The regulation of tyrosine aminotransferase (TAT) activity has been examined in two liver-derived heteroploid cell lines. One (hepatoma tissue culture cells [HTC]) was derived from a hepatoma, the other (rat liver culture cells [RLC]) was derived from normal liver. The two cell lines show the following striking similarities in the control of this specific protein: (a) The kinetics of TAT induction by dexamethasone phosphate (DxP) are similar in randomly growing cells of both lines; (b) During mitosis and early G1 phase of the cell cycle TAT activity cannot be induced by DxP in either cell line; (c) 2–3 h into G1, when both lines become sensitive to inducer, basal enzyme activity declines to a new steady-state level; (d) Preinduced cells collected in mitosis show approximately twice the level of TAT activity as fully induced, randomly growing cultures and this activity is maintained in early G1 with or without the inducer; and (e) Inhibition of RNA synthesis by 5 µg/ml of actinomycin D in preinduced, synchronized cells allows TAT activity to remain at constitutive levels throughout G1, even in the absence of inducer. These results are presented in support of a previously described model which states that glucocorticoid hormones exert posttranscriptional control of the synthesis of specific proteins in mammalian cells.

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