Attenuation of pulmonary fibrosis in type I collagen-targeted reporter mice with ALK-5 inhibitors

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal disease, and consequently, effective antifibrotic drugs are strongly desired. Although we have previously reported a validated Collal-Luc Tg rat model for fibrosis, there are only a few mouse models that enable the evaluation of fibrosis in a short time period and with high sensitivity. Therefore, we generated a Collal-internal ribosome entry site (IRES)-Luc knock-in (KI) mouse in which the IRES-luciferase gene construct was inserted into the 3'-UTR of the type I collagen alpha I gene (Collal). There was a high correlation between luciferase activity and hydroxyproline content in the KI mice, which is similar to the result that we have previously reported for the Collal-Luc Tg rat model, In a bleomycin (BLM)-induced lung fibrosis model, iuciferase activity in the lung showed a significant increase 3 days after BLM treatment, while only a slight increase was observed in the hydroxyproline content. An ALK-5 inhibitor-R268712-was effective in inhibiting the luciferase activity in both the in vivo BLM-induced lung fibrosis model and in vitro primary mouse lung fibroblasts. This suggests that fibroblasts are the major collagen-producing cells in lung fibrosis. In human lung fibroblasts, TGF-beta stimulation induced a-smooth muscle actin as observed by immunostaining, suggesting that myofibroblast transdifferentiation (MTD) plays an important role in lung fibrosis. Together, these results indicated that ALK-5 inhibitors might affect lung fibrosis mainly via the inhibition of MTD. Thus, the Collal-IRES-Luc RI mouse might be useful for the evaluation of antifibrotic effects and their underlying mechanisms.