Cooperative effect of thyroid and glucocorticoid hormones on the induction of hepatic phosphoenolpyruvate carboxykinase in vivo and in cultured hepatocytes

Abstract

The present study investigates the effect and interaction of glucocorticoid and thyroid hormones on the induction of phosphoenolpyruvate carboxykinase (PEPck) mRNA and enzyme protein under in vivo conditions and in serum-free cultured hepatocytes from hypothyroid rats.In hypothyroid/adrenalectomized rats T3 significantly enhanced the cAMP induced PEPck mRNA activity within 3 -6 h. This effect was further enhanced by the presence of glucocorticoids. The half-life of PEPck mKNA, as determined after administration of cordycepin, was not affected by hypothyroidism or hyperthyroidism ( t l l z w 45 min), but considerably prolonged by the absence of glucocorticoid hormones (tip < 80 min).In hepatocytes in culture Bt,cAMP (0.2 mM) provoked an increase in translatable PEPck mRNA within 2 h incubation time. Preincubation with either T3 (0.1 pM) or dexamethasone (0.1 pM) for 4 h significantly enhanced the cAMP response on PEPck mRNA. Addition of both, T3 plus dexamethasone further enhanced this Bt,cAMPmediated effect. By measurement of PEPck synthesis corresponding findings were observed.It is concluded that glucocorticoid and thyroid hormones predominantly enhance the CAMP-provoked induction of hepatic PEPck mRNA and, consequently, of PEPck synthesis. Their effect is rapid, significant and additive, indicating an independent action. While glucocorticoids, in addition, accelerate PEPck mRNA degradation, the PEPck mRNA decay rate is similar in the presence and absence of thyroid hormones.The synthesis of hepatic phosphoenolpyruvate carboxykinase (PEPck), an important rate-limiting enzyme for gluconeogenesis, is stimulated by glucocorticoids and the second messenger CAMP. Both act at the transcriptional level, thereby increasing the amount of translatable PEPck mRNA [l, 21. Recently we [3, 41 and others [5] were able to demonstrate that in addition thyroid hormones enhance the cAMP and glucagon-induced translatable PEPck mRNA and, consecutively, enzyme synthesis under in vivo conditions and in isolated liver systems.While the in vivo results of Hanson's group [6] favour a T3-mediated increase in the transcriptional activity of the PEPck gene, Hartong et al. [7], using a similar technique, could not detect such an effect: therefore, and from the close correlation between increasing nuclear T3 binding and increasing PEPck activity after application of varying T3 doses in vivo, these authors suggest an effect of thyroid hormones at a post-transcriptional level (e. g. stabilization of the specific mRNA). Ahhreviations. Bt2cAMP, N6,02'-dibutyryl-cAMP; T3, ~-3,3',5-triiodothyronine; PEPck, phosphoenolpyruvate carboxykinase; poly-(A)-rich RNA, polyadenylated RNA; SDS, sodium dodecyl sulfate.Enzymes. Tyrosine aminolransferase (EC 2.6.1.5); phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32).In the present study we, therefore, investigate the effect of T3 and cAMP on PEPck activity, synthesis, translatable mRNA and mRNA decay rate in the absence and presence of thyroid hormones. Furthermore we investigate the effect and interacti...

Specificity of a retinoic acid response element in the phosphoenolpyruvate carboxykinase gene promoter: consequences of both retinoic acid and thyroid hormone receptor binding.

“…several of these studies, T3 was seen to potentiate the effect of cAMP in inducing PEPCK mRNA levels (12,13,24). In our study, we observed no effect of T3 on expression of a transiently transfected PEPCK-CAT fusion gene containing 2,100 bp of the PEPCK promoter sequence (Fig.…”

“…2A). Alternatively, as has been previously suggested (12,13), T3 may potentiate cAMP responsiveness which is mediated by a cAMP response element located close to the PEPCK gene transcription start site (31,33).…”

“…Loose et al demonstrated a sixfold increase in PEPCK gene transcription in the livers of thyroidectomized rats following 7 to 9 days of T3 readministration (18). Other studies, using cultured hepatocytes, showed no effect of T3 alone on PEPCK mRNA levels (12), while a recent study using rat FAO hepatoma cells infected with a retrovirus expressing the chick TR-a gene demonstrated only a small increase in PEPCK mRNA (approximately 1.6-fold) in response to T3 (24 (Fig. 6).…”

Induction of Cytosolic Phosphoenolpyruvate Carboxykinase in the Ovine Fetal Liver by Chronic Fetal Hypoglycemia and Hypoinsulinemia

“…One such mechanism might involve regulation by corticosteroids. Dexamethasone has been shown to increase cytosolic PEPCK activity and mRNA in vivo and in vitro (30)(31)(32), although this is controversial (25). Because the HH fetal lamb may be a stressed animal, it is possible that the induction of cytosolic PEPCK in these animals is in part due to an increase in corticosteroid secretion.…”

Effects of insulin and cytosolic redox state on glucose production pathways in the isolated perfused mouse liver measured by integrated 2H and 13C NMR

“…However, in fasted conscious dogs, there was a non-significant decrease in GNG during a 3 h portal-venous infusion of insulin [17]. Insulin influences hepatic GNG by powerfully regulating the transcription of important gluconeogenic enzymes [18,19], but the effect of enzyme expression on glucose production may not be immediate, since the mRNA half-life of many gluconeogenic enzymes is close to 1 h [20] and the enzyme itself takes several hours to degrade [21]. On the other hand, the supply of gluconeogenic substrates to the liver has an immediate impact on glucose homoeostasis by increasing GNG [22] and decreasing GLY [23].…”