Permissive action of thyroid hormones in the cAMP-mediated induction of phosphoenolpyruvate carboxykinase in hepatocytes in culture.

Thyroid hormones are known to stimulate in vivo the synthesis of several liver proteins. In order to determine their role in the regulation of important hepatic proteins at the cell level, the effect of T3 on the induction of the glucoregulatory enzyme phosphoenolpyruvate carboxykinase (PEPck) was investigated in cultured hepatocytes. The cells were isolated from adult hypothyroid rats and exposed to a chemically defined medium, devoid of serum supplement and hormone-free over a period of 48 h. Addition of T3 to the medium had only a minor effect on PEPck induction. Dibutyryladenosine 3',5'-monophosphate (Bt2cAMP) or epinephrine provoked significant increase in PEPck synthesis within 4-6 h. Simultaneous addition of T3 was found significantly to promote the Bt2cAMP-mediated enzyme induction. Physiological concentrations of T3 (10 pM to 1 nM) were found to be effective in enhancing this cAMP-mediated signal. Exposure of the cells to T3 for only 90 min, 16 h prior to addition of Bt2cAMP was nearly as effective as continuous exposure to T3 in enhancing the cAMP-effect on PEPck synthesis. In contrast, no effect of T3 on the Bt2cAMP-induced tyrosine-aminotransferase activity could be detected. It is concluded that the role of T3 on the cAMP-elicited PEPck synthesis is selective, rapid, primarily permissive and significant within the physiological range of the hormone. In addition, these data indicate that such hepatocyte cultures prepared from hypothyroid rats are ideally suited to the analysis of the regulatory role of thyroid hormones on specific gene expression.