Induction of c-Erb A-AP-1 interactions and c-Erb A transcriptional activity in myoblasts by RXR - Consequences for muscle differentiation

We have previously shown that c-Erb A and v-Erb A display a cell-specific activity in avian myoblasts. In this work, we have compared the molecular basis of thyroid hormone action in HeLa cells and in QM7 myoblasts. The transcriptional activity of c-Erb A alpha 1 through a palindromic thyroid hormone response element (TRE) was similar in both cell types. However, c-Erb A did not activate gene transcription through a direct repeat sequence (DR) 4 TRE in myoblasts in contrast to results obtained in HeLa cells. Moreover, whereas retinoic acid receptor-AP-1 interactions were functional in both cell types, thyroid hormone receptor (T3R)-AP-1 interactions were only functional in HeLa cells. Using electrophoretic mobility shift assays, functional tests, and Northern blot experiments, we observed that RXR isoforms are not expressed in proliferating myoblasts. Expression of RXR gamma in these cells did not influence T3R transcriptional activity through a palindromic TRE but induced such an activity through a DR4 TRE. Moreover, it restored c-Erb A-AP-1 functionality in QM7 myoblasts and enhanced the myogenic influence of T3. We also observed that c-Jun overexpression in proliferating QM7 cells restored T3R transcriptional activity through a DR4 TRE. Therefore, alternative mechanisms are involved in the induction of T3R transcriptional activity according to the cell status (proliferation: c-Jun; differentiation: RXR). In addition we provide the first evidence that RXR is required to allow inhibition of AP-1 activity by ligand-activated T3R. Lastly, we demonstrate the importance of RXR in the regulation of myoblast differentiation by T3.