Zinc oxide nanoparticles induce migration and adhesion of monocytes to endothelial cells and accelerate foam cell formation

https://doi.org/10.1016/j.taap.2014.04.010Get rights and content

Highlights

  • Effects of metal oxide nanoparticles on foam cell formation were investigated.

  • Exposure to ZnO nanoparticles induced migration and adhesion of monocytes.

  • Exposure to ZnO nanoparticles increased macrophage cholesterol uptake.

  • Expression of membrane scavenger receptors of modified LDL was also increased.

  • These effects were not observed after exposure to TiO2 nanoparticles.

Abstract

Metal oxide nanoparticles are widely used in industry, cosmetics, and biomedicine. However, the effects of exposure to these nanoparticles on the cardiovascular system remain unknown. The present study investigated the effects of nanosized TiO2 and ZnO particles on the migration and adhesion of monocytes, which are essential processes in atherosclerogenesis, using an in vitro set-up of human umbilical vein endothelial cells (HUVECs) and human monocytic leukemia cells (THP-1). We also examined the effects of exposure to nanosized metal oxide particles on macrophage cholesterol uptake and foam cell formation. The 16-hour exposure to ZnO particles increased the level of monocyte chemotactic protein-1 (MCP-1) and induced the migration of THP-1 monocyte mediated by increased MCP-1. Exposure to ZnO particles also induced adhesion of THP-1 cells to HUVECs. Moreover, exposure to ZnO particles, but not TiO2 particles, upregulated the expression of membrane scavenger receptors of modified LDL and increased cholesterol uptake in THP-1 monocytes/macrophages. In the present study, we found that exposure to ZnO particles increased macrophage cholesterol uptake, which was mediated by an upregulation of membrane scavenger receptors of modified LDL. These results suggest that nanosized ZnO particles could potentially enhance atherosclerogenesis and accelerate foam cell formation.

Introduction

Evidence based on epidemiological and toxicological studies indicates that high concentrations of particle masses < 2.5 μm (PM2.5) are associated with high risk of pulmonary complications, cardiovascular events, and death from cardiovascular disease (Araujo et al., 2008, Mar et al., 2000, Miller et al., 2007, Mills et al., 2007, Peters et al., 2001). The recent explosion in the field of nanotechnology provides promising potential applications of manufactured nanomaterials in a variety of areas (Donaldson et al., 2004). It has been demonstrated that the relative surface area of ultrafine carbon black or titanium dioxide (TiO2) particles correlates with the degree of their toxicity (Hohr et al., 2002, Sager et al., 2008, Yamamoto et al., 2006). Some kinds of nanomaterials are also considered to generate reactive oxidant species, resulting in the induction of oxidative stress and inflammation (Nel et al., 2009, Xia et al., 2006). Oxidative stress is known to be involved in the pathogenesis of cardiovascular diseases, such as hypertension and atherosclerosis (Noma et al., 2007, Taniyama and Griendling, 2003). Therefore, there is a concern that nanomaterials could have a major impact on the cardiovascular system. However, the effects of exposure to newly developed metal oxide nanoparticles on the cardiovascular system remain elusive.

Atherosclerosis is a disease of the vasculature characterized by a chronic inflammation of the arterial wall and the formation of fibrotic plaques in the major arteries (Lusis, 2000). The process of atherosclerogenesis is initiated by the activation of endothelial cells, with subsequent migration of mononuclear cells and expression of adhesion molecules for inflammatory cells (Berk, 2008, Libby et al., 2009). In addition, a critical factor in the progression of atherosclerogenesis is the development of an oxidizing environment caused by the activation of macrophages that become loaded with oxidized low-density lipoprotein (LDL) and other lipids (Tsimikas and Miller, 2001). The formation of foam cells is crucial in the initiation and progression of atherosclerosis, and one of the critical steps in foam cell formation is the uptake of modified LDL by macrophages via scavenger receptors (Moore and Freeman, 2006).

TiO2 and zinc oxide (ZnO) are widely used in paints, pharmaceutical, and cosmetic industries. The rapidly developing field of nanotechnology becomes a source for human potential exposures to engineered nanoparticles by different routes: inhalation (respiratory tract), ingestion (gastrointestinal tract), dermal (skin), and injection (blood circulation) (Oberdörster et al., 2005). The present study investigated the effects of nanosized metal oxide particles on the migration and adhesion of monocytes, which are essential processes in atherosclerogenesis, using an in vitro set-up of human umbilical vein endothelial cells (HUVECs) and human monocytic leukemia cells (THP-1). We also examined the effects of exposure to nanosized metal oxide particles on macrophage cholesterol uptake and foam cell formation.

Section snippets

Nanoparticle preparation and characterization

TiO2 nanoparticles (AEROXIDE TiO2 P25; Degussa AG, Dusseldorf, Germany) with a primary diameter of 21 nm, and ZnO nanoparticles (MKN-ZnO-020; mkNANO, Mississauga, ONT, Canada) with a primary diameter of 20 nm were used in the present study. Nanoparticles were suspended in culture media and dispersed using sonicator (BRA NSON Sonifier model 450, Danbury, CT, 80% pulsed mode, 100 W, 15 min), as described previously (Wu et al., 2013). The hydrodynamic sizes of the particles in media were measured four

Characterization of suspensions of nanoparticles and cell viability

Both nanosized TiO2 and ZnO were dispersed in the respective culture medium of HUVECs and THP-1. The intensity-weighted hydrodynamic average diameter of dispersed nanoparticles was measured by DLS technology. Table 1 shows the mean hydrodynamic diameters and PdI of dispersed TiO2 or ZnO particles in each medium. Although DLS data provided the mean hydrodynamic diameters of > 150 nm, the presence of nano-sized particles was confirmed in the medium (Table 1).

HUVECs were exposed to TiO2 and ZnO

Discussion

The present study demonstrated that exposure to ZnO nanoparticles induced cell migration mediated by increased MCP-1 level and adhesion of THP-1 cells to HUVECs. We also demonstrated that exposure to the same nanoparticles increased cholesterol uptake in THP-1 monocytes/macrophages due to the upregulation of membrane scavenger receptors of modified LDL, leading to foam cell formation.

What is the mechanism of the effects of ZnO nanoparticles on these atherosclerogenesis processes? Previous

Conclusion

Exposure to ZnO particles induced cell migration and adhesion of THP-1 cells to HUVECs. The results also demonstrated that exposure to ZnO particles increased macrophage cholesterol uptake due to the upregulation of membrane scavenger receptors of modified LDL. Considered together, the findings suggest that nanosized ZnO particles accelerate foam cell formation, a process crucial in the initiation and progression of atherosclerosis.

Conflict of interest statement

The authors declare no financial conflict of interest.

Acknowledgments

The authors thank Kumi Nakao for the help in preparation of the manuscript. This work was supported in part by grants from the Japan Society for the Promotion of Science (grants-in aid for Scientific Research #22390122 and NEXT Program #LS056).

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