REGULATED EXPRESSION OF THE RETINOBLASTOMA GENE IN DIFFERENTIATING EMBRYONAL CARCINOMA-CELLS

The expression of the retinoblastoma susceptibility (RB) gene was inve stigated in P19 embryonal carcinoma cells and in these cells induced t o differentiate with retinoic acid (RA) or with dimethyl sulfoxide (DM SO). In undifferentiated cells very low levels of RB mRNA and protein were present. DMSO-treated P19 cell cultures develop into mesodermal a nd endodermal cells and RB expression increased only slightly in these differentiating celts. RA-treated P19 cells develop into neuroectoder m and this differentiation was accompanied by a marked increase in RB expression with mRNA levels increasing 15 fold by 4-6 days following i nitiation of RA treatment. No such increase occurred in mutant cells t hat fail to respond to RA. The RB promoter did not appear to be direct ly activated by RA. Nevertheless, the increase in RB expression in RA- treated cells appeared to be due to enhanced initiation of transcripti on because cells transfected with a reporter gene driven by the RB pro moter expressed the reporter gene with kinetics similar to that of the RB gene. Thus the activation of the RB gene appears to be achieved in directly by RA-induced factor(s) in differentiating neuroectodermal ce lls. The post-mitotic neurons that developed in RA-treated cultured co ntained only the hypophosphorylated form of the RB protein. Recent stu dies (Clarke et aL, 1992; Jacks et al., 1992; Lee et al., 1992) have s hown that mice lacking the RB gene have abnormalities in early brain d evelopment suggesting that the rapid rise in RB expression and the hyp ophosphorylation of the protein are essential for neuronal cell differ entiation.