Mitochondrial uncoupling protein 2 in pancreatic β-cells.

Pancreatic β-cells have remarkable bioenergetics in which increased glucose supply upregulates the cytosolic ATP/ADP ratio and increases insulin secretion. This arrangement allows glucose-stimulated insulin secretion (GSIS) to be regulated by the coupling efficiency of oxidative phosphorylation. Uncoupling protein 2 (UCP2) modulates coupling efficiency and may regulate GSIS. Initial measurements of GSIS and glucose tolerance in Ucp2(-/-) mice supported this model, but recent studies show confounding effects of genetic background. Importantly, however, the enhancement of GSIS is robustly recapitulated with acute UCP2 knockdown in INS-1E insulinoma cells. UCP2 protein level in these cells is dynamically regulated, over at least a fourfold concentration range, by rapid proteolysis (half-life less than 1 h) opposing regulated gene transcription and mRNA translation. Degradation is catalysed by the cytosolic proteasome in an unprecedented pathway that is currently known to act only on UCP2 and UCP3. Evidence for proteasomal turnover of UCP2 includes sensitivity of degradation to classic proteasome inhibitors in cells, and reconstitution of degradation in vitro in mitochondria incubated with ubiquitin and the cytosolic 26S proteasome. These dynamic changes in UCP2 content may provide a fine level of control over GSIS in β-cells.