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 belched C/EBP alpha/beta gene, BbC/EBP alpha/beta, in the hepatic caecum in a tissue-specific manner; and ioclopanoic 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/EBP alpha/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. (C) 2009 Elsevier Ireland Ltd. All rights reserved.