Glucocorticoid Regulation of Hepatic 6-Phosphofructo-2-kinase/Fructose-2,6-bisphosphatase Gene Expression

Immediate early genes of glucocorticoid action on the developing intestine

Prior studies have demonstrated that glucocorticoid hormones elicit functional maturation of the small intestine as evidenced by their ability to induce increases in the expression of various digestive hydrolases, such as sucrase-isomaltase and trehalase. However, these increases have a lag time of approximately 24 h, suggesting that they are secondary effects of hormone action. To identify candidate primary response genes, we performed microarray analysis on pooled RNA from jejunums of untreated postnatal day 8 mouse pups and from littermates who earlier received dexamethasone 2 h. Fluorescent dye-labeled samples were hybridized in quadruplicate to glass-spotted cDNA microarrays containing 15,000 cDNA clones from the National Institute of Aging cDNA clone set. Analysis of the resulting signals using relatively stringent criteria identified 66 transcripts upregulated and 36 downregulated by 2 h of glucocorticoid treatment. Among the upregulated transcripts, the magnitude of the increase detected by microarray ranged from 1.4- to 16-fold. Selected mRNAs from throughout the range were subsequently analyzed by Northern blot analysis. Of 11 mRNAs chosen all were confirmed, and there was a strong correlation between the magnitude of the increase observed from the microarray analysis and from Northern blot analysis. Additional time points showed that these transcripts peaked between 2 and 6 h and had returned to baseline by 24 h. Gene ontology analysis showed pleiotropic effects of dexamethasone on the developing intestine and pointed to genes in the development category as being likely candidates for mediation of glucocorticoid-induced maturation of intestinal function.

6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: suiting structure to need, in a family of tissue-specific enzymes

The present review addresses recent advances in research into a family of bifunctional enzymes that are responsible for the twofold task of synthesizing and hydrolyzing fructose-2,6-bisphosphate (Fru-2,6-P2), which in turn regulates the rate of glycolysis in most cells. The structure of the synthetic kinase, conjoined at its carboxyl-terminus to the phosphatase, is very highly conserved throughout evolution and differentiation, with isotypic expression arising from highly variable amino-terminal and carboxyl-terminal regulatory domains. These domains, which frequently contain protein-kinase-catalyzed phosphorylation motifs, are responsible for the widely divergent kinetics observed in various tissues and species, and for the hormonal modulation that alters intracellular levels of Fru-2,6-P2. The present review discusses recent advances in relating structure to function, and the identification of new pathways of transcriptional regulation of this important family of regulatory enzymes.

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.

In vivo protein-DNA interactions on a glucocorticoid response unit of a liver-specific gene: hormone-induced transcription factor binding to constitutively open chromatin

Transcription from the liver promoter of a 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) gene depends on the presence of glucocorticoids that act via a glucocorticoid response unit (GRU) located in the first intron. The promoter and the GRU are in a constitutively open chromatin configuration. To determine how glucocorticoids would affect factor binding to the GRU in absence of chromatin remodeling, we have used a combination of in vitro DNA-binding assays and in vivo genomic footprinting in rat hepatocytes and hepatoma cells. We found that, in the absence of glucocorticoids, the GRU binds nuclear factor-I (NF-I). Glucocorticoid treatment modified factor binding to the NF-I site and induced the binding of hepatocyte nuclear factor-3 (HNF-3). Transfection assays showed that HNF-3 cooperates with the glucocorticoid receptor in stimulating transcription. In contrast with the lack of effect of glucocorticoids on factor binding to constitutively open GRUs of other genes, HNF-3 binding to the open PFK-2 GRU was hormone-dependent. Therefore, the PFK-2 GRU behaves as a novel type of GRU.

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.

Inositol lipid phosphorylation and breakdown in rat liver nuclei is affected by hydrocortisone blood levels

The possibility that inositol lipid metabolism is related to nuclear events accompanying steroid hormone action has been investigated by comparing lipid phosphorylation and breakdown in normal rat liver nuclei and in hypo- and hypercortisolemic conditions. Lipid phosphorylation in vitro showed the presence of diacylglycerol (DAG)-, phosphatidylinositol (PI)- and phosphatidylinositol-4-phosphate (PIP)-kinase activity, with differences between total tissue homogenates and isolated nuclei, relevant to the treatment in vivo. Administration of hydrocortisone (HC) produced a marked decrease in the phosphorylated nuclear products without influencing the homogenate kinase activity. Under conditions which were optimal for the kinase activities, nuclear PIP-kinase was strongly increased in presence of a high blood level of HC whereas PI-kinase activity was reduced. From these observations it appears that the observed differences were due to specific modulation of kinase activities rather than to changes in the availability of substrates. The phosphoinositide-specific phospholipase C (PLC) activity was also investigated. In the presence of a high HC blood level, the phosphodiesteratic cleavage of PIP strongly increased, while that of phosphatidylinositol bisphosphate (PIP2) was similar in normal and hypercortisolemic conditions. Nuclear phosphoinositide hydrolysis was affected by PLC, beta and gamma isoforms, which were equally represented in all the conditions investigated, indicating that the observed changes of activity were due to a modulation rather than to a change in the amount of enzyme. These results suggest that inositol lipid metabolism plays a role in the nuclear modifications accompanying steroid hormone induction of transcriptional activity.

Insulin inhibits glucocorticoid-induced stimulation of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene transcription

6-Phosphofructo-2-kinase (PFK-2)/fructose-2,6-bisphosphatase (FBPase-2) catalyzes the synthesis and degradation of fructose 2,6-bisphosphate, a potent stimulator of glycolysis. Transcription of the mRNA encoding rat liver PFK-2/FBPase-2 is stimulated by insulin and by glucocorticoids in rat hepatoma cells. We show here that insulin can also prevent and reverse this glucocorticoid effect. The inhibitory effect of insulin is independent of extracellular glucose and does not require ongoing protein synthesis. We conclude that insulin exerts opposite effects on PFK-2/FBPase-2 gene transcription depending on the hormonal context.

Liver-specific DNase I-hypersensitive sites and DNA methylation pattern in the promoter region of a 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene

The mRNA for the liver isozyme of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is transcribed from the L promoter of gene A. We show here that L-promoter activity is tissue specific. To identify on the gene in situ potential cis-acting sequences, we have examined 15 kb of its 5' region for DNase I-hypersensitive sites detectable on chromatin. We have also evaluated the DNA methylation status of the 3.7-kb encompassing the L promoter. Five DNase I-hypersensitive sites were detected on liver chromatin, three upstream (M1 at position -4500, L2 at position -1000, L1 at position -200) and two downstream (I1 at position +3000, I2 at position +3500) from the L-type mRNA transcription initiation site. Their presence correlated with transcriptional activity as they were not observed on chromatin from kidney, a tissue where gene A is not expressed. Sites M1 and L1 corresponded to the M and L promoters, respectively, providing in vivo evidence for a promoter localization obtained earlier with cloned DNA only. Site I2 coincided with a glucocorticoid-responsive unit described by others, but its presence did not depend on glucocorticoids. Thus, sites L2 and I1 could correspond to novel control elements. While DNA was methylated around position -2000 both in liver and kidney, downstream from that position it was fully demethylated in liver but not in kidney. This pattern changed during development of fetal liver. The data suggest mechanisms for the lack of activity of the L promoter in kidney and for its activation in developing and adult liver.

Multihormonal regulation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression in primary cultures of rat hepatocytes

In the absence of hormone, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA rapidly declined to undetectable levels in primary cultures of rat hepatocytes. Dexamethasone increased mRNA levels and this effect was blocked by actinomycin D or cycloheximide. Insulin or thyroxine had no effect but potentiated the dexamethasone induction. Dibutyryl cyclic AMP decreased the stimulatory effect of dexamethasone at early times but after 24 hours the cyclic nucleotide increased mRNA levels compared with dexamethasone alone. It was concluded that expression of the bifunctional enzyme gene in hepatocytes is under a complex multihormonal control. Glucocorticoids are the most important transcriptional factor and their effect requires ongoing protein synthesis.

Sequence of the 5'-flanking region of the rat 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase gene: regulation by glucocorticoids

Dexamethasone addition to cultured hepatocytes caused a 90-fold increase in mRNA for 6-phosphofructo 2-kinase/fructose-2,6-bisphosphatase. Glucocorticoid administration in vivo also increased the enzyme's mRNA in skeletal muscle by 3-4-fold. The sequence of the 5'-flanking region of the enzyme's gene revealed at least one consensus glucocorticoid response element. The amino acid sequence derived from a partial cDNA clone for the rat skeletal muscle bifunctional enzyme was identical to that of the liver isozyme except for an undetermined amount of N-terminal sequence. It is concluded that the rat muscle and liver isozymes, which are postulated to be identical except for the N-terminal region, are both regulated by glucocorticoids.