Inhibition of hepatic stellate cell activation and liver fibrosis by fat-specific protein 27

Lipid droplets, the Holy Grail of hepatic stellate cells: In health and hepatic fibrosis

Lipid droplets (LDs) are distinct morphological markers of hepatic stellate cells (HSCs). They are composed of a core of predominantly retinyl esters and triacylglycerols surrounded by a phospholipid layer; the latter harbors perilipins 2, 3, and 5, which help control LD lipolysis. Electron microscopy distinguishes between Types I and II LDs. Type I LDs are surrounded by acid phosphatase-positive lysosomes, which likely digest LDs. LD count and retinoid concentration are modulated by vitamin A intake. Alcohol consumption depletes hepatic retinoids and HSC LDs, with concomitant transformation of HSCs to fibrogenic myofibroblast-like cells. LD loss and accompanying HSC activation occur in HSC cell culture models. Loss of LDs is a consequence of and not a prerequisite for HSC activation. LDs are endowed with enzymes for synthesizing retinyl esters and triacylglycerols as well as neutral lipases and lysosomal acid lipase for breaking down LDs. HSCs have two distinct metabolic LD pools: an "original" pool in quiescent HSCs and a "new" pool emerging in HSC activation; this two-pool model provides a platform for analyzing LD dynamics in HSC activation. Besides lipolysis, LDs are degraded by lipophagy; however, the coordination between and relative contributions of these two pathways to LD removal are unclear. While induction of autophagy accelerates LD loss in quiescent HSCs and promotes HSC activation, blocking autophagy impairs LD degradation and inhibits HSC activation and fibrosis. This article is a critique of five decades of investigations into the morphology, molecular structure, synthesis, and degradation of LDs associated with HSC activation and fibrosis.

Playing Jekyll and Hyde-The Dual Role of Lipids in Fatty Liver Disease

Lipids play Jekyll and Hyde in the liver. On the one hand, the lipid-laden status of hepatic stellate cells is a hallmark of healthy liver. On the other hand, the opposite is true for lipid-laden hepatocytes—they obstruct liver function. Neglected lipid accumulation in hepatocytes can progress into hepatic fibrosis, a condition induced by the activation of stellate cells. In their resting state, these cells store substantial quantities of fat-soluble vitamin A (retinyl esters) in large lipid droplets. During activation, these lipid organelles are gradually degraded. Hence, treatment of fatty liver disease is treading a tightrope—unsophisticated targeting of hepatic lipid accumulation might trigger problematic side effects on stellate cells. Therefore, it is of great importance to gain more insight into the highly dynamic lipid metabolism of hepatocytes and stellate cells in both quiescent and activated states. In this review, part of the special issue entitled “Cellular and Molecular Mechanisms underlying the Pathogenesis of Hepatic Fibrosis 2020”, we discuss current and highly versatile aspects of neutral lipid metabolism in the pathogenesis of non-alcoholic fatty liver disease (NAFLD).

Specific Overexpression of Mitofusin-2 in Hepatic Stellate Cells Ameliorates Liver Fibrosis in Mice Model

Liver fibrosis is a chronic liver disease that could further develop to cirrhosis and liver carcinoma. Hepatic stellate cells (HSCs) are primary effector cells to initiate liver fibrosis. We aimed to explore the function and underlying mechanisms of mitochondrial fusion protein Mitofusin-2 (MFN2) in liver fibrosis. First, we utilized an alpha-smooth muscle actin promoter to overexpress MFN2 specifically in HSCs using adeno-associated virus (AAV) vector (AAV-MFN2). Overexpression of MFN2 was specifically achieved in HSC-T6 cells, but not in murine bone marrow-derived macrophages or hepatocyte AML-12 cells. We found that high expression of MFN2 induced apoptosis of HSC-T6 cells. Mechanistically, we demonstrated that high level of MFN2 inhibited TGF-β1/Smad signaling pathway, triggered downregulation of type I, type III, and type IV collagen, and antagonized the formation of factors associated with liver fibrosis. Furthermore, we found that overexpression of MFN2 using AAV-MFN2 ameliorated CCl₄-induced liver fibrosis in vivo with significantly decreased immune cell infiltration. Taken together, our findings indicate that MFN2 is critical in regulating apoptosis and liver fibrosis in HSCs, which might be a useful therapeutic target to treat liver fibrosis.

Effects of Simultaneous Downregulation of PHD1 and Keap1 on Prevention and Reversal of Liver Fibrosis in Mice

Background and Aim: To investigate whether double-knockdown of PHD1 and Keap1 in mice could enhance the resolution of carbon tetrachloride (CCl₄)-induced liver fibrosis. Methods: The liver fibrosis model of mice was established by intraperitoneal injection of 25% CCl₄ in olive oil (4 ul/g) twice a week for 8 weeks. PHD1shRNA and Keap1shRNA eukaryotic expression plasmids were simultaneously administered from the beginning of the first to fourth week (preventive group) or from the fifth to eighth week of CCl₄ injection (therapeutic group) via hydrodynamic-based tail vein injection. Successful transfection was confirmed with the expression of red fluorescent protein and green fluorescent protein in hepatocytes. Western blot was used for determining the expression of PHD1 and Keap1, HE, Sirius red, and Masson staining for evaluating the histopathological stages of fibrosis. Immunohistochemical techniques were applied to evaluate the expression of a-SMA. Results: The fluorescence of red and green were observed mainly in hepatocytes, and downregulation of PHD1 and Keap1 expression in liver was detected by western blot. Meanwhile, double-knockdown of PHD1 and Keap1 in mice alleviated liver fibrosis, and the effect was further enhanced especially in the preventive group. Immunocytochemical staining showed decreased expression of a-SMA when both PHD1 and Keap1 were knockdown. Conclusion: Downregulation of PHD1 and Keap1 expression in the liver could be achieved via hydrodynamic injection of PHD1shRNA and Keap1shRNA, thereby, preventing liver fibrosis.

Molecular Pathways: Targeting the Microenvironment of Liver Metastases

Curative treatment for metastatic solid cancers remains elusive. The liver, which is nourished by a rich blood supply from both the arterial and portal venous systems, is the most common site of visceral metastases, particularly from cancers arising in the gastrointestinal tract, with colorectal cancer being the predominant primary site in Western countries. A mounting body of evidence suggests that the liver microenvironment (LME) provides autocrine and paracrine signals originating from both parenchymal and nonparenchymal cells that collectively create both pre- and prometastatic niches for the development of hepatic metastases. These resident cells and their molecular mediators represent potential therapeutic targets for the prevention and/or treatment of liver metastases (LM). This review summarizes: (i) the current therapeutic options for treating LM, with a particular focus on colorectal cancer LM; (ii) the role of the LME in LM at each of its phases; (iii) potential targets in the LME identified through preclinical and clinical investigations; and (iv) potential therapeutic approaches for targeting elements of the LME before and/or after the onset of LM as the basis for future clinical trials. Clin Cancer Res; 23(21); 6390-9. ©2017 AACR.

Comparison of bone marrow-vs. adipose tissue-derived mesenchymal stem cells for attenuating liver fibrosis

Mesenchymal stem cell (MSC) therapy has emerged as a potential novel method of treating liver fibrosis. To date, bone marrow-derived MSCs (BM-MSCs) and adipose tissue-derived MSCs (AD-MSCs) have not been analyzed with respect to their ability to combat liver fibrosis. The present study aimed to compare the capabilities of BM-MSCs and AD-MSCs in the treatment of liver fibrosis. BM-MSCs and AD-MSCs were taken from male Sprague-Dawley rats and cultured. Hepatic stellate cells (HSCs) were co-cultured with either BM-MSCs or AD-MSCs, and the effects of BM-MSCs or AD-MSCs on the proliferation, activation and apoptosis of HSCs were determined. The secretion of a selected group of cytokines by BM-MSCs and AD-MSCs was measured using enzyme-linked immunosorbent assays. Using a CCl₄-induced liver fibrosis animal model, the anti-inflammatory and anti-fibrotic effects of BM-MSCs or AD-MSCs against liver fibrosis in vivo were evaluated. The morphological examination and analysis of specific surface markers confirmed the successful preparation of BM-MSCs and AD-MSCs. Furthermore, the proliferation, activation and apoptosis of HSCs were significantly inhibited by BM-MSCs and AD-MSCs, with statistically greater reductions achieved by AD-MSCs compared with BM-MSCs. Direct comparison of the secretion of selected cytokines by BM-MSCs and AD-MSCs revealed that significantly higher levels of nerve growth factor and transforming growth factor-β1 were secreted in the AD-MSC culture medium, whereas levels of vascular endothelial growth factor and interleukin-10 did not differ significantly between AD-MSCs and BM-MSCs. In vivo studies using a CCl₄-induced liver fibrosis model demonstrated that inflammatory activity and fibrosis staging scores were significantly lower in the MSC-treated groups compared with controls. Although AD-MSCs improved anti-inflammatory and anti-fibrotic effects compared with BM-MSCs, these differences were not significant. Thus, the current study demonstrated that BM-MSCs and AD-MSCs are similarly effective at attenuating liver fibrosis by inhibiting the activation and proliferation of HSCs, as well as promoting the apoptosis of HSCs.

Low concentrations of bilirubin inhibit activation of hepatic stellate cells in vitro

Hepatic stellate cell (HSC) activation serves a key role in liver fibrosis, and is associated with chronic liver diseases. Bilirubin, a product of heme degradation, has been demonstrated to have antioxidant properties. The present study investigated the effects of physiological concentrations of bilirubin on rat HSC activation. Rat HSCs were isolated and cultured for several generations to induce activation. The activated HSCs were subsequently treated with 0, 1, 10 or 20 mg/l bilirubin and assayed for parameters of cell activation. As the bilirubin concentration increased, HSCs demonstrated reduced production of reactive oxygen species, reduced protein expression levels of α-smooth muscle actin, a decreased mRNA expression ratio of tissue inhibitor of matrix metalloproteinase-1/matrix metalloproteinase-2, decreased proliferation and increased apoptosis. In conclusion, elevated bilirubin levels, within its physiological concentration range, appeared to inhibit HSC activation. These findings suggested a potential role for bilirubin in the treatment of fibrosis that requires further investigation.

Prolyl oligopeptidase attenuates hepatic stellate cell activation through induction of Smad7 and PPAR-γ

Prolyl oligopeptidase (POP) is a serine endopeptidase widely distributed in vivo with high activity in the liver. However, its biological functions in the liver have remained largely elusive. A previous study by our group has shown that POP produced N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP) and thereby exerted an anti-fibrogenic effect on hepatic stellate cells (HSCs) in vitro. It was therefore hypothesized that POP may affect the activation state of HSCs and has an important role in liver fibrosis. The HSC-T6 immortalized rat liver stellate cell line was treated with the POP inhibitor S17092 or transfected with recombinant lentivirus to overexpress POP. Cell proliferation and apoptosis were determined using a Cell Counting Kit-8 and flow cytometry, respectively. The activation status of HSCs was determined by examination of the expression of α-smooth muscle actin (α-SMA), collagen I, monocyte chemoattractant protein-1 (MCP-1), transforming growth factor (TGF)-β-Smad signaling and peroxisome proliferator activated receptor-γ (PPAR-γ). Inhibition by S17092 decreased, whereas lentiviral expression increased the activity of POP and cell proliferation, while neither of the treatments affected cell apoptosis. Of note, S17092 significantly increased, whereas POP overexpression decreased the expression of α-SMA and MCP-1 without affecting the expression of collagen I and TGF-β1. Furthermore, S17092 caused a reduction, whereas POP overexpression caused an upregulation of Smad7 protein and PPAR-γ, but not phosphorylated-Smad2/3 expression. In conclusion, POP attenuated the activation of HSCs through inhibition of TGF-β signaling and induction of PPAR-γ, which may have therapeutic potential in liver fibrosis.

Fat-Specific Protein 27/CIDEC Promotes Development of Alcoholic Steatohepatitis in Mice and Humans

BACKGROUND & AIMS

Alcoholic steatohepatitis (ASH) is the progressive form of alcoholic liver disease and may lead to cirrhosis and hepatocellular carcinoma. We studied mouse models and human tissues to identify molecules associated with ASH progression and focused on the mouse fat-specific protein 27 (FSP-27)/human cell death-inducing DFF45-like effector C (CIDEC) protein, which is expressed in white adipose tissues and promotes formation of fat droplets.

METHODS

C57BL/6N mice or mice with hepatocyte-specific disruption of Fsp27 (Fsp27(Hep-/-) mice) were fed the Lieber-Decarli ethanol liquid diet (5% ethanol) for 10 days to 12 weeks, followed by 1 or multiple binges of ethanol (5 or 6 g/kg) during the chronic feeding. Some mice were given an inhibitor (GW9662) of peroxisome proliferator-activated receptor γ (PPARG). Adenoviral vectors were used to express transgenes or small hairpin (sh) RNAs in cultured hepatocytes and in mice. Liver tissue samples were collected from ethanol-fed mice or from 31 patients with alcoholic hepatitis (AH) with biopsy-proved ASH and analyzed histologically and immunohistochemically and by transcriptome, immunoblotting, and real-time PCR analyses.

RESULTS

Chronic-plus-binge ethanol feeding of mice, which mimics the drinking pattern of patients with AH, produced severe ASH and mild fibrosis. Microarray analyses revealed similar alterations in expression of many hepatic genes in ethanol-fed mice and humans with ASH, including up-regulation of mouse Fsp27 (also called Cidec) and human CIDEC. Fsp27(Hep-/-) mice and mice given injections of adenovirus-Fsp27shRNA had markedly reduced ASH following chronic-plus-binge ethanol feeding. Inhibition of PPARG and cyclic AMP-responsive element binding protein H (CREBH) prevented the increases in Fsp27α and FSP27β mRNAs, respectively, and reduced liver injury in this chronic-plus-binge ethanol feeding model. Overexpression of FSP27 and ethanol exposure had synergistic effects in inducing production of mitochondrial reactive oxygen species and damage to hepatocytes in mice. Hepatic CIDEC mRNA expression was increased in patients with AH and correlated with the degree of hepatic steatosis and disease severity including mortality.

CONCLUSIONS

In mice, chronic-plus-binge ethanol feeding induces ASH that mimics some histological and molecular features observed in patients with AH. Hepatic expression of FSP27/CIDEC is highly up-regulated in mice following chronic-plus-binge ethanol feeding and in patients with AH; this up-regulation contributes to alcohol-induced liver damage.

Inhibitory effects of capsaicin on hepatic stellate cells and liver fibrosis

Hepatic stellate cells (HSCs) play an important role in the process of liver fibrosis. In this study, we investigated the inhibitory effects of capsaicin on HSCs and liver fibrosis. Cultured HSCs were incubated with various concentrations of capsaicin. Cell proliferation was examined using a cell counting kit. Production of hydrogen peroxide was determined using a 2′,7′-dichlorofluorescin diacetate (DCFH-DA) assay. The mRNA and protein expression of target genes was analyzed by reverse transcription PCR and Western blot analysis, respectively. Cell apoptosis was evaluated by annexin V-FITC and propidium iodide (PI) costaining followed by flow cytometric analysis. A CCl4 rat liver fibrosis model was used to assess in vivo effects of capsaicin by histological examination and measurement of liver fibrosis markers, including hydroxyproline content, serum type III collagen, and hyaluronic acid (HA) levels. Our results show that capsaicin dose-dependently inhibited cell proliferation, suppressed cell activation, and decreased hydrogen peroxide production in cultured HSCs. Capsaicin reduced the mRNA levels of tissue inhibitors of metalloproteinase 1 (TIMP-1) and transforming growth factor-β1 (TGF-β1) in HSCs. Moreover, capsaicin-induced cell apoptosis was associated with increased expression of Bax, cytochrome c (cyt c), and caspase-3, but reduced levels of Bcl-2. The animal studies further revealed that capsaicin efficiently reduced the extent of liver fibrosis, inhibited HSC proliferation, and promoted cell apoptosis. Our findings suggest that capsaicin might inhibit fibrogenesis by inhibiting the activities of HSCs.

Targeting Hepatic Glycerolipid Synthesis and Turnover to Treat Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of metabolic abnormalities ranging from simple hepatic steatosis (accumulation of neutral lipid) to development of steatotic lesions, steatohepatitis, and cirrhosis. NAFLD is extremely prevalent in obese individuals and with the epidemic of obesity; nonalcoholic steatohepatitis (NASH) has become the most common cause of liver disease in the developed world. NASH is rapidly emerging as a prominent cause of liver failure and transplantation. Moreover, hepatic steatosis is tightly linked to risk of developing insulin resistance, diabetes, and cardiovascular disease. Abnormalities in hepatic lipid metabolism are part and parcel of the development of NAFLD and human genetic studies and work conducted in experimentally tractable systems have identified a number of enzymes involved in fat synthesis and degradation that are linked to NAFLD susceptibility as well as progression to NASH. The goal of this review is to summarize the current state of our knowledge on these pathways and focus on how they contribute to etiology of NAFLD and related metabolic diseases.