Cytokine-mediated down-regulation of the transcription factor cAMP-response element-binding protein in pancreatic β-cells

Abstract

Cytokines are known to induce apoptosis of pancreatic β-cells. Impaired expression of the anti-apoptotic gene bcl-2 is one of the mechanisms involved. In this study, we identified a defect involving transcription factor cAMP-response element-binding protein (CREB) in the expression of bcl-2. Exposure of mouse pancreatic β-cell line, MIN6 cells, to cytokines (interleukin-1β, tumor necrosis factor-α, and interferon-γ) led to a significant (p < 0.01) decrease in Bcl.2 protein and mRNA levels. Cytokines decreased (56%) the activity of the bcl-2 promoter that contains a cAMP-response element (CRE) site. Similar decreases were seen with a luciferase reporter gene driven by tandem repeats of CRE and a CREB-specific Gal4-luciferase reporter, suggesting a defect at the level of CREB. The active phospho form (serine 133) of CREB diminished significantly (p < 0.01) in cells exposed to cytokines. Examination of signaling pathways upstream of CREB revealed a reduction in the active form of Akt. Cytokine-induced decrease of bcl-2 promoter activity was partially restored when cells were cotransfected with a constitutively active form of Akt. Several end points of cytokine action including decreases in phospho-CREB, phospho-Akt, and BCl-2 levels and activation of caspase-9 were observed in isolated mouse islets. Overexpression of wild-type CREB in MIN6 cells by plasmid transfection and adenoviral infection led to protection against cytokine-induced apoptosis. Adenoviral transfer of dominant-negative forms of CREB, on the other hand, resulted in activation of caspase-9 and exaggeration of cytokine-induced β-cell apoptosis. Together, these results point to CREB as a novel target for strategies aimed at improving the survival of β-cells.