INDUCTION BY GLUCOSE OF GENES-CODING FOR GLYCOLYTIC-ENZYMES IN A PANCREATIC BETA-CELL LINE (INS-1)

Chronic elevation in glucose has pleiotropic effects on the pancreatic beta-cell including a high rate of insulin secretion at low glucose, beta-cell hypertrophy, and hyperplasia. These actions of glucose are e xpected to be associated with the modulation of the expression of a nu mber of glucose-regulated genes that need to be identified. To further investigate the molecular mechanisms implicated in these adaptation p rocesses to hyperglycemia, we have studied the regulation of genes enc oding key glycolytic enzymes in the glucose-responsive beta-cell line INS-1. Glucose (from 5 to 25 mM) induced phosphofructokinase-1 (PFK-1) isoform C, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (4-fold), and L-pyruvate kinase (L-PR) (7-fold) mRNAs. In contrast the expressi on level of the glucokinase (Gk) and 6-phosphofructo-2-kinase transcri pts remained unchanged. Following a 3-day exposure to elevated glucose , a similar induction was observed at the protein level for PFK-1 (iso forms C, M, and L), GAPDH, and L-PK, whereas M-PR expression only incr eased slightly. The study of the mechanism of GAPDH induction indicate d that glucose increased the transcriptional rate of the GAPDH gene bu t that both transcriptional and post transcriptional effects contribut ed to GAPDH mRNA accumulation, 2-Deoxyglucose did not mimic the induct ive effect of glucose, suggesting that increased glucose metabolism is involved in GAPDH gene induction. These changes in glycolytic enzyme expression were associated with a 2-3-fold increase in insulin secreti on at low (25 mM) glucose. The metabolic activity of the cells was als o elevated, as indicated by the reduction of the artificial electron a cceptor 4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium. A marked de position of glycogen, which was readily mobilized upon lowering of the ambient glucose, and increased DNA replication were also observed in cells exposed to elevated glucose. The results suggest that a coordina ted induction of key glycolytic enzymes as well as massive glycogen de position are implicated in the adaptation process of the beta-cell to hyperglycemia to allow for chronically elevated glucose metabolism, wh ich, in this particular fuel-sensitive cell, is linked to metabolic co upling factor production and cell activation.