Direct Regulation of Mitochondrial RNA Synthesis by Thyroid Hormone

Abstract

We have analyzed the influence of in vivo treatment and in vitro addition of thyroid hormone on in organello mitochondrial DNA (mtDNA) transcription and, in parallel, on the in organello footprinting patterns at the mtDNA regions involved in the regulation of transcription. We found that thyroid hormone modulates mitochondrial RNA levels and the mRNA/rRNA ratio by influencing the transcriptional rate. In addition, we found conspicuous differences between the mtDNA dimethyl sulfate footprinting patterns of mitochondria derived from euthyroid and hypothyroid rats at the transcription initiation sites but not at the mitochondrial transcription termination factor (mTERF) binding region. Furthermore, direct addition of thyroid hormone to the incubation medium of mitochondria isolated from hypothyroid rats restored the mRNA/rRNA ratio found in euthyroid rats as well as the mtDNA footprinting patterns at the transcription initiation area. Therefore, we conclude that the regulatory effect of thyroid hormone on mitochondrial transcription is partially exerted by a direct influence of the hormone on the mitochondrial transcription machinery. Particularly, the influence on the mRNA/rRNA ratio is achieved by selective modulation of the alternative H-strand transcription initiation sites and does not require the previous activation of nuclear genes. These results provide the first functional demonstration that regulatory signals, such as thyroid hormone, that modify the expression of nuclear genes can also act as primary signals for the transcriptional apparatus of mitochondria.Thyroid hormone regulates the expression of key nucleus encoded mitochondrial genes (23,37,38,47) and the steadystate concentration of all mitochondrial DNA (mtDNA)-encoded mRNAs (mt-mRNAs) (15-17, 32, 44). In rat liver, changes in mt-mRNA levels in response to in vivo treatment with thyroid hormones were well correlated with amounts of the corresponding polypeptides, while mtDNA copy number remained unmodified (33,47). Based on these observations, transcriptional control has been tentatively proposed as the main mechanism to explain the thyroid hormone effect on mt-mRNA steady-state levels and protein synthesis (33-36).Two alternative potential mechanisms for the regulation of mtDNA transcription by thyroid hormone have been suggested: through activation of mitochondrial transcription factor A (mtTFA) expression (18, 45), or by a direct action of the hormone through mitochondrial receptors (48). The second alternative is supported by earlier (42, 43) and recent (1, 48) reports that have proposed the existence of T 3 (3,3,5-triiodo-L-thyronine) binding proteins in mitochondria, which could interact with cis elements of mtDNA (7, 48). However, the lack of experimental evidence showing thyroid hormone-mediated transcription regulation in the absence of nuclear gene expression prevents any definitive conclusion about whether direct regulation of mtDNA transcription by this hormone occurs (38). Presently it is more generally believed that this transcripti...

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