Molecular mechanism of insulin regulated tyrosine aminotransferase expression in the cultured rat hepatoma cell line, FAO

The molecular mechanism(s) of induction of tyrosine aminotransferase (TAT) by insulin was studied in the well-differentiated rat hepatoma cell line Fao. Incubation of Fao with insuiin resulted in a 1.5-fold increase in TAT activity. The effect of insulin on TAT activity was maximal at 5 hr. In the presence of the transcriptional inhibitor DRB, insulin-stimulated TAT induction is only partially inhibited. At 4 hr following addition, DRB inhibits insulin-mediated induction of TAT activity 40%. In the same experiment DRB inhibited dexamethasonemediated induction of TAT activity 95%. Results of Northern blot analyses indicate a 2.6-fold maximal increase in TAT mRNA abundance between 4 - 8 hr following the addition of insulin. At early time points following the addition of db-cAMP and insulin together to the Fao cells, there is a greater increase in TAT mRNA levels than with db-cAMP treatment alone. Treatment of Fao cells with the phorbol ester TPA resulted in a rapid increase in the accumulation of TAT mRNA that is maximal 1 hr following treatment. This was followed by a rapid decay in TAT mRNA abundance to below basal levels at 4 hr with a siow recovery approaching basal levels 8 hr following treatment. Pretreatment of Fao celis for 16 hr with excess TPA did not affect the capability of insulin to induce TAT mRNA levels. These studies suggest that insulin may stimulate TAT activity by affecting post-transcriptionai processes. The author hypothesizes that the increased TAT activity in Fao cells following the addition of insulin is primarily a result of a selective stabilization of the TAT transcript combined with increased protein synthesis and decreased degradation of the short half-life aminotransferase.