Long-term exposure of isolated rat islets of langerhans to supraphysiologic glucose concentrations decreases insulin mRNA levels

Chronic hyperglycemia has been postulated to contribute to β-cell dysfunction in type 2 diabetic patients. A deleterious effect of prolonged exposure to high glucose concentrations on insulin gene expression has been demonstrated in insulin-secreting cell lines. This study was designed to investigate in isolated rat islets the effects of long-term exposure to supraphysiologic glucose concentrations on insulin, GLUT2, and glucokinase gene expression. The acute effects of glucose on gene expression were investigated by culturing rat islets in 2.8 or 16.7 mmol/L glucose for 24 hours. Insulin, GLUT2, and glucokinase mRNA levels were assessed by semiquantitative reverse transcriptase—polymerase chain reaction (RT-PCR). As expected, glucose acutely increased relative insulin and GLUT2 mRNA levels by 2.8- ± 0.5-fold (n = 5, P < .005) and 1.8- ± 0.3-fold (n = 5, P < .05), respectively, but had no effect on glucokinase gene expression (1.1- ± 0.1-fold increase, n = 4, NS). These results validate the use of semiquantitative RT-PCR to detect changes in gene expression in rat islets. Islets were then cultured in 5.6 or 16.7 mmol/L glucose for 2, 4, or 6 weeks. Relative insulin mRNA levels were higher in islets cultured in high glucose after 2 weeks (1.8 ± 0.1 v 1.0 ± 0.1, n = 4, P < .05), identical after 4 weeks (0.9 ± 0.1 v 1.00 ± 0.2, n = 4, NS), and significantly lower after 6 weeks (0.6 ± 0.1 v 1.0 ± 0.2, n = 6, P < .05). Relative GLUT2 mRNA levels were higher in islets cultured in high glucose after 2 weeks (1.7 ± 0.2 v 1.0 ± 0.2, n = 3, P < .05) and then identical in both groups after 4 weeks (1.0 ± 0.1 v 1.0 ± 0.1, n = 3, NS) and 6 weeks (1.0 ± 0.2 v 1.0 ± 0.1, n = 6, NS). Relative glucokinase mRNA levels were identical under both culture conditions at 2 (1.4 ± 0.4 v 1.0 ± 0.2, n = 3, NS), 4 (0.8 ± 0.5 v 1.0 ± 0.3, n = 3, NS), and 6 (0.9 ± 0.2 v 1.0 ± 0.1, n = 6, NS) weeks. These results indicate that a 6-week exposure of rat islets to supraphysiologic glucose concentrations decreases insulin mRNA levels without affecting GLUT2 and glucokinase gene expression. We conclude that the phenomenon of glucose toxicity decreasing insulin gene expression is not restricted to transformed cells, and might provide insight into the mechanisms by which chronic hyperglycemia adversely affects β-cell function.