Efeito citotóxico

A citrato liase de ATP (ACLY) é uma enzima chave da síntese de ácidos graxos de novo responsável pela geração de acetil-CoA e oxaloacetato citosólicos. O aumento do metabolismo de glicose e lipídios é uma das propriedades mais comuns das células malignas. A superexpressão ou ativação de ACLY tem sido relatada em tumores de bexiga, mama, fígado, estômago, cólon e próstata ( Szutowicz et al., 1979; Halliday et al., 1988; Yahagi et al., 2005 ). Também foi relatado que inibidores químicos ou RNAi ACLY suprimem o crescimento de células tumorais ( Hatzivassiliou et al., 2005; Bauer et al., 2005 ). O ACLY também é responsável pela fosforilação de outras quinases em diferentes locais, como a nucleosídeo difosfato quinase ( Wagner and Vu, 1995).) e proteína quinase dependente de AMP cíclico ( Pierce et al., 1981 ), que contribui para a estabilização de proteínas e crescimento de células tumorais. A inibição seletiva da ACLY pelo extrato de Garcinia / HCA pode ajudar na supressão do crescimento de células tumorais ( Migita et al., 2008 ). O garcinol e as gutiferonas presentes nos extratos de G. cambogia foram avaliados quanto aos efeitos antiproliferativos nos fibroblastos de camundongo Balb / c 3T3 e nos linfócitos periféricos humanos. Os resultados mostraram que os extratos de Garcinia inibiram os linfócitos e a sobrevivência das células 3T3 dos fibroblastos devido a efeitos citotóxicos pronunciados ( Varalakshmi et al., 2011).). Do mesmo modo, a gutiferona K tem potentes efeitos antitumorais na apoptose mediada por caspase-3 e na atividade antiproliferativa do cancro do cólon contra a linha celular de cancro do ovário humano A2780 ( Cao et al., 2007; Kan et al., 2013 ). Estudos mostram que o garcinol reprime a acetilação p300 do gene p53 e promove a apoptose através da ativação da caspase-3, além de diminuir a expressão gênica global em células HeLa ( Balasubramanyam et al., 2004 ). O Garcinol também exibe propriedades anticarcinogênicas e antiinflamatórias através da supressão seletiva da síntese de PGE2 e da formação do produto 5-lipoxigenase ( Koeberle et al., 2009).). A fruta também contém xantonas, que também inibem lesões pré-neoplásicas em câncer de mama e cólon. As xantonas também podem induzir apoptose em células de câncer de boca, leucemia, mama, estômago e pulmão in vitro ( Mazzio e Soliman, 2009 ). Vários estudos sobre a atividade anticancerígena de G. cambogia ou seus fitoconstituintes ativos são mostrados na Tabela 48.2 .

3.15.8.4 Garcinia cambogia

A Garcinia cambogia contém ácido hidroxicítrico, um inibidor da enzima de clivagem de citrato (ATP-citrato liase) que inibe a síntese de ácidos graxos dos carboidratos. O hidroxicitrato foi estudado por Hoffmann-LaRoche na década de 1970 e foi mostrado para reduzir a ingestão de alimentos e causar perda de peso em roedores. ⁹⁹

Embora tenha havido relatos de perda de peso bem-sucedida em pequenos estudos em humanos, alguns dos quais incluíam outras ervas, o maior e melhor projetado estudo controlado por placebo não demonstrou diferença na perda de peso em comparação com um placebo. ¹⁷² No geral, a evidência para G. cambogia não é convincente. ^171,172,184 .

5.1.4 Lipídios

Metabolismo lipídico é alterado nas células do cancro de tumores reactivar de novo a síntese de lípidos, a liase de ATP-citrato é necessário para transformação in vitro , a síntese de colesterol no cancro da próstata é aumentada, e a oxidação dos ácidos gordos é uma importante fonte de energia para as células de cancro da próstata ( Santos & Schulze, 2012 ). A autofagia na forma específica da lipofagia é importante para a degradação das gotículas lipídicas no tecido adiposo ( Singh & Cuervo, 2012 ) e a autofagia regula o metabolismo lipídico nos hepatócitos, uma vez que a hidrólise dos triglicérides é prejudicada nas células Atg5 ^{- / -} ( Singh et al. , 2009 ). Se esses processos afetam o metabolismo lipídico do tumor, é necessário um estudo mais aprofundado.

Além disso, a autofagia afeta o metabolismo lipídico, alterando o número mitocondrial. Células mutantes de pg7 deletadas, em um modelo NSCLC dirigido por KRAS, têm acúmulo lipídico intracelular devido ao aumento das mitocôndrias disfuncionais que comprometem a oxidação dos ácidos graxos, sugerindo que a autofagia é crucial para manter o metabolismo lipídico nas células mutantes KRAS e p53. Isso impede o crescimento eficiente das células tumorais e as transforma em cistos lipídicos em vez de tumores ( Guo et al., 2013 ).

2 Myasthenic Syndrome Associated With Defects in the Mitochondrial Citrate Carrier SLC25A1

The mitochondrial SLC25A1 mediates the exchange of mitochondrial citrate/isocitrate with cytosolic maleate that is cleaved into acetyl-CoA and oxaloacetate by ATP-citrate lyase. Mutations of SLC25A1 were shown to interfere with brain, eye, and psychomotor development. Exome sequencing of two siblings born to consanguineous parents with CMS and intellectual disability revealed homozygous missense mutation in SLC25A1. Another patient harboring two missense mutations in SLC25A1 had hypotonia, severe intellectual disability, epilepsy, postnatal microcephaly, as well hypoplastic optical nerves, agenesis of the corpus callosum, sensorineural deafness and 2-hydroxyglutaric aciduria. At 18 months of age the patient had no spontaneous voluntary movements and was shown to have neuromuscular transmission defect.

2.2.8 ATP citrate lyase

Lipogenesis, along with glycolysis, is one of the major metabolic changes in cancer cells that is required to satisfy the increasing demand for macromolecules as part of autonomous growth. ATP citrate lyase (ACLY) is a cytosolic enzyme that converts mitochondrially derived citrate into acetyl-CoA and oxaloacetate, thereby defining the first step in the cellular fatty acid synthesis pathway; ATP is hydrolyzed concomitantly. As the citrate substrate stems from the TCA cycle which is powered most often by glucose or glutamine, ACLY represents a linker between the extracellular carbon sources and lipid metabolic pathways. In the cytosol, oxaloacetate is reduced to malate. ACLY overexpression or activation has been reported in urinary, bladder, breast, brain, liver, stomach, colon, prostate and lung cancers, as is necessitated for cellular proliferation (Migita, 2008 and references therein). Furthermore, in lung cancers the expression level of ACLY correlates with tumor stage, differentiation grade and prognosis (Migita, 2008). Several reports have shown recently that the chemical or RNAi-mediated inhibition of ACLY leads to the suppression of tumor cell growth (Migita, 2008; Hatzivassiliou et al., 2005). ACLY knockdown experiments have shown a dramatic decrease in the conversion rate of glucose to lipid, and a decrease in cell proliferation, though with a parallel increase in intracellular lipid levels. Thus, it was hypothesized that ACLY inhibition could affect cell proliferation via an impairment of glucose metabolism rather than a depletion of lipid production. Although ACLY knockdown results in proliferation arrest, it does not lead to apoptosis. Despite ACLY function not being completely understood, it is considered a valuable therapeutic target, with a handful of suggested inhibitors, the most prominent examples being the substrate analog (4S)-hydroxycitrate (Beckner, 2010), the hydrophobically tailed (S)-2-((S)-8-(2,4-dichlorophenyl)-2-hydroxyoctyl)-2-hydroxysuccinic acid (SB-204990), and its corresponding γ-lactone, SB-201076, all of which have stemmed from research conducted at SmithKline Beecham. Examples of the inhibitor structures are provided in Figure 2.44. SB-204990, a cell-penetrating γ-lactone, was developed over 10 years ago for the regulation of plasma lipids (Beckner, 2010). It is a prodrug which has been shown to inhibit pancreatic cancer growth in nude mice (Hatzivassiliou et al., 2005; Kroemer and Pouyssegur, 2008), and is currently undergoing preclinical trials for its potential anti-tumor properties (Tennant et al., 2010 and references therein).

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Figure 2.44. Some ACLY inhibitors

7.11.5.4 Lyases: Using CoA to Break Carbon–Carbon bonds

In this section two enzymes that use CoA (or CoA-derived prosthetic groups) to cleave carbon–carbon bonds will be discussed: citrate lyase (EC 4.1.3.6) and ATP-citrate lyase, also called the citrate cleavage enzyme (EC 4.1.3.8, but recently transferred to 2.3.3.8). Although these are two of the few CoA-dependent activities that are labeled as carbon–carbon bond ‘lyases,’ it should be pointed out that many of the Claisen condensing reactions discussed in the previous section are reversible, and the enzymes that catalyze these reactions (most notably thiolase) are therefore also able to facilitate the cleavage of a C–C bond through similar retro-Claisen reactions. An interesting exception is HMG-CoA lyase (EC 4.1.3.4), which catalyzes the Claisen-type cleavage of HMG-CoA 32 to give acetyl-CoA 30 and oxaloacetate 35. Although this is the reverse reaction of the condensation catalyzed by HMG-CoA synthase (see Equation (21)) there is little similarity between these two enzymes both with regard to structure and mechanism. HMG-CoA lyase is an Mg²⁺-dependent enzyme with a mechanism similar to that of malate and IPMS. It is also structurally related to MS, as both are TIM barrel enzymes.^336,337

The ATP-independent citrate lyase that acts in the citrate fermentation pathways of mainly enterobacteria is one of the few enzyme activities that is dependent on the 2′-(5″-phosphoribosyl)-3′-dephospho-CoA prosthetic group for catalysis (see Section 7.11.4.1.2).^205,206 These enzymes are large complexes about 550 kDa in size, which consist of three different subunits in a 1:1:1 stoichiometry and with a native composition of α₆β₆γ₆.³³⁸ Although the α- and β-subunits each have distinct enzymatic activities, the γ-subunits of this complex are noncatalytic ACPs that carry the essential prosthetic group. The lyase reaction proceeds in distinct steps: first, a separate [citrate (pro-3S)-lyase] ligase enzyme (EC 6.2.1.22) catalyzes the ATP-dependent transfer of an acetyl group to the thiol of the prosthetic group. This primes the complex for catalysis, as enzymes in which the acetyl group has been removed loses all activity. Second, the α-subunit of the citrate lyase complex acts as a CoA-transferase (EC 2.8.3.10) and facilitates the exchange of the acetyl group for a citryl group. Finally, the β-subunit catalyzes the lyase reaction, the Mg²⁺-dependent retro-Claisen cleavage of citryl-ACP 43 to give acetyl-ACP 42 (which reenters the catalytic cycle) and oxaloacetate 35 (Scheme 11). Trivial detail of the kinetic mechanism of the lyase reaction is known, and the active site residues that are involved in the cleavage reaction have not been identified. Only two crystal structures of citrate lyase β-subunits have been determined: the apo-CitE protein from D. radiodurans(pdb: 1SGJ) on which no detailed crystallographic analysis has been published, and the apo-CitE protein from M. tuberculosis.³³⁹However, the latter protein is not associated with the usual α- and β-subunits of a typical citrate lyase, and based on its distant sequence similarity to a malyl-CoA lyase it has been suggested that it functions as a citryl-CoA lyase, that is, catalyzing the CS reaction in reverse and forming acetyl-CoA and oxaloacetate as products.

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Scheme 11. Reaction mechanism of the bacterial ATP-independent citrate lyases that are made up of three distinct subunits, each with its own function. The α-subunit acts as a CoA-transferase, the β-subunit has the actual citrate lyase activity, and the γ-subunit acts as an ACP which has a unique 2′-(5″-phosphoribosyl)-3′-dephospho-CoA prosthetic group. The lyase catalytic cycle can only take place after the ACP has been primed by condensation of an acetyl group to the thiol of the prosthetic group; this is catalyzed by a separate AMP-forming ligase enzyme.

A second ATP-dependent citrate lyase (ACL) activity is responsible for the formation of acetyl-CoA 30 and oxaloacetate 35 from citrate 39with concomitant hydrolysis of ATP to ADP and phosphate (Equation (22)).³⁴⁰ It is proposed to play a vital role in maintaining acetyl-CoA and oxaloacetate levels in most mammals, whereas in some bacteria it is an essential enzyme of the reductive tricarboxylic acid cycle (RTCA).

(22)

The reaction mechanism of ACL enzymes is complex and involves the formation of a phospho–enzyme intermediate by transfer of the γ-phosphate of ATP to a conserved histidine residue. The phospho-histidine subsequently transfers its phosphate to citrate, forming a tightly bound citryl–phosphate intermediate, which then reacts with CoA to give citryl-CoA.³⁴¹ Up to this point the enzyme shows great similarity in mechanism to the succinyl-CoA synthetase described in an earlier subsection (see Section 7.11.5.1.1). In fact, comparison of the primary sequence of the Chlorobium tepidum ACL enzyme with that of E. coli’s succinyl-CoA synthetase indicate up to 33% sequence identity, as well as the presence of a putative ATP grasp domain in the former similar to those found in other ADP-forming ACSs.³⁴²Also, in agreement with the reaction mechanism of these enzymes, no evidence currently exists suggesting the formation of a covalent citryl–enzyme intermediate during the ACL-catalyzed reaction as has been proposed in some earlier studies.³⁴¹ In the last step citryl-CoA is cleaved in a retro-Claisen reaction to give acetyl-CoA and oxaloacetate. The residues that are involved in this last step have not been unequivocally identified, despite some kinetic and site-directed mutagenesis studies of the Chlorobium ACL enzymes.^342,343 No structure of an ACL enzyme has been determined to date.

TCA Cycle Metabolites Are Required for Macromolecular Biosynthesis

In proliferating cells, citrate is not only oxidized in the TCA cycle but also can be exported to the cytosol where it is converted back to oxaloacetate and acetyl-CoA by the enzyme ATP citrate lyase (ACL). Cytosolic acetyl-CoA provides the substrate for the synthesis of fatty acids, cholesterol, and prostaglandins (Figure 13-7). Through this pathway, glucose acts as the major substrate for de novo lipogenesis; consequently, ACL inhibition blocks cell proliferation and inhibits tumor growth.^28-30

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Figure 13-7. Glucose-derived acetyl-CoA fuels lipid and sterol synthesis

Pyruvate derived from glycolysis can generate citrate in the mitochondrion. This citrate can be exported to the cytosol to provide acetyl-CoA for the synthesis of fatty acids, phospholipids, cholesterol, and isoprenoids, all of which are critical for membrane biogenesis and function. Metabolites are in black; enzymes are in blue. ACC, Acetyl-CoA carboxylase; Ac-CoA, acetyl-CoA; ACL, ATP-citrate lyase; CDP, cytidine diphosphate; CoA-SH, coenzyme A; CTP, cytidine triphosphate; FAS, fatty acid synthetase; glycerol-3-P, glycerol 3-phosphate; HCS, HMG-CoA synthetase; HMG-CoA, 3-hydroxy-3-methylglutaryl CoA; lyso PA, lysophosphatidic acid; Mal-CoA,malonyl-CoA; OAA, oxaloacetate; Pi, inorganic phosphate; R, acyl group on a lipid molecule.

Citrate is not the only TCA cycle metabolite that has an important biosynthetic role. Oxaloacetate and α-ketoglutarate can provide the carbon backbone for nonessential amino acids, which are used for protein and nucleic acid synthesis (see Figure 13-6). In this manner, multiple TCA cycle metabolites are diverted to other pathways that support cell growth.

Glutamine Plays Several Roles Supporting the TCA Cycle and Anabolic Metabolism

Although efflux of TCA cycle metabolites supports anabolic reactions, this results in depletion of TCA cycle intermediates from the mitochondrial matrix. Growing cells must draw on alternative sources to maintain the oxaloacetate required for citrate synthesis. Replenishment of TCA cycle intermediates, a process known as anaplerosis, is thus critical to maintain both a constant supply of metabolites for synthesis and appropriate ATP production through oxidative phosphorylation. Most proliferating cancer cells meet their anaplerotic needs through the catabolism of glutamine (see Figure 13-6). Glutamine is converted to the TCA cycle intermediate α-ketoglutarate through a metabolic pathway known as glutaminolysis. Tumor cells take up high levels of glutamine, often in great excess of any other amino acid, and studies using carbon labeling techniques have demonstrated that glutamine is the major source of carbon for the cellular oxaloacetate pool in cancer cell lines.²⁸

By maintaining flux through the TCA cycle—and therefore delivery of electrons to the electron transport chain—glutamine plays a critical role maintaining mitochondrial bioenergetics in many cancer cells. Consequently, many cancer cell lines are absolutely dependent on glutamine for survival. Addition of cell-permeable TCA cycle analogs can rescue death of glutamine-deprived cells, highlighting the importance of glutamine as an anaplerotic substrate in cancer cells.^31,32

Proliferating cells rely on glutamine to fulfill additional biosynthetic roles. First, glutamine provides an important source of nitrogen for synthesis of nonessential amino acids and nucleotides. Glutamine is required for two independent steps in purine nucleotide synthesis, and oxaloacetate-derived aspartate is required for a third (Figure 13-8). Similarly, two steps of pyrimidine synthesis require glutamine. In all cases, glutamine donates nitrogen in the form of an amide group and is converted to glutamic acid, which provides a major source of nitrogen for amino acid synthesis. Transaminases can transfer the amine group from glutamic acid to α-ketoacids, which are themselves derived from the catabolism of glucose or glutamine, producing alanine, serine, aspartate, and ornithine. In turn, these amino acids act as precursors for the synthesis of glycine, cysteine, arginine, and asparagine. Likewise, two enzymatic reactions directly convert glutamate to proline. In this manner, glucose and glutamine contribute to the synthesis of every nonessential amino acid except for tyrosine, which is directly produced from the essential amino acid phenylalanine.

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Figure 13-8. Nucleotide biosynthesis requires multiple metabolic inputs

Nucleotide biosynthesis requires inputs from several metabolic pathways, highlighting why cells must coordinately regulate multiple metabolic pathways during proliferation. The de novo synthesis of purines (shown) and pyrimidines requires glucose, several amino acids, and one-carbon groups from folate metabolism. The origin of individual carbons and nitrogens on inosine monophosphate (IMP), the precursor to GTP and ATP, are color-coded in the purple box. α-KG, α-ketoglutarate; N¹⁰ formyl THF, N¹⁰ formyl tetrahydrofolate; 3-PG, 3-phosphoglycerate; β 5-P-ribosylamine, β 5-phosphate-ribosylamine; PRPP, 5-phosphoribosyl pyrophosphate; ribose 5-P, ribose 5-phosphate; P, phosphate group.

Intriguingly, cancer cells may convert up to 60% of glutamine carbon into lactic acid, an ostensibly wasteful secretion analogous to the Warburg effect.²⁸ One possible explanation for this behavior is that the conversion of glutamine to lactate produces NADPH, which is absolutely required for many anabolic reactions. NADPH is produced when malic enzyme converts glutamine-derived malate to pyruvate, which is subsequently reduced to lactate and secreted. In this manner, glutamine may contribute to all three needs of proliferating cells: Glutamine can provide the carbon and nitrogen for most metabolic building blocks, maintain ATP production to support bioenergetics, and generate reducing equivalents required for many anabolic reactions.

TCA Cycle Rearrangements Highlight the Role of TCA Cycle Metabolites as Biosynthetic Precursors

In proliferating cancer cells, the TCA cycle can behave more as a source of anabolic substrates rather than a bona fide cycle. A prime example of how the TCA cycle prioritizes biosynthetic reactions is the reductive carboxylation of α-ketoglutarate to isocitrate. During conditions of hypoxia or mitochondrial dysfunction, NADH accumulates from a combination of increased glycolysis and reduced oxidation of NADH by the electron transport chain. Depletion of oxidized NAD⁺ poses a bioenergetic challenge for the cell: Both the conversion of glucose-derived pyruvate to acetyl-CoA and the production of oxaloacetate through the enzymes of the TCA cycle require NAD⁺. Furthermore, during periods of acute hypoxia, pyruvate is converted to lactate at the expense of acetyl-CoA. Both of these events would strongly impair citrate production and thus cell growth. How, then, do hypoxic cells proliferate?

Several groups have demonstrated that under these conditions, the TCA cycle can partially function in reverse (for examples, see Refs. 33-35). Normally, isocitrate dehydrogenase (IDH) catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate, consuming NAD⁺ and producing NADH. When NAD⁺ is limiting, both the mitochondrial and cytosolic isoforms of IDH can function in reverse, carboxylating α-ketoglutarate to isocitrate and producing oxidized NADP⁺ (Figure 13-9). In this manner, cells can use glutamine-derived α-ketoglutarate to generate citrate, providing a mechanism to maintain anabolic reactions even under hypoxic conditions that characterize the tumor microenvironment.

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Figure 13-9. Reductive carboxylation supports lipid synthesis during hypoxia

Citrate levels can be maintained even in cells with defective TCA cycles or during periods of hypoxia by the reductive metabolism of glutamine-derived α-ketoglutarate to citrate. Both mitochondrial and cytosolic isoforms of isocitrate dehydrogenase (IDH2 and IDH1, respectively) can function in reverse, reducing α-ketoglutarate to isocitrate and oxidizing NADPH to NADP+. Isocitrate is reversibly converted to citrate, which can be used to generate acetyl-CoA for lipid synthesis in the cytosol.

A CARBON METABOLISM

Acetyl-CoA is the principal building block for de novo synthesis of fatty acids (Weete, 1980). However, an additional series of metabolic events occurs prior to the formation of acetyl-CoA. A summary of carbon metabolism in oleaginous fungi is shown in Fig.3. The two key enzymes, ATP: citrate lyase and malic enzyme, are involved in lipid accumulation in oleaginous fungi (Ratledge, 1981). A correlation has been observed between the activity of the ATP:citrate lyase and the ability of yeast (Boulton and Ratledge, 1981) and fungi (Kendrick and Ratledge, 1992a) to accumulate more than 20% of their biomass as lipid. ATP:citrate lyase is located in the cytosol fraction of the oleaginous organisms and provides acetyl-CoA from citrate for fatty acid biosynthesis:

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Fig. 3. Carbon metabolism in oleaginous fungi. Key enzymes:1, aconitase; 2, isocitrate dehydrogenase; 3, ATP:citrate lyase; 4, malate dehydrogenase; 5, malic enzyme; 6, fatty acid synthetase.

$\mathrm{Citrate}+ATP+CoA\to \mathrm{Acetyl}-CoA+\mathrm{Oxaloacetate}+ADP+\mathrm{Pi}\text{.}$

Another key enzyme, malic enzyme, generates the NADPH by which the acetyl units can be reduced and used as the backbone of the fatty acids (Boulton and Ratledge, 1985). On the basis of a study of three oleaginous microorganisms, two yeast and one fungi, it has been postulated that lipid accumulation is a result of the concerted action of at least two separate metabolic events (Botham and Ratledge, 1979; Ratledge, 1981). First, the NAD-dependent isocitrate dehydrogenase of mitochondria has an absolute requirement for AMP, so that when AMP concentration is low, as occurs during nitrogen deprivation, citric acid will accumulate. Second, ATP:citrate lyase cleaves citrate to acetyl-CoA and oxaloacetate, so that fatty acid synthesis is constantly primed with substrate. The sites of biosynthesis of fatty acids are mainly in the cytoplasm outside the mitochondria. However, most of the acetyl-CoA is derived from the oxidation of pyruvate in mitochondria, and the mitochondrial membrane is relatively impermeable to acetyl-CoA (Gurr and Harwood, 1991). Therefore, oleaginous eukaryotic microorganisms accumulate citrate in the mitochondria, which is then transported into the cytoplasm and cleaved there by the ATP:citrate lyase. Since nonoleaginous organisms do not possess the citrate-cleaving enzyme and most rely on the less effective carnitine-mediated system for production of acetyl-CoA in the cytoplasm (Kohlow and Tan-Wilson, 1977), desaturation of fatty acids occurs with the fatty acyl groups attached to phospholipids. In fungi, the desaturation occurs with fatty acyl groups specifically attached to the sn-2 position of phosphatidylinositol (Ratledge, 1992). On the other hand, desaturation of fatty acyl groups attached to phosphatidylcholine has been reported in plants (Stumpf, 1987).

9.10.2.1 SREBP1 and Fatty Acid Synthesis

SREBPs are synthesized as 125 kD precursors embedded in the endoplasmic reticulum. Activation by proteolytic cleavage allows for the accumulation of active SREBP in the nucleus (Mizuno et al.1992). SREBP1a and 1c are important in the regulation of genes required for hepatic triglyceride synthesis including acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), stearoyl-CoA desaturase 1, malic enzyme, and ATP citrate lyase. SREBP2 is involved in the regulation of genes required for cholesterol synthesis, including HMG-CoA synthase, HMG-CoA reductase, and low-density lipoprotein receptor (Mizuno et al. 1992; White and Tsan 2001). SREBP1c and SREBP2 are expressed in liver, whereas SREBP1a is expressed only at very low levels in the liver of adult mice, rats, and humans (Mizuno et al. 1992).

Increased expression of SREBP1c has been observed in several models of obesity and diabetes, both in response to diet-induced and genetic models of obesity, such as the ob/ob mouse (Sano et al.2001). These models exhibit increased SREBP1c activity, as well as increased expression and activity of the lipogenic genes, ACC and FAS. Mice deficient in SREBP1c are protected from high-fat diet-induced fatty liver (Miyazaki et al. 2007). In rodent models of ALD, chronic ethanol feeding in mice also increases expression of the mature form of the activated SREBP1c protein, associated with increased expression of FAS, stearoyl-CoA desaturase, malic enzyme, ATP citrate lyase, and ACC (Ji and Kaplowitz 2003; You et al. 2002).

6.1.3 Changing Metabolism

Inulin intake reduces lipogenesis in the liver and fasting triglyceride levels in healthy individuals without affecting blood cholesterol levels (Fig. 5.1) [15]. The decrease in lipogenesis is due to dietary fiber-mediated suppression of lipogenic enzymes, such as acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), malic enzyme, ATP citrate lyase, and glucose-6-phosphate dehydrogenase in liver [58]. However, de novo lipogenesis is greatly influenced by background diet. Oligofructose induces secretion of glucose-dependent insulinotrophic polypeptide (GIP) in rats [22]. GIP is known to enhance activity of lipoprotein lipase (LPL), the key enzyme involved in the clearance of triglyceride-enriched lipoproteins following lipid intake, which will lead to a reduction in plasma triglyceride levels [59]. In animal studies, both β-glucagon and inulin decreased body weight. Interestingly, inulin decreased body adiposity without any effect on food intake, whereas, β-glucagon decreased food intake [60]. These findings suggest that different dietary fibers decrease cardiovascular risks by stimulating different mechanisms.