Genetic ablation of the Bach1 gene reduces hyperoxic lung injury in mice: Role of IL-6

Bach1 is a transcriptional repressor of the heme oxygenase (HO)-1 gene. Bach1-null (Bach1^-/-) mice are reported to be protected from myocardial ischemia/reperfusion injury; however, the effect of Bach1 disruption on another oxidative stress model of hyperoxic lung injury has yet to be determined. To investigate the role of Bach1 in hyperoxic lung injury, Bach1^-/- mice and wild-type (WT) mice were exposed to 90% O₂. During hyperoxic exposure, the survival of Bach1^-/- mice was significantly longer than that of WT mice. However, the administration of zinc protoporphyrin, an inhibitor of HO-1 activity, did not change the mortality in either of the mice, thus suggesting that this protective effect was not mediated by an HO-1 overexpression in Bach1^-/- mice. The indices of lung injury in the lungs of Bach1^-/- mice were lower than those of WT mice; unexpectedly, however, the levels of IL-6 in bronchoalveolar lavage (BAL) fluid from Bach1^-/- mice were significantly higher than those of WT mice. Interestingly, the intrapulmonary administration of small interfering RNA against IL-6 was shown to reduce the IL-6 levels in BAL fluids and shorten the survival in Bach1^-/- mice during hyperoxic exposure. In addition, a chromatin immunoprecipitation analysis revealed the binding of Bach1 to the IL-6 promoter and its detachment after oxidative stress. Considering the previous observation that the transgenic mice overexpressing IL-6 are protected from hyperoxic lung injury, these results therefore indicate that IL-6 mediates an increased survival in Bach1^-/- mice during hyperoxic exposure.