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A3807 - Alveolar Epithelial Cell Processing of Nanoparticles: Distribution Between Cytosolic and Vesicular Compartments
Author Block: A. Sipos1, K. Kim1, R. H. Chow2, Z. Borok1, E. D. Crandall1; 1Division of Pulmonary, Critical Care and Sleep Medicine, University of Southern California, Los Angeles, CA, United States, 2Zilkha Neurogenetic Institute, University of Southern California, Los Angeles, CA, United States.
Rationale: We previously reported that polystyrene nanoparticles (PNP) in apical fluid of primary rat alveolar epithelial cell (AEC) monolayers (RAECM) are taken up in a time-, dose- and size-dependent manner via a non-endocytic ‘diffusional’ process, trigger autophagic flux, and exit via both fast intracellular calcium-dependent lysosomal exocytosis and a slower (‘diffusional’) process. In this study, we investigated intracellular fate/distribution of PNP in cytosolic and vesicular compartments of AEC.
Methods: RAECM were apically exposed to near-infrared labeled PNP (80 μg/mL; carboxylated; 20 nm diameter). Serial z sections over the entire cell volume were acquired in live single cells by confocal microscopy. In each z plane, fluorescence intensity of vesicular and cytosolic PNP was determined and normalized to total intracellular PNP (PNPic). Volumes of endosomal, autophagosomal and lysosomal compartments, as well as colocalization of PNP in these compartments, were assessed using quinacrine, microtubule-associated protein 1 light chain 3 (LC3B) tagged with green fluorescent protein (GFP) (LC3B-GFP) and Magic Red, respectively. [PNPic] was determined using isotropic methods based on microfluorometry of cell lysates as a function of time. [PNP] in vesicles ([PNPv]) and cytosol ([PNPc]) were estimated from the respective fractions of compartmental volumes and co-localized PNP in each compartment. The role of autophagy in processing intracellular PNP was addressed using 3-methyladenine (3MA, 5 mM) and bafilomycin (0.5 μM), inhibitors of autophagosome formation and autophagosome-lysosome fusion, respectively.
Results: [PNPic] reached a plateau of 1.84 mg/mL after 12 hr of exposure to apical PNP. The amount of PNP in intracellular vesicles (primarily lysosomes) and cytosol was ~80% and ~20%, respectively, of PNPic at steady state, equivalent to 8.86 mg/ml and 0.58 mg/mL for [PNPv] and [PNPc], respectively. Inhibition of classical endocytosis pathways did not alter PNP uptake kinetics, while blockade of autophagosome formation by 3MA slowed PNP uptake and decreased steady state [PNPic] by ~75%. When autophagosome formation was inhibited, no vesicular PNP was detected and PNPc accounted for PNPic. Inhibition of autophagosome-lysosome fusion by bafilomycin led to lower steady state PNPic by ~20% compared to control, with ~81% in cytosol and ~19% in vesicles.
Conclusions: 1) [PNPv] exceeds both [PNPc] and [PNPic], with [PNPc]>>apical [PNP]; 2) cytosolic PNP triggers autophagosome formation, leading to PNP accumulation in lysosomes; and, 3) inhibition of autophagy leads to decreased PNP entry and [PNPic]. These findings suggest that intracellular [PNP] is regulatable, providing potential approaches to enhancement of nanoparticle-driven gene/drug delivery and/or amelioration of nanoparticle-related cytotoxicity in AEC.