Intravenous and gastric cerium dioxide nanoparticle exposure disrupts microvascular smooth muscle signaling.

Cerium dioxide nanoparticles (CeO2 NP) are an anthropogenic homogenous mixture of particles with one dimension less than or equal to 100 nm (Borm et al., 2006). Unique physical characteristics displayed at this size contribute to CeO2 NP diverse applications. Presently, CeO2 NP are widely used as a fuel additive because they increase the combustion efficiency of diesel engines (via enhanced catalytic activity) (Cassee et al., 2011). CeO2 NP may also potentially protect against tissue damage associated with radiation treatments and stroke in biomedical applications (Celardo et al., 2011). Tissue damage in these conditions is largely derived from increased reactive oxygen species (ROS) generation. CeO2 NP may prevent tissue damage from ROS due to their anti-oxidant capabilities (Kim et al., 2012). This anti-oxidant potential comes from the ability of CeO2 NP to react with ROS, which in turn alters its valence state (cycling between Ce4+ and Ce3+) (Heckert et al., 2008). The diverse and potentially widespread future applications mandate that the concept of intentional and unintentional CeO2 NP exposure (via multiple routes) be given full toxicological consideration. Currently, the lungs are the most common CeO2 NP exposure route. This is largely due to the occupational inhalation risk associated with the manufacturing process where CeO2 NP are easily aerosolized (Department of Health and Human Services, 2009). Furthermore, the risk of environmental pulmonary exposures may increase due to the presence of CeO2 NP in diesel engine emissions. The lack of governmental regulations in Europe and the United States further elevates this exposure risk (Cassee et al., 2011). Pulmonary CeO2 NP exposure has been associated with inflammation and granuloma formation (Ma et al., 2011). These exposures have also resulted in extra-pulmonary biological effects including inflammation, organ toxicity, and vascular impairments (Minarchick et al., 2013; Nalabotu et al., 2011; Wingard et al., 2011). Although pulmonary CeO2 NP exposure is associated with several untoward biological outcomes, neither these effects nor their intensities have been extensively studied or compared when the nanoparticles are given by alternate exposure routes. At this time, CeO2 NP exposure by non-pulmonary routes (intravenous and oral) is not an obvious risk. However, if CeO2 NP are to be fully developed for systemic therapeutic applications, the biological effects that follow CeO2 NP exposure by these clinically relevant exposure routes must be better understood. In vivo, studies investigating the outcomes of intravenous and oral CeO2 NP exposures have largely focused on long-term biological responses (1–3 months post-exposure), including organ distribution, and overt organ toxicity (Hirst et al., 2013; Yokel et al., 2012). Despite the fundamental role of the microcirculation in blood flow control, pressure regulation, and permeability in all tissues (Renkin, 1984; Zweifach, 1984), no investigation to date has analyzed the effects of intravenous and oral CeO2 NP exposures on normal microvascular function. In the microcirculation, the arterioles respond to a variety of chemical and mechanical stimuli (Gewaltig and Kojda, 2002). Under normal conditions, nitric oxide (NO) diffuses freely from endothelial cells to vascular smooth muscle (VSM) cells. This initiates a signaling cascade that activates soluble guanylyl cyclase (sGC), increases cyclic guanosine monophosphate (cGMP), and stimulates cGMP-protein kinase (PKG) (Schlossmann et al., 2003; Taylor et al., 2004). This signaling cascade decreases intracellular calcium (Ca2+) and, ultimately, relaxes VSM (Taylor et al., 2004). Disruptions in VSM signaling may compromise microvascular function and, if unresolved, may contribute to the development of numerous pathological conditions (Li and Forstermann, 2000). We have previously established that pulmonary CeO2 NP exposure results in systemic microvascular dysfunction (Minarchick et al., 2013). However, the presence of microvascular dysfunction following alternate exposure routes and the possible mechanism(s) of these impairments are currently unknown. Therefore, the aims of this study were two-fold. The first aim was to determine whether microvascular impairment followed intravenous injection and gastric gavage of CeO2 NP. The second aim was to provide mechanistic insight into the link between CeO2 NP exposure and microvascular function following three distinct exposure routes (intratracheal instillation, intravenous injection, and gastric gavage). Based on our pulmonary results, we hypothesized that intravenous and gastric exposures will also result in microvascular impairment, but to differing degrees, and the source of this dysfunction will be, at least partly, due to changes in NO bioavailability and/or VSM signaling. Microvascular impairment and NO bioavailability were assessed in freshly isolated arterioles 24 h following CeO2 NP exposure.