Fibroblasts play a critical role in the transition from the saccular to the alveolar stage of lung development, during which there is a four-fold increase in the number of interstitial fibroblasts in the neonatal rat lung [86
]. Perturbations such as hyperoxia, barotrauma and steroid therapy have been shown to interfere with alveolar development in the rat [87
], baboon [88
] and human [89
], the net result of which is a significant, often permanent, decrease in the number of alveoli.
Although the control of alveolar formation is poorly understood, a substantial body of evidence exists regarding events that coincide with alveolar septation, many of which may influence fibroblast proliferation. Elastic fibers are thought to be involved in septation by providing structural support for newly emerging secondary septa. Inhibition of elastic fiber assembly has been linked to impaired septation and alveolarization [90
]. In neonatal rat lung fibroblasts, elastin expression peaks during the second postnatal week, that is during the phase of rapid alveolarization, and declines rapidly thereafter [91
]. This means that interference with lung fibroblast integrity may result in impaired formation of alveoli, which may lead to a permanent reduction in the number of alveoli.
Exposure to cigarette smoke inhibits fibroblast proliferation and migration by increasing cell cycle transit time, thereby reducing the rate of alveolarization [92
]. Consequently the surface area available for gas exchange is reduced; another effect of reduced alveolarization is a reduction in the number of alveolar-bronchiolar attachments, which can lead to airway narrowing [93
]. Cigarette smoke exposure also compromises fibroblast-induced repair responses, and may be one of the factors that contributes to the development of smoke-induced lung diseases [94
]. Accumulation of nicotine in fibroblasts will affect glycolysis and plausibly fibroblast function too. However, in vitro
studies have shown that nicotine has no effect on fibroblasts from human fetal lungs [94
]. In vivo
studies also show that nicotine only has a transient effect on metabolism in lungs of adult animals, as opposed to a permanent suppression of energy metabolism of animals that were exposed to nicotine during lung development [75
]. The in vitro
studies on fibroblasts were performed on cells that were not metabolically permanently compromised as opposed to the fibroblasts of lung cells of neonatal rats that had been exposed to nicotine during gestation and lactation. Therefore, since nicotine exposure during gestation and lactation interferes with glucose metabolism and apoptosis in the fetal and neonatal lung, and since it may cause disruption of the interaction between lung fibroblast glucose metabolism and fibroblast function, it is plausible that it will also adversely affect the long-term maintenance of lung structure. It is interesting to note that lung fibroblasts from patients with emphysema show a reduced proliferation rate [95
] and premature aging [96
] and that this condition is characterized by slow degeneration of the lung parenchyma [97
]. Therefore, the gradual deterioration of the connective tissue framework of the lungs of nicotine exposed rat pups () may be partially due to inadequate fibroblast proliferation and function.
Figure 3. Effect of maternal nicotine exposure during pregnancy and lactation in rats on the connective tissue framework (stained black) of the lung of adult offspring. Note that the connective tissue framework of the control lungs (A) is more extensive than that (more ...)
Many agents that induce lung injury may do so by modifying key metabolic events for various cell populations in the lung. Type I alveolar epithelial cells for example, which cover more than 90% of the alveolar surface [98
], depend on glycolysis for energy [99
]. Glycolysis also supplies the ATP required to maintain the membrane-linked Na + -K + ATPase [100
]. The Na + -K + ATPase pump plays a vital role in maintaining cell volume. Therefore, reducing its activity by inhibition of glycolysis will result in the swelling of these cells and the formation of membrane “blebs” [101
Inhibition of glycolysis would therefore be expected to interfere with the ability of the type I alveolar epithelial cells to adapt to changes in the environment and to maintain cell volume. Since pulmonary glycolysis is irreversibly suppressed in animals that were exposed to nicotine during lung development, the activity of the Na + -K + ATPase pump will also be permanently lower in type I epithelial cells, and this could result in membrane blebbing and rupture of the cell membranes (). The type I epithelial cells are the most vulnerable to injury [102
] and the permanently reduced glycolytic activity will therefore make them even more susceptible to damage, especially when exposed to toxic substances in blood and inhaled air.
Figure 4. Scanning electron micrographs of the alveolar surface in postnatal rats showing (A) control lung, (B) blebbing of the alveolar type I cell membrane in a nicotine exposed animal and (C) rupture of the alveolar surface to reveal the underlying capillary (more ...)
Alveolar type II cell proliferation has been found to be increased in the lungs of nicotine exposed animals, which is a likely response to type I cell injury and death [103
]. Type II alveolar epithelial cells are critical for the maintenance of alveolar homeostasis by secreting surfactant and by proliferating and differentiating to replace damaged type I cells [104
]; in effect these cells act in defense of the alveolus. It has also been shown that maternal nicotine exposure during gestation and lactation induces rapid type II alveolar epithelial cell proliferation in response to type I cell damage [105
]. Since rapid cell proliferation is associated with rapid shortening of the telomeres [108
], it is conceivable that premature aging of the type II cells will occur in the lungs of the nicotine exposed rats. This may result in an increased vulnerability of the alveolus, which is supported by the observation that loss of type II cells has a detrimental effect on the alveolus [106
It appears that the negative impact of maternal nicotine exposure during gestation and lactation on the growth, development and repair processes of the lungs of the offspring causes lung structure to more rapidly deteriorate with age than in animals that were not exposed to nicotine. This is illustrated by the appearance of membrane blebs (), alveolar fenestrations [109
] and eventually microscopic emphysema (). The elastic tissue framework () of the lungs of nicotine exposed animals is also compromised [110
]. Exposure of fetal monkeys and rats to nicotine via the placenta during the late saccular/early alveolar phase of lung development results in an increase in the size of the primitive alveoli; as a consequence the alveolar surface area for gas exchange in the adult lung is decreased [109
]. Collectively, the structural changes in the lungs of these animals resembles faster aging of the lungs, and are likely to make the lungs more susceptible to respiratory disease.
The effect of nicotine exposure via the mother during pregnancy and lactation on the parenchyma of the lung tissue of adult offspring. The alveoli of the control lungs (A) are smaller than those of the lungs that were exposed to nicotine (B).
The gradual deterioration of the lung parenchyma with increasing age is clearly due to an inability of the lung epithelium and fibroblasts to maintain the structural integrity of the lungs. This effect is likely due to premature aging of the fibroblasts [96
] and alveolar epithelial cells [112
], which can be attributed to altered “programming” due to the changes in the in utero
The reason for the altered “programming” is not clear. It is known that nicotine induces peroxidation of membrane lipids. It also reduces the anti-oxidant capacity of the lungs [40
]. Since oxidants [33
] and nicotine [116
] can induce point mutations in DNA (), it is possible that the imbalance in the oxidant/antioxidant status of the nicotine-exposed developing lung results in the altered “programming” and consequently the lower glycolytic capacity of the lungs [70
], as well as the drastic increase in AMP [75
]. This theory is supported by the observation that maternal vitamin C supplementation during pregnancy and lactation prevents the lowering of the glycolytic capacity of the lungs of nicotine-exposed offspring [40
] as well as the development of microscopic emphysema (G Maritz, unpublished data). It is therefore plausible that restoration of the oxidant/antioxidant status of the mother and offspring will prevent altered “programming” and thus premature aging of the lungs of the offspring.
It has been demonstrated that in utero
exposure to nicotine increases DNA methylation and acetylation in the fetus. Nicotine also alters gene methylation in cultured human esophageal squamous epithelial cells [117
]. It is therefore plausible that some of the longer term effects of maternal nicotine exposure on the respiratory system of the offspring are due to epigenetic changes. It has been suggested that the rapid induction of insulin resistance in rats exposed to nicotine during gestation and lactation is a reflection of an acute epigenetic response and not a genetic predisposition [118
]. It is thus plausible that the effects of maternal nicotine exposure on the metabolism and lung structural integrity of the offspring are due to epigenetic changes rather than changes to the DNA.