Increased expression of latent TGF-β-binding protein 4 affects the fibrotic process in scleroderma by TGF-β/SMAD signaling

Abstract

Scleroderma is a fibrosis-related disorder characterized by cutaneous and internal organ fibrosis, and excessive collagen deposition in extracellular matrix (ECM) is a major cause of fibrosis. Transforming growth factor-β (TGF-β)/SMAD signaling has a central role in the pathogenesis of fibrosis by inducing abnormal collagen accumulation in ECM, and latent TGF-β-binding protein 4 (LTBP-4) affects the secretion of latent TGF-β to ECM. A previous study indicated that bleomycin (BLM) treatment increased LTBP-4 expression in lung fibroblasts of Thy-1 knockout mice with lung fibrosis, and LTBP-4 further promoted TGF-β bioavailability as well as SMAD3 phosphorylation. However, the expression and function of LTBP-4 in human scleroderma remain unclear. We aimed to investigate the potential role of LTBP-4 in scleroderma through clinical, in vivo and in vitro studies. LTBP-4 and TGF-β expressions were significantly upregulated in systemic scleroderma (SSc) patients' plasma compared with normal controls (LTBP-4, 1,215±100.2 vs 542.8±41.7 ng/ml, P<0.0001; TGF-β, 1.5±0.2 vs 0.7±0.1 ng/ml, P=0.0031), while no significant difference was found between localized scleroderma (LSc) and normal controls. The plasma concentrations of LTBP-4 and TGF-β were even higher in SSc patients with lung fibrosis (LTBP-4, 1462± 137.3 vs 892.8±113.4 ng/ml, P=0.0037; TGF-β, 2.0±0.4 vs 0.9±0.2 ng/ml, P=0.0212) and esophagus involvement (1390±134.4 vs 940.7±127.0 ng/ml, P=0.0269; TGF-β, 1.9±0.3 vs 0.9±0.2 ng/ml, P=0.0426). The area under receiver operating characteristics (ROC) curve of LTBP-4 was 0.86. Immunohistochemistry measurement also demonstrated a higher LTBP-4 expression in sclerotic skin tissue of LSc and SSc compared with normal controls. More positive fibroblasts were also found in BLM-induced scleroderma mouse model than the saline-treated group. In in vitro studies, knockdown of LTBP-4 in SSc skin fibroblasts prominently reduced downstream COL1A1, COL1A2, and COL3A1 mRNA level by 84%, 82%, and 43%, respectively, and other fibrosis-related genes' expression were also decreased. Furthermore, extracellular TGF-β level and the SMAD2/3 phosphorylation were inhibited through LTBP-4 knockdown treatment, suggesting that the knockdown of LTBP-4 reduced the collagen expression through TGF-β/SMAD signaling pathway. Taken together, these data suggest that LTBP-4 affects fibrotic process in scleroderma, and the high expression of LTBP-4 in SSc plasma may serve as a clinical biomarker in diagnosing this disease. In addition, this study also lays the theoretical foundation for targeting LTBP-4 as treatment of scleroderma.