Wright CJ, Zhuang T, La P, Yang G, Dennery PA. Hyperoxiainduced NF-B activation occurs via a maturationally sensitive atypical pathway. Am J Physiol Lung Cell Mol Physiol 296: L296 -L306, 2009. First published December 12, 2008 doi:10.1152/ajplung.90499.2008.-NF-B activation is exaggerated in neonatal organisms after oxidant and inflammatory insults, but the reason for this and the downstream effects are unclear. We hypothesized that specific phosphorylation patterns of IB␣ could account for differences in NF-B activation in hyperoxia-exposed fetal and adult lung fibroblasts. After exposure to hyperoxia (Ͼ95% O2), nuclear NF-B binding increased in fetal, but not adult, lung fibroblasts. Unique to fetal cells, phosphorylation of IB␣ on tyrosine 42, rather than serine 32/36 as seen in TNF-␣-exposed cells, preceded NF-B nuclear translocation. In fetal cells stably transfected with an NF-B-driven luciferase reporter, hyperoxia significantly suppressed reporter activity, in contrast to increased reporter activity after TNF-␣ incubation. Targeted gene profiling analysis showed that hyperoxia resulted in decreased expression of multiple genes, including proapoptotic factors. Transfection with a dominant-negative IB␣ (Y42F), which cannot be phosphorylated on tyrosine 42, resulted in upregulation of multiple proapoptotic genes. In support of this finding, caspase-3 activity and DNA laddering were specifically increased in fetal lung fibroblasts expressing Y42F after exposure to hyperoxia. These data demonstrate a unique pathway of NF-B activation in fetal lung fibroblasts after exposure to hyperoxia, whereby these cells are protected against apoptosis. Activation of this pathway in fetal cells may prevent the normal pattern of fibroblast apoptosis necessary for normal lung development, resulting in aberrant lung morphology in vivo.IB␣; apoptosis DESPITE ADVANCES IN NEONATAL care and improved survival of very low-birth-weight infants, the incidence of bronchopulmonary dysplasia (BPD) has remained at 25-35% (65). O 2 therapy has long been implicated in the pathogenesis of BPD (63). The toxic effects of O 2 depend not only on the nature of the exposure, but also on the maturational stage of the organism at the time of exposure. Hyperoxia causes severe lung injury and mortality in adult rodents, but this mortality is less pronounced in similarly exposed neonatal rodents (31). Nonetheless, neonatal animals exposed to hyperoxia develop lesions resembling BPD (5,26,33,43,66). In vitro studies have demonstrated that hyperoxia results in necrosis, apoptosis, and abnormal differentiation of lung cells (8,51,54), which may explain in part the abnormal lung development seen in these studies.NF-B has been implicated in the pathogenesis of multiple pulmonary diseases (22). This multisubunit transcription factor activates genes that regulate apoptosis and respond to inflammation and oxidative stress (4, 34, 37). Multiple models have shown increased NF-B activation in neonates compared with adults after exposure to inflammatory and oxi...