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N‑acetyl‑seryl‑aspartyl‑lysyl‑proline (Ac‑SDKP) is a natural tetrapeptide that is released from thymosin β4 by prolyl oligopeptides. It is hydrolyzed by the key enzyme of the renin‑angiotensin system, angiotensin‑converting enzyme (ACE). The aim of the present study was to investigate the alterations in Ac‑SDKP and the ACE/angiotensin II (Ang II)/angiotensin II type 1 (AT1) receptor axis and its impact on the pathogenesis and development of silicotic fibrosis. For in vivo studies, a HOPE MED 8050 exposure control apparatus was used to establish different stages of silicosis in a rat model treated with Ac‑SDKP. For in vitro studies, cultured primary lung fibroblasts were induced to differentiate into myofibroblasts by Ang II, and were pretreated with Ac‑SDKP and valsartan. The results of the present study revealed that, during silicosis development, ACE/Ang II/AT1 expression in local lung tissues increased, whereas that of Ac‑SDKP decreased. Ac‑SDKP and the ACE/AT1/Ang II axis were inversely altered in the development of silicotic fibrosis. Ac‑SDKP treatment had an anti‑fibrotic effect in vivo. Compared with the silicosis group, the expression of α‑smooth muscle actin (α‑SMA), Collagen (Col) I, Fibronectin (Fn) and AT1 were significantly downregulated, whereas matrix metalloproteinase‑1 (MMP‑1) expression and the MMP‑1/tissue inhibitor of metalloproteinases‑1 (TIMP‑1) ratio was increased in the Ac‑SDKP treatment group. In vitro, pre‑treatment with Ac‑SDKP or valsartan attenuated the expression of α‑SMA, Col I, Fn and AT1 in Ang II‑induced fibroblasts. In addition, MMP‑1 expression and the MMP‑1/TIMP‑1 ratio were significantly higher in Ac‑SDKP and valsartan pre‑treatment groups compared with the Ang II group. In conclusion, the results of the present study suggest that an imbalance between Ac‑SDKP and ACE/Ang II/AT1 molecules promotes the development of silicosis and that Ac‑SDKP protects against silicotic fibrosis by inhibiting Ang II‑induced myofibroblast differentiation and extracellular matrix production.

Influence of the interaction between Ac‑SDKP and Ang II on the pathogenesis and development of silicotic fibrosis