Involvement of alcohol and aldehyde dehydrogenase activities on hepatic retinoid metabolism and its possible participation in the progression of rat liver regeneration

Abnormal metabolism in hepatic stellate cells: Pandora's box of MAFLD related hepatocellular carcinoma

Metabolic associated fatty liver disease (MAFLD) is a significant risk factor for the development of hepatocellular carcinoma (HCC). Hepatic stellate cells (HSCs), as key mediators in liver injury response, are believed to play a crucial role in the repair process of liver injury. However, in MAFLD patients, the normal metabolic and immunoregulatory mechanisms of HSCs become disrupted, leading to disturbances in the local microenvironment. Abnormally activated HSCs are heavily involved in the initiation and progression of HCC. The metabolic disorders and abnormal activation of HSCs not only initiate liver fibrosis but also contribute to carcinogenesis. In this review, we provide an overview of recent research progress on the relationship between the abnormal metabolism of HSCs and the local immune system in the liver, elucidating the mechanisms of immune imbalance caused by abnormally activated HSCs in MAFLD patients. Based on this understanding, we discuss the potential and challenges of metabolic-based and immunology-based mechanisms in the treatment of MAFLD-related HCC, with a specific focus on the role of HSCs in HCC progression and their potential as targets for anti-cancer therapy. This review aims to enhance researchers' understanding of the importance of HSCs in maintaining normal liver function and highlights the significance of HSCs in the progression of MAFLD-related HCC.

Involvement of cell oxidant status and redox state in the increased non-enzymatic ethanol oxidation by the regenerating rat liver

Ethanol administration is capable of inhibiting or delaying the partial hepatectomy (PH)-induced liver regeneration, probably altering liver metabolism by means of its oxidative metabolism. Since the regenerating liver has increased capacity for oxidizing ethanol, the present study was aimed to address the contribution of the ethanol-oxidizing metabolic pathways in the regenerating liver cells. Isolated hepatocytes were prepared from control livers and from animals subjected to two-thirds PH. In both preparations, ethanol oxidation was largely increased by incubation with glucose and was highly sensitive to inhibitors of ethanol-oxidizing enzymatic pathways (alcohol dehydrogenase, catalase and cytochrome P-4502E1 activities). The latter led to a total blockade of ethanol disposal by control hepatocytes, while liver cells from PH-rats only showed an early 70-75% inhibition of ethanol catabolism with the inhibitors used. In regenerating hepatocytes, the enhanced ethanol oxidation was blocked by scavengers of reactive oxygen species, an effect that correlated with enhanced cytoplasmic lipid peroxidation by-products. Both cell preparations showed similar sensitivity to inhibitors for the malate-aspartate shuttle and mitochondrial electron transport chain; the shift of the cytoplasmic redox state was also quite similar after ethanol oxidation. A more oxidized mitochondrial redox state was found in hepatocytes from PH-rats and more shifted to the reduced state during ethanol oxidation this effect was not abolished by inhibiting alcohol dehydrogenase activity. In conclusion, data clearly show that an important fraction of ethanol is metabolized through a non-enzymatic-mediated oxidative event, which could largely contribute to the deleterious effect of ethanol on the proliferating liver.

Putrescine treatment reverses α-tocopherol-induced desynchronization of polyamine and retinoid metabolism during rat liver regeneration

Background

The pre-treatment with α-tocopherol inhibits progression of rat liver proliferation induced by partial hepatectomy (PH), by decreasing and/or desynchronizing cyclin D1 expression and activation into the nucleus, activation and nuclear translocation of STAT-1 and -3 proteins and altering retinoid metabolism. Interactions between retinoic acid and polyamines have been reported in the PH-induced rat liver regeneration. Therefore, we evaluated the effect of low dosage of α-tocopherol on PH-induced changes in polyamine metabolism.

Methods

This study evaluated the participation of polyamine synthesis and metabolism during α-tocopherol-induced inhibition of rat liver regeneration. In PH-rats (Wistar) treated with α-tocopherol and putrescine, parameters indicative of cell proliferation, lipid peroxidation, ornithine decarboxylase expression (ODC), and polyamine levels, were determined.

Results

Pre-treatment with α-tocopherol to PH-animals exerted an antioxidant effect, shifting earlier the increased ODC activity and expression, temporally affecting polyamine synthesis and ornithine metabolism. Whereas administration of putrescine induced minor changes in PH-rats, the concomitant treatment actually counteracted most of adverse actions exerted by α-tocopherol on the remnant liver, restituting its proliferative potential, without changing its antioxidant effect. Putrescine administration to these rats was also associated with lower ODC expression and activity in the proliferating liver, but the temporally shifting in the amount of liver polyamines induced by α-tocopherol, was also “synchronized” by the putrescine administration. The latter is supported by the fact that a close relationship was observed between fluctuations of polyamines and retinoids.

Conclusions

Putrescine counteracted most adverse actions exerted by α-tocopherol on rat liver regeneration, restoring liver proliferative potential and restituting the decreased retinoid levels induced by α-tocopherol. Therefore interactions between polyamines and retinol, mediated by the oxidant status, should be taken into consideration in the development of new therapeutic strategies for pathologies occurring with liver cell proliferation.

Blockade of Retinol Metabolism Protects T Cell-Induced Hepatitis by Increasing Migration of Regulatory T Cells

Retinols are metabolized into retinoic acids by alcohol dehydrogenase (ADH) and retinaldehyde dehydrogenase (Raldh). However, their roles have yet to be clarified in hepatitis despite enriched retinols in hepatic stellate cells (HSCs). Therefore, we investigated the effects of retinols on Concanavalin A (Con A)-mediated hepatitis. Con A was injected into wild type (WT), Raldh1 knock-out (Raldh1(-/-)), CCL2(-/-) and CCR2(-/-) mice. For migration study of regulatory T cells (Tregs), we used in vivo and ex vivo adoptive transfer systems. Blockade of retinol metabolism in mice given 4-methylpyrazole, an inhibitor of ADH, and ablated Raldh1 gene manifested increased migration of Tregs, eventually protected against Con A-mediated hepatitis by decreasing interferon-γ in T cells. Moreover, interferon-γ treatment increased the expression of ADH3 and Raldh1, but it suppressed that of CCL2 and IL-6 in HSCs. However, the expression of CCL2 and IL-6 was inversely increased upon the pharmacologic or genetic ablation of ADH3 and Raldh1 in HSCs. Indeed, IL-6 treatment increased CCR2 expression of Tregs. In migration assay, ablated CCR2 in Tregs showed reduced migration to HSCs. In adoptive transfer of Tregs in vivo and ex vivo, Raldh1-deficient mice showed more increased migration of Tregs than WT mice. Furthermore, inhibited retinol metabolism increased survival rate (75%) compared with that of the controls (25%) in Con A-induced hepatitis. These results suggest that blockade of retinol metabolism protects against acute liver injury by increased Treg migration, and it may represent a novel therapeutic strategy to control T cell-mediated acute hepatitis.

Treatment with 4-methylpyrazole modulated stellate cells and natural killer cells and ameliorated liver fibrosis in mice

Background & Aims

Accumulating evidence suggests that retinol and its metabolites are closely associated with liver fibrogenesis. Recently, we demonstrated that genetic ablation of alcohol dehydrogenase 3 (ADH3), a retinol metabolizing gene that is expressed in hepatic stellate cells (HSCs) and natural killer (NK) cells, attenuated liver fibrosis in mice. In the current study, we investigated whether pharmacological ablation of ADH3 has therapeutic effects on experimentally induced liver fibrosis in mice.

Methods

Liver fibrosis was induced by intraperitoneal injections of carbon tetrachloride (CCl₄) or bile duct ligation (BDL) for two weeks. To inhibit ADH3-mediated retinol metabolism, 10 μg 4-methylpyrazole (4-MP)/g of body weight was administered to mice treated with CCl₄ or subjected to BDL. The mice were sacrificed at week 2 to evaluate the regression of liver fibrosis. Liver sections were stained for collagen and α-smooth muscle actin (α-SMA). In addition, HSCs and NK cells were isolated from control and treated mice livers for molecular and immunological studies.

Results

Treatment with 4-MP attenuated CCl₄- and BDL-induced liver fibrosis in mice, without any adverse effects. HSCs from 4-MP treated mice depicted decreased levels of retinoic acids and increased retinol content than HSCs from control mice. In addition, the expression of α-SMA, transforming growth factor-β1 (TGF-β1), and type I collagen α1 was significantly reduced in the HSCs of 4-MP treated mice compared to the HSCs from control mice. Furthermore, inhibition of retinol metabolism by 4-MP increased interferon-γ production in NK cells, resulting in increased apoptosis of activated HSCs.

Conclusions

Based on our data, we conclude that inhibition of retinol metabolism by 4-MP ameliorates liver fibrosis in mice through activation of NK cells and suppression of HSCs. Therefore, retinol and its metabolizing enzyme, ADH3, might be potential targets for therapeutic intervention of liver fibrosis.

Retinoic acid regulates cell cycle genes and accelerates normal mouse liver regeneration

All-trans retinoic acid (RA) is a potent inducer of regeneration. Because the liver is the principal site for storage and bioactivation of vitamin A, the current study examines the effect of RA in mouse hepatocyte proliferation and liver regeneration. Mice that received a single dose of RA (25μg/g) by oral gavage developed hepatomegaly with increased number of Ki67-positive cells and induced expression of cell cycle genes in the liver. DNA binding data revealed that RA receptors retinoic acid receptor β (RARβ) and retinoid x receptor α (RXRα) bound to cell cycle genes Cdk1, Cdk2, Cyclin B, Cyclin E, and Cdc25a in mice with and without RA treatment. In addition, RA treatment induced novel binding of RARβ/RXRα to Cdk1, Cdk2, Cyclin D, and Cdk6 genes. All RARβ/RXRα binding sites contained AGGTCA-like motifs. RA treatment also promoted liver regeneration after partial hepatectomy (PH). RA signaling was implicated in normal liver regeneration as the mRNA levels of RARβ, Aldh1a2, Crabp1, and Crbp1 were all induced 1.5 days after PH during the active phase of hepatocyte proliferation. RA treatment prior to PH resulted in early up-regulation of RARβ, Aldh1a2, Crabp1, and Crbp1, which was accompanied by an early induction of cell cycle genes. Western blotting for RARβ, c-myc, Cyclin D, E, and A further supported the early induction of retinoid signal and cell proliferation by RA treatment. Taken together, our data suggest that RA may regulate cell cycle progression and accelerates liver regeneration. Such effect is associated with an early induction of RA signaling, which includes increased expression of the receptor, binding proteins, and processing enzyme for retinoids.

Proteomic analysis of visceral adipose tissue in pre-obese patients with type 2 diabetes

The mechanisms involved in the progression to overt diabetes in pre-obese subjects remain unclear. Therefore, a nontargeted evaluation of differences in the protein abundance of visceral adipose tissue (VAT) obtained from pre-obese diabetic subjects and pre-obese subjects showing normal glucose tolerance may provide novel insights on the molecular processes involved in the progression to overt diabetes in pre-obesity. Diabetic patients showed increased VAT abundance of glutathione S-transferase Mu 2, peroxiredoxin-2, antithrombin-III, apolipoprotein A-IV, Ig κ chain C region, mitochondrial aldehyde dehydrogenase and actin, and decreased abundance of annexin-A1, retinaldehyde dehydrogenase-1, and vinculin, compared with their non-diabetic counterparts. These proteins are involved in cytoskeleton function and structure, oxidative stress, inflammation and retinoid metabolism. The presence of diabetes influences the VAT abundance of several proteins. Hence, the proteins identified here could be considered candidate molecules in future studies addressing the role that VAT dysfunction plays in the development of type 2 diabetes.

Hepatic retinoid stores are required for normal liver regeneration

Preliminary studies of liver regeneration induced by partial hepatectomy (PHE) identified a substantial depletion of hepatic retinoid stores, by greater than 70%, in regenerating livers of wild-type C57Bl/6J mice. To understand this, we compared responses of wild-type and lecithin:retinol acyltransferase (Lrat)-deficient mice, which totally lack hepatic retinoid stores, to PHE. The Lrat-deficient livers showed delayed regeneration in the first 24 h after PHE. At 12 h after PHE, we observed significantly less mRNA expression for growth factors and cytokines implicated in regulating the priming phase of liver regeneration, specifically for Hgf and Tgfα, but not Tgfβ. Compared with wild-type mice, the changes in mRNA levels for p21 and cyclins E1, B1, and A2 mRNAs and for hepatocellular BrdU incorporation and mitoses were delayed (i.e., shifted to later times) in regenerating Lrat(-/-) livers. Concentrations of all-trans-retinoic acid were significantly lower in the livers of Lrat(-/-) mice following PHE, and this was accompanied by diminished expression of known retinoid-responsive genes. At later times after PHE, the rate of liver weight restoration for Lrat(-/-) mice was parallel to that of wild-type mice, although additional biochemical differences were observed. Thus, hepatic retinoid stores are required for maintaining expression of signaling molecules that regulate cell proliferation and differentiation immediately after hepatic injury, accounting for the delayed restoration of liver mass in Lrat(-/-) mice.

High dosing of α-tocopherol inhibits rat liver regeneration by modifying signal transducer and activator of transcription protein expression and its correlation with cell redox state and retinoid metabolism

Lipid peroxidation (LP) promoted by partial hepatectomy (PH) is qualitatively distinct among subcellular fractions and temporally transient, probably being a necessary physiological event for rat liver regeneration. In fact, α-tocopherol (vitamin E [VE]) exerts adverse effects, partially inhibiting PH-induced rat liver regeneration and inducing decreased cyclin D1 expression. The phosphorylation of signal transducer and activator of transcription (STAT) factors 1 and 3 are involved in DNA synthesis and cyclin D1 expression after PH, which is stimulated by production of retinoic acid (RA). Hence, this study was aimed at addressing these events, and its association with cell redox state and oxidative stress, probably underlying VE effects on rat liver regeneration. PH-enhanced activation of STAT proteins, mainly as activated STAT-3, significantly change the cytoplasmic pool for STATs. The latter was associated to a more reduced cytoplasmic redox state and increased alcohol dehydrogenase (ADH)-mediated retinol oxidation to RA. Whereas α-tocopherol promoted minor changes in the parameters tested when administered to sham (control)-animals, pretreatment with VE blocked the PH-induced increase of reactive oxygen species (ROS), altering the pattern of STAT protein activation, blunting RA formation by decreased ADH activity, inducing higher liver caspase-3 activity and increasing tumor necrosis factor- α concentrations, while levels of interleukin-6 were decreased; altogether coinciding with disturbed PH-promoted changes on the liver redox state. In conclusion, altered activation and translocation of STAT-1 and -3 proteins and inhibited retinoid metabolism seem to be involved in the VE-induced inhibition of rat liver regeneration. Data suggest that a PH-induced increase of ROS could participate in the activation of STAT factors, retinoid metabolism and changes in the cell redox state during proliferation of liver cells.

Retinoic acids and hepatic stellate cells in liver disease

Quiescent hepatic stellate cells (HSCs) in healthy liver store 80% of total liver retinols and release them depending on the extracellular retinol status. However, HSCs activated by liver injury lose their retinols and produce a considerable amount of extracellular matrix, subsequently leading to liver fibrosis. Emerging evidence suggests that retinols and their metabolites such as retinoic acids (RAs) contribute to liver regeneration, fibrosis and tumor. However, it is not clear yet why HSCs lose retinol, which enzymes are involved in the retinol metabolism of HSCs and what function of retinol metabolites on HSCs upon liver injury. Recently, our group and collaborators have demonstrated that during activation, HSCs not only lose retinols but also metabolize them into RAs by alcohol dehydrogenases and retinaldehyde dehydrogenases. As transcriptional factors, metabolized RAs induce retinoic acid early inducible-1 and suppressor of cytokine signaling 1 in HSCs, which plays an important role in the interaction between HSCs and natural killer cells. In addition, RAs released from HSCs may induce hepatic cannabinoid receptor 1 expression in alcoholic liver steatosis or regulate immune responses upon liver inflammation. The present review summarizes the role of endogenous metabolized RAs on HSCs themselves and on other liver cells including hepatocytes and immune cells. Moreover, the effects of exogenous retinol and RA treatments on HSCs and liver disease are discussed.

Peroxisome proliferator-activated receptor and retinoic x receptor in alcoholic liver disease

A growing number of new studies demonstrate that nuclear receptors are involved in the development of alcoholic liver disease (ALD). Ethanol metabolism and RXR/PPAR functions are tightly interconnected in the liver. Several ethanol metabolizing enzymes are potently regulated by RXR and PPARα after alcohol consumption. The increased ethanol metabolism, in turn, leads to alteration of the redox balance of the cells and impairment of RXR/PPAR functions by direct and indirect effects of acetaldehyde, resulting in deranged lipid metabolism, oxidative stress, and release of proinflammatory cytokines. The use of animal models played a crucial role in understanding the molecular mechanisms of ALD. In this paper we summarize the reciprocal interactions between ethanol metabolism and RXR/PPAR functions. In conclusion, RXR and PPAR play a central role in the onset and perpetuation of the mechanisms underling all steps of the clinical progression in ALD.

Adenosine administration accelerates progression of the cell cycle during rat liver regeneration induced by one-third hepatectomy

We have shown that adenosine administration is capable of reversing fibrosis in the carbon tetrachloride-induced rat cirrhotic liver, stimulating the diminished proliferative potential of the cirrhotic liver. To characterize adenosine actions on liver cellular proliferation, we used rats subjected to one-third partial hepatectomy (PH). In PH animals acutely administered with adenosine (25-200 mg/kg b.w.), parameters indicative of cell proliferation were determined. In addition, hepatocyte growth factor (HGF), epidermal growth factor, and transforming growth factor-alpha, cyclins, members of the E2F family, proto-oncogenes, and adenosine-receptors were determined through Western blot analyses. Adenosine (100 mg/kg body weight) induced an earlier increase in liver cell proliferation as evidenced by enhanced levels of proliferating cell nuclear antigen, nuclear Ki-67 antigen, and those for cyclins (D1, E, A, and B1), as well as by an increased mitotic index. These effects were also accompanied for a long-lasting increase of serum and liver levels of HGF and liver expression of c-Met and HGF liver activator. Adenosine effects on cell proliferation could be mediated by an early increase in E2F-1 and by that of c-Myc, despite the fact that phosphorylation of the Rb protein and expression of E2F-3 were decreased. Moreover, the liver amount of specific receptors for adenosine was not significantly changed by PH and/or adenosine treatment. In conclusion, these data suggest that adenosine actions can accelerate and increase proliferation in a "primed" liver, mainly through enhancing c-Myc, E2F family, cell-cycle cyclins, and HGF expression. Therefore, these pharmacological adenosine effects suggest a modulating role for the nucleoside on mitogenic events once the liver has been triggered to proliferate.

Exploring the priming mechanism of liver regeneration: proteins and protein complexes

The liver has the ability to restore its functional capacity following injury or resection and the priming of liver regeneration is a complex process that has not been completely elucidated. In the current research, to further reveal the priming mechanism of liver regeneration, hepatocyte total protein and hepatocyte cytosol of the rats at 4 h after 2/3 partial hepatectomy (PHx) were studied, respectively, by 2-DE and 2-D blue native gel electrophoresis. Seventeen unique differential proteins were identified in hepatocyte total protein samples. Nine differential protein complexes containing 41 protein components were identified in hepatocyte cytosol samples. For the first time, at the priming stage of liver regeneration, the variations of serine protease inhibitor 2c, sulfite oxidase and valosin-containing protein (VCP) were presented and validated by Western blotting, and the VCP complex was further validated by antibody super-shift experiments. The current results suggested that at 4 h after PHx, VCP complex was down-regulated in hepatocyte cytosol, apoptosis pathways were inhibited, nuclear factor-kappaB and interleukin 6 pathways worked together and triggered the liver regeneration.

Protein profilings in mouse liver regeneration after partial hepatectomy using iTRAQ technology

Liver is unique in its capability to regenerate after an injury. Liver regeneration after a 2/3 partial hepatectomy served as a classical model and is adopted frequently to study the mechanism of liver regeneration. In the present study, semiquantitative analysis of protein expression in mouse liver regeneration following partial hepatectomy was performed using an iTRAQ technique. Proteins from pre-PHx control livers and livers regenerating for 24, 48 and 72 h were extracted and inspected using 4-plex isotope labeling, followed by liquid chromatography fractionation, mass spectrometry and statistical differential analysis. A total of 827 proteins were identified in this study. There were 270 proteins for which quantitative information was available at all the time points in both biologically duplicate experiments. Among the 270 proteins, Car3, Mif, Adh1, Lactb2, Fabp5, Es31, Acaa1b and LOC100044783 were consistently down-regulated, and Mat1a, Dnpep, Pabpc1, Apoa4, Oat, Hpx, Hp and Mt1 were up-regulated by a factor of at least 1.5 from that of the controls at one time point or more. The regulation of each differential protein was also demonstrated by monitoring its time-dependent expression changes during the regenerating process. We believe this is the first report to profile the protein changes in liver regeneration utilizing the iTRAQ proteomic technique.

Alcohol and the liver

PURPOSE OF REVIEW

This review aims to acquaint the reader with advances in 2006 in the epidemiology, genetics, detection, pathogenesis and treatment of alcoholic liver disease.

RECENT FINDINGS

Important discoveries have been made in pathogenesis and mechanism of disease, with great emphasis on the many pathways leading to oxidative stress, and the novel mechanism of endoplasmic reticulum stress that is proving to be important in the pathogenesis of many liver diseases. The reliability of ethyl glucuronide and other biomarkers for the detection of alcohol abuse is being better established. There have been no treatment advances for alcoholic liver disease but, on balance, steroids are still favored for carefully selected patients with alcoholic hepatitis. Many compounds tested in rodents may now be available for consideration for clinical trials. Criteria for patient selection and refusal for liver transplantation are being established but the 6 months abstinence rule still holds.

SUMMARY

Insights are being made into the pathogenesis of alcoholic liver disease but safe and effective therapies for both alcoholic hepatitis and alcoholic cirrhosis have yet to be discovered.