Rapid action of insulin and cyclic AMP in the regulation of functional messenger RNA coding for glucokinase in rat liver

Morning Engagement of Hepatic Insulin Receptors Improves Afternoon Hepatic Glucose Disposal and Storage

Glucose tolerance improves significantly upon consuming a second, identical meal later in the day (second meal phenomenon). We previously established that morning hyperinsulinemia primes the liver for increased afternoon hepatic glucose uptake (HGU). Although the route of insulin delivery is an important determinant of the mechanisms by which insulin regulates liver glucose metabolism (direct hepatic vs indirect insulin action), it is not known if insulin's delivery route affects the second meal response. To determine whether morning peripheral insulin delivery (as occurs clinically (subcutaneous)) can enhance afternoon HGU, conscious dogs were treated in the morning with insulin delivered via the portal vein, or peripherally (leg vein), while glucose was infused to maintain euglycemia. Consequently, arterial insulin levels increased similarly in both groups, but relative hepatic insulin deficiency occurred when insulin was delivered peripherally. In the afternoon, all animals were challenged with the same hyperinsulinemic-hyperglycemic clamp to simulate identical postprandial-like conditions. The substantial enhancement of HGU in the afternoon caused by morning portal vein insulin delivery was lost when insulin was delivered peripherally. This indicates that morning insulin does not cause the second meal phenomenon via its indirect actions on the liver, but rather through direct activation of hepatic insulin signaling.

Article Highlights

Morning insulin delivery primes the liver for increased hepatic glucose uptake (HGU) later in the day, but the mechanism (direct hepatic and/or indirect insulin action) remains unclear.This study compared insulin infusion via physiologic (hepatic portal vein) and clinical (peripheral) routes to assess their impact on afternoon hepatic glucose disposal.Morning peripheral insulin delivery failed to induce a significant enhancing effect on afternoon HGU and glycogen storage, unlike morning hepatic portal vein insulin delivery, which did.These findings highlight the importance of achieving appropriate hepatic insulin exposure in the morning to effectively prime the liver for efficient glucose disposal.

Pancreatic regulation of glucose homeostasis

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

Nutritional regulation of glucokinase: a cross-species story

The glucokinase (GK) enzyme (EC 2.7.1.1.) is essential for the use of dietary glucose because it is the first enzyme to phosphorylate glucose in excess in different key tissues such as the pancreas and liver. The objective of the present review is not to fully describe the biochemical characteristics and the genetics of this enzyme but to detail its nutritional regulation in different vertebrates from fish to human. Indeed, the present review will describe the existence of the GK enzyme in different animal species that have naturally different levels of carbohydrate in their diets. Thus, some studies have been performed to analyse the nutritional regulation of the GK enzyme in humans and rodents (having high levels of dietary carbohydrates in their diets), in the chicken (moderate level of carbohydrates in its diet) and rainbow trout (no carbohydrate intake in its diet). All these data illustrate the nutritional importance of the GK enzyme irrespective of feeding habits, even in animals known to poorly use dietary carbohydrates (carnivorous species).

Factors affecting insulin-regulated hepatic gene expression

Obesity has become a major concern of public health. A common feature of obesity and related metabolic disorders such as noninsulin-dependent diabetes mellitus is insulin resistance, wherein a given amount of insulin produces less than normal physiological responses. Insulin controls hepatic glucose and fatty acid metabolism, at least in part, via the regulation of gene expression. When the liver is insulin-sensitive, insulin can stimulate the expression of genes for fatty acid synthesis and suppress those for gluconeogenesis. When the liver becomes insulin-resistant, the insulin-mediated suppression of gluconeogenic gene expression is lost, whereas the induction of fatty acid synthetic gene expression remains intact. In the past two decades, the mechanisms of insulin-regulated hepatic gene expression have been studied extensively and many components of insulin signal transduction pathways have been identified. Factors that alter these pathways, and the insulin-regulated hepatic gene expression, have been revealed and the underlying mechanisms have been proposed. This chapter summarizes the recent progresses in our understanding of the effects of dietary factors, drugs, bioactive compounds, hormones, and cytokines on insulin-regulated hepatic gene expression. Given the large amount of information and progresses regarding the roles of insulin, this chapter focuses on findings in the liver and hepatocytes and not those described for other tissues and cells. Typical insulin-regulated hepatic genes, such as insulin-induced glucokinase and sterol regulatory element-binding protein-1c and insulin-suppressed cytosolic phosphoenolpyruvate carboxyl kinase and insulin-like growth factor-binding protein 1, are used as examples to discuss the mechanisms such as insulin regulatory element-mediated transcriptional regulation. We also propose the potential mechanisms by which these factors affect insulin-regulated hepatic gene expression and discuss potential future directions of the area of research.

Identification of the ubiquitin-like domain of midnolin as a new glucokinase interaction partner

Glucokinase acts as a glucose sensor in pancreatic beta cells. Its posttranslational regulation is important but not yet fully understood. Therefore, a pancreatic islet yeast two-hybrid library was produced and searched for glucokinase-binding proteins. A protein sequence containing a full-length ubiquitin-like domain was identified to interact with glucokinase. Mammalian two-hybrid and fluorescence resonance energy transfer analyses confirmed the interaction between glucokinase and the ubiquitin-like domain in insulin-secreting MIN6 cells and revealed the highest binding affinity at low glucose. Overexpression of parkin, an ubiquitin E3 ligase exhibiting an ubiquitin-like domain with high homology to the identified, diminished insulin secretion in MIN6 cells but had only some effect on glucokinase activity. Overexpression of the elucidated ubiquitin-like domain or midnolin, containing exactly this ubiquitin-like domain, significantly reduced both intrinsic glucokinase activity and glucose-induced insulin secretion. Midnolin has been to date classified as a nucleolar protein regulating mouse development. However, we could not confirm localization of midnolin in nucleoli. Fluorescence microscopy analyses revealed localization of midnolin in nucleus and cytoplasm and co-localization with glucokinase in pancreatic beta cells. In addition we could show that midnolin gene expression in pancreatic islets is up-regulated at low glucose and that the midnolin protein is highly expressed in pancreatic beta cells and also in liver, muscle, and brain of the adult mouse and cell lines of human and rat origin. Thus, the results of our study suggest that midnolin plays a role in cellular signaling of adult tissues and regulates glucokinase enzyme activity in pancreatic beta cells.

Roles of Vitamin A Metabolism in the Development of Hepatic Insulin Resistance

The increase in the number of people with obesity- and noninsulin-dependent diabetes mellitus has become a major public health concern. Insulin resistance is a common feature closely associated with human obesity and diabetes. Insulin regulates metabolism, at least in part, via the control of the expression of the hepatic genes involved in glucose and fatty acid metabolism. Insulin resistance is always associated with profound changes of the expression of hepatic genes for glucose and lipid metabolism. As an essential micronutrient, vitamin A (VA) is needed in a variety of physiological functions. The active metablite of VA, retinoic acid (RA), regulates the expression of genes through the activation of transcription factors bound to the RA-responsive elements in the promoters of RA-targeted genes. Recently, retinoids have been proposed to play roles in glucose and lipid metabolism and energy homeostasis. This paper summarizes the recent progresses in our understanding of VA metabolism in the liver and of the potential transcription factors mediating RA responses. These transcription factors are the retinoic acid receptor, the retinoid X receptor, the hepatocyte nuclear factor 4α, the chicken ovalbumin upstream promoter-transcription factor II, and the peroxisome proliferator-activated receptor β/δ. This paper also summarizes the effects of VA status and RA treatments on the glucose and lipid metabolism in vivo and the effects of retinoid treatments on the expression of insulin-regulated genes involved in the glucose and fatty acid metabolism in the primary hepatocytes. I discuss the roles of RA production in the development of insulin resistance in hepatocytes and proposes a mechanism by which RA production may contribute to hepatic insulin resistance. Given the large amount of information and progresses regarding the physiological functions of VA, this paper mainly focuses on the findings in the liver and hepatocytes and only mentions the relative findings in other tissues and cells.

Roles of vitamin A status and retinoids in glucose and fatty acid metabolism

The rising prevalence of metabolic diseases, such as obesity and diabetes, has become a public health concern. Vitamin A (VA, retinol) is an essential micronutrient for a variety of physiological processes, such as tissue differentiation, immunity, and vision. However, its role in glucose and lipid metabolism has not been clearly defined. VA activities are mediated by the metabolite of retinol catabolism, retinoic acid, which activates the retinoic acid receptor and retinoid X receptor (RXR). Since RXR is an obligate heterodimeric partner for many nuclear receptors involved in metabolism, it is reasonable to assume that VA status and retinoids contribute to glucose and lipid homeostasis. To date, the impacts of VA and retinoids on energy metabolism in animals and humans have been demonstrated in some basic and clinical investigations. This review summarizes the effects of VA status and retinoid treatments on metabolism of the liver, adipocytes, pancreatic β-cells, and skeletal muscle. It proposes a mechanism by which the dietary and hormonal signals converge on the promoter of sterol regulatory element-binding protein 1c gene to induce its expression, and in turn, the expression of lipogenic genes in hepatocytes. Future research projects relevant to the VA's roles in metabolic diseases are also discussed.

Ameliorating effect of chromium administration on hepatic glucose metabolism in streptozotocin-induced experimental diabetes

Chromium has been recognized as an essential trace element that plays an important role in carbohydrate metabolism. However, the molecular mechanisms involved in its action are not clear. This study was undertaken to understand the mechanism of chromium action in experimental diabetes. Streptozotocin-induced diabetic animals were administered chromium as chromium picolinate (CrP) at a daily dose of 1 mg/kg body weight for a period of 4 weeks. It was observed that chromium complexed with picolinate was effective in lowering plasma glucose levels as well as was able to alleviate polyphagia, polydipsia, and weight loss in diabetic animals. Administration of chromium was also found to normalize glycogen content in liver of diabetic animals to near control levels. The reduction in plasma glucose levels by chromium was accompanied by increase in activity of glycolytic enzymes (e.g., glucokinase, phosphofructokinase, and pyruvate kinase) and by suppression in activity of gluconeogenic enzymes (e.g., glucose-6-phosphatase and phosphoenolpyruvate carboxykinase) in liver. Hepatic glucose uptake was found to be increased by chromium supplementation as demonstrated by decrease in Km and increase in Vmax values in diabetic animals. Chromium levels were lower in the liver of diabetic rats when compared with that of control rats. A negative correlation was observed between plasma glucose and chromium concentration in patients with diabetes. The data suggests that chromium supplementation as CrP is beneficial in correcting hyperglycemia, implying that the modulation of the glucose metabolism by chromium may be therapeutically beneficial in the treatment of diabetes.

PREB regulates transcription of pancreatic glucokinase in response to glucose and cAMP

Prolactin regulatory element binding (PREB) is a transcription factor that regulates prolactin promoter activity in rat anterior pituitary. The PREB protein is not only expressed in the anterior pituitary but also in the pancreas. We have recently reported that in pancreatic beta-cells, PREB regulates the transcription of the insulin gene in response to glucose stimulation. In the current study, we have examined the role of PREB in regulating glucokinase (GK) in pancreatic beta-cells. To analyse the effects of PREB on GK gene transcription, we employed a reporter gene assay. In the cells expressing or with knocked down PREB, GK expression was determined. GK expression was regulated by glucose and cAMP, and both glucose and cAMP stimulated the expression of PREB in a dose-dependent manner. Conversely, overexpression of PREB using a PREB-expressing adenovirus increased the expression of the GK protein. GK enzymatic activity was also significantly increased in the cells that stably expressed PREB. In addition, PREB induced GK promoter activity. Chromatin immunoprecipitation (ChIP) analyses showed that PREB mediated its transcriptional effect by binding to the PREB-responsive cis-element of the GK promoter. Finally, we used siRNA to inhibit PREB expression in cells and demonstrated that the knockdown of PREB attenuated the effects of glucose and cAMP on GK expression. Our data show that in pancreatic -cells, PREB regulates the transcription of the GK gene in response to glucose and cAMP.

Retinoids synergize with insulin to induce hepatic Gck expression

Hepatic GK (glucokinase) plays a key role in maintaining glucose homoeostasis. Many stimuli regulate GK activity by controlling its gene transcription. We hypothesized that endogenous lipophilic molecules modulate hepatic Gck expression. Lipophilic molecules were extracted from rat livers, saponified and re-constituted as an LE (lipophilic extract). LE synergized with insulin to induce primary hepatocyte, but not beta-cell, Gck expression in an SREBP-1c (sterol-regulatory-element-binding protein-1c)-independent manner. The dramatic induction of Gck mRNA resulted in a significant increase in GK activity. Subsequently, the active molecules were identified as retinol and retinal by MS after the purification of the active LE fractions. Retinoids synergized with insulin to induce Gck expression by the activation of both RAR [RA (retinoic acid) receptor] and RXR (retinoid X receptor). Inhibition of RAR activation completely abolished the effect of retinal. The hepatic GK specific activity and Gck mRNA levels of Zucker lean rats fed with a VAD [VA (vitamin A)-deficient] diet were significantly lower than those of rats fed with VAS (VA-sufficient) diet. Additionally, the hepatic Gck mRNA expression of Sprague-Dawley rats fed with a VAD diet was lower than that of rats fed with VA-marginal, -adequate or -supplemented diets. The reduced expression of Gck mRNA was increased after an intraperitoneal dose of RA in VAD rats. Furthermore, an intravenous injection of RA rapidly raised hepatic Gck expression in rats fed with a VAS control diet. Understanding the underlying mechanism that mediates the synergy may be helpful for developing a treatment strategy for patients with diabetes.

Induction of glucokinase in chicken liver by dietary carbohydrates

We recently provided evidence of the presence of glucokinase (GCK) in the chicken liver [Berradi, H., Taouis, M., Cassy, S., Rideau, N., 2005. Glucokinase in chicken (Gallus gallus). Partial cDNA cloning, immunodetection and activity determination. Comp. Biochem. Physiol. B Biochem. Mol. Biol. 141, 129-139]. In the present study we addressed the question of whether nutritional regulation of GCK occurs. Several nutritional conditions were compared in chickens (5 weeks old) previously trained to meal-feeding. One group was left in the fasted state (F: 24h) and one was tested at the end of the 2h meal (refed: RF). Two other 2h meal-refed groups received an acute oral saccharose load (6ml/kg BW) just before the 2h meal and were sacrificed either at the end of the meal (Saccharose refed, SRF) or 3h later (SRF+3). Liver GCK mRNA and protein levels did not differ between F, RF and SRF chickens but were significantly increased in SRF+3 chickens (2-fold, p<0.05). GCK activity did not differ between F and RF chickens but increased significantly in SRF and SRF+3 chickens (1.7-fold, p<0.05). Chicken liver GCK expression (mRNA and protein) and activity were therefore inducible in these chickens by feeding a meal with acute oral administration of carbohydrate. These and recent findings demonstrating insulin dependency of the liver GCK mRNA and protein strongly suggest that GCK may have an important role in carbohydrate metabolism, including that of the chicken. However, even in these highly stimulatory conditions, liver GCK activity remained relatively low in comparison with other species. The latter result may partly explain the high plasma glucose level in the chicken.

Overexpression of beta2-adrenergic receptors in mouse liver alters the expression of gluconeogenic and glycolytic enzymes

In the livers of humans and many other mammalian species, beta2-adrenergic receptors (beta2-ARs) play an important role in the modulation of glucose production by glycogenolysis and gluconeogenesis. In male mice and rats, however, the expression and physiological role of hepatic beta2-ARs are rapidly lost with development under normal physiological conditions. We previously described a line of transgenic mice, F28 (Andre C, Erraji L, Gaston J, Grimber G, Briand P, and Guillet JG. Eur J Biochem 241: 417-424, 1996), which carry the human beta2-AR gene under the control of its own promoter. In these mice, hepatic beta2-AR levels are shown to increase rapidly after birth and, as in humans, be maintained at an elevated level in adulthood. F28 mice display strongly enhanced adenylyl cyclase responses to beta-AR agonists in their livers and, compared with normal mice, have increased basal hepatic adenylyl cyclase activity. In this report we demonstrate that, under normal physiological conditions, this increased beta2-AR activity affects the expression of the gluconeogenic and glycolytic key enzymes phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and l-pyruvate kinase and considerably decreases hepatic glycogen levels. Furthermore, we show that the effects of beta-adrenergic ligands on liver glycogen observed in humans are reproduced in these mice: liver glycogen levels are strongly decreased by the beta2-AR agonist clenbuterol and increased by the beta-AR antagonist propranolol. These transgenic mice open new perspectives for studying in vivo the hepatic beta2-AR system physiopathology and for testing the effects of beta-AR ligands on liver metabolism.

Adenovirus-mediated expression of glucokinase in the liver as an adjuvant treatment for type 1 diabetes

Glucokinase (GK) plays a crucial role in hepatic glucose disposal. Its activity is decreased in patients with maturity-onset diabetes of the young and in some animal models of diabetes. We investigated the feasibility of manipulating GK expression as an adjuvant treatment for type 1 diabetes, using an E1/E3-deleted adenoviral vector (Ad.EF1(alpha)GK) delivered to the liver of streptozotocin-induced type 1 diabetic rats. First, we studied the metabolic impact of constitutive glucokinase expression in the absence of insulin. Normal blood glucose levels were observed after gene transfer, and glucose tolerance was substantially enhanced compared with diabetic control animals, suggesting that hepatic GK expression is a feasible mechanism to enhance glucose disposal. In a second study we administered Ad.EF1(alpha)GK together with subcutaneous insulin injections to determine whether the combined action of insulin plus GK activity would provide better glucose homeostasis than insulin treatment alone. This combination approach resulted in constant, near-normal glucose values under fed conditions. Furthermore, the animals stayed in the normoglycemic range after an overnight fast, indicating that the risk to develop hypoglycemia is not increased by expression of GK. Alterations of other metabolic routes were observed, suggesting that insulin-regulated expression of GK may be necessary to use the strategy as a treatment of type 1 diabetes.

Cyclic adenosine 3',5'-monophosphate increases pancreatic glucokinase activity and gene expression

Comparison of the pancreatic and hepatic glucokinase gene transcripts reveals tissue-specific control of expression and the existence of two distinct promoters in a single glucokinase gene. The existence of alternate promoters suggests that separate factors regulate glucokinase transcription in the two tissues. Hepatic glucokinase expression has been shown to be repressed by cAMP; however, in the pancreatic beta-cell it is unlikely that cAMP represses glucokinase activity, as cAMP is known to positively affect glucose-induced insulin secretion, a process that in mature islets requires pancreatic glucokinase activity. In this work we demonstrate that cAMP indeed has a stimulatory effect on pancreatic glucokinase. The cyclic nucleotide stimulates pancreatic glucokinase activity after 3-h incubation, and maximal effects are observed after 6 and 12 h of treatment. Using the bDNA assay, a sensitive signal amplification technique, we detected relative increases in glucokinase messenger RNA levels of 40.5 +/- 7.5% after 3-h incubation with cAMP. This stimulatory effect was increased to 106.3 +/- 22% after 6-h incubation and sustained up to 12 h of incubation. Inhibition of gene transcription by actinomycin D abolishes cAMP-induced glucokinase activity. In transfected fetal islets, cAMP increased the activity of the -1000 bp rat glucokinase promoter by 60 +/- 6%. These data demonstrate that cAMP has a stimulatory effect on pancreatic glucokinase gene expression and that the nucleotide has opposite effects on pancreatic and hepatic glucokinase, supporting the concept that glucokinase transcription in the liver and that in the beta-cell differ.

Glucokinase gene locus transgenic mice are resistant to the development of obesity-induced type 2 diabetes

Transgenic mice that overexpress the entire glucokinase (GK) gene locus have been previously shown to be mildly hypoglycemic and to have improved tolerance to glucose. To determine whether increased GK might also prevent or diminish diabetes in diet-induced obese animals, we examined the effect of feeding these mice a high-fat high-simple carbohydrate low-fiber diet (HF diet) for 30 weeks. In response to this diet, both normal and transgenic mice became obese and had similar BMIs (5.3 +/- 0.1 and 5.0 +/- 0.1 kg/m2 in transgenic and non-transgenic mice, respectively). The blood glucose concentration of the control mice increased linearly with time and reached 17.0 +/- 1.3 mmol/l at the 30th week. In contrast, the blood glucose of GK transgenic mice rose to only 9.7 +/- 1.2 mmol/l at the 15th week, after which it returned to 7.6 +/- 1.0 mmol/l by the 30th week. The plasma insulin concentration was also lower in the GK transgenic animals (232 +/- 79 pmol/l) than in the controls (595 +/- 77 pmol/l), but there was no difference in plasma glucagon concentrations. Together, these data indicate that increased GK levels dramatically lessen the development of both hyperglycemia and hyperinsulinemia associated with the feeding of an HF diet.

Vanadate restores glucose 6-phosphate in diabetic rats: a mechanism to enhance glucose metabolism

Vanadate mimics the metabolic actions of insulin. In diabetic rodents, vanadate also sensitizes peripheral tissues to insulin. We have analyzed whether this latter effect is brought about by a mechanism other than the known insulinomimetic actions of vanadium in vitro. We report that the levels of glucose 6-phosphate (G-6-P) in adipose, liver, and muscle of streptozotocin-treated (STZ)-hyperglycemic rats are 77, 50, and 58% of those in healthy control rats, respectively. Normoglycemia was induced by vanadium or insulin therapy or by phlorizin. Vanadate fully restored G-6-P in all three insulin-responsive peripheral tissues. Insulin did not restore G-6-P in muscle, and phlorizin was ineffective in adipose and muscle. Incubation of diabetic adipose explants with glucose and vanadate in vitro increased lipogenic capacity three- to fourfold (half-maximally effective dose = 11 +/- 1 microM vanadate). Lipogenic capacity was elevated when a threshold level of approximately 7.5 +/- 0.3 nmol G-6-P/g tissue was reached. In summary, 1) chronic hyperglycemia largely reduces intracellular G-6-P in all three insulin-responsive tissues; 2) vanadate therapy restores this deficiency, but insulin therapy does not restore G-6-P in muscle tissue; 3) induction of normoglycemia per se (i.e., by phlorizin) restores G-6-P in liver only; and 4) glucose and vanadate together elevate G-6-P in adipose explants in vitro and significantly restore lipogenic capacity above the threshold of G-6-P level. We propose that hyperglycemia-associated decrease in peripheral G-6-P is a major factor responsible for peripheral resistance to insulin. The mechanism by which vanadate increases peripheral tissue capacity to metabolize glucose and to respond to the hormone involves elevation of this hexose phosphate metabolite and the cellular consequences of this elevated level of G-6-P.

Lithium's effects on rat liver glucose metabolism in vivo

Oral administration of lithium carbonate to fed-healthy rats strongly decreased liver glycogen content, despite the simultaneous activation of glycogen synthase and the inactivation of glycogen phosphorylase. The effect seemed to be related to a decrease in glucose 6-phosphate concentration and to a decrease in glucokinase activity. Moreover, in these animals lithium markedly decreased liver fructose 2,6-bisphosphate, which could be a consequence of the fall in glucose 6-phosphate and of the inactivation of 6-phosphofructo-2-kinase. Liver pyruvate kinase activity and blood insulin also decreased after lithium administration. Lower doses of lithium carbonate had less intense effects. Lithium administration to starved-healthy and fed-streptozotocin-diabetic rats caused a slight increase in blood insulin, which was simultaneous with increases in liver glycogen, glucose 6-phosphate, and fructose 2, 6-phosphate. Glucokinase, 6-phosphofructo-2-kinase, and pyruvate kinase activities also increased after lithium administration in starved-healthy and fed-diabetic rats. Lithium treatment activated glycogen synthase and inactivated glycogen phosphorylase in a manner similar to that observed in fed-healthy rats. Glycemia was not modified in any group of animals. These results indicate that lithium acts on liver glycogen metabolism in vivo in at least two different ways: one related to changes in insulinemia, and the other related to the direct action of lithium on the activity of some key enzymes of liver glucose metabolism.

Effects of endothelin-1 and nitric oxide on glucokinase activity in isolated rat hepatocytes

To test the hypothesis that endothelin-1 (ET-1) and nitric oxide (NO) influence glucokinase (GK) activity in an opposite manner, we evaluated the effects of ET-1, L-NAME, an inhibitor of NO synthase, and L-arginine, a substrate for NO synthase, on GK activity and glycogen content in isolated rat hepatocytes. Moreover, to understand the receptor involved in the process, the effects of BQ 788, a specific antagonist of ETB receptor, and PD 142893, an antagonist of ETA-ETB receptors, were also evaluated. GK activity, cyclic guanosine monophosphate (cGMP), and glycogen intracellular content were measured on isolated hepatocytes, while glucose levels and NO as NO2-/NO3- were determined in the medium. High ET-1 levels induced a 20% decrease of NO2-/NO3- levels and cGMP intracellular content, followed by a 49% reduction of GK activity and a 15% decrease of glycogen. In parallel, a 10% increase of glucose in the medium was observed. In the presence of L-NAME, GK activity and glycogen levels showed analogous decrements as observed with ET-1. Also in this case, a significant decrease of the intracellular content of cGMP was observed. No synergistic effects of ET-1 and L-NAME were observed. L-Arginine was able to counteract the inhibitory effect of ET-1 on cGMP and GK activity. Glycogen content was slightly but not significantly reduced, and under those conditions, a significant decrease of glucose in the medium was observed. When hepatocytes were incubated with ET-1 plus BQ 788 or ET-1 plus PD 142893, GK activity was unchanged. Interestingly, no changes were observed in NO2-/NO3- levels and the intracellular content of cGMP was not modified when the antagonists of ET-1 receptors were added to the medium. In conclusion, the present study shows that the NO pathway seems to be an important regulator of GK activity and glycogen content through cGMP activity. In addition, ET-1 seems to be not active per se, but its activity seems mediated by a simultaneous decrease of NO levels.

Induction of the glucokinase gene by insulin in cultured neonatal rat hepatocytes. Relationship with DNase-I hypersensitive sites and functional analysis of a putative insulin-response element

Previous, in vivo experiments have shown that an appropriate hormonal environment (high plasma insulin, low plasma glucagon) was unable to induce the accumulation of glucokinase mRNA in term fetal rat liver, whereas it was very efficient in the newly born rat. We have confirmed in the present study that insulin induced the accumulation of glucokinase mRNA in cultured hepatocytes from 1-day-old newborn rats, but not in cultured hepatocytes from 21-day-old fetuses. To identify regulatory regions of the glucokinase gene involved in the insulin response, we have scanned the glucokinase locus for DNase I hypersensitive sites in its in vivo conformation. We confirmed the presence of four liver-specific DNase I hypersensitive sites located in the 5' flanking region of the gene. Moreover, two additional hypersensitive sites, located at 2.5 kb and 3.5 kb upstream of the cap site were found but none of these new sites displayed inducibility by insulin. Finally, an increase of the sensitivity of hypersensitive site-1 and hypersensitive site-2 to DNase I correlates with the ability of insulin to induce glucokinase gene expression in cultured hepatocytes from 1-day-old rats, as observed in previous in vivo studies. This suggests that neither a prior exposure to insulin nor a simple aging of the fetal cells in the presence of the hormone in culture are instrumental for the full DNase-I hypersensitivity of the two proximal sites necessary for the neonatal response of the glucokinase gene to insulin. The proximal hypersensitive site-1, which is close to the transcription start site in the liver, does coincide with a sequence (designated IRSL) that is 80% identical to the phosphoenolpyruvate carboxykinase IRS and with a DNase-I footprint that has been identified overlapping this sequence. Nevertheless, functional analysis of this sequence suggested that it is unlikely that the insulin-response sequence like alone is sufficient to mediate the transcriptional effect of insulin on the hepatic glucokinase gene.

Initial expression of glucokinase gene in cultured hepatocytes from suckling rats is linked to the synthesis of an insulin-dependent protein

The initial accumulation of glucokinase mRNA in response to insulin in cultured hepatocytes from 10-day-old suckling rats was characterized by a delay of 18-24 h with a maximal level reached after 48 h. This delay is not observed in cultured adult rat hepatocytes. When hepatocytes from 10-day-old suckling rats were cultured for 48 h in the presence of insulin (to obtain a maximal accumulation of glucokinase mRNA) and then deprived of insulin for 18 h, glucokinase mRNA returned to very low levels. Reexposure of these cultured hepatocytes to insulin allowed a rapid accumulation of glucokinase mRNA, with a maximal level reached after 8 h, as in adult rat hepatocytes. The aim of the present study was to investigate the factors responsible for the delay in insulin action during first exposure to insulin. The difference in the kinetics of glucokinase mRNA accumulation after the first and secondary exposure to insulin was due to differences in the rate of transcriptional activity of the glucokinase gene, as shown by a run-on assay on isolated nuclei. The half-life of glucokinase mRNA was similar after the first and second exposure to insulin. The delay in the initial accumulation of glucokinase mRNA in response to the first exposure to insulin was not due to elevated levels of cAMP (a potent inhibitor of glucokinase gene expression) or to a defect in insulin signalling (insulin inhibited without delay phosphoenolpyruvate carboxykinase gene expression). In contrast, it was markedly dependent upon whether glucokinase has been already expressed in vivo. Hepatocytes from rats that had already expressed glucokinase in vivo (suckling rats force-fed with glucose or rats weaned to a high-carbohydrate diet) showed no delay in their response to insulin in culture, whereas hepatocytes from rats that have never expressed glucokinase in vivo (suckling rats or rats weaned to a high-fat diet) showed a delay of 24 h. Two different inhibitors of protein synthesis (cycloheximide and puromycin) prevented the initial accumulation of glucokinase mRNA in response to the first exposure to insulin but not to the secondary accumulation of glucokinase mRNA in response to reexposure to insulin. This suggests that the synthesis of one or several insulin-dependent proteins is necessary for the first activation of glucokinase gene transcription in response to the first exposure to insulin.

Glucose administration induces the premature expression of liver glucokinase gene in newborn rats. Relation with DNase-I-hypersensitive sites

Glucokinase first appears in the liver of the rat 2 weeks after birth and its activity rapidly increases after weaning on to a high-carbohydrate diet. The appearance of glucokinase is principally due to the increase of plasma insulin and to the decrease of plasma glucagon concentrations. Oral glucose administration to 1- or 10-day-old suckling rats induced an increase in plasma insulin and a fall in plasma glucagon and allowed a rapid accumulation of liver glucokinase mRNA, secondarily to a stimulation of gene transcription. When unrestrained late pregnant rats were infused with glucose during 36 h to induce an increase in fetal plasma insulin and a decrease in fetal plasma glucagon concentrations, glucokinase mRNA was detectable in fetal liver but the level was 100-fold lower than that observed in 1- or 10-day-old suckling rats. It is suggested that the hormonal environment did not allow glucokinase gene expression to be induced in fetal liver and that the absence of expression of glucokinase in suckling rat liver is due to the presence of low plasma insulin and high plasma glucagon levels. The chromatin structure of the glucokinase gene was examined during development by identification of DNase-I-hypersensitive sites from the region comprised between -8 kb upstream and +4 kb downstream of the cap site. Five hypersensitive sites were found: four liver-specific sites upstream of the cap site and one non-specific site in the first intron. These sites are already present in term fetus but the intensity of the two proximal sites located upstream of the cap site increase markedly after birth. This suggests that these sites could be implicated in the regulation of glucokinase gene expression by insulin and glucagon. Full DNase-I-hypersensitivity of these two proximal sites seems necessary for the mature response of glucokinase gene in response to changes in pancreatic hormones concentrations.

Phlorizin treatment of diabetic rats partially reverses the abnormal expression of genes involved in hepatic glucose metabolism

Liver insulin resistance and glucagon-stimulated hepatic glucose production are characteristics of the diabetic state. To determine the potential role of glucose toxicity in these abnormalities, we examined whether phlorizin treatment of streptozotocin-diabetic rats resulted in altered expression of genes involved in key steps of hepatic glucose metabolism. By inhibiting renal tubular glucose reabsorption, phlorizin infusion to diabetic rats induced normoglycaemia, did not significantly alter low circulating insulinaemia, but caused a marked decrease in hyperglucagonaemia. Glucokinase and L-type pyruvate kinase mRNA levels were reduced respectively by 90% and 70% in fed diabetic rats, in close correlation with changes in enzyme activities. Eighteen days of phlorizin infusion partially restored glucokinase mRNA and activity (40% of control levels), but had no effect on L-type pyruvate kinase mRNA and activity. In contrast to the glycolytic enzymes, mRNA and activity of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase were increased (10- and 2.2-fold, respectively) in fed diabetic rats. Phlorizin administration decreased phosphoenolpyruvate carboxykinase mRNA to values not different from those in control rats, while phosphoenolpyruvate carboxykinase activity remained 50% higher than that in control rats. The 50% rise in liver glucose transporter (GLUT 2) mRNA and protein, produced by diabetes, was also corrected by phlorizin treatment. In conclusion, we propose that phlorizin treatment of diabetic rats may induce a partial shift of the predominating gluconeogenesis, associated with hepatic glucose overproduction, into glycolysis, by correction of impaired pre-translational regulatory mechanisms. This could be essentially mediated through improved pancreatic alpha-cell function and subsequent lowering of hyperglucagonaemia. These observations suggest that glucagon-stimulated hepatic glucose production may result, in part, from glucose toxicity.

In vitro stimulation of glucose utilization by insulin in primary cultures of rat hepatocytes

The effect of glucose concentration and insulin on glucose incorporation was studied in primary cultures of rat hepatocytes. The rate of glucose incorporation into hepatocytes was proportional to the medium glucose concentration from 100 to 800 mg/dl. At 800 mg/dl glucose the rate reached a plateau. Of the glucose taken up by hepatocytes, 16 and 18% was incorporated into glycogen and lipid, respectively, and 58% into the nucleotide fraction after incubation for 4 h. In the medium, lactate was the major product found. Insulin stimulates glucose incorporation by 20-112% into all the above pathways at glucose concentrations between 100 and 800 mg/dl. The insulin effect was noted as early as 2-4 h (early effect) and up to 24 h (delayed effect). This effect of insulin was observed to be dose dependent from 5 to 200 ng/ml insulin. While the delayed insulin effect was abolished by cycloheximide, the early effect of insulin was not affected. With respect to the key enzyme activities of glucose utilization, activation of glycogen synthase (increase of I-activity/total activity) and pyruvate kinase (activation at 0.2 mM phosphoenolpyruvate) was noted 4 h after insulin addition, and these effects were not abolished by cycloheximide. These two enzymes increased in total activity after 24 h. Both glucokinase and glucose-6-phosphate dehydrogenase activities increased by 30-35% and 65-93% at 4 and 24 h, respectively. The results indicate that hepatocytes directly utilize glucose in a dose-dependent manner with respect to glucose and insulin. A major early and delayed effect of insulin appeared due to the activation and induction of the key hepatic enzymes of glucose utilization, respectively.

Effect of diabetes and insulin treatment of diabetic rats on total RNA, poly(A)+ RNA, and mRNA in skeletal muscle

We have assessed the time course of alterations in several biochemical parameters and expression of specific mRNAs in gastrocnemius muscle following both the induction of diabetes and the administration of insulin to diabetic rats. Muscle mass, total RNA, and total protein were reduced, whereas poly(A)+ RNA relative to total RNA was increased following the induction of diabetes. All the above parameters, with the exception of poly(A)+ RNA, were reciprocally and rapidly altered following administration of insulin to 3-day diabetic animals. These changes suggest that during the induction of diabetes 1) total cellular protein is reduced at a rate that is less than the reduction in gastrocnemius mass, whereas RNA is reduced at a rate 1.5 times the reduction in tissue mass, and 2) poly(A)+ RNA is elevated relative to total RNA. After insulin administration, there appears to be coordinate synthesis of both poly(A)+ RNA and ribosomal RNA, assuming 85% of total RNA is ribosomal. Therefore, we conclude that poly(A)+ RNA is more stable than ribosomal RNA during diabetes, whereas the amounts of poly(A)+ RNA and ribosomal RNA are increased at the same rates following insulin administration to diabetic animals. Analysis of expression of specific gene products over the same time course, as assessed by in vitro translation of total RNA followed by two-dimensional gel analysis, suggests that there are a few mRNAs that are very rapidly altered in response to insulin administration. The mRNAs that are altered demonstrate variable temporal patterns of either repression or full or transient expression. These rapid, but limited, alterations in gene expression may prove important in the development of the defects that occur in skeletal muscle in response to diabetes.

The role of receptor kinase in insulin action and the effects of insulin on human hepatoma cells

Insulin has both short- and long-term effects on cellular metabolism. The short-term effects are known to involve the insulin receptor, a protein kinase capable of phosphorylating itself and other proteins. The role of the receptor was elucidated by studies of a mutant insulin receptor which lacked kinase activity and inhibited several actions of insulin. The long-term effects of insulin could be demonstrated by its growth-promoting effect on hepatoma cells, and by the suppression in transfected hepatoma cells of hepatitis B virus antigen production in a dose-dependent manner. The process whereby insulin appears to regulate gene expression is not clearly understood.

Expression of rat hepatic glucokinase in Escherichia coli

Rat liver glucokinase was expressed in Escherichia coli by using an expression system based on bacteriophage T7 RNA polymerase. The expressed protein starts with the predicted initiator methionine residue and ends at the appropriate carboxyl terminal residue. It was partially purified by ammonium sulfate precipitation and gel filtration and had kinetic and physical properties similar to the purified rat liver enzyme. The efficient expression of this low abundance hepatic protein in bacteria provides a system for in vitro analysis of mutations of the enzyme.

Characterization of the 5' flanking region of rat glucokinase gene

The rat glucokinase (GK) gene containing the first exon was isolated and its 5' flanking region was characterized by the bacterial chloramphenicol acetyltransferase (CAT) assay. A transient expression assay with a series of 5' deletion constructs (-5.5 k to -48) of GK-CAT fusion genes indicated that the 5' flanking sequence up to nucleotide -87 was sufficient for promoter activity in adult rat hepatocytes, but its activity was much weaker than that of the SV40 enhancer/promoter. Similar promoter activity was also detected in dRLh-84 hepatoma cells, which do not express glucokinase. Insulin treatment caused no change in the CAT activity of hepatocytes transfected with the fusion genes. These results suggest that the 5' flanking region of the glucokinase gene up to -5.5 k does not contain enhancer elements responsible for tissue-specific expression or insulin regulation.

Regulation of gene expression by insulin in adipose cells: opposite effects on adipsin and glycerophosphate dehydrogenase genes

Insulin is known to play the role of a positive effector both in vitro on the adipose conversion process and in vivo on the fatty acid synthesis and esterification processes in adipose tissue. The effects of insulin on the expression of two genes activated during adipose conversion, glycerol-3-phosphate dehydrogenase (GPDH) and adipsin genes, have been investigated in 3T3 F442A adipose cells. Within a physiological range of concentrations, insulin exerts opposite effects on the levels of GPDH (EC50 approximately 0.2 nM) and adipsin (EC50 approximately 1 nM) mRNAs. Its negative effect on the abundance of adipsin mRNA involves primarily a rapid inhibition of the transcriptional rate (less than 2 h). Its positive effect on the abundance of GPDH mRNA is due to a stimulation of the transcriptional rate accompanied by a delayed stabilization of GPDH mRNA. In addition, insulin exerts a specific effect on the length of the poly(A) tract of the adipsin mRNA. These results show that a single mechanism for the regulation of adipose-related genes by insulin can be excluded but rather suggest a complex phenomenon in which various levels of regulation take place.

Induction of an Alu-sequence containing transcript by insulin in human hepatoma cells

A cDNA clone, designated AF19-1, was isolated from a cDNA library derived from insulin-stimulated hepatoma cells. The nucleotide sequences of AF19-1 showed 83% homology to Alu consensus sequence. It contained a full-length 300-bp Alu family sequence followed in direct tandem by a partial sequence of Alu left monomer. Primer extension analysis confirmed that this Alu transcript was induced even in short-term insulin stimulation. The increase in Alu-transcripts in the early phase of insulin stimulation in hepatoma cells suggests that the Alu sequences may play some important regulatory roles in gene expression.

Insulin-mediated post-transcriptional regulation of hepatic malic enzyme and albumin mRNAs

Livers of insulin-treated diabetic rats accumulate albumin and malic enzyme mRNAs at very different rates. We now report that in normal rats insulin directs a specific increase in malic enzyme mRNA, while albumin mRNA levels remain unaltered. These studies support the contention that insulin regulates the accumulation of hepatic mRNAs in a highly specific manner. To evaluate whether or not albumin and malic enzyme mRNA levels are determined by altered rates of transcription, in vitro transcription assays were performed. The results of these studies demonstrate that increased malic enzyme mRNA levels in insulin-treated normal rats and increased malic enzyme and albumin mRNA levels in insulin-treated diabetic rats do not involve altered rates of transcription of the genetic sequences encoding these proteins. For these two specific proteins, insulin mediates changes in mRNA levels by a post-transcriptional mechanism.

Activation of low Km hexokinases in purified hepatocytes by binding to mitochondria

Hepatocytes were purified on a Percoll gradient. The cell membrane of these hepatocytes was disrupted by digitonin in the presence of albumin, glucose and physiological concentrations of monovalent and divalent cations. This treatment led to a separation between free and loosely structure-bound cytosolic enzymes which is not achieved by conventional subfractionation techniques. According to kinetic and immunological analyses, the free extractable cytosolic fraction contained high Km, hexokinase (glucokinase) and less than 10% of low Km hexokinases, while the hexokinase activity bound to the cell structures represented exclusively low Km isozymes. The total activity of the bound hexokinases was comparable to that observed in the supernate (approx. 1.0 U per g fresh weight). This activity decreased more than 10-fold upon desorption at higher digitonin concentrations. Such activation by binding, as well as inactivation by desorption, could also be demonstrated in intact hepatocytes correlated to different metabolic states, and also in vitro with isolated mitochondria and purified isozyme I. The binding of low Km hexokinases in hepatocytes was restricted to the mitochondrial fraction and there it was observed in the contact sites between the two mitochondrial boundary membranes. In view of these findings it appears that the binding-dissociation equilibrium of low Km hexokinases plays an important role in metabolic regulation of glucose uptake and glycogen synthesis in the liver and presumably in muscle.

Regulation of hepatic glucokinase gene expression. Role of carbohydrates, and glucocorticoid and thyroid hormones

The present study investigates the effect of thyroid and glucocorticoid hormones on the induction of hepatic glucokinase mRNA activity, enzyme synthesis and activity in starved/refed adrenalectomized, thyroidectomized and intact rats. In intact rats glucose refeeding resulted within 2 h in a more than tenfold increase in the functional messenger, followed by a corresponding increase in glucokinase synthesis and, a little later, in enzyme activity. Glucokinase mRNA and synthesis remained elevated at this level for about further 6 h. Then the mRNA activity and enzyme synthesis declined considerably to a new steady state (a factor of about 4 above the starvation level) within a further 8 h, while enzyme activity remained constantly elevated. The half-life of glucokinase mRNA, as determined after administration of cordycepin, was identical during the different refeeding periods. Thus the overshoot phenomenon, provoked by carbohydrate refeeding, in glucokinase mRNA is not explained by alteration of the glucokinase mRNA decay rates. In thyroidectomized or adrenalectomized rats, glucose refeeding resulted in only a small increase in glucokinase mRNA, synthesis and activity. Application of thyroid hormones in thyroidectomized rats, refed a carbohydrate-rich diet, enhanced the specific mRNA considerably within 8-10 h, while it took 20-24 h to enhance glucokinase mRNA by glucocorticoids in adrenalectomized rats refed a carbohydrate-rich diet. The decay in translatable glucokinase mRNA, as determined after administration of cordycepin, was identical in the hypothyroid and euthyroid fed state, while adrenalectomy resulted in a significant decrease in the specific mRNA half-life. We conclude that refeeding a carbohydrate-rich diet rapidly stimulates glucokinase mRNA regeneration showing overshoot kinetics. 3,3',5-Triiodothyronine in its physiological concentration significantly enhances the response in glucokinase mRNA at the nuclear level, while glucocorticoids in their physiological concentration predominantly stabilize the translatable glucokinase mRNA.

Mechanisms of hormonal regulation of hepatic glucose metabolism

Acute hormonal regulation of liver carbohydrate metabolism mainly involves changes in the cytosolic levels of cAMP and Ca2+. Epinephrine, acting through beta 2-adrenergic receptors, and glucagon activate adenylate cyclase in the liver plasma membrane through a mechanism involving a guanine nucleotide-binding protein that is stimulatory to the enzyme. The resulting accumulation of cAMP leads to activation of cAMP-dependent protein kinase, which, in turn, phosphorylates many intracellular enzymes involved in the regulation of glycogen metabolism, gluconeogenesis, and glycolysis. These are (1) phosphorylase b kinase, which is activated and, in turn, phosphorylates and activates phosphorylase, the rate-limiting enzyme for glycogen breakdown; (2) glycogen synthase, which is inactivated and is rate-controlling for glycogen synthesis; (3) pyruvate kinase, which is inactivated and is an important regulatory enzyme for glycolysis; and (4) the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase bifunctional enzyme, phosphorylation of which leads to decreased formation of fructose 2,6-P2, which is an activator of 6-phosphofructo-1-kinase and an inhibitor of fructose 1,6-bisphosphatase, both of which are important regulatory enzymes for glycolysis and gluconeogenesis. In addition to rapid effects of glucagon and beta-adrenergic agonists to increase hepatic glucose output by stimulating glycogenolysis and gluconeogenesis and inhibiting glycogen synthesis and glycolysis, these agents produce longer-term stimulatory effects on gluconeogenesis through altered synthesis of certain enzymes of gluconeogenesis/glycolysis and amino acid metabolism. For example, P-enolpyruvate carboxykinase is induced through an effect at the level of transcription mediated by cAMP-dependent protein kinase. Tyrosine amino-transferase, serine dehydratase, tryptophan oxygenase, and glucokinase are also regulated by cAMP, in part at the level of specific messenger RNA synthesis. The sympathetic nervous system and its neurohumoral agonists epinephrine and norepinephrine also rapidly alter hepatic glycogen metabolism and gluconeogenesis acting through alpha 1-adrenergic receptors. The primary response to these agonists is the phosphodiesterase-mediated breakdown of the plasma membrane polyphosphoinositide phosphatidylinositol 4,5-P2 to inositol 1,4,5-P3 and 1,2-diacylglycerol. This involves a guanine nucleotide-binding protein that is different from those involved in the regulation of adenylate cyclase. Inositol 1,4,5-P3 acts as an intracellular messenger for Ca2+ mobilization by releasing Ca2+ from the endoplasmic reticulum.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of lactate dehydrogenase gene expression by cAMP-dependent protein kinase subunits

The studies described in this report suggest a rather complex, albeit incomplete, sequence of molecular events that we believe form part of the cascade of reactions through which a series of hormones, via cAMP, regulates the expression of specific gene products. The majority of our own studies relate to cAMP-mediated induction of LDH. Some, if not all, of the molecular steps discussed in this paper may ultimately be recognized as part of a universal mechanism by which cAMP controls gene expression in higher eukaryotes. The idea of a functional role for cAMP-dependent protein kinase subunits in cAMP-mediated gene control has already had experimental support, but our identification of the regulatory subunit RII as a topoisomerase now more firmly points to a complex function for the kinase in regulating gene function at the DNA level. We look forward to the elucidation of the function of those nuclear proteins that serve as substrate for the catalytic subunit of cAMP-dependent protein kinase. Further studies related to the molecular interaction of RII with chromosomal DNA should be a fruitful area for future research.

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

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 mRNA, as determined after administration of cordycepin, was not affected by hypothyroidism or hyperthyroidism (t 1/2 approximately equal to 45 min), but considerably prolonged by the absence of glucocorticoid hormones (t 1/2 less than 80 min). In hepatocytes in culture Bt2cAMP (0.2 mM) provoked an increase in translatable PEPck mRNA within 2 h incubation time. Preincubation with either T3 (0.1 microM) or dexamethasone (0.1 microM) for 4 h significantly enhanced the cAMP response on PEPck mRNA. Addition of both, T3 plus dexamethasone further enhanced this Bt2cAMP-mediated 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.