The thyroid hormone receptor and the insulator protein CTCF: two different factors with overlapping functions

Thyroid hormones and thyroid hormone receptors (TRs) confer a fundamental regulation of critical genes involved in metabolism, differentiation, and development. A similar role is attributed to the highly conserved zinc-finger factor CTCF. Furthermore, a potential role in tumour suppression has been attributed to CTCF. In addition to promoter regulation, CTCF has also been shown to be involved in chromatin insulation or enhancer blocking. In several cases, binding sites for TR and for CTCF have been found next to each other. Functionally, these sites mediate synergistic repression or induction dependent on the type of binding site and on the presence or absence of thyroid hormone. Here we discuss functional similarities between TR and CTCF and their roles within these composite elements.     

Dual functions of thyroid hormone receptors during Xenopus development

Thyroid hormone (TH) plays a causative role in anuran metamorphosis. This effect is presumed to be manifested through the regulation of gene expression by TH receptors (TRs). TRs can act as both activators and repressors of a TH-inducible gene depending upon the presence and absence of TH, respectively. We have been investigating the roles of TRs during Xenopus laevis development, including premetamorphic and metamorphosing stages. In this review, we summarize some of the studies on the TRs by others and us. These studies reveal that TRs have dual functions in frog development as reflected in the following two aspects. First, TRs function initially as repressors of TH-inducible genes in premetamorphic tadpoles to prevent precocious metamorphosis, thus ensuring a proper period of tadpole growth, and later as activators of these genes to activate the metamorphic process. Second, TRs can promote both cell proliferation and apoptosis during metamorphosis, depending upon the cell type in which they are expressed.