patient-oriented and epidemiological research ABCG1 is deficient in alveolar macrophages of GM-CSF knockout mice and patients with pulmonary alveolar proteinosis

Patients with pulmonary alveolar proteinosis (PAP) display impaired surfactant clearance, foamy, lipidfilled alveolar macrophages, and increased cholesterol metabolites within the lung. Neutralizing autoantibodies to granulocyte-macrophage colony-stimulating factor (GMCSF) are also present, resulting in virtual GM-CSF deficiency. We investigated ABCG1 and ABCA1 expression in alveolar macrophages of PAP patients and GM-CSF knockout (KO) mice, which exhibit PAP-like pulmonary pathology and increased pulmonary cholesterol. Alveolar macrophages from both sources displayed a striking similarity in transporter gene dysregulation, consisting of deficient ABCG1 accompanied by highly increased ABCA1. Peroxisome proliferator-activated receptor g (PPARg), a known regulator of both transporters, was deficient, as reported previously. In contrast, the liver X receptor a, which also upregulates both transporters, was highly increased. GMCSF treatment increased ABCG1 expression in macrophages in vitro and in PAP patients in vivo. Overexpression of PPARg by lentivirus-PPARg transduction of primary alveolar macrophages, or activation by rosiglitazone, also increased ABCG1 expression. These results suggest that ABCG1 deficiency in PAP and GM-CSF KO alveolar macrophages is attributable to the absence of a GM-CSFmediated PPARg pathway. These findings document the existence of ABCG1 deficiency in human lung disease and highlight a critical role for ABCG1 in surfactant homeostasis.—Thomassen, M. J., B. P. Barna, A. G. Malur, T. L. Bonfield, C. F. Farver, A. Malur, H. Dalrymple, M. S. Kavuru, and M. Febbraio. ABCG1 is deficient in alveolar macrophages of GM-CSF knockout mice and patients with pulmonary alveolar proteinosis. J. Lipid Res. 2007. 48: 2762–2768. Supplementary key words liver X receptor a & peroxisome proliferatoractivated receptor g & ATP binding cassette transporter A1 & ATP binding cassette transporter G1 & foam cells & granulocyte-macrophage colonystimulating factor Pulmonary alveolar proteinosis (PAP) is a rare autoimmune disease in which surfactant clearance is defective and granulocyte-macrophage colony-stimulating factor (GMCSF) is deficient as a result of neutralizing autoantibodies (1). In patients with PAP, the lungs are filled with excess lipoproteinaceous material and alveolar macrophages are engorged with lipid, resulting in a foamy appearance (2). We have shown that alveolar macrophages from PAP patients are also severely deficient in peroxisome proliferatoractivated receptor g (PPARg), a key regulator of lipid metabolism (3). This deficiency is correctable by GM-CSF therapy (3). Subsequently, we reported that cholesterol and the cholesterol metabolites, cholestenoic acid and 27hydroxycholesterol, are highly increased in PAP bronchoalveolar lavage (BAL) fluid (4). Interestingly, a PAP-like pulmonary histopathology is displayed by GM-CSF knockout (KO)mice (5, 6). Exogenous or local overexpression of GM-CSF reverses this pathology (7, 8). Pulmonary cholesterol is also increased in theGM-CSFKOmouse lung (9), and GM-CSF KOmacrophages are deficient in PPARg (10, 11). Surfactant, which is produced by type II alveolar pneumocytes, is composed of 90% lipid, 10% protein, and a small amount of carbohydrate. Approximately 80–90% of surfactant lipid is phospholipid; the other lipids, in decreasing order, are cholesterol, triacylglycerol, and free fatty acids (12). The processes mediating and regulating surfactant clearance, recycling, and catabolism are not completely defined (13). Two primary surfactant catabolic pathways have been described (reviewed in Ref. 14). The first pathway is a recycling process requiring type II pneumocytes. The second pathway is a clearance pathway in which surfactant is phagocytosed and degraded by alveolar macrophages. Although a small amount of surfactant is degraded by type II pneumocytes, recent findings emManuscript received 18 June 2007 and in revised form 30 July 2007. Published, JLR Papers in Press, September 11, 2007. DOI 10.1194/jlr.P700022-JLR200 To whom correspondence should be addressed. e-mail: [email protected] Copyright D 2007 by the American Society for Biochemistry and Molecular Biology, Inc. This article is available online at http://www.jlr.org 2762 Journal of Lipid Research Volume 48, 2007 at P E N N S T A T E U N IV E R S IT Y , on F ebuary 3, 2013 w w w .j.org D ow nladed fom phasize the critical role of the alveolar macrophage in surfactant catabolism (15). The ABC transporters ABCA1 and ABCG1 are members of a group of transmembrane proteins that transport a wide variety of substrates across membranes (16–19). Macrophages are a rich source of both ABCA1 and ABCG1, which can be regulated by a number of pathways, including PPARg activation and the activation of liver X receptor a (LXRa) via the uptake of cholesterol or oxysterols (20–22). Cholesterol is esterified by macrophages, and accumulated cholesteryl esters are stored within the cell, giving it a foamy appearance (reviewed in Ref. 18). ABCA1 and ABCG1 play critical roles in mediating the cellular efflux of both cholesterol and phospholipids to lipoproteins (18). Recent findings have implicated these transporters in surfactant homeostasis (16, 23, 24). ABCA1 KO mice exhibit morphologic abnormalities in the lungs, including a massive accumulation of lipid-laden macrophages and type II pneumocytes (24). Similarly, lungs from ABCG1 KO mice show abnormal accumulation of lipid deposits in both alveolar macrophages and type II pneumocytes (16). Although the phenotype of GM-CSF KOmice appears to resemble that of PAP, the status of genes involved in alveolar macrophage lipid efflux has not been investigated in either case. We hypothesized that the ABC transporters might be dysregulated in PAP and in GM-CSF KO mice. Our results indicate that alveolar macrophages from PAP patients and GM-CSF KO mice display a striking similarity of transporter dysregulation characterized by deficient ABCG1 and increased ABCA1. MATERIALS AND METHODS