EVIDENCE FOR DIVERSE ROLES OF PROTEIN-KINASE-C IN THE INHIBITION OF GENE-EXPRESSION BY INSULIN - THE TYROSINE AMINOTRANSFERASE, ALBUMIN, AND PHOSPHOENOLPYRUVATE CARBOXYKINASE GENES

We have previously shown that insulin is less effective in inducing ex pression of several genes in H4 hepatoma cells with reduced functional protein kinase-C (PKC) activity. However, other reports suggest that insulin regulation of gene transcription is not PKC dependent. Insulin and phorbol 12-myristate 13-acetate (PMA) rapidly inhibit transcripti on of the tyrosine aminotransferase and albumin genes. Prolonged PMA p retreatment, to desensitize cells to PMA, resulted in a loss of insuli n ability to inhibit albumin transcription. Insulin was still able to inhibit tyrosine aminotransferase transcription, but less than in non- PMA-pretreated cells, and there was also a slight decrease in the abil ity of insulin to inhibit phosphoenolpyruvate carboxykinase transcript ion. We previously demonstrated decreased responsiveness of PMA-induce d gene expression in insulin-desensitized cells. In the present work, using insulin-desensitized H4 cells (insulin pretreatment for 24 h), s ubsequent treatment with PMA did not alter phosphoenolpyruvate carboxy kinase transcription rates, whereas PMA did inhibit tyrosine aminotran sferase transcription rates to an extent similar to observed in nonpre treated cells. Unexpectedly, there was a significant increase in album in transcription after PMA addition to insulin-pretreated cells. These findings support our hypothesis that the role of PKC in the regulatio n of gene expression by insulin varies for different insulin-regulated genes.