TMEM88 is a disheveled-binding protein belonging to TMEM family. TMEM88 has been demonstrated to be implicated in the organ fibrosis, such as liver fibrosis and kidney fibrosis. TMEM88 expression is decreased in the human liver fibrotic tissues. In vitro assays prove that TMEM88 inhibits activation and proliferation of hepatic stellate cells (HSCs) by blocking Wnt/β-catenin pathway, and promotes the activated HSCs apoptosis by initiating Bcl-2/Bax/Caspase3 pathway . TMEM88 was also found to be downregulated in renal fibrotic tissues and TGF-β1-treated HK2 cells. TMEM88 overexpression inhibits TGF-β1-induced cell proliferation and migration, and production of ECM proteins including α-SMA, Col I, and Col III in HK2 cells . In the current study, we investigated the role of TMEM88 in pleural fibrosis. Our results showed that the expression level of TMEM88 was downregulated in pleural fibrosis tissues and TGF-β1-treated Met-5A cells. The findings implied that TMEM88 might be involved in the progression of pleural fibrosis, which were consistent with the previous studies.
Fibrosis is the consequence of dysfunctional response of wound healing mechanisms after tissue injury . It is a basic connective tissue lesion accompanied by extensive expression and accumulation of ECM components . ECM deposition results in the disruption of the proper three-dimensional structure of tissue and homeostatic imbalance . During the fibrotic event, various regulators including growth factors, proteolytic enzymes, angiogenic factors, and fibrogenic cytokines have been observed to stimulate the deposition of ECM . Among these, TGF-β is a major pro-fibrotic mediator . EMT, an essential mechanism in embryonic development and tissue repair, governs the progression of many diseases, including organ fibrosis and cancer [26, 27, 28]. Transcription program switching in EMT is induced by various signaling pathways, including epidermal growth factor (EGF), fibroblast growth factor (FGF), TGF-β, and platelet-derived growth factor (PDGF) . In addition, ECM proteins induce various signaling mechanisms in the fibrotic event, and certain ECM proteins, such as Col I, provide a suitable microenvironment for EMT, thereby promoting the organ fibrosis [29, 30]. Further gain-of-function assays demonstrated that overexpression of TMEM88 inhibited the proliferation of Met-5A cells under TGF-β1 stimulation. Besides, overexpression of TMEM88 prevented TGF-β1-induced ECM accumulation and EMT in Met-5A cells with decreased expression levels of ECM proteins (Col I and fibronectin), increased levels of epithelial phenotypic markers (cytokeratin-8 and E-cadherin), as well as decreased levels of mesenchymal phenotypic markers (vimentin and α-SMA). These data indicated that TMEM88 suppressed the progression of pleural fibrosis.
According to the published articles, TGF-β signaling begins when the activated TGF-β ligand binds to the TβRII subunits on the cell surface, which leads to the recruitment of TβRI subunits . The TβRI subunits become phosphorylated and activated by TβRII, leading to the activation of Smad-dependent TGF-β1 signaling (canonical pathway) [19, 32]. The Smad2 and Smad3 are recruited and subsequently phosphorylated by TβRI, thereby assembles with co-Smad4 and translocates to the nucleus . Finally, the activation of TGF-β1-Smad results in the regulation of gene expression through interaction with co-activators and cell-specific transcription factors . In the current study, our results demonstrated that overexpression of TMEM88 inhibited the expression of TβRI and TβRII, as well as suppressed the phosphorylation of Smad2 and Smad3 in Met-5A cells. The findings suggested that TMEM88 prevented the activation of TGF-β1/Smad pathway in Met-5A cells, which might contribute to its anti-fibrotic activity.