Opposing functions of ATF2 and Fos-like transcription factors in e-Jun-mediated myogenin expression and terminal differentiation of avian myoblasts

With the aim to identify the oncoprotein partners implicated in the c-Jun m yogenic influence, we carried out stable transfection experiments of c-Jun and/or ATF2, Fra2, c-Fos overexpression in avian myoblasts. Before inductio n of differentiation, c-Jun repressed myoblast withdrawal from the cell cyc le, as did a TPA treatment. However, after serum removal, unlike TPA, c-Jun significantly stimulated myoblast differentiation. In search for specific partners involved in this dual influence, we found that a reduction in the amounts of c-Fos and Fra2 and an increase in c-Jun proteins occurred at cel l confluence, a situation likely to favor cooperation between c-Jun and ATF 2 during terminal differentiation. Whereas c-Fos and Fra2 cooperated with c -Jun to abrogate myoblast withdrawal from the cell cycle and terminal diffe rentiation, ATF2 co-expression potentiated the positive myogenic e-Jun infl uence. In addition, myogenin expression was a positive target of this coope ration and this regulation occurred through a stimulation of myogenin promo ter activity: (1) whereas c-Fos or Fra2 co-expression abrogated c-Jun stimu latory activity on this promoter, ATF2 co-expression potentiated this influ ence; (2) using a dominant negative ATF2 mutant, we established that c-Jun transcriptional activity required functionality of endogenous ATF2. These d ata suggest that through this dual myogenic influence due to cooperations w ith different partners, c-Jun is involved in the control of duration of myo blast proliferation and thereafter of fusion efficiency.