Regulation of Phosphoenolpyruvate Carboxykinase and Tyrosine Transaminase in Hepatoma Cell Cultures: I. EFFECTS OF GLUCOCORTICOIDS, N6,O2′-DIBUTYRYL CYCLIC ADENOSINE 3′,5′-MONOPHOSPHATE AND INSULIN IN REUBER H35 CELLS

Abstract

Abstract Reuber H35 hepatoma cells growing in monolayer cultures have been found to contain the soluble enzyme phosphoenolpyruvate carboxykinase. The activity of this enzyme can be increased 2- to 3-fold by the addition of either N6,O2'-dibutyryl cyclic adenosine 3',5'-monophosphate (DBcAMP) or the synthetic glucocorticoid dexamethasone (Δ1,2,9α-fluoro,16α-methyl cortisol). Tyrosine transaminase responds similarly to DBcAMP but its activity was elevated 5- to 8-fold by dexamethasone. The effects of each inducer on both enzymes were similar with respect to kinetics, dose-response relationships, analogues, and cycloheximide inhibition. The response of both enzymes to DBcAMP, however, was more rapid than to dexamethasone. The rate of transaminase synthesis was increased 2- to 3-fold by DBcAMP but no effect of the cyclic nucleotide could be detected on the rate of transaminase degradation. Elevation of carboxykinase activity appears to be due to an increase in the rate of synthesis of this enzyme based on immunological evidence and cycloheximide inhibition of the response to both dexamethasone and DBcAMP. Insulin completely suppressed the effects of both DBcAMP and dexamethasone on the carboxykinase at the same time that it produced additive increases in transaminase activity with these agents. On the other hand, progesterone completely prevented the effects of dexamethasone on both enzymes but did not inhibit the effects of DBcAMP on either enzyme.