RETINOIC ACID, DIBUTYRYL-CAMP, AND DIFFERENTIATION AFFECT THE EXPRESSION OF RETINOIC ACID RECEPTORS IN F9 CELLS

Expression of the retinoic acid receptors α and β (RAR-α and RAR-β) was examined in F9 cells, an embryonal carcinoma cell model established for the study of retinoid metabolism and function. Addition of retinoic acid to F9 cell medium caused a dose-dependent increase in RAR-β mRNA within 3 hr that reached 5- to 30-fold greater than the constitutively expressed mRNA by 24 hr. The elevation in mRNA resulted from increased transcription, as demonstrated by nuclear run-on transcription, did not require protein synthesis, and required the constant presence of retinoic acid. N6,O2'-Dibutyryl-cAMP attenuated the retinoic acid-induced increase in RAR-β mRNA by a post-transcriptional mechanism. In contrast, RAR-α mRNA in F9 stem cells was affected less (1.2- to 1.4-fold increase) by retinoic acid and decreased 3-fold transiently when fresh serum was added to the medium. Differentiation of F9 cells resulted in increased steady-state levels of RAR-β mRNA in primitive (4-fold), parietal (3-fold), and visceral (8-fold) endoderm but decreased steady-state levels of RAR-α mRNA in primitive (2-fold), parietal (3-fold), and visceral (1.4-fold) endoderm. These data demonstrate that RAR-β is a primary target gene for retinoic acid in a characterized model of retinoid function, indicate that constitutive expression of both RAR-β and RAR-α is dependent upon the differentiation state, and suggest hormonal modulation of RAR-β by cAMP and modulation of RAR-α by serum factors. These results distinguish the effects of serum, cAMP, and retinoic acid on the expression of RAR from the effects mediated by differentiation.