Reactive oxygen species (ROS) are generated as by-products of cellular metabolism, primarily in the mitochondria. When cellular production of ROS overwhelms its antioxidant capacity, damage to cellular macromolecules such as lipids, protein, and DNA may ensue. Such a state of "oxidative stress" is thought to contribute to the pathogenesis of a number of human diseases including those of the lung. Recent studies have also implicated ROS that are generated by specialized plasma membrane oxidases in normal physiological signaling by growth factors and cytokines. In this review, we examine the evidence for ligand-induced generation of ROS, its cellular sources, and the signaling pathways that are activated. Emerging concepts on the mechanisms of signal transduction by ROS that involve alterations in cellular redox state and oxidative modifications of proteins are also discussed.

In addition to its participation in a variety of other biochemical reactions, glutathione (GSH) is a major antioxidant. It is regularly generated intracellularly from its oxidized form by glutathione reductase activity that is coupled with a series of interrelated reactions. Synthesis of GSH also takes place intracellularly by a two-step reaction, the first of which is catalyzed by rate-limiting gamma-glutamylcysteine synthetase activity. Intracellular substrates for GSH are provided both by direct amino acid transport and by a gamma-glutamyl transpeptidase reaction that salvages circulating GSH by coupling the gamma-glutamyl moiety to a suitable amino acid acceptor for transport into the cell. Although the liver is a net synthesizer of circulating GSH, organs such as the kidney salvage GSH through the gamma-glutamyl transpeptidase reaction. Intracellular GSH may be consumed by GSH transferase reactions that conjugate GSH with certain xenobiotics. Elevation of cellular GSH levels in cultured cells in response to hyperoxia or electrophilic agents such as diethylmaleate is coupled with an increase in activity of the Xc- transport system for the amino acids cystine and glutamate. Strategies may be developed for protection against oxidant injury by enhancement of transport systems for precursor amino acids of GSH or by providing substrate that circumvents feedback inhibition of GSH synthesis.

Reactive oxygen species (ROS) play an important role in the pathogenesis of many human diseases, including the acute respiratory distress syndrome, Parkinson’s disease, pulmonary fibrosis, and Alzheimer’s disease. In mammalian cells, several genes known to be induced during the immediate early response to growth factors, including the protooncogenes c- fos and c- myc, have also been shown to be induced by ROS. We show that members of the STAT family of transcription factors, including STAT1 and STAT3, are activated in fibroblasts and A-431 carcinoma cells in response to H2O2. This activation occurs within 5 min, can be inhibited by antioxidants, and does not require protein synthesis. STAT activation in these cell lines is oxidant specific and does not occur in response to superoxide- or nitric oxide-generating stimuli. Buthionine sulfoximine, which depletes intracellular glutathione, also activates the STAT pathway. Moreover, H2O2stimulates the activity of the known STAT kinases JAK2 and TYK2. Activation of STATs by platelet-derived growth factor (PDGF) is significantly inhibited by N-acetyl-l-cysteine and diphenylene iodonium, indicating that ROS production contributes to STAT activation in response to PDGF. These findings indicate that the JAK-STAT pathway responds to intracellular ROS and that PDGF uses ROS as a second messenger to regulate STAT activation.

The cellular source(s) and mechanisms of generation of reactive oxygen species (ROS) in nonphagocytic cells stimulated by cytokines are unclear. In this study, we demonstrate that transforming growth factor ␤1 (TGF-␤1, 1 ng/ml) induces the release of H 2 O 2 from human lung fibroblasts within 8 h following exposure to this cytokine. Elevation in H 2 O 2 release peaked at 16 h (ϳ22 pmol/min/10 6 cells) and gradually declined to undetectable levels at 48 h after TGF-␤1 treatment. NADH consumption by these cells was stimulated by TGF-␤1 while that of NADPH remained unchanged. NADH oxidase activity as measured by diphenyliodonium (DPI)-inhibitable NADH consumption in TGF-␤1-treated cells followed a time course similar to that