Cigarette smoke inhibits lung fibroblast proliferation by translational mechanisms

Cigarette smoke is a major cause of chronic obstructive pulmonary disease (COPD) and emphysema. Although cigarette smoke represses cellular proliferation, the molecular mechanisms underlying this phenomenon are unknown. CCAAT/enhancer-binding proteins (C/EBPs) are key regulators of cell cycle progression, differentiation and pro-inflammatory gene expression, are regulated predominantly at the translational level and may be involved in the pathogenesis of COPD. The aim of this study was to assess the effect of cigarette smoke on proliferation and the expression and translational regulation of C/EBP&agr; and C/EBP&bgr; in nondiseased primary human lung fibroblasts. Fibroblasts were exposed to cigarette smoke-conditioned medium (10% and 20% for 24 h). Proliferation was determined by [3H]thymidine incorporation. Protein expression levels were determined by immunoblotting and translation was monitored using a translation control reporter system. Cigarette smoke significantly reduced fibroblast proliferation and significantly upregulated full-length C/EBP&agr; and C/EBP&bgr; proteins due to a shift in the translational control of CEBPA and CEBPB mRNAs. This shift involved the re-initiation of mRNA translation via the regulatory upstream open reading frame, which coincided with increased interleukin-8 release and a decrease in functional elastin level. These findings provide a novel mechanism to understanding the tissue remodelling observed in the lungs of COPD patients.