Regulation of the levels of phosphoenolpyruvate carboxykinase (GTP) mRNA in rat liver by insulin and glucagon

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD) alter body weight by decreasing insulin-like growth factor I (IGF-I) signaling

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) affects glycemia due to reduced gluconeogenesis; when combined with a reduction in feed intake, this culminates in decreased body weight. We investigated the effects of steady-state levels of TCDD (loading dose rates of 0.0125, 0.05, 0.2, 0.8, and 3.2 microg/kg) or approximately isoeffective dose rates of 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HxCDD) (loading dose rates of 0.3125, 1.25, 5, 20, and 80 microg/kg) on body weight, phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression and activity, and circulating concentrations of insulin, glucose, and insulin-like growth factor-I (IGF-I), and expression of hepatic phosphorylated AMP kinase-alpha (p-AMPK) protein in female Sprague-Dawley rats (approximately 250 gm) at 2, 4, 8, 16, 32, 64, and 128 days after commencement of treatment. At the 0.05 and 1.25 microg/kg loading dose rates of TCDD and HxCDD, respectively, there was a slight increase in body weight as compared to controls, whereas at the 3.2 and 80 microg/kg loading dose rates of TCDD and HxCDD, respectively, body weight of the rats was significantly decreased. TCDD and HxCDD also inhibited PEPCK activity in a dose-dependent fashion, as demonstrated by reductions in PEPCK mRNA and protein. Serum IGF-I levels of rats treated initially with 3.2 microg/kg TCDD or 80 microg/kg HxCDD started to decline at day 4 and decreased to about 40% of levels seen in controls after day 16, remaining low for the duration of the study. Eight days after initial dosing, hepatic p-AMPK protein was increased in a dose-dependent manner with higher doses of TCDD and HxCDD. There was no effect with any dose of TCDD or HxCDD on circulating insulin or glucose levels. In conclusion, doses of TCDD or HxCDD that began to inhibit body weight in female rats also started to inhibit PEPCK, inhibited IGF-I, while at the same time inducing p-AMPK.

S-adenosylmethionine decarboxylase gene expression in rat hepatoma cells: regulation by insulin and by inhibition of protein synthesis

We have investigated expression of the S-adenosylmethionine decarboxylase (AdoMetDC) gene in H4-II-E rat hepatoma cells treated with growth factors (epidermal growth factor and transforming growth factor beta 1) and inducers (cAMP and insulin). Treatment with insulin caused a marked increase in both RNA level and enzyme activity. The stability of AdoMetDC mRNA was not altered by insulin treatment: the accumulation of mRNA in hepatoma cells therefore seems to be due to an increase in the transcription rate. Cycloheximide was found to be a strong inducer of AdoMetDC mRNA transcription and the effects of insulin and cycloheximide were additive, suggesting that they increase expression by separate mechanisms. Chloramphenicol acetyltransferase assays in rat hepatoma cells using 5' flanking regions of different lengths revealed that the promoter region extending 337 bp upstream from the transcription start site contains elements involved in insulin response.

Detection and hormonal regulation of the mRNA for cytosolic phosphoenolpyruvate carboxykinase in rat lung

We have studied the presence of the messenger RNA (mRNA) for the cytosolic enzyme, phosphoenolpyruvate carboxykinase (PEPCK), in rat lung by Northern blot hybridization to a complementary DNA (cDNA) probe. Lung from normal rats contained substantial amounts of this mRNA, although its relative concentration was approximately six times lower than in liver. Fasting produced an eightfold increase in the content of the enzyme mRNA in lung, which could be reverted to normal values by glucose refeeding. Induced diabetes also resulted in a sevenfold increase of the levels of PEPCK mRNA in lung. Dexamethasone, thyroid hormone, dibutyryl cyclic adenosine monophosphate (cAMP), histamine, and serotonin also induced important accumulations of the enzyme mRNA without affecting the concentration of beta-tubulin mRNA measured as reference. Thus, the PEPCK gene appears to be regulated in a similar manner in lung and liver. The results suggest that PEPCK may be involved in lung metabolism in starvation, diabetes, and other specific hormonal situations.

The heterotopic effects of insulin and glucagon on the acinar activity pattern of phosphoenolpyruvate carboxykinase in male and female rat liver

The effects of the administration of insulin and glucagon on the intraacinar heterotopy of phosphoenolpyruvate carboxykinase (PEPCK) were investigated in male and female rat liver. Insulin did not noticeably influence PEPCK activity or its acinar distribution, either in males or in females. But it affected the activities of glucose-6-phosphate dehydrogenase and malic enzyme. Glucagon in supraphysiological concentrations led to an induction of PEPCK activity. Despite high glucagon concentration along the whole sinusoidal length, the inducing effect of glucagon was most pronounced in the periportal and intermediary parts of the acinus; thus indicating that there is no direct interrelationship between local glucagon concentration and PEPCK activity. In both experiments blood glucose levels were kept fairly constant.

Lipogenic enzyme activities and mRNA in rat adipose tissue at weaning

The activities and mRNA concentrations of two lipogenic enzymes, fatty acid synthetase and acetyl-CoA carboxylase, and one enzyme involved in glyceroneogenesis, phosphoenolpyruvate carboxykinase (PEPCK), were measured in rat white adipose tissue during the suckling-weaning transition. Activities and mRNA concentrations of lipogenic enzymes were low in suckling rats, whereas activity and mRNA concentration of PEPCK were high. At weaning to a high-carbohydrate diet, the rapid increase in lipogenic enzymes mRNA (10- to 20-fold) and decrease in PEPCK mRNA (10-fold) were followed by parallel changes in enzyme activities. In contrast, weaning to a high-fat diet prevented these modifications. Force feeding suckling rats with carbohydrates induced a rise in blood glucose and plasma insulin concentrations. During these experiments, mRNA concentrations increased 10- to 20-fold for lipogenic enzymes and decreased 5-fold for PEPCK in less than 6 h, whereas all enzyme activities did not vary. This suggests a pretranslational regulation of gene expression. Force feeding suckling rats with a mixture of fat devoid of carbohydrate induced a slight increase in plasma insulin concentration and a fall in PEPCK mRNA but was not accompanied by a rise in lipogenic enzyme mRNAs. This suggested that insulin is a prime regulator of PEPCK gene expression, whereas glucose and insulin act synergistically in the regulation of lipogenic enzyme gene expression.

Phosphoenolpyruvate carboxykinase activity patterns in the liver acinus of diabetic and diabetic and estrogen treated rats

The acinar activity pattern of phosphoenolpyruvate carboxykinase (PEPCK) was investigated in livers of streptozotocin diabetic male and female rats and in addition in livers of diabetic males, which had undergone estrogen treatment. In all diabetic animals blood glucose levels were supranormal and liver PEPCK activity was increased. This increase in activity was greatest in estrogen treated diabetic males and lowest in diabetic females. Plasma insulin levels were reduced after the application of streptozotocin to otherwise normal male and female rats. Yet, in males treated in addition with estrogens the plasma insulin levels reached the normal range again. The PEPCK activity showed a heterotopic distribution along the acinus. The periportal to perivenous gradient was steeper in males compared to females in the untreated as well as in the diabetic state. The application of estrogens to males resulted in a further steepening of the gradient.

cDNA sequence of rat liver fructose-1,6-bisphosphatase and evidence for down-regulation of its mRNA by insulin

A coding-length clone of rat liver fructose-1,6-bisphosphatase (EC 3.1.3.11) was isolated by immunological screening of a cDNA library in lambda gt11. Its identity was verified by comparing the deduced amino acid sequence with that obtained by direct sequencing of a complete set of CNBr and proteolytic peptides from the purified protein. The enzyme subunit is composed of 362 amino acids and has N-acetylvaline as the amino-terminal residue. The cDNA, 1255 base pairs (bp) long, consisted of 1086 bp of coding region, 15 bp of 5' untranslated sequence, and 154 bp at the 3' untranslated end. The 3' untranslated sequence contained a polyadenylylation signal (AATAAA) followed after 30 bp by a stretch of 7 adenines at the end of the clone. The deduced amino acid sequence was identical to the primary sequence of the protein and confirmed the alignment of five nonoverlapping peptides. It also confirmed the 27-residue extension, unique to the rat liver subunit, ending with a carboxyl-terminal phenylalanine. RNA blot analyses using the radiolabeled liver cDNA as a probe revealed a single band of fructose-1,6-bisphosphatase mRNA, 1.4 kilobases long, in liver and kidney but not in nongluconeogenic tissues. Fructose-1,6-bisphosphatase mRNA was increased 10-fold in livers from diabetic rats and was reduced to control levels after 24 hr of insulin treatment, suggesting that the changes in enzyme activity observed in diabetes and after insulin treatment are due to alterations in mRNA abundance.

Effect of hormones on transcription of the gene for cytosolic phosphoenolpyruvate carboxykinase (GTP) in rat kidney

The effect of hormones on the transcription rate of cytosolic phosphoenolpyruvate carboxykinase and level of mRNA for this enzyme in the rat kidney has been investigated. In renal nuclei isolated from rats given dibutyryladenosine cyclic 3',5'-phosphate (Bt2cAMP) or 8-bromoadenosine cyclic 3',5'-phosphate (8-Br-cAMP), [32P]UMP incorporation into hybridizable phosphoenolpyruvate carboxykinase mRNA increased severalfold within 1 h. Changes in the concentration of cytosolic phosphoenolpyruvate carboxykinase mRNA, measured by hybridization of [32P]cDNA to poly(A)+ mRNA, paralleled alterations in the transcription rate. Dexamethasone treatment of adrenalectomized rats increased the transcription rate and the level of phosphoenolpyruvate carboxykinase mRNA 3-4-fold after 4 h. Both parameters then declined to control values by 8 h. When dexamethasone (5 mg/kg) and Bt2cAMP (25 mg/kg) were given together, the rate of phosphoenolpyruvate carboxykinase RNA synthesis and the level of cytosolic mRNA were not increased more than those with either drug alone. Transcription of the gene for renal phosphoenolpyruvate carboxykinase was not affected by diabetes or glucose refeeding but was increased 2-fold after 24 h of starvation and reduced by bicarbonate feeding after 2 h. We conclude that glucocorticoids and cAMP change the rate of transcription of the phosphoenolpyruvate carboxykinase gene in rat kidney, leading to changes of similar magnitude in mRNA level and, hence, enzyme activity. The results presented here and in previous work [Lamers, W., Hanson, R. W., & Meisner, H. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 5137] indicate that the transcription rate of the gene for phosphoenolpyruvate carboxykinase in liver and kidney responds to hormones in a tissue-specific manner.

The long-term effect of glucagon on pyruvate kinase activity in primary cultures of hepatocytes

Glucagon caused a marked decrease in the total L-pyruvate kinase activity of control hepatocytes maintained in monolayer culture (t1/2 = 54 h), while the addition of insulin to hepatocytes isolated from a fasted rat caused a four- to fivefold increase in the total enzyme activity. Maintenance of L-pyruvate kinase in control cultures of hepatocytes was shown to require insulin. However, when 1 microM glucagon was present in the medium, the total L-pyruvate kinase activity was not maintained even in the presence of 1 microM insulin, but rather the total L-pyruvate kinase activity of the cells steadily declined from 12.1 to 5.7 units/mg DNA by the 6th day in culture. The increase in the total L-pyruvate kinase activity of fasted hepatocytes cultured in the presence of insulin was shown to result from an increase in protein synthesis, since actinomycin D and cycloheximide blocked the insulin-induced increase in the enzyme activity. The addition of 1 microM glucagon to cultures of fasted hepatocytes also blocked the insulin-induced increase in total L-pyruvate kinase activity. Since glucagon decreased the total L-pyruvate kinase activity in control hepatocytes and blocked the increase in L-pyruvate kinase activity in fasted hepatocytes, it is suggested that, in addition to the phosphorylation of L-pyruvate kinase by a cAMP-dependent protein kinase, glucagon also acts to decrease the synthesis of L-pyruvate kinase in vitro.

Isolation and characterization of the gene coding for cytosolic phosphoenolpyruvate carboxykinase (GTP) from the rat

The gene for cytosolic phosphoenolpyruvate carboxykinase (GTP) [GTP:oxaloacetate carboxy-lyase (transphosphorylating), EC 4.1.1.32] from the rat was isolated from a recombinant library containing the rat genome in phage lambda Charon 4A. The isolated clone, lambda PCK1, contains the complete gene for phosphoenolpyruvate carboxykinase and approximately equal to 7 kilobases (kb) of flanking sequence at the 5' end and 1 kb at the 3' terminus. Restriction endonuclease mapping, R-loop mapping, and partial DNA sequence assay indicate that the gene is approximately equal to 6.0 kb in length (coding for a mRNA of 2.8 kb) and contains eight introns. Southern blotting of rat DNA digested with various restriction enzymes shows a pattern predicted from the restriction map of lambda PCK1. A control region at the 5' end of the gene contained in a 1.2-kb restriction fragment was isolated and subcloned into pBR322. This segment of the gene contains the usual transcription start sequences and a 24-base sequence virtually identical to the sequence found in the 5'-flanking region of the human proopiomelonocortin gene, which is known to be regulated by glucocorticoids. The 1.2-kb fragment of the phosphoenolpyruvate carboxykinase gene can be transcribed into a unique RNA fragment of predicted size by an in vitro transcription assay.

Control of the activity of phosphoenolpyruvate carboxykinase and the level of its mRNA in livers of newborn rats. Effect of diabetes, glucose load and glucocorticoids

Streptozotocin treatment produces a typical experimental diabetes in neonates exhibiting hyperglycemia, glucosuria, ketonemia and increased level of fatty acids in the blood. The liver is affected as well, with reduced activity of glycogen synthase and a corresponding decrease in the content of liver glycogen. In contrast, the activity of liver cytosolic phosphoenolpyruvate carboxykinase and the level of its mRNA are not affected. Using a cDNA containing P-pyruvate carboxykinase sequence, the relative abundance of the enzyme mRNA was estimated. The level of the mRNA was readily observed increasing by glucocorticoid treatment or decreasing in response to administered load of glucose. These parallel the changes observed in the activity of the enzyme under these treatments, indicating that the level of P-pyruvate carboxykinase mRNA actually determines that of the enzyme. The failure of diabetes to increase the level of enzyme mRNA and the limited response to glucose loading strongly suggest that the mechanisms controlling the level of P-pyruvate carboxykinase mRNA in neonates are relatively resistant to insulin. This is unique to neonates, since in both the adult and the fetal liver. P-pyruvate carboxykinase readily responds to insulin. The minimal levels of glucocorticoids characteristic of neonates may be associated with this phenomenon.