Inflammation, increased fibroblast proliferation, and increased deposition of extracellular matrix (ECM) are hallmarks of early lung fibrosis and asthma. Transforming growth factor-β (TGF-􀀂) has been suggested as a key regulator of lung tissue homeostasis with several and often opposite effects on fibroblast proliferation and ECM production. In human and animal model systems, it has been shown that TGF-β induced several signaling cascades including Smads, p38 mitogen-activated protein (MAP) kinases, and extracellular signal-regulated kinases 1/2 (ERK). Information on how TGF-β regulates and controls normal primary lung fibroproliferation and ECM production is not present.

We sought to dissect the effects of TGF-β on fibroproliferation and ECM production of primary adult human lung fibroblasts and elucidate the involved signaling pathways.

Depending on the presence of fetal bovine serum (FBS; 10%), TGF-β exerted opposite effects on fibroproliferation. In the absence of FBS, low TGF-β concentrations (0.01 and 0.001 ng/ml) significantly induced fibroproliferation. In the presence of FBS, TGF-β (1 ng/ml, 10 ng/ml) significantly reduced fibroproliferation. TGF-β dose-dependently increased ECM deposition, which was independent of the presence of FBS. The anti-proliferative effect of TGF-β was associated with increased prostaglandin E₂ (PGE₂) production, that was induced via p38δ and ERK 1/2 MAP kinases. Indomethacin (2.5 µM) and a small interfering RNA specific for p38 MAP kinase completely reversed the TGF-β-dependent inhibition of fibroblast proliferation.

Both pro- and anti-proliferative cascades can be activated by TGF-β. In a mitogenic or inflammatory environment TGF-β induces PGE2 synthesis via activation of p38δ MAP kinase, which then exerts a strong antiproliferative effect. This dualistic nature of TGF-β may exist in order to maintain lung tissue integrity.