Rationale A Disintegrin And Metalloprotease (ADAM)33 is a susceptibility gene associated with asthma, bronchial hyperresponsiveness (BHR) and reduced lung function in young children. It is selectively expressed in mesenchymal cells, including bronchial smooth muscle and fibroblasts. We have previously shown that ADAM33 expression increases during lung development when spontaneous peristaltic contractions of the airways commence. Therefore we hypothesised that mechanical strain induces ADAM33 expression and affects smooth muscle/myofibroblast differentiation.
Methods Primary human embryonic fibroblasts from the pseudoglandular stage of lung development were cultured on flexible collagen-coated membranes and exposed to cyclical mechanical stretch (30% amplitude, 12 cycles per minute) for 48, 96 and 168 h. Control cells were cultured on the same membranes without mechanical stretch. Quantitative RT-PCR was performed for ADAM33 and a-smooth muscle actin (a-SMA), a marker of smooth muscle and myofibroblast differentiation. We also measured collagen III and IL-8 mRNA. Soluble collagen protein levels in culture supernatants were measured using soluble collagen assay.
Results ADAM33 and a-SMA mRNA expression were not significantly affected by mechanical strain. In contrast, collagen III mRNA expression was increased fourfold by cyclical mechanical strain and there was a threefold increase in soluble collagen proteins in culture supernatants of stretched cells. Unexpectedly, IL-8 expression was also increased by cyclical mechanical strain.
Conclusion Mechanical strain did not appear to influence markers of smooth muscle differentiation. However, the increase in ECM production may indicate a requirement for stiffening of the airways as the tubular structures develop. We postulate that IL-8 is not proinflammatory in the context of airway development, and may be a paracrine growth factor for developing epithelial cells.
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