Urokinase plasminogen activator released by alveolar epithelial cells modulates alveolar epithelial repair in vitro

Intra-alveolar fibrin is formed following lung injury and inflammation and may contribute to the development of pulmonary fibrosis. Fibrin turnover is altered in patients with pulmonary fibrosis, resulting in intra-alveolar fibrin accumulation, mainly due to decreased fibrinolysis. Alveolar type II epithelial cells (AEC) repair the injured alveolar epithelium by migrating over the provisional fibrin matrix. We hypothesized that repairing alveolar epithelial cells modulate the underlying fibrin matrix by release of fibrinolytic activity, and that the degree of fibrinolysis modulates alveolar epithelial repair on fibrin. To test this hypothesis we studied alveolar epithelial wound repair in vitro using a modified epithelial wound repair model with human A549 alveolar epithelial cells cultured on a fibrin matrix. In presence of the inflammatory cytokine interleukin-1beta, wounds increase by 800% in 24 hours mainly due to detachment of the cells, whereas in serum-free medium wound areas decreases by 22.4 +/- 5.2% (p < 0.01). Increased levels of D-dimer, FDP and uPA in the cell supernatant of IL-1beta-stimulated A549 epithelial cells indicate activation of fibrinolysis by activation of the plasmin system. In presence of low concentrations of fibrinolysis inhibitors, including specific blocking anti-uPA antibodies, alveolar epithelial repair in vitro was improved, whereas in presence of high concentrations of fibrinolysis inhibitors, a decrease was observed mainly due to decreased spreading and migration of cells. These findings suggest the existence of a fibrinolytic optimum at which alveolar epithelial repair in vitro is most efficient. In conclusion, uPA released by AEC alters alveolar epithelial repair in vitro by modulating the underlying fibrin matrix.

Nuclear Factor I-C acts as a regulator of hepatocyte proliferation at the onset of liver regeneration

BACKGROUND & AIMS: Knockout studies of the murine Nuclear Factor I-C (NFI-C) transcription factor revealed abnormal skin wound healing and growth of its appendages, suggesting a role in controlling cell proliferation in adult regenerative processes. Liver regeneration following partial hepatectomy (PH) is a well-established regenerative model whereby changes elicited in hepatocytes lead to their rapid and phased proliferation. Although NFI-C is highly expressed in the liver, no hepatic function was yet established for this transcription factor. This study aimed to determine whether NFI-C may play a role in hepatocyte proliferation and liver regeneration. METHODS: Liver regeneration and cell proliferation pathways following two-thirds PH were investigated in NFI-C knockout (ko) and wild-type (wt) mice. RESULTS: We show that the absence of NFI-C impaired hepatocyte proliferation because of plasminogen activator I (PAI-1) overexpression and the subsequent suppression of urokinase plasminogen activator (uPA) activity and hepatocyte growth factor (HGF) signalling, a potent hepatocyte mitogen. This indicated that NFI-C first acts to promote hepatocyte proliferation at the onset of liver regeneration in wt mice. The subsequent transient down regulation of NFI-C, as can be explained by a self-regulatory feedback loop with transforming growth factor beta 1 (TGF-ß1), may limit the number of hepatocytes entering the first wave of cell division and/or prevent late initiations of mitosis. CONCLUSION: NFI-C acts as a regulator of the phased hepatocyte proliferation during liver regeneration.