Regulation of lung fibroblast tropoelastin expression by alveolar epithelial cells

Into the Tissues: Extracellular Matrix and Its Artificial Substitutes: Cell Signalling Mechanisms

The existence of orderly structures, such as tissues and organs is made possible by cell adhesion, i.e., the process by which cells attach to neighbouring cells and a supporting substance in the form of the extracellular matrix. The extracellular matrix is a three-dimensional structure composed of collagens, elastin, and various proteoglycans and glycoproteins. It is a storehouse for multiple signalling factors. Cells are informed of their correct connection to the matrix via receptors. Tissue disruption often prevents the natural reconstitution of the matrix. The use of appropriate implants is then required. This review is a compilation of crucial information on the structural and functional features of the extracellular matrix and the complex mechanisms of cell-cell connectivity. The possibilities of regenerating damaged tissues using an artificial matrix substitute are described, detailing the host response to the implant. An important issue is the surface properties of such an implant and the possibilities of their modification.

Severe but not moderate hyperoxia of newborn mice causes an emphysematous lung phenotype in adulthood without persisting oxidative stress and inflammation

Background

Preterm newborns typically require supplemental oxygen but hyperoxic conditions also damage the premature lung. Oxygen-induced lung damages are mainly studied in newborn mouse models using oxygen concentrations above 75% and looking at short-term effects. Therefore, we aimed at the investigation of long-term effects and their dependency on different oxygen concentrations.

Methods

Newborn mice were exposed to moderate vs. severe hyperoxic air conditions (50 vs. 75% O₂) for 14 days followed by a longer period of normoxic conditions. Lung-related parameters were collected at an age of 60 or 120 days.

Results

Severe hyperoxia caused lower alveolar density, enlargement of parenchymal air spaces and fragmented elastic fibers as well as higher lung compliance with peak airflow limitations and higher sensitivity to ventilation-mediated damages in later life. However, these long-term lung structural and functional changes did not restrict the voluntary physical activity. Also, they were not accompanied by ongoing inflammatory processes, increased formation of reactive oxygen species (ROS) or altered expressions of antioxidant enzymes (superoxide dismutases, catalase) and lung elasticity-relevant proteins (elastin, pro-surfactant proteins) in adulthood. In contrast to severe hyperoxia, moderate hyperoxia was less lung damaging but also not free of long-term effects (higher lung compliance without peak airflow limitations, increased ROS formation).

Conclusions

Severe but not moderate neonatal hyperoxia causes emphysematous lungs without persisting oxidative stress and inflammation in adulthood. As the existing fragmentation of the elastic fibers seems to play a pivotal role, it indicates the usefulness of elastin-protecting compounds in the reduction of long-term oxygen-related lung damages.

IUGR decreases elastin mRNA expression in the developing rat lung and alters elastin content and lung compliance in the mature rat lung

Complications of intrauterine growth restriction (IUGR) include increased pulmonary morbidities and impaired alveolar development. Normal alveolar development depends upon elastin expression and processing, as well as the formation and deposition of elastic fibers. This is true of the human and rat. In this study, we hypothesized that uteroplacental insufficiency (UPI)-induced IUGR decreases mRNA levels of elastin and genes required for elastin fiber synthesis and assembly, at birth (prealveolarization) and postnatal day 7 (midalveolarization) in the rat. We further hypothesized that this would be accompanied by reduced elastic fiber deposition and increased static compliance at postnatal day 21 (mature lung). We used a well characterized rat model of IUGR to test these hypotheses. IUGR decreases mRNA transcript levels of genes essential for elastic fiber formation, including elastin, at birth and day 7. In the day 21 lung, IUGR decreases elastic fiber deposition and increases static lung compliance. We conclude that IUGR decreases mRNA transcript levels of elastic fiber synthesis genes, before and during alveolarization leading to a reduced elastic fiber density and increased static lung compliance in the mature lung. We speculate that the mechanism by which IUGR predisposes to pulmonary disease may be via decreased lung elastic fiber deposition.

Sustained changes in lung expansion alter tropoelastin mRNA levels and elastin content in fetal sheep lungs

Our objective was to determine the effects of sustained alterations in fetal lung expansion on pulmonary elastin synthesis. In fetal sheep, lung expansion was either decreased between 111 and 131 days' gestation (term approximately 147 days) by tracheal drainage or increased for 2, 4, 7, or 10 days by tracheal obstruction, ending at 128 days' gestation. Lung tropoelastin mRNA levels were assessed by Northern blot analysis, total elastin content was measured biochemically, and staining of lung sections was used to assess the localization and form of elastic fibers. Tracheal obstruction significantly elevated pulmonary tropoelastin mRNA levels 2.5-fold at 2 days, but values were not different from controls at 4, 7, and 10 days; elastin content tended to be increased at all time points. A sustained decrease in lung expansion by tracheal drainage reduced pulmonary tropoelastin mRNA levels 2.5-fold; elastin content was also decreased compared with controls, and tissue localization was altered. Our results indicate that the degree of lung expansion in the fetus influences elastin synthesis, content, and tissue deposition.

Rat alveolar type II cells inhibit lung fibroblast proliferation in vitro

Fibroblasts stimulate alveolar type II epithelial cell differentiation and proliferation in vitro and during lung development. However, little is known about the effects of adult type II cells on fibroblasts. We investigated the effect of adult rat type II cells on proliferation of adult human lung fibroblasts. Fibroblasts were suspended within rat tail collagen which was gelled on a floating polycarbonate filter, and type II cells were cultured on Matrigel. In this coculture system, alveolar type II cells inhibited fibroblast proliferation and indomethacin blocked the inhibitory effect on fibroblast growth. Prostaglandin (PG) E2, the major PG secreted by type II cells, inhibited fibroblast proliferation and was increased during the period of inhibition of fibroblast proliferation. Incubation with arachidonate showed that most of the PGE2 in the coculture system was produced by the fibroblasts. In addition, we found that rat type II cells also inhibited rat fibroblasts and that inhibition of fibroblast growth by type II cells could be stimulated by keratinocyte growth factor. We conclude that in this coculture system, type II cells inhibit fibroblast proliferation by secreting a factor(s) that stimulates PGE2 production by fibroblasts, and that PGE2 directly inhibits fibroblast proliferation.

Fibroblast tropoelastin and alpha-smooth-muscle actin expression are repressed by particulate-activated macrophage-derived tumor necrosis factor-alpha in experimental silicosis

Lung elastin synthesis is normally confined to periods of development, is maximal during alveolarization, and declines to low levels in mature lung. We have previously described an elastogenic response in the adult rat lung associated with experimental granulomatous disease induced by silica instillation. Reinitiated tropoelastin expression was identified throughout the lung in fibroblasts expressing alpha-smooth-muscle actin, whereas fibroblasts within the granulomatous lesions failed to express both tropoelastin and alpha-smooth- muscle actin (Mariani and colleagues, Am. J. Pathol. 1995;147:988-1000). We hypothesized that inflammatory cells within the granulomatous lesions produce factors that alter fibroblast phenotype. We found that macrophages accumulating within granulomatous lesions of silicotic rat lungs produce and secrete tumor necrosis factor (TNF)-alpha, a proinflammatory cytokine previously appreciated as a repressor of tropoelastin gene expression. In experimental cell systems, macrophages activated by particulates, either in vivo or in vitro, conditioned medium with a tropoelastin-repressing activity. This activity repressed both tropoelastin and alpha-smooth-muscle actin expression in primary cultures of rat lung fibroblasts in a time- dependent, transient manner. The particulate-activated macrophage-conditioned medium was found to contain TNF-alpha, which was both necessary and sufficient to induce these changes in lung fibroblast gene expression. These data indicate that macrophage-derived factors can modulate lung fibroblast tropoelastin expression in the diseased lung. Furthermore, the findings extend the association between expression by lung fibroblasts of tropoelastin and alpha-smooth-muscle actin.