Molecular basis of the cell-specific activity of v-erb A in quail myoblasts

We have previously shown that v-erb A expression strongly stimulates quail myoblast proliferation and differentiation without alteration of the triiodothyronine (T3) influence in this cell type. In order to understand the molecular basis of v-erb A action in myoblasts, we have studied the influence of this oncoprotein on c-erb A alpha1 encoded T3 nuclear receptor (TR alpha) activity. In transfection experiments, v-erb A did not inhibit the T3-dependent c-erb A alpha1 transcriptional activity in QM7 myoblasts in contrast to its action in HeLa cells. However, it repressed the retinoic acid receptor RAR alpha activity in both cell-types, indicating that v-erb A interactions with T3 or RA mediated transcription significantly differs. In EMSA experiments using a TREpa1 probe, T3R alpha binds as three complexes in HeLa cells. We have previously identified the slow migrating complex, undetectable in QM7 myoblasts, as a T3R/RXR heterodimer. Interestingly, v-erb A inhibited binding of this complex in HeLa cells, but did not affect binding of the two other complexes in QM7 myoblasts. Expression of RXR (gamma isoform), the TR alpha dimerization partner absent in proliferating QM7 cells, restored inhibition of c-erb A alpha1 transcriptional activity in these cells and abrogated the v-erb A myogenic influence. Lastly, v-erb A induced a T3-independent c-erb A alpha1 activity in QM7 cells when cotransfected in equimolar ratio with the receptor, by inhibiting AP-1 activity and stimulating transcription of a reporter gene driven by a TRE sequence.