Antioxidant Gene Expression in Rat Lung After Exposure to Cigarette Smoke

To investigate the effects of cigarette smoke on the expression of genes encoding intracellular antioxidant species, we exposed rats to whole cigarette smoke or air (control) daily for 1, 2, 7, or 14 days. After sacrifice, RNA was extracted from one lung and expression of mRNA for catalase (CAT), manganese superoxide dismutase (MnSOD), copper-zinc superoxide dismutase (CuZnSOD), glutathione peroxidase (GPX), and metallothionein (MT) was determined by Northern blots and dot blots. The anatomical distribution of expression of these genes was determined by in situ hybridization studies on sections of the contralateral lung. We found that expression of both MnSOD and MT was significantly increased (to levels 70 to 400% greater than in controls) at days 1 and 2 and returned to control levels by day 7. GPX expression was slightly but significantly increased at days 7 and 14 in smoke-exposed animals. CuZnSOD and CAT expression did not change from control levels. In control lungs, MnSOD was expressed in all cell types, with the highest expression seen in bronchial epithelial cells; a notable finding was a mosaic pattern of expression in the bronchial epithelium, with contiguous areas of bronchial epithelium composed of cells expressing MnSOD at high levels (hot spots), compared with the adjacent epithelium. In smoke-exposed lungs, the hot spots became less prominent after 1 and 2 days of exposure to smoke, but after 7 and 14 days the distribution of MnSOD expression was similar in control and smoke-exposed animals. CAT, CuZnSOD, GPX, and MT also showed widespread expression in the lung by in situ hybridization; GPX, CuZnSOD and MT were all most highly expressed in bronchial epithelium, whereas CAT expression levels were similar in all cell types. In contrast to MnSOD, expression of CAT, CuZnSOD, GPX, and MT was uniform within the bronchial epithelium, and the distribution of expression was the same in control and smoke-exposed animals at all time points. We conclude that most of these antioxidant enzymes and scavengers show prominent bronchial expression but that MnSOD shows a unique pattern, with intense hot spots in the epithelium of the small airways. This pattern is similar to the phenomenon of clonal heterogeneity described in other tissues but not previously reported in the lung. We conclude that cigarette smoke, like other forms of oxidant attack, transiently increases expression of MnSOD, and up-regulation of MnSOD expression appears to occur particularly in bronchial epithelial cells, which normally express MnSOD at relatively low levels. MT expression is also transiently increased by smoke whereas GPX expression increases after prolonged (7 to 14 days) exposure to cigarette smoke.