Abstract
The insulin promoter is both positively and negatively regulated in response to conditions to which pancreatic beta-cells are exposed. Exposure of intact rat islets and INS-1 pancreatic beta-cells to 11 mm glucose for minutes to hours results in an enhancement in the rate of insulin gene transcription assessed with a reporter linked to the insulin gene promoter. In contrast, chronic exposure of rat islets or beta-cells to 11 mm glucose results in loss of the glucose responsiveness of the insulin gene promoter. By 48 h, glucose inhibits insulin gene promoter activity. Here we show that not only the acute effect of elevated glucose to stimulate the insulin gene promoter but also the chronic effect of elevated glucose to inhibit the insulin gene promoter depend on ERK1/2 mitogen-activated protein kinase activity. In examining the underlying mechanism, we found that acute exposure to 11 mm glucose resulted in the binding of the transcription factors NFAT and Maf to the glucose-responsive A2C1 element of the insulin gene promoter. An NFAT and C/EBP-beta complex was observed in cells chronically exposed to 11 mm glucose. Formation of NFAT-Maf and NFAT-C/EBP-beta complexes was sensitive to inhibitors of ERK1/2 and calcineurin, consistent with our previous finding that activation of ERK1/2 by glucose required calcineurin activity and the well documented regulation of NFAT by calcineurin. These results indicate that the ERK1/2 pathway modulates partners of NFAT, which may either stimulate or repress insulin gene transcription during stimulatory and chronic exposure to elevated glucose.
Highlights
Pancreatic -cells of the islets of Langerhans produce and store insulin in response to physiological demand
ERK1/2 Stimulate and Repress the Activity of the Insulin Gene Promoter—We compared the ability of different concentrations of glucose to stimulate insulin transcription and found that there was little stimulation of transcription of a reporter linked to the rat I insulin gene promoter observed from 3.8 to 5.5 mM glucose
As the glucose concentration was increased to 11 mM glucose, increased activity of the reporter was observed; no further increase was noted at 16 mM glucose (Fig. 1 and data not shown)
Summary
C/EBP-, CAAT enhancer-binding protein ; ERK1/2, extracellular signal-regulated kinase 1/2; PBS, phosphate-buffered saline; MEK, mitogen-activated protein kinase/extracellular signal-regulated kinase kinase. Analysis of mRNA levels showed that blockade of ERK1/2 activity using kinase-defective ERK2 resulted in a substantial reduction in prepro-insulin mRNA content after 24 h, which along with insulin gene promoter reporter assays, suggested a role for ERK1/2 in insulin gene transcription [28]. Possible actions of this signaling pathway on other key steps of the biosynthetic process have not yet been thoroughly explored. We conclude that prolonged exposure of -cells to glucose results in an ERK1/2-dependent change in factors associated with the insulin gene promoter
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