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 beta-cell dysfun ction in type 2 diabetic patients. A deleterious effect of prolonged exposu re to high glucose concentrations on insulin gene expression has been demon strated in insulin-secreting cell lines. This study was designed to investi gate in isolated rat islets the effects of long-term exposure to supraphysi ologic glucose concentrations on insulin, GLUTS, and glucokinase gene expre ssion. 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, GLUTS, and glucokinase mRNA levels were assessed by semiquantitative revers e transcriptase-polymerase chain reaction (RT-PCR). As expected, glucose ac utely increased relative insulin and GLUTS mRNA levels by 2.8- +/- 0.5-fold (n = 5, P < .005) and 1.8- +/- 0.3-fold (n = 5, P < .05), respectively, bu t 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 d etect changes in gene expression in rat islets. Islets were then cultured i n 5.6 or 16.7 mmol/L glucose for 2, 4, or 6 weeks. Relative insulin mRNA le vels 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 GLUTS 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 con ditions 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. The se results indicate that a 6-week exposure of rat islets to supraphysiologi c glucose concentrations decreases insulin mRNA levels without affecting GL UTS and glucokinase gene expression. We conclude that the phenomenon of glu cose toxicity decreasing insulin gene expression is not restricted to trans formed cells, and might provide insight into the mechanisms by which chroni c hyperglycemia adversely affects beta-cell function. Copyright (C) 1999 by W.B. Saunders Company.