Metabolic Hormones, Apolipoproteins, Adipokines, and Cytokines in the Alveolar Lining Fluid of Healthy Adults: Compartmentalization and Physiological Correlates

Abstract

ObjectivesOur current understanding of hormone regulation in lung parenchyma is quite limited. We aimed to quantify a diverse array of biologically relevant protein mediators in alveolar lining fluid (ALF), compared to serum concentrations, and explore factors associated with protein compartmentalization on either side of the air-blood barrier.Research Design and MethodsParticipants were 24 healthy adult non-smoker volunteers without respiratory symptoms or significant medical conditions, with normal lung exams and office spirometry. Cell-free bronchoalveolar lavage fluid and serum were analyzed for 24 proteins (including enteric and metabolic hormones, apolipoproteins, adipokines, and cytokines) using a highly sensitive multiplex ELISA. Measurements were normalized to ALF concentrations. The ALF:serum concentration ratios were examined in relation to measures of protein size, hydrophobicity, charge, and to participant clinical and spirometric values.ResultsALF measurements from 24 individuals detected 19 proteins, including adiponectin, adipsin, apoA-I, apoA-II, apoB, apoC-II, apoC-III, apoE, C-reactive protein, ghrelin, glucose-dependent insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1), glucagon, insulin, leptin, monocyte chemoattractant protein-1, plasminogen activator inhibitor-1, resistin, and visfatin. C-peptide and serpin E1 were not detected in ALF for any individual, and IL-6, IL-10, and TNF-alpha were not detected in either ALF or serum for any individual. In general, ALF levels were similar or lower in concentration for most proteins compared to serum. However, ghrelin, resistin, insulin, visfatin and GLP-1 had ALF concentrations significantly higher compared to serum. Importantly, elevated ALF:serum ratios of ghrelin, visfatin and resistin correlated with protein net charge and isoelectric point, but not with molecular weight or hydrophobicity.ConclusionsBiologically relevant enteric and metabolic hormones, apolipoproteins, adipokines, and cytokines can be detected in the ALF of healthy individuals. For the proteins measured, charge may influence trafficking and compartmentalization to the alveolar airspace more than molecular weight or hydrophobicity. These data may have implications for homeostasis and drug delivery to the lung.

Apolipoprotein E is a concentration-dependent pulmonary danger signal that activates the NLRP3 inflammasome and IL-1β secretion by bronchoalveolar fluid macrophages from asthmatic subjects

“…74 For example, the concentration of APOE in ELF from healthy adults ranges from 10 to 80 ng/mL (0.3-2.4 nmol/L), with a median concentration of 60 ng/mL (1.8 nmol/L). 36 Here we show that concentrations of APOE similar to levels present in ELF from healthy subjects neither activated the NLRP3 inflammasome nor induced secretion of mature IL-1b from BALF macrophages from asthmatic subjects. In contrast, APOE at concentrations of 25 nmol/L or greater induced cleavage of pro-IL-1b to its mature 17-kDa form, which was secreted from cells.…”

“…8 Instead, we found that APOE activated the NLRP3 inflammasome in BALF macrophages and induced IL-1b secretion when concentrations exceeded levels that had previously been found in ELF from healthy subjects. 36 Furthermore, using a murine viral exacerbation model of HDM-induced neutrophilic airway inflammation, we show that APOE concentrations in mouse ELF are sufficiently high to activate the NLRP3 inflammasome in ex vivo cultures of BALF macrophages from asthmatic subjects.…”

“…In particular, we assessed whether APOE at concentrations similar to those present in lung ELF from healthy adults, which range from 0.3 to 2.4 nmol/L, can prime and activate the NLRP3 inflammasome. 36 APOE at concentrations of 0.25 and 2.5 nmol/L did not increase NLRP3 expression (Fig 3, A) nor did it activate cleavage of pro-IL-1b to a mature form (Fig 3, B) that is secreted (Fig 3, C) from BALF macrophages. APOE at a concentration of 2.5 nmol/L increased expression of pro-IL-1b, whereas 0.25 nmol/L APOE did not (Fig 3, B).…”

Glucagon-like peptide 1: A potential anti-inflammatory pathway in obesity-related asthma

“…Although there is no evidence of GLP-1 production within the lung, it appears to be in much higher concentrations within bronchoalveolar fluid as compared to the serum in one study of healthy adults (Mendivil et al, 2015; Richter et al, 1993). It is not known how these levels might change in unhealthy conditions, but reductions in GLP-1 production with obesity and diabetes likely are paralleled within the lung and may contribute to dysfunction.…”

Leptin Promotes cPLA2 Gene Expression through Activation of the MAPK/NF-κB/p300 Cascade

“…Plasma leptin concentrations and the leptin/adiponectin ratio exhibited a significantly inversed correlation with initial FEV1 in emphysema patients [ 37 ]. Leptin was detected in the alveolar lining fluid of healthy humans [ 38 ] and was induced in injured human and murine lungs [ 39 ]. In rat alveolar macrophages and murine peritoneal macrophages models, leptin increased phospholipase activity and enhanced the group IVC iPLA2 (cPLA2γ) protein expression, resulting in upregulated alveolar macrophage leukotriene synthesis [ 40 ].…”