Unilateral paraquat-induced lung fibrosis: evolution of changes in lung fibronectin and collagen after graded degrees of lung injury

Modeling paraquat-induced lung fibrosis in C. elegans reveals KRIT1 as a key regulator of collagen gene transcription

Paraquat poisoning causes lung fibrosis, which often results in long-term pulmonary dysfunction. Lung fibrosis has been attributed to collagens accumulation, but the underlying regulatory pathway remains unclear. Here we use the genetically tractable C. elegans as a model to study collagen gene transcription in response to paraquat. We find that paraquat robustly up-regulates collagen gene transcription, which is dependent on KRI-1, a poorly studied protein homologous to human KRIT1/CCM1. KRI-1 knockdown prevents paraquat from activating the oxidative stress response transcription factor SKN-1/Nrf2, resulting in reduced collagen transcription and increased paraquat sensitivity. Using human lung fibroblasts (MRC-5), we confirm that both KRIT1 and Nrf2 are required for collagen transcription in response to paraquat. Nrf2 hyper-activation by KEAP1 knockdown bypasses KRIT1 to up-regulate collagen transcription. Our findings on the regulation of collagen gene transcription by paraquat could suggest potential strategies to treat pulmonary fibrosis caused by paraquat poisoning.

Protective effect of the glial cell line-derived neurotrophic factor (GDNF) on human mesencephalic neuron-derived cells against neurotoxicity induced by paraquat

Paraquat is a cationic herbicide that causes acute cell injury by undergoing redox cycling. Oxidative stress is thought to be the crucial mechanism invoked by this redox-cycling compound. The cytotoxicity of paraquat was examined in an immortalized human mesencephalic neuron-derived cell line. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide reduction activity was examined as cytotoxicity indicator. Cells were seeded with densities at inoculation of 5 × 10(4)cells/ml and 10 × 10(4)cells/ml, and paraquat was added 24h later to give final concentrations from 10 to 500 μM. At 24 and 48 h of treatment, mitochondrial activity was determined with the MTT assay. To further understand the effect of paraquat exposure on human mesencephalic neuron-derived cells, the cells were differentiated and similar experiments were carried out. Supplementation of culture medium with dibutyryl cyclic AMP and GDNF significantly increased the resistance of the cultures to the paraquat-mediated cytotoxicity. These results confirm that GDNF confers protection against paraquat-mediated cytotoxicity and show that immortalized human mesencephalic neuron-derived cells are an adequate in vitro system for evaluating the cytoprotective effects of GDNF on oxidative injury caused by xenobiotics.

Green tea extract inhibits paraquat-induced pulmonary fibrosis by suppression of oxidative stress and endothelin-l expression

Paraquat-induced pulmonary fibrosis involves two factors, direct injury by oxygen free radicals and indirect injury by inflammatory cells and fibroblasts. Endothelin-1 (ET-1) has been shown to act as a mediator of pulmonary fibrosis, and its formation increases during oxidative stress. We investigated whether green tea extract (GTE), which has antioxidant properties, inhibits paraquat-induced pulmonary fibrosis and whether ET-1 is involved in this process. Paraquat (0.3 mg/kg) was instilled into the right lungs of rats, following which the rats were either not further treated (Group P, n = 7), or they were administered 1% GTE mixed with feed (Group PG; n = 7) or the ET(A) receptor antagonist ZD2574 (10 mg/kg through gavage; Group PZ; n = 7) for two weeks. As control, we used rats instilled with saline (Group N; n = 6). Two weeks after paraquat instillation, we assayed the degree of pulmonary fibrosis by light microscopic morphometry and hydroxyproline content; lipid peroxidation as a marker of oxidative stresses by measurement of malondialdehyde (MDA); ET-1 by immunohistochemistry; and prepro-ET-1 mRNA expression by reverse transcription-polymerase chain reaction. Compared with Group N, significant pulmonary fibrosis was observed in Group P, accompanied by increases in MDA, ET-1, and prepro-ET-1 mRNA expression. Compared with Group P, Group PG showed significant decreases in pulmonary fibrosis, along with decreases in MDA, ET-1, and prepro-ET-1 mRNA expression. We also observed significant decreases in pulmonary fibrosis in Group PZ compared with Group P. These findings suggest that GTE inhibits paraquat-induced pulmonary fibrosis by suppression of oxidative stress and ET-1 expression.

Transfection with human copper-zinc superoxide dismutase induces bidirectional alterations in other antioxidant enzymes, proteins, growth factor response, and paraquat resistance

Transfection of a pSV2 human copper-zinc superoxide dismutase expression vector into murine fibroblasts resulted in stable transgenic clones producing increased amounts of copper-zinc superoxide dismutase. Two classes of transfectants were observed and were characterized by the presence or absence of an increase in endogenous glutathione peroxidase activity. In addition, increases and decreases in individual clones in the activities of manganese superoxide dismutase, glutathione reductase, and NADPH-reductase were detected. In general, these alterations in enzyme activity correlated to the cellular glutathione peroxidase/copper-zinc superoxide dismutase ratio. Parameters of cellular physiological functions were also altered, including cell division time, FGF and EGF response, fibronectin content, paraquat resistance, hydrogen peroxide release into media, and sensitivity to radiation. Some of these cellular parameters were also bidirectional and reflected the cellular glutathione peroxidase/copper-zinc superoxide dismutase ratio. Our results indicate that small deviations from the normal physiological copper-zinc superoxide dismutase/seleno-glutathione peroxidase ratios can have pronounced effects on other antioxidant enzymes, growth rate, growth factor response, and expression of proteins normally not associated with oxygen metabolism.

Dynamic changes in the functional characteristics of the interstitial fibroblast during lung repair

To evaluate the mechanisms involved in the regulation of fibroblast function during the repair of fibrotic lung injury, we isolated lung fibroblasts from adult male Fischer-344 rats before the induction of severe unilateral paraquat lung injury, as well as 1 and 14 days later. Fibroblasts were utilized at an early generation time to avoid senescence. In general, fibroblasts of injured lungs displayed significant increases in proliferative and matrix synthesis properties, with more pronounced increases detected early after the induction of injury. This was true of DNA synthesis, which increased by 3- and 1.4-fold on days 1 and 14, respectively; tyrosine kinase activity, which increased by 4- and 3.5-fold; fibronectin synthesis, 14- and 8-fold, respectively; and glycosaminoglycans synthesis, 4.4- and 3-fold, respectively. The increase in function of fibroblasts isolated from the immediate influence of extrinsic growth factors suggests that fibroblast function during repair may be under intrinsic as well as extrinsic control. In the early phases of repair, intrinsic changes may be more dominant and may result in autoregulation of fibroblast function. In the later phases of repair, despite some reduction in intrinsic fibroblast activation, exposure to extrinsic growth factors may result in maintaining the state of activation and in sustaining the repair process.

The role of free radicals in asbestos-induced diseases

Asbestos exposure causes pulmonary fibrosis and malignant neoplasms by mechanisms that remain uncertain. In this review, we explore the evidence supporting the hypothesis that free radicals and other reactive oxygen species (ROS) are an important mechanism by which asbestos mediates tissue damage. There appears to be at least two principal mechanisms by which asbestos can induce ROS production; one operates in cell-free systems and the other involves mediation by phagocytic cells. Asbestos and other synthetic mineral fibers can generate free radicals in cell-free systems containing atmospheric oxygen. In particular, the hydroxyl radical often appears to be involved, and the iron content of the fibers has an important role in the generation of this reactive radical. However, asbestos also appears to catalyze electron transfer reactions that do not require iron. Iron chelators either inhibit or augment asbestos-catalyzed generation of the hydroxyl radical and/or pathological changes, depending on the chelator and the nature of the asbestos sample used. The second principal mechanism for asbestos-induced ROS generation involves the activation of phagocytic cells. A variety of mineral fibers have been shown to augment the release of reactive oxygen intermediates from phagocytic cells such as neutrophils and alveolar macrophages. The molecular mechanisms involved are unclear but may involve incomplete phagocytosis with subsequent oxidant release, stimulation of the phospholipase C pathway, and/or IgG-fragment receptor activation. Reactive oxygen species are important mediators of asbestos-induced toxicity to a number of pulmonary cells including alveolar macrophages, epithelial cells, mesothelial cells, and endothelial cells. Reactive oxygen species may contribute to the well-known synergistic effects of asbestos and cigarette smoke on the lung, and the reasons for this synergy are discussed. We conclude that there is strong evidence supporting the premise that reactive oxygen species and/or free radicals contribute to asbestos-induced and cigarette smoke/asbestos-induced lung injury and that strategies aimed at reducing the oxidant stress on pulmonary cells may attenuate the deleterious effects of asbestos.

Compensatory growth in fibrotic lung injury

We induced severe left-sided lung fibrosis by the unilateral endobronchial instillation of paraquat (1.0 mg/kg) into the left lungs of adult Fischer 344 male rats. Growth of the contralateral lung as well as its proliferative activity were measured 6 or 14 days later. Whereas the left lung underwent severe fibrosis and shrinkage with more than 85% reduction in lung volume, the right lung more than doubled in size. In addition, there was a significant increase in total protein content, DNA content, and DNA synthesis. We conclude that unilateral lung injury resulting in ipsilateral fibrosis and loss of parenchyma is associated with compensatory growth of the contralateral lung.

Tyrosine kinase activation during lung injury, fibrosis, and compensatory lung growth

Endobronchial instillation of paraquat into left lungs of rats resulted in ipsilateral fibrosis (evaluated by light and electron microscopy) and compensatory growth of the contralateral lung. Tyrosine kinase (Tyr-K) activity rose early in the fibrotic lung, peaked to a value 4 times the controls on day 3, and remained elevated on day 14. In the right lung, slight increase in activity occurred only on day 5. Cellular proliferative activity was inhibited in both lungs early after injury. In the right lung, however, DNA synthesis increased to levels 10 times the controls by day 7. In the left lung, the inhibition decreased gradually after day 7, indicating increased cellular proliferation. We conclude that activation of some cellular Tyr-K may play a role in regulating early cellular proliferation during lung injury, fibrosis, and compensatory growth.