Role of the transmembrane potential in the membrane proton leak

The oxidative stress in breast tumors of postmenopausal women is ERα/ERβ ratio dependent

Estrogen receptor status is a diagnostic parameter in breast cancer treatment. Estrogen receptor presence is related to a better prognosis because the principal treatments attacking breast cancer tumors have their action site directed at the estrogen receptor. However, the two different subtypes of estrogen receptor, ERα and ERβ, have different functions. In this work an alternative point of view focusing on oxidative stress is shown, given that estrogen receptors regulate several proteins related to this oxidative stress, such as antioxidant enzymes, sirtuins, and uncoupling proteins. Postmenopausal human breast tumors with different ERα/ERβ ratios were analyzed to characterize the amount of oxidative stress, mitochondrial function, and proliferation-related and oxidative stress-activated signaling pathways. Results showed that tumors with a low ERα/ERβ ratio have greater oxidative damage and higher antioxidant enzyme protein levels, as well as uncoupling protein (UCP) and sirtuin 3 (SIRT3), and have high studied signaling pathway activation. Glutathione peroxidase, Complex V, Complex III, Complex II, Complex IV, AKT, SAPK, and ERα were significantly and positively correlated with ERα/ERβ ratio. However, carbonyl groups, catalase, CuZn-superoxide dismutase, UCP5, SIRT3, and ERβ were significantly and negatively correlated with ERα/ERβ ratio. From the independent variables included in the step-by-step stepwise multiple linear regression analysis, only the ERα/ERβ ratio was independently associated with carbonyl groups. Surprisingly, these low ERα/ERβ ratio tumors have poor prognosis for the patient, and these results and those of other authors suggest that these tumors are adapted to conditions of increased oxidative stress.

Metabolic regulation by the mitochondrial phosphatase PTPMT1 is required for hematopoietic stem cell differentiation

The regulation and coordination of mitochondrial metabolism with hematopoietic stem cell (HSC) self-renewal and differentiation is not fully understood. Here we report that depletion of PTPMT1, a PTEN-like mitochondrial phosphatase, in inducible or hematopoietic-cell-specific knockout mice resulted in hematopoietic failure due to changes in the cell cycle and a block in the differentiation of HSCs. Surprisingly, the HSC pool was increased by ∼40-fold in PTPMT1 knockout mice. Reintroduction of wild-type PTPMT1, but not catalytically deficient PTPMT1 or truncated PTPMT1 lacking mitochondrial localization, restored differentiation capabilities of PTPMT1 knockout HSCs. Further analyses demonstrated that PTPMT1 deficiency altered mitochondrial metabolism and that phosphatidylinositol phosphate substrates of PTPMT1 directly enhanced fatty-acid-induced activation of mitochondrial uncoupling protein 2. Intriguingly, depletion of PTPMT1 from myeloid, T lymphoid, or B lymphoid progenitors did not cause any defects in lineage-specific knockout mice. This study establishes a crucial role of PTPMT1 in the metabolic regulation of HSC function.

Mapping the nucleotide binding site of uncoupling protein 1 using atomic force microscopy

A tight regulation of proton transport in the inner mitochondrial membrane is crucial for physiological processes such as ATP synthesis, heat production, or regulation of the reactive oxygen species as proposed for the uncoupling protein family members (UCP). Specific regulation of proton transport is thus becoming increasingly important in the therapy of obesity and inflammatory, neurodegenerative, and ischemic diseases. We and other research groups have shown previously that UCP1- and UCP2-mediated proton transport is inhibited by purine nucleotides. Several hypotheses have been proposed to explain the inhibitory effect of ATP, although structural details are still lacking. Moreover, the unresolved mystery is how UCP operates in vivo despite the permanent presence of high (millimolar) concentrations of ATP in mitochondria. Here we use the topographic and recognition (TREC) mode of an atomic force microscope to visualize UCP1 reconstituted into lipid bilayers and to analyze the ATP–protein interaction at a single molecule level. The comparison of recognition patterns obtained with anti-UCP1 antibody and ATP led to the conclusion that the ATP binding site can be accessed from both sides of the membrane. Using cantilever tips with different cross-linker lengths, we determined the location of the nucleotide binding site inside the membrane with 1 Å precision. Together with the recently published NMR structure of a UCP family member (Berardi et al. Nature, 2011, 476, 109–113), our data provide a valuable insight into the mechanism of the nucleotide binding and pave the way for new pharmacological approaches against the diseases mentioned above.

Antioxidant activity by a synergy of redox-sensitive mitochondrial phospholipase A2 and uncoupling protein-2 in lung and spleen

Mitochondrial uncoupling protein-2 (UCP2) has been suggested to participate in the attenuation of the reactive oxygen species production, but the mechanism of action and the physiological significance of UCP2 activity remain controversial. Here we tested the hypothesis that UCP2 provides feedback downregulation of oxidative stress in vivo via synergy with an H2O2-activated mitochondrial calcium-independent phospholipase A2 (mt-iPLA2). Tert-butylhydroperoxide or H2O2 induced free fatty acid release from mitochondrial membranes as detected by gas chromatography/mass spectrometry, which was inhibited by r-bromoenol lactone (r-BEL) but not by its stereoisomer s-BEL, suggesting participation of mt-iPLA2γ isoform. Tert-butylhydroperoxide or H2O2 also induced increase in respiration and decrease in mitochondrial membrane potential in lung and spleen mitochondria from control but not UCP2-knockout mice. These data suggest that mt-iPLA2γ-dependent release of free fatty acids promotes UCP2-dependent uncoupling. Upon such uncoupling, mitochondrial superoxide formation decreased instantly also in the s-BEL presence, but not when mt-iPLA2 was blocked by R-BEL and not in mitochondria from UCP2-knockout mice. Mt-iPLA2γ was alternatively activated by H2O2 produced probably in conjunction with the electron-transferring flavoprotein:ubiquinone oxidoreductase (ETFQOR), acting in fatty acid β-oxidation. Palmitoyl-d,l-carnitine addition to mouse lung mitochondria, respiring with succinate plus rotenone, caused a respiration increase that was sensitive to r-BEL and insensitive to s-BEL. We thus demonstrate for the first time that UCP2, functional due to fatty acids released by redox-activated mt-iPLA2γ, suppresses mitochondrial superoxide production by its uncoupling action. In conclusion, H2O2-activated mt-iPLA2γ and UCP2 act in concert to protect against oxidative stress.

Redox homeostasis in pancreatic β cells

We reviewed mechanisms that determine reactive oxygen species (redox) homeostasis, redox information signaling and metabolic/regulatory function of autocrine insulin signaling in pancreatic β cells, and consequences of oxidative stress and dysregulation of redox/information signaling for their dysfunction. We emphasize the role of mitochondrion in β cell molecular physiology and pathology, including the antioxidant role of mitochondrial uncoupling protein UCP2. Since in pancreatic β cells pyruvate cannot be easily diverted towards lactate dehydrogenase for lactate formation, the respiration and oxidative phosphorylation intensity are governed by the availability of glucose, leading to a certain ATP/ADP ratio, whereas in other cell types, cell demand dictates respiration/metabolism rates. Moreover, we examine the possibility that type 2 diabetes mellitus might be considered as an inevitable result of progressive self-accelerating oxidative stress and concomitantly dysregulated information signaling in peripheral tissues as well as in pancreatic β cells. It is because the redox signaling is inherent to the insulin receptor signaling mechanism and its impairment leads to the oxidative and nitrosative stress. Also emerging concepts, admiting participation of redox signaling even in glucose sensing and insulin release in pancreatic β cells, fit in this view. For example, NADPH has been firmly established to be a modulator of glucose-stimulated insulin release.

Quantification of uncoupling protein 2 reveals its main expression in immune cells and selective up-regulation during T-cell proliferation

Uncoupling protein 2 (UCP2) is an inner mitochondrial membrane protein. Although the protein was discovered in 1997, its function and even its tissue distribution are still under debate. Here we present a quantitative analysis of mRNA and protein expression in various mice tissues, revealing that UCP2 is mainly expressed in organs and cells associated with the immune system. Although the UCP2 gene is present in the brain, as demonstrated using quantitative RT-PCR, the protein was not detectable in neurons under physiological conditions. Instead, we could detect UCP2 in microglia, which act in the immune defense of the central nervous system. In lymphocytes, activation led to a ten-fold increase of UCP2 protein expression simultaneously to the increase in levels of other mitochondrial proteins, whereas lymphocyte re-stimulation resulted in the selective increase of UCP2. The highest detected level of UCP2 expression in stimulated T-cells (0.54 ng/(µg total cellular protein)) was approximately 200 times lower than the level of UCP1 in brown adipose tissue from room temperature acclimated mice. Both the UCP2 expression pattern and the time course of up-regulation in stimulated T-cells imply UCP2’s involvement in the immune response, probably by controlling the metabolism during cell proliferation.

Long-term Fgf23 deficiency does not influence aging, glucose homeostasis, or fat metabolism in mice with a nonfunctioning vitamin D receptor

It is still controversial whether the bone-derived hormone fibroblast growth factor-23 (FGF23) has additional physiological functions apart from its well-known suppressive actions on renal phosphate reabsorption and vitamin D hormone synthesis. Here we analyzed premature aging, mineral homeostasis, carbohydrate metabolism, and fat metabolism in 9-month-old male wild-type (WT) mice, vitamin D receptor mutant mice (VDR(Δ/Δ)) with a nonfunctioning vitamin D receptor, and Fgf23⁻/⁻/VDR(Δ/Δ) compound mutant mice on both a standard rodent chow and a rescue diet enriched with calcium, phosphorus, and lactose. Organ atrophy, lung emphysema, and ectopic tissue or vascular calcifications were absent in compound mutants. In addition, body weight, glucose tolerance, insulin tolerance, insulin secretory capacity, pancreatic beta cell volume, and retroperitoneal and epididymal fat mass as well as serum cholesterol and triglycerides were indistinguishable between vitamin D receptor and compound mutants. In contrast to VDR(Δ/Δ) and Fgf23⁻/⁻/VDR(Δ/Δ) mice, which stayed lean, WT mice showed obesity-induced insulin resistance. To rule out alopecia and concomitantly elevated energy expenditure present in 9-month-old VDR(Δ/Δ) and Fgf23⁻/⁻/VDR(Δ/Δ) mice as a confounding factor for the lacking effect of Fgf23 deficiency on fat mass, we analyzed whole-body composition in WT, Fgf23⁻/⁻, VDR(Δ/Δ), and Fgf23⁻/⁻/VDR(Δ/Δ) mice at the age of 4 wk, when the coat in VDR(Δ/Δ) mice is still normal. Whole-body fat mass was reduced in Fgf23⁻/⁻ mice but almost identical in WT, VDR(Δ/Δ), and Fgf23⁻/⁻/VDR(Δ/Δ) mice. In conclusion, our data indicate that Fgf23 has no molecular vitamin D-independent role in aging, insulin signaling, or fat metabolism in mice.

Role of mitochondrial uncoupling protein 4 in rat inner ear

The uncoupling protein 4 (UCP4) belongs to the mitochondrial anion transporter family. Protein tissue distribution and functions are still a matter of debate. Using an antibody we have previously shown that UCP4 appears in neurons and to a lesser extent in astrocytes of murine neuronal tissue as early as days 12-14 of embryonic development (Smorodchenko et al., 2009). Here we demonstrated for the first time that neurosensory cells such as hair cells of the inner ear and mechanosensitive Merkel cells in skin also express a significant amount of UCP4. We tested the hypothesis about whether UCP4 contributes to the regulation of oxidative stress using the model of oxygen deprivation. For this we compared the protein expression level in freshly isolated explants of organ of Corti, modiolus and stria vascularis from neonatal rats with explants cultured under hypoxia. Western blot analysis revealed that the UCP4 level was not increased under hypoxic conditions, when compared to the mitochondrial outer membrane protein VDAC or to the anti-oxidative enzyme SOD2. We moreover demonstrated that UCP4 expression is differently regulated during postnatal stages and is region-specific. We hypothesized that UCP4 may play an important role in functional maturation of the rat inner ear.

Test systems to study the structure and function of uncoupling protein 1: a critical overview

The discovery of active brown adipose tissue (BAT) in healthy adult humans has renewed interest in the biology of this organ. BAT is capable of distributing nutrient energy in the form of heat allowing small mammals to efficiently defend their body temperature when acutely exposed to the cold. On the other hand BAT might be a target for the treatment of obesity and related diseases, as its pharmacological activation could allow release of excess energy stored in white adipose tissue depots. Energy dissipation in BAT depends on the activity of uncoupling protein 1 (UCP1), therefore a BAT-based obesity therapy requires a detailed understanding of structure and function of UCP1. Although UCP1 has been in the focus of research since its discovery, central questions concerning its mechanistic function and regulation are not yet resolved. They have been addressed in native mitochondria but also in several test systems, which are generally used to lower inter-experimental variability and to simplify analysis conditions. Different test systems have contributed to our current knowledge about UCP1 but of course all of them have certain limitations. We here provide an overview about research on UCP1 structure and function in test systems. So far, these have nearly exclusively been employed to study rodent and not human UCP1. Considering that the amino acid sequence of mouse and human UCP1 is only 79% identical, it will be essential to test whether the human version has a similarly high catalytic activity, allowing a relevant amount of energy dissipation in human BAT. Besides the issue of comparable mechanistic function a sufficiently high expression level of human UCP1 is a further prerequisite for anti-obesity therapeutic potential. Treatments which induce BAT hyperplasia and UCP1 expression in humans might therefore be equally important to discover as mere activators of the thermogenic process.

Channel character of uncoupling protein-mediated transport

Mitochondrial uncoupling proteins (UCPs) are pure anion uniporters, which mediate fatty acid (FA) uniport leading to FA cycling. Protonated FAs then flip-flop back across the lipid bilayer. An existence of pure proton channel in UCPs is excluded by the equivalent flux-voltage dependencies for uniport of FAs and halide anions, which are best described by the Eyring barrier variant with a single energy well in the middle of two peaks. Experiments with FAs unable to flip and alkylsulfonates also support this view. Phylogenetically, UCPs took advantage of the common FA-uncoupling function of SLC25 family carriers and dropped their solute transport function.