Beta-catenin in the fibroproliferative response to acute lung injury

The GSK-3/beta-catenin-signalling axis in smooth muscle and its relationship with remodelling

beta-Catenin is a plasma membrane-associated protein that plays a dual role in cellular signalling by stabilizing cadherin mediated cell-cell contact and by regulating TCF-/LEF-mediated gene transcription. Traditionally, the role of beta-catenin in health and disease has mainly been studied in the context of development and uncontrolled cell growth in diseases such as cancer. Recent findings indicate, however, that beta-catenin also plays a significant role in fibro-proliferative diseases of several organ systems and that beta-catenin regulates mitogenic responses of smooth muscle cells. As several diseases of the internal organs are characterized by structural and phenotypic abnormalities of smooth muscle, including increased fibro-proliferative responses, these findings implicate that beta-catenin could play a broad pathophysiological role. This article will review this potential novel role for beta-catenin and associated intracellular signalling in smooth muscle and discuss the hypothesis that it plays a central role in smooth muscle remodelling.

Lung EC-SOD overexpression attenuates hypoxic induction of Egr-1 and chronic hypoxic pulmonary vascular remodeling

Although production of reactive oxygen species (ROS) such as superoxide (O(2)(.-)) has been implicated in chronic hypoxia-induced pulmonary hypertension (PH) and pulmonary vascular remodeling, the transcription factors and gene targets through which ROS exert their effects have not been completely identified. We used mice overexpressing the extracellular antioxidant enzyme extracellular superoxide dismutase (EC-SOD TG) to test the hypothesis that O(2)(.-) generated in the extracellular compartment under hypoxic conditions contributes to PH through the induction of the transcription factor, early growth response-1 (Egr-1), and its downstream gene target, tissue factor (TF). We found that chronic hypoxia decreased lung EC-SOD activity and protein expression in wild-type mice, but that EC-SOD activity remained five to seven times higher in EC-SOD TG mice under hypoxic conditions. EC-SOD overexpression attenuated chronic hypoxic PH, and vascular remodeling, measured by right ventricular systolic pressures, proliferation of cells in the vessel wall, muscularization of small pulmonary vessels, and collagen deposition. EC-SOD overexpression also prevented the early hypoxia-dependent upregulation of the redox-sensitive transcription factor Egr-1 and the procoagulant protein TF. These data provide the first evidence that EC-SOD activity is disrupted in chronic hypoxia, and increased EC-SOD activity can attenuate chronic hypoxic PH by limiting the hypoxic upregulation of redox-sensitive genes.

Inactivation of AP1 proteins by a nuclear serine protease precedes the onset of radiation-induced fibrosing alveolitis

Radiation-induced lung damage comprises inflammation (alveolitis) as well as disturbed regulation of cell differentiation and proliferation (fibrosis). The transcriptional regulation of this process is poorly understood. One key transcription factor involved in the regulation of proliferation and differentiation is AP1 (activator protein 1). The present study examined changes in the DNA-binding activity of AP1 after irradiation and defined the underlying molecular mechanisms in an animal model. The right lungs of Fischer rats received a single radiation dose of 20 Gy. Lung tissue was tested for AP1 DNA-binding activity, AP1 mRNA, and levels of AP1 proteins as well as for c-Jun specific proteolytic activity. After an initial increase, the AP1 DNA-binding activity was completely lost starting at 5.5 weeks after irradiation, which is 2.5 weeks before the onset of fibrosing alveolitis. This was not caused by reduction of mRNA levels or size. Instead, a selective nuclear cleavage of c-Jun by a serine protease caused the loss of AP1 activity. Considering the central role of AP1 in cell proliferation and differentiation and the strict timely correlation to the onset of the disease, the complete loss of AP1 function is likely to play a critical role in radiation-induced fibrosing alveolitis.

GSK-3/β-catenin signaling axis in airway smooth muscle: role in mitogenic signaling

β-Catenin plays a dual role in cellular signaling by stabilizing cadherin-mediated cell-cell contact and by regulating gene transcription associated with cell cycle progression. Nonetheless, its presence and function in airway smooth muscle have not been determined. We hypothesized a central role for β-catenin in mitogenic signaling in airway smooth muscle in response to growth factor stimulation. Immunocytochemical and biochemical analysis revealed that human airway smooth muscle cells indeed express abundant β-catenin, which was localized primarily to the plasma membrane in quiescent cells. Treatment of airway smooth muscle cells with PDGF or FBS induced sustained phosphorylation of glycogen synthase kinase-3 (GSK-3), a negative regulator in its unphosphorylated form that promotes β-catenin degradation. GSK-3 phosphorylation was also increased in airway smooth muscle cells with a proliferative phenotype compared with quiescent airway smooth muscle cells with a mature phenotype. Parallel with the increase in GSK-3 phosphorylation, growth factor treatment induced an increased expression and nuclear presence of β-catenin and activated promitogenic signaling in airway smooth muscle, including the phosphorylation of retinoblastoma protein, DNA synthesis ([3H]thymidine incorporation), and cell proliferation. Importantly, small interfering RNA knockdown of β-catenin strongly reduced retinoblastoma protein phosphorylation, [3H]thymidine incorporation, and cell proliferation induced by PDGF and FBS. Collectively, these data reveal the existence of a GSK-3/β-catenin signaling axis in airway smooth muscle that is regulated by growth factors and of central importance to mitogenic signaling.

Advances in mechanisms of repair and remodelling in acute lung injury

BACKGROUND

Acute respiratory distress syndrome (ARDS) is the most severe manifestation of acute lung injury (ALI). In patients who survive the acute injury the process of repair and remodelling may be an independent risk factor determining morbidity and mortality. This review explores recent advances in the field of fibroproliferative ARDS/ALI, with a special emphasis on (a) the primary contributing factors with a focus on cellular and soluble factors, and (b) mechanisms involved in repair and remodelling as they pertain to the importance of cell death, re-population, and matrix deposition.

DISCUSSION

Factors influencing progression to fibroproliferative ARDS vs. resolution and reconstitution of the normal pulmonary parenchymal architecture are poorly understood. Determinants of persistent injury and abnormal repair and remodelling may be profoundly affected by both environmental and genetic factors. Moreover, cumulative evidence suggests that acute inflammation and fibrosis may be in part independent and interactive processes that are autonomously regulated and thus amenable to individual and specific therapy.

CONCLUSIONS

Although our current understanding of these processes is limited by the inability to accurately replicate the complex human physiology in laboratory settings, it has recently become apparent that the process of repair and remodelling begins early in the course of ARDS/ALI and may be determined by the type of pulmonary injury. Understanding the mechanisms leading to and regulating fibroproliferative changes may contribute to the development of novel early therapeutic interventions in ARDS/ALI patients.

Angiogenesis in chronic lung disease

Chronic lung diseases like COPD, severe progressive pulmonary hypertension (PH), and interstitial lung diseases all have a lung vascular disease component. Cellular and molecular mechanisms of pulmonary vascular remodeling have been experimentally explored in many animal models, and it is now clear that microvessels are involved. In emphysema patients, there is a loss of lung microvessels, and in many forms of severe PH there is obliteration of precapillary arterioles by angioproliferation. Thus, COPD/emphysema and severe angioproliferative PH are on the opposite ends of a spectrum of vascular biology responses. Animal experiments have provided insight regarding some of the initiating events that shape the various forms of pulmonary vascular remodeling. In pulmonary fibrosis and in the postinjury phase of acute lung injury, the angiogenic/angiostatic balance is also affected. This review will therefore discuss angiogenesis in several chronic lung diseases and will speculate on how altered vascular homeostasis may contribute to lung disease development.

Transcriptional control of lung morphogenesis

The vertebrate lung consists of multiple cell types that are derived primarily from endodermal and mesodermal compartments of the early embryo. The process of pulmonary organogenesis requires the generation of precise signaling centers that are linked to transcriptional programs that, in turn, regulate cell numbers, differentiation, and behavior, as branching morphogenesis and alveolarization proceed. This review summarizes knowledge regarding the expression and proposed roles of transcription factors influencing lung formation and function with particular focus on knowledge derived from the study of the mouse. A group of transcription factors active in the endodermally derived cells of the developing lung tubules, including thyroid transcription factor-1 (TTF-1), beta-catenin, Forkhead orthologs (FOX), GATA, SOX, and ETS family members are required for normal lung morphogenesis and function. In contrast, a group of distinct proteins, including FOXF1, POD1, GLI, and HOX family members, play important roles in the developing lung mesenchyme, from which pulmonary vessels and bronchial smooth muscle develop. Lung formation is dependent on reciprocal signaling among cells of both endodermal and mesenchymal compartments that instruct transcriptional processes mediating lung formation and adaptation to breathing after birth.

Cell-specific Gene Expression in Patients with Usual Interstitial Pneumonia

RATIONALE

Usual interstitial pneumonia (UIP) is characterized by extracellular matrix deposition and the development of pulmonary fibrosis. Fibroblastic foci found in the lung are believed to represent an early stage in the evolution of this disease.

OBJECTIVES

To compare gene expression profiles in different components of lung tissue (fibroblastic foci, adjacent epithelium, and areas of type 2 pneumocyte hyperplasia) from patients with UIP, and contrast these profiles to distal, uninvolved (control) alveolar tissue from patients undergoing lung resection for cancer.

METHODS

Lung resection tissue (UIP, n = 11; controls, n = 11) was snap-frozen for subsequent laser capture microdissection, followed by mRNA extraction, linear amplification, and quantitative real-time polymerase chain reaction.

RESULTS

In patients with UIP, tissue inhibitor of matrix metalloprotease-1 and matrix metalloprotease (MMP)-2 gene expression was up-regulated within the fibroblastic foci compared with the overlying epithelium (p = 0.03, p = 0.02), and to control alveoli (p = 0.001, p = 0.04), respectively. MMP-9 and MMP-7, as well as osteopontin, were up-regulated in fibroblastic foci (p = 0.01, p = 0.08, p = 0.08), the adjacent epithelium (p = 0.001, p = 0.001, p = 0.03), and the hyperplastic type 2 pneumocytes (p = 0.02, p = 0.001, p = 0.08), respectively, compared with control alveoli.

CONCLUSION

Altered gene expression of important profibrotic mediators in the different cellular lung compartments in patients with UIP likely plays an important role in pathogenesis of the deranged extracellular matrix deposition and subsequent fibrosis in this condition.

Burying the dead: the impact of failed apoptotic cell removal (efferocytosis) on chronic inflammatory lung disease

Apoptosis and the removal of apoptotic cells (termed efferocytosis) are tightly coupled with the regulation of normal lung structure, both in the developing and adult organism. Processes that disrupt or uncouple this balance have the potential to alter normal cell turnover, ultimately resulting in the induction of lung pathology and disease. Apoptotic cells are increased in several chronic inflammatory lung diseases, including cystic fibrosis (CF), non-CF bronchiectasis, COPD, and asthma. While this may well be due to the enhanced induction of apoptosis, increasing data suggest that the clearance of dying cells is also impaired. Because efferocytosis appears to be a key regulatory checkpoint for the innate immune system, the adaptive immune system, and cell proliferation, the failure of this highly conserved process may contribute to disease pathogenesis by impeding both the resolution of inflammation and the maintenance of alveolar integrity. The recognition of impaired efferocytosis as a contributor to chronic inflammation may ultimately direct us toward the identification of new disease biomarkers, as well as novel therapeutic approaches.