Due to their high glucose permeability, insulin secreting pancreatic β-cells likely undergo strong intracellular protein glycation at high glucose concentrations. They may, however, be partly protected from the glucotoxic alterations of their survival and function by fructosamine-3-kinase (FN3K), a ubiquitous enzyme that initiates deglycation of intracellular proteins. To test that hypothesis, we cultured pancreatic islets from Fn3k knockout (Fn3k-/-) mice and their wild-type (WT) littermates for one to three weeks in the presence of 10 or 30 mmol/l glucose (G10-G30) and measured protein glycation, apoptosis, preproinsulin gene expression, and Ca2+ and insulin secretory responses to acute glucose stimulation. The more potent glycating agent D-ribose (25 mmol/l) was used as positive control for protein glycation. In WT islets, a one-week culture in G30 significantly increased the amount of soluble intracellular protein-bound fructose-ε-lysines and the glucose sensitivity of β-cells for changes in Ca2+ and insulin secretion while decreasing the islet insulin content. After three weeks, culture in G30 also strongly decreased β-cell glucose responsiveness and preproinsulin mRNA levels while increasing islet cell apoptosis. Although protein-bound fructose-ε-lysines were more abundant in Fn3k-/- vs. WT islets, islet cell survival and function and their glucotoxic alterations were almost identical in both types of islets, except for a lower level of apoptosis in Fn3k-/- islets cultured for three weeks in G30. In comparison, D-ribose (one week) similarly decreased preproinsulin expression and β-cell glucose responsiveness in both types of islets while increasing apoptosis to a larger extent in Fn3k-/- vs. WT islets. We conclude that, despite its ability to reduce the glycation of intracellular islet proteins, FN3K is neither required for the maintenance of β-cell survival and function under control conditions, nor involved in protection against β-cell glucotoxicity. The latter therefore occurs independently from the associated increase in the level of intracellular fructose-ε-lysines.
- pancreatic beta-cell
- Copyright © 2009, American Journal of Physiology - Endocrinology and Metabolism