Drinking coffee burns hepatic fat by inducing lipophagy coupled with mitochondrial β-oxidation

PPAR-gamma agonists: Potential modulators of autophagy in obesity

Autophagy pathways are involved in the pathogenesis of some obesity related health problems. As obesity is a nutrient sufficiency condition, autophagy process can be altered in obesity through AMP activated protein kinase (AMPK) inhibition. Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) as the main modulator of adipogenesis process can be effective in the regulation of obesity related phenotypes. As well, it has been revealed that PPAR-gamma and its agonists can regulate autophagy in different normal or cancer cells. However, their effects on autophagy modulation in obesity have been investigated in the limited number of studies. In the current comprehensive mechanistic review, we aimed to investigate the possible mechanisms of action of PPAR-gamma on the process of autophagy in obesity through narrating the effects of PPAR-gamma on autophagy in the non-obesity conditions. Moreover, mode of action of PPAR-gamma agonists on autophagy related implications comprehensively reviewed in the various studies. Understanding the different effects of PPAR-gamma agonists on autophagy in obesity can help to develop a new approach to management of obesity.

Autophagy in liver diseases

Autophagy is the liver cell energy recycling system regulating a variety of homeostatic mechanisms. Damaged organelles, lipids and proteins are degraded in the lysosomes and their elements are re-used by the cell. Investigations on autophagy have led to the award of two Nobel Prizes and a health of important reports. In this review we describe the fundamental functions of autophagy in the liver including new data on the regulation of autophagy. Moreover we emphasize the fact that autophagy acts like a two edge sword in many occasions with the most prominent paradigm being its involvement in the initiation and progress of hepatocellular carcinoma. We also focused to the implication of autophagy and its specialized forms of lipophagy and mitophagy in the pathogenesis of various liver diseases. We analyzed autophagy not only in well studied diseases, like alcoholic and nonalcoholic fatty liver and liver fibrosis but also in viral hepatitis, biliary diseases, autoimmune hepatitis and rare diseases including inherited metabolic diseases and also acetaminophene hepatotoxicity. We also stressed the different consequences that activation or impairment of autophagy may have in hepatocytes as opposed to Kupffer cells, sinusoidal endothelial cells or hepatic stellate cells. Finally, we analyzed the limited clinical data compared to the extensive experimental evidence and the possible future therapeutic interventions based on autophagy manipulation.

Autophagy and Liver Diseases

Autophagy plays an important role in the physiology and pathology of the liver. It is involved in the development of many liver diseases such as α-1-antitrypsin deficiency, chronic hepatitis virus infection, alcoholic liver disease, nonalcoholic fatty liver disease, and liver cancer. Autophagy has thus become a new target for the treatment of liver diseases. How to treat liver diseases by regulating autophagy has been a hot topic.

Granulocyte Colony Stimulating Factor Ameliorates Hepatic Steatosis Associated with Improvement of Autophagy in Diabetic Rats

BACKGROUND

We previously reported that the granulocyte colony stimulating factor (G-CSF) ameliorated hepatic steatosis with the enhancement of β-oxidation-related gene expression. However, the mechanisms underlying this process remain unclear. This study aimed to determine whether the improvement of hepatic steatosis by G-CSF was associated with autophagy in a rat model of diabetes.

METHODS

Eight rats were fed a standard diet, and 16 rats were fed high-fat diet (HFD) for 5 weeks. All HFD-fed rats were then injected with streptozotocin (STZ). One week later, HFD rats injected with STZ were randomly treated with either G-CSF (200 μg/kg/day; diabetes mellitus (DM)/G-CSF) or saline (DM/saline) for 5 consecutive days. Four weeks later, serum biochemical and histology analyses were conducted. The expression of autophagy-associated proteins was determined by Western blotting. The mRNA expression of β-oxidation-related genes was determined by quantitative real-time polymerase chain reaction. HepG2 cells were cultured under high glucose (HG) conditions with G-CSF treatment, followed by Oil Red O staining for quantification of lipids.

RESULTS

Histological analysis showed lower lipid accumulation in the DM/G-CSF group than in the DM/saline-treated rats. Protein levels of LC3 and beclin-1 were higher, and those of p62 were lower in the DM/G-CSF rats than in the DM/saline-treated rats. The mRNA expression of β-oxidation-related genes was higher in DM/G-CSF rats than in the DM/saline-treated rats. Quantification of lipid levels in HepG2 cells cultured with HG and G-CSF treatment revealed no significant differences.

CONCLUSIONS

Our data suggested that G-CSF potentially improves hepatic steatosis and autophagy in the liver of diabetic rats.

Autophagy in hepatic adaptation to stress

Autophagy is an evolutionarily ancient process whereby eukaryotic cells eliminate disposable or potentially dangerous cytoplasmic material, to support bioenergetic metabolism and adapt to stress. Accumulating evidence indicates that autophagy operates as a critical quality control mechanism for the maintenance of hepatic homeostasis in both parenchymal (hepatocytes) and non-parenchymal (stellate cells, sinusoidal endothelial cells, Kupffer cells) compartments. In line with this notion, insufficient autophagy has been aetiologically involved in the pathogenesis of multiple liver disorders, including alpha-1-antitrypsin deficiency, Wilson disease, non-alcoholic steatohepatitis, liver fibrosis and hepatocellular carcinoma. Here, we critically discuss the importance of functional autophagy for hepatic physiology, as well as the mechanisms whereby defects in autophagy cause liver disease.

[Role of lipophagy in the regulation of lipid metabolism and the molecular mechanism]

Recent studies have discovered a selective autophagy-lipophagy, which can selectively identify and degrade lipids and plays an important role in regulating cellular lipid metabolism and maintaining intracellular lipid homeostasis. The process of lipophagy can be directly or indirectly regulated by genes, enzymes, transcriptional regulators and other factors. This review examines the role of lipophagy in reducing liver lipid content, regulating pancreatic lipid metabolism, and regulating adipose tissue differentiation, and summarizes the findings of the molecules (Rab GTPase, enzymes, ion channels, transcription factors, small molecular substances) involved in the regulation of lipophagy, which points to new directions for the treatment of diseases caused by lipid accumulation.

Cardiomyocyte-Restricted Low Density Lipoprotein Receptor-Related Protein 6 (LRP6) Deletion Leads to Lethal Dilated Cardiomyopathy Partly Through Drp1 Signaling

Low density lipoprotein receptor-related protein 6 (LRP6), a wnt co-receptor, regulates multiple functions in various organs. However, the roles of LRP6 in the adult heart are not well understood.

Methods: We observed LRP6 expression in heart with end-stage dilated cardiomyopathy (DCM) by western blot. Tamoxifen-inducible cardiac-specific LRP6 knockout mouse was constructed. Hemodynamic and echocardiographic analyses were performed to these mice.

Results: Cardiac LRP6 expression was dramatically decreased in patients with end-stage dilated cardiomyopathy (DCM) compared to control group. Tamoxifen-inducible cardiac-specific LRP6 knockout mice developed acute heart failure and mitochondrial dysfunction with reduced survival. Proteomic analysis suggests the fatty acid metabolism disorder involving peroxisome proliferator-activated receptors (PPARs) signaling in the LRP6 deficient heart. Accumulation of mitochondrial targeting to autophagosomes and lipid droplet were observed in LRP6 deletion hearts. Further analysis revealed cardiac LRP6 deletion suppressed autophagic degradation and fatty acid utilization, coinciding with activation of dynamin-related protein 1 (Drp1) and downregulation of nuclear TFEB (Transcription factor EB). Injection of Mdivi-1, a Drp1 inhibitor, not only promoted nuclear translocation of TFEB, but also partially rescued autophagic degradation, improved PPARs signaling, and attenuated cardiac dysfunction induced by cardiac specific LRP6 deletion.

Conclusions: Cardiac LRP6 deficiency greatly suppressed autophagic degradation and fatty acid utilization, and subsequently leads to lethal dilated cardiomyopathy and cardiac dysfunction through activation of Drp1 signaling. It suggests that heart failure progression may be attenuated by therapeutic modulation of LRP6 expression.

Lipophagy and liver disease: New perspectives to better understanding and therapy

Intracellular lipid droplets (LDs) are remarkably dynamic and complex organelles that enact regulated storage and release of lipids to fulfil their fundamental roles in energy metabolism, membrane synthesis and provision of lipid-derived signaling molecules. The recent finding that LDs can be selectively degraded by the lysosomal pathway of autophagy through a process termed lipophagy has opened up a new understanding of how lipid metabolism regulates cellular physiology and pathophysiology. Many new functions for autophagic lipid metabolism have now been defined in various diseases including liver disease. Lipophagy was originally described in hepatocytes, where it is critical for maintaining cellular energy homeostasis in obesity and metabolic syndrome. In vitro and in vivo studies have demonstrated the selective uptake of LDs by autophagosomes, and inhibition of autophagy has been shown to reduce the β-oxidation of free fatty acids due to the increased accumulation of lipids and LDs. The identification of lipophagy as a new process dedicated to cellular lipid removal has mapped autophagy as an emerging player in cellular lipid metabolism. Pharmacological or genetic modulation of lipophagy might point to possible therapeutic strategies for combating a broad range of liver diseases. This review summarizes recent work focusing on lipophagy and liver disease as well as highlighting challenges and future directions of research. On the other hand, it also offers a glimpse into different strategies that have been used in experimental models to counteract excessive pathological lipophagy in the prevention and treatment of liver disease.

Formation kinetics of furfuryl alcohol in a coffee model system

The production of furfuryl alcohol from green coffee during roasting and the effect of multiple parameters on its formation were studied employing HPLC-DAD. Results show that coffee produces furfuryl alcohol in larger quantities (418µg/g) compared to other beans or seeds (up to 132µg/g) roasted under the same conditions. The kinetics of furfuryl alcohol production resemble those of other process contaminants (e.g., HMF, acrylamide) produced in coffee roasting, with temperature and time of roasting playing significant roles in quantities formed. Different coffee species yielded different amounts of furfuryl alcohol. The data point out that the amounts of furfuryl alcohol found in roasted coffee do not reflect the total amounts produced during roasting because great amounts of furfuryl alcohol (up to 57%) are evaporating and released to the atmosphere during roasting. Finally the effect of the moisture content on furfuryl alcohol formation was found to be of little impact.

Hepatic Lipophagy: New Insights into Autophagic Catabolism of Lipid Droplets in the Liver

The liver is a central fat‐storage organ, making it especially susceptible to steatosis as well as subsequent inflammation and cirrhosis. The mechanisms by which the liver mobilizes stored lipid for energy production, however, remain incompletely defined. The catabolic process of autophagy, a well‐known process of bulk cytoplasmic recycling and cellular self‐regeneration, is a central regulator of lipid metabolism in the liver. In the past decade, numerous studies have examined a selective form of autophagy that specifically targets a unique neutral lipid storage organelle, the lipid droplet, to better understand the function for this process in hepatocellular fatty acid metabolism. In the liver (and other oxidative tissues), this specialized pathway, lipophagy, likely plays as important a role in lipid turnover as conventional lipase‐driven lipolysis. In this review, we highlight several recent studies that have contributed to our understanding about the regulation and effects of hepatic lipophagy. (Hepatology Communications 2017;1:359–369)

Non-alcoholic fatty liver disease--a chronic disease of the 21^st century

Non-alcoholic fatty liver disease (NAFLD) comprises a spectrum of metabolic states ranging from simple steatosis to inflammation with associated fibrosis to cirrhosis. Though accumulation of hepatic fat is not associated with a significant increase in mortality rates, hepatic inflammation is, as this augments the risk of terminal liver disease, i.e., cirrhosis, hepatic decompensation (liver failure) and/or hepatocellular carcinoma. Disease progression is usually slow, over a decade or more and, for the most part, remains asymptomatic. Recent estimates suggest that the global prevalence of NAFLD is high, about one in four. In most cases, NAFLD overlaps with overweight, obesity, cardiovascular disease and the metabolic syndrome with numerous contributing parameters including a dysregulation of adipose tissue, insulin resistance, type 2 diabetes, changes in the gut microbiome, neuronal and hormonal dysregulation and metabolic stress. NAFLD is diagnosed incidentally, despite its high prevalence. Non-invasive imaging techniques have emerged, making it possible to determine degree of steatosis as well asfibrosis. Despite this, the benefit of routine diagnostics remains uncertain. A better understanding of the (molecular) pathogenesis of NAFLD is needed combined with long-term studies where benefits of treatment can be assessed to determine cost-benefit ratios. This review summarizes the current state of knowledge and possible areas of treatment.

Methodological and metabolic considerations in the study of caffeine-containing energy drinks

Caffeine-containing energy drinks are popular and widely available beverages. Despite large increases in consumption, studies documenting the nutritional, metabolic, and health implications of these beverages are limited. This review provides some important methodological considerations in the examination of these drinks and highlights their potential impact on the gastrointestinal system, liver, and metabolic health. The gastrointestinal system is important as it comes into contact with the highest concentration of energy drink ingredients and initiates a chain of events to communicate with peripheral tissues. Although energy drinks have diverse compositions, including taurine, ginseng, and carnitine, the most metabolically deleterious ingredients appear to be simple sugars (such as glucose and fructose) and caffeine. In combination, these last two ingredients have the greatest metabolic impact and potential influence on overall health.

Ethanol-induced oxidant stress modulates hepatic autophagy and proteasome activity

In this review, we describe research findings on the effects of alcohol exposure on two major catabolic systems in liver cells: the ubiquitin-proteasome system (UPS) and autophagy. These hydrolytic systems are not unique to liver cells; they exist in all eukaryotic tissues and cells. However, because the liver is the principal site of ethanol metabolism, it sustains the greatest damage from heavy drinking. Thus, the focus of this review is to specifically describe how ethanol oxidation modulates the activities of the UPS and autophagy and the mechanisms by which these changes contribute to the pathogenesis of alcohol-induced liver injury. Here, we describe the history and the importance of cellular hydrolytic systems, followed by a description of each catabolic pathway and the differential modulation of each by ethanol exposure. Overall, the evidence for an involvement of these catabolic systems in the pathogenesis of alcoholic liver disease is quite strong. It underscores their importance, not only as effective means of cellular recycling and eventual energy generation, but also as essential components of cellular defense.

Autophagy and non-alcoholic fatty liver disease

Autophagy, or cellular self-digestion, is a catabolic process that targets cell constituents including damaged organelles, unfolded proteins, and intracellular pathogens to lysosomes for degradation. Autophagy is crucial for development, differentiation, survival, and homeostasis. Important links between the regulation of autophagy and liver complications associated with obesity, non-alcoholic fatty liver disease (NAFLD), have been reported. The spectrum of these hepatic abnormalities extends from isolated steatosis to non-alcoholic steatohepatitis (NASH), steatofibrosis, which sometimes leads to cirrhosis, and hepatocellular carcinoma. NAFLD is one of the three main causes of cirrhosis and increases the risk of liver-related death and hepatocellular carcinoma. The pathophysiological mechanisms of the progression of a normal liver to steatosis and then more severe disease are complex and still unclear. The regulation of the autophagic flux, a dynamic response, and the knowledge of the role of autophagy in specific cells including hepatocytes, hepatic stellate cells, immune cells, and hepatic cancer cells have been extensively studied these last years. This review will provide insight into the current understanding of autophagy and its role in the evolution of the hepatic complications associated with obesity, from steatosis to hepatocellular carcinoma.