Mitochondrialuncouplingprotein-2isnotinvolvedin palmitate-induced impairmentofglucose-stimulatedinsulinsecretion in INS-1Einsulinomacellsandisnotneededfortheamplification of insulin release

WehaverecentlyshownthatovernightexposureofINS-1Einsulinomacellstopalmitateinthepresence of highglucosecausesdefectsinbothmitochondrialenergymetabolismandglucose-stimulatedinsulin secretion (GSIS).Herewereportexperimentsdesignedtotesttheinvolvementofmitochondrial uncoupling protein-2(UCP2)intheseglucolipotoxiceffects.Measuringreal-timeoxygenconsumption in siRNA-transfectedINS-1Ecells,weshowthatdeleteriouseffectsofpalmitateontheglucosesensitivity of mitochondrialrespirationandonthecouplingefficiency ofoxidativephosphorylationareindepen- dent ofUCP2.Consistently,palmitateimpairsGSIStothesameextentincellswithandwithoutUCP2. Furthermore,weknockeddownUCP2inspheroidINS-1Ecellclusters(pseudoislets)totestwhetheror not UCP2regulatesinsulinsecretionduringprolongedglucoseexposure.Wedemonstratethatthereare no differencesintemporalGSISkineticsbetweenperifusedpseudoisletswithandwithoutUCP2.We conclude thatUCP2isnotinvolvedinpalmitate-inducedimpairmentofGSISinINS-1Einsulinomacells and isnotneededfortheamplification ofinsulinrelease.Theseconclusionsinformongoingdebateon the disputedbiochemicalandphysiologicalfunctionsofthebetacellUCP2

Uncoupling protein-2 attenuates palmitoleate protection against the cytotoxic production of mitochondrial reactive oxygen species in INS-1E insulinoma cells.

High glucose and fatty acid levels impair pancreatic beta cell function. We have recently shown that palmitate-induced loss of INS-1E insulinoma cells is related to increased reactive oxygen species (ROS) production as both toxic effects are prevented by palmitoleate. Here we show that palmitate-induced ROS are mostly mitochondrial: oxidation of MitoSOX, a mitochondria-targeted superoxide probe, is increased by palmitate, whilst oxidation of the equivalent non-targeted probe is unaffected. Moreover, mitochondrial respiratory inhibition with antimycin A stimulates palmitate-induced MitoSOX oxidation. We also show that palmitate does not change the level of mitochondrial uncoupling protein-2 (UCP2) and that UCP2 knockdown does not affect palmitate-induced MitoSOX oxidation. Palmitoleate does not influence MitoSOX oxidation in INS-1E cells ±UCP2 and largely prevents the palmitate-induced effects. Importantly, UCP2 knockdown amplifies the preventive effect of palmitoleate on palmitate-induced ROS. Consistently, viability effects of palmitate and palmitoleate are similar between cells ±UCP2, but UCP2 knockdown significantly augments the palmitoleate protection against palmitate-induced cell loss at high glucose. We conclude that UCP2 neither mediates palmitate-induced mitochondrial ROS generation and the associated cell loss, nor protects against these deleterious effects. Instead, UCP2 dampens palmitoleate protection against palmitate toxicity

Emerging role of the calcium-activated, small conductance, SK3 K ⁺ channel in distal tubule function: Regulation by TRPV4

The Ca2+-activated, maxi-K (BK) K+ channel, with low Ca2+-binding affinity, is expressed in the distal tubule of the nephron and contributes to flow-dependent K+ secretion. In the present study we demonstrate that the Ca2+-activated, SK3 (KCa2.3) K + channel, with high Ca2+-binding affinity, is also expressed in the mouse kidney (RT-PCR, immunoblots). Immunohistochemical evaluations using tubule specific markers demonstrate significant expression of SK3 in the distal tubule and the entire collecting duct system, including the connecting tubule (CNT) and cortical collecting duct (CCD). In CNT and CCD, main sites for K+ secretion, the highest levels of expression were along the apical (luminal) cell membranes, including for both principal cells (PCs) and intercalated cells (ICs), posturing the channel for Ca2+- dependent K+ secretion. Fluorescent assessment of cell membrane potential in native, split-opened CCD, demonstrated that selective activation of the Ca2+-permeable TRPV4 channel, thereby inducing Ca2+ influx and elevating intracellular Ca2+ levels, activated both the SK3 channel and the BK channel leading to hyperpolarization of the cell membrane. The hyperpolarization response was decreased to a similar extent by either inhibition of SK3 channel with the selective SK antagonist, apamin, or by inhibition of the BK channel with the selective antagonist, iberiotoxin (IbTX). Addition of both inhibitors produced a further depolarization, indicating cooperative effects of the two channels on Vm. It is concluded that SK3 is functionally expressed in the distal nephron and collecting ducts where induction of TRPV4-mediated Ca2+ influx, leading to elevated intracellular Ca2+ levels, activates this high Ca2+- affinity K+ channel. Further, with sites of expression localized to the apical cell membrane, especially in the CNT and CCD, SK3 is poised to be a key pathway for Ca2+-dependent regulation of membrane potential and K+ secretion. © 2014 Berrout et al