The mechanisms underlying cell death during oxygen deprivation are unknown. We report here a model for oxygen deprivation-induced apoptosis. The death observed during oxygen deprivation involves a decrease in the mitochondrial membrane potential, followed by the release of cytochrome c and the activation of caspase-9. Bcl-X L prevented oxygen deprivation-induced cell death by inhibiting the release of cytochrome c and caspase-9 activation. The ability of Bcl-X L to prevent cell death was dependent on allowing the import of glycolytic ATP into the mitochondria to generate an inner mitochondrial membrane potential through the F 1 F 0 -ATP synthase. In contrast, although activated Akt has been shown to inhibit apoptosis induced by a variety of apoptotic stimuli, it did not prevent cell death during oxygen deprivation. In addition to Bcl-X L , cells devoid of mitochondrial DNA (°cells) that lack a functional electron transport chain were resistant to oxygen deprivation. Further, murine embryonic fibroblasts from bax ؊/؊ bak ؊/؊ mice did not die in response to oxygen deprivation. These data suggest that when subjected to oxygen deprivation, cells die as a result of an inability to maintain a mitochondrial membrane potential through the import of glycolytic ATP. Proapoptotic Bcl-2 family members and a functional electron transport chain are required to initiate cell death in response to oxygen deprivation.

The molecular mechanisms of pulmonary fibrosis are poorly understood. Previous reports indicate that activation of TGF-␤1 is essential for the development of pulmonary fibrosis. Here, we report that the proapoptotic Bcl-2 family member Bid is required for the development of pulmonary fibrosis after the intratracheal instillation of bleomycin. Mice lacking Bid exhibited significantly less pulmonary fibrosis in response to bleomycin compared with WT mice. The attenuation in pulmonary fibrosis was observed despite similar levels of inflammation, lung injury, and active TGF-␤1 in bronchoalveolar lavage fluid 5 days after the administration of bleomycin in mice lacking Bid and in WT controls. Bleomycin induced similar levels cell death in vitro in alveolar epithelial cells isolated from WT and bid ؊/؊ mice. By contrast, alveolar epithelial cells from bid ؊/؊ mice were resistant to TGF-␤1-induced cell death. These results indicate that Bcl-2 family members are critical regulators for the development of pulmonary fibrosis downstream of TGF-␤1 activation.apoptosis ͉ Bcl-2 ͉ TGF-␤

The intracellular signaling pathways that control O(2) deprivation (anoxia)-induced apoptosis have not been fully defined in lung epithelial cells. We show here that the lung epithelial cell line A549 releases cytochrome c and activates caspase-9 followed by DNA fragmentation and plasma membrane breakage in response to anoxia. The antiapoptotic protein Bcl-X(L) prevented the anoxia-induced cell death by inhibiting the release of cytochrome c and caspase-9 activation. A549 cells devoid of mitochondrial DNA (rho(o)-cells) and lacking a functional electron transport chain were resistant to anoxia-induced apoptosis. A549 cells preconditioned with either hypoxia (1.5% O(2)) or tumor necrosis factor-alpha, which activated the transcription factors hypoxia-inducible factor-1 or nuclear factor-kappaB, respectively, did not provide protection from anoxia-induced cell death. These results indicate that A549 cells require a functional electron transport chain and the release of cytochrome c for anoxia-induced apoptosis.

Exposure to bleomycin in rodents induces lung injury and fibrosis. Alveolar epithelial cell death has been hypothesized as an initiating mechanism underlying bleomycin-induced lung injury and fibrosis. In the present study we evaluated the contribution of mitochondrial and receptor-meditated death pathways in bleomycin-induced death of mouse alveolar epithelial cells (MLE-12 cells) and primary rat alveolar type II cells. Control MLE-12 cells and primary rat alveolar type II cells died after 48 h of exposure to bleomycin. Both MLE-12 cells and rat alveolar type II cells overexpressing Bcl-X(L) did not undergo cell death in response to bleomycin. Dominant negative Fas-associating protein with a death domain failed to prevent bleomycin-induced cell death in MLE-12 cells. Caspase-8 inhibitor CrmA did not prevent bleomycin-induced cell death in primary rat alveolar type II cells. Furthermore, fibroblast cells deficient in Bax and Bak, but not Bid, were resistant to bleomycin-induced cell death. To determine whether the stress kinase JNK was an upstream regulator of Bax activation, MLE-12 cells were exposed to bleomycin in the presence of an adenovirus encoding a dominant negative JNK. Bleomycin-induced Bax activation was prevented by the expression of a dominant negative JNK in MLE-12 cells. Dominant negative JNK prevented cell death in MLE-12 cells and in primary rat alveolar type II cells exposed to bleomycin. These data indicate that bleomycin induces cell death through a JNK-dependent mitochondrial death pathway in alveolar epithelial cells.

Nitric oxide (NO) can induce apoptosis in a variety of cell types. A non-toxic concentration of nitric oxide under normal oxygen conditions triggered cell death under hypoxic conditions (1.5% O 2 ) in fibroblasts. Nitric oxide administered during hypoxia induced the release of cytochrome c, caspase-9 activation, and the loss of mitochondrial membrane potential followed by DNA fragmentation and lactate dehydrogenase release (markers of cell death). Bcl-X L protected cells from nitric oxide-induced apoptosis during hypoxia by preventing the release of cytochrome c, caspase-9 activation, and by maintaining a mitochondrial membrane potential. Murine embryonic fibroblasts from bax ؊/؊ bak ؊/؊ mice exposed to nitric oxide during hypoxia did not die, indicating that pro-apoptotic Bcl-2 family members are required for NO-induced apoptosis during hypoxia. The nitric oxide-induced cell death during hypoxia was independent of cGMP and peroxynitrite. Cells devoid of mitochondrial DNA (°-cells) lack a functional electron transport chain and were resistant to nitric oxide-induced cell death during hypoxia, suggesting that a functional electron transport chain is required for nitric oxide-induced apoptosis during hypoxia.