TREM2 Promotes Liver Fibrosis by Enhancing Hepatic Stellate Cell Endocytosis and Activation via Phosphorylating ITGAV

Abstract

Objective: This study aims to explore the role of the triggering receptor expressed on myeloid cells 2 (TREM2) in liver fibrosis and the activation of hepatic stellate cells (HSCs). Methods: Tandem Mass Tag (TMT) proteomic analysis, immunohistochemical staining and immunofluorescence were conducted to examine the variations of TREM2 expression between healthy and fibrotic liver tissues. Sh-TREM2 adenovirus was delivered into the mouse portal vein, and a chronic liver fibrosis model was developed by injecting carbon tetrachloride (CCl₄). The expression of TREM2 in liver tissues was evaluated using immunohistochemistry and immunofluorescence staining. Next, primary mouse HSCs were extracted, and the TREM2 expression in cells was knocked down by adenovirus. Primary HSCs were exposed to transforming growth factor-β1 (TGF-β1), platelet-derived growth factor-BB (PDGF-BB), or epidermal growth factor (EGF) to observe the effects of cytokines on cells. The cells were incubated for 2 h in spleen tyrosine kinase (Syk) inhibitor R406 and serine/threonine-protein kinase (AKT) 1 inhibitor A-674563, and Western blot was used to detect the phosphorylation integrin αv (ITGAV) protein expression. Results: TREM2 expression was significantly increased in human liver fibrosis tissue. TREM2 knockdown markedly reduced liver fibrosis in the mouse fibrosis model. In addition, TREM2 knockdown affected the phagosome signal pathway. TREM2 knockdown failed to activate HSCs by cytokines, which could inhibit endocytosis and TGF-β signaling without impacting the TGF receptor. Interestingly, the integrin family (ITGAV, integrin αx (ITGAX) and integrin α4 (ITGA4)), especially ITGAV expression, decreased after the knockdown of TREM2. The overexpression or knockdown of TREM2 promoted or inhibited ITGAV and phosphorylation ITGAV protein expression, respectively. R406 treatment considerably reduced the phosphorylation ITGAV protein expression of HSCs compared to the control and A-674563 treatment groups. Conclusions: TREM2 phosphorylated ITGAV via Syk, which boosted the endocytosis of cytokines to aggravate liver fibrosis.