma and pericardial fluid samples. B type natriuretic peptide level was positively correlated with pLcn2/npLcn2 ratio in plasma and positively associated with both pLcn2 and npLcn2 in pericardial fluid. The heart failure patients with preserved ejection fraction (HFpEF) had increased levels of npLcn2 in plasma and pericardial fluid, and more accumulation of npLcn2 in their pericardial fluid than those heart failure patients with reduced ejection fraction (HFrEF). The pericardium biopsy samples were immuno-stained with antibodies against pLcn2 or npLcn2. Different patterns of distribution were observed for these two forms of lipocalin-2. Large amount of npLcn2 was detected in the mesothelial cells of the pericardium. In addition, npLcn2 was present in leukocytes and adipocytes. By contrast, pLcn2 was mainly observed in the leukocytes. Neutrophil activation-associated markers, including neutrophil elastase and proteinase 3, were positively correlated with npLcn2 in plasma. Increased levels of proteinase 3 was detected in pericardial fluid samples of patients with HFpEF, a pattern similar as that of npLcn2. Cellular studies suggested that neutrophils lipocalin-2 protein, indicating that their interactions with lipocalin-2 determined the distribution of this molecule in the pericardium of heart failure patients. In conclusion, npLcn2 was associated with unfavorable cardiometabolic characteristics. The accumulation of npLcn2 in the pericardial space, as well as its association with cardiac dysfunction and neutrophils-activation markers suggest a pivotal role of this form of lipocalin-2 in the development of heart failure.