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 À 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.

The mitochondrial tricarboxylate carrier (TCC) plays an important role in lipogenesis being TCC-responsible for the efflux from the mitochondria to the cytosol of acetylCoA, 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 V max , with no change of the K m . 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.

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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 ‫ف‬ 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. -Siculella, L., F. Damiano, S. Sabetta, and G. V. Gnoni. n-6 PUFAs downregulate expression of the tricarboxylate carrier in rat liver by transcriptional and posttranscriptional mechanisms. J. Lipid Res. 2004. 45: 1333-1340.

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 ϳ40% 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 posttranscriptional levels.Thyroid hormones have a profound influence on normal development, differentiation, and metabolism (1). Mitochondria are considered possible subcellular loci of thyroid hormone action in view of their crucial role in energy metabolism (2). Mitochondrial function can be regulated by thyroid hormones both indirectly in a "nuclear mediated" way and directly through interaction with some mitochondrial component (2, 3). Hypothyroidism is associated with a considerable decrease in basic metabolic rate, oxygen consumption, and rates of oxidation of glucose, fatty acids, and amino acids (4, 5).Moreover, thyroid hormones dramatically affect the extent to which liver contributes to total lipogenesis in rat. This contribution ranges from 5% in the hypothyroid animals to 34% in the hyperthyroid animals (6). Fatty acid synthesis in rat liver is extremely responsive to the thyroid status of the organism. Hypothyroidism in developing or adult rats depresses hepatic fatty acid synthase (FAS) 2 activity by 50% (7). Cultures of hepatocytes obtained from hypothyroid rats synthesize fatty acids from acetate half as fast as euthyroid hepatocytes (8). The enzymatic activities of de novo fatty acid synthesis, i.e. acetyl-CoA carboxylase (ACC) and FAS, are greatly increased in hyperthyroid rats (9, 10) and, conversely, strongly reduced in propylthiouracil-induced hypothyroid rats (10). Furthermore, rat thyroidectomy decreases the activities of FAS and other lipogenic enzymes such as malic enzyme and glucose-6-phosphate dehydrogenase (11). It has been shown that tri-i...