Toxicity pathway focused gene expression profiling of PEI-based polymers for pulmonary applications

Polyethylene imine (PEI) based polycations, successfully used for gene therapy or RNA interference in vitro as well as in vivo, have been shown to cause well-known adverse side effects, especially high cytotoxicity. Therefore, various modifications have been developed to improve safety and efficiency of these nonviral vector systems, but profound knowledge about the underlying mechanisms responsible for the high cytotoxicity of PEI is still missing. In this in vitro study, we focused on stress and toxicity pathways triggered by PEI-based vector systems to be used for pulmonary application and two well-known lung toxic particles: fine crystalline silica (CS) and nanosized ZnO (NZO). The cytotoxicity profiles of all stressors were investigated in alveolar epithelial-like type II cells (LA4) to define concentrations with matching toxicity levels (cell viability >60% and LDH release <10%) for subsequent qRT-PCR-based gene array analysis. Within the first 6 h pathway analysis revealed for CS an extrinsic apoptotic signaling (TNF pathway) in contrast to the intrinsic apoptotic pathway (mitochondrial signaling) which was induced by PEI 25 kDa after 24 h treatment. The following causative chain of events seems conceivable: reactive oxygen species derived from particle surface toxicity triggers TNF signaling in the case of CS, whereby endosomal swelling and rupture upon endocytotic PEI 25 kDa uptake causes intracellular stress and mitochondrial alterations, finally leading to apoptotic cell death at higher doses. PEG modification most notably reduced the cytotoxicity of PEI 25 kDa but increased proinflammatory signaling on mRNA and even protein level. Hence in view of the lung as a sensitive target organ this inflammatory stimulation might cause unwanted side effects related to respiratory and cardiovascular disorders. Thus further optimization of the PEI-based vector systems is still needed for pulmonary application.