Mammalian 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: a bifunctional enzyme that controls glycolysis

Cell-type specificity of inhibition of glycolysis by 5-amino-4-imidazolecarboxamide riboside. Lack of effect in rabbit cardiomyocytes and human erythrocytes, and inhibition in FTO-2B rat hepatoma cells

The nucleoside AICAriboside (5-amino-4-imidazolecarboxamide riboside) has been shown to inhibit glycolysis in isolated rat hepatocytes [Vincent, Bontemps and Van den Berghe (1992) Biochem. J. 281, 267-272]. The effect is mediated by AICA-ribotide (ZMP), the product of the phosphorylation of AICA-riboside by adenosine kinase. To assess the cell-type specificity of the effect, studies were conducted in rabbit cardiomyocytes, human erythrocytes and rat hepatoma FTO-2B cells. AICA-riboside had no effect on glycolysis in cardiomyocytes, and a slight stimulatory effect in erythrocytes, but inhibited glycolysis by 65% at 250 microM concentration in FTO-2B cells, although only when tissue-culture medium was replaced by Krebs-Ringer bicarbonate buffer. At 500 microM AICAriboside, ZMP remained undetectable in cardiomyocytes, but reached 0.65 mM in erythrocytes and 5 mM in FTO-2B cells. In the latter, AICAriboside provoked up to 2-fold elevations of glucose 6-phosphate and fructose 6-phosphate, accompanied by a decrease in fructose 1,6-bisphosphate. This indicated inhibition of 6-phosphofructo-1-kinase (PFK-1). Accordingly, in FTO-2B cell-free extracts, the activity of PFK-1, measured under physiological conditions, was inhibited by approx. 70% by 5 mM ZMP. ZMP had a less pronounced effect on the activity of PFK-1 in normal rat liver; it did not influence the activity of PFK-1 in rat muscle, rabbit heart and human erythrocytes. It is concluded that the inhibitory effect of AICAriboside on glycolysis is dependent on both (1) the capacity of the cells to accumulate ZMP and (2) the presence of target enzymes which are sensitive to ZMP.

Transcriptional control of genes that regulate glycolysis and gluconeogenesis in adult liver

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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.

Cloning and expression of novel isoforms of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from bovine heart

Distinct 6-phosphofructo-2-kinase (PFK-2)/fructose 2,6-bisphosphatase (FBPase-2) cDNAs were cloned from bovine heart, showing that PFK-2/FBPase-2 gene B, which contains 16 exons, codes for at least five mRNAs. Three of them (B1, B2, B4) could encode the 58,000-M(r) isozyme. In B2 mRNA, exon 15 encodes four more residues than in B1. In B4 mRNA, exon 15 encodes six more residues than in B1, but exon 16 (20 residues) is missing. B3 mRNA corresponds to the 54,000-M(r) isozyme. It lacks exon 15 and also differs from the other mRNAs in the 5' noncoding region. B5 mRNA encodes a truncated form. When expressed in E. coli, the recombinant isoforms corresponding to all these mRNAs except B5 exhibited PFK-2 activity.

Characterization of a hepatoma mRNA transcribed from a third promoter of a 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-encoding gene and controlled by ets oncogene-related products

6-Phosphofructo-2-kinase (EC 2.7.1.105)/fructose-2,6-bis-phosphatase (EC 3.1.3.46) catalyzes the synthesis and degradation of fructose 2,6-bisphosphate, a ubiquitous stimulator of glycolysis. The liver (L-type) and muscle (M-type) mRNAs for this bifunctional enzyme arise from distinct promoters of the same gene. We have now characterized in rat hepatoma FTO2B cells another mRNA, which is transcribed from a third promoter of that gene. This F-type mRNA is present in fetal rat liver and muscle, in rat placenta, and in several established rat cell lines. The F promoter contains no TATA box but contains several binding sites for Sp1 and for members of the ets oncogene family. Transfection of FTO2B cells with constructs containing the intact or mutagenized F promoter showed that its activity depends mainly on one of these sites. This site bound a heteromeric FTO2B cell protein indistinguishable from the ets-related GA binding protein alpha/ankyrin-repeats GA binding protein beta transcription factor.

Glycolysis revisited

Glycolysis is usually considered as a paradigm metabolic pathway, due to the fact that it is present in most organisms, and also because it is the pathway by which an important nutrient, glucose, is consumed. Far from being completely understood, the regulation of this pathway witnessed several important progresses during the last few years. One of these is the discovery of fructose 2,6-bisphosphate, a potent stimulator of phosphofructokinase and inhibitor of fructose-1,6-bisphosphatase. Originally found in the liver during the course of a study on the mechanism by which glucagon acts on gluconeogenesis, this compound is now recognized as a major element in the control of glycolysis and/or gluconeogenesis in many cell types and in various organisms. The other finding is that of a regulatory protein that modulates the activity of glucokinase, the enzyme that phosphorylates glucose in the liver and in the beta cells of pancreatic islets.

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.