Roles of v-erbA homodimers and heterodimers in mediating dominant negative activity by v-erbA

v-erbA, a viral oncogenic homolog of thyroid hormone receptor (TR), blocks the effect of T₃ in TR-mediated transcription. The mechanism(s) for this dominant negative effect by v-erbA on TRs is unknown but may involve competition between v-erbA and TR-containing complexes for binding to thyroid hormone response elements (TREs) and/or protein-protein interactions between v-erbA and TR. To investigate these potential mechanisms, we used the electrophoretic mobility shift assay to compare in vitro translated v-erbA and TRα binding to two TREs-chick lysozyme TRE (F2) and direct repeat TRE (DR4). v-erbA bound as a homodimer to these TREs, whereas TRα bound as a homodimer and monomer. T₃ decreased TRα homodimer binding to the TREs as we reported previously; however, surprisingly, high concentrations of T₃ (10^{- 6} M) also decreased v-erbA homodimer binding to the TREs. Additionally, v- erbA formed heterodimers with nuclear proteins such as retinoid X receptor and T₃ receptor auxiliary protein as well as with TRα. These dimers remained bound to DNA in the presence of T₃. Finally, v-erbA could not mediate ligand-dependent transcriptional activation even at 10^-6 M T₃ but could block ligand-dependent TR-mediated transactivation in co-transfection experiments. v-erbA also exhibited differential dominant negative activity on F2 and DR4 suggesting that half-site sequence and/or orientation may influence v-erbA-dominant negative activity. In sum, there are multiple v- erbA complexes that bind to TREs in the presence of T₃, which all may contribute to v-erbA's dominant negative effect on TR-mediated transcription by competing with TR-containing complexes for binding to TREs.