Molecular mechanisms of EGFR activation by carbon nanoparticles in lung epithelial cells

Background: Particulate air pollution induces pathogenic endpoints like proliferation, apoptosis, and pro-inflammatory reactions in lung epithelial cells. The activation of the epidermal growth factor receptor (EGFR) is a key step responsible for signaling events specific for these endpoints. Earlier experiments identified particle-derived reactive oxygen species (ROS) as one possible trigger eliciting the observed EGFR activation. As initial molecular mechanism responsible for the activation of this signaling event, we hypothesize a ligand-independent internalization of EGFR involving caveolin-1.

Methods: Membrane signaling events were studied in isolated lipid rafts from lung epithelial cells treated with environmental model particles (carbon nanoparticles) with regard to lipid and protein content of the signaling platforms. Using positive and negative intervention approaches, lipid raft changes, subsequent signaling events, sphingomyelinase activity, and lung inflammation were investigated in vitro in lung epithelial cells (RLE-6TN) and in vivo in exposed animals.

Results: As a typical pattern of cell reaction after exposure to carbon nanoparticles, EGFR activation and internalization were observed. First results indicate the involvement of caveolin-1 as a marker of ligand-indepedent activation of the receptor. Further analyses demonstrate that carbon nanoparticles triggered intracellular oxidative stress is responsible for an increase of ceramides in lipid raft structures and the subsequent EGFR signaling. In this context, the activity of neutral sphingomyelinase (nSmase) was determined. Interestingly, carbon particles rather inhibited the enzyme activity, indicating a non-enzymatic generation of ceramide by nanoparticle-induced oxidative stress.

Conclusion: The data identify the ligand-independent, mechanism of EGFR activation as a result of the accumulation of ceramides in lipid rafts of lung epithelial cells as initial molecular event contributing to the toxicity of inhaled carbon particles.