Mitochondrial uncoupling protein 2 in pancreatic β‐cells

Abstract

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. Keywords: glucose-stimulated insulin secretion, INS-1E cells, mitochondria, proteasome, protein degradation, turnover, UCP2, UCP3 Date submitted 25 March 2010; date of final acceptance 24 April 2010 IntroductionThe energy metabolism of pancreatic β-cells is extraordinary. Nearly all other mammalian cells rigorously regulate their bioenergetics using internal cues, and only take up fuels when they need them for oxidation and energy release or for storage. Pancreatic β-cells work the other way round, and use fuel supply to regulate their ATP production and cytosolic ATP/ADP ratio.A skeletal muscle cell, for example, will take up glucose from the bloodstream through a plasma membrane glucose transporter, glucose transporter type 4 (GLUT4), when insulin is high and glucose is abundant, otherwise it will use fatty acids as fuel [1]. It will store glucose as glycogen when insulin and ATP are high and calcium (the trigger for muscle contraction and energy demand) is low. Even after glucose enters the cytoplasm, a muscle cell will allow it to enter glycolysis only when that cell requires energy. Glucose 6-phosphate is a non-competitive inhibitor of muscle hexokinase, so when ATP is sufficiently high, glycolysis stalls, glucose 6-phosphate builds up, hexokinase activity is inhibited and glucose metabolism slows. When more ATP is needed because of internal ATP demand, phosphofructokinase and pyruvate kinase are activated, glucose 6-phosphate levels fall and hexokinase channels glucose into glycolysis and to the mitochondria. The mitochondrial electron transport chain pumps protons across the mitochondrial inner membrane, restoring a high protonmotive force and ATP production, until ATP rises and oxidativ...

UCP2 transports C4 metabolites out of mitochondria, regulating glucose and glutamine oxidation

“…1). On the contrary, the induction of UCP2 expression in β-cells has an adverse effect on hyperglycemia, because a reduced ATP:ADP ratio and ROS production together with increased release of lactate would inhibit insulin secretion (28)(29)(30)(31). Consistent with this explanation, we have found that replacement of Gly-268 with Ala in UCP2, a mutation associated with congenital hyperinsulinism (32), causes a marked decrease in the transport activity of UCP2 (Fig.…”

The Pancreatic β-Cell: A Bioenergetic Perspective

“…While UCP2 has a wide tissue distribution (60), it is expressed at very low levels, comparable to that of most metabolite transporters. The UCP2 content of mitochondria isolated from INS-1E cells depended on the prior nutritional status of the cells, and varied from 2.7 ng/mg mitochondrial protein in the absence of both fetal calf serum and glutamine to 14.5 ng/mg mitochondrial protein in full culture medium containing 20 mM glucose, responding with a half-life of ϳ1 h (31,63). Even this latter figure is Ͻ0.1% of the content of UCP1 in cold-adapted brown fat.…”

Mechanisms of glucose sensing in the pancreatic β-cell

“…Given their effects on mitochondrial bioenergetics, uncoupling proteins have been implicated in the physiology and pathophysiology of pancreatic β-cells. 5,31,32 Our simulation on the basis of model showed (see Fig. 4 from ref. 17) that at a constant glucose level increased uncoupling protein activity leads to decreases in Ψ m and [Ca 2+ ] c .…”

“…This effect of increased activity of uncoupling proteins has been described experimentally. 5,32 However, our simulation also shows a novel aspect of this problem: an ability of uncoupling proteins (and other uncoupling agents) to shift the glucose dependence of the ATP/ADP ] c at different leak activity. To simulate increased leak activity we magnified the proton leak rate two-fold.…”