CREB regulates TNF-α-induced GM-CSF secretion via p38 MAPK in human lung fibroblasts

Abstract

Background: Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine that mediates eosinophilic differentiation, migration and survival, causing respiratory tract inflammation. GM-CSF is also known to be secreted from respiratory tract structural cells. However, the mechanisms of GM-CSF secretion have not been well established. Methods: Human fetal lung fibroblasts and human primary asthmatic lung fibroblasts were used for the study of tumor necrosis factor alpha (TNF-α)-induced GM-CSF secretion. GM-CSF secretion and mRNA expression were measured by enzyme-linked immunosorbent assay and quantitative real-time reverse transcription polymerase chain reaction, respectively. Knockdown of cAMP response element-binding protein (CREB) in fibroblasts was carried out by using specific small interfering RNAs of CREB. Results: Among respiratory tract structural cells, pulmonary fibroblasts exhibited increased GM-CSF secretion and mRNA expression after stimulation with TNF-α in a concentration-dependent manner. Moreover, a p38 mitogen-αctivated protein kinase (MAPK) inhibitor controlled TNF-α-induced GM-CSF secretion, and roflumilast and rolipram, inhibitors of phosphodiesterase-4, suppressed TNF-α-induced GM-CSF secretion. Consistent with this, forskolin also completely blocked GM-CSF secretion, and similar results were observed in response to cAMP treatment, suggesting that cAMP signaling suppressed TNF-α- induced GM-CSF secretion in human lung fibroblasts. Furthermore, CREB was phosphorylated through p38 MAPK but not cAMP signaling after TNF-α stimulation, and GM-CSF secretion was inhibited by CREB knockdown. Finally, these effects were also demonstrated in human primary lung fibroblasts in a patient with asthma. Conclusions: CREB signaled independent of cAMP signaling and was phosphorylated by p38 MAPK following TNF-α stimulation, playing a critical role in GM-CSF secretion in human lung fibroblasts.