Fatty liver choline deficiency

One type of fatty liver that has been smdied extensively in rats is due to a deficiency of choline, which has therefore been called a lipotropic factor. The antibiotic puromycin, ethionine (a-amino-y-mercaptobu-tyric acid), carbon tetrachloride, chloroform, phosphorus, lead, and arsenic all cause fatty liver and a marked reduction in concentration of VLDL in rats. Choline will not protect the organism against these agents but appears to aid in recovery. The action of carbon tetrachloride probably involves formation of free radicals... 

The physiologic sequelae of biotin deficiency are almost unexplored. Severe skin lesions, especially seborrheic dermatitis and Leiner s disease (Erythroderma desquamativum or exfoliative dermatitis), were increased in young infants bom of mothers on a restricted diet low in eggs, livers, and other biotin-rich foods. After biotin administration the lesions healed. There are claims that excess biotin produces a fatty liver characterized by heightened cholesterol content. Choline has no effect in the prevention of biotin-fatty livers (G2, M2). In mice with transplanted tumors, both the tumors and the blood levels of biotin are below normal (R8). More recent studies established a protection with avidin, the biotin-binding fraction of egg white, against tumor formation (K4). More data along these lines are still needed for confirmation. 

Orotic acid in the diet (usually at a concentration of 1 per cent) can induce a deficiency of adenine and pyridine nucleotides in rat liver (but not in mouse or chick liver). The consequence is to inhibit secretion of lipoprotein into the blood, followed by the depression of plasma lipids, then in the accumulation of triglycerides and cholesterol in the liver (fatty liver) [141 — 161], This effect is not prevented by folic acid, vitamin B12, choline, methionine or inositol [141, 144], but can be prevented or rapidly reversed by the addition of a small amount of adenine to the diets [146, 147, 149, 152, 162]. The action of orotic acid can also be inhibited by calcium lactate in combination with lactose [163]. It was originally believed that the adenine deficiency produced by orotic acid was caused by an inhibition of the reaction of PRPP with glutamine in the de novo purine synthesis, since large amounts of PRPP are utilized for the conversion of orotic acid to uridine-5 -phosphate. However, incorporation studies of glycine-1- C in livers of orotic acid-fed rats revealed that the inhibition is caused rather by a depletion of the PRPP available for reaction with glutamine than by an effect on the condensation itself [160]. 

Why does a dietary deficiency of choline in humans induce a fatty liver, i.e., a liver in which the hepatocytes contain excess triglycerides ... 

Methionine is intimately related to lipid metabolism in the liver. Methionine deficiency is one of the causes of the fatty liver syndrome. Lack of methionine prevents the methylation of phosphatidylethanolamine to phosphatidylcholine, resulting in an ability by the liver to build and export very low density lipoprotein. The syndrome can be treated by the administration of choline, and for this reason, choline has often been referred to as the lipotropic factor. 

Liver health. As noted above, a biomarker of choline deficiency is elevated serum ALT levels, which is an indication of liver damage. One of the many functions of the liver is its role in fat metabolism. Without PC, the liver is unable to synthesize lipoproteins. Of particular importance in liver is the synthesis of very low-density lipoproteins (VLDL). With diminished VLDL production, the liver is not able to export lipid. This results in an accumulation of fat in the liver. Lipid accumulation in the liver leads to various stages of liver disease such as liver cell death, fibrosis, cirrhosis, and liver cancer (248-250). The role of choline in liver disease was underscored in the early 1990s when it was determined that patients on extended total parental nutrition (TPN) treatment developed fatty livers (251). At that time, TPN formulas did not include choline. Adding choline (in the form of lecithin) to TPN formulas reversed fatty buildup in these patients, and a... 

The effects of choline deficiency can be demonstrated easily using animals. One of the earliest is a fatty liver. Feeding rats a choline-free diet for 1 day doubles the fat (triglyceride) content of their livers. Rats that survive the continued consumption of such diets can accumulate over 50 times the normal level of hepatic fat. This condition results from impairment of the normal secretion of fatthin shell of phospholipid and protein. 

Fatty acid synthetase, 183 Fatty acid transport, 215-216,12D, 777-774 Falty liver, 293 alcoholism, 250-251 choline deficiency, 3l7 Fatty streak, 360, 636 Fecal blood test, 84 Feedback inhibition, 256 Feeding center, brain, 103-104 Fenton reaction, 627.635,903 Fermentative metabolism, 159,181-182, 233-243... 

Lombardi and colleagues (1968) studied the uptake of fatty acids by the liver, packaging as TGs in lipoproteins, and release back into the bloodstream of fatty acids in choline-sufficient and choline-deficient rats. This study involved injection of a radioactive fatty add (tagged with K ) into a blood vessel. After a 30-minute waiting period, the lipoproteins of the plasma were analyzed. The appearance of radioactivity in the lipoproteins was impaired in the deficient rats. This experiment is illustrated in Figure 6.8. Additional material on the lipoproteins is presented later in this chapter. 

FIGURE 6.8 Packaging of fatty acids by the liver. (1) The radioactive fatty acid is injected into a vein and enters the cells of the liver. Most of the cells of the liver manufacture lipoproteins (circles). (2) Fatty acids in the Uver, including the injected radioactive fatty acid, are converted to triglycerides and packaged into the lipoproteins. (3) Lipoproteins are secreted continuously by the liver into the bloodstream. Most of the lipoproteins in the blood have been in the circulatory system for several hours. Blood sample removal after various time periods following injection of the radioactive tracer facilitates generating a minute-by-minute picture of the secretory processes, both in normal animals and in those that are choline deficient. 

Synthesis of phosphatidylcholine. The rate-limiting reaction is that catalyzed by cytidylyltransferase (reaction 2) which appears to be active only when attached to the endoplasmic reticulum, although it is also found free in the cytosol. Cytidylyltransferase is inactivated by a cAMP-dependent protein kinase and activated by a phosphatase. Translocation to the endoplasmic reticulum can be stimulated by substrates such as fatty acyl Coenzyme A (CoA). Choline deficiency can result in deposition of triacylglycerol in liver and reduced phospholipid synthesis. Enzymes (1) choline kinase ... 

Rats fed a choline-deficient diet rapidly develop a fatty liver. It was of interest to investigate whether rats fed such a choline-deficient diet would respond to treatment with L-tryptophan. Rats fed the control (choline-supplemented) diet but not the choline-deficient diet for 1 week and tube-fed L-tryptophan 10 min before being killed revealed enhanced labeled hepatic nuclear RNA release in vitro.174 When rats were fed elevated L-tryptophan (2%) in the diets (choline-deficient (CD) or choline-supplemented (CS)) for 1 week, labeled hepatic nuclear RNA release was increased with the CS + tryptophan diet but not with the CS + tryptophan diet groups. 3H-tryptophan binding to hepatic nuclei in vitro revealed no change in the CS + tryptophan group,... 

A second lipothrophic factor is betaine, which is effective because the transfer of at least one of its methyl groups to homocysteine is very efficient and can replenish methionine for choline formation. In the absence of sufficient lipotrophic factors, a fatty liver develops, and there is insufficient movement of fats either ingested or synthesized in the liver to the adipose tissue. As fats enter or are synthesized in the liver, they are repackaged or packaged as VLDLs to be moved out for transport from the blood to adipose tissue. The VLDLs contain protein, triacylglycerol, cholesterol, cholesterol esters, and phospholipids, especially phosphatidylcholine (lecithin). If one has either a protein deficiency or a lipotrophic factor deficiency, the movement of triacylglycerol s from the liver to adipose is ineffective and a fatty liver can develop. Choline can be present in the diet and need not be synthesized de novo. Phospholipid synthesis has been discussed previously (Chapter 15). 

Decreased Complex I activity in fatty liver mitochondria isolated from rats fed with a choline-deficient diet to model in animals nonalcoholic fatty liver disease could also be completely restored to the level of control livers by exogenously added CL (Petrosillo et al., 2007). Under conditions of a choline-deficient diet the mitochondrial content of CL decreased due to reactive oxygen species-induced CL oxidation. Although no high-resolution crystal structure of the entire Complex I is available, these findings strongly suggest the presence of functionally important CL molecules in the complex. 

Phosphatidylcholine is the major phospholipid on the surface monolayer of all lipoproteins, including VLDLs. In the liver, phosphatidylcholine is synthesized by two biosynthetic pathways the CDP-choline pathway and the phosphatidylethanolamine A -methyltransferase pathway (Chapter 8). Choline is an essential biosynthetic precursor of phosphatidylcholine via the CDP-choline pathway. When cells or animals are deprived of choline, plasma levels of TG and apo B are markedly reduced and TG accumulates in the liver, resulting in fatty liver. These observations led to the widely held view that the fatty liver caused by choline deficiency is due to inhibition of PC synthesis, which in turn would decrease VLDL secretion. This hypothesis was tested in primary rat hepatocytes cultured in medium lacking choline. Upon removal of choline/methionine from culture medium, the TG content of hepatocytes was increased 6-fold, and the secretion of TG and apo B in VLDL was markedly reduced. The interpretation of these experiments was that hepatic VLDL secretion requires the synthesis of phosphatidylcholine from either the CDP-choline or methylation pathways which require choline or methionine, respectively, as precursors (D.E. Vance, 1988). However, since choline deprivation was induced in a background of methionine insufficiency, it was not clear whether the lack of choline per se, and inhibition of the choline pathway for phosphatidylcholine synthesis, decreased VLDL secretion. More recent experiments have shown, surprisingly, that deficiency of choline in primary mouse hepatocytes does not reduce, but increases, phosphatidylcholine synthesis via the CDP-choline pathway, and does not decrease VLDL secretion (J.E. Vance, 2004). Thus, a deficiency of dietary choline reduces plasma TG and apo B levels by a mechanism that does not involve reduction of phosphatidylcholine synthesis. 

Because choline is widely distributed in the food supply, primarily in phosphatidylcholine (lecithin), deficiencies have not been observed in humans on a normal diet. Deficiencies may occur, however, in patients on total parental nutrition (TPN), i.e., supported solely by intravenous feeding. The fatty livers that have been observed in these patients probably result from a decreased ability to synthesize phospholipids for VLDL formation. 

The fatty infiltration of the liver which accompanies the ingestion of orotic acid does not seem to be accompanied by serious pathological disturbances [293] and is readily reversible, unlike the development of fatty liver induced by a choline deficient diet. Supplementation of the orotic acid diet with adenine essentially modifies the effect of orotic acid [294]. Since PRPP is required for both the synthesis of purines and the metabolism of orotic acid, the decrease in the pool of adenine nucleotides is caused [295,296] by an inhibition of purine synthesis de novo due to extensive depletion of PRPP during the conversion of orotic acid to UMP. After the disappearance of orotic acid from the liver of animals previously fed a diet containing orotic acid, stimulation of the synthesis of adenine nucleotides occurred. 

Simon, J. B., Scheig, R. and Jvlatskin, G. Protection by orotic acid against the renal necrosis and fatty liver of choline deficiency. Proc. Soc. Exp. Biol. Med., 129,874-877... 

The source of fatty hvers due to deficiency of choline in the diet is much less clear. Choline is a constituent of lecithin and as such might be supposed to aid in phospholipid synthesis. It has actually been shown to increase the turnover rate of phospholipids in the liver, but did not cause any net increase in plasma and liver phospholipids. No increased secretion of phospholipids into the blood could be found, following choline administration to deficient rats (Entenman et al. 1946 ZiLVERSMiT and Dilitzio 1958). The relationship between increased synthesis of phospholipids in the liver and the action of choline on the prevention and cure of fatty livers ( lipotropic action ) is not at all clear. 

In addition to this effect choline seems to have an accelerating effect on fatty acid oxidation (Artom 1958). The mechanism of this action is unknown, nor is it clear whether the quantitative effect on fatty acid oxidation could explain the accumulation of fat in the liver of choline deficient animals. 

Fatty livers are also formed, in the presence of adequate choline, by diets deficient in amino acids other than methionine. This has been shown for threonine (SiNGAL et al. 1954) lysine and tryptophan (Vennart et al. 1958). Threonine deficiency, like choline deficiency, also leads to an increased synthesis of fatty acid from acetate (Yoshida and Harper 1960). An increase in synthesis has also been observed when cystine is added to a low protein diet. The relative importance of these effects for the formation of fatty livers is still uncertain. 

Fig. 18. Liver of a rat suffering from choline deficiency (low-protein, high-fat diet). Note extensive fatty infiltration of cells. Fibrous tissue has developed which divides organ up into lobules which bear little relation to the normal anatomical lobular arrangement.

Cirrhosis of the liver in man is often found in chronic alcoholism and is probably due to dietary deficiency. In active fatty alcoholic cirrhosis, choline administration has been shown to lead to a decrease in liver fat. An increase in the rate of phospholipid turnover, following administration of 10 g. of choline or methionine, has been demonstrated in patients with cirrhosis who had evidence of fatty infiltration of the liver as shown by biopsy. In animals, vitamin B12 and folic acid are intimately related to choline and methionine metabolism and are important in the prevention of fatty livers under certain conditions. Whether these vitamins are related to accumulation of fat in the liver and cirrhosis in man remains to be ascertained. The value of high protein diets in the prevention and treatment of experimental dietary cirrhosis in animals is well established there is much evidence that such is also true in man (see also p. 521). 

Choline is considered to be an important member of the vitamin B complex, since most animals, when given diets low in this compound, develop deficiency characterized by fatty livers and hemorrhagic lesions of the kidney. Choline is believed to function in at least three ways (1) as an integral part of acetylcholine, (2) as a source of labile methyl groups, and (3) in stimulating the formation of phospholipids. 

Choline is a source of methyl groups for metabolic activity. It is not always grouped with the water-soluble B vitamins. It can be made in the body, but under some conditions it might become essential in the diet. In various species choline deficiency has been associated with fatty liver, cirrhosis, hemorrhagic kidney, and later development of a renal type of hypertension. The significance of these findings in man is not established, and there is, as yet, no clear therapeutic value of supplying choline in human diets, as distinct from other dietary improvement. Therefore any dietary requirement can not be estimated. 

Patterson and his associates found that choline deficiency resulted in decreased phosphatide turnover as well as in decreased concentration of phosphatides in liver (137) and kidneys (138) of rats. The fatty liver and... 

Animal experiments have shown that faulty nutrition, i.e. > 90% fat, < 10% protein and < 2 mg choline per day, leads to pronounced fatty fiver and even fatty cirrhosis within a few weeks. The same changes could be observed when the protein intake remained more or less normal, while extremely little methionine and choline was offered. With a partial surplus of certain foodstuffs, the special nature of the excessive nutritional components is also of considerable importance. The term partial malnutrition may, for example, be associated with a pronounced protein deficiency (and thus possibly inadequate production of lipoproteins) or a lack of lipotropic substances (such as methionine, choline, cystine, glycocoUbetaine, pyridoxine, casein and various N- or S-methylated substances). Protein deficiency has particularly severe consequences when toxic substances are absorbed at the same time or when the organism has to fight bacterial or parasitic infections. A diseased liver reacts to both a serious deficiency in and an excessive supply of different nutrients (e.g. proteins, certain kinds of amino acids, various lipids, trace elements) with unfavourable or even complicative developments during the course of disease. 

However the activation of the A6 desaturation is shown later on. At 15 days it is very important. The apparent Km and Vm of the enz)mie are modified. This activation recovers the double bond index saturated acid ratio to 2.2 and is correlative to an increase of the triacylglycerol phosphatidyl choline ratio of the microsomes. Undoubtedly it is not correlative to the modification of the fatty acid composition of the membrane. Besides Ayala and Brenner have shown that the effect is not due to substrate deprivation (linoleate or a-linolenate) since rats fed on diets containing fish oil during 4 or 6 weeks have even lower A6 desaturation activity in liver microsomes than animals fed on sunflower seed oil compared to rats fed on EFA free diets (Table 2), Therefore the increase of the A6 desaturase activity in EFA deficiency is a physiological response of the cell to maintain the unsaturated/saturated acid ratio and fluidity of the membrane. 

Detoxification Of Poisonous Substances. A healthy liver has at its disposal many chemical reactions for the detoxification of poisonous substances. These reactions rely on the presence of enzymes whose activity depends in part on the level of dietary protein, and in part on the need to detoxify substances such as alcohol and various drugs. Therefore, protein-deficient persons may have heightened susceptibility to the undesirable side effects of certain medications. Chronic abuse of alcohol may also impair liver function since it may promote fatty degeneration. In addition to protein, other nutrients which promote liver function are the B-com-plex vitamins, methionine, choline, and lecithin. 

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