The transfer of
Morris hepatoma cells induced by the
hormone within 10-60 min in to a
hormone-free medium is associated with the augmentation of
tyrosine aminotransferase synthesis. The kinetics of this process does not differ from that of the
hormone-induced
enzyme. The return of
tyrosine aminotransferase synthesis to the basal level occurs 15-20 hours after the
hormone withdrawal from the medium, although the concentration of the intranuclear
hormone sharply decreases already after 3 hours. It was demonstrated that the presence in the
hepatoma cell nuclei of 20-25% of the initially bound
hormone for at least 20 hours after the cell transfer to the
hormone-free medium is not sufficient for maintaining a high level of
tyrosine aminotransferase gene expression. Using two-dimensional electrophoresis of 3H-labeled
hepatoma cell
proteins, it was demonstrated that the observed high activity of
tyrosine aminotransferase is due to the de novo synthesis of
enzyme molecules rather than to the existence of preformed long-living
tyrosine aminotransferase molecules inside the cell. Study of [14C]
uridine incorporation into non-ribosomal
nuclear RNA of
hepatoma cells showed a long-term presence of the label in the
RNA throughout the chase experiment. It was assumed that the high activity of the
enzyme for 10-15 hours after the
hormone release from the
hepatoma cell nuclei is due to the accumulation in the nuclei of long-living
pre-mRNA molecules synthesized after the
hormone addition to the cells and during the first hours after the cell transfer to the
hormone-free medium.(ABSTRACT TRUNCATED AT 250 WORDS)