Elevation of AP1 activity during F9 cell differentiation is due to increased c-jun transcription

The regulation of jun family genes and AP1 activity during the course of differentiation of F9 embryonal carcinoma stem cells was investigated. The induction of differentiation by retinoic acid (RA) leads to an accumulation of c-jun mRNA caused by increased c-jun transcription. This induction is an indirect response to RA and requires a functional AP1 binding site within the c-jun promoter. Expression of jun-B mRNA, however, is transiently induced but at a later time point is repressed by RA. The third member of the family, jun-D, is already active in undifferentiated cells and is only slightly induced after differentiation. Differentiation also converts c-jun from being refractory to phorbol esters to a highly inducible state. The development of this response is correlated with increased AP1 activity in RA-treated cells. By contrast, the induction of c-fos by phorbol esters or cAMP is greatly diminished after RA treatment. Transfection experiments indicate that, in the absence of c-Fos, only c-Jun is an effective transactivator. Hence, the major increase in AP1 activity is due to elevated c-jun expression and probably involves positive autoregulation by the c-Jun protein. Furthermore, these results demonstrate that AP1 activity can be stimulated by phorbol ester without concomitant c-fos induction. Forced expression of c-Jun and v-Jun results in activation of at least two differentiation marker genes, EndoB and tissue plasminogen activator, whose regulatory regions contain AP1 binding sites. Thus, the induction of c-jun transcription by RA, although indirect, can have an important role in the differentiation process.