Increased expression of uncoupling protein 2 in HepG2 cells attenuates oxidative damage and apoptosis

UCP2 is highly expressed in pancreatic alpha-cells and influences secretion and survival

In pancreatic beta-cells, uncoupling protein 2 (UCP2) influences mitochondrial oxidative phosphorylation and insulin secretion. Here, we show that alpha-cells express significantly higher levels of UCP2 than do beta-cells. Greater mitochondrial UCP2-related uncoupling was observed in alpha-cells compared with beta-cells and was accompanied by a lower oxidative phosphorylation efficiency (ATP/O). Conversely, reducing UCP2 activity in alpha-cells was associated with higher mitochondrial membrane potential generated by glucose oxidation and with increased ATP synthesis, indicating more efficient metabolic coupling. In vitro, the suppression of UCP2 activity led to reduced glucagon secretion in response to low glucose; however, in vivo, fasting glucagon levels were normal in UCP2(-/-) mice. In addition to its effects on secretion, UCP2 played a cytoprotective role in islets, with UCP2(-/-) alpha-cells being more sensitive to specific death stimuli. In summary, we demonstrate a direct role for UCP2 in maintaining alpha-cell function at the level of glucose metabolism, glucagon secretion, and cytoprotection.

The mitochondrial uncoupling protein-2 promotes chemoresistance in cancer cells

Cancer cells acquire drug resistance as a result of selection pressure dictated by unfavorable microenvironments. This survival process is facilitated through efficient control of oxidative stress originating from mitochondria that typically initiates programmed cell death. We show this critical adaptive response in cancer cells to be linked to uncoupling protein-2 (UCP2), a mitochondrial suppressor of reactive oxygen species (ROS). UCP2 is present in drug-resistant lines of various cancer cells and in human colon cancer. Overexpression of UCP2 in HCT116 human colon cancer cells inhibits ROS accumulation and apoptosis after exposure to chemotherapeutic agents. Tumor xenografts of UCP2-overexpressing HCT116 cells retain growth in nude mice receiving chemotherapy. Augmented cancer cell survival is accompanied by altered NH(2)-terminal phosphorylation of the pivotal tumor suppressor p53 and induction of the glycolytic phenotype (Warburg effect). These findings link UCP2 with molecular mechanisms of chemoresistance. Targeting UCP2 may be considered a novel treatment strategy for cancer.

Mitochondrial uncoupling protein-2 mediates steatotic liver injury following ischemia/reperfusion

Steatotic livers are not used for transplantation because they have a reduced tolerance for ischemic events with reduced ATP levels and greater levels of cellular necrosis, which ultimately result in total organ failure. Mitochondrial uncoupling protein-2 (UCP2) is highly expressed in steatotic livers and may be responsible for liver sensitivity to ischemia through mitochondrial and ATP regulation. To test this hypothesis, experiments were conducted in lean and steatotic (ob/ob), wild-type, and UCP2 knock-out mice subjected to total warm hepatic ischemi-a/reperfusion. Although ob/ob UCP2 knock-out mice and ob/ob mice have a similar initial phenotype, ob/ob UCP2 knock-out animal survival was 83% when compared with 30% in ob/ob mice 24 h after reperfusion. Serum alanine aminotransferase concentrations and hepatocellular necrosis were decreased in the ob/ob UCP2 knock-out mice when compared with ob/ob mice subjected to ischemia. Liver ATP levels were increased in the ob/ob UCP2 knock-out animals after reperfusion when compared with the ob/ob mice but remained below the concentrations from lean livers. Lipid peroxidation (thiobarbituric acid-reactive substances) increased after reperfusion most significantly in the steatotic groups, but the increase was not affected by UCP2 deficiency. These results reveal that UCP2 expression is a critical factor, which sensitizes steatotic livers to ischemic injury, regulating liver ATP levels after ischemia and reperfusion.

Angiotensin II Type 1 receptor antagonism mediates uncoupling protein 2-driven oxidative stress and ameliorates pancreatic islet beta-cell function in young Type 2 diabetic mice

We recently identified a local pancreatic islet renin-angiotensin system (RAS), and demonstrated that it is upregulated in an animal model of obesity-induced type 2 diabetes mellitus (T2DM). Moreover, angiotensin II type 1 receptor (AT1R) antagonism improves beta-cell function and glucose tolerance in young T2DM mice and delays the onset of diabetes. Meanwhile, obesity-induced T2DM results in oxidative stress-mediated activation of uncoupling protein 2 (UCP2), a negative regulator of islet function. In the present study, we postulated that some of the protective effects of AT1R antagonism might be mediated through interference with this pathway and tested this hypothesis in a T2DM animal model. Losartan, an AT1R antagonist, was given to 4-week-old obese db/db mice for a period of 8 weeks. UCP2-driven oxidative damage and apoptosis were then analyzed in isolated islets. Losartan selectively inhibited oxidative stress via downregulation of NADPH oxidase; this in turn suppressed UCP2 expression, thus improving beta-cell insulin secretion and decreasing apoptosis-induced beta-cell mass loss in db/db mouse islets. These data indicate that islet AT1R activation in young diabetic mice can generate progressive islet beta-cell failure through UCP-driven oxidative damage.

Overexpression of uncoupling protein 4 promotes proliferation and inhibits apoptosis and differentiation of preadipocytes

Uncoupling proteins are a family of mitochondrial proteins involved in energy metabolism. We previously showed that uncoupling protein 4 (UCP4) is differentially expressed in omental adipose tissue in diet-induced obese and normal rats. However, the effect of UCP4 on adipocytes is unclear. In this work, we established a stable preadipocyte cell line overexpressing UCP4 to observe the direct effect of UCP4 on adipocytes. Cells overexpressing UCP4 showed significantly attenuated differentiation of preadipocytes into adipocytes. During differentiation, expression of adipogenesis-associated markers such as fatty acid synthetase, peroxisome proliferator-activated receptor gamma, CCAAT enhancer binding protein alpha, adipocyte lipid binding protein and lipoprotein lipase were downregulated. Preadipoctes expressing UCP4 grew faster and more of them stayed in S phase compared to control cells. In addition, UCP4 overexpression protected preadipocytes from apoptosis induced by serum deprivation. Our results demonstrate that overexpression of UCP4 can promote proliferation and inhibit apoptosis and differentiation of preadipocytes.

Aluminum impairs rat neural cell mitochondria in vitro

Exposure to aluminum has been reported to lead to neurotoxicity. Mitochondria are important organelles involved in maintaining cell function. This study investigates the effect of aluminum on mitochondria in rat neural cells. The ultrastructure of mitochondria was observed, and the cell death rate (CDR), reactive oxygen species (ROS), mitochondrial membrane potential (MMP) and 3-[4,5demethyl-2-thiazalyl]-2,-5diphenyl-2H-tetrazolium bromide (MTT) were measured to investigate the effect of aluminum on the mitochondrial structure and its function in neural cells. Results observed from the mitochondrial ultrastructure show that aluminum may impair the mitochondrial membrane and cristae. Increased CDR, enhanced ROS, decreased MMP, and decreased enzyme activity in mitochondria were observed in the Al-exposed neurons (100 500 microM). The present study demonstrates that alteration in the mitochondrial structure and function plays an important role in neurotoxic mechanisms induced by aluminum.

Enhanced colon tumor induction in uncoupling protein-2 deficient mice is associated with NF-kappaB activation and oxidative stress

Oxidative stress has a complex effect on cancer development. To further study this process, we induced colon tumors with azoxymethane (AOM) in mice deficient for uncoupling protein-2 (UCP2). UCP2 has recently emerged as a negative regulator of mitochondrial oxidant production. When overexpressed, UCP2 protects cells from oxidative stress, while its absence may cause abundance of reactive oxygen species, release of pro-inflammatory cytokines and persistent activation of nuclear factor kappaB (NF-kappaB), a pleiotropic transcription factor with an increasingly recognized role in cancer. Here we show that Ucp2-/- mice develop more aberrant crypt foci and colon tumors than Ucp2+/+ littermates when examined 24 weeks after the completion of treatment with AOM (10 mg/kg i.p. weekly for a total of 6 weeks, n = 8-12). This effect is primarily seen in the proximal colon of Ucp2-/- mice (P < 0.05), in association with changes indicative of increased oxidative stress (increased staining for malondialdehyde and inducible nitric oxide synthase), enhanced NF-kappaB activation (increased levels of phosphorylated IkappaB and increased nuclear presence of p65) and a disrupted balance between intestinal epithelial cell proliferation (greater 5-bromo-2'-deoxy-uridine incorporation rates and increased phosphorylation of ERK1/2 and AKT) and apoptosis (decreased number of terminal deoxynucleotidyltransferase-mediated nick-end-labeling (TUNEL)-positive cells and increased expression of Bcl-2). In conclusion, our findings provide the first in vivo evidence for a link between UCP2 and tumorigenesis and indicate the need for additional studies to assess the role of mitochondrial uncoupling in cancer development.

The emerging functions of UCP2 in health, disease, and therapeutics

The uncoupling proteins (UCPs) are attracting an increased interest as potential therapeutic targets in a number of important diseases. UCP2 is expressed in several tissues, but its physiological functions as well as potential therapeutic applications are still unclear. Unlike UCP1, UCP2 does not seem to be important to thermogenesis or weight control, but appears to have an important role in the regulation of production of reactive oxygen species, inhibition of inflammation, and inhibition of cell death. These are central features in, for example, neurodegenerative and cardiovascular disease, and experimental evidence suggests that an increased expression and activity of UCP2 in models of these diseases has a beneficial effect on disease progression, implicating a potential therapeutic role for UCP2. UCP2 has an important role in the pathogenesis of type 2 diabetes by inhibiting insulin secretion in islet beta cells. At the same time, type 2 diabetes is associated with increased risk of cardiovascular disease and atherosclerosis where an increased expression of UCP2 appears to be beneficial. This illustrates that therapeutic applications involving UCP2 likely will have to regulate expression and activity in a tissue-specific manner.