The fat metabolism lecture—lecture 3 in the RuG Metabolism & Nutrition course—is really two lectures: lecture 3A and 3B are delivered by Janine Kruit and Uwe Tietge, respectively.

Lipid nomenclature

Lipids are basically molecules that are fat-soluble and include fat-soluble vitamins (A, D, E, and K), but here we’re interested in fats.

Most of the fat we eat, comes to us in the form of triglycerides. These neutral fats are also how we store fat in our adipose tissue. We distinguish 3 types of triglycerides:

  • MCT, medium chain triglycerides, C₈, C₁₀;
  • LCT, long chain triglycerides; and
  • SCFA, short chain triglycerides (which are the breakdown products of carbohydrates, not fats).

Fosfolipids (e.g. lecithine) are found mostly in plasma membranes.

There are saturated and unsaturated fatty acids. Most fats can be synthesized by our body, but there are two essential fatty acids (linolenic and linoleic acid), which are necessary to synthesize ω-3 and ω-6. Animal fats are mostly saturated, plant fats more often unsaturated. Trans-fats are fats with a trans instead of cis connection.

Fat digestion

Triglycerides first have to be split in fatty acids and monoacylglycerols in the lumen by lipases (from pancreatic juice), to be able to be absorbed into the mucosal cells, where triglycerides are reassembled and incorporated into chylomicrons and transported into the lymph system.

Lipases are present on the tong, in the stommach, the pancreas, and in milk (to aid the baby in digestion when it doesn’t produce its own lipases yet). The breakdown speed dependends on triglyceride structure, cofactors and pH.

Short-chain fatty acids (SCFAs)

Short-chain fatty acids (SCFAs) are breakdown products of carbohydrates. They are transported to the liver via the hepatic portal vein. In the liver, SCFAs are completely broken down or incorporated in LTFAs.

Medium-chain fatty acids (MCFAs)

Medium-chain fatty acids (MCFAs) come from medium Chain triglycerides (MCT): C₈, C₁₀. They undergo the same treatment as SCFAs.

Long-chain fatty acids (LCFAs)

Long-chain fatty acids (LCFAs) come from long chain triglycerides (LCT).
In intestinal endothele cells, they are resynthesized into triglycerides and form chylomicrons (= lipoprotein). These chylomicrons are transported to the blood stream and the tissues through the lymph system.

Lipoproteins

  • The largest lipopoproteins are chylomicrons, which consist for 86% of fat. Their structure and function is provided by apoplipoproteins. Their task is fat transport to other tissues.
  • Very low density lipoprotein (VLDL).
  • Intermediate density lipoprotein (IDL).
  • Low density lipoprotein (LDL)
  • High density lipoproteins (HDLs) consist of a hydrophobic core of triglyceride and cholesteryl esters. [Uwe slide (at least 2)]

From the intestine, fat enters the lymf system, where its built into chylomicrons (and thus doesn’t need to pass the liver). After that, the chylomicrons journey from the lymf system into the blood system. Here,lipoprotein lipases (LPLs) hydrolize the triglycerides in chylomicrons into free fatty acids (FFAs) that can be absorbed by tissues. Chylomicron remnants (containing high cholesterol) are produced. Some FFAs enter the adipose tissues, after which they can be mobilized to the liver. The chylomicron remnants pass directly to the liver.

In lipoprotein lipase defient patient, the blood turns into a milky substance, and fat is stored in the weirdest of places.

Fatty acids and triglycerides

Mobilisation of triglycerides

Fat in adipose tissue (adipocytes) can be mobilized when glucagon (or adrenaline) enters the cells.

In fat cells (adipocytes), Triacylglycerol (=triglyceride) is converted into glycerol and fatty acids.
Free fatty acids first go to the liver, where they are built into VLDL particles, adding an extra layer of control. In the liver, glycerol is converted into pyruvate by glycolysis or into glucose by gluconeogenesis. VLDL particles transport FAs and cholesterol to other tissues, where FFAs are oxidized into Acetyl CoA, which enters the cytric acid cycle to produce CO₂ and H₂O.

Biological function of triglycerides

Triglycerides can be very efficiently stored (9 kcal/g), compared to glycogen and protein (both 4 kg/g).
A 70kg man only has about 480g. glycogen, on which he could survive for about a day. The 6000g protein in muscle tissue allows him to surive fasting for about 12 days, whereas his 12kg of fat can last him 60 days.

Triglycerides are:

  • energy-rich molecules;
  • chemically inert;
  • don’t play a prominent functional role; and are
  • hydrophobic: glycogen and protein attract water (3g H2O / 1g glycogen); storing the energy of 12kg fat in glycogen would require the storage of about 100kg of water.

Fat functions:

  • Energy is stored in triglycerides in white adipose tissue (WAT).
  • Phospholipid are a structural element of cell membranes.
  • Lipoproteins provide transport.
  • Brown adipose tissue provided temperature regulation.
  • Fat tissue protects and isolates.
  • Omega fatty acids metabolites provide essential biological activity.

Dietary triglycerides also provide physiological functions. They

  • delay stommach emptying;
  • increase satiety;
  • are tasteless;
  • provide a solvent for taste substances;
  • are essential for brain functioning; and
  • act as signalling molecules.

    • Fat and chronic disease

    Food rich in unsatured (n-3) fatty acids protects against CVD, because they make HDL go up and VLDL go down.

    Trans-fats increase the chance of CVD. Found in fried foods, commercial baked goods, processed foods and margarine. Used a lot in processed foods because it increases shelf-life. HDL down; LDL up.

    Too much saturated fats increases the risk of CVD, (colonic) cancer and type II diabetes.

    Insufficient intake of unsaturated fats poses a risk during the development of brain functions and, like too much saturated fat, increases the risk of type 2 diabetes.

    Read the following sections in Berg’s Biochemistry (7th ed) for better understanding: 22.1, 22.2 and 26.3.

    Cholesterol

    In vertebrates, cholesterol is essential for synthesizing cell membranes and for metabolites, including:

    • steroid hormones (androgens, estrogens, progestins, glucocorticoids, and mineralocorticoids);
    • bile acids (detergent function for lipid absorption and biliary secretion, signaling function).

    Cholesterol has many apolar structures (only one OH), which makes is hardly water-soluble (~2 mg/l).

    About 1% of the total cholesterol pool is turned over each day. In a typical 70kg man, with a total cholesterol pool of ~140g, the turnover rate would be ~1200 mg/day. Around 300–500 mg/day new cholesterol comes in through food. The rest (~1000 mg/day) is synthesized de novo. The production of bile salts utilizes ~ 800 mg/day, and ~ 400 mg is excreted in fecal matter. Because the amount of cholesterol that is consumed can vary wildly, de novo synthesis has to be tightly regulated.

    The molecular regulation of cholesterol synthesis follows a general feedback regulation system, wherein, when a precursor is converted into a product by an enzyme, decreasing [product] upregulates the enzyme (through a sensor) and increasing [product] downregulates the enzyme (through another sensor). The cholesterol sensor is SCAP (SREBP cleavage activating protein), which, in the presence of sufficient cholesterol, binds to an ER protein called Insig. SCAP escorts SREBP (sterol regulatory element binding protein) from the ER to the Golgi apparatus in the absense of cholesterol.

    Reverse cholesterol transport

    Cholesterol in macrophages in periphial tissues is transported as HDL to the liver, which excretes it as bile into the intestine.

    Transintestinal cholesterol transport (TICE)

    As an alternative reverse pathway, TICE is not yet demonstrated on a molecular level, but it involves a direct route of HDL from the intestine to the periphial tissues. This was found in a landmark study van van der Velde et al. (in Gastroenterology 2007, 133:967-975), who found that intestine-derived cholesterol contributes to fecal neutral sterol output in mice, which implies that there is an active payway facilitating the direct excretion of cholesterol from the enterocyte.

    TICE is likely (mostly) mediated by apoB-containing lipoproteins. Molecular characterization of TICE holds the promise of finding ways to substantially lower VLDL/LDL-cholesterol.

    Lipoprotein structure

    CE/TG are encapsulated in phospholipids interwoven with apolipoprotein, which form a hydrophilic surface to the outside and and hydrophobic surface to the inside.

    HDL cholesterol is inversely correlated with the risk of CHD (PROCAM study). Incidence of coronary events per 1000 in 8 years over [HDL-cholesterol] * [cholesterol] show an increase of coronary events with higher [Chol], but protection against these events by >[HDL]

    Cholesterol is costly to synthesize. It better to absorb it from nutrition.

    Niemann-Pick C1-like protein 1 (NPC1L1) is a key regulator of intestinal cholesterol absorption and is also highly expressed in human (but not in rodent) liver. (Beware of the (non-)applicability of mouse models.) In the liver, NPC1L1 facilitates the re-uptake of newly secreted cholesterol.

    ABCG5/G8 are dimerizing ABC half-transporters that are expressed in enterocytes and hepatocytes. Mutations in either ABCG5 or ABCG8 cause sitosterolemia (hyperabsorption of plant sterols xanthomas, accelerated atherosclerosis). Quantitatively, ABCG5/G8 are major transporter of of cholesterol as bile from the liver.

    The uptake of plant sterols in enterocytes (intestine) is regulated by NPC1L1. Some sterols can be transported back into the lumen by ABCG5/G8. Other sterols are transported to the liver, where they are excreted as bile.

    LDL adn atherosclerotic lesion development

    Hypercholesterolemia and inflammation are required for the development of atherosclerosis (Rader DJ & Daugherty A, Nature 2008, 451: 904-913). LDL becomes particularly bad when it binds to the extracellular matrix. Monocytes visit the blood vessel wall. Signalled by VLA4 and VCAM1 (inflammation signals on the wall). The monocytes enter the cell and develop into macrophages, which start eating modified LDL, forming foam cells.

    HDL metabolism

    HDL-associated proteins are not only linked to lipid metabolism, but also to inflammation and innate immunity.

    The production of HDL requires activity of ABCA1 transporter and apoA-I working together. (ABCA1 = ATP-binding cassette transporter A1.) ABCA1 is expressed ubiquitously (in every cell in every organ), while ApoA-I is expressed almost exclusively in liver and intestine. ABCA1 mediates cholesterol efflux to apoA-I. ABCA1 transcription is controlled by LXR nuclear receptor. Mutations in ABCA1 gene caused HDL-deficiency in Tangier disease (TD). Low HDL in TD heterozygotes.

    Contribution of hepatic and intestinal ABCA1 to plasma HDL assessed using organ-specific knock-out mice
    (Brunham et al. J. Clin. Invest. 2006; Circ. Res. 2006) (% HDL of control): Intestine KO: -30%; Liver KO: -80%; Intestine + Liver KO: -90%; Whole body KO: -95%.

    The scavenger receptor SR-BI selectively binds native as well as modified LDL and is expressed in the liver, in steroidogenic tissues and in macrophages. (It also binds anionic phospholipids.) SR-BI is the most important uptake receptor for HDL-CE, and, although hepatic SR-BI expression results in decreased plasma [HDL], it is anti-atherogenic.

    HDL is beneficial, in that it promotes cholesterol efflux and reverse cholestoral transport, inhibits LDL oxidation, inhibits endothelial inflammation, promotes endothelial NO production, promotes prostacyclin availability, and inhibits platelet aggregation.

    Cholesterol in a nutshell

    HDL are good and protect against atherosclerotic CVD, which is more likely to develop from LDL-C. HDL is synthesized in the hepatocytes of the liver and in the enterocytes of the intestine.