An In Vitro Study Of Vascular Endothelial Toxicity Of CdTe Quantum Dots And The Underlying Mechanism

As a class of semiconductor nanocrystals with unique electronic and optical properties, quantum dots (QDs) were widely used in biomedical imaging, drug tracing as well as gene technologies. However, with the spring up of the nascent QD industry, human being is inevitably exposed to these engineered nanomaterials. Therefore, these novel techniques raised concerns about QDs toxicity on the human health and their potential environmental risks. QDs could directly or indirectly enter the vascular through inhalation, dermal exposure or injection; and thus the endothelial cells (ECs) that line the luminal surface of all blood vessels would be directly exposed to or even be impaired by QDs circulating in the blood stream. Because the endothelial injury plays a key role in the development of cardiovascular diseases, the studies on the potential toxicity of QDs on vascular ECs become urgent and crucial. Although recent studies show that QDs was toxic to some types of cells, the question of whether QDs can directly damage vascular ECs remains unanswered.In this study, using confocal laser scanning microscopy, flow cytometry, and immunofluroscence and immunoblotting technologies, we investigated the vascular endothelial toxicity of the mercaptosuccinic acid (MSA) capped cadmium tellurium (CdTe) QDs, which were produced by hydrothermal method, as well as the underlying molecular mechanisms. We found that CdTe QDs could significantly decreased the cell viability (TC50=10.60μg/mL) and the cellular electrode impedance of endothelial monolayer. Meanwhile, CdTe QDs induced a decrease of dense peripheral actin bands, an increase of stress fibers, changes in the structure and expression of VE-Cadherin, and inhibition of cell proliferation. As a result, the endothelial barrier function was markedly impaired by CdTe QDs.Our results also showed that vascular ECs could uptake CdTe QDs in a dose-and time-dependent manner; and the saturation of uptake of CdTe QDs by ECs was achieved in 12 h. Moreover, the mechanistic data indicated that CdTe QDs were internalized by vascular ECs though both caveolae/raft- and clathrin-dependent endocytosis. Then, they were distributed to and cause damage of multiple organelles, including the lysosomal membrane permeabilization (LMP) and cytoplasmic acidification; the up-regulation of endoplasmic reticulum (ER) stress markers (GRP78/GRP94) and dilation of ER; the fragmentation of mitochondrial network and disruption of mitochondrial membrane potential.In addition, results revealed that CdTe QDs induced remarkable apoptotic cell death. They activatied both mitochondria- and ER-dependent apoptosis pathways as they induced down-regulation of Bcl-2, up-regulation of Bax, release of mitochondrial cytochrome c, cleavage of caspase-9/caspase-3/caspasc-4 and activation of CHOP. Salubrinal (an ER stress inhibitor), Z-VAD-FMK (a pan-caspase inhibitor) and Ac-DEVD-CHO (a caspase-3 inhibitor) could effectively inhibit endothelial apoptosis in the presence of CdTe QDs, indicating that ER and caspase family were indeed involved in CdTe QDs-induced endothelial apoptosis. Besides that, CdTe QDs elicited significant oxidative stress; whereas reactive oxygen species (ROS) scavenger could protect ECs from QDs-induced injuries in mitochondria, and apoptosis, suggesting ROS plays an important role in mediating QD-induced cellular damage.Taking together, our present study achieved the following novel findings:1) MSA-capped CdTe QDs have favorable optical properties and thus present promising prospects in the biomedical applications; 2) Using the real-time cell-impedance sensing (RT-CES) assay, we demonstrated that CdTe QDs could impair endothelial barrier functions and thus were toxic to vascular ECs; 3) CdTe QDs were taken up by ECs through both clathrin- and caveolae/rafts-dependent endocytosis pathways, and then distributed to and caused injuries of multiple organelles, like lysosomes, ER and mitochonria; 4) CdTe QDs finally activated mitochondril-dependent apoptosis pathways and induced apoptotic cell death in ECs. We reported for the first time that QDs could also induced pro-apoptotic ER stress and activated ER-dependent apoptosis pathways. CdTe QDs-induced increased intracellular ROS generation may play important role in mediating QD-induced cellular damage.Collectively, in this study, we systematically analyzed the vascular endothelial toxicity of CdTe QDs, their intracellular targets and the underlying molecular mechanisms. Our results provide strong evidences of the direct toxic effects of QDs on human vascular ECs, and reveal that exposure to QDs is a significant risk for the development of cardiovascular diseases. These results also provide helpful guidance on the future safe use and manipulation of QDs to make them more suitable tools in nanomedicine.