Thyroid hormone induction of keratin genes: a two-step activation of gene expression during development

To determine the mechanism of action of the thyroid hormone triiodothyronine (T3) during metamorphosis of the amphibian epidermis, we have investigated the developmental activation of the 63-kD keratin genes in the frog Xenopus laevis. These genes code for three closely related keratins that first appear in the larval epidermis and accumulate during metamorphosis to become the most abundant proteins in the adult epidermis. We report here that the 63-kD keratins and their mRNAs first appear at stages 48-52. The level of 63-kD keratin gene expression remains relatively low until stage 56 and then increases dramatically. Quantitative analysis of the concentration of 63-kD keratin mRNA demonstrates low levels until stage 55/56, followed by an increase greater than 300-fold from stage 55/56 to the adult. Each adult cell contains approximately 55,000 molecules of the 63-kD keratin mRNAs. T3 is not required for the initial activation of the 63-kD keratin genes, but high-level expression is absolutely dependent on T3. High-level expression is prevented by propylthiouracil, which inhibits thyroid hormone synthesis and can be induced precociously both in vivo and in vitro with exogenous T3 as early as stage 48, but not prior to that time. Thus, the full activation of the 63-kD keratin genes during development requires two regulatory steps, one independent and one dependent on T3.