Leptin and liver tissue repair: do rodent models provide the answers?

Diverting hepatic lipid fluxes with lifestyles revision and pharmacological interventions as a strategy to tackle steatotic liver disease (SLD) and hepatocellular carcinoma (HCC)

Steatotic liver disease (SLD) and Hepatocellular Carcinoma (HCC) are characterised by a substantial rewiring of lipid fluxes caused by systemic metabolic unbalances and/or disrupted intracellular metabolic pathways. SLD is a direct consequence of the interaction between genetic predisposition and a chronic positive energy balance affecting whole-body energy homeostasis and the function of metabolically-competent organs. In this review, we discuss how the impairment of the cross-talk between peripheral organs and the liver stalls glucose and lipid metabolism, leading to unbalances in hepatic lipid fluxes that promote hepatic fat accumulation. We also describe how prolonged metabolic stress builds up toxic lipid species in the liver, and how lipotoxicity and metabolic disturbances drive disease progression by promoting a chronic activation of wound healing, leading to fibrosis and HCC. Last, we provide a critical overview of current state of the art (pre-clinical and clinical evidence) regarding mechanisms of action and therapeutic efficacy of candidate SLD treatment options, and their potential to interfere with SLD/HCC pathophysiology by diverting lipids away from the liver therefore improving metabolic health.

Noninvasive biomarkers in pediatric nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the leading cause of chronic liver disease worldwide among children and adolescents. It encompasses a spectrum of disease, from its mildest form of isolated steatosis, to nonalcoholic steatohepatitis (NASH) to liver fibrosis and cirrhosis, or end-stage liver disease. The early diagnosis of pediatric NAFLD is crucial in preventing disease progression and in improving outcomes. Currently, liver biopsy is the gold standard for diagnosing NAFLD. However, given its invasive nature, there has been significant interest in developing noninvasive methods that can be used as accurate alternatives. Here, we review noninvasive biomarkers in pediatric NAFLD, focusing primarily on the diagnostic accuracy of various biomarkers as measured by their area under the receiver operating characteristic, sensitivity, and specificity. We examine two major approaches to noninvasive biomarkers in children with NAFLD. First, the biological approach that quantifies serological biomarkers. This includes the study of individual circulating molecules as biomarkers as well as the use of composite algorithms derived from combinations of biomarkers. The second is a more physical approach that examines data measured through imaging techniques as noninvasive biomarkers for pediatric NAFLD. Each of these approaches was applied to children with NAFLD, NASH, and NAFLD with fibrosis. Finally, we suggest possible areas for future research based on current gaps in knowledge.

Metabolic dysfunction and cancer in HCV: Shared pathways and mutual interactions

HCV hijacks many host metabolic processes in an effort to aid viral replication. The resulting hepatic metabolic dysfunction underpins many of the hepatic and extrahepatic manifestations of chronic hepatitis C (CHC). However, the natural history of CHC is also substantially influenced by the host metabolic status: obesity, insulin resistance and hepatic steatosis are major determinants of CHC progression toward hepatocellular carcinoma (HCC). Direct-acting antivirals (DAAs) have transformed the treatment and natural history of CHC. While DAA therapy effectively eradicates the virus, the long-lasting overlapping metabolic disease can persist, especially in the presence of obesity, increasing the risk of liver disease progression. This review covers the mechanisms by which HCV tunes hepatic and systemic metabolism, highlighting how systemic metabolic disturbance, lipotoxicity and chronic inflammation favour disease progression and a precancerous niche. We also highlight the therapeutic implications of sustained metabolic dysfunction following sustained virologic response as well as considerations for patients who develop HCC on the background of metabolic dysfunction.

Role of Leptin in the Digestive System

Leptin is a pluripotent peptide hormone produced mainly by adipocytes, as well as by other tissues such as the stomach. Leptin primarily acts on the central nervous system, particularly the hypothalamus, where this hormone regulates energy homeostasis and neuroendocrine function. Owing to this, disruption of leptin signaling has been linked with numerous pathological conditions. Recent studies have also highlighted the diverse roles of leptin in the digestive system including immune regulation, cell proliferation, tissue healing, and glucose metabolism. Of note, leptin acts differently under physiological and pathological conditions. Here, we review the current knowledge on the functions of leptin and its downstream signaling in the gastrointestinal tract and accessory digestive organs, with an emphasis on its physiological and pathological implications. We also discuss the current therapeutic uses of recombinant leptin, as well as its limitations.

Non-alcoholic fatty liver disease as metabolic consequence of obstructive sleep apnea

Obstructive sleep apnea (OSA) as a worldwide prevalent condition carries risk for cardiovascular and metabolic diseases, ultimately increasing overall mortality rates. Non-alcoholic fatty liver disease (NAFLD) can be considered as the primary metabolic disease, but also as a coexisting OSA comorbidity. Although prevalence of NAFLD covers quarter of world population, it increases with OSA presence. It can be speculated that chronic intermittent hypoxia (CIH) and sympathetic nervous system overactivity are involved in NAFLD pathogenesis and progression from simple steatosis through steatohepatitis to fibrosis. CIH provides the environment for liver oxidative stress, inflammation and increases the expression of genes involved in cholesterol and fatty acids synthesis. Catecholamines increase b-oxidation in liver and release free fatty acids from adipose tissue in plasma which inhibit insulin effects. Obesity and insulin resistance as key players in NAFLD development and advancement, deepen vicious circle of oxidative stress, inflammation and dyslipidemia. If not treated, OSA in NAFLD patients has been associated with inflammation, hepatocytes' necrosis, and fibrosis. Continuous positive airway pressure (CPAP) represents gold standard for OSA therapy, allowing the unimpeded air passage through upper parts of respiratory system. However, it has been demonstrated that CPAP therapy have beneficial effects on cardiometabolic outcomes and slow liver degeneration.

Recent Insights into the Pathogenesis of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is a burgeoning health problem worldwide and an important risk factor for both hepatic and cardiometabolic mortality. The rapidly increasing prevalence of this disease and of its aggressive form nonalcoholic steatohepatitis (NASH) will require novel therapeutic approaches based on a profound understanding of its pathogenesis to halt disease progression to advanced fibrosis or cirrhosis and cancer. The pathogenesis of NAFLD involves a complex interaction among environmental factors (i.e., Western diet), obesity, changes in microbiota, and predisposing genetic variants resulting in a disturbed lipid homeostasis and an excessive accumulation of triglycerides and other lipid species in hepatocytes. Insulin resistance is a central mechanism that leads to lipotoxicity, endoplasmic reticulum stress, disturbed autophagy, and, ultimately, hepatocyte injury and death that triggers hepatic inflammation, hepatic stellate cell activation, and progressive fibrogenesis, thus driving disease progression. In the present review, we summarize the currently available data on the pathogenesis of NAFLD, emphasizing the most recent advances. A better understanding of NAFLD/NASH pathogenesis is crucial for the design of new and efficient therapeutic interventions.

Cellular and molecular mechanisms in the pathogenesis of liver fibrosis: An update

There have been considerable recent advances towards a better understanding of the complex cellular and molecular network underlying liver fibrogenesis. Recent data indicate that the termination of fibrogenic processes and the restoration of deficient fibrolytic pathways may allow the reversal of advanced fibrosis and even cirrhosis. Therefore, efforts have been made to better clarify the cellular and molecular mechanisms that are involved in liver fibrosis. Activation of hepatic stellate cells (HSCs) remains a central event in fibrosis, complemented by other sources of matrix-producing cells, including portal fibroblasts, fibrocytes and bone marrow-derived myofibroblasts. These cells converge in a complex interaction with neighboring cells to provoke scarring in response to persistent injury. Defining the interaction of different cell types, revealing the effects of cytokines on these cells and characterizing the regulatory mechanisms that control gene expression in activated HSCs will enable the discovery of new therapeutic targets. Moreover, the characterization of different pathways associated with different etiologies aid in the development of disease-specific therapies. This article outlines recent advances regarding the cellular and molecular mechanisms involved in liver fibrosis that may be translated into future therapies. The pathogenesis of liver fibrosis associated with alcoholic liver disease, non-alcoholic fatty liver disease and viral hepatitis are also discussed to emphasize the various mechanisms involved in liver fibrosis.

Cellular and molecular mechanisms in liver fibrogenesis

Liver fibrogenesis is a dynamic and highly integrated molecular, tissue and cellular process, potentially reversible, that drives the progression of chronic liver diseases (CLD) towards liver cirrhosis and hepatic failure. Hepatic myofibroblasts (MFs), the pro-fibrogenic effector cells, originate mainly from activation of hepatic stellate cells and portal fibroblasts being characterized by a proliferative and survival attitude. MFs also contract in response to vasoactive agents, sustain angiogenesis and recruit and modulate activity of cells of innate or adaptive immunity. Chronic activation of wound healing and oxidative stress as well as derangement of epithelial-mesenchymal interactions are "major" pro-fibrogenic mechanisms, whatever the etiology. However, literature has outlined a complex network of pro-fibrogenic factors and mediators proposed to modulate CLD progression, with some of them being at present highly debated in the field, including the role of epithelial to mesenchymal transition and Hedgehog signaling pathways. Hypoxia and angiogenesis as well as inflammasomes are recently emerged as ubiquitous pro-inflammatory and pro-fibrogenic determinants whereas adipokines are mostly involved in CLD related to metabolic disturbances (metabolic syndrome and/or obesity and type 2 diabetes). Finally, autophagy as well as natural killer and natural killer-T cells have been recently proposed to significantly affect fibrogenic CLD progression.

Hepatitis C virus core protein stimulates fibrogenesis in hepatic stellate cells involving the obese receptor

Hepatitis C virus core protein (HCVcp), which is secreted by infected cells, is reported as an immunomodulator in immune cells. However, the effects of HCVcp on hepatic stellate cells (HSCs), the key cells in liver fibrosis, still remain unclear. In this study, we investigated the effects of HCVcp on obese receptor (ObR) related downstream signaling pathways and fibrogenic gene expression in HSCs. LX-2, a human HSC line, was incubated with HCVcp. Inhibitors and short interfering RNAs were used to interrogate the mechanisms of HCVcp action on HSCs. HCVcp (20-100 ng/ml) concentration-dependently stimulated α-smooth muscle actin (α-SMA) protein expression and mRNA expression of α-SMA, procollagen α2(I) and TGF-β1 genes, with a plateau of 220% of controls at 100 ng/ml. HCVcp induced mRNA and protein expression of ObR. Blocking of Ob-Rb with a neutralizing antibody inhibited phosphorylation of signal transducer and activator of transcription 3 (STAT3) and AMPKα stimulated by HCVcp. Furthermore, knockdown of Ob-Rb down-regulated HCVcp-induced STAT3, AKT, and AMPKα phosphorylation, and reversed HCVcp-suppressed mRNA expression of matrix metalloproteinase (MMP)-1, peroxisome proliferator-activated receptor (PPAR)γ and sterol regulatory element binding protein-1c (SREBP-1c) genes. AMPKα signaling blockade reversed HCVcp-suppressed SREBP-1c mRNA expression. HCVcp stimulated reactive oxygen species formation and gp91(phox) (a component of NADPH oxidase) protein expression, together with AKT phosphorylation, leading to suppression of PPARγ and SREBP-1c genes. Our results provide a new finding that HCVcp induced ObR-dependent Janus Kinase (JAK) 2-STAT3, AMPKα, and AKT signaling pathways and modulated downstream fibrogenetic gene expression in HSCs.

Retinoids ameliorate insulin resistance in a leptin-dependent manner in mice

Transgenic mice expressing dominant-negative retinoic acid receptor (RAR) α specifically in the liver exhibit steatohepatitis, which leads to the development of liver tumors. Although the cause of steatohepatitis in these mice is unknown, diminished hepatic expression of insulin-like growth factor-1 suggests that insulin resistance may be involved. In the present study, we examined the effects of retinoids on insulin resistance in mice to gain further insight into the mechanisms responsible for this condition. Dietary administration of all-trans-retinoic acid (ATRA) significantly improved insulin sensitivity in C57BL/6J mice, which served as a model for high-fat, high-fructose diet-induced nonalcoholic fatty liver disease (NAFLD). The same effect was observed in genetically insulin-resistant KK-A(y) mice, occurring in concert with activation of leptin-signaling pathway proteins, including signal transducer and activator of transcription 3 (STAT3) and Janus kinase 2. However, such an effect was not observed in leptin-deficient ob/ob mice. ATRA treatment significantly up-regulated leptin receptor (LEPR) expression in the livers of NAFLD mice. In agreement with these observations, in vitro experiments showed that in the presence of leptin, ATRA directly induced LEPR gene expression through RARα, resulting in enhancement of STAT3 and insulin-induced insulin receptor substrate 1 phosphorylation. A selective RARα/β agonist, Am80, also enhanced hepatic LEPR expression and STAT3 phosphorylation and ameliorated insulin resistance in KK-A(y) mice.

CONCLUSION

We discovered an unrecognized mechanism of retinoid action for the activation of hepatic leptin signaling, which resulted in enhanced insulin sensitivity in two mouse models of insulin resistance. Our data suggest that retinoids might have potential for treating NAFLD associated with insulin resistance.

Progress in understanding the role of leptin in the pathogenesis and treatment of chronic hepatitis C

Leptin is an adipokine that is abundantly expressed in adipose tissue and has multiple biological effects related to the development of human diseases, such as diabetes mellitus, obesity, metabolism syndrome, and cancer. Pegylated interferon plus ribavirin has been considered as fist-line therapy in patients with chronic hepatitis C (CHC). Because this therapy is associated with many side effects, further studies on the mechanism and treatment of CHC are needed. More and more studies have demonstrated that leptin plays an important role in the pathogenesis of CHC and provides a new target for the treatment of this disease. This paper reviews the recent advances in understanding the role of leptin in the pathogenesis and treatment of CHC.

Dietary protein restriction induces steatohepatitis and alters leptin/signal transducers and activators of transcription 3 signaling in lactating rats

Dietary protein restriction during lactation affects lipid metabolism and food intake in rats. The goals of this study were to determine the effect of a low-protein diet on a liver damage in lactating rats, to determine whether dietary protein restriction of lactating dams affects the liver health of their offspring and to elucidate the molecular mechanisms underlying the development of hepatic damage. Lactating Sprague-Dawley rats were fed either a control 20% protein diet or an 8% low-protein diet for 11 or 23 days, respectively. After weaning, the offspring were continuously fed either the same control diet or the low-protein diet for an additional 22 days. Feeding a low-protein diet during lactation caused steatohepatitis with severe steatosis, lobular inflammation, ballooning degeneration and fibrosis. Offspring nourished by dams fed a low-protein diet showed simple hepatic steatosis. Combined effects of increased lipogenesis, decreased fatty acid oxidation and impaired very-low-density lipoprotein secretion were responsible for the development of hepatic steatosis. Hepatic up-regulation of genes linked to oxidative stress including nicotinamide adenine dinucleotide phosphate oxidase, inflammation and fibrogenesis supports the development of steatohepatitis in protein-restricted lactating rats. Furthermore, protein-restricted lactating rats showed activation of the leptin/signal transducers and activators of the transcription 3 signaling pathway. Taken together, oxidative stress induced by up-regulation of nicotinamide adenine dinucleotide phosphate oxidase with activation of leptin/signal transducers and activators of the transcription 3 signaling was responsible for development of steatohepatitis in protein-restricted lactating rats. Our findings suggest that protein malnutrition has a potential to induce steatohepatitis/hepatic steatosis in lactating mothers and infants during breast-feeding.

Mammalian target of rapamycin mediates the angiogenic effects of leptin in human hepatic stellate cells

Leptin modulates the angiogenic properties of hepatic stellate cells (HSC), but the molecular mechanisms involved are poorly understood. We investigated the pathways regulating hypoxia-inducible factor 1α (HIF-1α) and vascular endothelial growth factor (VEGF) in leptin-stimulated myofibroblastic HSC. Exposure to leptin enhanced the phosphorylation of TSC2 on T1462 residues and of p70 S6 kinase and the translational inhibitor 4E-binding protein-1, indicating the ability of leptin to activate the mammalian target of rapamycin (mTOR) pathway. Similar findings were observed when HSC were exposed to PDGF. Both leptin and PDGF increased the expression of HIF-1α and VEGF in HSC. In the presence of rapamycin, a specific mTOR inhibitor, leptin and PDGF were no longer able to activate mTOR, and expression of VEGF was reduced, whereas HIF-1α abundance was not affected. Moreover, knockdown of Raptor, a component of the mTORC1 complex, reduced the ability of leptin to increase VEGF. mTOR was also necessary for leptin- and PDGF-dependent increase in HSC migration. Leptin increased the generation of reactive oxygen species in HSC, which was reduced by NADP(H) oxidase inhibitors. Both N-acetyl cysteine and diphenylene iodonium, a NADP(H) inhibitor, inhibited the expression of HIF-1α and VEGF stimulated by leptin or PDGF. Finally, conditioned media from HSC treated with leptin or PDGF induced tube formation in cultured human umbilical vein endothelial cells. In conclusion, in HSC exposed to leptin or PDGF, increased expression of VEGF requires both activation of mTOR and generation of reactive oxygen species via NADPH-oxidase. Induction of HIF-1α requires NADP(H) oxidase but not mTOR activation.

Role of adiponectin in the pathogenesis and treatment of nonalcoholic fatty liver disease

Adiponectin is an insulin-sensitizing adipokine possessing multiple beneficial effects on nonalcoholic fatty liver disease. This adipokine is secreted from adipocytes into the circulation as three oligomeric isoforms: trimer, hexamer and the high molecular weight (HMW) oligomeric complex. Adiponectin binds to its receptor to exert its effects on target organs. The hepato-protective activities of adiponectin have been demonstrated by many clinical and experimental studies. Decreased level of serum adiponectin represents an independent risk factor for (NAFLD and liver dysfunction in humans. In animals, elevation of circulating adiponectin by either pharmacological or genetic approaches leads to a significant alleviation of hepatomegaly, steatosis and necro-inflammation associated with various liver diseases. In adiponectin knockout mice, there is a pre-existing condition of hepatic steatosis and mitochondrial dysfunction, which might contribute to increased vulnerability of these mice to secondary liver injuries induced by obesity and other conditions. This review aims to summarize the recent advances in research of the structural, molecular and cellular mechanisms underlying the hepato-protective properties of adiponectin.

Pathophysiology of non-alcoholic steatohepatitis and basis for treatment

Non-alcoholic fatty liver disease (NAFLD) is likely the most common cause of liver disease in adults as well as in children and adolescents. Its occurrence is closely associated with obesity and insulin resistance. NAFLD may lead to non-alcoholic steatohepatitis (NASH) with possible evolution towards cirrhosis and hepatocellular carcinoma. In addition to steatosis, NASH is characterized by necroinflammation and fibrosis. While the presence of simple steatosis can be assessed by imaging studies, the occurrence of NASH and its staging requires a liver biopsy. Along these lines, major efforts are directed at identifying non-invasive methodologies able to discriminate simple NAFLD from NASH and to predict the stage of fibrotic evolution. Current treatment relies on weight loss and exercise, although various insulin-sensitizing agents, antioxidants and anti-inflammatory and antifibrogenic agents are under evaluation.

Potential role of leptin, adiponectin and three novel adipokines--visfatin, chemerin and vaspin--in chronic hepatitis

Chronic hepatitis C (CHC) is generally a slowly progressive disease, but some factors associated with rapid progression have been identified. Steatosis, independently of its metabolic or viral origin, leads to liver injury and fibrosis. It is suggested that hepatitis C virus may contribute to a wide spectrum of metabolic disturbances-namely, steatosis, insulin resistance, increased prevalence of impaired glucose tolerance, type 2 diabetes mellitus and lipid metabolism abnormalities. Adipokines, which are produced mainly by adipose tissue, may influence the inflammatory response and insulin sensitivity and contribute to the development of metabolic abnormalities in CHC and also regulate fibrogenesis and angiogenesis. Visfatin was described as an adipokine with immunomodulating and proinflammatory properties that promotes B-cell maturation and enhances activation of leukocytes, synthesis of adhesion molecules and production of proinflammatory cytokines. Visfatin exerts insulin-mimetic effects, decreases plasma glucose levels and regulates cell energy balance. Chemerin stimulates chemotaxis of dendritic cells, macrophages and natural killer (NK) cells toward the site of inflammation. On the other hand, it inhibits synthesis of proinflammatory mediators and enhances adiponectin production, influences adipocyte differentiation and maturation and regulates glucose uptake in adipocytes. Vaspin expression in human adipose tissue seems to be a compensatory mechanism associated with obesity and insulin resistance. Vaspin suppresses leptin, tumor necrosis factor (TNF)-α and resistin expression. Leptin protects against liver steatosis but accelerates fibrosis progression and exacerbates the inflammatory process. In contrast, adiponectin exerts a hepatoprotective effect. In this report, data indicating a possible role of these adipokines in the pathogenesis of chronic hepatitis are summarized.

Pathogenesis of liver fibrosis

Liver fibrosis is a major cause of morbidity and mortality worldwide due to chronic viral hepatitis and, more recently, from fatty liver disease associated with obesity. Hepatic stellate cell activation represents a critical event in fibrosis because these cells become the primary source of extracellular matrix in liver upon injury. Use of cell-culture and animal models has expanded our understanding of the mechanisms underlying stellate cell activation and has shed new light on genetic regulation, the contribution of immune signaling, and the potential reversibility of the disease. As pathways of fibrogenesis are increasingly clarified, the key challenge will be translating new advances into the development of antifibrotic therapies for patients with chronic liver disease.

Serum surrogate markers of liver fibrosis in primary biliary cirrhosis

BACKGROUND

Hyaluronan, leptin, laminin and collagen IV have been used extensively for the assessment of liver fibrosis. The aim of this study was to assay these markers in the peripheral and hepatic vein blood of primary biliary cirrhosis (PBC) patients and to study their ability to discriminate early from advanced disease.

METHODS

Sera from 62 PBC patients were compared to 60 controls, 44 chronic Hepatitis C, 38 hepatocellular carcinoma and 34 viral cirrhosis patients. Serum from the hepatic vein of 15 cirrhotic PBC patients and 17 patients with viral cirrhosis was also assayed.

RESULTS

All disease groups had significantly increased levels of hyaluronan and collagen IV, compared to controls, while laminin was significantly increased only in viral cirrhosis. Hyaluronan levels were statistically different between early (54.5 ng/ml; 95%CI 27.3-426.9) and late PBC (154.5 ng/ml; 95%CI 55.3-764.4, p<0.05). The area under the curve (AUC) for the identification of late PBC was 0.74 for hyaluronan, 0.63 for leptin, 0.59 for laminin and 0.70 for collagen IV. Hyaluronan had high sensitivity and NPV in identifying late stages of PBC (96% and 90%, respectively). Short term UDCA had no effect on these markers.

CONCLUSION

No single measurement can differentiate between advanced and early fibrosis in PBC. However serum hyaluronan is a promising single serum marker for longitudinal studies in PBC.

Advances in understanding the roles of leptin and adiponectin in the pathogenesis of chronic liver diseases

Leptin and adiponectin are adipokines that are abundantly expressed in adipose tissue and have multiple biological effects related to the development of human diseases. More and more studies have demonstrated that adipokines play important roles in the pathogenesis of steatosis, steatohepatitis and liver fibrosis, especially nonalcoholic fatty liver disease (NAFLD) and chronic hepatitis C (CHC). This paper reviews the recent advances in understanding the roles of leptin and adiponectin in the pathogenesis of liver diseases.

Insulin resistance, adipose depots and gut: interactions and pathological implications

This review article focuses on the many metabolic actions of insulin at the level of muscle, liver and adipose tissue. In terms of pathogenetic mechanisms, the condition of insulin resistance is complex, as multiple genetic and environmental factors, among which an increasingly sedentary lifestyle associated with high-fat diet, mutually interact according to variable patterns in time in any given individual. It is well recognized that obesity (in particular abdominal obesity) favours the development of insulin resistance. Here we evaluate the impact of obesity and ectopic fat accumulation (visceral and hepatic) on insulin resistance at the level of different target organs, i.e., muscle, liver and adipose tissue. The roles of the gut and the liver, in particular of bile acids and gut microflora, are also discussed as possible determinants of energy balance and glucose metabolism.

Protective roles of adiponectin in obesity-related fatty liver diseases: mechanisms and therapeutic implications

Adiponectin is an insulin-sensitizing adipokine possessing multiple beneficial effects on obesity-related medical complications. This adipokine is secreted from adipocytes into the circulation as three oligomeric isoforms, including trimer, hexamer and the high molecular weight (HMW) oligomeric complex. Each oligomeric isoform of adiponectin possesses distinct biological properties and activates different signaling pathways in various target tissues. The hepato-protective activities have been demonstrated by many clinical and experimental studies. The decreased level of serum adiponectin represents an independent risk factor for nonalcoholic fatty liver disease (NAFLD) and liver dysfunctions in humans. In animals, elevation of circulating adiponectin by either pharmacological or genetic approaches leads to a significant alleviation of hepatomegaly, steatosis and necro-inflammation associated with various liver diseases. In adiponectin knockout mice, there is a pre-existing condition of hepatic steatosis and mitochondria dysfunction, which might contribute to the increased vulnerabilities of these mice to the secondary liver injuries induced by obesity and other conditions. This review aims to summarize recent advances on delineation of the structural, molecular and cellular mechanisms underlying the hepato-protective properties of adiponectin.

Metformin in patients with non-alcoholic fatty liver disease: a randomized, controlled trial

OBJECTIVE

The antidiabetic agent metformin is regularly discussed as a promising treatment for non-alcoholic fatty liver disease (NAFLD), which is characterized by insulin resistance. However, the evidence for its beneficial effects is limited, and conflicting reports have been published. The purpose of this study was to conduct a randomized, double-blind, placebo-controlled trial to test whether metformin improves liver histology in patients with non-alcoholic fatty liver disease.

MATERIAL AND METHODS

Forty-eight patients with biopsy-proven NAFLD were randomized to treatment with metformin (n=24) or placebo (n=24) for 6 months. A second liver biopsy was obtained in all subjects who completed the trial (n=44). Data analyses are restricted to this group (per-protocol analyses). The primary outcome was changes in histologically assessed liver steatosis. Secondary outcomes were changes in NAFLD activity (NAS)-score, liver steatosis assessed by computed tomography (CT), liver transaminases, body-weight, metabolic variables and inflammatory markers.

RESULTS

No significant differences between treatment with metformin or placebo were observed for changes in liver steatosis, assessed either histologically or by CT, NAS-score, liver transaminases or on markers of insulin resistance or inflammation. In contrast, beneficial effects of metformin were observed on changes in body-weight (p<0.001), serum levels of cholesterol (p=0.004), LDL-cholesterol (p<0.001), glucose (p=0.032) and on HbA1c (p=0.020).

CONCLUSIONS

Treatment with metformin for 6 months was no better than placebo in terms of improvement in liver histology in patients with NAFLD. Nevertheless, the use of metformin could still be beneficial in this group as it is associated with a reduction in serum levels of lipids and glucose. (ClinicalTrials.gov number, NCT00303537).

Adipokines in liver diseases

Adipokines are polypeptides secreted in the adipose tissue in a regulated manner. While some of these molecules are expressed only by adipocytes, resident and infiltrating macrophages and components of the vascular stroma markedly contribute to expression of other adipokines. As a result, adipose tissue inflammation is associated with a modification in the pattern of adipokine secretion. Leptin, adiponectin, and resistin are the best-studied molecules in this class, but cytokines such as tumor necrosis factor or interleukin-6 are also secreted at high levels by the adipose tissue. Several other molecules have been recently identified and are actively investigated. Adipokines interfere with hepatic injury associated with fatty infiltration, differentially modulating steatosis, inflammation, and fibrosis. Several studies have investigated plasma levels of adiponectin in patients with nonalcoholic fatty liver disease, to establish correlations with the underlying state of insulin resistance and with the type and severity of hepatic damage. Hepatitis C is another disease where adipokines may represent a link between viral infection, steatosis, and metabolic disturbances. Identification of the mediators secreted by expanded adipose tissue and their pathogenic role is pivotal in consideration of the alarming increase in the prevalence of obesity and of the detrimental role that this condition exerts on the course of liver diseases.

Mechanisms of non-alcoholic steatohepatitis: pathophysiology and molecular biology

Non-alcoholic steatohepatitis (NASH), characterized by liver fatty infiltration, inflammation, hepatocellular injury and fibrosis, may easily develop into liver cirrhosis and hepatocellular carcinoma. The increased flow of FFAs (free fatty acids) to the liver and the de novo lipogenesis in the liver lead to fat overload. Lipotoxicity can induce oxidative stress, inflammatory reaction and apoptosis. Subsequently chronic liver injury activates a fibrogenic response that accelerates the evolution of NASH towards end-stage liver disease. Further research on pathophysiology and molecular biology is beneficial to clinical diagnosis and management of NASH.

Egr-1 is involved in the inhibitory effect of leptin on PPARgamma expression in hepatic stellate cell in vitro

AIMS

Hepatic stellate cell (HSC) activation is a key step in the hepatic fibrogenic process. Increasing evidence demonstrates the pro-fibrogenic action of leptin in rodent liver. Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a potential molecular target for inhibition of HSC activation. Our previous study suggested that leptin markedly down-regulated PPARgamma gene expression in HSCs. The aim of this study is to explore the molecular mechanisms underlying the inhibitory effect of leptin on PPARgamma expression in rat HSCs in vitro.

MAIN METHODS

The effects of leptin on the expression and trans-activation activity of early growth response-1 (Egr-1) are examined by using real-time PCR, Western blotting analysis, transient transfection, and electrophoretic mobility shift assay. The role of Egr-1 in PPARgamma gene expression is demonstrated by co-transfection approach, Western blotting analysis and real-time PCR.

KEY FINDINGS

We document that leptin increases Egr-1 expression at protein and mRNA levels, and significantly stimulates Egr-1 trans-activation activity. Moreover, leptin induces the expression and activity of Egr-1 through activation of extracellular signal-regulated kinase (ERK) or phosphatidylinositol 3-kinase/AKT signaling (PI-3K/AKT) pathway. Further investigation reveals that Egr-1 exerts a clear inhibitory effect on the promoter activity and expression of PPARgamma gene and demonstrates that Egr-1 increases the expression of HSC activation markers and promotes HSC growth. Taken together, these findings suggest that Egr-1 is involved in the inhibitory effect of leptin on PPARgamma expression in rat HSCs in vitro.

SIGNIFICANCE

Our results provide novel insights into the mechanisms of leptin-induced inhibition of PPARgamma expression in HSCs in vitro.

Glycogenic induction of thyroid hormone conversion and leptin system activation in the liver of postpartum dairy cows

In the regulation of energy metabolism, the liver plays an important role in the reinforcement of energy production. In periparturient cows the energy homeostasis turns into a negative energy balance that may shift the physiological regulation of energy balance towards pathological processes. Propylene glycol (PG), as a complementary source of energy used in the nutrition of dairy cows, alters systemic thyroid hormone economy; however, the exact mechanism through which highly glycogenic feed supplements impact liver metabolism is little known. Previous studies showed that only leptin receptors are expressed in the liver of cows, and now we report that leptin mRNA is expressed in the liver of cows as well. The present results show that the mRNA of leptin and its receptors are differentially modulated by the increased energy content of the feed consumed. Simultaneous changes in hepatic type I deiodinase activity suggest that hepatic modulation of the leptin system by PG supplementation may be mediated by an increased local thyroxine-triiodothyronine conversion. Since PG supplementation with simultaneous T4-T3 turnover and increased hepatic leptin- and short-form leptin receptor mRNA were not associated with a significant change in hepatic total lipid levels, it is suggested that the leptin system, directly or indirectly modulated by thyroid hormones, may represent a local defence mechanism to prevent fatty liver formation.

Identification of leptin gene expression in sinusoidal endothelial rat liver cells

Sinusoidal endothelial liver cells (SECs) have a key role in the pathophysiology of chronic liver disease. Leptin is an important profibrogenic and proinflammatory cytokine whose expression in sinusoidal endothelial liver has not been documented. The authors studied the potential of rat SECs to express the leptin and leptin receptor genes. Two cell lines of rat SECs were generated from a male rat liver by pronase-collagenase perfusion and dilution cloning. They were characterized according to morphology, ploidy, von Willebrand antigen immunoreactivity, CD31 transcription, matrix metalloproteinase secretion, and pseudocapillary formation. Expression of the leptin and leptin receptor genes was studied using qualitative reverse transcriptase-polymerase chain reaction. Both cell lines fulfilled the accepted criteria for consideration as being derived from the liver sinusoidal endothelium. Confluent monolayers of both cell lines transcribed leptin and leptin receptor genes. This work demonstrated that SECs can transcribe the leptin gene in vitro, cotranscribing with the leptin receptor gene. Leptin production and signaling at this level could be of paramount importance in liver physiopathology; further studies of this issue are warranted because it represents a potential intervention point during chronic liver diseases.

Partial leptin deficiency favors diet-induced obesity and related metabolic disorders in mice

Partial leptin deficiency is not uncommon in the general population. We hypothesized that leptin insufficiency could favor obesity, nonalcoholic steatohepatitis (NASH), and other metabolic abnormalities, particularly under high calorie intake. Thus, mice partially deficient in leptin (ob/+) and their wild-type (+/+) littermates were fed for 4 mo with a standard-calorie (SC) or a high-calorie (HC) diet. Some ob/+ mice fed the HC diet were also treated weekly with leptin. Our results showed that, when fed the SC diet, ob/+ mice did not present significant metabolic abnormalities except for elevated levels of plasma adiponectin. Under high-fat feeding, increased body fat mass, hepatic steatosis, higher plasma total cholesterol, and glucose intolerance were observed in +/+ mice, and these abnormalities were further enhanced in ob/+ mice. Furthermore, some metabolic disturbances, such as blunted plasma levels of leptin and adiponectin, reduced UCP1 expression in brown adipose tissue, increased plasma liver enzymes, beta-hydroxybutyrate and triglycerides, and slight insulin resistance, were observed only in ob/+ mice fed the HC diet. Whereas de novo fatty acid synthesis in liver was decreased in +/+ mice fed the HC diet, it was disinhibited in ob/+ mice along with the restoration of the expression of several lipogenic genes. Enhanced expression of several genes involved in fatty acid oxidation was also observed only in ob/+ animals. Leptin supplementation alleviated most of the metabolic abnormalities observed in ob/+ fed the HC diet. Hence, leptin insufficiency could increase the risk of obesity, NASH, glucose intolerance, and hyperlipidemia in a context of calorie overconsumption.

Molecular basis and mechanisms of progression of non-alcoholic steatohepatitis

Non-alcoholic steatohepatitis (NASH), a cause of cirrhosis and hepatocellular carcinoma, is characterized by fatty infiltration of the liver, inflammation, hepatocellular damage and fibrosis. Progress has been made in understanding the molecular and cellular mechanisms implicated in the pathogenesis of this condition, therefore, we here review recent developments regarding the basic mechanisms of NASH development. Accumulation of triglycerides in the hepatocytes is the result of increased inflow of free fatty acids and de novo lipogenesis. Steatosis leads to lipotoxicity, which causes apoptosis, necrosis, generation of oxidative stress and inflammation. The resulting chronic injury activates a fibrogenic response that leads eventually to end-stage liver disease. A better understanding of these mechanisms is crucial for the design of novel diagnostic and therapeutic strategies.

In vivo confocal laser laparoscopy allows real time subsurface microscopy in animal models of liver disease

BACKGROUND/AIMS

Histopathology is essential in the diagnostic workup of most liver diseases. However, biopsy sampling might carry risks, is subject to sampling error, and does not provide dynamic tissue imaging. Therefore a newly developed miniaturised confocal probe was evaluated for in vivo microscopic imaging in rodent models of human liver diseases.

METHODS

The handheld laparoscopy probe used a 488nm single line laser for fluorophore excitation. Optical slice thickness was 7 microm, lateral resolution 0.7 microm. Imaging depth was 0-250 microm below the tissue surface. Imaging using different fluorescent staining protocols was performed in healthy mice, IFN-gamma- and IL-12-induced hepatitis, after bile duct ligation, and in ob/ob mice.

RESULTS

Confocal imaging permitted microscopic imaging of the liver in vivo at high resolution. Landmarks of liver diseases such as inflammatory infiltration, steatosis, bile duct proliferation, pericellular fibrosis and perfusion anomalies could be characterised, localised and differentiated in vivo.

CONCLUSIONS

In vivo confocal microscopy with this newly developed probe allows real-time subsurface imaging and characterisation of normal and pathologic liver tissue at high resolution. Since this technology is suited for laparoscopy in humans, it has the potential to impact on the diagnostic workup of liver diseases and to permit dynamic monitoring of (patho-)physiologic events in vivo.