Changes in the glucose-6-phosphatase complex in hepatomas

Differential role of hepatocyte nuclear factor-1 in the regulation of glucose-6-phosphatase catalytic subunit gene transcription by cAMP in liver- and kidney-derived cell lines

In liver and kidney, the terminal step in gluconeogenesis is catalyzed by glucose-6-phosphatase. To examine the effect of the cAMP signal transduction pathway on transcription of the gene encoding the catalytic subunit of glucose-6-phosphatase (G6Pase), G6Pase-chloramphenicol acetyltransferase (CAT) fusion genes were transiently transfected into either the liver-derived HepG2 or kidney-derived LLC-PK cell line. Co-transfection of an expression vector encoding the catalytic subunit of cAMP-dependent protein kinase (PKA) markedly stimulated G6Pase-CAT fusion gene expression, and mutational analysis of the G6Pase promoter revealed that multiple regions are required for this PKA response in both the HepG2 and LLC-PK cell lines. A sequence in the G6Pase promoter that resembles a cAMP response element is required for the full PKA response in both HepG2 and LLC-PK cells. However, in LLC-PK cells, but not in HepG2 cells, a hepatocyte nuclear factor-1 (HNF-1) binding site was critical for the full induction of G6Pase-CAT expression by PKA. Changing this HNF-1 motif to that for the yeast transcription factor GAL4 reduces the PKA response in LLC-PK cells to the same degree as deleting the HNF-1 site. However, co-transfection of this mutated construct with chimeric proteins comprising the GAL4-DNA binding domain ligated to the coding sequence for HNF-1alpha, HNF-1beta, HNF-3, or HNF-4 completely restored the PKA response. Thus, we hypothesize that, in LLC-PK cells, HNF-1 is acting as an accessory factor to enhance PKA signaling through the cAMP response element by altering G6Pase promoter conformation or accessibility rather than specifically affecting some component of the PKA signal transduction pathway.

19F NMR of 2-deoxy-2-fluoro-D-glucose for tumor diagnosis in mice. An NDP-bound hexose analog as a new NMR target for imaging

A well-known radiopharmaceutical 2-deoxy-2-fluoro-D-glucose widely used for positron emission tomography diagnosis in terms of glucose utilization, was re-evaluated here as a nuclear magnetic resonance pharmaceutical for cancer detection. The uptake and metabolism of FDG in the experimental tumor, MH134, transplanted to the peritoneum of C3H mice as an ascitic tumor was studied extensively by ex vivo 19F NMR. Prolonged retention of FDG and its metabolites over 2 days was confirmed in the tumor cells as well as in the heart. In these tissues, the 6-phosphate of the injected compound was converted reversibly to its epimer 2-deoxy-2-fluoro-D-mannose and further to their NDP bound forms. The metabolites were almost cleared within a day from the other healthy organs where the formation of NDP-2-deoxy-2-fluoro-D-mannose was low. Thus, the 19F NMR signal of NDP-FDM detected 1 day after the FDG injection could be used as a target signal for tumor detection. Through the use of in vivo 19F NMR spectra and 19F chemical shift images, the feasibility of this proposal was demonstrated. It was concluded that FDG-NMR has a potential for tumor diagnosis in animals.

Histone II-A stimulates glucose-6-phosphatase and reveals mannose-6-phosphatase activities without permeabilization of liver microsomes

The effect of histone II-A on glucose-6-phosphatase and mannose-6-phosphatase activities was investigated in relation to microsomal membrane permeability. It was found that glucose-6-phosphatase activity in histone II-A-pretreated liver microsomes was stimulated to the same extent as in detergent-permeabilized microsomes, and that the substrate specificity of the enzyme for glucose 6-phosphate was lost in histone II-A-pretreated microsomes, as [U-14C]glucose-6-phosphate hydrolysis was inhibited by mannose 6-phosphate and [U-14C]mannose 6-phosphate hydrolysis was increased. The accumulation of [U-14C]glucose from [U-14C]glucose 6-phosphate into untreated microsomes was completely abolished in detergent-treated vesicles, but was increased in histone II-A-treated microsomes, accounting for the increased glucose-6-phosphatase activity, and demonstrating that the microsomal membrane was still intact. The stimulation of glucose-6-phosphatase and mannose-6-phosphatase activities by histone II-A was found to be reversed by EGTA. It is concluded that the effects of histone II-A on glucose-6-phosphatase and mannose-6-phosphatase are not caused by the permeabilization of the microsomal membrane. The measurement of mannose-6-phosphatase latency to evaluate the intactness of the vesicles is therefore inappropriate.