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Comprehensive Physiology Dec 2017Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. The liver is the central organ for fatty acid... (Review)
Review
Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. The liver is the central organ for fatty acid metabolism. Fatty acids accrue in liver by hepatocellular uptake from the plasma and by de novo biosynthesis. Fatty acids are eliminated by oxidation within the cell or by secretion into the plasma within triglyceride-rich very low-density lipoproteins. Notwithstanding high fluxes through these pathways, under normal circumstances the liver stores only small amounts of fatty acids as triglycerides. In the setting of overnutrition and obesity, hepatic fatty acid metabolism is altered, commonly leading to the accumulation of triglycerides within hepatocytes, and to a clinical condition known as nonalcoholic fatty liver disease (NAFLD). In this review, we describe the current understanding of fatty acid and triglyceride metabolism in the liver and its regulation in health and disease, identifying potential directions for future research. Advances in understanding the molecular mechanisms underlying the hepatic fat accumulation are critical to the development of targeted therapies for NAFLD. © 2018 American Physiological Society. Compr Physiol 8:1-22, 2018.
Topics: Biological Transport; Fatty Acids; Humans; Lipid Metabolism; Lipogenesis; Lipolysis; Liver; Non-alcoholic Fatty Liver Disease; Triglycerides
PubMed: 29357123
DOI: 10.1002/cphy.c170012 -
Clinical Science (London, England :... Sep 2022The metabolic-associated fatty liver disease (MAFLD) is a condition of fat accumulation in the liver in combination with metabolic dysfunction in the form of overweight... (Review)
Review
The metabolic-associated fatty liver disease (MAFLD) is a condition of fat accumulation in the liver in combination with metabolic dysfunction in the form of overweight or obesity and insulin resistance. It is also associated with an increased cardiovascular disease risk, including hypertension and atherosclerosis. Hepatic lipid metabolism is regulated by a combination of the uptake and export of fatty acids, de novo lipogenesis, and fat utilization by β-oxidation. When the balance between these pathways is altered, hepatic lipid accumulation commences, and long-term activation of inflammatory and fibrotic pathways can progress to worsen the liver disease. This review discusses the details of the molecular mechanisms regulating hepatic lipids and the emerging therapies targeting these pathways as potential future treatments for MAFLD.
Topics: Fatty Acids; Humans; Lipid Metabolism; Lipogenesis; Liver; Non-alcoholic Fatty Liver Disease; Triglycerides
PubMed: 36148775
DOI: 10.1042/CS20220572 -
Cell Metabolism Oct 2020Mammalian organs are nourished by nutrients carried by the blood circulation. These nutrients originate from diet and internal stores, and can undergo various...
Mammalian organs are nourished by nutrients carried by the blood circulation. These nutrients originate from diet and internal stores, and can undergo various interconversions before their eventual use as tissue fuel. Here we develop isotope tracing, mass spectrometry, and mathematical analysis methods to determine the direct sources of circulating nutrients, their interconversion rates, and eventual tissue-specific contributions to TCA cycle metabolism. Experiments with fifteen nutrient tracers enabled extensive accounting for both circulatory metabolic cycles and tissue TCA inputs, across fed and fasted mice on either high-carbohydrate or ketogenic diet. We find that a majority of circulating carbon flux is carried by two major cycles: glucose-lactate and triglyceride-glycerol-fatty acid. Futile cycling through these pathways is prominent when dietary content of the associated nutrients is low, rendering internal metabolic activity robust to food choice. The presented in vivo flux quantification methods are broadly applicable to different physiological and disease states.
Topics: Animals; Citric Acid Cycle; Fatty Acids; Glucose; Glycerol; Lactic Acid; Mice; Mice, Inbred C57BL; Triglycerides
PubMed: 32791100
DOI: 10.1016/j.cmet.2020.07.013 -
Current Protein & Peptide Science 2018Adipose triglyceride lipase (ATGL) is the key-enzyme for the release of fatty acids (FAs) from triacylglycerol (TG) stores during intracellular lipolysis producing FAs... (Review)
Review
Adipose triglyceride lipase (ATGL) is the key-enzyme for the release of fatty acids (FAs) from triacylglycerol (TG) stores during intracellular lipolysis producing FAs used for energy production. There is growing evidence that the products and intermediates from lipolytic breakdown during the FA mobilization process also have fundamental regulatory functions affecting cell signaling, gene expression, metabolism, cell growth, cell death, and lipotoxicity. Regulation of ATGL is therefore vital for maintaining a defined balance between lipid storage and mobilization. This review addresses the regulation of ATGL activity at the post-translational level with special emphasis on protein-mediated interaction at the site of hydrolytic action, namely to the lipid droplet.
Topics: Animals; Fatty Acids; Humans; Lipase; Lipid Metabolism; Lipids; Lipolysis; Molecular Structure; Protein Conformation; Signal Transduction; Triglycerides
PubMed: 28925902
DOI: 10.2174/1389203718666170918160110 -
Hormone and Metabolic Research =... Apr 2022Fibroblast growth factor (FGF) 21 is a recently recognized metabolic regulator that evokes interest due to its beneficial action of maintaining whole-body energy balance... (Review)
Review
Fibroblast growth factor (FGF) 21 is a recently recognized metabolic regulator that evokes interest due to its beneficial action of maintaining whole-body energy balance and protecting the liver from excessive triglyceride production and storage. Together with FGF19 and FGF23, FGF21 belongs to the FGF family with hormone-like activity. Serum FGF21 is generated primarily in the liver under nutritional stress stimuli like prolonged fasting or the lipotoxic diet, but also during increased mitochondrial and endoplasmic reticulum stress. FGF21 exerts its endocrine action in the central nervous system and adipose tissue. Acting in the ventromedial hypothalamus, FGF21 diminishes simple sugar intake. In adipose tissue, FGF21 promotes glucose utilization and increases energy expenditure by enhancing adipose tissue insulin sensitivity and brown adipose tissue thermogenesis. Therefore, FGF21 favors glucose consumption for heat production instead of energy storage. Furthermore, FGF21 specifically acts in the liver, where it protects hepatocytes from metabolic stress caused by lipid overload. FGF21 stimulates hepatic fatty acid oxidation and reduces lipid flux into the liver by increasing peripheral lipoprotein catabolism and reducing adipocyte lipolysis. Paradoxically, and despite its beneficial action, FGF21 is elevated in insulin resistance states, that is, fatty liver, obesity, and type 2 diabetes.
Topics: Adipose Tissue, Brown; Animals; Diabetes Mellitus, Type 2; Energy Metabolism; Fibroblast Growth Factors; Glucose; Humans; Insulin Resistance; Lipid Metabolism; Liver; Triglycerides
PubMed: 35413740
DOI: 10.1055/a-1778-4159 -
Proceedings of the National Academy of... Mar 2003Excess lipid accumulation in non-adipose tissues is associated with insulin resistance, pancreatic beta-cell apoptosis and heart failure. Here, we demonstrate in...
Excess lipid accumulation in non-adipose tissues is associated with insulin resistance, pancreatic beta-cell apoptosis and heart failure. Here, we demonstrate in cultured cells that the relative toxicity of two common dietary long chain fatty acids is related to channeling of these lipids to distinct cellular metabolic fates. Oleic acid supplementation leads to triglyceride accumulation and is well tolerated, whereas excess palmitic acid is poorly incorporated into triglyceride and causes apoptosis. Unsaturated fatty acids rescue palmitate-induced apoptosis by channeling palmitate into triglyceride pools and away from pathways leading to apoptosis. Moreover, in the setting of impaired triglyceride synthesis, oleate induces lipotoxicity. Our findings support a model of cellular lipid metabolism in which unsaturated fatty acids serve a protective function against lipotoxicity though promotion of triglyceride accumulation.
Topics: Animals; Apoptosis; CHO Cells; Cell Line; Cricetinae; Drug Resistance; Fatty Acid Desaturases; Fatty Acids; Lipid Metabolism; Mice; Models, Biological; Oleic Acid; Palmitic Acid; Triglycerides
PubMed: 12629214
DOI: 10.1073/pnas.0630588100 -
Sports Medicine (Auckland, N.Z.) May 2014Fat and carbohydrate are important fuels for aerobic exercise and there can be reciprocal shifts in the proportions of carbohydrate and fat that are oxidized. The... (Review)
Review
Fat and carbohydrate are important fuels for aerobic exercise and there can be reciprocal shifts in the proportions of carbohydrate and fat that are oxidized. The interaction between carbohydrate and fatty acid oxidation is dependent on the intracellular and extracellular metabolic environments. The availability of substrate, both from inside and outside of the muscle, and exercise intensity and duration will affect these environments. The ability of increasing fat provision to downregulate carbohydrate metabolism in the heart, diaphragm and peripheral skeletal muscle has been well studied. However, the regulation of fat metabolism in human skeletal muscle during exercise in the face of increasing carbohydrate availability and exercise intensity has not been well studied until recently. Research in the past 10 years has demonstrated that the regulation of fat metabolism is complex and involves many sites of control, including the transport of fat into the muscle cell, the binding and transport of fat in the cytoplasm, the regulation of intramuscular triacylglycerol synthesis and breakdown, and the transport of fat into the mitochondria. The discovery of proteins that assist in transporting fat across the plasma and mitochondrial membranes, the ability of these proteins to translocate to the membranes during exercise, and the new roles of adipose triglyceride lipase and hormone-sensitive lipase in regulating skeletal muscle lipolysis are examples of recent discoveries. This information has led to the proposal of mechanisms to explain the downregulation of fat metabolism that occurs in the face of increasing carbohydrate availability and when moving from moderate to intense aerobic exercise.
Topics: Animals; Carbohydrate Metabolism; Carnitine; Dietary Fats; Exercise; Fatty Acids; Glucose; Humans; Lipid Metabolism; Mitochondria, Muscle; Muscle, Skeletal; Oxidation-Reduction; Oxygen Consumption; Triglycerides
PubMed: 24791920
DOI: 10.1007/s40279-014-0154-1 -
Proceedings of the National Academy of... Mar 2022In response to inflammatory activation by pathogens, macrophages accumulate triglycerides in intracellular lipid droplets. The mechanisms underlying triglyceride...
In response to inflammatory activation by pathogens, macrophages accumulate triglycerides in intracellular lipid droplets. The mechanisms underlying triglyceride accumulation and its exact role in the inflammatory response of macrophages are not fully understood. Here, we aim to further elucidate the mechanism and function of triglyceride accumulation in the inflammatory response of activated macrophages. Lipopolysaccharide (LPS)-mediated activation markedly increased triglyceride accumulation in macrophages. This increase could be attributed to up-regulation of the hypoxia-inducible lipid droplet–associated (HILPDA) protein, which down-regulated adipose triglyceride lipase (ATGL) protein levels, in turn leading to decreased ATGL-mediated triglyceride hydrolysis. The reduction in ATGL-mediated lipolysis attenuated the inflammatory response in macrophages after ex vivo and in vitro activation, and was accompanied by decreased production of prostaglandin-E2 (PGE2) and interleukin-6 (IL-6). Overall, we provide evidence that LPS-mediated activation of macrophages suppresses lipolysis via induction of HILPDA, thereby reducing the availability of proinflammatory lipid precursors and suppressing the production of PGE2 and IL-6.
Topics: Humans; Inflammation; Lipid Droplets; Lipid Metabolism; Lipids; Macrophages; Neoplasm Proteins; Triglycerides
PubMed: 35302892
DOI: 10.1073/pnas.2114739119 -
Biochimica Et Biophysica Acta.... Mar 2023Fasting and starvation were common occurrences during human evolution and accordingly have been an important environmental factor shaping human energy metabolism. Humans... (Review)
Review
Fasting and starvation were common occurrences during human evolution and accordingly have been an important environmental factor shaping human energy metabolism. Humans can tolerate fasting reasonably well through adaptative and well-orchestrated time-dependent changes in energy metabolism. Key features of the adaptive response to fasting are the breakdown of liver glycogen and muscle protein to produce glucose for the brain, as well as the gradual depletion of the fat stores, resulting in the release of glycerol and fatty acids into the bloodstream and the production of ketone bodies in the liver. In this paper, an overview is presented of our current understanding of the effects of fasting on adipose tissue metabolism. Fasting leads to reduced uptake of circulating triacylglycerols by adipocytes through inhibition of the activity of the rate-limiting enzyme lipoprotein lipase. In addition, fasting stimulates the degradation of stored triacylglycerols by activating the key enzyme adipose triglyceride lipase. The mechanisms underlying these events are discussed, with a special interest in insights gained from studies on humans. Furthermore, an overview is presented of the effects of fasting on other metabolic pathways in the adipose tissue, including fatty acid synthesis, glucose uptake, glyceroneogenesis, autophagy, and the endocrine function of adipose tissue.
Topics: Humans; Adipose Tissue; Fasting; Adipocytes; Lipolysis; Triglycerides
PubMed: 36521736
DOI: 10.1016/j.bbalip.2022.159262 -
Journal of Lipid Research Aug 2020The risk for metabolic disease, including metabolic syndrome, insulin resistance, and diabetes, increases with age. Altered plasma TG metabolism and changes in fatty... (Review)
Review
The risk for metabolic disease, including metabolic syndrome, insulin resistance, and diabetes, increases with age. Altered plasma TG metabolism and changes in fatty acid partitioning are also major contributors to metabolic disease. Plasma TG metabolism itself is altered by age in humans and rodents. As discussed in this review, the age-induced changes in human TG metabolism include increased plasma TG levels, reduced postprandial plasma TG clearance rates, reduced postheparin LPL activity, decreased adipose tissue lipolysis, and elevated ectopic fat deposition, all of which could potentially contribute to age-associated metabolic diseases. Similar observations have been made in aged rats. We highlight the limitations of currently available data and propose that mechanistic studies are needed to understand the extent to which age-induced alterations in TG metabolism contribute to metabolic disease. Such mechanistic insights could aid in therapeutic strategies for preventing or managing metabolic disease in older individuals.
Topics: Adipose Tissue; Aging; Animals; Humans; Triglycerides
PubMed: 32586846
DOI: 10.1194/jlr.R120000922