Acute respiratory distress syndrome (ARDS) is characterized by an excessive pulmonary inflammatory response. Removal of excess cholesterol from the plasma membrane of inflammatory cells helps reduce their activation. The secreted apolipoprotein A-I binding protein (AIBP) has been shown to augment cholesterol efflux from endothelial cells to the plasma lipoprotein HDL. Here, we find that AIBP was expressed in inflammatory cells in the human lung and was secreted into the bronchoalveolar space in mice subjected to inhalation of LPS. AIBP bound surfactant protein B and increased cholesterol efflux from alveolar macrophages to calfactant, a therapeutic surfactant formulation. In vitro, AIBP in the presence of surfactant reduced LPS-induced p65, ERK1/2 and p38 phosphorylation, and IL-6 secretion by alveolar macrophages. In vivo, inhalation of AIBP significantly reduced LPS-induced airspace neutrophilia, alveolar capillary leak, and secretion of IL-6. These results suggest that, similar to HDL in plasma, surfactant serves as a cholesterol acceptor in the lung. Furthermore, lung injury increases pulmonary AIBP expression, which likely serves to promote cholesterol efflux to surfactant and reduce inflammation.

Apolipoprotein A-I binding protein (AIBP) reduces lipid raft abundance by augmenting the removal of excess cholesterol from the plasma membrane. Here, we report that AIBP prevents and reverses processes associated with neuroinflammatory-mediated spinal nociceptive processing. The mechanism involves AIBP binding to Toll-like receptor-4 (TLR4) and increased binding of AIBP to activated microglia, which mediates selective regulation of lipid rafts in inflammatory cells. AIBP-mediated lipid raft reductions downregulate LPS-induced TLR4 dimerization, inflammatory signaling, and expression of cytokines in microglia. In mice, intrathecal injections of AIBP reduce spinal myeloid cell lipid rafts, TLR4 dimerization, neuroinflammation, and glial activation. Intrathecal AIBP reverses established allodynia in mice in which pain states were induced by the chemotherapeutic cisplatin, intraplantar formalin, or intrathecal LPS, all of which are pro-nociceptive interventions known to be regulated by TLR4 signaling. These findings demonstrate a mechanism by which AIBP regulates neuroinflammation and suggest the therapeutic potential of AIBP in treating preexisting pain states.

Apolipoprotein A-I binding protein (AIBP) has been shown to augment cholesterol efflux from endothelial cells and macrophages. In zebrafish and mice, AIBP-mediated regulation of cholesterol levels in the plasma membrane of endothelial cells controls angiogenesis. The goal of this work was to evaluate metabolic changes and atherosclerosis in AIBP loss-of-function and gain-of-function animal studies. Here, we show that Apoa1bp^-/-Ldlr^-/- mice fed a high-cholesterol, high-fat diet had exacerbated weight gain, liver steatosis, glucose intolerance, hypercholesterolemia, hypertriglyceridemia, and larger atherosclerotic lesions compared with Ldlr^-/- mice. Feeding Apoa1bp^-/-Ldlr^-/- mice a high-cholesterol, normal-fat diet did not result in significant differences in lipid levels or size of atherosclerotic lesions from Ldlr^-/- mice. Conversely, adeno-associated virus-mediated overexpression of AIBP reduced hyperlipidemia and atherosclerosis in high-cholesterol, high-fat diet-fed Ldlr^-/- mice. Injections of recombinant AIBP reduced aortic inflammation in Ldlr^-/- mice fed a short high-cholesterol, high-fat diet. Conditional overexpression of AIBP in zebrafish also reduced diet-induced vascular lipid accumulation. In experiments with isolated macrophages, AIBP facilitated cholesterol efflux to HDL, reduced lipid rafts content, and inhibited inflammatory responses to lipopolysaccharide.jlr Our data demonstrate that AIBP confers protection against diet-induced metabolic abnormalities and atherosclerosis.

Introduction

Chronic low-grade inflammation is associated with obesity and diabetes. However, what causes and mediates chronic inflammation in metabolic disorders is not well understood. Toll-like receptor 4 (TLR4) mediates both infection-induced and sterile inflammation by recognizing pathogen-associated molecular patterns and endogenous molecules, respectively. Saturated fatty acids can activate TLR4, and TLR4-deficient mice were protected from high fat diet (HFD)-induced obesity and insulin resistance, suggesting that TLR4-mediated inflammation may cause metabolic dysfunction, such as obesity and insulin resistance.

Methods

We generated two transgenic (TG) mouse lines expressing a constitutively active TLR4 in adipose tissue and determined whether these TG mice would show increased insulin resistance.

Results

TG mice fed a high fat or a normal chow diet did not exhibit increased insulin resistance compared to their wild-type controls despite increased localized inflammation in white adipose tissue. Furthermore, females of one TG line fed a normal chow diet had improved insulin sensitivity with reduction in both adiposity and body weight when compared with wild-type littermates. There were significant differences between female and male mice in metabolic biomarkers and mRNA expression in proinflammatory genes and negative regulators of TLR4 signaling, regardless of genotype and diet.

Conclusions

Together, these results suggest that constitutively active TLR4-induced inflammation in white adipose tissue is not sufficient to induce systemic insulin resistance, and that high fat diet-induced insulin resistance may require other signals in addition to TLR4-mediated inflammation.