New anti-fibrotic mechanisms of n-acetyl-seryl-aspartyl-lysyl-proline in silicon dioxide-induced silicosis

AimsWe previously reported that tetrapeptide N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) inhibited pulmonary inflammation and fibrosis in SiO2-induced silicosis. This study aimed to explore the precise mechanism involved.Main methodsRats were divided into 3 groups: 1) sham (saline), 2) silicosis + vehicle, and 3) silicosis + Ac-SDKP [800 μg/(kg d)]. SiO2 particles or saline were administered by tracheal instillation and Ac-SDKP or vehicle (saline) via a miniosmotic pump planted into the abdominal cavity 48 h before instillation. Animals were observed for 4 weeks. Silicotic nodule fraction (SNF) and macrophage infiltration (ED-1 positive cells) were measured by hematoxylin and eosin (H.E.) and immunohistochemical staining respectively. Collagen I and III, transforming growth factor-β1 (TGF-β1) proteins and monocyte chemotactic protein-1 (MCP-1) mRNA were detected by Western Blot (WB) and real-time RT-PCR respectively. In vitro, pulmonary fibroblasts were stimulated by TGF-β1 (5 μg/ml) with or without Ac-SDKP. Phosphorylated c-Jun N-terminal Kinase (p-JNK) was detected by WB and p-JNK nuclear translocation by confocal analysis.Key findingsSiO2 significantly increased the SNF, collagen I and III proteins, TGF-β1, MCP-1 mRNA and macrophage infiltration. All these pathological changes were inhibited by Ac-SDKP. TGF-β1 resulted in fibroblast proliferation, increased expression of collagen I and III proteins, p-JNK and its subsequent nuclear translocation. Addition of Ac-SDKP markedly suppressed these changes.SignificanceThese data indicate that the anti-fibrotic effect of Ac-SDKP in silicosis is mediated by inhibiting chronic inflammation, TGF-β1 production, and TGF-β1-induced pulmonary fibroblast proliferation and collagen synthesis.