| Summary: | <p>Abstract</p> <p>Background</p> <p>Inorganic particles, such as drug carriers or contrast agents, are often introduced into the vascular system. Many key components of the <it>in vivo</it> vascular environment include monocyte-endothelial cell interactions, which are important in the initiation of cardiovascular disease. To better understand the effect of particles on vascular function, the present study explored the direct biological effects of particles on human umbilical vein endothelial cells (HUVECs) and monocytes (THP-1 cells). In addition, the integrated effects and possible mechanism of particle-mediated monocyte-endothelial cell interactions were investigated using a coculture model of HUVECs and THP-1 cells. Fe<sub>3</sub>O<sub>4</sub> and SiO<sub>2</sub> particles were chosen as the test materials in the present study.</p> <p>Results</p> <p>The cell viability data from an MTS assay showed that exposure to Fe<sub>3</sub>O<sub>4</sub> or SiO<sub>2</sub> particles at concentrations of 200 μg/mL and above significantly decreased the cell viability of HUVECs, but no significant loss in viability was observed in the THP-1 cells. TEM images indicated that with the accumulation of SiO<sub>2</sub> particles in the cells, the size, structure and morphology of the lysosomes significantly changed in HUVECs, whereas the lysosomes of THP-1 cells were not altered. Our results showed that reactive oxygen species (ROS) generation; the production of interleukin (IL)-6, IL-8, monocyte chemoattractant protein 1 (MCP-1), tumor necrosis factor (TNF)-α and IL-1β; and the expression of CD106, CD62E and tissue factor in HUVECs and monocytes were significantly enhanced to a greater degree in the SiO<sub>2</sub>-particle-activated cocultures compared with the individual cell types alone. In contrast, exposure to Fe<sub>3</sub>O<sub>4</sub> particles had no impact on the activation of monocytes or endothelial cells in monoculture or coculture. Moreover, using treatment with the supernatants of SiO<sub>2</sub>-particle-stimulated monocytes or HUVECs, we found that the enhancement of proinflammatory response by SiO<sub>2</sub> particles was not mediated by soluble factors but was dependent on the direct contact between monocytes and HUVECs. Furthermore, flow cytometry analysis showed that SiO<sub>2</sub> particles could markedly increase CD40L expression in HUVECs. Our data also demonstrated that the stimulation of cocultures with SiO<sub>2</sub> particles strongly enhanced c-Jun NH<sub>2</sub>-terminal kinase (JNK) phosphorylation and NF-κB activation in both HUVECs and THP-1 cells, whereas the phosphorylation of p38 was not affected.</p> <p>Conclusions</p> <p>Our data demonstrate that SiO<sub>2</sub> particles can significantly augment proinflammatory and procoagulant responses through CD40–CD40L-mediated monocyte-endothelial cell interactions via the JNK/NF-κB pathway, which suggests that cooperative interactions between particles, endothelial cells, and monocytes may trigger or exacerbate cardiovascular dysfunction and disease, such as atherosclerosis and thrombosis. These findings also indicate that the monocyte-endothelial cocultures represent a sensitive <it>in vitro</it> model system to assess the potential toxicity of particles and provide useful information that may help guide the future design and use of inorganic particles in biomedical applications.</p>
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