SMAD-signaling in chronic obstructive pulmonary disease: transcriptional down-regulation of inhibitory SMAD 6 and 7 by cigarette smoke

Transforming growth factor-beta1 is a potent mediator of fibrosis stimulating the secretion of extracellular matrix proteins and is involved in airway remodeling in chronic obstructive pulmonary disease (COPD). Signals from the TGF superfamily are mediated by the SMAD group of transcription factors. Here, the expression of the regulatory SMAD2, 3, the co-SMAD4 and the inhibitory SMAD6 and 7 was assessed in bronchial biopsies of COPD patients and controls by quantitative RT-PCR. While SMAD2 was not expressed and SMAD3 and 4 displayed no change, the inhibitory SMAD6 and 7 were significantly down-regulated in COPD. To reveal the molecular basis of tobacco smoke-induced airway remodeling and to test whether it may interfere with intracellular SMAD signaling, the airway epithelial cell line A549 was incubated with cigarette smoke extract (1% and 10%) for 48 hours, which led to down-regulation of SMAD6 and 7 at both concentrations tested. It can be concluded that TGF-beta-mediated effects in COPD are influenced by a disturbed intracellular feedback mechanism of inhibitory SMADs. Also, the effects of non-volatile components in tobacco smoke may partly be regulated via a smoke-induced down-regulation of inhibitory SMADs.