Sterol regulatory element-binding proteins are regulators of the rat thyroid peroxidase gene in thyroid cells.

Sterol regulatory element-binding proteins (SREBPs)-1c and -2, which were initially discovered as master transcriptional regulators of lipid biosynthesis and uptake, were recently identified as novel transcriptional regulators of the sodium-iodide symporter gene in the thyroid, which is essential for thyroid hormone synthesis. Based on this observation that SREBPs play a role for thyroid hormone synthesis, we hypothesized that another gene involved in thyroid hormone synthesis, the thyroid peroxidase (TPO) gene, is also a target of SREBP-1c and -2. Thyroid epithelial cells treated with 25-hydroxycholesterol, which is known to inhibit SREBP activation, had about 50% decreased mRNA levels of TPO. Similarly, the mRNA level of TPO was reduced by about 50% in response to siRNA mediated knockdown of both, SREBP-1 and SREBP-2. Reporter gene assays revealed that overexpression of active SREBP-1c and -2 causes a strong transcriptional activation of the rat TPO gene, which was localized to an approximately 80 bp region in the intron 1 of the rat TPO gene. In vitro- and in vivo-binding of both, SREBP-1c and SREBP-2, to this region in the rat TPO gene could be demonstrated using gel-shift assays and chromatin immunoprecipitation. Mutation analysis of the 80 bp region of rat TPO intron 1 revealed two isolated and two overlapping SREBP-binding elements from which one, the overlapping SRE+609/InvSRE+614, was shown to be functional in reporter gene assays. In connection with recent findings that the rat NIS gene is also a SREBP target gene in the thyroid, the present findings suggest that SREBPs may be possible novel targets for pharmacological modulation of thyroid hormone synthesis.

Sterol regulatory element-binding proteins are regulators of the NIS gene in thyroid cells.

The uptake of iodide into the thyroid, an essential step in thyroid hormone synthesis, is an active process mediated by the sodium-iodide symporter (NIS). Despite its strong dependence on TSH, the master regulator of the thyroid, the NIS gene was also reported to be regulated by non-TSH signaling pathways. In the present study we provide evidence that the rat NIS gene is subject to regulation by sterol regulatory element-binding proteins (SREBPs), which were initially identified as master transcriptional regulators of lipid biosynthesis and uptake. Studies in FRTL-5 thyrocytes revealed that TSH stimulates expression and maturation of SREBPs and expression of classical SREBP target genes involved in lipid biosynthesis and uptake. Almost identical effects were observed when the cAMP agonist forskolin was used instead of TSH. In TSH receptor-deficient mice, in which TSH/cAMP-dependent gene regulation is blocked, the expression of SREBP isoforms in the thyroid was markedly reduced when compared with wild-type mice. Sterol-mediated inhibition of SREBP maturation and/or RNA interference-mediated knockdown of SREBPs reduced expression of NIS and NIS-specific iodide uptake in FRTL-5 cells. Conversely, overexpression of active SREBPs caused a strong activation of the 5'-flanking region of the rat NIS gene mediated by binding to a functional SREBP binding site located in the 5'-untranslated region of the rat NIS gene. These findings show that TSH acts as a regulator of SREBP expression and maturation in thyroid epithelial cells and that SREBPs are novel transcriptional regulators of NIS.

Mouse γ-butyrobetaine dioxygenase is regulated by peroxisome proliferator-activated receptor α through a PPRE located in the proximal promoter.

Convincing evidence from studies with peroxisome proliferator-activated receptor (PPAR)α-deficient mice suggested that the carnitine biosynthetic enzyme γ-butyrobetaine dioxygenase (BBD) is regulated by PPARα. However, the identification of BBD as a direct PPARα target gene as well as its exact regulation remained to be demonstrated. In silico-analysis of the mouse BBD promoter revealed seven putative peroxisome proliferator response elements (PPRE) with high similarity to the consensus PPRE. Luciferase reporter gene assays using mutated and non-mutated serial 5'-truncation BBD promoter reporter constructs revealed that one PPRE located at -75 to -87 relative to the transcription start site in the proximal BBD promoter is probably functional. Using gel shift assays we observed in vitro-binding of PPARα/RXRα heterodimer to this PPRE confirming that it is functional. In conclusion, the present study clearly shows that mouse BBD is a direct PPARα target gene and that transcriptional up-regulation of mouse BBD by PPARα is likely mediated by binding of the PPARα/RXR heterodimer to one PPRE located in its proximal promoter region. The results confirm emerging evidence from recent studies that PPARα plays a key role in the regulation of carnitine homeostasis by controlling genes involved in both, carnitine synthesis and carnitine uptake.