Periportal zonation of the cytosolic acetyl-CoA synthetase of male rat liver

Peroxisomal oxidation of erucic acid suppresses mitochondrial fatty acid oxidation by stimulating malonyl-CoA formation in the rat liver

Feeding of rapeseed (canola) oil with a high erucic acid concentration is known to cause hepatic steatosis in animals. Mitochondrial fatty acid oxidation plays a central role in liver lipid homeostasis, so it is possible that hepatic metabolism of erucic acid might decrease mitochondrial fatty acid oxidation. However, the precise mechanistic relationship between erucic acid levels and mitochondrial fatty acid oxidation is unclear. Using male Sprague-Dawley rats, along with biochemical and molecular biology approaches, we report here that peroxisomal β-oxidation of erucic acid stimulates malonyl-CoA formation in the liver and thereby suppresses mitochondrial fatty acid oxidation. Excessive hepatic uptake and peroxisomal β-oxidation of erucic acid resulted in appreciable peroxisomal release of free acetate, which was then used in the synthesis of cytosolic acetyl-CoA. Peroxisomal metabolism of erucic acid also remarkably increased the cytosolic NADH/NAD⁺ ratio, suppressed sirtuin 1 (SIRT1) activity, and thereby activated acetyl-CoA carboxylase, which stimulated malonyl-CoA biosynthesis from acetyl-CoA. Chronic feeding of a diet including high-erucic-acid rapeseed oil diminished mitochondrial fatty acid oxidation and caused hepatic steatosis and insulin resistance in the rats. Of note, administration of a specific peroxisomal β-oxidation inhibitor attenuated these effects. Our findings establish a cross-talk between peroxisomal and mitochondrial fatty acid oxidation. They suggest that peroxisomal oxidation of long-chain fatty acids suppresses mitochondrial fatty acid oxidation by stimulating malonyl-CoA formation, which might play a role in fatty acid-induced hepatic steatosis and related metabolic disorders.

Possible involvement of ACSS2 gene in alcoholism

Alcoholism is a psychiatric disorder that composes one of the principal causes of health disabilities in the world population. Furthermore, the available pharmacotherapy is limited. Therefore, this research was carried out to better understand the basis of the underlying neurobiological processes of this disorder and to discover potential therapeutic targets. Real-time PCR analysis was performed in the amygdala nuclei region of the brain of mice exposed to a chronic three-bottle free-choice model (water, 5 and 10% v/v ethanol). Based on individual ethanol intake, the mice were classified into three groups: "compulsive-like" (i.e., ethanol intake not affected by quinine adulteration), "ethanol-preferring" and "ethanol non-preferring". A fourth group had access only to tap water (control group). The candidate gene ACSS2 was genotyped in human alcoholics by real-time polymerase chain reaction using the markers rs6088638 and rs7266550. Seven genes were picked out (Acss2, Acss3, Acat1, Acsl1, Acaa2, Hadh, and Hadhb) and the mRNA level of the Acss2 gene was increased only in the "compulsive-like" group (p = 0.004). The allele frequency of rs6088638 for the gene ACSS2 was higher in the Alcoholic human group (p = 0.03), although sample size was very small. The gene ACSS2 is associated with alcoholism, suggesting that biochemical pathways where it participates may have a role in the biological mechanisms susceptible to the ethanol effects.

Zonation of hepatic fatty acid metabolism - The diversity of its regulation and the benefit of modeling

A pronounced heterogeneity between hepatocytes in subcellular structure and enzyme activities was discovered more than 50years ago and initiated the idea of metabolic zonation. In the last decades zonation patterns of liver metabolism were extensively investigated for carbohydrate, nitrogen and lipid metabolism. The present review focuses on zonation patterns of the latter. We review recent findings regarding the zonation of fatty acid uptake and oxidation, ketogenesis, triglyceride synthesis and secretion, de novo lipogenesis, as well as bile acid and cholesterol metabolism. In doing so, we expose knowledge gaps and discuss contradictory experimental results, for example on the zonation pattern of fatty acid oxidation and de novo lipogenesis. Thus, possible rewarding directions of further research are identified. Furthermore, recent findings about the regulation of metabolic zonation are summarized, especially regarding the role of hormones, nerve innervation, morphogens, gender differences and the influence of the circadian clock. In the last part of the review, a short collection of models considering hepatic lipid metabolism is provided. We conclude that modeling, despite its proven benefit for understanding of hepatic carbohydrate and ammonia metabolisms, has so far been largely disregarded in the study of lipid metabolism; therefore some possible fields of modeling interest are presented.

In vitro modeling of fatty acid synthesis under conditions simulating the zonation of lipogenic [13C]acetyl-CoA enrichment in the liver

In the companion report (Bederman, I. R., Reszko, A. E., Kasumov, T., David, F., Wasserman, D. H., Kelleher, J. K., and Brunengraber, H. (2004) J. Biol. Chem. 279, 43207-43216), we demonstrated that, when the hepatic pool of lipogenic acetyl-CoA is labeled from [13C]acetate, the enrichment of this pool decreases across the liver lobule. In addition, estimates of fractional synthesis calculated by isotopomer spectral analysis (ISA), a nonlinear regression method, did not agree with a simpler algebraic two-isotopomer method. To evaluate differences between these methods, we simulated in vitro the synthesis of fatty acids under known gradients of precursor enrichment, and known values of fractional synthesis. First, we synthesized pentadecanoate from [U-13C3]propionyl-CoA and four gradients of [U-13C3]malonyl-CoA enrichment. Second, we pooled the fractions of each gradient. Third, we diluted each pool with pentadecanoate prepared from unlabeled malonyl-CoA to simulate the dilution of the newly synthesized compound by pre-existing fatty acids. This yielded a series of samples of pentadecanoate with known values of (i) lower and upper limits for the precursor enrichment, (ii) the shape of the gradient, and (iii) the fractional synthesis. At each step, the mass isotopomer distributions of the samples were analyzed by ISA and the two-isotopomer method to determine whether each method could correctly (i) detect gradients of precursor enrichment, (ii) estimate the gradient limits, and (iii) estimate the fractional synthesis. The two-isotopomer method did not identify gradients of precursor enrichment and underestimated fractional synthesis by up to 2-fold in the presence of gradients. ISA uses all mass isotopomers, correctly identified imposed gradients of precursor enrichment, and estimated the expected values of fractional synthesis within the constraints of the data.

Zonation of labeling of lipogenic acetyl-CoA across the liver: implications for studies of lipogenesis by mass isotopomer analysis

Measurement of fractional lipogenesis by condensation polymerization methods assumes constant enrichment of lipogenic acetyl-CoA in all hepatocytes. mass isotopomer distribution analysis (MIDA) and isotopomer spectral analysis (ISA) represent such methods and are based on the combinatorial analyses of mass isotopomer distributions (MIDs) of fatty acids and sterols. We previously showed that the concentration and enrichment of [13C]acetate decrease markedly across the dog liver because of the simultaneous uptake and production of acetate. To test for zonation of the enrichment of lipogenic acetyl-CoA, conscious dogs, prefitted with transhepatic catheters, were infused with glucose and [1,2-13C2]acetate in a branch of the portal vein. Analyses of MIDs of fatty acids and sterols isolated from liver, bile, and plasma very low density lipoprotein by a variant of ISA designed to detect gradients in precursor enrichment revealed marked zonation of enrichment of lipogenic acetyl-CoA. As control experiments where no zonation of acetyl-CoA enrichment would be expected, isolated rat livers were perfused with 10 mm [1,2-13C2]acetate. The ISA analyses of MIDs of fatty acids and sterols from liver and bile still revealed a zonation of acetyl-CoA enrichment. We conclude that zonation of hepatic acetyl-CoA enrichment occurs under a variety of animal models and physiological conditions. Failure to consider gradients of precursor enrichment can lead to underestimations of fractional lipogenesis calculated from the mass isotopomer distributions. The degree of such underestimation was modeled in vitro, and the data are reported in the companion paper (Bederman, I. R., Kasumov, T., Reszko, A. E., David, F., Brunengraber, H., and Kelleher, J. K. (2004) J. Biol. Chem. 279, 43217-43226).

Enhanced expression of hepatic lipogenic enzymes in an animal model of sedentariness

The hindlimb-suspended rat was used as animal model to investigate the effects induced by immobilization of the skeletal muscle in the expression of the genes encoding hepatic lipogenic enzymes. Following a 14-day period of immobilization, rats were injected intraperitoneally with radioactive acetate, and the labeling of hepatic lipids and cholesterol was evaluated 15 min after the isotope injection. The incorporation of labeled acetate in lipids and cholesterol was almost three times higher in the liver of immobilized rats than in control animals as a consequence of the enhanced transcription of the genes encoding acetyl-CoA synthase, acetyl-CoA carboxylase, fatty acid synthase, and 3-hydroxy-3-methylglutaryl-CoA reductase. The high expression of the key enzymes for fatty acid and cholesterol synthesis induced by immobilization was not paralleled by an increase of the hepatic sterol-regulatory element binding protein (SREBP)-1 and SREBP-2 mRNA content. However, the expression of the mature form of SREBP-1 and SREBP-2 was higher in the nuclear fraction of immobilized rat liver than in controls due to a significant increase of the cleavage of the native proteins. Immobilization also affected the expression of proteins involved in lipid degradation. In fact, the hepatic content of peroxisome proliferator-activated receptor-alpha (PPARalpha) mRNA and of PPARalpha target genes encoding carnitine palmitoyl transferase-1 and acyl-CoA oxidase were significantly increased upon immobilization.

Zonation of acetate labeling across the liver: implications for studies of lipogenesis by MIDA

Measurement of fractional lipogenesis by mass isotopomer distribution analysis (MIDA) of fatty acids or cholesterol labeled from [(13)C]acetate assumes constant enrichment of lipogenic acetyl-CoA in all hepatocytes. This would not be the case if uptake and release of acetate by the liver resulted in transhepatic gradients of acetyl-CoA enrichment. Conscious dogs, prefitted with transhepatic catheters, were infused with glucose and [1, 2-(13)C(2)]acetate. Stable concentrations and enrichments of acetate were measured in artery (17 microM, 36%), portal vein (61 microM, 5. 4%), and hepatic vein (17 microM, 1.0%) and were computed for mixed blood entering the liver (53 microM, 7.4%). We also measured balances of propionate and butyrate across gut and liver. All gut release of propionate and butyrate is taken up by the liver. The threefold decrease in acetate concentration and the sevenfold decrease in acetate enrichment across the liver strongly suggest that the enrichment of lipogenic acetyl-CoA decreases across the liver. Thus fractional hepatic lipogenesis measured in vivo by MIDA may be underestimated.

Ethanol inhibits acetate metabolism in rat hepatocytes

The metabolism of acetate at concentrations of 1, 2.5, 5 and 10 mM was investigated in freshly isolated hepatocytes from 48 hr fasted, female rats in the absence and presence of 10 mM ethanol. The maximal capacity for acetate metabolism was 0.85 mumol/(10(8) cells.min). Ethanol caused a 20% decrease in the apparent Vmax for acetate metabolism and an increase in the apparent Km for acetate from 3.0 to 4.6 mM. At physiological concentration of acetate (approximately 1 mM) and in the absence of an inhibitory effect of ethanol, the capacity for acetate metabolism was 15-20% of the rate of acetate formation from ethanol and the inhibitory effect of ethanol further reduced it to 10-15%. The results thus explain the well-known but hitherto not understood fact that only a small fraction of acetate produced in the liver during ethanol oxidation is further metabolized by the liver, while the majority is exported for oxidation in other tissues. Finally, a new method for calculation of liver acetate uptake in the presence of ethanol is presented.

Garlic reduces plasma lipids by inhibiting hepatic cholesterol and triacylglycerol synthesis

Prompted by the reported hypolipidemic activity of garlic, the present study was undertaken to elucidate the mechanism(s) underlying the cholesterol-lowering effects of garlic. Rat hepatocytes in primary culture were used to determine the short-term effects of garlic preparations on [1-14C]acetate and [2-3H]glycerol incorporation into cholesterol, fatty acids and glycerol lipids. When compared with the control group, cells treated with a high concentration of garlic extracts [i.e., petroleum ether- (PEF), methanol- (MEF) and water-extractable (WEF) fractions from fresh garlic] showed decreased rates of [1-14C]acetate incorporation into cholesterol (by 37-64%) and into fatty acids (by 28-64%). Kyolic containing S-allyl cysteine and organosulfur compounds inhibited cholesterogenesis in a concentration dependent manner with a maximum inhibition of 87% at 0.4 mM. At this concentration, Kyolic decreased [1-14C]acetate incorporation into fatty acids by 67%. S-allyl cysteine at 2.0 and 4.0 mM inhibited cholesterogenesis by 20-25%. PEF, MEF and WEF depressed the rates of [2-3H]glycerol incorporation into triacylglycerol, diacylglycerol and phospholipids in the presence of acetate, but not in the presence of oleate. The results suggest that the hypocholesterolemic effect of garlic stems, in part, from decreased hepatic cholesterogenesis, whereas the triacylglycerol-lowering effect appears to be due to inhibition of fatty acid synthesis. Primary hepatocyte cultures as used in the present study have been proven useful as tools for screening the anticholesterogenic properties of garlic principles.

Application of dual-digitonin-pulse perfusion to the study of hepatic mRNA zonation

Heterogeneous zonation of hepatic protein expression over the liver lobule has been recognized by using several analytical techniques, including microdissection, selective cell isolation, immunohistochemistry and hybridization of mRNA in situ. We previously employed the technique of dual-digitonin-pulse perfusion for the highly selective collection and analysis of periportal and perivenous soluble protein. In the present work we have now documented the feasibility of the application of this technique to the study of zonal distribution of mRNA. By using a split-stream design, both protein and RNA fractions can be simultaneously collected from hepatic zones. High-quality RNA (average yield approximately 9-33 micrograms of total RNA per mg of eluted protein) is obtained for analysis. As analysed by immunoblotting and Northern-blot analysis, the zonal distribution of several important cytosolic metabolic enzymes and their mRNAs can be documented. This technique is also applicable to the study of mRNAs for organelle- and membrane-associated proteins that are not recoverable with this digitonin-lysis technique. The application of this experimental technique should allow further molecular insight into the mechanisms underlying zonation of hepatic function.