Uncoupling Protein 2 and Islet Function Catherine B. Chan,1 Monique C. Saleh,1 Vasilij Koshkin,2 and Michael B. Wheeler2

Stressors such as chronic hyperglycemia or hyperlipid- ␤-cell to increased nutrient availability and contribute to emia may lead to insufficient insulin secretion in sus- the suppression of glucose-stimulated insulin secretion ceptible individuals, contributing to type 2 diabetes. (GSIS) (5). The molecules mediating this effect are just beginning to be identified. Uncoupling protein (UCP)-2 may be one such negative modulator of insulin secretion. Accumu- UCP2 lating evidence shows that ␤-cell UCP2 expression is The existence of UCP2 was first described in 1997 in upregulated by glucolipotoxic conditions and that in- multiple tissues (6–8), including the pancreas (6). It was creased activity of UCP2 decreases insulin secretion. subsequently localized in rat (4,5) and human pancreatic Mitochondrial superoxide has been identified as a post- (9,10) islets. In general, UCPs function to decrease meta- translational regulator of UCP2 activity in islets; thus, bolic efficiency by dissociating substrate oxidation in the UCP2 may provide protection to ␤-cells at one level mitochondrion from ATP synthesis. This is thought to be while simultaneously having detrimental effects on in- accomplished by promoting net translocation of protons sulin secretion. Interestingly, the latter appears to be from the intermembrane space, across the inner mitochon- the dominant outcome, because UCP2 knockout mice display an increased ␤-cell mass and retained insulin drial membrane to the matrix, thereby dissipating the secretion capacity in the face of glucolipotoxicity. potential energy available for conversion of ADP to ATP Diabetes 53 (Suppl. 1):S136–S142, 2004 despite continued oxidation of fuels (11). This uncoupling effect then leads to homologue- and tissue-specific func- tions such as thermogenesis for UCP1 (12), regulation of free fatty acid (FFA) metabolism and transport for UCP2 ealthy pancreatic ␤-cells are poised to respond and UCP3 (11,13,14), decreasing reactive oxygen species rapidly and efficiently to acute changes in cir- (ROS) formation (UCP1 and UCP2) (15,16), and inhibition culating nutrient availability to maintain meta- of insulin secretion for UCP2 (9). Delineation of the Hbolic homeostasis. However, it is well recog- common versus unique properties of the UCP family nized that chronic exposure to overnutrition, such as what members continues to fuel much research. occurs in obesity, results in a blunting of the insulin response to an acute stimulus. The mechanisms by which UCP2 NEGATIVELY MODULATES INSULIN SECRETION this adaptation occurs are hotly debated but, based on Initially controversial because of a dissenting report (17), recent analyses of gene expression using oligonucleotide the bulk of evidence now shows that an increase in UCP2 microarray technology, include multiple enzymes involved expression results in suppression of GSIS (5,9,18), in glucose and fat metabolism (1,2). Key rate-limiting whereas absence of Ucp2 enhances GSIS (19,20). enzymes, such as carnitine palmitoyl transferase (CPT)-1, To test the hypothesis that UCP2 would inhibit insulin have been identified as crucial mediators of altered me- secretion, we created an adenovirus vector containing the tabolism of fat and glucose leading to impaired insulin full-length human Ucp2 cDNA to overexpress the protein secretion (3). Until recently, regulatory proteins that par- (5). Such upregulation completely suppressed GSIS (5), an ticipate specifically in downregulation of insulin secretion effect confirmed by others using similar technology (18). have received little attention. The discovery that uncou- The effects could be partially overcome by adding calcium pling protein (UCP)-2 is present in pancreatic islets and ionophore to a stimulatory glucose concentration (5) or ␤ -cell lines (4) has led to the suggestion that such mole- normalized with sulfonylurea or KCl (9). Upregulation of cules can participate in the long-term adaptation of the UCP3 did not inhibit GSIS (18), whereas induction of UCP1 was suppressive (21). These studies provided the From the 1Department of Biomedical Sciences, University of Prince Edward first direct evidence that UCP2 could negatively regulate Island, Charlottetown, Prince Edward, Canada; and the 2Departments of insulin secretion but could be criticized because the Medicine and Physiology, University of Toronto, Toronto, Ontario, Canada. Address correspondence and reprint requests to Dr. C.B. Chan, Department degree of upregulation was likely supraphysiological. This of Biomedical Sciences, University of Prince Edward Island, 550 University criticism was difficult to refute because of the continuing Ave., Charlottetown, PE C1A 4P3 Canada. E-mail: [email protected]. lack of a suitable antibody for unambiguous immunoblot- Received for publication 13 March 2003 and accepted 28 April 2003. This article is based on a presentation at a symposium. The symposium and ting of UCP2. Development of the UCP2 knockout mouse the publication of this article were made possible by an unrestricted educa- provided another method for directly assessing the role of tional grant from Les Laboratoires Servier. CPT, carnitine palmitoyl transferase; DIC, dicarboxylate carrier; FFA, free UCP2 in insulin secretion. We showed that isolated cul- fatty acid; GSIS, glucose-stimulated insulin secretion; KATP channel, ATP- tured islets from UCP2 knockout mice had increased dependent Kϩ channel; PPAR, peroxisome proliferator–activated receptor; responsiveness to glucose characterized by a left-shift in ROS, reactive oxygen species; SRE, sterol regulatory element; UCP, uncou- pling protein. the concentration-response curve (20). The knockout mice © 2004 by the American Diabetes Association. demonstrated an increase in the insulin-to-glucose ratio in

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86 ϩ We examined the function of KATP channels using Rb as a marker for Kϩ flux. As predicted, when UCP2 was induced and cellular ATP content reduced by 50%, KATP channels failed to close in response to elevated glucose, although sulfonylurea was effective (9). Likewise, an in- crease in KATP channel activity of UCP1-overexpressing ␤-cells has been reported (21), leading to a reduction in voltage-dependent Ca2ϩ influx. The reduction in insulin secretion secondary to the decrease in Ca2ϩ entry was only partially overcome by use of calcium ionophore A23187 (5), suggesting that ATP depletion affects regula- tory sites other than KATP channels. Other sites modulated by ATP or ATP/ADP have been less well characterized after UCP2 overexpression. However, our unpublished data suggest that cAMP formation is not altered, and elevation of FIG. 1. Generation and disposition of ATP in ␤-cells. ATP is both generated and consumed in glycolysis, leaving oxidative phosphoryla- cAMP does not potentiate GSIS to the same extent in UCP2 tion (OxPhos) to produce >90% of cellular ATP. An increase in the knockout as in wild-type mice (T.S. McQuaid, J.W. Joseph, ATP/ADP ratio leads to inactivation of the KATP channel, the initiating event in voltage-dependent insulin exocytosis. Aspects of the so-called M.C.S., M.B.W., C.B.C., unpublished data). cAMP is important

KATP-independent pathway, including protein kinase A– and protein in distal events of signaling (26); coincidentally, the energy kinase C–catalyzed protein phosphorylation (Pr-P), as well as cAMP requirement for such distal steps is estimated to be lower generation, depend on ATP. Insulin biosynthesis and processing are also energy-dependent processes. than that for KATP channel closure (27). Increasing mitochondrial uncoupling is predicted to promote glucose metabolism by virtue of releasing the vivo that was not explained by changes in peripheral back pressure on the electron transport chain caused by a insulin sensitivity. Moreover, a gene-dose effect was ap- buildup in the proton gradient. While this phenomenon of parent, such that heterozygotes for the Ucp2 gene dis- increased glucose usage and oxidation has been reported played insulin secretion intermediate between wild-type in ZDF rat islets overexpressing UCP2, thus leading to an and knockout animals (19). increase in insulin secretion (17), it has not been repro- duced in either UCP2-overexpressing (18; C.B.C., M.C.S., UCP2 MECHANISM OF ACTION IN ␤-CELLS unpublished data) or UCP1-overexpressing (21) ␤-cells. The secretion of insulin depends on ATP at multiple steps Moreover, incubating islets with chemical uncouplers has in metabolism-secretion coupling (Fig. 1), and an esti- classically been shown to inhibit insulin secretion (28). mated 98% of ␤-cell ATP production depends on mitochon- drial oxidative processes (22). That UCP2 expression inversely correlates with ␤-cell ATP has been established REGULATION OF UCP2 EXPRESSION BY NUTRIENTS in both overexpression (9,17,18) and null expression mod- Emphasis in current investigations of the regulation of els (19,20). The decrement in ATP of UCP2-upregulated UCP2 expression focuses on the two culprits of glucolipo- ␤-cells is accompanied by a failure of glucose to hyperpo- toxicity: glucose and FFA. In the case of glucose, there are larize the mitochondrial inner membrane, as assessed by conflicting results. Using isolated islets or INS1 cells, changes in rhodamine 123 fluorescence (9). State 4 respi- exposure to high glucose for at least 48 h had no effect on ration (i.e., when ADP is absent and ATP synthesis is (29), reduced (30), or increased UCP2 mRNA and/or inhibited with oligomycin) is increased in UCP2-overex- protein expression (10,31). When rats were made hyper- pressing insulinoma cells (18), consistent with an uncou- glycemic by means of partial pancreatectomy (32) or pling effect. Interestingly, UCP3 overexpression produces glucose infusion (33), Ucp2 mRNA expression was upregu- similar effects, but of a lower magnitude than UCP2 on lated—an effect reversed by normalizing the plasma glu- ATP, and these effects are not accompanied by inhibition cose concentrations with phlorizin (32). Conversely, of insulin secretion (18). On the other hand, UCP1 over- hyperglycemic Zucker diabetic fatty rats were reported to expression in ␤-cells totally suppressed the glucose-stim- have low expression of islet Ucp2 mRNA (4). ulated increment in ATP content and insulin secretion The hypothesis that Ucp2 mRNA expression could be (21). Altogether, these studies support the idea that UCP2 induced by FFA was quickly established in nonislet tis- catalyzes a proton translocation in ␤-cells, as suggested by sues, such as skeletal muscle and adipocytes (34). That studies in isolated yeast (8) or mammalian (24) mitochon- FFAs also upregulate islet UCP2 has now been shown both dria and intact thymocytes (25), leading to depression of by effects of in vivo feeding of high-fat diets on mRNA mitochondrially generated ATP. (9,20) and in vitro exposure of isolated islets or clonal Because ATP participates at multiple rate-limiting or ␤-cells to palmitic or oleic acid on UCP2 (29,31,35,36). rate-potentiating steps of the insulin secretion pathway, Despite consensus on the final outcome, the mechanism one might predict that agents that cause a decrease in by which FFAs stimulate UCP2 transcription in islets is ␤-cell ATP would affect numerous downstream events. An unclear. Conflicting evidence has been presented regard- ϩ obvious target is the ATP-dependent K (KATP) channel, ing the necessity for fat oxidation (29,35). Support for which permits the egress of Kϩ in resting ␤-cells. When induction of UCP2 secondary to fat oxidation is demon- ATP (or, specifically, ATP/ADP) rises in response to strated by use of etomoxir to inhibit CPT-1 and block entry substrate metabolism, the KATP channels close and the of fatty acyl-CoA into the mitochondria. In this study, it cells depolarize, leading to Ca2ϩ-dependent exocytosis. was shown that oleic acid upregulated Ucp2 mRNA only in

DIABETES, VOL. 53, SUPPLEMENT 1, FEBRUARY 2004 S137 UCP2 AND ISLET FUNCTION the presence of submaximal glucose concentrations (29). further increased fivefold by the high-fat diet. Assuming Because high glucose inhibits CPT-1 by generating malo- that such compensation occurs in other tissues, such as nyl CoA (37), the importance of FFA oxidation seems skeletal muscle, to permit a large increase in FFA oxida- apparent. These results would imply that the mediator tive capacity, this phenomenon may explain the normal regulating UCP2 transcription is a metabolite of FFA. On plasma triglyceride and FFA concentrations seen in the the other hand, studies showing similar results of nonme- high fat–fed UCP2 knockout mice (20). Other studies have tabolizable versus metabolizable analogs of FFAs (e.g., implicated FFA-induced uncoupling by UCP2 in the failure bromopalmitic acid) provide evidence that FFA oxidation of fat-laden islets to increase ATP content in response to is not necessary for Ucp2 gene transcription (35). glucose (31,35). The data from UCP2 knockout mice Interestingly, the UCP2 gene promoter region contains support this assertion. Isolated islets from UCP2 knockout peroxisome proliferator response elements that are acti- mice exposed to palmitate for 48 h had ATP content vated by FFAs. When islets from Zucker diabetic fatty rats similar to those from BSA-exposed controls, whereas the were treated with the peroxisome proliferator–activated ATP content of palmitate-exposed wild-type islets was receptor (PPAR)-␥ activator troglitazone, Ucp2 mRNA was reduced (20). increased (38), supporting the idea that nonmetabolized FFAs directly induce UCP2 via PPARs. Unfortunately, ENDOGENOUS REGULATION OF UCP2 ACTIVITY these results were not replicated with another PPAR-␥ Recent work by our group has sought to elucidate how activator, rosiglitazone (29). Laybutt et al. (32) showed UCP2 activity in ␤-cells may be post-translationally regu- concomitant induction of PPAR-␥ and UCP2 but had no lated. Like UCP1 (42) and as reported by others for UCP2 direct evidence of a causal link between the two. However, (43) in a variety of models, our data showed that mito- culture of rat islets for 72 h with supraphysiological chondrial uncoupling in clonal MIN6 cells was caused by concentrations of FFAs (2 mmol/l) induced UCP2 and FFAs and inhibited by GDP and BSA. Moreover, when the suppressed GSIS by mechanisms that were decreased with ␥ MIN6 cells were treated with 0.4 mmol/l oleic acid for 72 h a PPAR- antagonist (31). A study specifically examining before mitochondrial assay, the uncoupling effects of ␣ the role of PPAR- by use of the ligand clofibrate demon- acute FFA application were magnified, consistent with strated concomitant induction of CPT-1 and UCP2 in upregulation of UCP2 expression by fat. These data sug- islets. Whereas inhibition of FFA oxidation with etomoxir gest that FFAs are the physiological endogenous regula- reversed the negative effects of clofibrate on insulin secre- tors of ␤-cell UCP2 (44) and explain the partial uncoupling tion, the effects of etomoxir on UCP2 expression were not of ␤-cells observed after chronic FFA exposure (35,45). reported (39). Our work further identified superoxide radicals as the Another region of the Ucp2 promoter contains the sterol direct mediators of FFA-evoked uncoupling. Endogenous regulatory element (SRE) (36). Deletion of the SRE elim- superoxide has been shown to activate UCP2 in kidney inated the effects of oleic acid on Ucp2 gene transcription mitochondria (46). Acute FFA exposure induced ROS (36), whereas overexpression of the SRE binding protein 1 production to a similar degree in both control MIN6 cells caused a marked induction of Ucp2 promoter activity and and those exposed chronically to oleic acid. ROS were expression (36,40). Therefore, it appears that FFAs may produced primarily at complex I of the electron transport activate UCP2 gene transcription via SRE and possibly chain, and conditions stimulating this process caused peroxisome proliferator response elements, but there may mitochondrial depolarization, which depended on the also be potentiating effects exerted by FFA metabolites. presence of FFAs and was inhibited by a superoxide When consensus is reached as to what nutrients or their dismutase mimetic and GDP. Thus, FFAs induce superox- metabolites regulate islet UCP2 expression, the outcome ide production during their oxidation, leading to increased may well be that, for maximal expression (and therefore UCP2 activity to lower the mitochondrial membrane po- negative effects on insulin secretion), both glucose and tential. Because ␤-oxidation depends on a highly polarized FFAs need to be elevated. When high-fat diets were fed to membrane, these conditions serve as negative feedback on normal Wistar versus hyperglycemic Goto-Katazaki (GK) further superoxide production to limit ROS-mediated cell rats, only the GK rats exhibited enhanced islet UCP2 damage during the lipotoxic insult (44). Unfortunately, expression concomitant with impaired GSIS (41). More- FFAs appear to exert independent and distinct effects on over, if the high fat–fed GK rats were treated with insulin the respiratory chain to inhibit electron transport (47), to normalize their plasma glucose, the changes in UCP2 leading to increased ROS production. This concept is and insulin secretion were prevented (41). Recently, we supported by our data showing that FFAs stimulate ROS compared the effects of a high-fat diet on insulin secretion production (44). Other proteins may also function as in UCP2 knockout versus wild-type mice. After Ͼ4 months carriers of FFA anions and contribute to reduction of the feeding of a diet high in saturated fat, the wild-type mice mitochondrial membrane potential. The dicarboxylate car- exhibited a mild diabetes, with fasting hyperglycemia, rier (DIC) was shown to be induced by chronic FFA hyperinsulinemia, and hyperlipidemia in vivo. In vitro, exposure (44). Moreover, when malate (a natural substrate GSIS was impaired, while both Ucp2 and CPT-1 mRNA of DIC) was added along with acute oleate, the mitochon- expression were elevated. In contrast, the UCP2 knockout drial membrane became more polarized, showing that DIC mice fed the high-fat diet maintained normal fasting glu- contributes to uncoupling. To summarize, if FFAs are in cose, insulin, triglyceride, and FFA concentrations in vivo excess, the concentration of FFA anions in the mitochon- and maintained in vitro GSIS within normal limits. Inter- dria are projected to rise, either by “flip-flopping” of estingly, the UCP2 knockout mice on the control diet neutral FFAs across the mitochondrial membranes (11) or exhibited increased CPT-1 mRNA expression that was via CPT-1–mediated transfer followed by deacylation of

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FIG. 3. Palmitate oxidation by islets. Islets were cultured for 48 h in the absence (fatty acid–free BSA, Ⅺ) or presence of 0.5 mmol/l palmitate (with 1% fatty acid–free BSA carrier, s) and 8.3 mmol/l glucose. Overexpression of UCP2 in the absence of culture FFAs FIG. 2. Model of UCP2 and DIC catalyzed transport of FFA anions out reduced palmitate oxidation in the presence of 2.8 mmol/l glucose. In of the mitochondrial matrix. Under conditions of FFA excess, concen- the presence of culture FFAs, palmitate oxidation was enhanced in all trations of FFA anions in the mitochondria are projected to rise. The islet groups, but UCP2-overexpressing islets still oxidized significantly mechanism may either be through the flip-flop of neutral FFAs across less palmitate than control islets at both 2.8 and 25 mmol/l glucose. The the mitochondrial membranes (11) or through deacylation of long- number of experiments performed for each condition is 8–15. *P < 0.05 chain CoA by means of mitochondrial thioesterases (14). Increased compared with control islets exposed to the same glucose concentra- oxidation of long-chain acylCoA is expected to increase ROS produc- tion during palmitate oxidation assessment. §P < 0.05 comparing 25 tion, leading to activation of UCP2 by superoxide. In addition, both mmol/l glucose with 2.8 mmol/l glucose condition. UCP2 and DIC mRNA transcription is upregulated by exposure of ␤-cells to chronic FFAs. enzyme that facilitates transfer of long chain acyl-CoA into the mitochondrial matrix, thus promoting ␤-oxidation. long-chain CoA catalyzed by mitochondrial thioesterases High rates of FFA oxidation generate oxygen radicals (48) (14). Increased oxidation of long-chain CoA is expected to that are potentially damaging to ␤-cells, leading to so- increase ROS production, leading to activation of UCP2 by called lipoapoptosis. In our investigation of uncoupling in superoxide (see below). In addition, both UCP2 and DIC permeabilized ␤-cells, we found that oleate treatment for mRNA transcription is upregulated by exposure of ␤-cells 72 h increased succinate-dependent ROS production by to chronic FFAs (Fig. 2). nearly twofold. Moreover, acute application of oleate Metabolic data also obtained in our laboratory corrob- caused a fivefold increase in ROS production in both orate the hypothesis that induction of UCP2 will limit FFA control cells and ␤-cells chronically exposed to oleate. oxidation. Palmitate oxidation was examined in rat islets. When ␤-cells were induced to produce superoxide, the In control islets, increasing the glucose concentration mitochondria became more depolarized only in the pres- during the oxidation experiment suppressed palmitate ence of FFAs. Depolarization was greater in cells preincu- oxidation, as expected. In UCP2-overexpressing islets in bated with FFAs for 72 h (44). Thus, concomitant the presence of basal glucose, palmitate oxidation was induction of UCP2 by FFAs would at least partially negate significantly lower than that in control islets and not the potential to increase ROS production. changed when glucose was increased. When the islets Less physiological induction of oxidative stress by use were cultured for 48 h in the presence of palmitic acid, its of exogenous hydrogen peroxide elevated UCP2 mRNA oxidation was increased in both the wild-type and UCP2- expression in clonal ␤-cells. Furthermore, when UCP2 was overexpressing groups. However, the oxidation by UCP2- transiently overexpressed, cell survival was enhanced overexpressing islets was still 50% lower than that in the after peroxide exposure (49). These and other data show- wild-type islets (Fig. 3). This suggests that FFA oxidation ing that UCP2 knockout mice have increased ROS pro- is limited by the increased uncoupling exerted by UCP2, duction from macrophages (15) whereas UCP2 overex- which would increase the export of fatty acid anions out of pressing transgenic mice have protection from lesions the mitochondria, making them unavailable for oxidative inducing apoptosis (50) suggest that a physiological role of metabolism. When coupled with the inhibition of glucose UCP2 might be to modulate free radical production. Be- metabolism caused by high FFAs, this would explain the cause superoxide induces UCP2 expression and activity, lower cellular ATP observed in fat-exposed islets (20). its modulatory effect is expected to be enhanced when most needed, i.e., when free radical production is elevated. INTERACTION WITH ROS Because FFA treatment of islets has been shown to induce UCP2 AND TYPE 2 DIABETES UCP2 expression, the upregulation of UCP2 may be a Initially, the discovery of UCP1 homologues spurred inter- protective mechanism against excessive lipid exposure. est in their potential energy-wasting capabilities, a func- The concept of lipotoxicity in pancreatic islets is sup- tion associated with leanness and thus antidiabetic status. ported by evidence that high FFA levels induce a variety of Therefore, the finding that UCP2 suppressed insulin secre- genes that influence fat metabolism (1,2,47a). One key tion from islets and the hypothesis that an increase in modulator that undergoes upregulation after FFA expo- UCP2 could participate in insulin insufficiency in type 2 sure (1) or high-fat diet (20) is CPT-1, the rate-limiting diabetes was difficult to accept at first. The “yin-yang” of

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UCP2 and diabetes was reviewed (51,52) after the publi- the contention that lipotoxic effects are contingent upon cation of the UCP2 knockout mouse article (19). Other the coexistence of hyperglycemia (56). commentators have listed UCP2 as a potential “diabetes Upregulation of UCP2 expression and activity leading to gene” because of its negative effects in ␤-cells (53). Of suppression of GSIS as a contributor to diabetes develop- interest is a recent report of a functional polymorphism in ment is relatively straightforward to rationalize. However, the UCP2 promoter that increases the risk of obesity but it becomes more difficult to reconcile an increase in ␤-cell decreases the risk of type 2 diabetes (54). UCP2 with the fasting hyperinsulinemia observed in obese What is the functional evidence, at least in the rodent and obese-diabetic rodents such as fa/fa and ZDF rats. ␤-cell, that UCP2 is associated with type 2 diabetes? UCP2 This hypersecretion of insulin has been shown to be expression has been quantified in islets of several rodent dependent on mitochondrial ATP production and KATP models of type 2 diabetes. Ucp2 mRNA or protein is channel inactivation (57) and may be related to altered elevated in fa/fa rats (33) and ob/ob mice (19) but de- kinetics of glucokinase or other metabolic enzymes (58), creased in Zucker diabetic fatty rats (4,17). Induction of which strongly depends on the deranged hypothalamo- type 2 diabetes by high-fat feeding also upregulates UCP2 pituitary-adrenal axis in genetically obese rodent models gene expression, which is associated in all cases with (59). The increase in overall metabolism is sufficient to elevated plasma lipids (9,20,41). Likewise, hyperglycemic induce KATP channel closure at lower than normal glucose ␤ models also display an increase in islet UCP2 transcripts, (60). We propose that when fuel levels in -cells are at least temporarily (32,33). These animal models differ in relatively low, the mitochondrial membrane potential will the degree of insulin insufficiency and thus hyperglycemia. be commensurately reduced. Because activation of UCP2 None of these studies demonstrated a strong causal effect depends on generation of superoxide radicals, which depends on mitochondrial membrane hyperpolarization, of UCP2 induction on insulin insufficiency. Also, no sys- uncoupling will be minimized (44). tematic studies following the development of diabetes and correlating changes in UCP2 expression have been pub- lished. SIGNIFICANCE The development of the UCP2 knockout mouse pro- The overwhelming majority of research on ␤-cell metabo- vided an important model for studying causal relationships lism-secretion coupling has concentrated on stimulatory of UCP2 and various phenomena related to regulation of pathways and their modulation. UCP2 represents a novel insulin secretion. When the UCP2 gene was knocked out negative modulator of insulin secretion that has the poten- in the ob/ob mouse model by cross-breeding, the progeny tial to play a role in the pathogenesis of diet-related type 2 were not less obese than the parent ob/ob animals, but they diabetes. By determining how its endogenous expression displayed a reduction in plasma glucose that was associ- and activity is regulated, new methods for improving ated with an improvement in GSIS (19). Recently, we insulin secretion in diabetes may be realized. studied the effects of a chronic high-fat diet on glucose homeostasis and insulin secretion in the UCP2 knockout ACKNOWLEDGMENTS mice compared with wild-type controls (20). Both groups of mice gained the same amount of weight after 4.5 months Work conducted in the authors’ laboratories was funded on the lard-based diet. The UCP2 knockout mice displayed by grants to C.B.C. and M.B.W. from the Canadian Insti- partial resistance to the diet, in that they maintained lower tutes of Health Research (MOP 12898 and MOP 43987) and fasting and fed blood glucose levels. Both in vivo and in the Canadian Diabetes Association (the Neil M. Miller and vitro GSIS were enhanced. The maintenance of a high Doris Evangeline Bradley grants). capacity for insulin secretion was at least partially ac- Contributions to the work from T.S. McQuaid, P.E. counted for by higher islet insulin content and an increase MacDonald, J.W. Joseph, and D. De Leo are gratefully in ␤-cell area relative to the total pancreatic area. The acknowledged. We also thank D.C. Johns for constructing expansion of the ␤-cell mass appeared to be due to a more the UCP2 adenovirus and B.B. Lowell for supplying the favorable proliferation to apoptosis incidence in the UCP2 UCP2 knockout mice. knockout mice. 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