Fructose impairs fat oxidation: Implications for the mechanism of western diet-induced NAFLD

Abstract

Increased fructose intake from sugar-sweetened beverages and highly processed sweets is a well-recognized risk factor for the development of obesity and its complications. Fructose strongly supports lipogenesis on a normal chow diet by providing both, a substrate for lipid synthesis and activation of lipogenic transcription factors. However, the negative health consequences of dietary sugar are best observed with the concomitant intake of a HFD. Indeed, the most commonly used obesogenic research diets, such as "Western diet", contain both fructose and a high amount of fat. In spite of its common use, how the combined intake of fructose and fat synergistically supports development of metabolic complications is not fully elucidated. Here we present the preponderance of evidence that fructose consumption decreases oxidation of dietary fat in human and animal studies. We provide a detailed review of the mitochondrial β-oxidation pathway. Fructose affects hepatic activation of fatty acyl-CoAs, decreases acylcarnitine production and impairs the carnitine shuttle. Mechanistically, fructose suppresses transcriptional activity of PPARα and its target CPT1α, the rate limiting enzyme of acylcarnitine production. These effects of fructose may be, in part, mediated by protein acetylation. Acetylation of PGC1α, a co-activator of PPARα and acetylation of CPT1α, in part, account for fructose-impaired acylcarnitine production. Interestingly, metabolic effects of fructose in the liver can be largely overcome by carnitine supplementation. In summary, fructose decreases oxidation of dietary fat in the liver, in part, by impairing acylcarnitine production, offering one explanation for the synergistic effects of these nutrients on the development of metabolic complications, such as NAFLD.

Keywords: Fatty acid oxidation (FAO); Fructose; Non-alcoholic fatty liver disease (NAFLD); Sugar; Western diet.

Fig. 1.

The effect of fructose on lipogenesis and mitochondrial oxidation. In the liver fructose is taken up by solute carrier family 2, facilitated glucose transporter, member 2 (SLC2A2 aka GLUT2) and phosphorylated by ketohexokinase (KHK) to fructose-1 phosphate (Fruct-1 Phos). KHK mediated fructose phosphorylation is rapid leading to depletion of adenosine triphosphate (ATP), accumulation of adenosine diphosphate (ADP) and eventually production of uric acid. Fruct-1 Phos is further metabolized by aldolase b (ALDOB) into dihydroxyacetone phosphate (DHAP) and glyceraldehyde (GA). GA is then phosphorylated by triokinase and FMN cyclase (TKFC) into glyceraldehyde-3 phosphate (GA3P). DHAP and GA3P are also intermediates of glycolysis pathway and they are metabolized into pyruvate. Pyruvate enters the tricarboxylic acid cycle (TCA) to produce electrons for energy production. When the cellular energy stores are plentiful citrate is transported out of mitochondria into the cytosol to serve as a substrate for de novo lipogenesis (DNL). Uric acid produced by fructose metabolism inhibits aconites, an enzyme in TCA cycle, to further increase citrate production. In the cytosol, citrate is converted to acetyl-CoA by the enzyme ATP citrate lyase (ACLY). Acetyl-CoA carboxylase (ACC1), then catalyzes the production of malonyl-CoA and fatty acid synthase (FASN) extends the growing fatty acid chain to synthesize palmitoyl-CoA, a major building block of triglyceride (TG) formation. Malonyl-CoA, an intermediate in DNL also inhibits carnitine palmitoyltransferase 1 alpha (CPT1α) the rate limiting enzyme of mitochondrial fatty acid oxidation (FAO). In addition to providing substrate for DNL fructose through KHK affects the function of transcription factors mediating DNL and FAO. KHK and carbohydrate responsive element binding protein (ChREBP) regulate each other via a bidirectional loop, and upregulated ChREBP increases the expression of genes involved in DNL. Fructose through KHK also upregulates another lipogenic transcription factor, sterol regulatory element-binding protein 1c (SREBP1c), either directly via PPARG coactivator 1 beta (PGC1β) or indirectly by inducing selective insulin resistance and hyperinsulinemia. We present additional evidence that fructose through KHK decreases FAO. These effects are mediated, in part, via lower peroxisome proliferator activated receptor alpha (PPARα), a transcription factor that regulates expression of FAO enzymes, leading to decreased CTP1α protein. Additionally, fructose decreases PGC1α, a cofactor required for optimal PPARα activity via increasing PGC1α acetylation. Lastly, our research indicates that fructose through KHK increases CPT1α acetylation and decreases its protein stability.

Fig. 2.

The Pathway of Mitochondrial Fatty Acid Oxidation in the Liver. Long-chain fatty acids are transported into the liver via a complex of proteins consisting of fatty acid translocase (FAT, aka CD36), liver-fatty acid binding protein (L-FABP), caveolins and phospholipase A2 (PL). In the cytosol fatty acids are activated into fatty acyl-CoA by the actions of long-chain acyl-CoA synthetases (ACSL). Acyl-CoAs are unable to diffuse out of the cell and are committed to different cellular processes such as lipid synthesis or oxidation. The metabolic fate of acyl-CoAs is likely mediated by unique ACSL isoforms. Another pathway that mediates fatty acid import into the hepatocytes is via fatty acid transport protein 5 (FATP5). This enzyme complex additionally contains very long-chain acyl-CoA synthetase 6 (ACSVL6) activity to generate Acyl-CoA. Acyl-CoAs designated for energy production are conjugated to carnitine by carnitine palmitoyltransferase 1 alpha (CPA1a). This is the rate limiting enzyme of fatty acid oxidation accounting for 80% of the control over the pathway. Carnitine is transported into the hepatocytes by OCTN2. Acyl-carnitines can cross outer mitochondrial membrane and are transported across the inner mitochondrial membrane by carnitine-acylcarnitine translocase (CACT) in an exchange for free carnitine transported from mitochondrial matrix into inter membrane space. Inside the mitochondrial matrix carnitine palmitoyltransferase 2 (CPT2) converts acyl-carnitines into free carnitine and acyl-CoA. Beta oxidation involves a four-step process where long chain acyl-CoA are progressively shortened by two carbons to generate acetyl-CoA and two electrons. The first step dehydrogenates acyl-CoA into 2-enoly-CoA, and is mediated by acyl-CoA dehydrogenases (ACADs) of different chain lengths. This dehydrogenation step yields FADH₂, which donates electrons to complex II of the mitochondrial electron transport (ETC) chain for ATP production. The next three steps are mediated by mitochondrial trifunctional protein (MTP). The second step is hydration step and it produces 3-hydroxyacyl-CoA by the action of 2-enoyl-CoA hydratases (ECH). The third step is another dehydrogenation step mediated by 3-hydroxyacyl-CoA dehydrogenase (HADH) to produce 3-ketoacyl-CoA. This dehydrogenation step yields NADH, which donates electrons to complex I of the mitochondrial ETC chain for ATP production. The fourth step is mediated by acetyl-CoA acyltransferase 2 (ACAA2) to produce shortened acyl-CoA chain and acetyl-CoA. Acetyl-CoA can be further reduced in tricarboxylic acid (TCA) cycle and it yields citrate when energy stores are plentifully. During fasting, acetyl-CoA may be converted to ketone bodies that are used during fasting as energy source in other tissue.