Despite the considerable use of magnetic ferroferric oxide nanoparticles (Fe(3)O(4)NPs) worldwide, their safety is still an important topic of debate. In the present study, we detected the toxicity and biological behavior of bare-Fe(3)O(4)NPs (B-Fe(3)O(4)NPs) on human umbilical vascular endothelial cells (HUVECs). Our results showed that B-Fe(3)O(4)NPs did not induce cell death within 24 h even at concentrations up to 400 mu g/ml. The level of nitric oxide (NO) and the activity of endothelial NO synthase (eNOS) were decreased after exposure to B-Fe(3)O(4)NPs, whereas the levels of proinflammatory cytokines were elevated. Importantly, B-Fe(3)O(4)NPs increased the accumulation of autophagosomes and LC3-II in HUVECs through both autophagy induction and the blockade of autophagy flux. The levels of Beclin I and VPS34, but not phosphorylated mTOR, were increased in the B-Fe3O4NP-treated HUVECs. Suppression of autophagy induction or stimulation of autophagy flux, at least partially, attenuated the B-Fe3O4NP-induced HUVEC dysfunction. Additionally, enhanced autophagic activity might be linked to the B-Fe3O4NP-induced production of proinflammatory cytokines. Taken together, these results demonstrated that B-Fe(3)O(4)NPs disturb the process of autophagy in HUVECs, and eventually lead to endothelial dysfunction and inflammation. (C) 2015 Elsevier B.V. All rights reserved.