REDD1 attenuates hepatic stellate cell activation and liver fibrosis via inhibiting of TGF-β/Smad signaling pathway

Liver fibrosis is caused by repetitive hepatic injury. Regulated in development and DNA damage response 1 (REDD1) gene is induced by various stresses and has been studied in cell proliferation and survival. However, the role of REDD1 in hepatic stellate cell activation and hepatic fibrogenesis has not yet been investigated. In the current study, we examined the effect of REDD1 on hepatic fibrogenesis and the underlying molecular mechanism. REDD1 protein was upregulated in the activated primary hepatic stellate cells and transforming growth factor-β (TGF-β)-treated LX-2 cells. REDD1 mRNA levels were also elevated by TGF-β treatment. TGF-β signaling is primarily transduced via the activation of the Smad transcription factor. However, TGF-β-mediated REDD1 induction was not Smad-dependent. Thus, we investigated the transcription factors that influence the REDD1 expression by TGF-β. We found that c-JUN, a component of AP-1, upregulated the REDD1 expression that was specifically suppressed by p38 inhibitor. In silico analysis of the REDD1 promoter region showed putative AP-1-binding sites; additionally, its deletion mutants demonstrated that the AP-1-binding site between -716 and -587 bp within the REDD1 promoter is critical for TGF-β-mediated REDD1 induction. Moreover, REDD1 overexpression markedly inhibited TGF-β-induced plasminogen activator inhibitor-1 (PAI-1) expression and Smad phosphorylation. REDD1 adenovirus infection inhibited CCl4-induced hepatic injury in mice, which was demonstrated by reduced ALT/AST levels and collagen accumulation. In addition, we observed that REDD1 inhibited CCl4-induced fibrogenic gene induction and restored GSH and malondialdehyde levels. Our findings implied that REDD1 has the potential to inhibit HSC activation and protect against liver fibrosis.