Niemann‐Pick type C1 (NPC1) and cholesterol transport in type II pneumocytes

Abstract

Although cholesterol comprises 10% of the total lipid content of lung surfactant, few studies address the regulation of surfactant cholesterol levels. Surfactant is synthesized by type II cells and stored in specialized organelles, lamellar bodies (LBs), prior to secretion into the alveolar space. LBs are considered to be part of the late endosome/lysosome compartment, the same location as NPC1, a transmembrane protein shown to modulate intracellular cholesterol transport. Deficiency of NPC1 results in abnormal lysosomal accumulation of cholesterol and other lipids. Thus, we examined the role of NPC1 in pneumocytes. Using a Western blot protocol and equal protein loading, NPC1 was found in the intact rat lung, enriched (1.6‐fold) in primary cultures of type II cells and greatly enriched (11.6‐fold) in LBs isolated from the lung. By immunocytochemistry of type II cells, NPC1 colocalized with ABCA3 on the limiting membrane of LBs. U‐18666A (UA) is a compound that inhibits cholesterol trafficking from the late endosomes to the endoplasmic reticulum, thus mimicking NPC1 deficiency. UA treatment of isolated type II cells resulted in a 50% increase in free cholesterol content and the accumulation of cholesterol (filipin‐stain) in LB‐like structures. The data are consistent with a role for NPC1 in removal of cholesterol during the maturation of LBs, thereby regulating the surfactant cholesterol content. [HL 19737]