Triiodothyronine stimulates and cyclic AMP inhibits transcription of the gene for malic enzyme in chick embryo hepatocytes in culture.

In chick embryo hepatocytes in culture, insulin and triiodothyronine (T3) increase malic enzyme activity and the abundance of malic enzyme mRNA by at least 50-fold, and glucagon or cAMP blocks this effect. Steps regulated by these hormones were defined by measuring transcriptional activity with the nuclear run-on assay and multiple fragments of the malic enzyme gene as probes. T3 alone caused a significant increase in transcription within 1 h, with a maximal increase of 30-40-fold occurring by 24 h. When T3 was added with insulin, 80% of the maximum rate was reached in 1 h. Insulin alone had no effect on transcription of the malic enzyme gene; it amplified the response to T3 in the first few hours after adding T3 but did not alter T3's maximal effect. Cyclic AMP for 1 h completely inhibited the increase in transcription caused by T3. The size and speed of the responses of the malic enzyme gene to T3 and cAMP suggest regulation of transcription initiation. T3-stimulated transcription of the malic enzyme gene did not require ongoing protein synthesis despite the fact that inhibitors of protein synthesis inhibited the T3-stimulated accumulation of its mRNA. T3 may directly activate transcription of this gene via its receptor. The pattern of DNase I hypersensitivity of the malic enzyme gene in chick embryo hepatocytes was the same as that in fed chick liver. Insulin, T3, and cAMP had no effect on that pattern. In chick embryo hepatocytes in culture, factors involved in regulation of transcription by insulin, T3, and cAMP may be bound to DNA independently of hormonal treatment.