TGF-β₂ decreases baseline and IL-13-stimulated mucin production by primary human bronchial epithelial cells

ABSTRACT

Introduction: Mucus hypersecretion is a major contributor to asthma pathology and occurs as part of a spectrum of structural changes termed airway wall remodeling. Transforming growth factor (TGF)-β is proposed to play a key role in regulating airway matrix remodeling although less is known about the specific action of TGF-β isoforms in regulating mucus production. Methods: Primary human bronchial epithelial (HBE) cells cultured at air–liquid interface were treated with exogenous TGF-β₁, TGF-β₂, and/or a Th2 cytokine, interleukin (IL)-13. Expression and production of respiratory mucins, MUC5AC and MUC5B, were analyzed by real-time PCR, agarose gel electrophoresis, and western blotting. A murine-transformed Clara cell line (mtCC1-2) transfected with a luciferase reporter driven by the Muc5ac promoter containing Smad4 site-mutated cis sequences was used to determine whether exogenous TGF-β₂ affects Muc5ac promoter function. Results: Surprisingly, TGF-β₁ showed no measurable effect on MUC5AC or MUC5B production by HBE cells whereas TGF-β₂ caused a decrease in both MUC5AC and MUC5B mRNA and protein. Dual treatment with TGF-β₂ and IL-13 partially attenuated the increase in mucin production found with IL-13 alone. This effect was confirmed by using mtCC1-2 cells where addition of TGF-β₂ reduced the ability of IL-13/EGF to induce Muc5ac promoter expression in wild-type cells; however, this decrease was absent in mutant promoter-transfected cells. Discussion and conclusion: Findings suggest that normal regulation of MUC5AC and MUC5B production by HBE cells is TGF-β isoform-specific and that TGF-β₂ downregulates both MUC5AC and MUC5B. Furthermore, TGF-β₂ controls baseline and IL-13-driven Muc5ac promoter function in murine Clara cells via an endogenous Smad4 recognition motif.

KEYWORDS :

• airway disease
• Clara cells
• culture model
• mucus
• promoter