Nicotine and endotoxin in airway hyperreactivity and inflammation

Abstract

Active, as well as passive cigarette smoke exposure is detrimental to the human health, negatively affecting the cardiovascular, respiratory and immune systems. In addition, it is also carcinogenic. The present thesis focused on the effects of cigarette smoke and two of its major components nicotine and endotoxin (lipopolysaccharide, LPS) on airway hyperreactivity (AHR) and inflammation, in both the lower and upper airways. The roles of nicotine and LPS were analyzed in murine in vitro and in vivo models. Further, the effects of smoking on the expressions of activin receptor-like kinases (ALK) were analyzed in tonsils from heavy smokers and matching non-smokers.

The first four papers of this thesis illustrated the highly complex interactions induced by nicotine and LPS involving direct airway smooth muscle contractions, indirect nerve-mediated airway contractions and epithelium-dependent relaxations. Short-term exposure (1 day) to nicotine in an in vitro organ culture model resulted in decreased epithelium-dependent airway relaxations via cyclooxygenase (COX) and microsomal prostaglandin E synthase 1 (mPGES-1) pathways. Long-term exposure (4 days) in the same model caused increased smooth-muscle-mediated airway contractions via transcriptional upregulation of kinin-receptors, involving mitogen-activated protein kinase (MAPK) c-Jun N-terminal kinases (JNK) and phosphodiesterase (PDE) 4 pathways. In parallel, JNK-mediated transcriptional upregulation of toll-like receptors (TLR) was also seen. This resulted in enhanced contractile responses to the prototypical TLR4 agonist LPS, but at the same time decreased LPS-mediated AHR to electric field stimulation (EFS). In analogy with the latter, in vivo experiments using subcutaneously implanted pumps that delivered nicotine during 24 consecutive days, followed by 3 days of intranasally instilled LPS, demonstrated that the AHR induced by LPS was markedly decreased in the nicotine-treated mice. This highlighted the role of neuronal mechanisms in the development of lower airway AHR. The fifth paper used human tonsils to investigate the effect of chronic cigarette smoke exposure on the expression of ALK. A novel cotinine assay was developed to quantify the level of cigarette smoke exposure in tonsil homogenates. Results showed a positive correlation between increased levels of smoke exposure and increased tonsillar mRNA expressions of IL-8, ALK1 and 2.

In conclusion, both nicotine and LPS contribute to the development of AHR. Nicotine modulates the response to LPS in murine models both in vitro and in vivo. Altered ALK expressions in smokers may contribute to the poor prognosis that characterizes tonsillar cancer among smokers. These results may contribute to the development of specific and personalized treatment strategies for patients that are actively or passively exposed to nicotine or cigarette smoke.