Elevated Glucose Represses Liver Glucokinase and Induces Its Regulatory Protein to Safeguard Hepatic Phosphate Homeostasis

Abstract

OBJECTIVEThe 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 METHODSThe 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.RESULTSAccumulation 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.CONCLUSIONSThe 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.

Dietary carbohydrate and control of hepatic gene expression: mechanistic links from ATP and phosphate ester homeostasis to the carbohydrate-response element-binding protein

“…The proposed metabolites include: G6P (85,86) , xylulose 5-P (8) , the regulatory metabolite F2,6P 2 ( 43,87) , ketone bodies (88) and the product of the hexosamine pathway UDP-N-acetylglucosamine (79,80) which is the substrate for O-GlcNAc-modification of proteins. The induction of ChREBP target genes by high glucose is abolished by glucokinase inhibitors and markedly enhanced by inhibitors of G6P hydrolysis (42,32,53) . These inhibitors abolish and enhance, respectively, the elevation in PE pool in hepatocytes comprising G6P and downstream metabolites (42,32,53) .…”

“…The induction of ChREBP target genes by high glucose is abolished by glucokinase inhibitors and markedly enhanced by inhibitors of G6P hydrolysis (42,32,53) . These inhibitors abolish and enhance, respectively, the elevation in PE pool in hepatocytes comprising G6P and downstream metabolites (42,32,53) . This rules out an effect of glucose and points to mechanism(s) consequent to PE elevation.…”

“…G6pc) also influence the elevation of G6P in response to glucose, as shown by overexpression of G6pc which lowers the G6P elevation by high glucose (52) and by inhibitors of G6P hydrolysis, such as chlorogenic derivatives that target the G6P transporter, which markedly enhance the elevation in G6P (42,53) . The induction by high glucose via Mlx-ChREBP of G6pc and Gckr (41) therefore has a restraining effect on the elevation in G6P (42,43) . …”

“…Patients with glycogen storage disease type 1, who have gene defects in G6pc or the G6P transporter, respond to glucagon challenge by acute depletion of Pi and ATP and elevated uric acid production (68) , because Pi incorporation into PE during activation of glycogen phosphorylase is not balanced by Pi regeneration by G6pc. The induction of G6pc and the G6P transporter (SLC37A4) by high glucose can be rationalised as an adaptive mechanism to maintain Pi and ATP homeostasis (41,42) . The induction of Gckr (and the decrease in glucokinase:Gckr ratio) by glucose can also be rationalised as an adaptive response for ATP homeostasis, as shown by hepatocyte studies where an experimentally induced elevation in the glucokinase:Gckr ratio leads to compromised ATP homeostasis and increased uric acid production in response to high glucose challenge (42) .…”

“…The induction of G6pc and the G6P transporter (SLC37A4) by high glucose can be rationalised as an adaptive mechanism to maintain Pi and ATP homeostasis (41,42) . The induction of Gckr (and the decrease in glucokinase:Gckr ratio) by glucose can also be rationalised as an adaptive response for ATP homeostasis, as shown by hepatocyte studies where an experimentally induced elevation in the glucokinase:Gckr ratio leads to compromised ATP homeostasis and increased uric acid production in response to high glucose challenge (42) .…”