Unliganded T₃R, but not its oncogenic variant, v-erbA, suppresses RAR-dependent transactivation by titrating out RXR

V-erbA is thought to be an antagonist of thyroid hormone receptor (T₃R) function. Here we show that unliganded T₃R, but not v-erbA, suppresses retinoic acid (RA)-dependent induction of the RAR-β₂ promoter by competing for the common dimerization partner, the retinoid X receptor (RXR). Firstly, T₃R suppression can be alleviated by co-transfection of RXR. Secondly, T₃R, but not v-erbA, competes with RAR for RXR and causes the dissociation of a preformed RAR/RXR-RARE ternary complex in vitro. A single point mutation located in the dimerization interface of v-erbA (Pro349 to Ser) abolishes the transdominant phenotype when introduced at the respective position in T₃R. The hypertransforming v-erbA variant r12, in which this mutation is reversed (Ser349 to Pro) suppresses RA-induced differentiation in chicken erythroid progenitors, while v-erbA does not. Our data thus suggest that unliganded T₃R and v-erbA act as dominant suppressors through mechanistically distinct pathways.