Covariance of tricarboxylate carrier activity and lipogenesis in liver of polyunsaturated fatty acid (n-6) fed rats

Multiple roles played by the mitochondrial citrate carrier in cellular metabolism and physiology

The citrate carrier (CIC) is an integral protein of the inner mitochondrial membrane which catalyzes the efflux of mitochondrial citrate (or other tricarboxylates) in exchange with a cytosolic anion represented by a tricarboxylate or a dicarboxylate or phosphoenolpyruvate. In this way, the CIC provides the cytosol with citrate which is involved in many metabolic reactions. Several studies have been carried out over the years on the structure, function and regulation of this metabolite carrier protein both in mammals and in many other organisms. A lot of data on the characteristics of this protein have therefore accumulated over time thereby leading to a complex framework of metabolic and physiological implications connected to the CIC function. In this review, we critically analyze these data starting from the multiple roles played by the mitochondrial CIC in many cellular processes and then examining the regulation of its activity in different nutritional and hormonal states. Finally, the metabolic significance of the citrate flux, mediated by the CIC, across distinct subcellular compartments is also discussed.

3,5-diiodo-L-thyronine increases de novo lipogenesis in liver from hypothyroid rats by SREBP-1 and ChREBP-mediated transcriptional mechanisms

Hepatic de novo lipogenesis (DNL), the process by which carbohydrates are converted into lipids, is strictly controlled by nutritional and hormonal status. 3,5-Diiodo-L-thyronine (T2), a product of the 3,5,3'-triiodo-L-thyronine (T3) peripheral metabolism, has been shown to mimic some T3 effects on lipid metabolism by a short-term mechanism independent of protein synthesis. Here, we report that T2, administered for 1 week to hypothyroid rats, increases total fatty acid synthesis from acetate in isolated hepatocytes. Studies carried out on liver subcellular fractions demonstrated that T2 not only increases the activity and the expression of acetyl-CoA carboxylase and fatty acid synthase but also of other proteins linked to DNL such as the mitochondrial citrate carrier and the cytosolic ATP citrate lyase. Parallelly, T2 stimulates the activities of enzymes supplying cytosolic NADPH needed for the reductive steps of DNL. With respect to both euthyroid and hypothyroid rats, T2 administration decreases the hepatic mRNA level of SREBP-1, a transcription factor which represents a master regulator of DNL. However, when compared to hypothyroid rats T2 significantly increases, without bringing to the euthyroid value, the content of both mature (nSREBP-1), and precursor (pSREBP-1) forms of the SREBP-1 protein as well as their ratio. Moreover, T2 administration strongly augmented the nuclear content of ChREBP, another crucial transcription factor involved in the regulation of lipogenic genes. Based on these results, we can conclude that in the liver of hypothyroid rats the transcriptional activation by T2 of DNL genes could depend, at least in part, on SREBP-1- and ChREBP-dependent mechanisms. © 2019 IUBMB Life, 2019.

Dietary long-chain unsaturated fatty acids acutely and differently reduce the activities of lipogenic enzymes and of citrate carrier in rat liver

The activities of lipogenic enzymes appear to fluctuate with changes in the level and type of dietary fats. Polyunsaturated fatty acids (PUFAs) are known to induce on hepatic de novo lipogenesis (DNL) the highest inhibitory effect, which occurs through a long-term adaptation. Data on the acute effects of dietary fatty acids on DNL are lacking. In this study with rats, the acute 1-day effect of high-fat (15 % w/w) diets (HFDs) enriched in saturated fatty acids (SFAs) or unsaturated fatty acids (UFAs), i.e., monounsaturated (MUFA) and PUFA, of the ω-6 and ω-3 series on DNL and plasma lipid level was investigated; a comparison with a longer time feeding (21 days) was routinely carried out. After 1-day HFD administration UFA, when compared to SFA, reduced plasma triacylglycerol (TAG) level and the activities of the lipogenic enzymes acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), a decreased activity of the citrate carrier (CIC), a mitochondrial protein linked to lipogenesis, was also detected. In this respect, ω-3 PUFA was the most effective. On the other hand, PUFA maintained the effects at longer times, and the acute inhibition induced by MUFA feeding on DNL enzyme and CIC activities was almost nullified at 21 days. Mitochondrial fatty acid composition was slightly but significantly changed both at short- and long-term treatment, whereas the early changes in mitochondrial phospholipid composition vanished in long-term experiments. Our results suggest that in the early phase of administration, UFA coordinately reduced both the activities of de novo lipogenic enzymes and of CIC. ω-3 PUFA showed the greatest effect.

Nutritional and Hormonal Regulation of Citrate and Carnitine/Acylcarnitine Transporters: Two Mitochondrial Carriers Involved in Fatty Acid Metabolism

The transport of solutes across the inner mitochondrial membrane is catalyzed by a family of nuclear-encoded membrane-embedded proteins called mitochondrial carriers (MCs). The citrate carrier (CiC) and the carnitine/acylcarnitine transporter (CACT) are two members of the MCs family involved in fatty acid metabolism. By conveying acetyl-coenzyme A, in the form of citrate, from the mitochondria to the cytosol, CiC contributes to fatty acid and cholesterol synthesis; CACT allows fatty acid oxidation, transporting cytosolic fatty acids, in the form of acylcarnitines, into the mitochondrial matrix. Fatty acid synthesis and oxidation are inversely regulated so that when fatty acid synthesis is activated, the catabolism of fatty acids is turned-off. Malonyl-CoA, produced by acetyl-coenzyme A carboxylase, a key enzyme of cytosolic fatty acid synthesis, represents a regulator of both metabolic pathways. CiC and CACT activity and expression are regulated by different nutritional and hormonal conditions. Defects in the corresponding genes have been directly linked to various human diseases. This review will assess the current understanding of CiC and CACT regulation; underlining their roles in physio-pathological conditions. Emphasis will be placed on the molecular basis of the regulation of CiC and CACT associated with fatty acid metabolism.

Modulation of hepatic lipid metabolism by olive oil and its phenols in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) represents the most common chronic liver disease in western countries, being considered the hepatic manifestation of metabolic syndrome. Cumulative lines of evidence suggest that olive oil, used as primary source of fat by Mediterranean populations, may play a key role in the observed health benefits on NAFLD. In this review, we summarize the state of the art of the knowledge on the protective role of both major and minor components of olive oil on lipid metabolism during NAFLD. In particular, the biochemical mechanisms responsible for the increase or decrease in hepatic lipid content are critically analyzed, taking into account that several studies have often provided different and/or conflicting results in animal models fed on olive oil-enriched diet. In addition, new findings that highlight the hypolipidemic and the antisteatotic actions of olive oil phenols are presented. As mitochondrial dysfunction plays a key role in the pathogenesis of NAFLD, the targeting of these organelles with olive oil phenols as a powerful therapeutic approach is also discussed.

Mitochondrial tricarboxylate and dicarboxylate-tricarboxylate carriers: from animals to plants

The citrate carrier (CiC), characteristic of animals, and the dicarboxylate-tricarboxylate carrier (DTC), characteristic of plants and protozoa, belong to the mitochondrial carrier protein family whose members are responsible for the exchange of metabolites, cofactors, and nucleotides between the cytoplasm and the mitochondrial matrix. Most of the functional data on these transporters are obtained from the studies performed with the protein purified from rat, eel yeast, and maize mitochondria or recombinant proteins from different sources incorporated into phospholipid vesicles (liposomes). The functional data indicate that CiC is responsible for the efflux of acetyl-CoA from the mitochondria to the cytosol in the form of citrate, the primer for fatty acid, cholesterol synthesis, and histone acetylation. Like the CiC, the citrate exported by DTC from the mitochondria to the cytosol in exchange for oxaloacetate can be cleaved by citrate lyase to acetyl-CoA and oxaloacetate and used for fatty acid elongation and isoprenoid synthesis. In addition to its role in fatty acid synthesis, CiC is involved in other processes such as gluconeogenesis, insulin secretion, inflammation, and cancer progression, whereas DTC is involved in the production of glycerate, nitrogen assimilation, ripening of fruits, ATP synthesis, and sustaining of respiratory flux in fruit cells. This review provides an assessment of the current understanding of CiC and DTC structural and biochemical characteristics, underlying the structure-function relationship of these carriers. Furthermore, a phylogenetic relationship between CiC and DTC is proposed.

Dietary fat and hepatic lipogenesis: mitochondrial citrate carrier as a sensor of metabolic changes

Citrate carrier (CIC) is an integral protein of the inner mitochondrial membrane that has a fundamental role in hepatic intermediary metabolism. Its primary function is to catalyze the transport of citrate from mitochondria, where this molecule is formed, to cytosol, where this molecule is used for fatty acid (FA) and cholesterol synthesis. Therefore, mitochondrial CIC acts upstream of cytosolic lipogenic reactions, and its regulation is particularly important in view of the modulation of hepatic lipogenesis. Although a great deal of data are currently available on the dietary modulation of cytosolic lipogenic enzymes, little is known about the nutritional regulation of CIC transport activity. In this review, we describe the differential effects of distinct FAs present in the diet on the activity of mitochondrial CIC. In particular, polyunsaturated FAs were powerful modulators of the activity of mitochondrial CIC by influencing its expression through transcriptional and posttranscriptional mechanisms. On the contrary, saturated and monounsaturated FAs did not influence mitochondrial CIC activity. Moreover, variations in CIC activity were connected to similar alterations in the metabolic pathways to which the transported citrate is channeled. Therefore, CIC may be considered as a sensor for changes occurring inside the hepatocyte and may represent an important target for the regulation of hepatic lipogenesis. The crucial role of this protein is reinforced by the recent discovery of its involvement in other cellular processes, such as glucose-stimulated insulin secretion, inflammation, tumorigenesis, genome stability, and sperm metabolism.

Modulation of hepatic steatosis by dietary fatty acids

Non-alcoholic fatty liver disease (NAFLD) describes a range of conditions caused by fat deposition within liver cells. Liver fat content reflects the equilibrium between several metabolic pathways involved in triglyceride synthesis and disposal, such as lipolysis in adipose tissue and de novo lipogenesis, triglyceride esterification, fatty acid oxidation and very-low-density lipoprotein synthesis/secretion in hepatic tissue. In particular, it has been demonstrated that hepatic de novo lipogenesis plays a significant role in NAFLD pathogenesis. It is widely known that the fatty acid composition of the diet influences hepatic lipogenesis along with other metabolic pathways. Therefore, dietary fat may not only be involved in the pathogenesis of hepatic steatosis, but may also prevent and/or reverse hepatic fat accumulation. In this review, major data from the literature about the role of some dietary fats as a potential cause of hepatic fat accumulation or as a potential treatment for NAFLD are described. Moreover, biochemical mechanisms responsible for an increase or decrease in hepatic lipid content are critically analyzed. It is noteworthy that both quantitative and qualitative aspects of dietary fat influence triglyceride deposition in the liver. A high-fat diet or the dietary administration of conjugated linoleic acids induced hepatic steatosis. In contrast, supplementation of the diet with krill oil or pine nut oil helped in the prevention and/or in the treatment of steatotic liver. Quite interesting is the “case” of olive oil, since several studies have often provided different and⁄or conflicting results in animal models.

Streptozotocin-induced diabetes affects in rat liver citrate carrier gene expression by transcriptional and posttranscriptional mechanisms

Citrate carrier (CiC), also known as tricarboxylate carrier, is an integral protein of the mitochondrial inner membrane. It is an essential component of the shuttle system by which mitochondrial acetyl-CoA, primer for both fatty acid and cholesterol synthesis, is transported into the cytosol, where lipogenesis occurs. Here, we report the effect of streptozotocin-induced diabetes on the activity and expression of CiC in rat liver mitochondria. A significant reduction of CiC activity and a parallel decline in the abundance of CiC mRNA were found in liver from diabetic rats. Diabetes did not influence CiC mRNA stability, whereas nuclear run-on assay revealed that the transcriptional rate of CiC mRNA decreased, when compared to control, in the nuclei from diabetic rats. The ratio of mature to precursor CiC RNA decreased in diabetic animals, indicating that the splicing of CiC RNA was also affected. The 3'-end processing rate of CiC mRNA was not altered in diabetes. These results suggest that diabetes affects CiC expression at both transcriptional and posttranscriptional levels. In addition, by in vitro transfection experiments in rat hepatocytes cultured in the absence of insulin, a reduction of CiC promoter activity was observed, and this was ascribed to a decreased expression of sterol regulatory element-binding protein-1 transcriptional factor. Furthermore, the binding of sterol regulatory element-binding protein-1 to the CiC promoter was reduced in STZ-diabetic rats with respect to control ones, and it was restored to the control values after insulin treatment.

Reduced activity and expression of mitochondrial citrate carrier in streptozotocin-induced diabetic rats

Citrate carrier (CiC), an integral protein of the mitochondrial inner membrane, plays an important role in hepatic intermediary metabolism, supplying the cytosol with acetyl-coenzyme A for fatty acid and cholesterol synthesis. Here, the effect of streptozotocin-induced diabetes on CiC activity and expression in rat liver was investigated. The rate of citrate transport was reduced by about 35% in mitochondria from diabetic vs. control rats. Kinetic studies in mitochondria from diabetic rats showed a reduction in maximum velocity and almost unchanged Michaelis-Menten constant of the CiC protein. Mitochondrial phospholipid amount was not significantly affected, whereas an increase in the cholesterol content and in the cholesterol/phospholipid ratio was observed. To thoroughly investigate the mechanism responsible for the reduced CiC activity in the diabetic state, molecular studies were performed. Ribonuclease protection assays and Western blotting analysis indicated that both hepatic CiC mRNA accumulation and protein level decreased similarly to the CiC activity. The reduced mRNA level and the lower content of the mitochondrial CiC protein, might account for the decline of CiC activity in diabetic animals. To discriminate between the role played by hyperglycemia from that of hypoinsulinemia in the reduction of CiC activity and expression, studies were conducted administrating phlorizin or insulin to streptozotocin-diabetic rats. Our data indicated that both insulin and glucose affect CiC activity and expression in diabetic rats, although they act at different regulatory steps.

The mitochondrial citrate carrier: metabolic role and regulation of its activity and expression

The citrate carrier (CiC), a nuclear-encoded protein located in the mitochondrial inner membrane, is a member of the mitochondrial carrier family. CiC plays an important role in hepatic lipogenesis, which is responsible for the efflux of acetyl-CoA from the mitochondria to the cytosol in the form of citrate, the primer for fatty acid and cholesterol synthesis. In addition, CiC is a key component of the isocitrate-oxoglutarate and the citrate-malate shuttles. CiC has been purified from various species and its reconstituted function characterized as well as its cDNA isolated and sequenced. CiC mRNA and/or CiC protein levels are high in liver, pancreas, and kidney, but are low or absent in brain, heart, skeletal muscle, placenta, and lungs. A reduction of CiC activity was found in diabetic, hypothyroid, starved rats, and in rats fed on a polyunsaturated fatty acid (PUFA)-enriched diet. Molecular analysis suggested that the regulation of CiC activity occurs mainly through transcriptional and post-transcriptional mechanisms. This review begins with an assessment of the current understanding of CiC structural and biochemical characteristics, underlying the structure-function relationship. Emphasis will be placed on the molecular basis of the regulation of CiC activity in coordination with fatty acid synthesis.

Dietary fatty acid composition differently influences retinoylation reaction in rat testes mitochondria

All-trans-retinoic acid (atRA) is incorporated covalently into proteins of rat testes mitochondria. In this study, the effect of three diets with different fatty acid composition on the retinoylation of proteins of rat testes mitochondria has been investigated. Different groups of rats were fed on a basal diet supplemented with 15% of either coconut oil (CO), olive oil (OO) or fish oil (FO). We found that, when compared with CO, the binding of retinoic acid was decreased in FO- and OO-fed rats. Mitochondrial phospholipids composition was differently influenced by dietary treatments; minor changes were observed in fatty acid composition of phospholipids. Few differences were observed in the Arrhenius plots among the three groups of rats. Kinetic analysis revealed a decrease in the V (max) value in FO- and OO- as compared with CO-fed rats. No difference among the three groups were observed in the K ( M ) value. The retinoylation reaction was inhibited by 13-cis-RA and 9-cis-RA.

Changes in lipid composition and lipogenic enzyme activities in liver of lambs fed omega-6 polyunsaturated fatty acids

Twenty-four lambs (Ovis aries) were used in a 45-day finishing study to evaluate the effects of feeding diets high in linoleic acid (C(18:2), omega-6) on liver lipid composition and on lipogenic enzyme activities in subcellular fractions of liver. Lambs were fed either a 5% safflower oil (SO, high linoleic acid) supplemented diet or a control diet without added oil. SO feeding caused a reduction in the amount of serum and liver triacylglycerols and cholesterol, whereas the level of phospholipids in both tissues was hardly affected. In liver of SO-treated lambs an increase in the levels of C(18:2) and arachidonic acid (C(20:4), omega-6), together with a simultaneous decrease of saturated fatty acids, was observed. In comparison to rat liver, rather low activities of enzymes in the pathway for de novo fatty acid synthesis, i.e. acetyl-CoA carboxylase and fatty acid synthase, were found in lamb-liver cytosol. Both enzyme activities, as well as those of the NADPH-furnishing enzymes, were significantly reduced by SO feeding. In contrast, microsomal and especially mitochondrial fatty acid chain elongation activity, the latter being much higher than that of rat liver, were significantly increased in SO-treated lambs. In these animals, a stimulation of triangle up(9)-desaturase activity was observed in liver microsomes.

Transcription of the mitochondrial citrate carrier gene: role of SREBP-1, upregulation by insulin and downregulation by PUFA

In this study we investigated the transcriptional role of the sterol regulatory element (SRE) present in the promoter of the mitochondrial citrate carrier (CIC). We show that wild-type (but not mutated) CIC SRE cloned in front of the luciferase promoter confers transcriptional activation of the gene reporter. We also demonstrate that insulin activates, and polyunsaturated fatty acids (PUFA) inhibit, the gene reporter activity driven by the CIC promoter containing wild-type (but not mutated) SRE. Finally, both insulin treatment and overexpression of SRE binding protein (SREBP-1) increase the CIC transcript and protein levels, whereas PUFA have an opposite effect. These results show that SRE/SREBP-1 play a role in the transcriptional regulation of CIC by insulin and PUFA.

Hypothyroidism Reduces Tricarboxylate Carrier Activity and Expression in Rat Liver Mitochondria by Reducing Nuclear Transcription Rate and Splicing Efficiency

The tricarboxylate carrier (TCC), also known as citrate carrier, is an integral protein of the mitochondrial inner membrane. It is an essential component of the shuttle system by which mitochondrial acetyl-CoA, primer for both fatty acid and cholesterol synthesis, is transported into the cytosol, where lipogenesis occurs. The effect of hypothyroidism on the activity and expression of the hepatic mitochondrial TCC was investigated in this study. TCC activity was significantly decreased in hypothyroid rats as compared with euthyroid animals. This hormone deficiency effect was due to a reduction in the amount of carrier protein, which resulted from a proportionate decrease of the specific mRNA. Hypothyroidism did not influence TCC mRNA stability. On the other hand, nuclear run-on assay revealed that the transcriptional rate of TCC mRNA decreased by approximately 40% in the nuclei from hypothyroid versus euthyroid rats. In addition, the ribonuclease protection assay showed that, in the nuclei of hypothyroid rats, the ratio of mature to precursor RNA decreased, indicating that the splicing of TCC RNA is affected. Furthermore, we found that the ratio of polyadenylated/unpolyadenylated TCC RNA as well as the length of the TCC RNA poly(A) tail were similar in both euthyroid and hypothyroid rats. Thus, the rate of formation of the TCC 3'-end is not altered in hypothyroidism. These results suggest that hypothyroidism affects TCC expression at both the transcriptional and post-transcriptional levels.

Conjugated linoleic acid and hepatic lipogenesis in mouse: role of the mitochondrial citrate carrier

Conjugated linoleic acid (CLA) is able to reduce adiposity by affecting lipid metabolism. In particular, CLA administration to mice reduces body fat mass with a concomitant lipid accumulation in the liver. We investigated the effects of CLA on the activity of the mitochondrial citrate carrier (CIC), which is implicated in hepatic lipogenesis. The transport activity of the CIC, measured both in intact mitochondria and in the proteoliposomes, progressively increased with the duration of CLA feeding. An increase in the CIC activity of approximately 1.7-fold was found in 16 week CLA-treated mice with respect to control animals. A kinetic analysis showed a 1.6-fold increase in the V(max) of citrate transport but no change in the K(m) value. Western blot experiments revealed an increase of approximately 1.7-fold in the expression of CIC after CLA treatment. A strict correlation between the increase in CIC activity and the stimulation of the cytosolic lipogenic enzymes was also found. These data indicate that the CIC may play a role in the onset of hepatic steatosis in CLA-fed mice by supplying the carbon source for de novo fatty acid synthesis.

Hypothyroidism down-regulates mitochondrial citrate carrier activity and expression in rat liver

The effect of hypothyroidism on citrate carrier (CiC) activity has been investigated in rat-liver mitochondria. The rate of citrate transport was reduced by approximately 50% in mitochondria from hypothyroid as compared with euthyroid rats. In parallel, a decrease in the rate of de novo fatty acid synthesis was observed in the cytosol of the former animals. Kinetic analysis of citrate transport revealed that only the Vmax was reduced by hypothyroidism, while Km was almost unaffected. Hypothyroidism increased the mitochondrial percentage of phosphatidylcholine while decreased that of phosphatidylethanolamine; an altered fatty acid pattern but no significant difference in the sum of saturated and unsaturated fatty acids as well as in the unsaturation index was observed. The CiC Arrhenius plot did not show appreciable difference between the two groups of rats. However, Western blot analysis associated with mRNA quantitation indicated that both protein level and mRNA accumulation of hepatic CiC were noticeably decreased in hypothyroid state. Therefore, a reduced content of the carrier protein can represent a plausible mechanism to explain the decline in the CiC activity observed in rat liver mitochondria of hypothyroid rats.

Different dietary fatty acids have dissimilar effects on activity and gene expression of mitochondrial tricarboxylate carrier in rat liver

The tricarboxylate carrier (TCC), an integral protein of the mitochondrial inner membrane, transports mitochondrial acetyl-CoA into the cytosol, where lipogenesis occurs. We investigated in rat liver mitochondria the effect of diets enriched with saturated fatty acids (beef tallow, BT), monounsaturated fatty acids (olive oil, OO) or n-3 polyunsaturated fatty acids (fish oil, FO), respectively, on the activity and expression of TCC. TCC activity decreased, in parallel with TCC mRNA abundance, only upon FO-feeding. The TCC transcription rate, mRNA turnover and RNA processing indicated that FO administration regulates TCC gene at transcriptional and post-transcriptional steps, whereas BT- and OO-feeding do not seem to affect either TCC activity or gene expression.

n-6 PUFAs downregulate expression of the tricarboxylate carrier in rat liver by transcriptional and posttranscriptional mechanisms

The tricarboxylate (citrate) carrier (TCC), a protein of the mitochondrial inner membrane, is an obligatory component of the shuttle system by which mitochondrial acetyl-CoA is transported into the cytosol, where lipogenesis occurs. The aim of this study was to investigate the molecular basis for the regulation of TCC gene expression by a high-fat, n-6 PUFA-enriched diet. Rats received for up to 4 weeks a diet enriched with 15% safflower oil (SO), which is high in linoleic acid (70.4%). We found a gradual decrease of TCC activity and a parallel decline in the abundance of TCC mRNA, the maximum effect occurring after 4 weeks of treatment. At this time, the estimated half-life of TCC mRNA was the same in the hepatocytes from rats on both diets, whereas the transcriptional rate of TCC mRNA, tested by nuclear run-on assay, was reduced by approximately 38% in the rats on the SO-enriched diet. The RNase protection assay showed that the ratio of mature to precursor RNA, measured in the nuclei, decreased with the change to the n-6 PUFA diet. These results suggest that administration of n-6 PUFAs to rats leads to changes not only in the transcriptional rate of the TCC gene but also in the processing of the nuclear precursor for TCC RNA.

Structural and oxidative modifications of erythrocyte ghosts in patients with primary biliary cirrhosis: relation with the disease stage and effect of bile acid treatment

BACKGROUND

Erythrocyte membrane modifications in patients with cholestasis are supposed to reflect those of hepatocytes.

METHODS

Erythrocyte membrane composition (cholesterol, phospholipids, fatty acids, protein sulphydrils and carbonyls) was assessed and related to the stage of liver disease in patients with primary biliary cirrhosis before and after 1 year of ursodeoxycholate treatment.

RESULTS

Compared with controls, patients showed lower levels of protein sulphydrils (28.9 +/- 7.1 vs. 65.6 +/- 1.8 nmol mg(-1) prot) and accumulation of carbonyls (4.7 +/- 1.7 vs. 1.4 +/- 0.1 nmol mg(-1) prot). Phosphatidylethanolamine level was lower in stage III-IV cirrhosis while phosphatidylcholine and cholesterol levels were higher; as a consequence the phosphatidylcholine/sphingomyelin ratio was higher than in controls (4.25 +/- 0.55 in the I-II stage and 2.89 +/- 0.44 in the stage III-IV vs. 1.61 +/- 0.30). These changes were particularly evident in patients with more advanced stages of liver disease. Protein sulphydrils and carbonyls, phosphatidylethanolamine and cholesterol levels correlated (P<0.05) with the histological stage of the liver disease, serum and membrane cholesterol levels were significantly related (r=0.66, P<0.05). One year of ursodeoxycholate administration was accompanied by major changes of the membrane lipid composition, partial reversal of protein oxidation, and improvement of serum parameters.

CONCLUSIONS

This study indicates that major alterations in protein status and lipid composition occur in erythrocyte membrane of patients with primary biliary cirrhosis. These changes were more pronounced in patients with advanced liver disease. Ursodeoxycholate was able to revert in part serum and erythrocyte alterations, especially in patients with early stages of liver disease.

Differential effects of coconut oil- and fish oil-enriched diets on tricarboxylate carrier in rat liver mitochondria

The mitochondrial tricarboxylate carrier (TCC) plays an important role in lipogenesis being TCC-responsible for the efflux from the mitochondria to the cytosol of acetyl-CoA, the primer for fatty acid synthesis. In this study, we investigated the effects of two high-fat diets with different fatty acid composition on the hepatic TCC activity. Rats were fed for 3 weeks on a basal diet supplemented with 15% of either coconut oil (CO), abundant in medium-chain saturated fatty acids, or fish oil (FO), rich in n-3 polyunsaturated fatty acids. Mitochondrial fatty acid composition was differently influenced by the dietary treatments, while no appreciable change in phospholipid composition and cholesterol level was observed. Compared with CO, the TCC activity was markedly decreased in liver mitochondria from FO-fed rats; kinetic analysis of the carrier revealed a decrease of the Vmax, with no change of the Km. No difference in the Arrhenius plot between the two groups was observed. Interestingly, the carrier protein level and the corresponding mRNA abundance decreased following FO treatment. These data indicate that FO administration markedly decreased the TCC activity as compared with CO. This effect is most likely due to a reduced gene expression of the carrier protein.

Starvation-induced posttranscriptional control of rat liver mitochondrial citrate carrier expression

Starvation has been associated with a reduced citrate carrier (CTP) activity in rat liver mitochondria. In the present study the molecular mechanism responsible for this reduction was investigated. Northern blot analysis performed with hepatic total RNA showed a decrease of about 40% in the CTP mRNA abundance in starved rats, when compared to fed animals. Nuclear run-on assay did not reveal any appreciable difference in the rate of CTP mRNA synthesis between the two groups of animals, while the apparent half-life of CTP mRNA in hepatocytes from fed and starved rats was 11 and 6h, respectively. Therefore, these results suggest that in starved rats the regulation of hepatic CTP expression occurs at posttranscriptional level. Moreover, the reduced CTP activity in starved animals gradually increased by refeeding. The carrier activity reached fed rat values 6-9h following refeeding. Interestingly, the accumulation of CTP mRNA raised in parallel with the transport activity.

The mitochondrial tricarboxylate carrier of silver eel: dimeric structure and cytosolic exposure of both N- and C-termini

The mitochondrial tricarboxylate carrier plays a fundamental role in the hepatic fatty acid synthesis. In this study, we investigated the transmembrane organization of this protein in the inner membrane of eel liver mitochondria using anti-N-terminal and anti-C-terminal antibodies. These antibodies recognized the N- and C-termini of the tricarboxylate carrier in intact mitoplasts, thus suggesting a cytosolic exposure of these regions in the membrane-bound protein. This structural arrangement of the tricarboxylate carrier was further confirmed by protease treatment of intact mitoplasts. Moreover, the oligomeric state of the native tricarboxylate carrier was investigated by blue native electrophoresis. A dimeric form of the carrier protein was found when eel liver mitochondria were solubilized with the mild detergent digitonin. These findings suggest an arrangement of the dimeric tricarboxylate carrier into an even number of membrane-spanning domains, with the N-terminal and C-terminal regions oriented toward the intermembrane space of fish mitochondria.

Citrate carrier activity and cardiolipin level in eel (Anguilla anguilla) liver mitochondria

The activity of the tricarboxylate (citrate) carrier has been assayed in intact liver mitochondria from yellow eel (Anguilla anguilla) and compared to that from rat. The eel-citrate carrier specific activity was approximately 1.7-fold higher than that assayed in rat-liver mitochondria. The content of the main mitochondrial phospholipids, phosphatidylethanolamine and phosphatidylcholine, did not show a significant difference between the two species, while in eel a higher cardiolipin level was observed. Fatty acid composition of eel-liver mitochondrial phospholipids was characterised by a large amount of unsaturated fatty acids, dominated by octadecaenoic acid (C(18:1) (n-9)) and docosahexaenoic acid (C(22:6) (n-3)). The cardiolipin fatty acid pattern of eel-liver mitochondria showed, with respect to the rat, a higher C(20:5) (n-3) and C(22:6) (n-3) content and a lower amount of C(18:2) (n-6) and C(20:4) (n-6). A noticeable activity of lipogenic enzymes was also detected in eel liver cytosol. The results of this study suggest that the remarkable activity of the citrate carrier in eel-liver mitochondria can most likely be ascribed to a considerable cardiolipin level. A covariance of citrate carrier and lipogenic enzyme activities was observed.