Studies on the mechanism of induction of embryonal carcinoma cell differentiation by retinoic acid.

On the basis of our studies, we conclude that RA is a potent promoter of differentiation of EC cells. We have demonstrated that the tumorigenicity of PCC4-azalR EC cultures can be effectively reduced following exposure of the cells to RA. This is presumably a result of differentiation of the EC cells to nontumorigenic derivatives. However, even after several weeks of exposure to RA, there remains in the culture a subpopulation of unresponsive EC cells. The reason why these cells do not differentiate in the presence of RA is currently under investigation. We have also derived by mutagen treatment and clonal selection EC cells that fail to respond to RA. Preliminary indications are that these cells have lost the capacity to differentiate when subjected to other manipulations which stimulate differentiation of the parental EC cells. This is an important observation since the mutants were selected only by their lack of response to RA. Unlike the parental cells, which have relatively large amounts of RABP, dif- cells appear either to lack RABP or to possess an altered binding protein which has a greatly reduced affinity for RA. These observations are consistent with the view that some function of the RA-RABP complex is critical in the sequence of events leading to differentiation of EC cells. However, further studies are required before we can establish the generality of this proposal. We are presently investigating whether the level and/or function of RA-RABP complexes in cells from the EC lines listed in TABLE 1 can explain their varying tendencies to differentiate.