Further studies on the recoupling effect of 6-ketocholestanol upon oxidative phosphorylation in uncoupled liver mitochondria

Inhibition of respiratory complex I by 6-ketocholestanol: Relevance to recoupling action in mitochondria

6-Ketocholestanol (kCh) is known as a mitochondrial recoupler, i.e. it abolishes uncoupling of mitochondria by such potent agents as carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and 3,5-di(tert-butyl)-4-hydroxybenzylidenemalononitril (SF6847) [Starkov et al., 1997]. Here, we report data on the kCh-induced inhibition of both NADH-oxidase and NADH-ubiquinone oxidoreductase activities of the respiratory complex I in bovine heart submitochondrial particles (SMP). Based on the absence of such inhibition with hexaammineruthenium (III) (HAR) as the complex I electron acceptor, the kCh effect could be associated with the ubiquinone-binding centre of this respiratory enzyme. In isolated rat liver mitochondria (RLM), kCh inhibited oxygen consumption with the glutamate/malate, substrates of NAD-linked dehydrogenases, while no inhibition of RLM respiration was observed with succinate, in agreement with the absence of the kCh effect on the succinate oxidase activity in SMP. Three kCh analogs (cholesterol, 6α-hydroxycholesterol, and 5α,6α-epoxycholesterol) exhibited no effect on the NADH oxidase activities in both SMP and RLM. Importantly, the kCh analogs were ineffective in the recoupling of RLM treated with CCCP or SF6847. Therefore, interaction of kCh with the complex I may be involved in the kCh-mediated mitochondrial recoupling.

Role of respiratory uncoupling in drug-induced mitochondrial permeability transition

Mitochondrial injury contributes to severe drug-induced liver injury. Particularly, mitochondrial permeability transition (MPT) is thought to be relevant to cytolytic hepatitis. However, the mechanism of drug-induced MPT is unclear and prediction of MPT is not adequately evaluated in the preclinical stage. In a previous study, we found that troglitazone, a drug withdrawn due to liver injury, induced MPT via mild depolarization probably resulting from uncoupling. Herein, we investigated whether other drugs that induce MPT share similar properties as troglitazone, using isolated mitochondria from rat liver. Of the 22 test drugs examined, six drugs, including troglitazone, induced MPT and showed an uncoupling effect. Additionally, receiver operating characteristic analysis was conducted to predict the MPT potential from the respiratory control ratio, an indicator of uncoupling intensity. Results showed that 2.5 was the best threshold that exhibited high sensitivity (1.00) and high specificity (0.81), indicating that uncoupling was correlated with MPT potential. Activation of calcium-independent phospholipase A2 appeared to be involved in uncoupling-induced MPT. Furthermore, a strong relationship between MPT intensity and the uncoupling effect among similar compounds was confirmed. These results may help in predicting MPT potential using cultured cells and modifying the chemical structures of the drugs to reduce MPT risk.

The mitochondria-targeted derivative of the classical uncoupler of oxidative phosphorylation carbonyl cyanide m-chlorophenylhydrazone is an effective mitochondrial recoupler

The synthesis of a mitochondria-targeted derivative of the classical mitochondrial uncoupler carbonyl cyanide-m-chlorophenylhydrazone (CCCP) by alkoxy substitution of CCCP with n-decyl(triphenyl)phosphonium cation yielded mitoCCCP, which was able to inhibit the uncoupling action of CCCP, tyrphostin A9 and niclosamide on rat liver mitochondria, but not that of 2,4-dinitrophenol, at a concentration of 1–2 μM. MitoCCCP did not uncouple mitochondria by itself at these concentrations, although it exhibited uncoupling action at tens of micromolar concentrations. Thus, mitoCCCP appeared to be a more effective mitochondrial recoupler than 6-ketocholestanol. Both mitoCCCP and 6-ketocholestanol did not inhibit the protonophoric activity of CCCP in artificial bilayer lipid membranes, which might compromise the simple proton-shuttling mechanism of the uncoupling activity on mitochondria.

48Biochemical mechanisms of the anti-obesity effect of a triterpenoid-enriched extract of Cynomorium songaricum in mice with high-fat-diet-induced obesity

BACKGROUND

HCY2, a triterpenoid-enriched extract of Cynomorii Herba, has been shown to reduce body weight and adiposity and attenuate manifestations of the associated metabolic syndrome in high-fat-diet (HFD)-fed mice.

PURPOSE

The current study aimed to investigate the biochemical mechanism underlying the anti-obesity effect produced by HCY2.

STUDY DESIGN

An HCY2-containing extract was examined for its effects on the regulation of adenosine monophosphate-activated protein kinase (AMPK)/peroxisome proliferator-activated receptor gamma co-activator-1 (PGC1) pathways and the protein expression related to mitochondrial uncoupling and biogenesis in skeletal muscle using an HFD-induced obese mouse model.

METHODS

The obese mouse model was produced by providing HFD (60% kcal from fat) ad libitum. The effects and signaling mechanisms of HCY2 were examined using analytical procedures which included enzyme-linked immunosorbent assay kits, Western blot analysis, and the use of a Clark-type oxygen electrode.

RESULTS

The current study revealed that the weight reduction produced by HCY2 is associated with the activation of the AMPK signaling pathway, with resultant increases in mitochondrial biogenesis and expression of uncoupling protein 3 in skeletal muscle in vivo. The use of a recoupler, ketocholestanol, delineated the precise role of mitochondrial uncoupling in the anti-obesity effect afforded by HCY2 in obese mice.

CONCLUSION

Our experimental findings offer a promising prospect for the use of HCY2 in the management of obesity through the regulation of AMPK/PGC1 pathways.

6-Ketocholestanol suppresses lipid accumulation by decreasing FASN gene expression through SREBP-dependent regulation in HepG2 cells

Nuclear receptors, such as liver X receptors (LXRs) and sterol regulatory element-binding proteins (SREBPs), are key regulators of lipogenic genes, including fatty acid synthase (FASN). It has been reported that several oxycholesterols (OCs) act as LXR ligands; however, it is unclear whether all OC molecular species act as ligands. We previously demonstrated that the absorption rate of dietary 6-ketocholestanol (6-keto), an oxycholesterol, is the highest of all the OCs using thoracic lymph duct-cannulated rats. However, limited information is available about the physiological significance of 6-keto. In this study, we investigated whether treatment with 6-keto increases intracellular triacylglycerol (TAG) levels through up-regulation of lipogenic gene expression in HepG2 cells. 6-Keto treatment significantly reduced intracellular TAG levels through down-regulation of lipogenic genes including FASN. Although 6-keto significantly suppressed FASN gene promoter activities, the action was completely diminished when mutations were present in the SREBP promoter site. TO901317 (TO) significantly increased FASN gene promoter activities, whereas simultaneous treatment with TO and 6-keto significantly reduced this activity. We also compared the effects of several OCs that are oxidized at the carbon-6 and -7 in the B-ring of cholesterol on FASN gene promoter activities. Similar to 6-keto, 6α-OH and 6β-OH significantly reduced the activity of the FASN gene promoter, which suggests that oxidation of carbon-6 in the B-ring may play an important role in the reduction of FASN expression. Our results indicate that 6-keto suppresses lipid accumulation by decreasing FASN gene expression through SREBP-dependent regulation in HepG2 cells.

FR58P1a; a new uncoupler of OXPHOS that inhibits migration in triple-negative breast cancer cells via Sirt1/AMPK/β1-integrin pathway

Highly malignant triple-negative breast cancer (TNBC) cells rely mostly on glycolysis to maintain cellular homeostasis; however, mitochondria are still required for migration and metastasis. Taking advantage of the metabolic flexibility of TNBC MDA-MB-231 cells to generate subpopulations with glycolytic or oxidative phenotypes, we screened phenolic compounds containing an ortho-carbonyl group with mitochondrial activity and identified a bromoalkyl-ester of hydroquinone named FR58P1a, as a mitochondrial metabolism-affecting compound that uncouples OXPHOS through a protonophoric mechanism. In contrast to well-known protonophore uncoupler FCCP, FR58P1a does not depolarize the plasma membrane and its effect on the mitochondrial membrane potential and bioenergetics is moderate suggesting a mild uncoupling of OXPHOS. FR58P1a activates AMPK in a Sirt1-dependent fashion. Although the activation of Sirt1/AMPK axis by FR58P1a has a cyto-protective role, selectively inhibits fibronectin-dependent adhesion and migration in TNBC cells but not in non-tumoral MCF10A cells by decreasing β1-integrin at the cell surface. Prolonged exposure to FR58P1a triggers a metabolic reprograming in TNBC cells characterized by down-regulation of OXPHOS-related genes that promote cell survival but comprise their ability to migrate. Taken together, our results show that TNBC cell migration is susceptible to mitochondrial alterations induced by small molecules as FR58P1a, which may have therapeutic implications.

Small structural changes on a hydroquinone scaffold determine the complex I inhibition or uncoupling of tumoral oxidative phosphorylation

Mitochondria participate in several distinctiveness of cancer cell, being a promising target for the design of anti-cancer compounds. Previously, we described that ortho-carbonyl hydroquinone scaffold 14 inhibits the complex I-dependent respiration with selective anti-proliferative effect on mouse mammary adenocarcinoma TA3/Ha cancer cells; however, the structural requirements of this hydroquinone scaffold to affect the oxidative phosphorylation (OXPHOS) of cancer cells have not been studied in detail. Here, we characterize the mitochondrial metabolism of TA3/Ha cancer cells, which exhibit a high oxidative metabolism, and evaluate the effect of small structural changes of the hydroquinone scaffold 14 on the respiration of this cell line. Our results indicate that these structural changes modify the effect on OXPHOS, obtaining compounds with three alternative actions: inhibitors of complex I-dependent respiration, uncoupler of OXPHOS and compounds with both actions. To confirm this, the effect of a bicyclic hydroquinone (9) was evaluated in isolated mitochondria. Hydroquinone 9 increased mitochondrial respiration in state 4o without effects on the ADP-stimulated respiration (state 3ADP), decreasing the complexes I and II-dependent respiratory control ratio. The effect on mitochondrial respiration was reversed by 6-ketocholestanol addition, indicating that this hydroquinone is a protonophoric uncoupling agent. In intact TA3/Ha cells, hydroquinone 9 caused mitochondrial depolarization, decreasing intracellular ATP and NAD(P)H levels and GSH/GSSG ratio, and slightly increasing the ROS levels. Moreover, it exhibited selective NAD(P)H availability-dependent anti-proliferative effect on cancer cells. Therefore, our results indicate that the ortho-carbonyl hydroquinone scaffold offers the possibility to design compounds with specific actions on OXPHOS of cancer cells.

Kinetics of the H+-ATPase from dry and 5-hours-imbibed maize embryos in its native, solubilized, and reconstituted forms

Membranes undergo recovery upon rehydration in seed germination. Previous work has described that the plasma membrane H+-ATPase from maize embryos adopts two different forms at 0 and 5 h of imbibition. We investigated how the kinetics of these two forms could be affected by alterations in the plasma membrane (PM). In comparison to the 0-h, PMs from the 5-h imbibed embryos showed changes in glycerophospholipid composition, decrease in leakage, and increase in fluidity. Kinetics of the PM H+-ATPase from 0 and 5-h imbibed embryos showed negative cooperativity. With the removal of the membrane environment, the activity of the enzymes shifted to a more complex kinetics, displaying two enzyme components. Lipid reconstitution produced one component with positive cooperativity. In all cases, enzymes from 0 and 5-h imbibed embryos presented similar kinetics with some quantitative differences. These results indicate that the two enzyme forms have the potential ability to respond to changes in the membrane environment, but the fact that they do not show differences in the native membranes at 0 or 5 h implies that modifications in the membrane are not drastic enough to alter their kinetics, or that they are able to preserve their boundary lipids or associated proteins and thus retain the same kinetic behavior.