Expression of a thyroid hormone-responsive recombinant gene introduced into adult mice livers by replication-defective adenovirus can be regulated by endogenous thyroid hormone receptor.

We constructed a recombinant adenovirus carrying the firefly luciferase gene driven by the thymidine kinase promoter and controlled by the palindromic thyroid hormone/retinoic acid-responsive element. The same adenovirus vector without the hormone-responsive element was used as control. Regulation of the luciferase gene expression was tested in pituitary-derived GH cells and hepatoma-derived HepG2 cells infected with the recombinant adenoviruses. Administration of triiodothyronine to GH cells and all-trans-retinoic acid to HepG2 cells resulted in 8.0 +/- 0.3-fold and 4.6 +/- 0.5-fold induction of luciferase activity, respectively. No significant increase was observed with the control adenovirus. Hormonal regulation was also examined in adult mice. Mice depleted of thyroid hormone were injected intravenously with the recombinant adenoviruses and given 4 times the replacement dose of triiodothyronine or vehicle only for 4 days. Hormone administration resulted in 4.2-fold increase of luciferase activity in liver homogenates. No significant effect was observed in animals injected with the control adenovirus. This recombinant adenovirus provides a new experimental system in the study of thyroid hormone and retinoic acid action and offers the potential to regulate by physiological means the expression of genes transferred for the purpose of therapy.