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.