Interleukin-6–Induced Protection in Hyperoxic Acute Lung Injury

Hyperoxic lung injury is commonly encountered in patients who require treatment with high concentrations of inspired oxygen. To determine whether interleukin (IL)-6 is protective in oxygen toxicity, we compared the effects of 100% O 2 in transgenic mice that overexpress IL-6 in the lung and transgene ( 2 ) controls. IL-6 markedly enhanced survival, with 100% of transgene ( 2 ) animals dying within 72 to 96 h, 100% of transgene ( 1 ) animals living for more than 8 d and more than 90% of transgene ( 1 ) animals living longer than 12 d. This protection was associated with markedly diminished alveolar-capillary protein leak, endothelial and epithelial membrane injury, and lung lipid peroxidation. Hyperoxia also caused cell death with DNA fragmentation in the lungs of transgene ( 2 ) animals and IL-6 markedly diminished this cytopathic response. The protective effects of IL-6 were not associated with significant alterations in the activities of copper/ zinc superoxide dismutase (SOD) or manganese SOD. They were, however, associated with the enhanced accumulation of the cell-death inhibitor Bcl-2, but not the cell-death stimulator BAX, and with the heightened accumulation of the cell-death regulator tissue inhibitor of metalloproteinase-1 (TIMP-1). These studies demonstrate that IL-6 markedly diminishes hyperoxic lung injury and that this protection is associated with a marked diminution in hyperoxia-induced cell death and DNA fragmentation. They also demonstrate that this protection is not associated with significant alterations in SOD activity, but is associated with the induction of Bcl-2 and TIMP-1. Supplemental oxygen is commonly given to patients with cardiopulmonary disorders to enhance tissue oxygenation. Unfortunately, the prolonged administration of fractional inspired concentrations of oxygen greater than 50 to 60% leads to a variety of forms of tissue damage, including acute lung injury. The pathophysiology of this pulmonary toxicity has been characterized in animal models (1‐4). These studies demonstrated that toxic concentrations of O 2 generate oxygen-derived free radicals that damage lung epithelial and endothelial cells, leading to a protein-rich fluid that floods the alveolar space. Recent researchers have also demonstrated that this injury is associated with a cell-death response with features of both cell necrosis and apoptosis (4, 5). Interleukin (IL)-6 is a pleiotropic cytokine that is produced at sites of tissue inflammation. It is classified as an IL-6‐type cytokine with IL-11, leukemia inhibitory factor, cardiotrophin-1, oncostatin M, and ciliary neurotrophic factor on the basis of the overlapping effector profiles of these cytokines and their shared use of gp130 as the b -subunit in their multimeric receptor complexes (6). IL-6 induces fever, activates B and T lymphocytes, and stimulates hepatocytes to produce acute phase proteins. Recent studies have shown that IL-6 also has potent anti-inflammatory and protective properties. These include the ability to inhibit the production of tumor necrosis factor (TNF), IL-1, and macrophage inflammatory protein-2; decrease neutrophil sequestration; increase levels of IL-1 receptor antagonist and TNF soluble receptor; stimulate the production of metalloproteinase inhibitors; reduce intracellular superoxide production; reduce tissue matrix degradation; and inhibit cellular apoptosis (7‐ 14). Surprisingly, the ability of IL-6 to regulate hyperoxic lung injury has not been adequately investigated. We hypothesized that the anti-inflammatory and cytoproptective effects of IL-6 could ameliorate hyperoxic lung injury. To test this hypothesis we compared the injury induced by 100% O 2 in transgenic mice in which IL-6 was selectively overexpressed in the lung (CC10‐IL-6 mice) with appropriate transgene ( 2 ) littermate controls. These experiments demonstrated that CC10‐IL-6 transgene ( 1 ) mice have an impressive ability to tolerate 100% O 2 . This tolerance manifests as enhanced survival, decreased pulmonary edema and alveolar-capillary protein leakage, and decreased lung lipid peroxidation when compared with transgene ( 2 ) controls. Further, transgene ( 2 ) mice manifest an impressive cell-death response associated with DNA fragmentation which was inhibited in the IL-6 ( 1 ) animals. This protection was associated with the enhanced accumulation of the regulatory proteins Bcl-2 and tissue inhibitor of metalloproteinase-1 (TIMP-1).