Deubiquitinase USP18 inhibits hepatic stellate cells activation and alleviates liver fibrosis  via regulation of TAK1 activity

Background & Aims: Activation of hepatic stellate cells (HSCs) is the key process underlying liver fibrosis. Unveiling its molecular mechanism may provide an effective target for inhibiting liver fibrosis. Like other post-translational modifications, protein ubiquitination modification is a dynamic and reversible process. Deubiquitinases (DUBs) catalyze the removal of ubiquitin chains from substrate proteins, thereby inhibiting the biological processes regulated by ubiquitination modification signals. However, there are currently few studies revealing the role of deubiquitination in the activation of HSCs. Methods & Results: Single-cell RNA sequencing (scRNA-seq) revealed significantly decreased USP18 expression in activated HSCs when compared to quiescent HSCs. In mouse primary HSCs, continuous activation of HSCs led to a gradual decrease in USP18 expression whilst restoration of USP18 expression significantly inhibited HSC activation. Injection of USP18 lentivirus into the portal vein of a CCl ₄ -induced liver fibrosis mouse model confirmed that overexpression of USP18 can significantly reduce the degree of liver fibrosis. In terms of mechanism, we screened some targets of USP18 in mouse primary HSCs and found that USP18 could directly bind to TAK1. Furthermore, we demonstrated that USP18 can inhibit TAK1 activity by interfering with the K63 ubiquitination modification of TAK1. Conclusions Our study demonstrated that USP18 inhibited HSC activation and alleviated liver fibrosis in mice via modulation of TAK1 activity; this may prove to be an effective target for inhibiting liver fibrosis.