CONSTITUTIVE ACTIVATION OF GENE-EXPRESSION BY THYROID-HORMONE RECEPTOR RESULTS FROM REVERSAL OF P53-MEDIATED REPRESSION

Thyroid hormone receptor (T3R) is a member of the steroid hormone rece ptor gene family of nuclear hormone receptors, In most cells T3R activ ates gene expression only in the presence of its ligand, L-triiodothyr onine (T3), However, in certain cell types (e.g., GH4C1 cells) express ion of T3R leads to hormone-independent constitutive activation, This activation by unliganded T3R occurs with a variety of gene promoters a nd appears to be independent of the binding of T3R to specific thyroid hormone response elements (TREs), Previous studies indicate that this constitutive activation results from the titration of an inhibitor of transcription, Since the tumor suppresser p53 is capable of repressin g a wide variety of gene promoters, we considered the possibility that the inhibitor is p53. Evidence to support this comes from studies ind icating that expression of p53 blocks T3R-mediated constitutive activa tion in GH4C1 cells, In contrast with hormone-independent activation b y T3R, p53 had little or no effect on T3-dependent stimulation which r equires TREs, In addition, p53 mutants which oligomerize with wild-typ e p53 and interfere with its function also increase promoter activity, This enhancement is of similar magnitude to but is not additive with the stimulation mediated by unliganded T3R, suggesting that they targe t the same factor, Since p53 mutants are known to target wild-type p53 in the cell, this suggests that T3R also interacts-with p53 in vivo a nd that endogenous levels of p53 act to suppress promoter activity, Ev idence supporting both functional and physical interactions of T3R and p53 in the cell is presented. The DNA binding domain (DBD) of T3R is important in mediating constitutive activation, and the receptor DBD a ppears to functionally interact with the N terminus of p53 in the cell , In vitro binding studies indicate that the T3R DBD is important for interaction of T3R with p53 and that this interaction is reduced by T3 , These findings are consistent with the in vivo studies indicating th at p53 blocks constitutive activation but not ligand-dependent stimula tion, These studies provide insight into mechanisms by which unligande d nuclear hormone receptors can modulate gene expression and may provi de an explanation for the mechanism of action of the v-erbA oncoprotei n, a retroviral homolog of chicken T3R alpha.