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

Luisa Siculella,Simona Sabetta,Anna M Giudetti,Gabriele V Gnoni

doi:10.1074/jbc.m507237200

Abstract

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.

Highlights

Thyroid hormones dramatically affect the extent to which liver contributes to total lipogenesis in rat
It exports from mitochondria to the cytosol acetyl-CoA mainly derived from sugar sources, providing the carbon units for fatty acid and cholesterol biosyntheses
Hyperthyroid compared with hypothyroid rats, showing that In this study, we report the effect of hypothyroidism on the most of the T3 effect on hepatic lipogenesis can be explained at hepatic activity and expression of tricarboxylate carrier (TCC), a mitochondrial transthe pretranslational level [12]

Summary

Introduction

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]. 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, strongly reduced in propylthiouracil-induced hypothyroid rats [10]. It has been shown that tri-iodothyronine (T3) regulates fatty acid synthesis by altering the expression of the genes coding for the lipogenic enzymes [12]. There has been no report assessing mitochondrial TCC activity relative to the hypothyroid state, and no study so far has gone deep into the hormonal regulation at the molecular level of any gene for a mitochondrial carrier protein. We showed that the reduced TCC activity observed in the hypothyroid state can be ascribed to either a transcriptional and a post-transcriptional TCC gene regulation

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