Vitamin E does not prevent Western diet-induced NASH progression and increases metabolic flux dysregulation in mice.

Fatty liver involves ectopic lipid accumulation and dysregulated hepatic oxidative metabolism, which can progress to a state of elevated inflammation and fibrosis referred to as nonalcoholic steatohepatitis (NASH). The factors that control progression from simple steatosis to NASH are not fully known. Here, we tested the hypothesis that dietary vitamin E (VitE) supplementation would prevent NASH progression and associated metabolic alterations induced by a Western diet (WD). Hyperphagic melanocortin-4 receptor-deficient (MC4R-/-) mice were fed chow, chow+VitE, WD, or WD+VitE starting at 8 or 20 weeks of age. All groups exhibited extensive hepatic steatosis by the end of the study (28 weeks of age). WD feeding exacerbated liver disease severity without inducing proportional changes in liver triglycerides. Eight weeks of WD accelerated liver pyruvate cycling, and 20 weeks of WD extensively upregulated liver glucose and oxidative metabolism assessed by 2H/13C flux analysis. VitE supplementation failed to reduce the histological features of NASH. Rather, WD+VitE increased the abundance and saturation of liver ceramides and accelerated metabolic flux dysregulation compared with 8 weeks of WD alone. In summary, VitE did not limit NASH pathogenesis in genetically obese mice, but instead increased some indicators of metabolic dysfunction.

One-Carbon Metabolism in Fatty Liver Disease and Fibrosis: One-Carbon to Rule Them All.

Nonalcoholic fatty liver disease (NAFLD) is a term used to characterize a range of disease states that involve the accumulation of fat in the liver but are not associated with excessive alcohol consumption. NAFLD is a prevalent disease that can progress to organ damage like liver cirrhosis and hepatocellular carcinoma. Many animal models have demonstrated that one-carbon metabolism is strongly associated with NAFLD. Phosphatidylcholine is an important phospholipid that affects hepatic lipid homeostasis and de novo synthesis of this phospholipid is associated with NAFLD. However, one-carbon metabolism serves to support all cellular methylation reactions and catabolism of methionine, serine, glycine, choline, betaine, tryptophan, and histidine. Several different pathways within one-carbon metabolism that play important roles in regulating energy metabolism and immune function have received less attention in the study of fatty liver disease and fibrosis. This review examines what we have learned about hepatic lipid metabolism and liver damage from the study of one-carbon metabolism thus far and highlights unexplored opportunities for future research.

Creatine reduces hepatic TG accumulation in hepatocytes by stimulating fatty acid oxidation.

Non-alcoholic fatty liver disease encompasses a wide spectrum of liver damage including steatosis, non-alcoholic steatohepatitis, fibrosis and cirrhosis. We have previously reported that creatine supplementation prevents hepatic steatosis and lipid peroxidation in rats fed a high-fat diet. In this study, we employed oleate-treated McArdle RH-7777 rat hepatoma cells to investigate the role of creatine in regulating hepatic lipid metabolism. Creatine, but not structural analogs, reduced cellular TG accumulation in a dose-dependent manner. Incubating cells with the pan-lipase inhibitor diethyl p-nitrophenylphosphate (E600) did not diminish the effect of creatine, demonstrating that the TG reduction brought about by creatine does not depend on lipolysis. Radiolabeled tracer experiments indicate that creatine increases fatty acid oxidation and TG secretion. In line with increased fatty acid oxidation, mRNA analysis revealed that creatine-treated cells had increased expression of PPARα and several of its transcriptional targets. Taken together, this study provides direct evidence that creatine reduces lipid accumulation in hepatocytes by the stimulation of fatty acid oxidation and TG secretion.

Betaine supplementation prevents fatty liver induced by a high-fat diet: effects on one-carbon metabolism.

The purpose of this study was to examine the effects of betaine supplementation on the regulation of one-carbon metabolism and liver lipid accumulation induced by a high-fat diet in rats. Rats were fed one of three different liquid diets: control diet, high-fat diet and high-fat diet supplemented with betaine. The control and high-fat liquid diets contained, respectively, 35 and 71 % of energy derived from fat. Betaine supplementation involved the addition of 1 % (g/L) to the diet. After three weeks on the high-fat diet the rats had increased total liver fat concentration, liver triglycerides, liver TBARS and plasma TNF-α. The high-fat diet decreased the hepatic S-adenosylmethionine concentration and the S-adenosylmethionine/S-adenosylhomocysteine ratio compared to the control as well as altering the expression of genes involved in one-carbon metabolism. Betaine supplementation substantially increased the hepatic S-adenosylmethionine concentration (~fourfold) and prevented fatty liver and hepatic injury induced by the high-fat diet. It was accompanied by the normalization of the gene expression of BHMT, GNMT and MGAT, which code for key enzymes of one-carbon metabolism related to liver fat accumulation. In conclusion, the regulation of the expression of MGAT by betaine supplementation provides an additional and novel mechanism by which betaine supplementation regulates lipid metabolism and prevents accumulation of fat in the liver.

Choline supplementation protects against liver damage by normalizing cholesterol metabolism in Pemt/Ldlr knockout mice fed a high-fat diet.

Dietary choline is required for proper structure and dynamics of cell membranes, lipoprotein synthesis, and methyl-group metabolism. In mammals, choline is synthesized via phosphatidylethanolamine N-methyltransferase (Pemt), which converts phosphatidylethanolamine to phosphatidylcholine. Pemt(-/-) mice have impaired VLDL secretion and developed fatty liver when fed a high-fat (HF) diet. Because of the reduction in plasma lipids, Pemt(-/-)/low-density lipoprotein receptor knockout (Ldlr(-/-)) mice are protected from atherosclerosis. The goal of this study was to investigate the importance of dietary choline in the metabolic phenotype of Pemt(-/-)/Ldlr(-/-) male mice. At 10-12 wk of age, Pemt(+/+)/Ldlr(-/-) (HF(+/+)) and half of the Pemt(-/-)/Ldlr(-/-) (HF(-/-)) mice were fed an HF diet with normal (1.3 g/kg) choline. The remaining Pemt(-/-)/Ldlr(-/-) mice were fed an HF diet supplemented (5 g/kg) with choline (HFCS(-/-) mice). The HF diet contained 60% of calories from fat and 1% cholesterol, and the mice were fed for 16 d. HF(-/-) mice lost weight and developed hepatomegaly, steatohepatitis, and liver damage. Hepatic concentrations of free cholesterol, cholesterol-esters, and triglyceride (TG) were elevated by 30%, 1.1-fold and 3.1-fold, respectively, in HF(-/-) compared with HF(+/+) mice. Choline supplementation normalized hepatic cholesterol, but not TG, and dramatically improved liver function. The expression of genes involved in cholesterol transport and esterification increased by 50% to 5.6-fold in HF(-/-) mice when compared with HF(+/+) mice. Markers of macrophages, oxidative stress, and fibrosis were elevated in the HF(-/-) mice. Choline supplementation normalized the expression of these genes. In conclusion, HF(-/-) mice develop liver failure associated with altered cholesterol metabolism when fed an HF/normal choline diet. Choline supplementation normalized cholesterol metabolism, which was sufficient to prevent nonalcoholic steatohepatitis development and improve liver function. Our data suggest that choline can promote liver health by maintaining cholesterol homeostasis.

Synthesis of guanidinoacetate and creatine from amino acids by rat pancreas.

Creatine is an important molecule involved in cellular energy metabolism. Creatine is spontaneously converted to creatinine at a rate of 1·7% per d; creatinine is lost in the urine. Creatine can be obtained from the diet or synthesised from endogenous amino acids via the enzymes arginine:glycine amidinotransferase (AGAT) and guanidinoacetate N-methyltransferase (GAMT). The liver has high GAMT activity and the kidney has high AGAT activity. Although the pancreas has both AGAT and GAMT activities, its possible role in creatine synthesis has not been characterised. In the present study, we examined the enzymes involved in creatine synthesis in the pancreas as well as the synthesis of guanidinoacetate (GAA) and creatine by isolated pancreatic acini. We found significant AGAT activity and somewhat lower GAMT activity in the pancreas and that pancreatic acini had measurable activities of both AGAT and GAMT and the capacity to synthesise GAA and creatine from amino acids. Creatine supplementation led to a decrease in AGAT activity in the pancreas, though it did not affect its mRNA or protein abundance. This was in contrast with the reduction of AGAT activity and mRNA and protein abundance in the kidney, suggesting that the regulatory mechanisms that control the expression of this enzyme in the pancreas are different from those in the kidney. Dietary creatine increased the concentrations of GAA, creatine and phosphocreatine in the pancreas. Unexpectedly, creatine supplementation decreased the concentrations of S-adenosylmethionine, while those of S-adenosylhomocysteine were not altered significantly.

The Dietary Supplement Creatyl-l-Leucine Does Not Bioaccumulate in Muscle, Brain or Plasma and Is Not a Significant Bioavailable Source of Creatine.

Creatine is an important energy metabolite that is concentrated in tissues such as the muscles and brain. Creatine is reversibly converted to creatine phosphate through a reaction with ATP or ADP, which is catalyzed by the enzyme creatine kinase. Dietary supplementation with relatively large amounts of creatine monohydrate has been proven as an effective sports supplement that can enhances athletic performance during acute high-energy demand physical activity. Some side effects have been reported with creatine monohydrate supplementation, which have stimulated research into new potential molecules that could be used as supplements to potentially provide bioavailable creatine. Recently, a popular supplement, creatyl-l-leucine, has been proposed as a potential dietary ingredient that may potentially provide bioavailable creatine. This study tests whether creatyl-l-leucine is a bioavailable compound and determines whether it can furnish creatine as a dietary supplement. Rats were deprived of dietary creatine for a period of two weeks and then given one of three treatments: a control AIN-93G creatine-free diet, AIN-93G supplemented with creatine monohydrate or AIN-93G with an equimolar amount of creatyl-l-leucine supplement in the diet for one week. When compared to the control and the creatine monohydrate-supplemented diet, creatyl-l-leucine supplementation resulted in no bioaccumulation of either creatyl-l-leucine or creatine in tissue.

Creatine supplementation protects against diet-induced non-alcoholic fatty liver but exacerbates alcoholic fatty liver.

AIMS: This work investigated the effects of creatine supplementation on different pathways related to the pathogenesis of non-alcoholic fatty liver disease and alcoholic liver disease. MAIN METHODS: To induce alcoholic liver disease, male Swiss mice were divided into three groups: control, ethanol and ethanol supplemented with creatine. To induce non-alcoholic fatty liver disease, mice were divided into three groups: control, high-fat diet and high-fat diet supplemented with creatine. Each group consisted of eight animals. In both cases, creatine monohydrate was added to the diets (1 %; weight/vol). KEY FINDINGS: Creatine supplementation prevented high-fat diet-induced non-alcoholic fatty liver disease progression, demonstrated by attenuated liver fat accumulation and liver damage. On the other hand, when combined with ethanol, creatine supplementation up-regulated key genes related to ethanol metabolism, oxidative stress, inflammation and lipid synthesis, and exacerbated ethanol-induced liver steatosis and damage, demonstrated by increased liver fat accumulation and histopathological score, as well as elevated oxidative damage markers and inflammatory mediators. SIGNIFICANCE: Our results clearly demonstrated creatine supplementation exerts different outcomes in relation to non-alcoholic fatty liver disease and alcoholic liver disease, namely it protects against high-fat diet-induced non-alcoholic fatty liver disease but exacerbates ethanol-induced alcoholic liver disease. The exacerbating effects of the creatine and ethanol combination appear to be related to oxidative stress and inflammation-mediated up-regulation of ethanol metabolism.

Creatine supplementation prevents the accumulation of fat in the livers of rats fed a high-fat diet.

The aim of the present study was to examine the effects of creatine supplementation on liver fat accumulation induced by a high-fat diet in rats. Rats were fed 1 of 3 different diets for 3 wk: a control liquid diet (C), a high-fat liquid diet (HF), or a high-fat liquid diet supplemented with creatine (HFC). The C and HF diets contained, respectively, 35 and 71% of energy derived from fat. Creatine supplementation involved the addition of 1% (wt:v) of creatine monohydrate to the liquid diet. The HF diet increased total liver fat concentration, liver TG, and liver TBARS and decreased the hepatic S-adenosylmethionine (SAM) concentration. Creatine supplementation normalized all of these perturbations. Creatine supplementation significantly decreased the renal activity of l-arginine:glycine amidinotransferase and plasma guanidinoacetate and prevented the decrease in hepatic SAM concentration in rats fed the HF diet. However, there was no change in either the phosphatidylcholine:phosphatidylethanolamine (PE) ratio or PE N-methyltransferase activity. The HF diet decreased mRNA for PPARα as well as 2 of its targets, carnitine palmitoyltransferase and long-chain acylCoA dehydrogenase. Creatine supplementation normalized these mRNA levels. In conclusion, creatine supplementation prevented the fatty liver induced by feeding rats a HF diet, probably by normalization of the expression of key genes of ß-oxidation.

The metabolic burden of creatine synthesis.

Creatine synthesis is required in adult animals to replace creatine that is spontaneously converted to creatinine and excreted in the urine. Additionally, in growing animals it is necessary to provide creatine to the expanding tissue mass. Creatine synthesis requires three amino acids: glycine, methionine and arginine, and three enzymes: L-arginine:glycine amidinotransferase (AGAT), methionine adenosyltransferase (MAT) and guanidinoacetate methyltransferase (GAMT). The entire glycine molecule is consumed in creatine synthesis but only the methyl and amidino groups, respectively, from methionine and arginine. Creatinine loss averages approximately 2 g (14.6 mmol) for 70 kg males in the 20- to 39-year age group. Creatinine loss is lower in females and in older age groups because of lower muscle mass. Approximately half of this creatine lost to creatinine can be replaced, in omnivorous individuals, by dietary creatine. However, since dietary creatine is only provided in animal products, principally in meat and fish, virtually all of the creatine loss in vegetarians must be replaced via endogenous synthesis. Creatine synthesis does not appear to place a major burden on glycine metabolism in adults since this amino acid is readily synthesized. However, creatine synthesis does account for approximately 40% of all of the labile methyl groups provided by S-adenosylmethionine (SAM) and, as such, places an appreciable burden on the provision of such methyl groups, either from the diet or via de novo methylneogenesis. Creatine synthesis consumes some 20-30% of arginine's amidino groups, whether provided in the diet or synthesized within the body. Creatine synthesis is, therefore, a quantitatively major pathway in amino acid metabolism and imposes an appreciable burden on the metabolism of methionine and of arginine.

Systematic deletion of adenosine receptors reveals novel roles in inflammation and pyroptosis in THP-1 macrophages.

Macrophages perform the fundamental function of sensing cellular damage, initiating and mediating immune response and tissue repair. Adenine nucleotides are in relatively high abundance in cells and are released from cells during tissue damage that are converted to adenosine in the extracellular environment. The A1, A2A, A2B and A3 adenosine receptors serve to regulate immune function. Despite characterization of the adenosine receptors, a comprehensive examination of adenosine receptor signaling in THP-1 macrophage cells has not been done. Moreover, previous studies employed chemical agonists and antagonists that have the potential for off-target affects. Here we systematically knockdown each of the four known adenosine receptors in THP-1 macrophages using validated siRNA and investigated their function under LPS stimulation. We demonstrate that the A1 receptor is required for adenosine-stimulated IL-10 and IL-1ß secretion indicating an important role of this receptor during resolution of inflammation and tissue repair in these cells. The A1 and A3 receptor were required for IL-6 and IL-1ß secretion showing a net pro-inflammatory role for these receptors. Finally, we present the novel finding that THP-1 macrophages lacking the A2B receptor undergo pyroptosis when exposed to LPS, demonstrating a novel role of the A2B receptor in regulation of programmed cell death during inflammation. This work underscores the fundamental importance of adenosine signaling and provides insight into the independent roles of the adenosine receptors in modulating cytokine signaling.

Targeted and untargeted metabolomics provide insight into the consequences of glycine-N-methyltransferase deficiency including the novel finding of defective immune function.

Fatty liver disease is increasing along with the prevalence of obesity and type-2 diabetes. Hepatic fibrosis is a major health complication for which there are no efficacious treatment options available. A better understanding of the fundamental mechanisms that contribute to the accumulation of fibrosis is needed. Glycine-N-methyltransferase (GNMT) is a critical enzyme in one-carbon metabolism that serves to regulate methylation and remethylation reactions. GNMT knockout (GNMT-/- ) mice display spontaneous hepatic fibrosis and later develop hepatocellular carcinoma. Previous literature supports the idea that hypermethylation as a consequence of GNMT deletion contributes to the hepatic phenotype observed. However, limited metabolomic information is available and the underlying mechanisms that contribute to hepatic fibrogenesis in GNMT-/- mice are still incomplete. Therefore, our goals were to use dietary intervention to determine whether increased lipid load exacerbates steatosis and hepatic fibrosis in this model and to employ both targeted and untargeted metabolomics to further understand the metabolic consequences of GNMT deletion. We find that GNMT mice fed high-fat diet do not accumulate more lipid or fibrosis in the liver and are in fact resistant to weight gain. Metabolomics analysis confirmed that pan-hypermethylation occurs in GNMT mice resulting in a depletion of nicotinamide intermediate metabolites. Further, there is a disruption in tryptophan catabolism that prevents adequate immune cell activation in the liver. The chronic cellular damage cannot be appropriately cleared due to a lack of immune checkpoint activation. This mouse model is an excellent example of how a disruption in small molecule metabolism can significantly impact immune function.

Disruption of hepatic one-carbon metabolism impairs mitochondrial function and enhances macrophage activity in methionine-choline-deficient mice.

One-carbon metabolism is a collection of metabolic cycles that supports methylation and provides one-carbon bound folates for the de novo synthesis of purine and thymidine nucleotides. The methylation of phosphatidylethanolamine to form choline has been extensively studied in the context of fatty liver disease. However, the role of one-carbon metabolism in supporting nucleotide synthesis during liver damage has not been addressed. The objective of this study is to determine how the disruption of one-carbon metabolism influences nucleotide metabolism in the liver after dietary methionine and choline restriction. Mice (n=8) were fed a methionine-choline-deficient or control diet for 3 weeks. We treated mice with the compound alloxazine (0.5 mg/kg), a known adenosine receptor antagonist, every second day during the final week of feeding to probe the function of adenosine signaling during liver damage. We found that concentrations of several hepatic nucleotides were significantly lower in methionine- and choline-deficient mice vs. controls (adenine: 13.9±0.7 vs. 10.1±0.6, guanine: 1.8±0.1 vs. 1.4±0.1, thymidine: 0.0122±0.0027 vs. 0.0059±0.0027 nmol/mg dry tissue). Treatment of alloxazine caused a specific decrease in thymidine nucleotides, decrease in mitochondrial content in the liver and exacerbation of steatohepatitis as shown by the increased hepatic lipid content and altered macrophage morphology. This study demonstrates a role for one-carbon metabolism in supporting de novo nucleotide synthesis and mitochondrial function during liver damage.

Dietary creatine supplementation lowers hepatic triacylglycerol by increasing lipoprotein secretion in rats fed high-fat diet.

Recent studies have shown that dietary creatine supplementation can prevent lipid accumulation in the liver. Creatine is a small molecule that plays a large role in energy metabolism, but since the enzyme creatine kinase is not present in the liver, the classical role in energy metabolism does not hold in this tissue. Fat accumulation in the liver can lead to the development of nonalcoholic fatty liver disease (NAFLD), a progressive disease that is prevalent in humans. We have previously reported that creatine can directly influence lipid metabolism in cell culture to promote lipid secretion and oxidation. Our goal in the current study was to determine whether similar mechanisms that occur in cell culture were present in vivo. We also sought to determine whether dietary creatine supplementation could be effective in reversing steatosis. Sprague-Dawley rats were fed a high-fat diet or a high-fat diet supplemented with creatine for 5 weeks. We found that rats supplemented with creatine had significantly improved rates of lipoprotein secretion and alterations in mitochondrial function that were consistent with greater oxidative capacity. We also find that introducing creatine into a high-fat diet halted hepatic lipid accumulation in rats with fatty liver. Our results support our previous report that liver cells in culture with creatine secrete and oxidize more oleic acid, demonstrating that dietary creatine can effectively change hepatic lipid metabolism by increasing lipoprotein secretion and oxidation in vivo. Our data suggest that creatine might be an effective therapy for NAFLD.

Tissue Specific Effects of Dietary Carbohydrates and Obesity on ChREBPα and ChREBPß Expression.

Carbohydrate response element binding protein (ChREBP) regulates insulin-independent de novo lipogenesis. Recently, a novel ChREBPß isoform was identified. The purpose of the current study was to define the effect of dietary carbohydrates (CHO) and obesity on the transcriptional activity of ChREBP isoforms and their respective target genes. Mice were subjected to fasting-refeeding of high-CHO diets. In all three CHO-refeeding groups, mice failed to induce ChREBPα, yet ChREBPß increased 10- to 20-fold. High-fat fed mice increased hepatic ChREBPß mRNA expression compared to chow-fed along with increased protein expression. To better assess the independent effect of fructose on ChREBPα/ß activity, HepG2 cells were treated with fructose ± a fructose-1,6-bisphosphatase inhibitor to suppress gluconeogenesis. Fructose treatment in the absence of gluconeogenesis resulted in increased ChREBP activity. To confirm the existence of ChREBPß in human tissue, primary hepatocytes were incubated with high-glucose and the expression of ChREBPα and -ß was determined. As with the animal models, glucose induced ChREBPß expression while ChREBPα was decreased. Taken together, ChREBPß is more responsive to changes in dietary CHO availability than the -α isoform. Diet-induced obesity increases basal expression of ChREBPß, which may increase the risk of developing hepatic steatosis, and fructose-induced activation is independent of gluconeogenesis.

Choline deficiency impairs intestinal lipid metabolism in the lactating rat.

Choline is a precursor to phosphatidylcholine (PC), a structural molecule in cellular membranes that is crucial for cell growth and function. PC is also required for the secretion of lipoprotein particles from liver and intestine. Choline requirements are increased during lactation when maternal choline is supplied to the offspring through breast milk. To investigate the effect of dietary choline on intestinal lipid metabolism during lactation, choline-supplemented (CS), phosphatidylcholine-supplemented (PCS) or choline-deficient (CD) diets were fed to dams during the suckling period. CD dams had lower plasma triacylglycerol, cholesterol and apoB in the fasted state and following a fat-challenge (P < .05). There was a higher content of neutral lipids and lower content of PC in the intestine of CD dams, compared with CS and PCS fed animals (P < .05). In addition, there was lower (P < .05) villus height in CD dams, which indicated a reduced absorptive surface area in the intestine. Choline is critical for the absorption of fat in lactating rats and choline deficiency alters intestinal morphology and impairs chylomicron secretion by limiting the supply of PC.

Novel insights on interactions between folate and lipid metabolism.

Folate is an essential B vitamin required for the maintenance of AdoMet-dependent methylation. The liver is responsible for many methylation reactions that are used for post-translational modification of proteins, methylation of DNA, and the synthesis of hormones, creatine, carnitine, and phosphatidylcholine. Conditions where methylation capacity is compromised, including folate deficiency, are associated with impaired phosphatidylcholine synthesis resulting in non-alcoholic fatty liver disease and steatohepatitis. In addition, folate intake and folate status have been associated with changes in the expression of genes involved in lipid metabolism, obesity, and metabolic syndrome. In this review, we provide insight on the relationship between folate and lipid metabolism, and an outlook for the future of lipid-related folate research.

The concentration of phosphatidylethanolamine in mitochondria can modulate ATP production and glucose metabolism in mice.

Phosphatidylethanolamine (PE) N-methyltransferase (PEMT) catalyzes the synthesis of phosphatidylcholine (PC) in the liver. Mice lacking PEMT are protected against diet-induced obesity and insulin resistance. We investigated the role of PEMT in hepatic carbohydrate metabolism in chow-fed mice. A pyruvate tolerance test revealed that PEMT deficiency greatly attenuated gluconeogenesis. The reduction in glucose production was specific for pyruvate; glucose production from glycerol was unaffected. Mitochondrial PC levels were lower and PE levels were higher in livers from Pemt(-/-) compared with Pemt(+/+) mice, resulting in a 33% reduction of the PC-to-PE ratio. Mitochondria from Pemt(-/-) mice were also smaller and more elongated. Activities of cytochrome c oxidase and succinate reductase were increased in mitochondria of Pemt(-/-) mice. Accordingly, ATP levels in hepatocytes from Pemt(-/-) mice were double that in Pemt(+/+) hepatocytes. We observed a strong correlation between mitochondrial PC-to-PE ratio and cellular ATP levels in hepatoma cells that expressed various amounts of PEMT. Moreover, mitochondrial respiration was increased in cells lacking PEMT. In the absence of PEMT, changes in mitochondrial phospholipids caused a shift of pyruvate toward decarboxylation and energy production away from the carboxylation pathway that leads to glucose production.

Creatine synthesis: hepatic metabolism of guanidinoacetate and creatine in the rat in vitro and in vivo.

Since creatinine excretion reflects a continuous loss of creatine and creatine phosphate, there is a need for creatine replacement, from the diet and/or by de novo synthesis. Creatine synthesis requires three amino acids, methionine, glycine, and arginine, and two enzymes, l-arginine:glycine amidinotransferase (AGAT), which produces guanidinoacetate acid (GAA), and guanidinoacetate methyltransferase (GAMT), which methylates GAA to produce creatine. In the rat, high activities of AGAT are found in the kidney, whereas high activities of GAMT occur in the liver. Rat hepatocytes readily convert GAA to creatine; this synthesis is stimulated by the addition of methionine, which increases cellular S-adenosylmethionine concentrations. These same hepatocytes are unable to produce creatine from methionine, arginine, and glycine. (15)N from (15)NH(4)Cl is readily incorporated into urea but not into creatine. Hepatic uptake of GAA is evident in vivo by livers of rats fed a creatine-free diet but not when rats were fed a creatine-supplemented diet. Rats fed the creatine-supplemented diet had greatly decreased renal AGAT activity and greatly decreased plasma [GAA] but no decrease in hepatic GAMT or in the capacity of hepatocytes to produce creatine from GAA. These studies indicate that hepatocytes are incapable of the entire synthesis of creatine but are capable of producing it from GAA. They also illustrate the interplay between the dietary provision of creatine and its de novo synthesis and point to the crucial role of renal AGAT expression in regulating creatine synthesis in the rat.