Uncoupling of protein-3 induces an uncontrolled uncoupling of mitochondria after expression in muscle derived L6 cells

The regulation and physiology of mitochondrial proton leak

Mitochondria couple respiration to ATP synthesis through an electrochemical proton gradient. Proton leak across the inner membrane allows adjustment of the coupling efficiency. The aim of this review is threefold: 1) introduce the unfamiliar reader to proton leak and its physiological significance, 2) review the role and regulation of uncoupling proteins, and 3) outline the prospects of proton leak as an avenue to treat obesity, diabetes, and age-related disease.

Targeted expression of uncoupling protein 2 to mouse liver increases the susceptibility to lipopolysaccharide/galactosamine-induced acute liver injury

Normal hepatocytes do not express endogenous uncoupling protein 2 (UCP2) in adult liver, although Kupffer cells do, and it is strikingly induced in hepatocytes in steatotic liver and obese conditions. However, the direct link of UCP2 with the pathogenic development of liver diseases and liver injury remains elusive. Here we report that targeted expression of UCP2 to mouse liver increases susceptibility to acute liver injury induced by lipopolysaccharide (LPS) and galactosamine (GalN). UCP2 appears to enhance proton leak, leading to mild uncoupling in a guanosine diphosphate-repressible manner. Indeed, mitochondria from the genetically manipulated mouse liver have increased state 4 respiration, lower respiratory control ratio, and reduced adenosine triphosphate (ATP) levels, which altered mitochondrial physiology. To address the underlying mechanism of how UCP2 and the reduced energy coupling efficiency enhance cell death in mouse liver, we show that the reduced ATP levels lead to activation of 5'AMP-activated protein kinase (AMPK) and its downstream effector, c-Jun N-terminal kinase; thus, the increased sensitivity toward LPS/GalN-induces apoptosis. Importantly, we show that inhibition of UCP2 activity by its pharmacological inhibitor genipin prevents LPS/GalN-induced ATP reduction, AMPK activation, and apoptosis. Also, inhibition of ATP production by oligomycin promotes LPS/GalN-induced cell death both in vivo and in vitro.

CONCLUSION

Our results clearly show that targeted expression of UCP2 in liver may result in compromised mitochondrial physiology that contributes to enhanced cell death and suggests a potential role of UCP2 in the development of liver diseases.

Fatty acids do not activate UCP2 in pancreatic beta cells: comparison with UCP1

UCP2 is expressed in pancreatic beta cells where its postulated uncoupling activity will modulate glucose-induced changes in ATP/ADP ratio and insulin secretion. The consequences of UCP2 over/underexpression on beta-cell function has mainly been studied in the basal state; however, a UCP has no uncoupling activity unless stimulated by fatty acids and/or reactive oxygen species. Here, UCP2 was overexpressed in INS-1 cells and parameters reflecting mitochondrial coupling measured in the basal state and after stimulation by fatty acids. For comparison, UCP1 was expressed to similar levels and the same parameters measured. Neither UCP1 expression nor UCP2 overexpression modified basal or glucose-stimulated metabolic changes. Upon addition of fatty acids, UCP1-expressing cells displayed the expected mitochondrial uncoupling effect, while UCP2 did not elicit any measurable change in mitochondrial function. Taken together, our data demonstrate that, in pancreatic beta-cells, UCP2 has no uncoupling activity in the basal state or after fatty acid stimulation.

The reactions catalysed by the mitochondrial uncoupling proteins UCP2 and UCP3

The mitochondrial uncoupling proteins UCP2 and UCP3 may be important in attenuating mitochondrial production of reactive oxygen species, in insulin signalling (UCP2), and perhaps in thermogenesis and other processes. To understand their physiological roles, it is necessary to know what reactions they are able to catalyse. We critically examine the evidence for proton transport and anion transport by UCP2 and UCP3. There is good evidence that they increase mitochondrial proton conductance when activated by superoxide, reactive oxygen species derivatives such as hydroxynonenal, and other alkenals or their analogues. However, they do not catalyse proton leak in the absence of such acute activation. They can also catalyse export of fatty acid and other anions, although the relationship of anion transport to proton transport remains controversial.

Uncoupling protein-3 sensitizes cells to mitochondrial-dependent stimulus of apoptosis

The mitochondrial uncoupling protein-3 is a member of the mitochondrial carrier protein family. As a homologue of the thermogenic brown fat uncoupling protein-1, it possesses a mitochondrial uncoupling activity and thus can influence cell energy metabolism but its exact biological function remains unclear. In the present study, uncoupling protein-3 was expressed in 293 cells using the tetracycline-inducible system and its impact on cell bioenergetics and responsiveness to the apoptotic stimulus was determined. The induction of uncoupling protein-3 expression in mitochondria did not lead to uncontrolled respiratory uncoupling in intact cells. However, it caused a GDP-inhibition of state 4 respiration and a GDP-induced re-polarization of the inner mitochondrial membrane in the presence of fatty acids, in agreement with its expected physiological behavior as an uncoupling protein (UCP). Uncoupling protein-3 expression did not cause apoptosis per se but increased the responsiveness of the cells to a mitochondrial apoptotic stimulus (i.e., addition of staurosporine in the culture medium). It enhanced caspase 3 and caspase 9 activation and favored cytochrome c release. Moreover, cells in which uncoupling protein-3 expression had been induced showed a higher mitochondrial Bax/Bcl-2 ratio essentially due to enhanced translocation of Bax from cytosol to mitochondria. Finally, the induction of uncoupling protein-3 also increased the sensitivity of mitochondria to open the permeability transition pore in response to calcium. It is concluded that the presence of uncoupling protein-3 in mitochondria sensitizes cells to apoptotic stimuli involving mitochondrial pathways.

Taurine increases glucose sensitivity of UCP2-overexpressing beta-cells by ameliorating mitochondrial metabolism

A low-taurine diet during fetal or early postnatal life causes abnormal pancreatic beta-cell development. Tissue and plasma taurine concentrations can also be low in diabetic patients. We examined the effect of taurine on impaired glucose responses in diabetic rat beta-cells adenovirally overexpressing uncoupling protein (UCP)2, which is upregulated in obesity-related type 2 diabetes. We found that taurine pretreatment restored the ATP-to-ADP (ATP/ADP) ratio and glucose-stimulated insulin secretion in UCP2-infected islets. ATP-sensitive K(+) channel sensitivity to dihydroxyacetone, another insulin secretagogue, was similar in both UCP2-infected and control beta-cells. In freshly isolated mitochondria from UCP2-overexpressing insulin-secreting (INS)-1 beta-cells, methyl pyruvate-mediated mitochondrial Ca(2+) increase was significantly ameliorated by taurine. A mitochondrial Ca(2+) uniporter blocker, ruthenium red, inhibited the action of taurine. This study suggests that taurine enhances the glucose sensitivity of UCP2-overexpressing beta-cells, probably by increasing mitochondrial Ca(2+) influx through the Ca(2+) uniporter, thereby enhancing mitochondrial metabolic function and increasing the ATP/ADP ratio.

Positive correlation of skeletal muscle UCP3 mRNA levels with overweight in male, but not in female, rats

The objective of this study was to investigate the sex-dependent regulation of skeletal muscle uncoupling protein (UCP)3 mRNA expression in response to overweight and its relationship with serum levels of free fatty acids, leptin, and insulin. Two obesity models were used: rats made obese by feeding them with a cafeteria diet for 14 wk, and postcafeteria overweight rats fed a chow diet for 10 wk after consuming the cafeteria diet for 14 wk. The effects of 24-h fasting were studied in postcafeteria rats and their age-matched controls. The cafeteria rats ate a high-fat diet and attained an excess body weight that was higher in females (+59%) than in males (+39%). A trend to higher induction of abdominal muscle UCP3 mRNA in male rats than in females after cafeteria diet was apparent (+116% increase vs. +26% increase). Postcafeteria male but not female rats still showed the tendency to have increased UCP3 mRNA levels relative to their age-matched controls. A linear regression analysis showed a significant positive correlation of the UCP3 mRNA levels with overweight and with serum levels of leptin and insulin in males, but not in females, and no correlation with serum free fatty acid levels. A subsequent correlation analysis and a multiple linear regression analysis showed that overweight was the only parameter actually related to UCP3 mRNA levels in males. Fasting-induced upregulation of muscle UCP3 mRNA levels was higher in males (5- to 7-fold) than in females (3- to 4-fold). Our results point to the existence of sex-associated differences in the control of muscle UCP3 expression in response to overweight and fasting, with an impaired induction in female rats under both conditions. The correlation of abdominal muscle UCP3 mRNA expression with overweight in males could be related to their relative resistance to gain weight after chronic overeating of a cafeteria diet, by the purported role of UCP3 in the regulation of lipid utilization.

Superoxide activates mitochondrial uncoupling protein 2 from the matrix side. Studies using targeted antioxidants

Superoxide activates nucleotide-sensitive mitochondrial proton transport through the uncoupling proteins UCP1, UCP2, and UCP3 (Echtay, K. S., et al. (2002) Nature 415, 1482-1486). Two possible mechanisms were proposed: direct activation of the UCP proton transport mechanism by superoxide or its products and a cycle of hydroperoxyl radical entry coupled to UCP-catalyzed superoxide anion export. Here we provide evidence for the first mechanism and show that superoxide activates UCP2 in rat kidney mitochondria from the matrix side of the mitochondrial inner membrane: (i) Exogenous superoxide inhibited matrix aconitase, showing that external superoxide entered the matrix. (ii) Superoxide-induced uncoupling was abolished by low concentrations of the mitochondrially targeted antioxidants 10-(6'-ubiquinonyl)decyltriphenylphosphonium (mitoQ) or 2-[2-(triphenylphosphonio)ethyl]-3,4-dihydro-2,5,7,8-tetramethyl-2H-1-benzopyran-6-ol bromide (mitoVit E), which are ubiquinone (Q) or tocopherol derivatives targeted to the matrix by covalent attachment to triphenylphosphonium cation. However, superoxide-induced uncoupling was not affected by similar concentrations of the nontargeted antioxidants Q(o), Q(1), decylubiquinone, vitamin E, or 6-hydroxy-2,5,7,8-tetramethylchroman 2-carboxylic acid (TROLOX) or of the mitochondrially targeted but redox-inactive analogs decyltriphenylphosphonium or 4-chlorobutyltriphenylphosphonium. Thus matrix superoxide appears to be necessary for activation of UCP2 by exogenous superoxide. (iii) When the reduced to oxidized ratio of mitoQ accumulated by mitochondria was increased by inhibiting cytochrome oxidase, it induced nucleotide-sensitive uncoupling that was not inhibited by external superoxide dismutase. Under these conditions quinols are known to produce superoxide, and because mitoQ is localized within the mitochondrial matrix this suggests that production of superoxide in the matrix was sufficient to activate UCP2. Furthermore, the superoxide did not need to be exported or to cycle across the inner membrane to cause uncoupling. We conclude that superoxide (or its products) exerts its uncoupling effect by activating the proton transport mechanism of uncoupling proteins at the matrix side of the mitochondrial inner membrane.