Alveolar type 2 cell LDL receptor associated protein 1 regulates surfactant homeostasis and pulmonary function

Abstract

Surfactant metabolism is associated with infant and adult pulmonary disease. The mechanisms for surfactant involvement in adult disease are unknown. Pulmonary type 2 cells (T2C) synthesize surfactant and sustain an active and strictly regulated lipid metabolism, since surfactant is composed by phospholipid, and minor amounts of other lipids and specific proteins. The LDL receptor related protein 1 (LRP1) partakes in lipid metabolism, and in signaling processes. In T2C, the role of LRP1 has not been explored, despite these being the pulmonary cells with most active lipid metabolism. We hypothesized that LRP1 in T2C is required for intracellular lipid homeostasis, surfactant metabolism and respiratory function. A549 cells were stably transfected with LRP1 shRNA (LRP1 KD) and cultured at the air-liquid interface on collagen inserts. LRP1 KD cells showed decreased surfactant phospholipid secretion associated with decreased intracellular phospholipid. mRNA expression of proteins involved in surfactant synthesis and secretion was downregulated. However, the intracellular content of triglyceride and cholesterol esters was increased. Basolateral uptake of free fatty acids or LDL-derived phosphatidylcholine but the expression of cholesterol and phospholipid exporters ABCA1 and ABCG1 was decreased. We generated tamoxifen inducible T2C-specific LRP1 knockout mice (SPC-LRP1-/-) and knockout was induced at 6 weeks of age. SPC-LRP1-/- mice had decreased pulmonary compliance, forced vital capacity and forced expiratory volumes. Surfactant phospholipid in their bronchoalveolar lavage fluid was decreased and consistently, surface tension was increased. These data show that LRP1 in T2C regulates pulmonary function through surfactant lipid metabolism.