AUTOACTIVATION OF XENOPUS THYROID-HORMONE RECEPTOR-BETA GENES CORRELATES WITH LARVAL EPITHELIAL APOPTOSIS AND ADULT-CELL PROLIFERATION

The thyroid hormone (T-3)-dependent amphibian metamorphosis involves d egeneration of larval tissues through programmed cell death (apoptosis ) and concurrent proliferation and differentiation of adult cell types . As the mediators of the causative effects of T-3 on metamorphosis, b oth thyroid hormone receptor (TR) alpha and beta genes have been found to be expressed in different tissues during this process. In particul ar, the Xenopus TR beta genes have been shown to be regulated by T-3 a t the transcriptional level and their expression correlates with organ -specific metamorphosis. We demonstrate here by in situ hybridization that the Xenopus TR beta genes are regulated in a cell-type specific m anner that correlates with tissue transformation. In particular, they are found to be expressed in the larval intestinal epithelial cells pr ior to their apoptotic degeneration and in the proliferating cells of the adult epithelium, connective tissue, and muscles. However, they ar e repressed again upon the differentiation of these adult cells. These results implicate that TR beta participates both in inducing apoptosi s and stimulating cell proliferation during development.