Statin as a novel pharmacotherapy of pulmonary alveolar proteinosis

Abstract

Pulmonary alveolar proteinosis (PAP) is a syndrome of reduced GM-CSF-dependent, macrophage-mediated surfactant clearance, dysfunctional foamy alveolar macrophages, alveolar surfactant accumulation, and hypoxemic respiratory failure for which the pathogenetic mechanism is unknown. Here, we examine the lipids accumulating in alveolar macrophages and surfactant to define the pathogenesis of PAP and evaluate a novel pharmacotherapeutic approach. In PAP patients, alveolar macrophages have a marked increase in cholesterol but only a minor increase in phospholipids, and pulmonary surfactant has an increase in the ratio of cholesterol to phospholipids. Oral statin therapy is associated with clinical, physiological, and radiological improvement in autoimmune PAP patients, and ex vivo statin treatment reduces cholesterol levels in explanted alveolar macrophages. In Csf2rb-/- mice, statin therapy reduces cholesterol accumulation in alveolar macrophages and ameliorates PAP, and ex vivo statin treatment increases cholesterol efflux from macrophages. These results support the feasibility of statin as a novel pathogenesis-based pharmacotherapy of PAP.

Fig. 1

Resolution of PAP associated with oral statin therapy. a HRCT chest at diagnosis and after WLL therapy or statin therapy. HRCT image illustrating quantitative categorical parenchymal-pattern analysis (green-masking: uninvolved/normal lung parenchyma, yellow-masking: PAP-involved/abnormal lung comprising ground glass and reticular changes). Glyphs showing parenchymal-pattern analysis of total lung parenchyma segmented by right, left; upper, middle, and lower zones. b Lung histology at diagnosis. Haematoxylin and eosin. c Percentage of lung affected by PAP determined by parenchymal-pattern analysis. Supplemental oxygen requirement (d), FVC (e), and DLCO (f) before and after statin therapy. Data are mean ± SD, statistical differences determined by Student’s t test or Mann–Whitney test. P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001

Fig. 2

Cholesterol accumulation in human PAP alveolar macrophages and correction by statin. a Human alveolar macrophages (AMs) stained with Diff-Quick (DQ), Oil-red O (ORO), periodic acid–Schiff (PAS), or electron microscopy (EM). b Thin-layer chromatography of AM total lipids from healthy people (control) or PAP patients (PAP). c AM cholesterol levels in PAP or controls (n = 8 PAP/4 control). d The ratio of cholesterol to total phospholipids in pulmonary surfactant of PAP or controls (n = 7 PAP/5 control). e Total cholesterol and f mRNA levels for SREBP2, NCEH, ABCA1, ABCG1 in PAP AMs without and after statin treatment for 24 h ex vivo (n = 3–6 per group). Data are mean ± SD, statistical differences determined by ANOVA with Bonferroni’s post hoc test. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001

Fig. 3

Statin therapy improves cholesterol efflux from macrophages and ameliorates PAP in Csf2rb−/− mice. a–g Mice received oral statin therapy for 6 weeks or untreated age-matched Csf2rb−/− or wild type (WT) mice. Disease severity evaluated by bronchoalveolar lavage a turbidity and b total cholesterol. c Alveolar macrophage cholesterol and mRNA levels for Srebp2 d, Nceh e, Abca1 f, and Abcg1 g. Cholesterol efflux capacity of Csf2rb−/− AMs (h, i) or BMDMs (j, k), treated with statin for 24 hours, measured by percentage of [³H]-cholesterol transferred to Apo-A1 (h, j) or HDL (i, k). Data are mean ± SD (3–6 mice/group), statistical differences determined by ANOVA with Bonferroni’s post hoc test. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001

Fig. 4

Proposed mechanisms for the pathogenesis and statin therapy of PAP. In the absence of GM-CSF signaling, surfactant-derived cholesterol accumulates progressively in lipid droplets resulting in foamy alveolar macrophages (red arrows indicate the effects of reduced GM-CSF signaling). Statin therapy results in increased cholesterol clearance from macrophages in PAP (green arrows represent the effects of statin in foamy alveolar macrophages)