TGF-beta suppresses EGF-induced MAPK signaling and proliferation in asthmatic epithelial cells

Epithelial damage is an important pathophysiologic feature of asthma. Bronchial epithelium damage results in release of growth factors such as transforming growth factor (TGF)-beta(1) that may affect epithelial cell proliferation. The objective of our study is to evaluate the importance of TGF-beta(1) in regulating epithelial cell repair in asthma. We evaluated the effect of TGF-beta(1) on epidermal growth factor (EGF)-induced proliferation and downstream signaling in epithelial cells obtained from subjects with asthma compared with cells from healthy subjects. Cell proliferation was evaluated by bromodeoxyuridine incorporation. EGF receptor (EGFR), mitogen-activated protein kinase, TGF-beta receptors, Smads, Smad anchor for receptor activation (SARA), and cyclin-dependant kinase inhibitors were evaluated by Western blot. TGF-beta(1) and receptor expression were measured by RT-PCR and by enzyme-linked immunosorbent assay. Proliferation of epithelial cells at baseline and after EGF stimulation was significantly reduced in cells derived from subjects with asthma compared with cells obtained from healthy control subjects. EGF-induced ERK1/2 phosphorylation was reduced in epithelial cells from subjects with asthma compared with cells from healthy control subjects. This was paralleled with a reduced EGFR phosphorylation. Addition of TGF-beta(1) significantly decreased EGF-induced cell proliferation. TGF-beta(1) production was higher in asthmatic epithelial cells compared with normal cells. This was supported by a high expression of pSmad 3 and SARA in cells derived from individuals with asthma compared with normal subjects. Cycline-dependent kinase inhibitors were highly expressed in asthmatic compared with normal cells. Inhibition of TGF-beta(1) signaling in asthmatic epithelial cells restored EGFR, ERK1/2 phosphorylation, and cell proliferation induced by EGF. Our results suggest that TGF-beta restrains EGFR phosphorylation and downstream signaling in bronchial epithelial cells.