Lipofectamine 2000/siRNA complexes cause endoplasmic reticulum unfolded protein response in human endothelial cells

Lipofectamine 2000 (Lipo2000) delivery system is commonly used for short interfering RNA (siRNA) transfection, whereas the cellular responses have attracted little attention. The purpose of this study is to evaluate the effect of siRNA transfection using Lipo2000 on cellular functions and the possible underlying mechanism. Primary human umbilical vein endothelial cells (HUVECs) and adult human coronary artery endothelial cell line (HCAECs) were treated with different concentrations of a Lipo2000/negative control siRNA (NC siRNA) complex or Lipo2000 for specific durations. The cell proliferation, apoptosis rate, and protein expression of claudin5 (CLDN5) and ETS-related gene (ERG) were analyzed as indicators of cellular function. The effects of the Lipo2000/NC siRNA complex on cellular autophagy and endoplasmic reticulum (ER) unfolded protein response (UPR) were investigated by western blot and real-time polymerase chain reaction analyses; autophagy was also evaluated by transmission electron microscopy. The Lipo2000/NC siRNA complex inhibited proliferation, downregulated various proteins, and increased the apoptosis in both HUVECs and HCAECs. Both autophagy and UPR were observed in HUVECs treated with the Lipo2000/NC siRNA complex, ER stress-induced autophagy acted as a cellular protective factor against apoptosis, as inhibition of autophagy by chemical inhibitors increased the cell apoptosis rate. Chemical chaperones failed to prevent the Lipo2000/siRNA complex-induced UPR. However, knockdown of protein kinase RNA-like ER kinase and inositol-requiring protein 1, instead of activating transcription factor-6, partially ameliorated the UPR and reversed the protein level of CLDN5 and ERG downregulated by Lipo2000/NC siRNA complex. This study provides the first evidence that the Lipo2000-mediated transport of siRNA leads to an increase in UPR and ER stress-related apoptosis in endothelial cells.