Abstract

Objective- In type 2 diabetes, chronic hyperglycemia is detrimental to β-cells, causing apoptosis and impaired insulin secretion. The transcription factor CREB is crucial for β-cell survival and function. We investigated whether prolonged exposure of β-cells to high glucose affects the functional integrity of CREB.

Research Design and Methods- INS-1E cells, rat and human islets were used. Gene expression was analyzed by RT-PCR and Western blotting. Apoptosis was detected by cleaved caspase-3 emergence, DNA fragmentation and electron microscopy.

Results- Chronic exposure of INS-1E cells, rat and human islets to high glucose resulted in decreased CREB protein expression, phosphorylation and transcriptional activity associated with apoptosis and impaired β-cell function. High glucose treatment increased CREB polyubiquitination, while treatment of INS-1E cells with the proteasome inhibitor MG-132 prevented the decrease in CREB content. The emergence of apoptosis in INS-1E cells with decreased CREB protein expression knocked down by siRNA suggested that loss of CREB protein content induced by high glucose contributes to β-cell apoptosis. Loading INS-1E cells or human islets with a cell permeable peptide mimicking the proteasomal targeting sequence of CREB blocked CREB degradation and protected INS-1E cells and human islets from apoptosis induced by high glucose. The insulin secretion in response to glucose and the insulin content were preserved in human islets exposed to high glucose and loaded with the peptide.

Conclusions- These studies demonstrate that the CREB degradation by the ubiquitin-proteasome pathway contributes to β-cell dysfunction and death upon glucotoxicity, and provide new insight into the cellular mechanisms of glucotoxicity.