NO inhalation enhances asthma susceptibility in a rat model.

Nitrogen dioxide (NO) is a major air pollutant. Epidemiologic studies have found that NO exposure is associated with an increased risk of asthma. Nevertheless, the potential molecular mechanisms remain unclear. In this study, we investigated the effect of NO inhalation on the occurrence of allergic airway inflammation and its underlying mechanisms. Firstly, male Wistar rats were exposed to 2 and 5 mg/m NO (28 days, 5 h/day). The results showed that NO exposure could induce pulmonary inflammatory response, mucus formation, and Th1/Th2 imbalance in the lung of normal rats, resulting in allergic asthma-like features. Secondly, male Wistar rats were exposed to 5 mg/m NO (42 days, 5 h/day), sensitized with ovalbumin (OVA), challenged with aerosolized OVA, and characterized in asthma models. Results showed that NO exposure aggravated lung inflammation in the OVA-sensitized rats, accompanied by the increase in inflammatory cell infiltration, mucus hypersecretion, and collagen deposition. Furthermore, NO exposure promoted the increase in the expression of mucin gene (MUC5AC) and pro-inflammatory factors [interleukin (IL)-1β, intercellular adhesion molecule-1 (ICAM-1), and IL-6] as well as serum OVA-specific immunoglobulin E (IgE) production. Taken together, we established that NO exposure promotes allergic airway inflammation and increases the asthma susceptibility. The underlying mechanisms involve the promotion of activation of interleukin-4/signal transducer and activator of transcription-6 (IL-4/STAT6) pathway [IL-4 receptor (IL-4R) α, janus kinase (JAK) 1, JAK 3, and STAT6] and related transcription factor [T cell-specific protein-tyrosine kinase (Lck), extracellular-regulated kinase (ERK)1/2, and nuclear factor-κB (NF-κB)]. In particular, the imbalance of Th1/Th2 cell differentiation [IL-4, interferon (IFN)-γ, GATA-binding protein-3 (GATA-3), and T-box expressed in T cells (T-bet)] plays a pivotal role in NO-induced inflammatory responses. These findings may provide a better understanding of mechanism of NO-associated respiratory diseases.