GLUCOSE RAPIDLY AND REVERSIBLY DECREASES INS-1 CELL INSULIN GENE-TRANSCRIPTION VIA DECREMENTS IN STF-1 AND C1 ACTIVATOR TRANSCRIPTION FACTOR ACTIVITY

We have reported that chronic exposure of HIT-T15 cells to supraphysio logical concentrations of glucose over many months leads to decreased insulin gene transcription and decreased binding activities of two bet a-cell-specific transcription factors, STF-1 and C1 activators, and ha ve postulated that these events may provide a mechanism for glucose to xicity on beta-cell function, We now report that culturing the highly differentiated rat insulinoma cell line, INS-1, in glucose concentrati ons above 8.0 mM caused a marked decrease in insulin mRNA levels withi n 24 h. The decrease in insulin mRNA levels was reversed by further in cubation of the cells in 4.0 mM glucose, Transient transfection of a c hloramphenicol acetyltransferase reporter gene regulated by the 5'-reg ulatory sequences of the human insulin gene showed that elevated gluco se concentrations caused a large decrease in insulin gene promoter act ivity, The decrease in insulin gene promoter activity was associated w ith reductions in the binding activities of both STF-1 and C1 activato r, and these were partially reversed by lowering the glucose concentra tion, The decrease in STF-1 binding activity was associated with decre ased STF-1 mRNA and occurred independently of changes in STF-1 promote r activity, suggesting a posttranscriptional regulatory mechanism, Fur thermore, the decrease in insulin gene expression was found to occur i ndependently of changes in cell proliferation, We conclude that physio logically relevent elevations in glucose can reversibly diminish insul in gene transcription by reducing the expression and/or binding activi ty of two critical beta-cell transcription factors.