موجز عن البحث:
OBJECTIVE—The induction of hepatic glucose
6-phosphatase (G6pc) by glucose presents a paradox of glucose-induced glucose
intolerance. We tested whether glucose regulation of liver gene expression is
geared toward intracellular homeostasis.
RESEARCH DESIGN AND METHODS—The effect of
glucoseinduced accumulation of phosphorylated intermediates on expression of
glucokinase (Gck) and its regulator Gckr was determined in hepatocytes. Cell
ATP and uric acid production were measured as indices of cell phosphate
RESULTS—Accumulation of phosphorylated
intermediates in hepatocytes incubated at elevated glucose induced rapid and
inverse changes in Gck (repression) and Gckr (induction) mRNA concomitantly
with induction of G6pc, but had slower effects on the Gckr-to-Gck protein
ratio. Dynamic metabolic labeling in mice and liver proteome analysis
confirmed that Gckr and Gck are low-turnover proteins. Involvement of Max-like
protein X in glucose-mediated Gck-repression was confirmed by chromatin
immunoprecipitation analysis. Elevation of the Gck-to-Gckr ratio in
hepatocytes was associated with glucose-dependent ATP depletion and elevated
urate production confirming compromised phosphate homeostasis.
CONCLUSIONS—The lowering by glucose of the
Gck-to-Gckr ratio provides a potential explanation for the impaired hepatic
glucose uptake in diabetes. Elevated uric acid production at an elevated
Gck-to-Gckr ratio supports a role for glucose regulation of gene expression
in hepatic phosphate homeostasis.
موجز عن البحث:
In the liver, a high glucose
concentration activates transcription of genes encoding glucose 6-phosphatase
and enzymes for glycolysis and lipogenesis by elevation in phosphorylated
intermediates and recruitment of the transcription factor ChREBP (carbohydrate
response element binding protein) and its partner, Mlx, to gene promoters. A
proposed function for this mechanism is intracellular phosphate homeostasis.
In extrahepatic tissues, MondoA, the paralog of ChREBP, partners with Mlx in
transcriptional induction by glucose. We tested for glucose induction of
regulatory proteins of the glycogenic pathway in hepatocytes and identified
the glycogen-targeting proteins, GL and PTG (protein targeting to glycogen),
as being encoded by Mlx-dependent glucose- inducible genes. PTG induction by
glucose was MondoA dependent but ChREBP independent and was enhanced by
forced elevation of fructose 2,6-bisphosphate and by additional
xylitol-derived metabolites. It was counteracted by selective depletion of
fructose 2,6-bisphosphate with a bisphosphatase-active kinase-deficient
variant of phosphofructokinase 2/fructosebisphosphatase 2, which prevented
translocation of MondoA to the nucleus and recruitment to the PTG promoter.
We identify a novel role for MondoA in the liver and demonstrate that
elevated fructose 2,6-bisphosphate is essential for recruitment of MondoA to
the PTG promoter. Phosphometabolite activation of MondoA and ChREBP and their
recruitment to target genes is consistent with a mechanism for gene
regulation to maintain intracellular phosphate homeostasis.