Alveolar epithelial cell processing of nanoparticles activates autophagy and lysosomal exocytosis

Over the last decade, advancements in nanoparticle-based delivery systems have brought safer, more effective new treatment approaches for lung diseases that have allowed for the rapid clinical translation of many drugs.

Nanoparticle have substantial advantages, such as easy tunability of the physico-chemical properties to suit in vivo delivery of nucleic acids, a good safety profile, and the potential for targeted delivery, for example in gene lung delivery applications. However, there are still challenges to overcome to meet clinical safety, it is very crucial to study the effect of nanoparticles materials on different lung cell types and their detailed uptake mechanisms by Alveroli epithelial cells.

In general, non-toxic nanoparticles that are less than 10 in dimension are often translocated paracellularly while large nanoparticles that are ≥20 in dimension are often excluded from tight junctional pathways in the cell. However, cytotoxic nanoparticles may disrupt tight junctions, thereby resulting in its unrestricted paracellular translocation. Alternatively, epithelial cells may take in nanoparticles via endocytosis and/or alternative transmembrane pathways and expel nanoparticles via exocytosis and/or other transport mechanisms. The processing of internalized nanoparticles may involve a protective intracellular mechanism known as autophagy. This involves the encapsulation of damaged proteins and organelles in autophagosomes that fuse with lysosomes to degrade and recycle amino acids. Despite the frequent exposure of the lungs to airborne nanoparticles, the role of nanoparticles -induced autophagy in the pathogenesis of lung disease and alveolar epithelial cell injury is still unknown. Recent studies have indicated that live-cell imaging can be used for the quantitative assessment of the interactions of fluorescently labeled nanoparticles with cells. This assessment facilitates the detailed analysis of the kinetics of efflux and uptake as well as the mechanisms underlying the processing of intracellular nanoparticles within lung cells.

Scientists at University of South California led by Professor Edward Crandall used live-cell imaging to investigate the interactions of near-infrared dye-labeled polystyrene nanoparticles (PNP) with alveolar epithelial cells. In their studies the research team demonstrated the mechanisms of apical uptake, intracellular processing, distribution, and efflux of PNP in alveolar epithelial cells. The research work is published in the American Journal of Physiology-Lung Cellular and Molecular Physiology.

The authors reported detailed uptake kinetics and steady-state intracellular content of PNP reduced as its diameter increased from 20 to 200 nm. However, at a diameter of 20 nm, the rate of uptake and steady-state intracellular content of PNP increased as apical PNP concentration increased. This increase was not affected by the inhibition of endocytic pathways in the cells. They also observed an increase in the spatial overlap of intracellular PNP with autophagosomes and/or lysosomes over time.

The authors also found that the efflux of PNP can occur via a rapid cytosolic Ca²⁺ concentration-dependent release or via a parallel-slow diffusion process that does not involve Ca²⁺. The inhibition of microtubule polymerization affected the rapid efflux of PNP and resulted in high intracellular and vesicular PNP content. The inhibition of autophagosome formation was found to result in slower PNP uptake and a significant reduction in the steady-state intracellular content of PNP. Furthermore, it was observed that at a steady state, the cytosolic concentration of PNP was higher than the apical concentration of PNP while the vesicular PNP concentration exceeded both intracellular and cytosolic PNP concentration.

The findings by Professor Edward Crandall and his colleagues provide compelling evidence that the uptake of PNP in alveolar epithelial cells is independent of the classical endocytic pathway and dependent on its physicochemical properties. They also concluded the following from their study that autophagic processing of nanoparticles is required for the maintenance of alveolar epithelial cell integrity, altered autophagy and/or lysosomal exocytosis may cause alveolar epithelial cell injury and that cytosolic concentration of PNP in alveolar epithelial cells can be regulated.

These results will stimulate further studies on how the modulation of autophagy/lysosomal exocytosis may regulate intracellular PNP content and prevent PNP-induced cellular injury. Moreover, with major advances in understanding detailed uptake of variety of nanoparticles by the aleveoli and the knowledge of how drug delivery nanosystems can affect the successful delivery of the payload to the desired diseased site in the lung, rapid clinical translation of these nanoparticle gene/drug delivery systems could be possible in the future.