Methionine and S-adenosylmethionine levels are critical regulators of PP2A activity modulating lipophagy during steatosis

Corticosterone-Induced Lipogenesis Activation and Lipophagy Inhibition in Chicken Liver Are Alleviated by Maternal Betaine Supplementation

Background

We have shown previously that in ovo betaine injection can prevent nonalcoholic fatty liver induced by glucocorticoid exposure in chickens; yet it remains unknown whether feeding betaine to laying hens may exert similar effects in their progeny.

Objective

In this study, we fed laying hens a betaine-supplemented diet, and the progeny were later exposed chronically to corticosterone (CORT) to test hepatoprotective effects and further elucidate underlying mechanisms.

Methods

Rugao yellow-feathered laying hens (n = 120) were fed a basal (control, C) diet or a 0.5% betaine-supplemented (B) diet for 28 d before their eggs were collected for incubation. At 49 d of age, male chickens selected from each group were daily injected subcutaneously with solvent (15% ethanol; vehicle, VEH) or CORT (4.0 mg/kg body mass) for 7 d to establish a fatty liver model. Chickens in the 4 groups (C-VEH, C-CORT, B-VEH, and B-CORT) were killed at day 57. Plasma and hepatic triglyceride (TG) concentrations, as well as the hepatic expression of genes involved in lipogenesis and lipophagy, were determined.

Results

CORT induced a 1.6-fold increase in the plasma TG concentration (P < 0.05) and a 1.8-fold increment in the hepatic TG concentration (P < 0.05), associated with activation of lipogenic genes (70-780%). In contrast, lipophagy and mitochondrial β-oxidation genes were inhibited by 30-60% (P < 0.05) in CORT-treated chickens. These CORT-induced changes were completely normalized by maternal betaine supplementation or were partially normalized to intermediate values that were significantly different from those in the C-VEH and C-CORT groups. These effects were accompanied by modifications in CpG methylation and glucocorticoid receptor binding to the promoters of major lipogenic and lipophagic genes (P < 0.05).

Conclusions

These results indicate that maternal betaine supplementation protects male juvenile chickens from CORT-induced TG accumulation in the liver via epigenetic modulation of lipogenic and lipophagic genes.

Quercetin ameliorates HFD-induced NAFLD by promoting hepatic VLDL assembly and lipophagy via the IRE1a/XBP1s pathway

The consumption of a quercetin-rich diet has been well-established as a feasible method against non-alcoholic fatty liver disease (NAFLD); however, the molecular mechanisms underlying the progression of NAFLD and its intervention by quercetin remain largely obscure. Male Sprague-Dawley rats fed high-fat diet (HFD), and HepG2 cells stimulated with free fatty acid, were treated with quercetin and various pharmacological reagents to explore the effect of signaling pathways involved in endoplasmic reticulum stress on very low-density lipoprotein (VLDL) assembly and lipophagy. Quercetin intake decreased hepatic TG content by 39%, with a 1.5-fold increase in VLDL, and up-regulated spliced X-box binding protein 1 (XBP1s) expression compared with the HFD group. Thapsigargin or STF-083010 (an IRE1α endonuclease inhibitor) decreased VLDL content in a dose-dependent manner, partially counteracting the protective effects of quercetin, 4-PBA or APY-29 (an IRE1α endonuclease activator). Additionally, microsomal TG-transfer protein complex expression was increased with quercetin-treated and down-regulated by STF-083010. Moreover, quercetin increased co-localization of lysosomes with lipid droplets (LDs) accompanied by decreased p62 accumulation. STF-083010 partially abolished the effect of quercetin on LDs autophagy in an mTOR-independent manner. Collectively, these findings demonstrate that hepatic VLDL assembly and lipophagy are the main targets of quercetin against NAFLD via the IRE1a/XBP1s pathway.

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.

Role of methionine on epigenetic modification of DNA methylation and gene expression in animals

DNA methylation is one of the main epigenetic phenomena affecting gene expression. It is an important mechanism for the development of embryo, growth and health of animals. As a key nutritional factor limiting the synthesis of protein, methionine serves as the precursor of S-adenosylmethionine (SAM) in the hepatic one-carbon metabolism. The dietary fluctuation of methionine content can alter the levels of metabolic substrates in one-carbon metabolism, e.g., the SAM, S-adenosylhomocysteine (SAH), and change the expression of genes related to the growth and health of animals by DNA methylation reactions. The ratio of SAM to SAH is called ‘methylation index’ but it should be carefully explained because the complexity of methylation reaction. Alterations of methylation in a specific cytosine-guanine (CpG) site, rather than the whole promoter region, might be enough to change gene expression. Aberrant methionine cycle may provoke molecular changes of one-carbon metabolism that results in deregulation of cellular hemostasis and health problems. The importance of DNA methylation has been underscored but the mechanisms of methionine affecting DNA methylation are poorly understood. Nutritional epigenomics provides a promising insight into the targeting epigenetic changes in animals from a nutritional standpoint, which will deepen and expand our understanding of genes, molecules, tissues, and animals in which methionine alteration influences DNA methylation and gene expression.

Breaking fat: The regulation and mechanisms of lipophagy

Lipophagy is defined as the autophagic degradation of intracellular lipid droplets (LDs). While the field of lipophagy research is relatively young, an expansion of research in this area over the past several years has greatly advanced our understanding of lipophagy. Since its original characterization in fasted liver, the contribution of lipophagy is now recognized in various organisms, cell types, metabolic states and disease models. Moreover, recent studies provide exciting new insights into the underlying mechanisms of lipophagy induction as well as the consequences of lipophagy on cell metabolism and signaling. This review summarizes recent work focusing on LDs and lipophagy as well as highlighting challenges and future directions of research as our understanding of lipophagy continues to grow and evolve. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

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)

Autophagy in chronic liver diseases: the two faces of Janus

Alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD) are the leading causes of cirrhosis and increase the risk of hepatocellular carcinoma and liver-related death. ALD and NAFLD share common pathogenic features extending from isolated steatosis to steatohepatitis and steatofibrosis, which can progress to cirrhosis and hepatocellular carcinoma. The pathophysiological mechanisms of the progression of NAFLD and ALD are complex and still unclear. Important links between the regulation of autophagy (macroautophagy and chaperone-mediated autophagy) and chronic liver diseases have been reported. Autophagy may protect against steatosis and progression to steatohepatitis by limiting hepatocyte injury and reducing M1 polarization, as well as promoting liver regeneration. Its role in fibrosis and hepatocarcinogenesis is more complex. It has pro- and antifibrogenic properties depending on the hepatic cell type concerned, and beneficial and deleterious effects on hepatocarcinogenesis at initiating and late phases, respectively. This review summarizes the latest advances on the role of autophagy in different stages of fatty liver disease progression and describes its divergent and cell-specific effects during chronic liver injury.

Consensus recommendations for the diagnosis, treatment and follow-up of inherited methylation disorders

Inherited methylation disorders are a group of rarely reported, probably largely underdiagnosed disorders affecting transmethylation processes in the metabolic pathway between methionine and homocysteine. These are methionine adenosyltransferase I/III, glycine N-methyltransferase, S-adenosylhomocysteine hydrolase and adenosine kinase deficiencies. This paper provides the first consensus recommendations for the diagnosis and management of methylation disorders. Following search of the literature and evaluation according to the SIGN-methodology of all reported patients with methylation defects, graded recommendations are provided in a structured way comprising diagnosis (clinical presentation, biochemical abnormalities, differential diagnosis, newborn screening, prenatal diagnosis), therapy and follow-up. Methylation disorders predominantly affect the liver, central nervous system and muscles, but clinical presentation can vary considerably between and within disorders. Although isolated hypermethioninemia is the biochemical hallmark of this group of disorders, it is not always present, especially in early infancy. Plasma S-adenosylmethionine and S-adenosylhomocysteine are key metabolites for the biochemical clarification of isolated hypermethioninemia. Mild hyperhomocysteinemia can be present in all methylation disorders. Methylation disorders do not qualify as primary targets of newborn screening. A low-methionine diet can be beneficial in patients with methionine adenosyltransferase I/III deficiency if plasma methionine concentrations exceed 800 μmol/L. There is some evidence that this diet may also be beneficial in patients with S-adenosylhomocysteine hydrolase and adenosine kinase deficiencies. S-adenosylmethionine supplementation may be useful in patients with methionine adenosyltransferase I/III deficiency. Recommendations given in this article are based on general principles and in practice should be adjusted individually according to patient's age, severity of the disease, clinical and laboratory findings.