Abstract
Hepatocyte nuclear factor-4 alpha (HNF-4α) is an orphan nuclear receptor with important roles in hepatic metabolism. Protein phosphorylation plays a functional role in its nuclear localization, DNA binding, and transactivation. Thyroid-stimulating hormone (TSH) is a hormone produced by the anterior pituitary gland, whose direct effect on the metabolic pathway has been observed. Our previous study demonstrated that TSH significantly decreases hepatic nuclear HNF-4α expression. However, whether TSH can influence HNF-4α phosphorylation is unclear. Here, we discovered that TSH can increase HNF-4α phosphorylation and modulate its subcellularlocalization. When HepG2 cells were treated with TSH, the phosphorylation of HNF-4α increased and its nuclear localization was interrupted. Cytoplasmic HNF-4α increased, while nuclear HNF-4α decreased. When the cAMP/PKA pathway was inhibited by the PKA inhibitor H89 and the adenylate cyclase (AC) inhibitor SQ22536, the TSH-mediated phosphorylation of HNF-4α was disrupted. When Tshr was silenced in mice, the phosphorylation of HNF-4α decreased, and cytoplasmic HNF-4α decreased while nuclear HNF-4α increased. In conclusion, our study revealed a novel mechanism by which TSH regulated the hepatic HNF-4α subcellular localization, suggesting the possibility that one of the effects of TSH is to reduce the expression of HNF-4α target genes.
Highlights
Hepatocyte nuclear factor-4α (HNF-4α ) is an orphan nuclear receptor (NR) that plays a critical role in hepatocyte differentiation[1,2,3], as well as the maintenance of homeostasis of the adult liver, intestines, and pancreatic β cells[4,5,6,7]
We demonstrated that Thyroid-stimulating hormone (TSH) can increase the phosphorylation of HNF-4α via the cAMP/ protein kinase A (PKA) pathway, which reduces the nuclear translocation of HNF-4α and attenuates its transcriptional activity in the liver
Besides the change of nuclear HNF-4α protein, higher cytoplasmic levels of HNF-4α were found in the TSH-treated cells (Fig. 1A), suggesting that TSH might interfere with the nuclear localization of HNF-4α
Summary
Hepatocyte nuclear factor-4α (HNF-4α ) is an orphan nuclear receptor (NR) that plays a critical role in hepatocyte differentiation[1,2,3], as well as the maintenance of homeostasis of the adult liver, intestines, and pancreatic β cells[4,5,6,7]. Protein phosphorylation is a common post-translational modification among transcription factors and has been shown to play a functional role in nuclear localization, DNA binding, and transactivation[13,14]. A recent report showed that phosphorylation of that site in HNF-4α resulted in impaired nuclear localization and DNA binding, which would decrease the transcriptional effects of HNF-4α 15,16. Our recent study demonstrated that TSH significantly decreases the expression of hepatic nuclear HNF-4α in vivo and in vitro[20]. It is still not clear whether TSH can influence the phosphorylation of HNF-4α
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