Up-regulation of C/EBP by thyroid hormones: A case demonstrating the vertebrate-like thyroid hormone signaling pathway in amphioxus

Amphioxus has been shown to possess a thyroid-like organ (endostyle) as well a liver-like organ (hepatic caecum), but a definitive link between these organs remains to be established. In this study, we confirm that the endostyle abound in thyroxine (T4) and triiodothyronine (T3), and demonstrate that as in vertebrates, T4 and T3 and their derivative triiodothyroacetic acid (TRIAC) can significantly enhance the expression of the amphioxus Branchiostoma belcheri C/EBPα/β gene, BbC/EBPα/β, in the hepatic caecum in a tissue-specific manner; and iodopanoic acid (IOP), a deiodinase inhibitor, is able to inhibit only T4 action, suggesting the presence of the conversion of T4 to T3. Moreover, the recombinant peptide of B. belcheri nuclear thyroid hormone receptor (TR) binds to both T3 and TRIAC, implicating that the expression of BbC/EBPα/β is mediated through interaction between T3/TRIAC-TR. Altogether these results suggest the presence in amphioxus of a signaling flow from the endostyle to the hepatic caecum.