Nanoparticles-induced apoptosis of human airway epithelium is mediated by proNGF/p75^NTR signaling

ABSTRACT

Environmental and occupational exposures to respirable ultrafine fractions of particulate matter (PM) have been implicated in the initiation and exacerbation of lung diseases. However, the precise mechanisms underlying production of cell damage and death attributed to nanoparticles (NP) on human airway epithelium are not fully understood. This study examined the role of neurotrophic pathways in NP-induced airway toxicity. Size and agglomeration of TiO₂ nanoparticles (TiO₂-NP) and fine (TiO₂-FP) particles were measured by dynamic light scattering. Expression and signaling of key neurotrophic factors and receptors were assessed by real-time polymerase chain reaction, flow cytometry, immunostaining, and Western blot in various respiratory epithelial cells after exposure to TiO₂-NP or TiO₂-FP. Particle-induced cell death was measured by flow cytometry after annexin V/propidium iodide staining. The role of neurotrophin-dependent apoptotic pathways was analyzed with specific blocking antibodies or siRNAs. Exposure of human epithelial cells to TiO₂-NP enhanced interleukin (IL)-1α synthesis, as well as nerve growth factor (NGF) gene expression and protein levels, specifically the precursor form (proNGF). TiO₂-NP exposure also increased expression of p75^NRF receptor genes. These neurotropic factor and receptor responses were stimulated by IL-1α and abolished by its specific receptor antagonist (IL-1-ra). TiO₂-NP also increased JNK phosphorylation and apoptosis, which was prevented by anti-p75^NRF or NGFsiRNA. Data demonstrated that TiO₂-NP exerted adverse effects in the respiratory tract by inducing unbalanced overexpression of immature neurotrophins, which led to apoptotic death of epithelial cells signaled through the death receptor p75^NTR. This may result in airway inflammation and hyperreactivity after exposure to TiO₂-NP.

Funding

This work was supported in part by the U.S. National Institutes of Health (NIH) grant RO1-HL61007 to Dr. Giovanni Piedimonte. Image acquisition and data analysis were performed at the West Virginia University Imaging Core Facility, which was supported in part by NIH grant P20RR016440. FACS experiments were performed in the WVU Flow Cytometry Core Facility, which was supported in part by NIH grants RR106440 and RR020866. We also thank Drs. Min Ding and Sreekumar Othumpangat (NIOSH, Morgantown, WV), as well as Lennie Samsell and Cheryl Walton (WVU Pediatric Research Institute, Morgantown, WV), for providing the titanium dioxide particles and technical help.

Additional information

Funding

This work was supported in part by the U.S. National Institutes of Health (NIH) grant RO1-HL61007 to Dr. Giovanni Piedimonte. Image acquisition and data analysis were performed at the West Virginia University Imaging Core Facility, which was supported in part by NIH grant P20RR016440. FACS experiments were performed in the WVU Flow Cytometry Core Facility, which was supported in part by NIH grants RR106440 and RR020866. We also thank Drs. Min Ding and Sreekumar Othumpangat (NIOSH, Morgantown, WV), as well as Lennie Samsell and Cheryl Walton (WVU Pediatric Research Institute, Morgantown, WV), for providing the titanium dioxide particles and technical help.