Superinduction of tyrosine aminotransferase by actinomycin D in rat hepatoma (HTC) cells

Certain steroid hormones induce a tenfold increase in tyrosine aminotransferase synthesis in HTC cells, a tissue culture line derived from a rat hepatoma. Actinomycin D,given after maximum induction, provokes an additional rise in enzyme activity (“superinduction”), although the same dose of the drug almost entirely blocks the steroid-mediated induction if given to basal cells with or before steroid. This paper sets forth the conditions used to obtain optimal actinomycin super-induction and presents studies which suggest that superinduction is the result of blocking RNA synthesis. Superinduction, under the conditions given here, seems to be due to an increase in tyrosine aminotransferase synthesis and not to any alteration in enzyme decay. These experiments can be explained satisfactorily by assuming that inducing steroids act at a post-transcriptional site.