Lung emphysema induced by cigarette smoke: Studies in mice

Abstract

The experiments described in this thesis were designed to shed some more light on the mechanisms underlying cigarette smoke-induced lung emphysema. We used elastase instillation to induce lung emphysema, and subsequently perfused the lungs ex-vivo with buffer at a range of flows to measure changes in the resulting pressure. Unfortunately there was no clear relation between morphometric measures of lung damage and the flow/pressure profiles. We investigated whether exposure of mice to cigarette smoke in our experimental set-up led to the predisposition of emphysematous lesions towards the upper parts of the lungs that is seen in smoking humans. We showed that this was indeed the case, and, therefore, concluded that exposure of mice in our experimental set-up could serve as a valid model for cigarette smoke-induced emphysema in humans, at least with regard to the distribution of damage throughout the lungs. After having shown this, we applied the model to investigate the effects of one of the less well studied components of cigarette smoke: humic acid (HA). We hypothesised that deposition of HA would potentiate smoke-induced damage to the lungs, and thus the development of emphysema. Indeed, when mice were exposed to cigarette smoke for 2 months, after HA instillation in to the lungs, they had developed emphysematous lesions, whereas this was not the case in the mice that had been exposed to smoke or HA only. Our observation that HA potentiated the lung-damaging effects of smoking provides an explanation for the fact that many pack-years of smoking are generally required for symptoms to become clearly manifest, supposedly the time needed to get sufficient HA accumulation in the airways in order to catalyse the cytotoxic effects of smoking. Based on findings that cigarette smoke acts as a double-edged sword in the lungs, both inflicting tissue damage and depleting acid retinoids (molecules implied in tissue regeneration), the following hypothesis was formulated: a low vitamin A status can increase the susceptibility to cigarette smoke-induced lung emphysema. We exposed mice with a normal and a low, but not deficient, vitamin A status to cigarette smoke to test this, and found the hypothesis to be true. Unexpectedly, levels of acid retinoids remained normal in the lungs of the smoke-exposed animals, irrespective of their vitamin A status. Levels of retinol and retinyl palmitate were decreased in the lungs of animals with the low status. This could provide a possible explanation for the increased susceptibility, given their potent anti-oxidant activity, which could protect against cigarette smoke-induced damage. Involuntarily exposure to cigarette smoke in a nose-only system causes stress in experimental animals, through a combination of factors. We speculated that stress-induced hyperthermia might explain the high levels of mortality we experienced in both controls and smoke-exposed animals during our nose-only exposure experiments. Surprisingly, we found that mice instead showed a pronounced hypothermia during their stay in the nose-only exposure tubes, which we could attribute to the massive metal nosepieces. Unfortunately, preventing hypothermia by heating the nosepieces did not decrease parameters of stress, nor did it reduce mortality.