THYROID-HORMONE POTENTIATES ESTROGEN ACTIVATION OF VITELLOGENIN GENES AND AUTOINDUCTION OF ESTROGEN-RECEPTOR IN ADULT XENOPUS HEPATOCYTES

Although the important role of thyroid hormones in regulating metamorphosis of amphibian larvae is well known, it has not been clearly established if thyroid hormones have any function in the activities of adult amphibian tissues. We now describe a strong effect of 3,3',5-triiodothyronine (T3) on adult Xenopus liver cells. Low doses of T3 rapidly (within 6-12 h) potentiate the activation of vitellogenin (Vit) genes by estradiol-17beta (E2) in primary cultures of adult male and female Xenopus hepatocytes. This effect is developmentally regulated and is first manifested during metamorphic climax. In an attempt to explain this potentiation, we find that T3 also upregulates thyroid hormone receptor beta, but not alpha, transcripts and rapidly enhances the autoinduction of estrogen receptor (ER) mRNA in adult Xenopus hepatocytes. In transient transfection of the Xenopus cell line XTC-2 with an estrogen response element - chloramphenicol transacetylase (ERE-CAT) construct T3 was found to potentiate the transcription by E2 from the transfected ERE, thus suggesting that it enhances the accumulation of functional ER. We conclude that T3 can function in adult amphibian tissues, and discuss the significance of thyroid hormone potentiation of responses to estrogen in reproductive processes.