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Induction of glycation suppresses glucokinase gene expression in HIT-T15 cells
Y Kajimoto1, T Matsuoka, H Kaneto
1Department of Internal Medicine and Therapeutics, Osaka University Graduate School of Medicine, Suita, Japan.
Aims/Hypothesis:
Chronic hyperglycaemia in patients with Type II (non-insulin-dependent) diabetes mellitus often leads to a decline in glucose-responsive insulin secretion from pancreatic beta cells, a phenomenon called glucose toxicity. Upon hyperglycaemia, glycation reaction occurs in the beta cells and induces oxidative stress. To understand the molecular basis of the beta-cell glucose toxicity, we investigated the possible effects of glycation on the expression and enzymatic activity of glucokinase, which plays a crucial part in glucose-responsive insulin secretion.
Methods:
Glycation and reactive oxygen species were induced in HIT-T15 cells by treatment with D-ribose and effects on glucokinase gene transcription, glucokinase protein amount, glucose phosphorylation activity, and DNA-binding activities of putative glucokinase gene transcription factors were evaluated.
Results:
When glycation was induced in HIT-T15 cells, the activity of the human glucokinase gene beta-cell-type promoter was suppressed substantially (83% reduction at 60 mmol/l D-ribose). Also, similar reductions in mRNA and protein amounts of glucokinase and in the Vmax of its enzymatic activity were observed. In agreement with the reduction in the promoter activity, the two major transcription factors of the glucokinase gene, the Pal-binding factor and PDX-1, reduced their binding to their target sequences in the glucokinase gene promoter in glycation-induced HIT cells. Because these effects of D-ribose were counteracted by aminoguanidine or N-acetylcysteine, reactive oxygen species, generated by the glycation reaction, appears to be involved in the phenomena.
Conclusion/Interpretation:
The induction of the glycation reaction, which is known to occur in pancreatic beta cells in chronic hyperglycaemia, suppresses the glucokinase gene transcription and its enzymatic activity. Thus, hyperglycaemia-dependent inhibition of glucokinase activity could in part explain beta-cell glucose toxicity.
Insights
Chronic hyperglycemia in type 2 diabetes impairs pancreatic beta cells by reducing glucokinase activity. This study shows glycation suppresses glucokinase gene expression and function, contributing to glucose toxicity.
Area of Science:
- Endocrinology
- Molecular Biology
- Diabetes Research
Background:
- Chronic hyperglycemia in type 2 diabetes leads to pancreatic beta-cell dysfunction, known as glucose toxicity.
- Glycation and oxidative stress within beta cells are implicated in this decline of glucose-responsive insulin secretion.
Purpose of the Study:
- To investigate the molecular mechanisms by which glycation affects glucokinase (GCK) expression and enzymatic activity.
- To understand GCK's role in beta-cell glucose toxicity during hyperglycemia.
Main Methods:
- HIT-T15 cells were treated with D-ribose to induce glycation and reactive oxygen species.
- Evaluated effects on GCK gene transcription, protein levels, glucose phosphorylation activity, and transcription factor binding.
Main Results:
- Glycation significantly suppressed human GCK gene promoter activity (83% reduction at 60 mmol/l D-ribose).
- Observed parallel reductions in GCK mRNA, protein, and enzymatic activity (Vmax).
- Glycation decreased binding of transcription factors Pal-binding factor and PDX-1 to the GCK promoter, an effect counteracted by antioxidants.
Conclusions:
- Glycation suppresses GCK gene transcription and enzymatic activity in pancreatic beta cells.
- Hyperglycemia-induced inhibition of GCK activity may partially explain beta-cell glucose toxicity in type 2 diabetes.