Most obese patients develop hyperleptinaemia. Leptin, mainly produced by adipocytes, demonstrates a promotional role in liver fibrosis. Hepatic stellate cell (HSC) activation, a key step in liver fibrogenesis, requires global reprogramming of gene expression. The remodeling of DNA methylation is a mechanism of the epigenetic regulation of gene expression. The biosynthesis of S‐adenosylmethionine, a principle biological methyl donor, is catalyzed by methionine adenosyltransferase (MAT) such as MATⅡ which has been shown to promote HSC activation in vitro. This study was mainly aimed to determine the effect of leptin on MAT2A expression (the catalytic subunit of MATⅡ) in HSCs. Results showed that MAT2A knockdown reduced leptin‐induced HSC activation and liver fibrosis in the leptin‐deficient mouse model. Leptin promoted MAT2A expression in HSCs and increased MAT2A promoter activity. The axis of the β‐catenin pathway/E2F‐4 mediated the effect of leptin on MAT2A expression. Leptin‐induced β‐catenin signaling reduced E2F‐4 expression and thus abated E2F‐4 binding to MAT2A promoter at a site around −2779 bp, leading to an increase in the MAT2A promoter activity. These data might shed more light on the mechanisms responsible for liver fibrogenesis in obese patients with hyperleptinaemia.

Obese patients, often accompanied by hyperleptinemia, are prone to develop liver fibrosis. A large body of data including the results from human studies suggested the promotion role of leptin, an adipocyte-derived hormone, in liver fibrosis. Hepatic stellate cell (HSC) activation, a crucial step in liver fibrogenesis, requires global reprogramming of gene expression which is regulated by multiple mechanisms including epigenetic regulation such as methylation of DNA. S-Adenosylmethionine is a principal biological methyl donor and its biosynthesis is catalyzed by a methionine adenosyltransferase (MAT) such as MATII. MATII consists of the catalytic subunit MAT2A and regulatory subunit MAT2B which are essential for HSC activation. The present research investigated the effect of leptin on the expression of Mat2b in HSCs in vitro and in a leptin-deficient mouse model. Results demonstrated that leptin significantly increased Mat2b expression. Leptin-induced Mat2b expression required the PI3K/AKT signaling pathway. c-Jun, a component of activator protein (AP1), was phosphorylated by leptin-induced PI3K/AKT signaling and thus potentiated its binding to the element around −964 bp in the Mat2b promoter. MAT2B was involved in leptininduced HSC activation and liver fibrosis in a leptin-deficient mouse model. These results might broaden understanding of the mechanisms underlying the liver fibrogenesis in obese patients with hyperleptinemia.

Sterol regulatory element‐binding protein 1c (SREBP1c) plays key roles in maintenance of hepatic stellate cell (HSC) quiescence. The present researches investigated the mechanisms underlying the effects of SREBP1c on HSCs and liver fibrogenesis by HSC‐targeted overexpression of the active SREBP1c using adenovirus in vitro and in vivo. Results demonstrated that SREBP1c exerted inhibitory effects on TAA‐induced liver fibrosis. SREBP1c down‐regulated TGFβ1 level in liver, reduced the receptors for TGFβ1 and PDGFβ, and interrupted the signalling pathways of Smad3 and Akt1/2/3 but not ERK1/2 in HSCs. SREBP1c also led to the decreases in the protein levels of the bromodomain‐containing chromatin‐modifying factor bromodomain protein 4, methionine adenosyltransferase 2B (MAT2B) and TIMP1 in HSCs. In vivo activated HSCs did not express cyclin D1 and cyclin E1 but SREBP1c down‐regulated both cyclins in vitro. SREBP1c elevated PPARγ and MMP1 protein levels in the model of liver fibrosis. The effect of SREBP1c on MAT2B expression was associated with its binding to MAT2B1 promoter. Taken together, the mechanisms underlying the effects of SREBP1c on HSC activation and liver fibrosis were involved in its influences on TGFβ1 level, the receptors for TGFβ1 and PDGFβ and their downstream signalling, and the molecules for epigenetic regulation of genes.

Upon chronic damage to the liver, multiple cytokines stimulate hepatic stellate cells (HSCs), causing the alterations of gene expression profiles and thus leading to HSC activation, a key step in liver fibrogenesis. Activated HSCs are the dominant contributors to liver fibrosis. Bromodomain containing protein 4 (BrD4), an important epigenetic reader, was demonstrated to concentrate on hundreds of enhancers associated with genes involved in multiple profibrotic pathways, thereby directing HSC activation and the fibrotic responses. The present studies were designed to examine the effect of transforming growth factor beta‐1 (TGFβ1), the most potent pro‐fibrotic cytokine, on BrD4 expression in HSCs and, if so, elucidated the underlying mechanisms in vitro and in vivo. The experiments employed the heterogeneous TGFβ1 knockout (TGFβ1+/−) mice, gene knockdown in vivo, and a model of thioacetamide (TAA)‐induced liver injury. The results revealed that TGFβ1 enhanced BrD4 expression in HSCs, which was mediated, at least, by Smad3 signaling and early‐immediate gene Egr1 (early growth response‐1). TGFβ1‐induced Smad3 signaling increased Egr1 expression and promoted Egr1 binding to BrD4 promoter at a site around −111 bp, promoting BrD4 expression. Egr1 knockdown reduced BrD4 expression in HSCs in a mouse model of TAA‐induced liver injury and lessened liver fibrosis. Double fluorescence staining demonstrated a strong increase in BrD4 expression in activated HSCs in fibrotic areas of the human livers, paralleling the upregulation of p‐Smad3 and Egr1. This research suggested novel molecular events underlying the roles of the master pro‐fibrotic cytokine TGFβ1 in HSC activation and liver fibrogenesis.