Elevated glucose represses liver glucokinase and induces its regulatory protein to safeguard hepatic phosphate homeostasis.
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.


METHODS
The effect of glucose-induced 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 homeostasis.


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.
Institute of Cellular Medicine, Newcastle University, Newcastle upon Tyne, U.K.
Catherine
C
Arden
John L
JL
Petrie
Susan J
SJ
Tudhope
Ziad
Z
Al-Oanzi
Amy J
AJ
Claydon
Robert J
RJ
Beynon
Howard C
HC
Towle
Loranne
L
Agius
D000818:Animals; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors / Q000235:genetics / Q000378:metabolism; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002478:Cells, Cultured; D047369:Chromatin Immunoprecipitation; D005941:Glucokinase / Q000235:genetics / Q000378:metabolism*; D005947:Glucose / Q000494:pharmacology*; D005952:Glucose-6-Phosphatase / Q000235:genetics / Q000378:metabolism*; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism; D006706:Homeostasis / Q000187:drug effects / Q000235:genetics; D047908:Intracellular Signaling Peptides and Proteins; D008099:Liver / Q000187:drug effects* / Q000378:metabolism*; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D051381:Rats; D017208:Rats, Wistar; D060888:Real-Time Polymerase Chain Reaction
D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D002352:Carrier Proteins; C499484:Gckr protein, rat; D047908:Intracellular Signaling Peptides and Proteins; D005941:Glucokinase; D005952:Glucose-6-Phosphatase; D005947:Glucose