Hormonal control of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression in rat hepatoma cells

Fructose-2,6-bisphosphate and control of carbohydrate metabolism in eukaryotes

Fructose-2,6-bisphosphate is an important intracellular biofactor in the control of carbohydrate metabolic fluxes in eukaryotes. It is generated from ATP and fructose-6-phosphate by 6-phosphofructo-2-kinase and degraded to fructose-6-phosphate and phosphate ion by fructose-2,6-bisphosphatase. In most organisms these enzymatic activities are contained in a single polypeptide. The reciprocal modulation of the kinase and bisphosphatase activities by post-translational modifications places the level of the biofactor under the control of extra-cellular signals. In general, these signals are generated in response to changing nutritional states, therefore, fructose-2,6-bisphosphate plays a role in the adaptation of organisms, and the tissues within them, to changes in environmental and metabolic states. Although the specific mechanism of fructose-2,6-bisphosphate action varies between species and between tissues, most involve the allosteric activation of 6-phosphofructo-1-kinase and inhibition of fructose-1,6-bisphosphatase. These highly conserved enzymes regulate the fructose-6-phosphate/fructose-1,6-bisphosphate cycle, and thereby, determine the carbon flux. It is by reciprocal modulation of these activities that fructose-2,6-bisphosphate plays a fundamental role in eukaryotic carbohydrate metabolism.

Rat primary hepatocytes and H4 hepatoma cells display differential sensitivity to cyclic AMP at the level of expression of tyrosine aminotransferase

We have shown that the sensitivity of isolated hepatocytes and H4 hepatoma cells to cyclic AMP is different. In terms of activation of tyrosine aminotransferase at mRNA and activity level in response to cyclic AMP, isolated hepatocytes are 10-fold more sensitive. Hepatocytes and H4 hepatoma cells show similar sensitivities to cyclic AMP at the level of protein kinase A activation and phosphorylation of cyclic AMP response element binding protein (CREB) and the differential sensitivity must reside at other sites. The consequences of these findings to studies of control phenomena at the transcriptional level is discussed.

Insulin stimulates cAMP-response element binding protein activity in HepG2 and 3T3-L1 cell lines

Earlier studies from our laboratory demonstrated an insulin-mediated increase in cAMP-response element binding protein (CREB) phosphorylation. In this report, we show that insulin stimulates both CREB phosphorylation and transcriptional activation in HepG2 and 3T3-L1 cell lines, models of insulin-sensitive tissues. Insulin stimulated the phosphorylation of CREB at serine 133, the protein kinase A site, and mutation of serine 133 to alanine blocked the insulin effect. Many of the signaling pathways known to be activated by insulin have been implicated in CREB phosphorylation and activation. The ability of insulin to induce CREB phosphorylation and activity was efficiently blocked by PD98059, a potent inhibitor of mitogen-activated protein kinase kinase (MEK1), but not significantly by rapamycin or wortmannin. Likewise, expression of dominant negative forms of Ras or Raf-1 completely blocked insulin-stimulated CREB transcriptional activity. Finally, we demonstrate an essential role for CREB in insulin activation of fatty-acid synthase and fatty acid binding protein (FABP) indicating the potential physiologic relevance of insulin regulation of CREB. In summary, insulin regulates CREB transcriptional activity in insulin-sensitive tissues via the Raf --> MEK pathway and has an impact on physiologically relevant genes in these cells.

Glucose response elements in a gene that codes for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

We have shown previously that rat hepatoma FTO-2B cells express two mRNAs, called F (fetal) and L (liver), from distinct promoters of the same gene coding for 6-phosphofructo-2-kinase (PFK-2). This enzyme catalyzes the synthesis of fructose 2,6-bisphosphate, an allosteric stimulator of glycolysis. We have now found that glucose, as well as lactate and pyruvate, increases the concentration of the F and L mRNAs. The effect of glucose was mimicked by xylitol, a precursor of xylulose 5-phosphate, and hence of intermediates of the pentose phosphate and glycolytic pathways, and was inhibited by okadaic acid, an inhibitor of protein phosphatases. Transfection experiments showed that the F promoter region is a target of the glucose effect, with glucose stimulating F promoter activity in a way probably similar to mitogens. Another region of the gene, located between the F and L promoters, also behaved as a glucose-sensitive element. This region corresponds to a cluster of DNase I-hypersensitive sites that were induced in chromatin following glucose treatment. The sequence organization of this region is very similar to the functional architecture of the glucose-sensitive insulin gene promoter. We propose a model for the concerted regulation by glucose metabolites of three pathways that lead to increased PFK-2 activity.

Expression of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA during liver regeneration

The expression of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA was studied during liver regeneration by three independent assays: Northern blot analysis, reverse transcription-polymerase chain reaction and ribonuclease protection. We demonstrate the presence of F-type mRNA in foetal and adult rat livers and a transient increase in its levels with a maximum at 12 h after partial liver resection. The time course of F-type mRNA induction differs from that reported for the L-type isoform, suggesting differences in the regulation of the expression of F- and L-type isoforms of the bifunctional enzyme during liver regeneration.

Multiple forms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase are expressed in perinatal rat liver

Fetal rat liver expresses a 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/Fru-2,6-Pase2) form that differs from the adult liver enzyme in the inhibition by phosphorylation by the adenosine 3',5'-cyclic monophosphate-dependent protein kinase and in the recognition by an antibody specific for the NH2-terminal domain of the adult liver enzyme. Northern blot analysis shows that fetal hepatocytes contain a species of mRNA that is 2.2 kb in size and that exhibits the maximal levels after delivery. PFK-2/Fru-2,6-Pase2 mRNA analysis using a sensitive ribonuclease protection assay reveals the presence of nearly similar amounts of adult liver-specific and skeletal muscle-specific mRNA in fetal liver and hepatocytes during the last days of gestation, as well as a 233-bp protected fragment present in fetal liver. These results were confirmed by polymerase chain reaction using specific oligonucleotide pairs. Primer extension of fetal liver cDNA suggests the presence of two initiation sites of transcription. Analysis of the adult liver PFK-2/Fru-2,6-Pase2 protein during the perinatal transition using a specific antibody shows a marked accumulation of this form immediately after birth.

Differential regulation of the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and pyruvate kinase by cyclic adenosine 3',5'-monophosphate in fetal and adult hepatocytes

Incubation of fetal hepatocytes from 21-day-old rats with permeant derivatives of cyclic AMP (cAMP) or glucagon, increased the mRNA levels of 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase (PFK-2/FBPase-2), L-pyruvate kinase (L-PK) and phosphoenolpyruvate carboxykinase (PEPCK). Contrary to this behavior, adult hepatocytes exhibited a decrease in the PFK-2/FBPase-2 and L-PK mRNA levels when incubated under equivalent experimental conditions. Dexamethasone also increased the PFK-2/FBPase-2 mRNA levels and costimulation of fetal hepatocytes with dexamethasone and a permeant analogue of cyclic AMP enhanced the levels of PFK-2/FBPase-2 mRNA, a situation opposite to that exhibited by adult hepatocytes. Treatment of the hepatocytes with transcriptional and translational inhibitors also produced differential responses in both types of cells. The PFK-2/FBPase-2 mRNA in fetal hepatocytes was more stable than in the adult cells. These results suggest that specific transcriptional factors and regulatory pathways differentially operate in fetal and adult hepatocytes in the control of the responses of carbohydrate metabolism to cAMP.

The class I alcohol dehydrogenase gene is glucocorticoid-responsive in the rat hepatoma microcell hybrid cell line, 11-3

Expression of the class I alcohol dehydrogenase (ADH) gene in the rat hepatoma microcell hybrid cell line, 11-3, was examined. The steady-state level of ADH mRNA in 11-3 was approximately 2-fold higher than that or rat liver and Fao, the parental cell line of 11-3. Removal of steroid hormones by activated charcoal from the serum in which 11-3 cells were maintained resulted in a significant decrease in the level of ADH transcript. Dexamethasone at a concentration of 1 muM increased the ADH mRNA content in 11-3 in a time-dependent fashion, up to 48 hr after its addition to cells that had first been deprived of steroid hormones. In addition, levels of ADH transcript in cells treated with dexamethasone increased in a dose-dependent manner, and the concentration of dexamethasone required to achieve half-maximal activation was 5 nM. By using the techniques of reverse transcription and polymerase chain reaction, and by taking advantage of a restriction polymorphism present between the rat and mouse ADH cDNA, we found that 11-3 contained both the rat and mouse class I ADH transcripts, although the rat sequence accounted for the great majority. Moreover, levels of both rat and mouse class I ADH transcripts increased in a similarly time-dependent manner in cells treated with dexamethasone. These results indicate that expression of class I ADH gene in 11-3 is high and is regulated by glucocorticoids, making the cell line an excellent model for the in vitro study of ADH expression.

Glucokinase expression in rat hepatoma cells induces glucose uptake and is rate limiting in glucose utilization

In contrast to hepatocytes, hepatoma cells lack glucokinase activity and show increased aerobic glycolysis. FTO-2B and H4IIE rat hepatoma cell lines were obtained in which the rat glucokinase gene was expressed (FTOGK and H4GK). These lines were generated by infection of the hepatoma cells with a retroviral vector carrying the phosphoenolpyruvate carboxykinase (PEPCK)-glucokinase chimeric gene. Both the FTOGK and H4GK cells expressed the chimeric gene in a regulated manner, like the endogenous PEPCK gene. Glucokinase activity was detected in both FTOGK and H4GK. These cells lines showed a marked increase in glucose uptake with 18.5 mM glucose in the incubation medium. FTOGK and H4GK showed an increase in the content of glucose 6-phosphate, and were able to accumulate high levels of glycogen, in contrast to FTO-2B cells, which were unable to store the polysaccharide. In addition, cells expressing glucokinase showed high concentration of fructose 2,6-bisphosphate and substantial lactate production, which was related to the glucose concentration in the medium and the time of incubation. These results suggest that glucose phosphorylation is rate limiting for glucose uptake and utilization in FTO-2B and H4IIE cells.

Vanadate treatment of diabetic rats reverses the impaired expression of genes involved in hepatic glucose metabolism: effects on glycolytic and gluconeogenic enzymes, and on glucose transporter GLUT2

The trace element vanadium is a potent insulinomimetic agent in vitro. Oral administration of vanadate to rats made diabetic by streptozotocin (45 mg/kg i.v.) caused a 65% fall in plasma glucose levels without modifying low insulinemia. We studied whether the hypoglycemic effect of vanadate was associated with altered expression of genes involved in key steps of hepatic glucose metabolism. Glucokinase (GK) and L-type pyruvate kinase (L-PK) mRNA levels were decreased respectively by 90% and 70% in fed diabetic rats, in close correlation with changes in enzyme activities. Eighteen days of vanadate treatment partially restored GK mRNA and activity (40% of control levels), and totally restored L-PK parameters. In contrast to the glycolytic enzymes, mRNA levels and activity of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK) were increased (15- and 2-fold, respectively) in fed diabetic rats. Vanadate treatment normalized both PEPCK mRNA and activity in diabetic rat liver. The 2-fold increase in liver glucose transporter (GLUT2) mRNA and protein, produced by diabetes, was also corrected by this treatment. In conclusion, oral vanadate given to diabetic rats induces a shift of the predominating gluconeogenic flux, with subsequent high hepatic glucose production, into a glycolytic flux by pretranslational regulatory mechanisms.

Fructose 2,6-bisphosphate and the control of glycolysis by growth factors, tumor promoters and oncogenes

Tumor and proliferating cells maintain a high glycolytic rate even under aerobic conditions. The discovery of fructose 2,6-bisphosphate, a potent stimulator of glycolysis, has prompted a re-investigation of this phenomenon. Rat hepatoma cells and fibroblasts stimulated by mitogens or transformed by the Rous sarcoma virus carrying the v-src oncogene were used as models. The results indicate that in established lines of hepatoma cells the biochemical properties of the bifunctional enzyme, PFK-2/FBPase-2, involved in the synthesis and degradation of fructose 2,6-bisphosphate, differ from those of the enzyme from normal liver. In addition, the stimulation of glycolysis induced by phorbol esters and pp60v-src can be explained by an increase in the concentration of fructose 2,6-bisphosphate and an activation of PFK-2. The mechanism of stimulation involves the transcription of a gene whose product activates PFK-2 or is a distinct PFK-2 isozyme. Finally, mercaptopurines were found to block fructose 2,6-bisphosphate synthesis in vitro and in lymphocytes and lymphoblastic cells. In these cells, this resulted in an inhibition of glycolysis.

Oral administration of vanadate to diabetic rats restores liver 6-phosphofructo-2-kinase content and mRNA

Vanadate and insulin were administered to diabetic (streptozotocin) rats to compare their effects on the activity and mRNA content of 6-phosphofructo-2-kinase and L-type pyruvate kinase in the liver. The activity of 6-phosphofructo-2-kinase in livers of diabetic rats was about 40% of that found in normal rats. A similar decrease was found for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase content, measured by immunoprecipitation, and for mRNA, measured by hybridization of Northern blots. Administration of vanadate to the diabetic rats led to a progressive recovery of 6-phosphofructo-2-kinase activity, and 6-phosphofructo-2-kinase/fructose- 2,6-bisphosphatase content and mRNA. This recovery, which was complete after 15 days of oral treatment, was also obtained after 60 h of insulin administration. L-type pyruvate kinase activity and mRNA were also decreased by about 70% in livers of diabetic rats. Both parameters normalized after 15 days of vanadate treatment, whereas insulin administration (60 h) raised L-pyruvate kinase mRNA three-fold above control values. Oral treatment for 15 days with vanadate can thus mimic the effect of insulin on both pyruvate kinase and 6-phosphofructo-2-kinase/fructose-2,6- bisphosphatase in livers of diabetic rats.