Fat paradox of steatohepatitis

Exploring the impact of lipid droplets on the evolution and progress of hepatocarcinoma

Over the past 10 years, the biological role of lipid droplets (LDs) has gained significant attention in the context of both physiological and pathological conditions. Considerable progress has been made in elucidating key aspects of these organelles, yet much remains to be accomplished to fully comprehend the myriad functions they serve in the progression of hepatic tumors. Our current perception is that LDs are complex and active structures managed by a distinct set of cellular processes. This understanding represents a significant paradigm shift from earlier perspectives. In this review, we aim to recapitulate the function of LDs within the liver, highlighting their pivotal role in the pathogenesis of metabolic dysfunction-associated steatotic liver disease (MASLD) (Hsu and Loomba, 2024) and their contribution to the progression towards more advanced pathological stages up to hepatocellular carcinoma (HC) (Farese and Walther, 2009). We are aware of the molecular complexity and changes occurring in the neoplastic evolution of the liver. Our attempt, however, is to summarize the most important and recent roles of LDs across both healthy and all pathological liver states, up to hepatocarcinoma. For more detailed insights, we direct readers to some of the many excellent reviews already available in the literature (Gluchowski et al., 2017; Hu et al., 2020; Seebacher et al., 2020; Paul et al., 2022).

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.

Perilipin 5 Ameliorates Hepatic Stellate Cell Activation via SMAD2/3 and SNAIL Signaling Pathways and Suppresses STAT3 Activation

Comprehending the molecular mechanisms underlying hepatic fibrogenesis is essential to the development of treatment. The hallmark of hepatic fibrosis is the development and deposition of excess fibrous connective tissue forcing tissue remodeling. Hepatic stellate cells (HSC) play a major role in the pathogenesis of liver fibrosis. Their activation via the transforming growth factor-β1 (TGF-β1) as a key mediator is considered the crucial event in the pathophysiology of hepatic fibrogenesis. It has been shown that Perilipin 5 (PLIN5), known as a lipid droplet structural protein that is highly expressed in oxidative tissue, can inhibit such activation through various mechanisms associated with lipid metabolism. This study aimed to investigate the possible influence of PLIN5 on TGF-β1 signaling. Our findings confirm the importance of PLIN5 in maintaining HSC quiescence in vivo and in vitro. PLIN5 overexpression suppresses the TGF-β1-SMAD2/3 and SNAIL signaling pathways as well as the activation of the signal transducers and activators of transcription 3 (STAT3). These findings derived from experiments in hepatic cell lines LX-2 and Col-GFP, in which overexpression of PLIN5 was able to downregulate the signaling pathways SMAD2/3 and SNAIL activated previously by TGF-β1 treatment. Furthermore, TGF-β1-mediatedinduction of extracellular matrix proteins, such as collagen type I (COL1), Fibronectin, and α-smooth muscle actin (α-SMA), was suppressed by PLIN5. Moreover, STAT3, which is interrelated with TGF-β1 was already basally activated in the cell lines and inhibited by PLIN5 overexpression, leading to a further reduction in HSC activity shown by lowered α-SMA expression. This extension of the intervening mechanisms presents PLIN5 as a potent and pleiotropic target in HSC activation.

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).

Hepatocyte and stellate cell deletion of liver fatty acid binding protein reveals distinct roles in fibrogenic injury

Liver fatty acid binding protein (L-Fabp) modulates lipid trafficking in enterocytes, hepatocytes, and hepatic stellate cells (HSCs). We examined hepatocyte vs. HSC L-Fabp deletion in hepatic metabolic adaptation and fibrotic injury. Floxed L-Fabp mice were bred to different transgenic Cre mice or injected with adeno-associated virus type 8 (AAV8) Cre and fed diets to promote steatosis and fibrosis or were subjected to either bile duct ligation or CCl4 injury. Albumin-Cre-mediated L-Fabp deletion revealed recombination in hepatocytes and HSCs; these findings were confirmed with 2 other floxed alleles. Glial fibrillary acid protein-Cre and platelet-derived growth factor receptor β-Cre-mediated L-Fabp deletion demonstrated recombination only in HSCs. Mice with albumin promoter-driven Cre recombinase (Alb-Cre)-mediated or AAV8-mediated L-Fabp deletion were protected against food withdrawal-induced steatosis. Mice with Alb-Cre-mediated L-Fabp deletion were protected against high saturated fat-induced steatosis and fibrosis, phenocopying germline L-Fabp-/- mice. Mice with HSC-specific L-Fabp deletion exhibited retinyl ester depletion yet demonstrated no alterations in fibrosis. On the other hand, fibrogenic resolution after CCl4 administration was impaired in mice with Alb-Cre-mediated L-Fabp deletion. These findings suggest cell type-specific roles for L-Fabp in mitigating hepatic steatosis and in modulating fibrogenic injury and reversal.-Newberry, E. P., Xie, Y., Lodeiro, C., Solis, R., Moritz, W., Kennedy, S., Barron, L., Onufer, E., Alpini, G., Zhou, T., Blaner, W. S., Chen, A., Davidson, N. O. Hepatocyte and stellate cell deletion of liver fatty acid binding protein reveal distinct roles in fibrogenic injury.

Sex-specific differences in hepatic steatosis in obese spontaneously hypertensive (SHROB) rats

Background

Patients with metabolic syndrome, who are characterized by co-existence of insulin resistance, hypertension, hyperlipidemia, and obesity, are also prone to develop non-alcoholic fatty liver disease (NAFLD). Although the prevalence and severity of NAFLD is significantly greater in men than women, the mechanisms by which gender modulates the pathogenesis of hepatic steatosis are poorly defined. The obese spontaneously hypertensive (SHROB) rats represent an attractive model of metabolic syndrome without overt type 2 diabetes. Although pathological manifestation caused by the absence of a functional leptin receptor has been extensively studied in SHROB rats, it is unknown whether these animals elicited sex-specific differences in the development of hepatic steatosis.

Methods

We compared hepatic pathology in male and female SHROB rats. Additionally, we examined key biochemical and molecular parameters of signaling pathways linked with hyperinsulinemia and hyperlipidemia. Finally, using methods of quantitative polymerase chain reaction (qPCR) and western blot analysis, we quantified expression of 45 genes related to lipid biosynthesis and metabolism in the livers of male and female SHROB rats.

Results

We show that all SHROB rats developed hepatic steatosis that was accompanied by enhanced expression of SREBP1, SREBP2, ACC1, and FASN proteins. The livers of male rats also elicited higher induction of Pparg, Ppara, Slc2a4, Atox1, Skp1, Angptl3, and Pnpla3 mRNAs. In contrast, the livers of female SHROB rats elicited constitutively higher levels of phosphorylated JNK and AMPK and enhanced expression of Cd36.

Conclusion

Based on these data, we conclude that the severity of hepatic steatosis in male and female SHROB rats was mainly driven by increased de novo lipogenesis. Moreover, male and female SHROB rats also elicited differential severity of hepatic steatosis that was coupled with sex-specific differences in fatty acid transport and esterification.

Electronic supplementary material

The online version of this article (10.1186/s13293-018-0202-x) contains supplementary material, which is available to authorized users.

Perilipin 5 and liver fatty acid binding protein function to restore quiescence in mouse hepatic stellate cells

Hepatic stellate cell (HSC) activation occurs along with decreased Perilipin5 (Plin5) and liver fatty acid-binding protein (L-Fabp) expression and coincident lipid droplet (LD) depletion. Conversely, the activated phenotype is reversible in WT HSCs upon forced expression of Plin5. Here, we asked if L-Fabp expression is required for Plin5-mediated rescue of the quiescent phenotype. Lentiviral Plin5 transduction of passaged L-Fabp^-/- HSCs failed to reverse activation markers or restore lipogenic gene expression and LD formation. However, adenoviral L-Fabp infection of lentiviral Plin5 transduced L-Fabp^-/- HSCs restored both the quiescent phenotype and LD formation, an effect also mediated by adenoviral intestine-Fabp or adipocyte-Fabp. Expression of exogenous Plin5 in activated WT HSCs induced a transcriptional program of lipogenic gene expression including endogenous L-Fabp, but none of the other FABPs. We further demonstrated that selective, small molecule inhibition of endogenous L-Fabp also eliminated the ability of exogenous Plin5 to rescue LD formation and reverse activation of WT HSCs. This functional coordination of L-Fabp with Plin5 was 5'-AMP-activated protein kinase (AMPK)-dependent and was eliminated by AMPK inhibition. Taken together, our results indicate that L-Fabp is required for Plin5 to activate a transcriptional program that restores LD formation and reverses HSC activation.

Perilipin 5 restores the formation of lipid droplets in activated hepatic stellate cells and inhibits their activation

Hepatic stellate cells (HSC) are major effectors during hepatic fibrogenesis. The activation of HSC is coupled to the loss of lipid droplets (LDs), which are specialized organelles composed of neutral lipids surrounded by perilipins. LDs have emerged as a focal point of interest in understanding the metabolic regulation of intrahepatic lipids during lipid-mediated liver fibrogenesis. Perilipin 5 (Plin5) is a newly identified LD protein in the perilipin family, which plays a key role in regulating aspects of intracellular trafficking, signaling, and cytoskeletal organization in hepatocytes. Recent work in Plin5 knockout mice suggests a role in high fat diet-induced hepatic lipotoxicity. The current report is to evaluate the impact of Plin5 on HSC activation and to elucidate the underlying mechanisms. We now show that high fat diet-induced liver fibrosis is accompanied by an approximate 75% reduction in Plin5 in HSC, and that spontaneous activation of primary HSC produces temporally coincident loss of Plin5 expression and LD depletion. As modulating lipid content in HSC is a suggested strategy for inhibition of HSC activation and treatment of hepatic fibrosis, we asked whether exogenous Plin5 expression in primary HSC would reverse the activation phenotype and promote LD formation. Recombinant lentiviral Plin5 expression in primary mouse HSC restored the formation of LDs, increased lipid content by inducing expression of pro-lipogenic genes and suppressing expression of pro-lipolytic genes, and suppressed HSC activation (~two fold reduction in expression of procollagen and α-smooth muscle actin, two unique biomarkers for activated HSC). In addition, the expression of exogenous Plin5 in HSC attenuated cellular oxidative stress by reducing cellular reactive oxygen species, elevating cellular glutathione, and inducing gene expression of glutamate-cysteine ligase. Taken together, our results indicate that expression of Plin5 plays a critical role in the formation of LDs, the elevation of lipid content in HSC, and the inhibition of the activation of HSC.

Role of Transcription Factors in Steatohepatitis and Hypertension after Ethanol: The Epicenter of Metabolism

Chronic alcohol consumption induces multi-organ damage, including alcoholic liver disease (ALD), pancreatitis and hypertension. Ethanol and ethanol metabolic products play a significant role in the manifestation of its toxicity. Ethanol metabolizes to acetaldehyde and produces reduced nicotinamide adenine dinucleotide (NADH) by cytosolic alcohol dehydrogenase. Ethanol metabolism mediated by cytochrome-P450 2E1 causes oxidative stress due to increased production of reactive oxygen species (ROS). Acetaldehyde, increased redox cellular state and ROS activate transcription factors, which in turn activate genes for lipid biosynthesis and offer protection of hepatocytes from alcohol toxicity. Sterol regulatory element binding proteins (SREBPs) and peroxisome proliferator activated-receptors (PPARs) are two key lipogenic transcription factors implicated in the development of fatty liver in alcoholic and non-alcoholic steatohepatitis. SREBP-1 is activated in the livers of chronic ethanol abusers. An increase in ROS activates nuclear factor erythroid-2-related factor-2 (Nrf2) and hypoxia inducible factor (HIF) to provide protection to hepatocytes from ethanol toxicity. Under ethanol exposure, due to increased gut permeability, there is release of gram-negative bacteria-derived lipopolysaccharide (LPS) from intestine causing activation of immune response. In addition, the metabolic product, acetaldehyde, modifies the proteins in hepatocyte, which become antigens inviting auto-immune response. LPS activates macrophages, especially the liver resident macrophages, Kupffer cells. These Kupffer cells and circulating macrophages secrete various cytokines. The level of tumor necrosis factor-α (TNFα), interleukin-1beta (IL-1β), IL-6, IL-8 and IL-12 have been found elevated among chronic alcoholics. In addition to elevation of these cytokines, the peripheral iron (Fe(2+)) is also mobilized. An increased level of hepatic iron has been observed among alcoholics. Increased ROS, IL-1β, acetaldehyde, and increased hepatic iron, all activate nuclear factor-kappa B (NF-κB) transcription factor. Resolution of increased reactive oxygen species requires increased expression of genes responsible for dismutation of increased ROS which is partially achieved by IL-6 mediated activation of signal transducers and activators of transcription 3 (STAT3). In addition to these transcription factors, activator protein-1 may also be activated in hepatocytes due to its association with resolution of increased ROS. These transcription factors are central to alcohol-mediated hepatotoxicity.

Morphogen-related therapeutic targets for liver fibrosis

Recent research on hepatic stellate cells (HSCs) has spotlighted the involvement of morphogens in their cell fate determination in liver fibrosis. Temporally and spatially expressed during embryonic development, morphogens are involved in regulation of cell proliferation and differentiation, and tissue patterning. In normal adult liver, morphogens are generally expressed at low levels. However, in liver disease, myofibroblastic HSCs express morphogens such as Wnt, Shh, Necdin, DLK1, and Notch as part of their participation in fibrogenesis and wound healing. Liver regeneration involves cell proliferation and differentiation akin to embryonic liver development where the cells appear to undergo similar fates, and not surprisingly the morphogens are re-activated for the regenerative purpose in adult liver injury. Evidence also points to crosstalk of these morphogens in regulation of HSC fate determination. Genetic ablation or pharmacologic inhibition of morphogens reverts activated HSC to quiescent cells in culture and attenuates progression of hepatic fibrosis. However, positive regulation of liver regeneration by the morphogens needs to be spared. Therapeutically, manipulation of morphogen activities in a cell type and phase-specific manner should offer new modalities for chronic liver disease.

Rhein Elicits In Vitro Cytotoxicity in Primary Human Liver HL-7702 Cells by Inducing Apoptosis through Mitochondria-Mediated Pathway

Objective. To study rhein-induced apoptosis signaling pathway and to investigate its molecular mechanisms in primary human hepatic cells. Results. Cell viability of HL-7702 cells treated with rhein showed significant decrease in dose-dependent manner. Following rhein treatment (25 μM, 50 μM, and 100 μM) for 12 h, the detection of apoptotic cells was significantly analyzed by flow cytometry and nuclear morphological changes by Hoechst 33258, respectively. Fatty degeneration studies showed upregulation level of the relevant hepatic markers (P < 0.01). Caspase activities expressed significant upregulation of caspase-3, caspase-9, and caspase-8. Moreover, apoptotic cells by rhein were significantly inhibited by Z-LEHD-FMK and Z-DEVD-FMK, caspase-9 inhibitor, and caspase-3 inhibitor, respectively. Overproduction of reactive oxygen species, lipid peroxidation, and loss of mitochondrial membrane potential were detected by fluorometry. Additionally, NAC, a ROS scavenger, significantly attenuated rhein-induced oxidative damage in HL-7702 cells. Furthermore, real-time qPCR results showed significant upregulation of p53, PUMA, Apaf-1, and Casp-9 and Casp-3 mRNA, with no significant changes of Fas and Cytochrome-c. Immunoblotting revealed significant Cytochrome-c release from mitochondria into cytosol and no change in Fas expression. Conclusion. Taken together, these observations suggested that rhein could induce apoptosis in HL-7702 cells via mitochondria-mediated signal pathway with involvement of oxidative stress mechanism.

Peroxisome proliferator-activated receptors as targets to treat non-alcoholic fatty liver disease

Lately, the world has faced tremendous progress in the understanding of non-alcoholic fatty liver disease (NAFLD) pathogenesis due to rising obesity rates. Peroxisome proliferator-activated receptors (PPARs) are transcription factors that modulate the expression of genes involved in lipid metabolism, energy homeostasis and inflammation, being altered in diet-induced obesity. Experimental evidences show that PPAR-alpha is the master regulator of hepatic beta-oxidation (mitochondrial and peroxisomal) and microsomal omega-oxidation, being markedly decreased by high-fat (HF) intake. PPAR-beta/delta is crucial to the regulation of forkhead box-containing protein O subfamily-1 expression and, hence, the modulation of enzymes that trigger hepatic gluconeogenesis. In addition, PPAR-beta/delta can activate hepatic stellate cells aiming to the hepatic recovery from chronic insult. On the contrary, PPAR-gamma upregulation by HF diets maximizes NAFLD through the induction of lipogenic factors, which are implicated in the fatty acid synthesis. Excessive dietary sugars also upregulate PPAR-gamma, triggering de novo lipogenesis and the consequent lipid droplets deposition within hepatocytes. Targeting PPARs to treat NAFLD seems a fruitful approach as PPAR-alpha agonist elicits expressive decrease in hepatic steatosis by increasing mitochondrial beta-oxidation, besides reduced lipogenesis. PPAR-beta/delta ameliorates hepatic insulin resistance by decreasing hepatic gluconeogenesis at postprandial stage. Total PPAR-gamma activation can exert noxious effects by stimulating hepatic lipogenesis. However, partial PPAR-gamma activation leads to benefits, mainly mediated by increased adiponectin expression and decreased insulin resistance. Further studies are necessary aiming at translational approaches useful to treat NAFLD in humans worldwide by targeting PPARs.

Vitamin a deficiency and alterations in the extracellular matrix

Vitamin A or retinol which is the natural precursor of several biologically active metabolites can be considered the most multifunctional vitamin in mammals. Its deficiency is currently, along with protein malnutrition, the most serious and common nutritional disorder worldwide. It is necessary for normal embryonic development and postnatal tissue homeostasis, and exerts important effects on cell proliferation, differentiation and apoptosis. These actions are produced mainly by regulating the expression of a variety of proteins through transcriptional and non-transcriptional mechanisms. Extracellular matrix proteins are among those whose synthesis is known to be modulated by vitamin A. Retinoic acid, the main biologically active form of vitamin A, influences the expression of collagens, laminins, entactin, fibronectin, elastin and proteoglycans, which are the major components of the extracellular matrix. Consequently, the structure and macromolecular composition of this extracellular compartment is profoundly altered as a result of vitamin A deficiency. As cell behavior, differentiation and apoptosis, and tissue mechanics are influenced by the extracellular matrix, its modifications potentially compromise organ function and may lead to disease. This review focuses on the effects of lack of vitamin A in the extracellular matrix of several organs and discusses possible molecular mechanisms and pathologic implications.

Multi-SNP analysis of GWAS data identifies pathways associated with nonalcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a common liver disease; the histological spectrum of which ranges from steatosis to steatohepatitis. Nonalcoholic steatohepatitis (NASH) often leads to cirrhosis and development of hepatocellular carcinoma. To better understand pathogenesis of NAFLD, we performed the pathway of distinction analysis (PoDA) on a genome-wide association study dataset of 250 non-Hispanic white female adult patients with NAFLD, who were enrolled in the NASH Clinical Research Network (CRN) Database Study, to investigate whether biologic process variation measured through genomic variation of genes within these pathways was related to the development of steatohepatitis or cirrhosis. Pathways such as Recycling of eIF2:GDP, biosynthesis of steroids, Terpenoid biosynthesis and Cholesterol biosynthesis were found to be significantly associated with NASH. SNP variants in Terpenoid synthesis, Cholesterol biosynthesis and biosynthesis of steroids were associated with lobular inflammation and cytologic ballooning while those in Terpenoid synthesis were also associated with fibrosis and cirrhosis. These were also related to the NAFLD activity score (NAS) which is derived from the histological severity of steatosis, inflammation and ballooning degeneration. Eukaryotic protein translation and recycling of eIF2:GDP related SNP variants were associated with ballooning, steatohepatitis and cirrhosis. Il2 signaling events mediated by PI3K, Mitotic metaphase/anaphase transition, and Prostanoid ligand receptors were also significantly associated with cirrhosis. Taken together, the results provide evidence for additional ways, beyond the effects of single SNPs, by which genetic factors might contribute to the susceptibility to develop a particular phenotype of NAFLD and then progress to cirrhosis. Further studies are warranted to explain potential important genetic roles of these biological processes in NAFLD.

Liver fatty acid binding protein (L-Fabp) modulates murine stellate cell activation and diet-induced nonalcoholic fatty liver disease

Activation of hepatic stellate cells (HSCs) is crucial to the development of fibrosis in nonalcoholic fatty liver disease. Quiescent HSCs contain lipid droplets (LDs), whose depletion upon activation induces a fibrogenic gene program. Here we show that liver fatty acid-binding protein (L-Fabp), an abundant cytosolic protein that modulates fatty acid (FA) metabolism in enterocytes and hepatocytes, also modulates HSC FA utilization and in turn regulates the fibrogenic program. L-Fabp expression decreased 10-fold following HSC activation, concomitant with depletion of LDs. Primary HSCs isolated from L-FABP(-/-) mice contain fewer LDs than wild-type (WT) HSCs, and exhibit up-regulated expression of genes involved in HSC activation. Adenoviral L-Fabp transduction inhibited activation of passaged WT HSCs and increased both the expression of prolipogenic genes and also augmented intracellular lipid accumulation, including triglyceride and FA, predominantly palmitate. Freshly isolated HSCs from L-FABP(-/-) mice correspondingly exhibited decreased palmitate in the free FA pool. To investigate whether L-FABP deletion promotes HSC activation in vivo, we fed L-FABP(-/-) and WT mice a high-fat diet supplemented with trans-fatty acids and fructose (TFF). TFF-fed L-FABP(-/-) mice exhibited reduced hepatic steatosis along with decreased LD abundance and size compared to WT mice. In addition, TFF-fed L-FABP(-/-) mice exhibited decreased hepatic fibrosis, with reduced expression of fibrogenic genes, compared to WT mice.

CONCLUSION

L-FABP deletion attenuates both diet-induced hepatic steatosis and fibrogenesis, despite the observation that L-Fabp paradoxically promotes FA and LD accumulation and inhibits HSC activation in vitro. These findings highlight the importance of cell-specific modulation of hepatic lipid metabolism in promoting fibrogenesis in nonalcoholic fatty liver disease. (Hepatology 2013).

Clearance of activated stellate cells for hepatic fibrosis regression: molecular basis and translational potential

Hepatic fibrosis, characterized by abnormal accumulation of extracellular matrix (ECM), is a common pathological process of many chronic liver diseases. A growing number of studies have shown that the activation of hepatic stellate cells (HSCs) plays an important role in the pathogenesis of hepatic fibrosis. Inhibiting the activation of HSCs and accelerating the clearance of activated HSCs may be effective strategies for resolution of hepatic fibrosis. Therefore, understanding the underlying mechanisms of clearance of activated HSCs and the therapeutic implications is an active subject of research. Studies have shown that apoptosis, immune clearance, phenotype reversion and senescence are involved in clearance of activated HSCs. In this review, we will discuss the mechanisms of clearance of activated HSCs and their potential in resolution of hepatic fibrosis.

Mitochondrial uncouplers inhibit hepatic stellate cell activation

Background

Mitochondrial dysfunction participates in the progression of several pathologies. Although there is increasing evidence for a mitochondrial role in liver disease, little is known about its contribution to hepatic stellate cell (HSC) activation. In this study we investigated the role of mitochondrial activity through mild uncoupling during in vitro activation of HSCs.

Methods

Cultured primary human and mouse HSCs were treated with the chemical uncouplers FCCP and Valinomycin. ATP levels were measured by luciferase assay and production of reactive oxygen species was determined using the fluorescent probe DCFH-DA. Possible cytotoxicity by uncoupler treatment was evaluated by caspase 3/7 activity and cytoplasmic protease leakage. Activation of HSCs and their response to the pro-fibrogenic cytokine TGF-β was evaluated by gene expression of activation markers and signal mediators using RT-qPCR. Proliferation was measured by incorporation of EdU and protein expression of α-smooth muscle actin was analyzed by immunocytochemistry and western blot.

Results

FCCP and Valinomycin treatment mildly decreased ATP and reactive oxygen species levels. Both uncouplers increased the expression of mitochondrial genes such as Tfam and COXIV while inducing morphological features of quiescent mouse HSCs and abrogating TGF-β signal transduction. Mild uncoupling reduced HSC proliferation and expression of pro-fibrogenic markers of mouse and human HSCs.

Conclusions

Mild mitochondrial uncoupling inhibits culture-induced HSC activation and their response to pro-fibrogenic cytokines like TGF-β. These results therefore suggest mitochondrial uncoupling of HSCs as a strategy to reduce progression of liver fibrosis.

Antilipogenic and anti-inflammatory activities of Codonopsis lanceolata in mice hepatic tissues after chronic ethanol feeding

This study evaluated the antilipogenic and anti-inflammatory effects of Codonopsis lanceolata (C. lanceolata) root extract in mice with alcohol-induced fatty liver and elucidated its underlying molecular mechanisms. Ethanol was introduced into the liquid diet by mixing it with distilled water at 5% (wt/v), providing 36% of the energy, for nine weeks. Among the three different fractions prepared from the C. lanceolata root, the C. lanceolata methanol extract (CME) exhibited the most remarkable attenuation of alcohol-induced fatty liver with respect to various parameters such as hepatic free fatty acid concentration, body weight loss, and hepatic accumulations of triglyceride and cholesterol. The hepatic gene and protein expression levels were analysed via RT-PCR and Western blotting, respectively. CME feeding significantly restored the ethanol-induced downregulation of the adiponectin receptor (adipoR) 1 and of adipoR2, along with their downstream molecules. Furthermore, the study data showed that CME feeding dramatically reversed ethanol-induced hepatic upregulation of toll-like receptor- (TLR-) mediated signaling cascade molecules. These results indicate that the beneficial effects of CME against alcoholic fatty livers of mice appear to be with adenosine- and adiponectin-mediated regulation of hepatic steatosis and TLR-mediated modulation of hepatic proinflammatory responses.

Mitochondria and redox signaling in steatohepatitis

Alcoholic and nonalcoholic fatty liver diseases are potentially pathological conditions that can progress to steatohepatitis, fibrosis, and cirrhosis. These conditions affect millions of people throughout the world in part through poor lifestyle choices of excess alcohol consumption, overnutrition, and lack of regular physical activity. Abnormal mitochondrial and cellular redox homeostasis has been documented in steatohepatitis and results in alterations of multiple redox-sensitive signaling cascades. Ultimately, these changes in signaling lead to altered enzyme function and transcriptional activities of proteins critical to mitochondrial and cellular function. In this article, we review the current hypotheses linking mitochondrial redox state to the overall pathophysiology of alcoholic and nonalcoholic steatohepatitis and briefly discuss the current therapeutic options under investigation.

Pathogenesis of alcohol-induced liver disease: classical concepts and recent advances

Alcoholic liver disease (ALD) is a primary consequence of heavy and prolonged drinking. ALD contributes to the bulk of liver disease burden worldwide. Progression of ALD is a multifactorial and multistep process that includes many genetic and environmental risk factors. The molecular pathogenesis of ALD involves alcohol metabolism and secondary mechanisms such as oxidative stress, endotoxin, cytokines and immune regulators. The histopathological manifestation of ALD occurs as an outcome of complex but controlled interactions between hepatic cell types. Hepatic stellate cells (HSCs) are the key drivers of fibrogenesis, but transformation of hepatocytes to myofibroblastoids also implicate parenchymal cells as playing an active role in hepatic fibrogenesis. Recent discoveries indicate that lipogenesis during the early stages of ALD is a risk for advancement to cirrhosis. Other recently identified novel molecules and physiological/cell signaling pathways include fibrinolysis, osteopontin, transforming growth factor-β-SMAD and hedgehog signaling, and involvement of novel cytokines in hepatic fibrogenesis. The observation that ALD and non-alcoholic steatohepatitis share common pathways and genetic polymorphisms suggests operation of parallel pathogenic mechanisms. Future research involving genomics, epigenomics, deep sequencing and non-coding regulatory elements holds promise to identify novel diagnostic and therapeutic targets for ALD. There is also a need for adequate animal models to study pathogenic mechanisms at the molecular level and targeted therapy.

Role of lipid rafts in liver health and disease

Liver diseases are an increasingly common cause of morbidity and mortality; new approaches for investigation of mechanisms of liver diseases and identification of therapeutic targets are emergent. Lipid rafts (LRs) are specialized domains of cellular membranes that are enriched in saturated lipids; they are small, mobile, and are key components of cellular architecture, protein partition to cellular membranes, and signaling events. LRs have been identified in the membranes of all liver cells, parenchymal and non-parenchymal; more importantly, LRs are active participants in multiple physiological and pathological conditions in individual types of liver cells. This article aims to review experimental-based evidence with regard to LRs in the liver, from the perspective of the liver as a whole organ composed of a multitude of cell types. We have gathered up-to-date information related to the role of LRs in individual types of liver cells, in liver health and diseases, and identified the possibilities of LR-dependent therapeutic targets in liver diseases.

Role for PPARγ in obesity-induced hepatic steatosis as determined by hepatocyte- and macrophage-specific conditional knockouts

Peroxisome proliferator-activated receptor (PPAR) γ is a nuclear receptor central to glucose and lipid homeostasis. PPARγ role in nonalcoholic fatty liver disease is controversial because PPARγ overexpression is a general property of steatotic livers, but its activation by thiazolidinediones reduces hepatic steatosis. Here, we investigated hepatic PPARγ function by using Cre-loxP technology to generate hepatocyte (PPARγ(Δhep))- and macrophage (PPARγ(Δmac))-specific PPARγ-knockout mice. Targeted deletion of PPARγ in hepatocytes, and to a lesser extent in macrophages, protected mice against high-fat diet-induced hepatic steatosis. Down-regulated expression of genes involved in lipogenesis (SCD1, SREBP-1c, and ACC), lipid transport (CD36/FAT, L-FABP, and MTP), and β-oxidation (PPARα and ACO) was observed in PPARγ(Δhep) mice. Moreover, PPARγ(Δhep) mice showed improved glucose tolerance and reduced PEPCK expression without changes in Pcx, Fbp1, and G6Pc expression and CREB and JNK phosphorylation. In precision-cut liver slices (PCLSs) and hepatocytes, rosiglitazone either alone or in combination with oleic acid increased triglyceride accumulation, an effect that was blocked by the PPARγ antagonist biphenol A diglycidyl ether (BADGE). PCLSs and hepatocytes from PPARγ(Δhep) mice showed blunted responses to rosiglitazone and oleic acid, whereas the response to these compounds remained intact in PCLSs from PPARγ(Δmac) mice. Collectively, these findings establish PPARγ expression in hepatocytes as a prosteatotic factor in fatty liver disease.

Fatty liver and global cardiometabolic risk

A nem alkoholos zsírmáj betegség (NAFLD = nonalcoholic fatty liver disease) az „iparilag fejlett” országok felnőtt lakosságának 30%-ában észlelhető. Előrehaladott, súlyosabb formája a nem alkoholos steatohepatitis (NASH = nonalcoholic steatohepatitis), amelyből az esetek 20%-ában cirrhosis alakul ki, a kialakult cirrhosis mintegy 30–40%-a májeredetű halálhoz, elsősorban hepatocellularis carcinomához vezethet. Az NAFLD-t sokan a metabolikus szindróma májbeli manifesztációjának tartják, e két jelenség kapcsolatát tekintjük át, különös tekintettel a hasi elhízás, az inzulinrezisztencia, az oxidatív stressz és a csökkent antioxidáns védelem kóroki szerepére. A májsejtek trigliceridtartalmának növekedése együtt jár az egész szervezet, illetve a máj inzulinrezisztenciájával. Klasszikus tyúk–tojás probléma: a lipidlerakódás oka az inzulinrezisztencia, vagy az inzulinrezisztencia az ectopiás lipidfelhalmozódás következménye? A patogenetikai történések egy lehetséges sorrendje: megnövekedett zsírsavfluxus, a májbeli zsírsavkínálat növekedése → VLDL-túltermelődés, atherogen dyslipidaemia → a zsírsavak fokozott oxidációja és peroxidációja, enormis szabadgyök-terhelés → az antioxidáns védekezőrendszer kimerülése → a gyulladást és az immunválaszt kiváltó mediátorok „cunamiszerű” kiáramlása → a fibrosis progresszióját elősegítő transzkripciós és transzlációs változások → carcinogenesis. Az NAFLD és a metabolikus szindróma egyaránt része a globális kardiometabolikus kockázatnak, így lényeges a korai felismerés, és – amennyiben lehetséges a kezelés – ez ma a metabolikus szindróma részelemeinek kezelési ajánlásaiban testesül meg. Orv. Hetil., 2010, 47, 1946–1950.

Wnt signalling and the control of cellular metabolism

At the cellular level, the biological processes of cell proliferation, growth arrest, differentiation and apoptosis are all tightly coupled to appropriate alterations in metabolic status. In the case of cell proliferation, this requires redirecting metabolic pathways to provide the fuel and basic components for new cells. Ultimately, the successful co-ordination of cell-specific biology with cellular metabolism underscores multicellular processes as diverse as embryonic development, adult tissue remodelling and cancer cell biology. The Wnt signalling network has been implicated in all of these areas. While each of the Wnt-dependent signalling pathways are being individually delineated in a range of experimental systems, our understanding of how they integrate and regulate cellular metabolism is still in its infancy. In the present review we reassess the roles of Wnt signalling in functionally linking cellular metabolism to tissue development and function.

Recent advances in understanding the pathogenesis of nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a pathological condition characterized by macrovesicular steatosis, necroinflammation, loss of hepatocytes and fibrosis. NASH is often associated with type 2 diabetes mellitus, hypertension, hyperlipoproteinemia and obesity. Recent studies emphasize the role of insulin resistance, oxidative stress and subsequent lipid peroxidation, proinflammatory cytokines, adipokines and mitochondrial dysfunction in the development and progression of NASH. In this article, we will review the role of insulin resistance, oxidative stress and subsequent lipid peroxidation, mitochondrial dysfunction, proinflammatory cytokines, adipokines such as resistin, leptin, adiponectin and PPAR-α, apoptosis, NF kappa B, SREBP-1c, endotoxaemia, and iron overload in the pathogenesis of NASH. The pathogenesis of NASH is thought to be related mainly with insulin resistance and oxidative stress and subsequent lipid peroxidation. Adipocytokines also play an important role in the pathogenesis of NASH through complex and interactive paracrine and endocrine mechanisms. Understanding of the mechanisms responsible for the pathogenesis of NASH has important implications for the treatment of NASH.

"Hypothesis of seven balances": molecular mechanisms behind alcoholic liver diseases and association with PPARalpha

OBJECTIVES

The purpose of this review to collate current leading scientific advances of molecular mechanisms in alcoholic liver diseases and to propose a working "hypothesis of seven balances" in relation to peroxisome proliferator activated receptor alpha (PPARalpha), which has important roles in fatty acid oxidation, oxidative stress, inflammatory responses, and possibly liver fibrosis.

METHODS

We conducted an extensive literature review of over a hundred publications and collated the findings with evidence generated in our laboratory.

RESULTS

Our research points to a working hypothesis of seven balances for alcoholic liver diseases consisting of: 1) ethanol oxidation balance in hepatocytes; 2) PPAR alpha activities in liver; 3) fatty acid metabolism balance in hepatic mitochondria; 4) gastrointestinal response to ethanol, acetaldehyde and lipopolysaccharide (LPS); 5) Kupffer cells response to LPS, oxidative stress and inflammatory cytokines; 6) adiponectin levels in plasma interchangeably regulated by tumor necrosis factor-alpha (TNF-alpha); and 7) stellate cells response to all of the above promoting hepatic fibrosis. Cellular mechanisms behind alcoholic liver diseases reveal close temporal associations of PPARalpha, adiponectin, TNF-alpha, cellular inflammation, proliferation, and potentially fibrosis as illustrated in "the hypothesis of seven balances."

CONCLUSIONS

The regulation and adjustment of PPARalpha activation underlying the balance of molecular cascades might resolve the progression of alcoholic liver diseases by reducing oxidative stress and inflammatory effects induced by nuclear factor-kappaB as well as the associated adiponectin pathway. Further elucidation of these pathways would reveal exciting new prospects for treating alcoholic liver diseases and other related liver disorders.

MicroRNA expression profile in Lieber-DeCarli diet-induced alcoholic and methionine choline deficient diet-induced nonalcoholic steatohepatitis models in mice

BACKGROUND

Alcoholic and nonalcoholic steatohepatitis are leading causes of liver diseases worldwide. While of different etiology, these share common pathophysiological mechanisms and feature abnormal fat metabolism, inflammation and fibrosis. MicroRNAs (miRNA) are highly conserved noncoding RNAs that control gene expression at the post-transcriptional level either via the degradation of target mRNAs or the inhibition of translation. Each miRNA controls the expression of multiple targets; miRNAs have been linked to regulation of lipid metabolism and inflammation.

METHODS

We fed Lieber-DeCarli alcohol or methionine-choline-deficient (MCD) diets to C57Bl6 and analyzed livers for histopathology, cytokines by ELISA, alanine aminotransferase (ALT) by biochemical assay, and microRNA profile by microarray.

RESULTS

Both Lieber-DeCarli and MCD diets lead to development of liver steatosis, liver injury, indicated by increased ALT, and elevated levels of serum TNFalpha, suggesting that animal models portray the pathophysiological features of alcoholic and nonalcoholic fatty liver, respectively. We identified that Lieber-deCarli diet up-regulated 1% and down-regulated 1% of known miRNA; MCD diet up-regulated 3% and down-regulated 1% of known miRNA, compared to controls. Of miRNAs that changed expression levels, 5 miRNAs were common in alcoholic and nonalcoholic fatty livers: the expression of both miR-705 and miR-1224 was increased after Lieber-DeCarli or MCD diet feeding. In contrast, miR-182, miR-183, and miR-199a-3p were down-regulated in Lieber-deCarli feeding, while MCD diet lead to their up-regulation, compared to corresponding controls.

CONCLUSIONS

Our findings indicate etiology-specific changes in miRNA expression profile during steatohepatitis models, which opens new avenues for research in the pathophysiology of alcoholic and nonalcoholic fatty liver disease.

MicroRNA expression profile in Lieber-DeCarli diet-induced alcoholic and methionine choline deficient diet-induced nonalcoholic steatohepatitis models in mice

BACKGROUND

Alcoholic and nonalcoholic steatohepatitis are leading causes of liver diseases worldwide. While of different etiology, these share common pathophysiological mechanisms and feature abnormal fat metabolism, inflammation and fibrosis. MicroRNAs (miRNA) are highly conserved noncoding RNAs that control gene expression at the post-transcriptional level either via the degradation of target mRNAs or the inhibition of translation. Each miRNA controls the expression of multiple targets; miRNAs have been linked to regulation of lipid metabolism and inflammation.

METHODS

We fed Lieber-DeCarli alcohol or methionine-choline-deficient (MCD) diets to C57Bl6 and analyzed livers for histopathology, cytokines by ELISA, alanine aminotransferase (ALT) by biochemical assay, and microRNA profile by microarray.

RESULTS

Both Lieber-DeCarli and MCD diets lead to development of liver steatosis, liver injury, indicated by increased ALT, and elevated levels of serum TNFalpha, suggesting that animal models portray the pathophysiological features of alcoholic and nonalcoholic fatty liver, respectively. We identified that Lieber-deCarli diet up-regulated 1% and down-regulated 1% of known miRNA; MCD diet up-regulated 3% and down-regulated 1% of known miRNA, compared to controls. Of miRNAs that changed expression levels, 5 miRNAs were common in alcoholic and nonalcoholic fatty livers: the expression of both miR-705 and miR-1224 was increased after Lieber-DeCarli or MCD diet feeding. In contrast, miR-182, miR-183, and miR-199a-3p were down-regulated in Lieber-deCarli feeding, while MCD diet lead to their up-regulation, compared to corresponding controls.

CONCLUSIONS

Our findings indicate etiology-specific changes in miRNA expression profile during steatohepatitis models, which opens new avenues for research in the pathophysiology of alcoholic and nonalcoholic fatty liver disease.