LOXL2, a copper-dependent monoamine oxidase, activates lung fibroblasts through the TGF-β/Smad pathway

A previous study demonstrated that Lysyl oxidase-like 2 (LOXL2) serves an essential role in matrix remodeling and fibrogenesis, thus indicating its involvement in fibrosis-associated diseases. Our previous studies revealed a novel association between LOXL2 expression and pulmonary fibrosis in mice. However, the exact role and mechanisms of LOXL2 in interstitial lung disease remain poorly understood. The present study aimed to detect LOXL2 expression in mice with bleomycin (BLM)-induced pulmonary fibrosis, and explore the effects of silencing LOXL2 on the proliferation, activation and fibrosis process of mouse lung fibroblasts (MLFs). In addition, the present study investigated the association between LOXL2 and the transforming growth factor-beta (TGF-beta)/Smad signaling pathway to identify the mechanism underlying the role of LOXL2 in fibrosis progression. An animal model of pulmonary fibrosis was established by administering an intratracheal injection of 5 mg/kg BLM to C57BL/6 mice. ELISA and immunohistochemical examination were used to detect the LOXL2 level in the serum, lung homogenate and pulmonary tissues in mice. Pulmonary tissues of mice were extracted to culture primary MLFs, and a LOXL2 small interfering RNA adenovirus vector was established to silence LOXL2 in MLFs. Cell proliferation was detected using the cell counting kit-8 assay. Reverse transcription-quantitative polymerase chain reaction and western blotting were used to measure the expression of LOXL2, TGF-beta 1, Smad2/3, phosphorylated (p)Smad2/3, Smad4, and Smad7 and Snail in cells. Interleukin-6 (IL-6) and type 1 collagen alpha 1 (COL1A1) in the supernatant of cells were analyzed by ELISA. It was demonstrated that LOXL2 expression was significantly increased in serum, lung homogenate and pulmonary tissues of mice with BLM-induced pulmonary fibrosis compared with control mice. Furthermore, silencing LOXL2 significantly decreased MLF proliferation, and the levels of IL-6 and COL1A1 in the supernatant of cells. Furthermore, silencing LOXL2 inhibited the expression of pSmad2/3, Smad4 and Snail, while it promoted Smad7 expression. The present data provides a comprehensive analysis of the LOXL2 in pulmonary fibrosis and indicates prominent roles for LOXL2 in fibrogenesis via regulation of the TGF-beta/Smad signaling pathway.