Thyroid hormone induces constitutive keratin gene expression during Xenopus laevis development.

We have used in vitro explant cultures of Xenopus laevis skin to investigate the role that the thyroid hormone triiodothyronine (T3) plays in activating the 63-kilodalton (kDa) keratin genes. The activation of these genes in vivo requires two distinct steps, one independent of T3 and one dependent on T3. In this report we have shown that the same two steps are required to fully activate the 63-kDa keratin genes in skin explant cultures, and we have characterized the T3-mediated step in greater detail. Unlike the induction of transcription by T3 or steroid hormones in adult tissues, there was a long latent period of approximately 2 days between the addition of T3 to skin cultures and an increase in concentration of keratin mRNA. While the T3 induction of 63-kDa keratin gene transcription cannot occur until age 48, a short transient exposure of stage 40 skin cultures to T3 resulted in high-level expression of these genes 5 days later, when normal siblings had reached stage 48. This result indicates that T3 induces a stable change in epidermal cells which can be expressed much later, after extensive cell proliferation has occurred in the absence of T3. Once the 63-kDa keratin genes were induced, they were stably expressed, and by the end of metamorphosis T3 had no further effect on their expression. The results suggest that T3 induces constitutive expression of the 63-kDa keratin genes during metamorphosis.