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), bl ocks the effect of T3 in TR-mediated transcription. The mechanism(s) f or this dominant negative effect by v-erbA on TRs is unknown but may i nvolve competition between v-erbA and TR-containing complexes for bind ing to thyroid hormone response elements (TREs) and/or protein-protein interactions between v-erbA and TR. To investigate these potential me chanisms, we used the electrophoretic mobility shift assay to compare in vitro translated v-erbA and TRalpha binding to two TREs-chick lysoz yme TRE (F2) and direct repeat TRE (DR4). v-erbA bound as a homodimer to these TREs, whereas TRalpha bound as a homodimer and monomer. T3 de creased TRalpha homodimer binding to the TREs as we reported previousl y; however, surprisingly, high concentrations of T3 (10(-6) M) also de creased v-erbA homodimer binding to the TREs. Additionally, v-erbA for med heterodimers with nuclear proteins such as retinoid X receptor and T3 receptor auxiliary protein as well as with TRalpha. These dimers r emained bound to DNA in the presence of T3. Finally, v-erbA could not mediate ligand-dependent transcriptional activation even at 10(-6) M T 3 but could block ligand-dependent TR-mediated transactivation in co-t ransfection experiments. v-erbA also exhibited differential dominant n egative 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 presenc e of T3, which all may contribute to v-erbA's dominant negative effect on TR-mediated transcription by competing with TR-containing complexe s for binding to TREs.