Posttranscriptional control of tyrosine aminotransferase synthesis by insulin

Adrenal steroid hormones induce the synthesis of tyrosine amino-transferase (TAT) in HTC cells, an established line of rat hepatoma cells in tissue culture. The addition of insulin to cells previously induced in a serum-free medium results in a rapid two- to threefold further increase in the rate of synthesis of TAT and a small increase in total amino acid incorporation. These changes do not require concomitant RNA synthesis, suggesting that insulin acts at a step in protein synthesis beyond that of gene transcription. Although the effects of insulin on HTC cells are similar to those caused by dialyzed bovine serum, evidence is presented that insulin and serum affect different aspects of TAT synthesis. Unlike the glucocorticoids, insulin does not cause the accumulation of TAT mRNA, nor the sustained induction of TAT. The continued presence of the inducing steroid is required to permit maximal expression of the insulin effect.