Acetylation promotes thyroid hormone receptor ubiquitination and degradation by nuclear proteasomes

Thyroid hormone receptor (TR) activation of thyroid hormone (T3)-responsive gene expression is enhanced by receptor acetylation, but paradoxically, we previously showed that TR acetylation mimics have a more cytoplasmic localization compared to nonacetylation mimics. Here, we showed that a substitution mutation at any of the three acetylation sites is sufficient for altered localization and transcriptional activity in transfected human cells. Inhibition of select deacetylases showed no cytoplasmic shift for TR, whereas inhibition of the acetyltransferase CBP/p300 significantly increased TR nuclear localization, suggesting that acetylation, not deacetylation, drives localization. Using acetyl-lysine immunoprecipitation assays, we observed a greater population of acetylated TRα1 in the nucleus compared to the cytoplasm; however, acetylated TRα1 was more susceptible to nuclear proteases, suggesting that shuttling to the cytoplasm is a protective mechanism for modulating the extent of nuclear degradation of acetylated TR. Ubiquitin immunoprecipitation assays revealed significantly elevated polyubiquitination of the acetylation mimic, and biochemical fractionation revealed that polyubiquitinated wild-type TRα1 is enriched in the nuclear matrix, with no detectable modified TRα1 in the cytoplasm. Further, proteasomes shift toward a more nuclear localization in TRα1-expressing cells. Resistance to Thyroid Hormone syndrome α (RTHα) mutant C392X had significantly increased acetylation levels but a wild-type polyubiquitination profile, suggesting that aberrant acetylation is an additional characteristic of the molecular pathology RTHα. Our findings provide novel insights into TR's life cycle supporting a model in which TR exists primarily in the nonacetylated state, while acetylation promotes T3-responsive gene expression, followed by TR polyubiquitination, and rapid T3-dependent degradation by nuclear proteasomes.