Figure 1. (A) Fatty acid carbon atoms are numbered often starting at the carboxyl end. (B) The position of a double bond can be denoted by counting from the distal end, with the ω carbon atom (the methyl carbon) as number 1 (green). (C) The carboxylic acid group (red) is shown in its ionized form. (D) The International Union of Pure and Applied Chemists (IUPAC) Δ and common ω numbering systems.

Figure 2. Molecular structures of (A) saturated, (B) monounsaturated, (C) trans fatty, and (D) polyunsaturated fatty acids. The standard chemical formulae (left panel), the perspective formulae (middle panel), and the space‐filling models (right panel) of various fatty acids are shown. Both (B) oleic acid (cis 18:1ω9) and (C) elaidic acid (trans 18:1ω9) are 18‐carbon fatty acids with a single double bond. However, oleic acid has a cis double bond (hydrogen atoms are on the same side of the bond), whereas elaidic acid has a trans double bond (hydrogen atoms are on opposite sides of the bond). Of note, in a cis monounsaturated fatty acid (oleic acid), the double bond induces a degree of structural rigidity and creates a kink in the chain while the rest of the chain is free to rotate about the other C–C bonds. In a trans monounsaturated fatty acid (elaidic acid), a more linear rigid structure is created and this diminishes membrane fluidity when incorporated into membrane lipids. The trans bond imparts a structure more similar to that of saturated fats, altering the physiological properties and effects of the fatty acid.

Figure 3. (A) Triacylglycerols are triesters of glycerol, and each of the three hydroxyl (–OH) groups of glycerol forms an ester group by reaction with the carboxyl (–COOH) group of a fatty acid to form the triacylglycerol molecule. R ₁, R ₂, and R ₃ are fatty acids located at stereospecific numbers (sn)‐1, ‐2 and ‐3, respectively. (B) Diacylglycerols and (C) monoacylglycerols contain two and one fatty acids, respectively. R = hydrocarbon chain.

Figure 4. The general features of lipid balance across the body. There are three sources for lipids entering the small intestine for intestinal absorption: (i) dietary lipids; (ii) biliary lipids; and (iii) desquamated epithelial cells of the gastrointestinal tract. Likewise, there are two major pathways for the excretion of lipid from the body: the excretion of lipids from the body through (i) the gastrointestinal tract and (ii) skin. Because total input of lipids into the body must equal total output in the steady state, the body pool of lipids is kept constant. As a result, normal metabolic homeostasis prevents a potential accumulation of fat and cholesterol in the body. Of note is that in children, there is necessarily a greater input of fat and cholesterol into the body than output since there is a net accumulation of fat and cholesterol allowing for body weight gain with growth.

Figure 5. Putative pathways for uptake of fatty acids by the enterocytes based on the current understanding of fatty acid transport across the apical membranes of enterocytes. Because of their less hydrophobic nature, (A) short‐chain fatty acids may traverse the apical membrane by simple passive diffusion and may be absorbed into the mesenteric venous blood and then the portal vein. (B) Long‐chain fatty acids can be transported by fatty acid transport protein 4 (FATP4). (C) Alternatively, CD36 (also referred to as fatty acid translocase; 88 kDa), alone or together with the peripheral membrane protein plasma membrane‐associated fatty acid‐binding protein (FABP_pm; 43 kDa) accepts fatty acids at the cell surface to increase their local concentrations. This could help CD36 actively transport fatty acids across the apical membrane of the enterocyte. Once at the inner side of the membrane, fatty acids are bound by cytoplasmic FABP (FABP_c) before entering metabolic pathways. Some fatty acids may be transported by fatty acid transport proteins and rapidly thioesterified by plasma membrane acyl‐CoA synthetase 1 (ACS1) to form acyl‐CoA esters. Acyl‐CoA is used for triacylglycerol synthesis in the enterocyte, which is then a substrate for chylomicron formation and secretion into the lymph.

Figure 6. Elongation and unsaturation of fatty acids from a saturated fatty acid palmitic acid (16:0) in the liver. De novo lipogenesis from glucose as a substrate generates saturated fatty acids such as palmitic acid. Palmitic acid is further elongated and desaturated to form the abundant monounsaturated fatty acids such as oleic acid (18:1ω9). Oleic acid is incorporated into triacylglycerol.

Figure 7. Pathway of fatty acid elongation in mitochondria. In humans, the preferred elongation substrate is palmitoyl‐CoA, which is converted exclusively to stearic acid (18:0) in most tissues including the liver.

Figure 8. Positions in the fatty acid chain where desaturation can occur in humans. The human fatty acid desaturase systems can desaturate various chain lengths at Δ⁴, Δ⁵, Δ⁶, and Δ⁹ positions. However, humans cannot introduce double bonds beyond carbons 9 and 10 and must have the polyunsaturated fatty acids linoleic (18:2 cis‐Δ^9,12), linolenic (18:3 cis‐Δ^9,12,15), and arachidonic (20:4 cis‐Δ^5,8,11,14) acids provided in the diet. These fatty acids are thus essential fatty acids in humans.

Figure 9. Transfer of a fatty acid from the adipose tissues to the liver and into the mitochondrial matrix for β‐oxidation. The rate of fatty acid release from the adipose tissues affects the total amount of fatty acid available as a fuel for the liver. Abbreviation: ATGL, adipose triglyceride lipase; FAD, flavin adenine dinucleotide; FADH₂, the reduced form of FAD; HSL, hormone‐sensitive lipase; NAD⁺, nicotinamide adenine dinucleotide; NADH, the reduced form of NAD⁺. See text for details.

Figure 10. Pathways of triacylglycerol biosynthesis in the liver. Both glucose and fructose generate triose phosphate intermediates that form the glycerol backbone of triacylglycerol. R = hydrocarbon chain.

Figure 11. The regulation of fatty acid and triacylglycerol biosynthesis by sterol regulatory element‐binding protein‐1c (SREBP‐1c). In the liver, SREBP‐1c preferentially activates the genes involved fatty acid and triacylglycerol metabolism.

Figure 12. This diagram shows fatty acid balance across the liver, indicating three major (solid lines) and two minor (dashed lines) sources of fatty acids entering the hepatocyte (blue lines) and three main pathways for their utilization (brown lines) for triacylglycerol synthesis, oxidation, and phospholipid synthesis in the hepatocyte. Dietary fatty acids go to the liver due to “spillover” of fatty acids released by lipoprotein lipase and hepatic lipase mediated lipolysis of lipoprotein triacylglycerols in capillaries of adipose tissues and other tissues. Triacylglycerols are packaged with other lipids and apolipoproteins to produce very‐low‐density lipoproteins (VLDL). Triacylglycerols accumulate in the liver when their synthesis exceeds VLDL formation and export, thus leading to hepatic steatosis. See text for details.

Figure 13. Very‐low‐density lipoprotein (VLDL) metabolism. The cycle begins with the hepatic synthesis of nascent VLDL particles. These particles contain apolipoproteins (apo)B‐100 and apoE. Hepatic VLDL assembly involves the lipidation of a newly synthesized apoB‐100 molecule with triacylglycerols (TG). This step is achieved through the action of microsomal triglyceride transfer protein. A further step is the formation of mature VLDL particles, which are enriched with cholesteryl esters and possibly other apolipoproteins, some of which are derived from HDL catabolism. After secretion into the circulation, contact of mature VLDL with the lipolytic action of lipoprotein lipase (apoC‐II acting as primary ligand) results the partial delipidation of VLDL into VLDL remnants which are smaller and enriched in apoB‐100 and apoE. The resulting fatty acids are mostly taken up locally at the site of release from VLDL. The destiny of the VLDL remnants is to be cleared in the liver (LDL and remnant receptors) or to undergo delipidation by hepatic triglyceride lipase to yield LDL particles containing apoB‐100.

Figure 14. Multiple biologically active lipid metabolites are generated during the metabolism of fatty acids and production of triacylglycerols. Many of these have been implicated in causing lipotoxicity manifested as endoplasmic reticulum stress, mitochondrial dysfunction, apoptosis, inflammation, and necrosis. Abbreviations: ACSL, acyl‐CoA synthase; AGPAT, acyl‐glycerolphosphate acyltransferase; ATGL, adipose triglyceride lipase; CPT, choline phosphotransferase; DAG, diacylglycerol; DAGK, diacylglycerol kinase; DGATs, diacylglycerol acyltransferases; FA, fatty acids; GPAT, glycerol monophosphate acyltransferase; HSL, hormone‐sensitive lipase; LPA, lysophosphatidic acid; LPAAT, lysophosphatidic acid acyltransferase; LPAP, lysophosphatidic acid phosphatase; LPC, lysophosphatidylcholine; LysoPLD, lysophospholipase D; MAG, monoacylglycerol; MAGK, monoacylglycerol kinase; MGL, monoacylglycerol lipase; MOGAT, monoacylglycerol acyltransferase; PA, phosphatidic acid; PAP, phosphatidic acid phosphatase; PC, phosphatidylcholine; PLA2, phospholipase A2; PLD, phospholipase D; TG, triacylglycerols.

Figure 15. Lipid droplets are enclosed by a monolayer of phospholipid and droplet‐associated proteins which stabilize them within the cytoplasm of adipocytes (left panel) and hepatocytes (right panel). In obese humans, reduced expression of cell death‐inducing DNA fragmentation factor 45‐like effector proteins (CIDEs) and perilipin 1 (PLIN1) allows increased amounts of fatty acids to be released by lipolysis. These fatty acids act locally and enter the bloodstream, where they activate inflammatory pathways, promote ectopic lipid deposition in peripheral tissues, and impair insulin signaling. Fatty acids from adipocyte lipolysis or the diet lead to a large amount of neutral lipid accumulation in lipid droplets in hepatocytes and incorporation of CIDE, PLIN, adipose triglyceride lipase (ATGL) and patatin‐like phospholipase containing 3 (PNPLA3) on the surface of lipid droplets. In the liver, increased fatty acid accumulation and lipid droplet formation are often associated with increased diacylglycerol and inflammatory cytokine production. Diacylglycerol stimulates atypical protein kinase C (PKC), and fatty acids and cytokines activate inflammatory signaling pathways. These alterations can impair insulin signaling and thus contribute to insulin resistance. In hepatocytes, insulin resistance is marked by increased hepatic gluconeogenesis and reduced glycogen formation. Notably, mutations in the phospholipase PNPLA3 result in hepatic steatosis.

Figure 16. The proposed models of the cell death‐inducing DNA fragmentation factor 45‐like effector protein (CIDE)‐mediated lipid transfer and lipid droplet growth. (A) CIDE proteins localized in lipid droplets protects against lipolysis by adipose triglyceride lipase (ATGL) or hormone‐sensitive lipase (HSL) and promotes triacylglycerol accumulation. (B) When clustered and enriched at the lipid droplet contacting site, CIDE proteins may provide a tethering force for stable lipid droplet attachment and recruit other proteins to form a complex at the lipid droplet contacting site. CIDE‐initiated protein complex may deform phospholipid monolayer to generate a pore (or channel‐like) structure at the lipid droplet contacting site, resulting in neutral lipid exchange among contacted lipid droplets and net triacylglycerol transfer from smaller to larger lipid droplets due to the internal pressure difference. The inset indicates an enlarged portion of the lipid droplet contacting site at where CIDE proteins are focally enriched and shows a directional net lipid transfer from a small to a large lipid droplet by a white arrow, thus leading to lipid droplet growth.

Figure 17. The proposed models promote the development of steatosis in the liver by a signaling pathway regulated by the nuclear receptor peroxisome proliferator‐activated receptor γ (PPARγ) and the cell death‐inducing DNA fragmentation factor 45‐like effector protein (CIDE). After being activated by PPARγ in the nucleus of the heaptocyte, (A) CIDE proteins promote lipid droplet clustering, (B) protect against lipolysis by lipases such as adipose triglyceride lipase (ATGL) or hormone‐sensitive lipase (HSL), and (C) inhibit mitochondrial β‐oxidation. AMP‐activated protein kinase (AMPK) may be involved in this inhibitory action of CIDE proteins. (D) In addition, CIDE proteins may mediate VLDL lipidation in the endoplasmic reticulum and Golgi through the direct delivery of triacylglycerol from cytosolic lipid droplets to pre‐VLDL particles that are attached to the membrane of the endoplasmic reticulum and Golgi. When triacylglycerol‐rich VLDL secretion cannot remove lipids from the liver, lipid droplet formation allows excess lipid accumulation in the liver in a relatively benign form, thus leading to hepatic steatosis and preventing lipotoxic injury and apoptosis induced by other fatty acid metabolites.

Figure 18. In the context of insulin resistance, excessive fatty acid (FA) flow through the liver following lipolysis in the adipose tissues and also lipophagy and hepatic de novo lipogenesis (DNL) following a carbohydrate‐enriched diet (fructose is especially implicated). Fatty acids are also derived from lipoprotein remnants and from chylomicrons resulting from intestinal fat absorption followed by spillover into the circulation during intravascular lipolysis. The hepatic pool of fatty acids is therefore obtained via DNL, influx following lipolysis and lysosomal breakdown of triacylglycerol‐rich lipoprotein remnants. The fate of fatty acids is normally to undergo oxidation mainly in mitochondria, and partially in peroxisomes and the smooth endoplasmic reticulum (ER). Formation of reactive oxygen species (ROS), that is, hydrogen peroxide and superoxide, and oxidant stress following oxidation is normally counteracted by specific antioxidant buffering systems (e.g., glutathione). Fatty acids undergo esterification with glycerol to form triacylglycerols (TG), which represents a lipid storage system in the liver, eventually leading to lipid droplets and steatosis. Alternatively, triacylglycerols can be exported into VLDL particles. Cytosolic lipases such as adipose triglyceride lipase can transfer additional fatty acids from lipid droplets to the fatty acid pool. Lipid droplet breakdown also occurs by autophagy (lipophagy), a process in which lipid droplets are sequestered in autophagosomes that fuse with lysosomes resulting in the breakdown of lipid droplet components by lysosomal enzymes. This pathway is regulated by changes in gene expression and increases when the cell is stressed either by nutrient deprivation or an excess of lipids. Increased autophagy generates more fatty acids, which contribute to the fatty acid pool. The lipotoxicity model of liver injury suggests that fatty acids are transformed into active metabolites able to orchestrate the hepatocellular damage, that is, ER stress, inflammation, necrosis, apoptosis, cellular ballooning, and formation of Mallory‐Denk bodies that characterize NASH.