Aggregation and cell cycle dependent retinoic acid receptor mRNA expression in P19 embryonal carcinoma cells

Differentiation of PI9 EC cells along different pathways into derivatives resembling cells of the three embryonic germ layers is accompanied by characteristic differences in modulation of expression of each of the three retinoic acid receptor genes, RAR α , - β and - γ . Differentiation induced by addition of RA to P19 EC cells cultured in monolayer is accompanied by a rapid increase in expression of both RAR α and - β . Induction of RAR β occurs in a characteristic biphasic manner, suggesting that multiple factors and/or different mechanisms are involved in controlling its expression. RAR β mRNA is induced to a far higher level during early aggregation in the presence of RA than during early differentiation in monolayer, suggesting that the direction of differentiation depends on the number and/or ratio of α and β type of RA receptors. Aggregation of P19 EC cells in the presence of RA, but not DMSO, is accompanied by repression of RAR γ , suggesting that the expression of RAR β and RAR γ during neuroectodermal differentiation is mutually exclusive. The effects of RA on RAR expression are significantly greater in G1 than in S-phase of the cell cycle. These results extend previous observations that commitment to differentiation is cell cycle dependent and indicates that critical target gene regulation in response to RA has to take place in G1 for differentiation to occur.