Inflammation and oxidative stress induced by cigarette smoke in Lewis rat brains

Abstract

Exposure to cigarette smoke has been associated with an increased risk of neurological diseases such as stroke, Alzheimer's disease and multiple sclerosis. In these studies, serum and brain sections from Lewis rats or those exposed to cigarette smoke and control rats were examined for evidence of increased inflammation and oxidative stress. Immunocytochemical staining of brain sections from CS-exposed rats showed increased expression of class II MHC and, in ELISA, levels of IFN-gamma and TNF-α were higher than for non-exposed rats. In polymerase chain reaction assays there was increased interferon-gamma, TNF-α, IL-1α, IL-1β, IL-23, IL-6, IL-23, IL-17, IL-10, TGF-β, T-bet and FoxP3 gene expression with CS exposure. There was also markedly elevated MIP-1α/CCL3, less prominent MCP-1/CCL2 and no elevation of SDF-1α gene expression. Analysis of samples from CS-exposed and control rats for anti-oxidant expression showed no significant difference in serum levels of glutathione and, in brain, similar levels of superoxide dismutase and decreased thioredoxin gene expression. In contrast, there was increased brain gene expression for the pro-oxidants iNOS and the NADPH components NOX4, dual oxidase 1 and p22^phox. Nrf2 expression, which is typically triggered as a secondary response to oxidative stress, was also increased in brains from CS-exposed rats with nuclear translocation of this protein from cytoplasm demonstrated in astrocytes in association with increased expression of the aryl hydrocarbon receptor gene, an Nrf2 target. These studies, therefore, demonstrate that CS exposure in these animals can trigger multiple immune and oxidative responses that may have important roles in the pathogenesis of CNS inflammatory neurological diseases.

Introduction

Links between cigarette smoking and neurological complications have been demonstrated in disease states such as stroke and Alzheimer's disease, and more recently in MS (Sundstrom et al., 2008, Cataldo et al., 2010, Shah and Cole, 2010). Underlying these associations are animal studies which show that smoking can increase levels of circulating proinflammatory markers and markers of oxidative stress, including levels of reactive oxygen species (ROS), and decrease levels of antioxidants (Churg et al., 2002, D'Hulst et al., 2005, Moerloose et al., 2005, Khanna et al., 2009). Among studies performed in humans are those which have shown that urinary levels of nucleic acid and lipid oxidative products were increased with smoking and that serum and urine levels of superoxide dismutase (SOD) and glutathione peroxidase (GPx) correlated inversely with levels of the nicotine metabolite cotinine in smokers (Harman et al., 2003, Sobczak et al., 2004). In addition to activating proinflammatory responses and cellular stress, CS and nicotine have effects that can result in immunosuppression (Geng et al., 1995, Sopori and Kozak, 1998, Sopori et al., 1998, Chen et al., 2007). Therefore, using a Lewis rat model we examined these possible effects of cigarette smoke on the expression of markers of proinflammatory and anti-inflammatory responses. We also examined the effects on the expression of NADPH oxidase, pro-oxidant and antioxidant genes and on the activation of Nrf2. These studies showed that a number of these responses increased in brains of rats with significant CS exposure. This study, which represents the first demonstration of these effects from CS in the brain, provides a basis for future investigations of the specific mechanisms by which cigarette smoking may trigger the development of inflammatory and degenerative nervous system diseases.