Biochemical and Biophysical Research Communications, Vol.423, No.2, 259-264, 2012
Marked over expression of uncoupling protein-2 in beta cells exerts minor effects on mitochondrial metabolism
Evidence is conflicting as to the impact of elevated levels of uncoupling protein-2 (UCP-2) on insulin-producing beta cells. Here we investigated effects of a fourfold induction of UCP-2 protein primarily on mitochondrial parameters and tested for replication of positive findings at a lower level of induction. We transfected INS-1 cells to obtain a tet-on inducible cell line. A 48 h exposure to 1 mu g/ml of doxycydine (dox) induced UCP-2 fourfold (424 +/- 113%, mean SEM) and 0.1 mu g/ml twofold (178 +/- 29%, n = 3). Fourfold induced cells displayed normal viability (MTT, apoptosis), normal cellular insulin contents and, glucose-induced insulin secretion (+27 +/- 11%) as well as D-[U-C-14]-glucose oxidation (+5 +/- 9% at 11 mM glucose). Oxidation of [1-C-14]-oleate was increased from 4088 to 5797 fmol/mu g prot/2 h at 3.3 mM glucose, p < 0.03. Oxidation of L-[C-14(U)]-glutamine was unaffected. Induction of UCP-2 did not significantly affect measures of mitochondria] membrane potential (Rhodamine 123) or mitochondrial mass (Mitotracker Green) and did not affect ATP levels. Oligomycin-inhibited oxygen consumption (a measure of mitochondrial uncoupling) was marginally increased, the effect being significant in comparison with dox-only treated cells, p < 0.05. Oxygen radicals, assessed by dichlorofluorescin diacetate, were decreased by 30%, p < 0.025. Testing for the lower level of UCP-2 induction did not reproduce any of the positive findings. A fourfold induction of UCP-2 was required to exert minor metabolic effects. These findings question an impact of moderately elevated UCP-2 levels in beta cells as seen in diabetes. (C) 2012 Elsevier Inc. All rights reserved.