Follistatin-Like 1 Promotes Bleomycin-Induced Pulmonary Fibrosis through the Transforming Growth Factor Beta 1/Mitogen-Activated Protein Kinase Signaling Pathway

Background: Follistatin-like 1 (FSTL1) is a novel profibrogenic factor that induces pulmonary fibrosis (PF) through the transforming growth factor-beta 1 (TGF-β1)/Smad signaling. Little is known about its effects on PF through the non-Smad signaling, like the mitogen-activated protein kinase (MAPK) pathway. Therefore, this study aimed to investigate the role of FSTL1 in PF through the MAPK signaling pathway and its mechanisms in lung fibrogenesis.

Methods: PF was induced in Fstl1^+/-and wild-type (WT) C57BL/6 mice with bleomycin. After 14 days, the mice were sacrificed, and lung tissues were stained with hematoxylin and eosin; the hydroxyproline content was measured to confirm PF. The mRNA and protein level of FSTL1 and the change of MAPK phosphorylation were measured by quantitative polymerase chain reaction and Western blotting. The effect of Fstl1 deficiency on fibroblasts differentiation was measured by Western blotting and cell immunofluorescence. MAPK signaling activation was measured by Western blotting in Fstl1^+/- and WT fibroblasts treated with recombinant human FSTL1 protein. We pretreated mouse lung fibroblast cells with inhibitors of the extracellular signal-regulated kinase (ERK), p38, and Jun N-terminal kinase (JNK) signaling and analyzed their differentiation, proliferation, migration, and invasion by Western blotting, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide analysis, and transwell assays. The Student's t-test was used to compare the differences between two groups.

Results: Fstl1 deficiency attenuated phosphorylation of the ERK, p38, and JNK signaling in bleomycin-induced fibrotic lung tissue 14 days after injury (0.67 ± 0.05 vs. 1.22 ± 0.03, t = 14.92, P = 0.0001; 0.41 ± 0.01 vs. 1.15 ± 0.07; t = 11.19; P = 0.0004; and 0.41 ± 0.01 vs. 1.07 ± 0.07, t = 8.92, P = 0.0009; respectively), compared with WT lungs at the same time and in primary lung fibroblasts (0.82 ± 0.01 vs. 1.01 ± 0.04, t = 4.06, P = 0.0150; 1.04 ± 0.03 vs. 1.24 ± 0.03, t = 4.44, P = 0.0100; and 0.76 ± 0.05 vs. 0.99 ± 0.05, t = 4.48, P = 0.0100; respectively), compared with TGF-β1-stimulated WT group. Recombinant human FSTL1 protein in lung fibroblasts enhanced TGF-β1-mediated phosphorylation of the ERK (1.19 ± 0.08 vs. 0.55 ± 0.04, t = 6.99, P = 0.0020), p38 (1.18 ± 0.04 vs. 0.66 ± 0.03, t = 11.20, P = 0.0020), and JNK (1.11 ± 0.01 vs. 0.84 ± 0.04, t = 6.53, P = 0.0030), compared with the TGF-β1-stimulated WT group. Fstl1-deficient fibroblasts showed reduced alpha-smooth muscle actin (α-SMA) expression (0.70 ± 0.06 vs. 1.28 ± 0.11, t = 4.65, P = 0.0035, compared with the untreated WT group; 1.40 ± 0.05 vs. 1.76 ± 0.02, t = 6.31, P = 0.0007; compared with the TGF-β1-treated WT group). Compared with the corresponding condition in the control group, the TGF-β1/FSTL1-mediated α-SMA expression was significantly suppressed by pretreatment with an inhibitor of p38 (0.73 ± 0.01 vs. 1.13 ± 0.10, t = 3.92, P = 0.0078) and JNK (0.78 ± 0.03 vs. 1.08 ± 0.06, t = 4.40, P = 0.0046) signaling. The proliferation of mouse lung fibroblast cells (MLgs) significantly decreased after treatment of an inhibitor of p38 (0.30 ± 0.01 vs. 0.46 ± 0.03, t = 4.64, P = 0.0009), JNK (0.30 ± 0.01 vs. 0.49 ± 0.01, t = 12.84, P = 0.0001), and Smad2/3 (0.18 ± 0.02 vs. 0.46 ± 0.02, t = 12.69, P = 0.0001) signaling compared with the dimethylsulfoxide group. The migration and invasion cells of MLgs significantly decreased in medium pretreated with an inhibitor of p38 (70.17 ± 3.28 vs. 116.30 ± 7.11, t = 5.89, P = 0.0042 for the migratory cells; 19.87 ± 0.84 vs. 32.70 ± 0.95, t = 10.14, P = 0.0005 for the invasive cells), JNK (72.30 ± 3.85 vs. 116.30 ± 7.11, t = 5.44, P = 0.0056 for the migratory cells; 18.03 ± 0.94 vs. 32.70 ± 0.95, t = 11.00, P = 0.0004 for the invasive cells), and Smad2/3 (64.76 ± 1.41 vs. 116.30 ± 7.11, t = 7.11, P = 0.0021 for the migratory cells; 18.03 ± 0.94 vs. 32.70 ± 0.95, t = 13.29, P = 0.0002 for the invasive cells) signaling compared with the corresponding condition in the dimethylsulfoxide group.

Conclusion: FSTL1 affects lung fibroblast differentiation, proliferation, migration, and invasion through p38 and JNK signaling, and in this way, it might influence the development of PF.