Dual functions of thyroid hormone receptors during Xenopus development

Thyroid hormone (TH) plays a causative role in anuran metamorphosis. This e ffect is presumed to be manifested through the regulation of gene expressio n 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, respectiv ely We have been investigating the roles of TRs during Xenopus laevis devel opment, including premetamorphic and metamorphosing stages. In this review, we summarize some of the studies on the TRs by others and us. These studie s reveal that TRs have dual functions in frog development as reflected in t he following two aspects. First, TRs function initially as repressors of TH -inducible genes in premetamorphic tadpoles to prevent precocious metamorph osis, thus ensuring a proper period of tadpole growth, and later as activat ors of these genes to activate the metamorphic process. Second, TRs can pro mote both cell proliferation and apoptosis during metamorphosis, depending upon the cell type in which they are expressed. (C) 2000 Elsevier Science I nc. All rights reserved.