HIF-1α was knocked down before treating vascular endothelial cells with CoNPs. (A) PCR and (B) Western blot assays were performed to detect the mRNA level of HIF-1α in HUVEC and HMEC-1 cells. MTT (C), apoptosis (D) and LDH (E) assays were performed to evaluate the cell viability, apoptosis rate and damage in cell membrane, respectively. HIF-1α knockdown attenuated the toxic effect of CoNPs on vascular endothelial cells. **P<0.01 and ***P<0.001 vs. control group; ^#P<0.05 vs. CoNPs group.