Retinoid Binding Protein Activities in Murine Embryonal Carcinoma Cells and Their Differentiated Derivatives

It has been proposed that cellular retinoic acid binding protein is essential for retinoid-induced differentiation of embryonal carcinoma line PCC4.aza1R. To assess the generality of this proposal, we have tested for the presence of cellular retinoic acid binding protein activities in several other embryonal carcinoma lines. Cytosolic extracts from all cells were found to possess binding proteins for retinoic acid and also for retinol, although levels varied widely among the different lines. There was no clear quantitative relationship between binding protein activities and the propensity of the cells for differentiation in tumor form or under various in vitro conditions. Our results suggest that other factors might modulate the response of embryonal carcinoma cells to retinoids and/or that alternate pathways for differentiation which do not involve retinoids and retinoid binding proteins exist in these cells. When embryonal carcinoma cells are stimulated to differentiate, the derivatives can possess higher, lower, or similar levels of retinoic acid binding protein activity. These levels appear to reflect the phenotype of the differentiated cells rather than the conversion from a tumorigenic to a nontumorigenic state.