Phorbol ester inhibition of hormonal induction of tyrosine aminotransferase.

The liver specific enzyme, tyrosine aminotransferase, can be induced by glucocorticoids, cAMP analogs, or insulin. Each of these different inducing agents is believed to act through a separate pathway. The tumor promoting phorbol esters have been reported to stimulate phosphorylation of the insulin receptor and thereby decrease the ability of insulin to induce tyrosine aminotransferase. Our results demonstrate that TPA will not only inhibit the insulin stimulated increase in tyrosine aminotransferase, but will also inhibit induction of the enzyme by glucocorticoids or by cAMP.