Abstract
Cigarette smoking is the major aetiological factor in the development of Chronic
Obstructive Pulmonary Disease (COPD), a collection of diseases encompassing chronic
bronchitis and emphysema. Emphysema is characterised by enlargement of the distal
airspaces in the lungs due to destruction of alveolar walls, and was initially thought to
be the result of matrix destruction from a protease-antiprotease and oxidant-antioxidant
imbalance, leading to detachment of alveolar cells. However, recently apoptosis has
been implicated in alveolar cell loss; increased numbers of apoptotic epithelial and
endothelial cells have been observed in the lungs of emphysema patients. Thus the
effect of cigarette smoke on apoptotic cell death was investigated.
Unexpectedly, cigarette smoke condensate (CSC) did not induce apoptosis in either an
alveolar epithelial type II cell line (A549) or primary human umbilical vein endothelial
cells (HUVECs), but instead it induced necrosis and inhibited staurosporine-induced
apoptosis. The anti-apoptotic, pro-necrotic, effect of CSC was reproduced in a model
system using Jurkat T cells, when either staurosporine or Fas ligation was used as an
apoptotic stimulus. Additional studies indicated that these effects might be oxidantmediated as the antioxidant compounds glutathione and dithiothreitol prevented CSCmediated apoptosis inhibition, and necrosis. Time course experiments revealed that
CSC inhibited an early step in the caspase cascade, whereby caspase-3 was not
activated. Moreover, reconstitution of the apoptosome in cytoplasmic extracts from
CSC-treated cells, by addition of cytochrome-c and dATP, did not result in activation of
caspases-3 or -9. Thus, smoke treatment may alter the levels of pro- and antiapoptogenic factors downstream of the mitochondria to inhibit active apoptosome
formation. Therefore these data demonstrate that CSC treatment did not induce
apoptosis as previously reported. More interestingly, CSC inhibited apoptosis by
preventing activation of caspases, resulting in necrotic cell death. Thus, cell death in
response to cigarette smoke by necrosis, and not apoptosis, may be responsible for the
loss of alveolar walls observed in emphysema.
