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 (T(4)) and triiodothyronine (T(3)), and demonstrate that as in vertebrates, T(4) and T(3) and their derivative triiodothyroacetic acid (TRIAC) can significantly enhance the expression of the amphioxus Branchiostoma belcheri C/EBPalpha/beta gene, BbC/EBPalpha/beta, in the hepatic caecum in a tissue-specific manner; and iodopanoic acid (IOP), a deiodinase inhibitor, is able to inhibit only T(4) action, suggesting the presence of the conversion of T(4) to T(3). Moreover, the recombinant peptide of B. belcheri nuclear thyroid hormone receptor (TR) binds to both T(3) and TRIAC, implicating that the expression of BbC/EBPalpha/beta is mediated through interaction between T(3)/TRIAC-TR. Altogether these results suggest the presence in amphioxus of a signaling flow from the endostyle to the hepatic caecum.