Molecular cloning and tissue-specific expression of mouse kidney 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

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

Fru-2,6-P2 (fructose 2,6-bisphosphate) is a signal molecule that controls glycolysis. Since its discovery more than 20 years ago, inroads have been made towards the understanding of the structure-function relationships in PFK-2 (6-phosphofructo-2-kinase)/FBPase-2 (fructose-2,6-bisphosphatase), the homodimeric bifunctional enzyme that catalyses the synthesis and degradation of Fru-2,6-P2. The FBPase-2 domain of the enzyme subunit bears sequence, mechanistic and structural similarity to the histidine phosphatase family of enzymes. The PFK-2 domain was originally thought to resemble bacterial PFK-1 (6-phosphofructo-1-kinase), but this proved not to be correct. Molecular modelling of the PFK-2 domain revealed that, instead, it has the same fold as adenylate kinase. This was confirmed by X-ray crystallography. A PFK-2/FBPase-2 sequence in the genome of one prokaryote, the proteobacterium Desulfovibrio desulfuricans, could be the result of horizontal gene transfer from a eukaryote distantly related to all other organisms, possibly a protist. This, together with the presence of PFK-2/FBPase-2 genes in trypanosomatids (albeit with possibly only one of the domains active), indicates that fusion of genes initially coding for separate PFK-2 and FBPase-2 domains might have occurred early in evolution. In the enzyme homodimer, the PFK-2 domains come together in a head-to-head like fashion, whereas the FBPase-2 domains can function as monomers. There are four PFK-2/FBPase-2 isoenzymes in mammals, each coded by a different gene that expresses several isoforms of each isoenzyme. In these genes, regulatory sequences have been identified which account for their long-term control by hormones and tissue-specific transcription factors. One of these, HNF-6 (hepatocyte nuclear factor-6), was discovered in this way. As to short-term control, the liver isoenzyme is phosphorylated at the N-terminus, adjacent to the PFK-2 domain, by PKA (cAMP-dependent protein kinase), leading to PFK-2 inactivation and FBPase-2 activation. In contrast, the heart isoenzyme is phosphorylated at the C-terminus by several protein kinases in different signalling pathways, resulting in PFK-2 activation.

Hypoxic regulation of the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene family (PFKFB-1-4) expression in vivo

When oxygen becomes limiting, cells shift primarily to a glycolytic mode for generation of energy. A key regulator of glycolytic flux is fructose-2,6-bisphosphate (F-2,6-BP), a potent allosteric regulator of 6-phosphofructo-1-kinase (PFK-1). The levels of F-2,6-BP are maintained by a family of bifunctional enzymes, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFKFB or PFK-2), which have both kinase and phosphatase activities. Each member of the enzyme family is characterized by their phosphatase:kinase activity ratio (K:B) and their tissue-specific expression. Previous work demonstrated that one of the PFK-2 isozyme genes, PFKFB-3, was induced by hypoxia through the hypoxia-inducible factor-1 (HIF-1) pathway. In this study we examined the basal and hypoxic expression of three members of this family in different organs of mice. Our findings indicate that all four isozymes (PFKFB-1-4) are responsive to hypoxia in vivo. However, their basal level of expression and hypoxia responsiveness varies in the different organs studied. Particularly, PFKFB-1 is highly expressed in liver, heart and skeletal muscle, with the highest response to hypoxia found in the testis. PFKFB-2 is mainly expressed in the lungs, brain and heart. However, the highest hypoxia responses are found only in liver and testis. PFKFB-3 has a variable low basal level of expression in all organs, except skeletal muscle, where it is highly expressed. Most importantly, its hypoxia responsiveness is the most ample of all three genes, being strongly induced in the lungs, liver, kidney, brain, heart and testis. Further studies showed that PFKFB-1 and PFKFB-2 were highly responsive to hypoxia mimics such as transition metals, iron chelators and inhibitors of HIF hydroxylases, suggesting that the hypoxia responsiveness of these genes is also regulated by HIF proteins. In summary, our data demonstrate that PFK-2 genes are responsive to hypoxia in vivo, indicating a physiological role in the adaptation of the organism to environmental or localized hypoxia/ischemia.

Characterization of glucokinase-binding protein epitopes by a phage-displayed peptide library. Identification of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase as a novel interaction partner

The low affinity glucose-phosphorylating enzyme glucokinase shows the phenomenon of intracellular translocation in beta cells of the pancreas and the liver. To identify potential binding partners of glucokinase by a systematic strategy, human beta cell glucokinase was screened by a 12-mer random peptide library displayed by the M13 phage. This panning procedure revealed two consensus motifs with a high binding affinity for glucokinase. The first consensus motif, LSAXXVAG, corresponded to the glucokinase regulatory protein of the liver. The second consensus motif, SLKVWT, showed a complete homology to the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2), which acts as a key regulator of glucose metabolism. Through yeast two-hybrid analysis it became evident that the binding of glucokinase to PFK-2/FBPase-2 is conferred by the bisphosphatase domain, whereas the kinase domain is responsible for dimerization. 5'-Rapid amplification of cDNA ends analysis and Northern blot analysis revealed that rat pancreatic islets express the brain isoform of PFK-2/FBPase-2. A minor portion of the islet PFK-2/FBPase-2 cDNA clones comprised a novel splice variant with 8 additional amino acids in the kinase domain. The binding of the islet/brain PFK-2/ FBPase-2 isoform to glucokinase was comparable with that of the liver isoform. The interaction between glucokinase and PFK-2/FBPase-2 may provide the rationale for recent observations of a fructose-2,6-bisphosphate level-dependent partial channeling of glycolytic intermediates between glucokinase and glycolytic enzymes. In pancreatic beta cells this interaction may have a regulatory function for the metabolic stimulus-secretion coupling. Changes in fructose-2,6-bisphosphate levels and modulation of PFK-2/FBPase-2 activities may participate in the physiological regulation of glucokinase-mediated glucose-induced insulin secretion.

Molecular cloning of a cDNA encoding 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase from liver of Sparus aurata: nutritional regulation of enzyme expression

A cDNA clone encoding full-length 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF-2-K/Fru-2, 6-P2ase) was isolated and sequenced from a Sparus aurata liver cDNA library. The 2527 bp nucleotide sequence of the cDNA contains a 73 bp 5'-untranslated region (5'-UTR), an open reading frame that encodes a 469 amino acid protein and 1041 bp at the 3'-UTR. The deduced amino acid sequence is the first inferred 6PF-2-K/Fru-2, 6-P2ase in fish. The kinase and bisphosphatase domains, where the residues described as crucial for the mechanism of reaction of the bifunctional enzyme are located, present a high degree of homology with other liver isoenzymes. However, within the first 30 amino acids at the N-terminal regulatory domain of the fish enzyme a low homology is found. Nutritional regulation of the 6-phosphofructo-2-kinase activity, together with immunodetectable protein and mRNA levels of 6PF-2-K/Fru-2,6-P2ase, was observed after starvation and refeeding. In contrast to results previously described for rat liver, the decrease in immunodetectable protein and kinase activity caused by starvation was associated in the teleostean fish to a decrease in mRNA levels.

Expression and regulation of 6-phosphofructo-2-kinase/fructose- 2,6-bisphosphatase isozymes in white adipose tissue

The aim of this work was to identify the 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) isozyme(s) present in white adipose tissue. Ion-exchange chromatography of PFK-2 from rat epididymal fat pads yielded an elution pattern compatible with the presence of both the L (liver) and M (muscle) isozymes. This was consistent with a study of the phosphorylation of the purified adipose tissue enzyme by cAMP-dependent protein kinase, by specific labelling of the preparation with [2-32P]fructose 2,6-bisphosphate and by reaction with antibodies. Characterization of the PFK-2/FBPase-2 mRNAs showed that mature adipocytes express the mRNA that codes for the L isozyme and the two mRNAs that code for the M isozyme. Preadipocytes expressed mRNA that codes for the M isozyme. Incubation of rat epididymal fat pads with adrenaline stimulated glycolysis but decreased fructose 2,6-bisphosphate concentrations without significant inactivation of PFK-2. These results support previous findings showing that fructose 2,6-bisphosphate is not involved in the adrenaline-induced stimulation of glycolysis in white adipose tissue.