Antioxidant action of ubiquinol homologues with different isoprenoid chain length in biomembranes

Location and interaction of idebenone and mitoquinone in a membrane similar to the inner mitochondrial membrane. Comparison with ubiquinone 10

Oxygen is essential for aerobic life on earth but it is also the origin of harmful reactive oxygen species (ROS). Ubiquinone is par excellence the endogenous cellular antioxidant, but a very hydrophobic one. Because of that, other molecules have been envisaged, such as idebenone (IDE) and mitoquinone (MTQ), molecules having the same redox active benzoquinone moiety but higher solubility. We have used molecular dynamics to determine the location and interaction of these molecules, both in their oxidized and reduced forms, with membrane lipids in a membrane similar to that of the mitochondria. Both IDE and reduced IDE (IDOL) are situated near the membrane interface, whereas both MTQ and reduced MTQ (MTQOL) locate in a position adjacent to the phospholipid hydrocarbon chains. The quinone moieties of both ubiquinone 10 (UQ10) and reduced UQ10 (UQOL10) in contraposition to the same moieties of IDE, IDOL, MTQ and MTQOL, located near the membrane interphase, whereas the isoprenoid chains remained at the middle of the hydrocarbon chains. These molecules do not aggregate and their functional quinone moieties are located in the membrane at different depths but near the hydrophobic phospholipid chains whereby protecting them from ROS harmful effects.

Endogenous chemicals guard health through inhibiting ferroptotic cell death

Ferroptosis is a new form of regulated cell death caused by iron-dependent accumulation of lethal polyunsaturated phospholipids peroxidation. It has received considerable attention owing to its putative involvement in a wide range of pathophysiological processes such as organ injury, cardiac ischemia/reperfusion, degenerative disease and its prevalence in plants, invertebrates, yeasts, bacteria, and archaea. To counter ferroptosis, living organisms have evolved a myriad of intrinsic efficient defense systems, such as cyst(e)ine-glutathione-glutathione peroxidase 4 system (cyst(e)ine-GPX4 system), guanosine triphosphate cyclohydrolase 1/tetrahydrobiopterin (BH4) system (GCH1/BH4 system), ferroptosis suppressor protein 1/coenzyme Q10 system (FSP1/CoQ10 system), and so forth. Among these, GPX4 serves as the only enzymatic protection system through the reduction of lipid hydroperoxides, while other defense systems ultimately rely on small compounds to scavenge lipid radicals and prevent ferroptotic cell death. In this article, we systematically summarize the chemical biology of lipid radical trapping process by endogenous chemicals, such as coenzyme Q10 (CoQ10), BH4, hydropersulfides, vitamin K, vitamin E, 7-dehydrocholesterol, with the aim of guiding the discovery of novel ferroptosis inhibitors.

Higher levels of the lipophilic antioxidants coenzyme Q10 and vitamin E in long-lived termite queens than in short-lived workers

Termite queens and kings live longer than nonreproductive workers. Several molecular mechanisms contributing to their long lifespan have been investigated; however, the underlying biochemical explanation remains unclear. Coenzyme Q (CoQ), a component of the mitochondrial electron transport chain, plays an essential role in the lipophilic antioxidant defense system. Its beneficial effects on health and longevity have been well studied in several organisms. Herein, we demonstrated that long-lived termite queens have significantly higher levels of the lipophilic antioxidant CoQ10 than workers. Liquid chromatography analysis revealed that the levels of the reduced form of CoQ10 were 4 fold higher in the queen's body than in the worker's body. In addition, queens showed 7 fold higher levels of vitamin E, which plays a role in antilipid peroxidation along with CoQ, than workers. Furthermore, the oral administration of CoQ10 to termites increased the CoQ10 redox state in the body and their survival rate under oxidative stress. These findings suggest that CoQ10 acts as an efficient lipophilic antioxidant along with vitamin E in long-lived termite queens. This study provides essential biochemical and evolutionary insights into the relationship between CoQ10 concentrations and termite lifespan extension.

Redox-crippled MitoQ potently inhibits breast cancer and glioma cell proliferation: A negative control for verifying the antioxidant mechanism of MitoQ in cancer and other oxidative pathologies

Mitochondria-targeted coenzyme Q10 (Mito-ubiquinone, Mito-quinone mesylate, or MitoQ) was shown to be an effective antimetastatic drug in patients with triple-negative breast cancer. MitoQ, sold as a nutritional supplement, prevents breast cancer recurrence. It potently inhibited tumor growth and tumor cell proliferation in preclinical xenograft models and in vitro breast cancer cells. The proposed mechanism of action involves the inhibition of reactive oxygen species by MitoQ via a redox-cycling mechanism between the oxidized form, MitoQ, and the fully reduced form, MitoQH2 (also called Mito-ubiquinol). To fully corroborate this antioxidant mechanism, we substituted the hydroquinone group (-OH) with the methoxy group (-OCH3). Unlike MitoQ, the modified form, dimethoxy MitoQ (DM-MitoQ), lacks redox-cycling between the quinone and hydroquinone forms. DM-MitoQ was not converted to MitoQ in MDA-MB-231 cells. We tested the antiproliferative effects of both MitoQ and DM-MitoQ in human breast cancer (MDA-MB-231), brain-homing cancer (MDA-MB-231BR), and glioma (U87MG) cells. Surprisingly, DM-MitoQ was slightly more potent than MitoQ (IC50 = 0.26 μM versus 0.38 μM) at inhibiting proliferation of these cells. Both MitoQ and DM-MitoQ potently inhibited mitochondrial complex I-dependent oxygen consumption (IC50 = 0.52 μM and 0.17 μM, respectively). This study also suggests that DM-MitoQ, which is a more hydrophobic analog of MitoQ (logP: 10.1 and 8.7) devoid of antioxidant function and reactive oxygen species scavenging ability, can inhibit cancer cell proliferation. We conclude that inhibition of mitochondrial oxidative phosphorylation by MitoQ is responsible for inhibition of breast cancer and glioma proliferation and metastasis. Blunting the antioxidant effect using the redox-crippled DM-MitoQ can serve as a useful negative control in corroborating the involvement of free radical-mediated processes (e.g., ferroptosis, protein oxidation/nitration) using MitoQ in other oxidative pathologies.

Recent Advances in Mitochondrial Diseases: from Molecular Insights to Therapeutic Perspectives

Mitochondria are double-membraned cytoplasmic organelles that are responsible for the production of energy in eukaryotic cells. The process is completed through oxidative phosphorylation (OXPHOS) by the respiratory chain (RC) in mitochondria. Thousands of mitochondria may be present in each cell, depending on the function of that cell. Primary mitochondria disorder (PMD) is a clinically heterogeneous disease associated with germline mutations in mitochondrial DNA (mtDNA) and/or nuclear DNA (nDNA) genes, and impairs mitochondrial structure and function. Mitochondrial dysfunction can be detected in early childhood and may be severe, progressive and often multi-systemic, involving a wide range of organs. Understanding epigenetic factors and pathways mutations can help pave the way for developing an effective cure. However, the lack of information about the disease (including age of onset, symptoms, clinical phenotype, morbidity and mortality), the limits of current preclinical models and the wide range of phenotypic presentations hamper the development of effective medicines. Although new therapeutic approaches have been introduced with encouraging preclinical and clinical outcomes, there is no definitive cure for PMD. This review highlights recent advances, particularly in children, in terms of etiology, pathophysiology, clinical diagnosis, molecular pathways and epigenetic alterations. Current therapeutic approaches, future advances and proposed new therapeutic plans will also be discussed.

Redox Pioneer: Professor Valerian Kagan

Professor Valerian Kagan (PhD, 1972, MV Lomonosov Moscow State University; DSci, 1981, USSR, Academy of Sciences, Moscow) is recognized as a Redox Pioneer because he has published 4 articles in the field of redox biology that have been cited >1000 times and 138 articles in this field have been cited between 100 and 924 times. The central and most important impact of Dr. Kagan's research is in the field of redox lipidomics-a term coined for the first time by Dr. Kagan in 2004-and consequently the definition of signaling pathways by oxidatively modified phospholipids; this acquires further significance considering that oxygenated phospholipids play multifunctional roles as essential signals coordinating metabolism and physiology. Some examples are the selective oxidation of cardiolipin (CL) by a cytochrome c peroxidase activity leading to the activation of the intrinsic apoptotic pathway; the hydroperoxy-arachidonoyl/adrenoyl phosphatidylethanolamine (PE) species, driven by 15-lipoxygenases (15-LOX), as death signals leading to ferroptotic cell death; the regulation of ferroptosis by iNOS/NO^• in pro-inflammatory conditions by a novel mechanism (realized via interactions of 15-LOX reaction intermediates formed from arachidonoyl phosphatidylethanolamine [PE] species) and Ca²⁺-independent phospholipase A2 (iPLA₂β; via elimination of peroxidized PE); the involvement of oxygenated (phospho)lipids in immunosuppression by myeloid cells in the tumor microenvironment; hydrolysis of peroxidized CL by Ca²⁺-independent phospholipase A2 (iPLA₂γ) leading to pro- and anti-inflammatory signals and lipid mediators. Kagan continues his investigations to decipher the roles of enzyme-linked oxygenated phospholipids. Antioxid. Redox Signal. 36, 813-823.

Role of lipoic acid in multiple sclerosis

Lipoic acid (LA) is an endogenous antioxidant that exists widely in nature. Supplementation with LA is a promising approach to improve the outcomes of patients with multiple sclerosis (MS). This systematic review aimed to provide a comprehensive overview of both in vitro and in vivo studies describing the pharmacokinetics, efficacy, safety, and mechanism of LA in MS‐related experiments and clinical trials. A total of 516 records were identified by searching five databases, including PubMed, Web of Science, Embase, Scopus, and Cochrane Library. Overall, we included 20 studies reporting LA effects in cell and mouse models of MS and 12 studies reporting LA effects in patients with MS. Briefly, cell experiments revealed that LA protected neurons by inhibiting the expression of inflammatory mediators and activities of immune cells. Experimental autoimmune encephalomyelitis mouse experiments demonstrated that LA consistently reduced the number of infiltrating immune cells in the central nervous system and decreased the clinical disability scores. Patients with MS showed relatively stable Expanded Disability Status Scale scores and better walking performance with few adverse events after the oral administration of LA. Notably, heterogeneity of this evidence existed among modeling methods, LA usage, MS stage, and trial duration. In conclusion, this review provides evidence for the anti‐inflammatory and antioxidative effects of LA in both in vitro and in vivo experiments; therefore, patients with MS may benefit from LA administration. Whether LA can be a routine supplementary therapy warrants further study.

Tuning the reduction potentials of benzoquinone through the coordination to Lewis acids

Electron transfer promoted by the coordination of a substrate molecule to a Lewis acid or hydrogen bonding group is a critical step in many biological and catalytic transformations. This computational study investigates the nature of the interaction between benzoquinone and one and two Lewis acids by examining the influence of Lewis acid strength on the ability to alter the two reduction potentials of the coordinated benzoquinone molecule. To investigate this interaction, the coordination of the neutral (Q), singly reduced ([Q]˙-), and doubly reduced benzoquinone ([Q]2-) molecule to eight Lewis acids was analyzed. Coordination of benzoquinone to a Lewis acid became more favorable by 25 kcal mol-1 with each reduction of the benzoquinone fragment. Coordination of benzoquinone to a Lewis acid also shifted each of the reduction potentials of the coordinated benzoquinone anodically by 0.50 to 1.5 V, depending on the strength of the Lewis acid, with stronger Lewis acids exhibiting a larger effect on the reduction potential. Coordination of a second Lewis acid further altered each of the reduction potentials by an additional 0.70 to 1.6 V. Replacing one of the Lewis acids with a proton resulted in the ability to modify the pKa of the protonated Lewis acid-Q/[Q]˙-/[Q]2- adducts by about 10 pKa units, in addition to being able to alter the ability to transfer a hydrogen atom by 10 kcal mol-1, and the capacity to transfer a hydride by about 30 kcal mol-1.

Exploring Ubiquinone Biosynthesis Inhibition as a Strategy for Improving Atovaquone Efficacy in Malaria

Atovaquone (AV) acts on the malaria parasite by competing with ubiquinol (UQH₂) for its union to the mitochondrial bc₁ complex, preventing the ubiquinone-8 and ubiquinone-9 (UQ-8 and UQ-9) redox recycling, which is a necessary step in pyrimidine biosynthesis. This study focused on UQ biosynthesis in Plasmodium falciparum and adopted proof-of-concept research to better elucidate the mechanism of action of AV and improve its efficacy. Initially, UQ biosynthesis was evaluated using several radioactive precursors and chromatographic techniques. This methodology was suitable for studying the biosynthesis of both UQ homologs and its redox state. Additionally, the composition of UQ was investigated in parasites cultivated at different oxygen saturations or in the presence of AV. AV affected the redox states of both UQ-8 and UQ-9 homologs by increasing the levels of the respective reduced forms. Conversely, low-oxygen environments specifically inhibited UQ-9 biosynthesis and increased the antimalarial efficacy of AV. These findings encouraged us to investigate the biological importance and the potential of UQ biosynthesis as a drug target based on its inhibition by 4-nitrobenzoate (4-NB), a 4-hydroxybenzoate (4-HB) analog. 4-NB effectively inhibits UQ biosynthesis and enhances the effects of AV on parasitic growth and respiration rate. Although 4-NB itself exhibits poor antimalarial activity, its 50% inhibitory concentration (IC₅₀) value increased significantly in the presence of a soluble UQ analog, p-aminobenzoic acid (pABA), or 4-HB. These results indicate the potential of AV combined with 4-NB as a novel therapy for malaria and other diseases caused by AV-sensitive pathogens.

Coenzyme Q10 Analogues: Benefits and Challenges for Therapeutics

Coenzyme Q₁₀ (CoQ₁₀ or ubiquinone) is a mobile proton and electron carrier of the mitochondrial respiratory chain with antioxidant properties widely used as an antiaging health supplement and to relieve the symptoms of many pathological conditions associated with mitochondrial dysfunction. Even though the hegemony of CoQ₁₀ in the context of antioxidant-based treatments is undeniable, the future primacy of this quinone is hindered by the promising features of its numerous analogues. Despite the unimpeachable performance of CoQ₁₀ therapies, problems associated with their administration and intraorganismal delivery has led clinicians and scientists to search for alternative derivative molecules. Over the past few years, a wide variety of CoQ₁₀ analogues with improved properties have been developed. These analogues conserve the antioxidant features of CoQ₁₀ but present upgraded characteristics such as water solubility or enhanced mitochondrial accumulation. Moreover, recent studies have proven that some of these analogues might even outperform CoQ₁₀ in the treatment of certain specific diseases. The aim of this review is to provide detailed information about these Coenzyme Q₁₀ analogues, as well as their functionality and medical applications.

Extracellular coenzyme Q10 (CoQ10) is reduced to ubiquinol-10 by intact Hep G2 cells independent of intracellular CoQ10 reduction

Coenzyme Q₁₀ (CoQ₁₀) is an essential factor in the mitochondrial respiratory chain and is closely associated with ATP production in humans. It is known that orally administered CoQ₁₀ in humans is rapidly reduced, and most is detected as a reduced form, ubiquinol-10 (CoQ₁₀H₂), in serum. However, the mechanism of exogenous CoQ₁₀ reduction in vivo is unclear. Therefore, in order to clarify how CoQ₁₀ is reduced to CoQ₁₀H₂, we conducted a study using human liver cancer cell line Hep G2 cells, which show strong intracellular CoQ₁₀-reducing activity. When intact cells were incubated with CoQ₁₀, the exogenously added CoQ₁₀ was incorporated into the cells, time-, concentration-, and temperature-dependently, and 50-80% of that was detected as CoQ₁₀H₂. On the other hand, a part of the extracellular CoQ₁₀ was also detected as CoQ₁₀H₂, and the amount was greater than that of the intracellular CoQ₁₀H₂. Furthermore, the CoQ₁₀-loaded cells did not leak the intracellular CoQ₁₀H₂ (or CoQ₁₀) to the outside of the cells, and modulation of the extracellular CoQ₁₀H₂ amount had little effect on the intracellular CoQ₁₀ or CoQ₁₀H₂ contents, suggesting the existence of an individual mechanism of CoQ₁₀ reduction inside and outside the cells. Moreover, intact cells could reduce CoQ₁₀ in low-density lipoprotein to CoQ₁₀H₂. Therefore, we concluded that a novel CoQ₁₀-reducing mechanism may exist in the plasma membrane, probably the outer surface, of Hep G2 cells, and it may work to reduce extracellular CoQ₁₀ and/or maintain extracellular CoQ₁₀H₂.

Intracellular reduction of coenzyme Q homologues with a short isoprenoid side chain induces apoptosis of HeLa cells

Coenzyme Q (CoQ) is an essential factor of the mitochondrial respiratory chain. CoQ homologues with different lengths of the isoprenoid side chain are widely distributed in nature, but little is known about the relationship between the isoprenoid side chain length and biological function; therefore, we examined the effects of CoQ homologues on HeLa cells. When CoQ homologues with a shorter isoprenoid side chain than CoQ4 were added to HeLa cells, they induced cell death, and the order of cytotoxic intensity was as follows: CoQ0 ≫ CoQ3 ≈ CoQ1 > CoQ2 ≫ CoQ4. Furthermore, we found that CoQ1, CoQ2 and CoQ3 could induce caspase-mediated apoptosis, and the order of intensity was as follows: CoQ3 > CoQ2 ≥ CoQ1. We could not identify the participation of reactive oxygen species in the apoptosis induction, but observed that an NAD(P)H dehydrogenase (quinone) 1 (NQO1) inhibitor, dicumarol, could inhibit not only the intracellular reduction of the homologues but also apoptosis. However, because dicumarol did not affect well-known apoptosis inducers, such as anti-Fas IgG, tumor necrosis factor (TNF)-α, TNF-related apoptosis-inducing ligand, UV-B and H2O2 of HeLa cells at all, we concluded that NQO1-related intracellular reduction of CoQ, or its reduced product, ubiquinol, may participate in the apoptosis induction of HeLa cells.

Mass Transport Phenomena in Lipid Oxidation and Antioxidation

In lipid dispersions, the ability of reactants to move from one lipid particle to another is an important, yet often ignored, determinant of lipid oxidation and its inhibition by antioxidants. This review describes three putative interparticle transfer mechanisms for oxidants and antioxidants: (a) diffusion, (b) collision-exchange-separation, and (c) micelle-assisted transfer. Mechanism a involves the diffusion of molecules from one particle to another through the intervening aqueous phase. Mechanism b involves the transfer of molecules from one particle to another when the particles collide with each other. Mechanism c involves the solubilization of molecules in micelles within the aqueous phase and then their transfer between particles. During lipid oxidation, the accumulation of surface-active lipid hydroperoxides (LOOHs) beyond their critical micelle concentration may shift their mass transport from the collision-exchange-separation pathway (slow transfer) to the micelle-assisted mechanism (fast transfer), which may account for the transition from the initiation to the propagation phase. Similarly, the cut-off effect governing antioxidant activity in lipid dispersions may be due to the fact that above a certain hydrophobicity, the transfer mechanism for antioxidants changes from diffusion to collision-exchange-separation. This hypothesis provides a simple model to rationalize the design and formulation of antioxidants and dispersed lipids.

Antioxidant defenses in human blood plasma and extra-cellular fluids

I had the fortune to be introduced to Helmut Sies during the mid 1980s, while working as a post-doctoral scientist at the University of California, Berkeley. At that time, Helmut was a frequent visitor of the Bruce Ames' laboratory and a leading authority in antioxidants and oxidative stress. His concepts, ideas and willingness to listen and make constructive suggestions have been far-reaching and visionary. Moreover, they have also been highly infectious, so much so that much of my research to this day has been on the same topic. The following is a personal recount on how the field of antioxidants has evolved since those exciting days in Berkeley.

Lipoic acid stimulates bone formation in ovariectomized rats in a dose-dependent manner

This study was undertaken to determine the osteotropic effect of different doses of lipoic acid (LA) on the mineralization of bone tissue in female Wistar rats with experimental osteopenia induced by bilateral ovariectomy. Fifty-six rats were randomly selected and submitted to either a sham operation (n = 8) or an ovariectomy (n = 48). The ovariectomized rats were randomly placed into two control groups, treated subcutaneously with either physiological saline or 17β-estradiol in the dose of 4 μg/kg body mass per day, and four experimental groups that received LA subcutaneously in the doses of 12.5, 25, 50, and 100 mg/kg body mass per day (n = 8 in each group). After 28 days of experimental treatment, the rats were sacrificed, and body mass, total skeletal density, and body composition were recorded. Blood serum and isolated femora were stored for further analysis. Our results revealed that the osteoprotective effect of LA was dose-dependent and was observed in rats treated with 50 and 100 mg/kg of LA. Moreover, the LA applied to the ovariectomized rats in the dose of 50 mg/kg not only stopped the bone resorption, but stimulated its formation.

Knockdown of the coenzyme Q synthesis gene Smed-dlp1 affects planarian regeneration and tissue homeostasis

The freshwater planarian is a model organism used to study tissue regeneration that occupies an important position among multicellular organisms. Planarian genomic databases have led to the identification of genes that are required for regeneration, with implications for their roles in its underlying mechanism. Coenzyme Q (CoQ) is a fundamental lipophilic molecule that is synthesized and expressed in every cell of every organism. Furthermore, CoQ levels affect development, life span, disease and aging in nematodes and mice. Because CoQ can be ingested in food, it has been used in preventive nutrition. In this study, we investigated the role of CoQ in planarian regeneration. Planarians synthesize both CoQ9 and rhodoquinone 9 (RQ9). Knockdown of Smed-dlp1, a trans-prenyltransferase gene that encodes an enzyme that synthesizes the CoQ side chain, led to a decrease in CoQ9 and RQ9 levels. However, ATP levels did not consistently decrease in these animals. Knockdown animals exhibited tissue regression and curling. The number of mitotic cells decreased in Smed-dlp1 (RNAi) animals. These results suggested a failure in physiological cell turnover and stem cell function. Accordingly, regenerating planarians died from lysis or exhibited delayed regeneration. Interestingly, the observed phenotypes were partially rescued by ingesting food supplemented with α-tocopherol. Taken together, our results suggest that oxidative stress induced by reduced CoQ9 levels affects planarian regeneration and tissue homeostasis.

Partition, orientation and mobility of ubiquinones in a lipid bilayer

Ubiquinone is the universal mobile charge carrier involved in biological electron transfer processes. Its redox properties and biological function depend on the molecular partition and lateral diffusion over biological membranes. However, ubiquinone localization and dynamics within lipid bilayers are long debated and still uncertain. Here we present molecular dynamics simulations of several ubiquinone homologs with variable isoprenoid tail lengths complexed to phosphatidylcholine bilayers. Initially, a new force-field parametrization for ubiquinone is derived from and compared to high level quantum chemical data. Free energy profiles for ubiquinone insertion in the lipid bilayer are obtained with the new force-field. The profiles allow for the determination of the equilibrium location of ubiquinone in the membrane as well as for the validation of the simulation model by direct comparison with experimental partition coefficients. A detailed analysis of structural properties and interactions shows that the ubiquinone polar head group is localized at the water-bilayer interface at the same depth of the lipid glycerol groups and oriented normal to the membrane plane. Both the localization and orientation of ubiquinone head groups do not change significantly when increasing the number of isoprenoid units. The isoprenoid tail is extended and packed with the lipid acyl chains. For ubiquinones with long tails, the terminal isoprenoid units have high flexibility. Calculated ubiquinone diffusion coefficients are similar to that found for the phosphatidylcholine lipid. These results may have further implications for the mechanisms of ubiquinone transport and binding to respiratory and photosynthetic protein complexes.

Improved spatial resolution of matrix-assisted laser desorption/ionization imaging of lipids in the brain by alkylated derivatives of 2,5-dihydroxybenzoic acid

RATIONALE

Matrix-assisted laser desorption/ionization (MALDI) is one of the major techniques for mass spectrometry imaging (MSI) of biological systems along with secondary-ion mass spectrometry (SIMS) and desorption electrospray mass spectrometry (DESI). The inherent variability of MALDI-MSI signals within intact tissues is related to the heterogeneity of both the sample surface and the matrix crystallization. To circumvent some of these limitations of MALDI-MSI, we have developed improved matrices for lipid analysis based on structural modification of the commonly used matrix 2,5-dihydroxybenzoic acid (DHB).

METHODS

We have synthesized DHB containing -C6H13 and -C12H25 alkyl chains and applied these matrices to rat brain using a capillary sprayer. We utilized a Bruker Ultraflex II MALDI-TOF/TOF mass spectrometer to analyze lipid extracts and tissue sections, and examined these sections with polarized light microscopy and differential interference contrast microscopy.

RESULTS

O-alkylation of DHB yields matrices, which, when applied to brain sections, follow a trend of phase transition from crystals to an oily layer in the sequence DHB → DHB-C6H13 → DHB-C12H25 . MALDI-MSI images acquired with DHB-C12H25 exhibited a considerably higher density of lipids than DHB.

CONCLUSIONS

Comparative experiments with DHB and DHB-C12H25 are presented, which indicate that the latter matrix affords higher lateral resolution than the former.

Vitamin E management of oxidative damage-linked dysfunctions of hyperthyroid tissues

INTRODUCTION

Thyroid hormones affect growth, development, and metabolism of vertebrates, and are considered the major regulators of their homeostasis. On the other hand, elevated circulating levels of thyroid hormones are associated with modifications in the whole organism (weight loss and increased metabolism and temperature) and in several body regions. Indeed, tachycardia, atrial arrhythmias, heart failure, muscle weakness and wasting, bone mass loss, and hepatobiliary complications are commonly found in hyperthyroid animals and humans.

RESULTS

Most thyroid hormone actions result from influences on transcription of T3-responsive genes, which are mediated through nuclear receptors. However, there is significant evidence that tissue oxidative stress underlies some dysfunctions produced by hyperthyroidism.

DISCUSSION

During the last decades, increasing interest has been turned to the use of antioxidants as therapeutic agents in various diseases and pathophysiological disorders believed to be mediated by oxidative stress. In particular, because elevated circulating levels of thyroid hormones are associated with tissue oxidative injury, more attention has been paid to explore the application of antioxidants as therapeutic agents in thyroid related disorders.

CONCLUSIONS

At present, vitamin E is among the most commonly consumed dietary supplements due to the belief that it, as an antioxidant, may attenuate morbidity and mortality. This is due to the results of numerous scientific studies, which demonstrate that vitamin E has a primary function to destroy peroxyl radicals, thus protecting polyunsaturated fatty acids biological membranes from oxidative damage. However, results are also available indicating that protective vitamin E effects against oxidative damage can be obtained even through different mechanisms.

Oxidative stress in patients with essential hypertension: a comparison of dippers and non-dippers

BACKGROUND

Oxidative stress seems to play an important role in the pathophysiology of essential hypertension. We aimed to examine serum MDA, NO, 8-OHdG, ADMA, NT, CoQ10 and TAC as biomarkers of oxidative stress in dipper and non-dipper hypertensive patients.

METHODS

Eighteen dipper hypertensives, 20 non-dipper hypertensives and 22 healthy control subjects were included in the study. Clinical assessment and ambulatory blood pressure monitoring were performed in patients. Serum MDA, TAC and NO levels were measured by using spectrophotometric methods. CoQ10 levels were measured by HPLC method. 8-OHdG, ADMA and NT were quantitated by ELISA methods.

RESULTS

MDA levels were significantly higher in dipper and non-dipper groups compared to controls (p<0.05 and p<0.01, respectively). TAC levels were found at low level in patients dipper and non-dipper patients compared to control group (p<0.01). Higher ADMA and NT levels but lower CoQ10 levels were found in non-dipper group compared to healthy controls (p<0.01, p<0.05, and p<0.05, respectively). ADMA levels were found higher in non-dipper group than those of dipper group (p<0.01).

DISCUSSION

Increased ADMA, NT levels and decreased CoQ10 levels in non-dipper hypertensive patients might indicate more severe oxidative stres compared with dipper hypertensive patients, which plays an important role in the development of cardiovascular diseases. Increased MDA and reduced TAC levels might be considered as prospective prognostic markers of the development of cardiovascular diseases in dipper and non-dipper hypertensive patients.

Aged garlic extract and coenzyme Q10 have favorable effect on inflammatory markers and coronary atherosclerosis progression: A randomized clinical trial

Background:

Aged garlic extract (AGE) and coenzyme Q10 (CoQ10) have been shown to affect multiple cardiovascular risk factors. The current study evaluates the effect of AGE combined with CoQ10 on inflammatory markers and progression of coronary atherosclerosis compared with placebo.

Methods and Results:

In this placebo-controlled, double-blind, randomized trial, 65 intermediate risk firefighters (age 55 ± 6 years) were treated with a placebo capsule or a capsule containing AGE and CoQ10 (AGE+CoQ10, 1200 and 120 mg, respectively) daily for 1 year. All participants underwent coronary artery calcium (CAC) scanning and C-reactive protein (CRP) at baseline and at 12 months. At 1 year, mean CAC progression was significantly lower in AGE+CoQ10 (32 ± 6 vs. 58 ± 8, P = 0.01) than placebo. Similarly, CRP were significantly decreased in AGE+CoQ10 compared with placebo (-0.12 ± 0.24 vs. 0.91 ± 0.56 mg/L, P < 0.05). After adjustment for age, gender, conventional cardiac risk factors, and statin therapy, AGE+CoQ10 was associated with 3.99 fold (95% 1.3–12.2, P = 0.01) lack of CAC progression compared with the placebo.

Conclusion:

AGE+CoQ10 are associated with beneficial effects on inflammatory markers and reduced progression of coronary atherosclerosis.

Actions of "antioxidants" in the protection against atherosclerosis

This review addresses the role of oxidative processes in atherosclerosis and its resulting cardiovascular disease by focusing on the outcome of antioxidant interventions. Although there is unambiguous evidence for the presence of heightened oxidative stress and resulting damage in atherosclerosis, it remains to be established whether this represents a cause or a consequence of the disease. This critical question is complicated further by the increasing realization that oxidative processes, including those related to signaling, are part of normal cell function. Overall, the results from animal interventions suggest that antioxidants provide benefit neither generally nor consistently. Where benefit is observed, it appears to be achieved at least in part via modulation of biological processes such as increase in nitric oxide bioavailability and induction of protective enzymes such as heme oxygenase-1, rather than via inhibition of oxidative processes and lipid oxidation in the arterial wall. Exceptions to this may be situations of multiple/excessive stress, the relevance of which for humans is not clear. This interpretation is consistent with the overall disappointing outcome of antioxidant interventions in humans and can be rationalized by the spatial compartmentalization of cellular oxidative signaling and/or damage, complex roles of oxidant-producing enzymes, and the multifactorial nature of atherosclerosis.

The quality control assessment of commercially available coenzyme q(10)-containing dietary and health supplements in Japan

Coenzyme Q₁₀ (CoQ₁₀) has been widely commercially available in Japan as a dietary and health supplement since 2001 and is used for the prevention of lifestyle-related diseases induced by free radicals and aging. We evaluated CoQ₁₀ supplements to ensure that these supplements can be used effectively and safely. Commercially available products were selected and assessed by the quality control tests specified in the Japanese Pharmacopoeia XV. When the disintegration time of CoQ₁₀ supplements was measured, a few tested supplements did not completely disintegrate even after incubation in water for an hour at 37°C. In the content test, many samples were well controlled. However, a few supplements showed low recovery rates of CoQ₁₀ as compared to manufacturer’s indicated contents. Among soft capsule and liquid supplements, the reduced form of CoQ₁₀ (H₂CoQ₁₀), as well as the oxidized form, was detected by HPLC with electrochemical detector. The results for experimental formulated CoQ₁₀ supplements demonstrated that H₂CoQ₁₀ was produced by the interaction of CoQ₁₀ with vitamins E and/or C. From these results, we concluded that quality varied considerably among the many supplement brands containing CoQ₁₀. Additionally, we also demonstrated that H₂CoQ₁₀ can be detected in some foods as well as in CoQ₁₀ supplements.

Protective effects of coenzyme q(10) on decreased oxidative stress resistance induced by simvastatin

The effects of simvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reductase), on oxidative stress resistance and the protective effects of coenzyme Q (CoQ) were investigated. When simvastatin was administered orally to mice, the levels of oxidized and reduced CoQ₉ and CoQ₁₀ in serum, liver, and heart, decreased significantly when compared to those of control. The levels of thiobarbituric acid reactive substances induced by Fe²⁺-ascorbate in liver and heart mitochondria also increased significantly with simvastatin. Furthermore, cultured cardiac myocytes treated with simvastatin exhibited less resistance to oxidative stress, decreased time to the cessation of spontaneous beating in response to H₂O₂ addition, and decreased responsiveness to electrical field stimulation. These results suggested that oral administration of simvastatin suppresses the biosynthesis of CoQ, which shares the same biosynthesis pathway as cholesterol up to farnesyl pyrophosphate, thus compromising the physiological function of reduced CoQ, which possesses antioxidant activity. However, these undesirable effects induced by simvastatin were alleviated by coadministering CoQ₁₀ with simvastatin to mice. Simvastatin also reduced the activity of NADPH-CoQ reductase, a biological enzyme that converts oxidized CoQ to the corresponding reduced CoQ, while CoQ₁₀ administration improved it. These findings may also support the efficacy of coadministering CoQ₁₀ with statins.

Lipid antioxidants: free radical scavenging versus regulation of enzymatic lipid peroxidation

The essentiality of polyunsaturated lipids makes membranes susceptible to peroxidative modifications. One of the most contemporary examples includes selective peroxidation of cardiolipin in mitochondria of cells undergoing apoptosis. Cardiolipin peroxidation products are required for the mitochondrial membrane permeabilization, release of pro-apoptotic factors and completion of the cell death program. Therefore, search for effective inhibitors of cardiolipin peroxidation is critical to discovery and development of anti-apoptotic antioxidants. Mitochondria contain significant amounts of α-tocopherol, a well known scavenger of reactive free radicals. In the present study, we used an oxidative lipidomics approach to evaluate the effect of α-tocopherol and its homologues with different lengths of the side-chain such as 2,5,7,8,-tetramethyl-2(4-methylpentyl)-6-chromanol and 2,2,5,7,8-pentamethyl-6-chromanol, on oxidation of tetralinoleoyl cardiolipin induced by cytochrome c in the presence of hydrogen peroxide. Our data indicate that vitamin E homologues inhibit not only accumulation of tetralinoleoyl cardiolipin hydroperoxides but also hydroxy-derivatives of tetralinoleoyl cardiolipin formed in the enzymatic peroxidase half-reaction catalyzed by cytochrome c. This suggests that protective effects of vitamin E homologues against tetralinoleoyl cardiolipin peroxidation catalyzed by cytochrome c/hydrogen peroxide are realized largely due to their effects on the peroxidase activity of cytochrome c towards tetralinoleoyl cardiolipin rather than via their scavenging activity.

The lipophilic antioxidants alpha-tocopherol and coenzyme Q10 reduce the replicative lifespan of Saccharomyces cerevisiae

Reactive oxygen species contribute to cellular ageing and an increased level of oxidative stress is often associated with ageing in many organisms. Supplementation of antioxidants has been advocated to decrease cellular oxidative stress and potentially extend lifespan. A genetically modified K6001 strain of Saccharomyces cerevisiae was employed to determine the effect of several antioxidants, including D-erythroascorbic acid, alpha-tocopherol and coenzyme Q(10) on yeast cell replicative ageing. The replicative lifespan of the K6001 strain was assessed by absorbance change as cells exhibited a linear growth in glucose medium. In this study, water-soluble D-erythroascorbic acid had little effect on cell replicative lifespan. However, supplementation of the growth medium with the lipophilic antioxidants alpha-tocopherol increased oxidative stress and decreased cell lifespan. The use of alpha-tocopherol analogues revealed that the antioxidant activity and the membrane retention ability of alpha-tocopherol were involved in the lifespan reduction effect. Supplementation with either coenzyme Q(10) alone, or in combination with alpha-tocopherol also led to a reduction in yeast replicative lifespan. This study highlights a potential pro-oxidant action of antioxidants.

Ockham's razor gone blunt: coenzyme Q adaptation and redox balance in tropical reef fishes

The ubiquitous coenzyme Q (CoQ) is a powerful antioxidant defence against cellular oxidative damage. In fishes, differences in the isoprenoid length of CoQ and its associated antioxidant efficacy have been proposed as an adaptation to different thermal environments. Here, we examine this broad contention by a comparison of the CoQ composition and its redox status in a range of coral reef fishes. Contrary to expectations, most species possessed CoQ(8) and their hepatic redox balance was mostly found in the reduced form. These elevated concentrations of the ubiquinol antioxidant are indicative of a high level of protection required against oxidative stress. We propose that, in contrast to the current paradigm, CoQ variation in coral reef fishes is not a generalized adaptation to thermal conditions, but reflects species-specific ecological habits and physiological constraints associated with oxygen demand.

The effect of different ubiquinones on lifespan in Caenorhabditis elegans

Ubiquinone (UQ, Coenzyme Q, CoQ) transfers electrons from complexes I and II to complex III in the mitochondrial electron transport chain. Depending on the degree of reduction, UQ can act as either a pro- or an antioxidant. Mutations disrupting ubiquinone synthesis increase lifespan in both the nematode (clk-1) and the mouse (mclk-1). The mutated nematodes survive using exogenous ubiquinone from bacteria, which has a shorter isoprenyl tail length (UQ(8)) than the endogenous nematode ubiquinone (UQ(9)). The mechanism underlying clk-1s increased longevity is not clear. Here we directly measure the effect of different exogenous ubiquinones on clk-1 lifespan and mitochondrial function. We fed clk-1 engineered bacteria that produced UQ(6), UQ(7), UQ(8), UQ(9) or UQ(10), and measured clk-1s lifespan, mitochondrial respiration, ROS production, and accumulated ROS damage to mitochondrial protein. Regardless of dietary UQ, clk-1 animals have increased lifespan, decreased mitochondrial respiration, and decreased ROS damage to mitochondrial protein than N2. However, clk-1 mitochondria did not produce less ROS than N2. The simplest explanation of our results is that clk-1 mitochondria scavenge ROS more effectively than wildtype due to the presence of DMQ(9). Moreover, when compared to other dietary quinones, UQ(10) further decreased mitochondrial oxidative damage and extended adult lifespan in clk-1.

Pharmacokinetics of different formulations of tioctic (alpha-lipoic) acid in healthy volunteers

This study was designed to evaluate in healthy volunteers plasma and cellular (in erythrocytes) of three formulations of alpha-lipoic acid (ALA) available in Italy with different rates of absorption, two with a claimed high absorption rate (Byodinoral 600 QR, Tiocronal 600 HR) and one with a claimed prolonged absorption rate (Tiobec 600 retard). These formulations were compared with the registered ethic formulation of the compound (Thioctacid 600 mg HR), available in Germany. Area under the curve from time 0 to last measured time (AUC(t)), peak plasma concentration (C(max)) of ALA, and time (T(max) ) at which C(max) was observed were the plasma kinetic parameters measured. Concentration of ALA at different sampling times was the only parameter assessed for erythrocytes. The AUC(t) values were similar for the four formulations of ALA tested. C(max) was significantly higher for Byodinoral 600 QR, Tiocronal 600 HR compared to Thioctacid 600 mg HR or Tiobec 600 retard, whereas T(max) value was significantly shorter for Byodinoral 600 QR in the order by Tiocronal 600 HR, Thioctacid 600 mg HR, and Tiobec 600 retard. ALA concentrations that accumulated in erytrocytes after the administration of the different formulations of the antioxidant are directly proportional to the plasma levels of each formulation. Because antioxidant capabilities of ALA depend on the glutathione regeneration the compound induces in cells, the most rationale approach for eliciting antioxidant activity at the cellular level is probably in the use of a formulation allowing the compound to reach its target at highest concentrations and in the shortest time.

Comparative crossover, randomized, open-label bioequivalence study on the bioequivalence of two formulations of thioctic acid in healthy volunteers

An open-label, randomized, crossover single-dose study, using two periods and two sequences with a washout period of seven days was conducted to assess the comparative bioavailability of thioctic (alpha-lipoic) acid (ALA) 600 mg formulation and that of a reference formulation. Blood samples were collected up to +6 h post dosing, the plasma was separated, and thioctic acid concentrations were determined by high-performance liquid chromatographic method with single mass spectrometry detection (HPLC-MS) and a lower limit of quantification of 190.1 ng/ml. Mean values of the individual C(max) were 1338.6 +/- 751.8 ng/ml and 1215.8 +/- 560.5 ng/ml for the test and reference preparations, respectively. Mean +/- standard deviation (SD) total area under the curve up to the last measurable concentration (AUC(t)) was 3510.9 +/- 1088.6 ng x h/ml for the test formulation and 3563.5 +/- 1374.1 ng x h/ml for the reference formulation. Mean +/- SD total area under the curve (AUC(inf)) was 6925.6 +/- 4045.8 ng x h/ml for the test formulation and 7797.1 +/- 5963.1 ng x h/ml for the reference preparation. Terminal elimination half-life was 5.68 +/- 5.05 h for the test and 6.11 +/- 6.15 h for the reference formulations. Time of maximum concentration (t(max)) was 1.24 +/- 1.23 h for the test and 2.05 +/- 1.21 h for the reference formulations. Ninety percent confidence intervals were comprised within the bioequivalence acceptance criteria (80-125%) for all of the parameters analyzed except t(max). The comparison between males and females showed no significant difference for the two drug treatment.

Effect of CoQ homologues on reactive oxygen generation by mitochondria

Effect of CoQ compounds (Qs) on reactive oxygen (ROS) generation by mitochondrial complex I was studied using rat liver mitochondria and chemiluminescence probe L012. Kinetic analysis revealed that short chain Qs, such as Q2 and idebenone enhanced ROS generation by mitochondrial NADH oxidase system by a succinate-inhibitable mechanism. Lipid peroxidation in mitochondrial membranes induced by NADH and iron was inhibited by short chain Qs. The inhibitory activity was enhanced by co-oxidation of succinate as determined by chemiluminescence method and by electron spin resonance spectroscopy. These results suggested that the reduced form of short chain Qs inhibited mitochondrial ROS generation and lipid peroxidation.

Fluorescence Lifetimes Study of α-Tocopherol and Biological Prenylquinols in Organic Solvents and Model Membranes

We have found that for biological prenyllipids, such as plastoquinol-9, alpha-tocopherol quinol, and alpha-tocopherol, the shortest fluorescence lifetimes were found in aprotic solvents (hexane, ethyl acetate) whereas the longest lifetimes were those of ubiquinonol-10 in these solvents. For all the investigated prenyllipids, fluorescence lifetime in alcohols increased along with an increase in solvent viscosity. In a concentrated hexane solution, the lifetimes of prenylquinols considerably decreased. This contrasts with methanol solutions, which is probably due to the self-association of these compounds in aprotic solvents. We have also found a correlation of the Stokes shift of prenyllipids fluorescence with the orientation polarizability of the solvents. Based on data obtained in organic solvents, measurements of the fluorescence lifetimes of prenyllipids in liposomes allowed an estimation of the relative distance of their fluorescent rings from the liposome membrane surface, and was found to be the shortest for alpha-tocopherol quinol in egg yolk phosphatidylcholine liposomes, and increased in the following order: alpha-tocopherol in dipalmitoyl phosphatidylcholine liposomes < alpha-tocopherol < plastoquinol-9 < ubiquinol-10 in egg-yolk phosphatidylcholine liposomes.

Coenzyme Q2 induced p53-dependent apoptosis

Coenzyme Q functions as an electron carrier and reversibly changes to either an oxidized (CoQ), intermediate (CoQ.-), or reduced (CoQH2) form within a biomembrane. The CoQH2 form also acts as an antioxidant and prevents cell death, and thus has been successfully used as a supplement. On the other hand, the value of the CoQ/CoQH2 ratio has been shown to increase in a number of diseases, presumably due to an anti-proliferative effect involving CoQ. In the present study, we examined the effect of CoQ and its isoprenoid side chain length variants on the growth of cells having different p53 statuses. Treatment with CoQs having shorter isoprenoid chains, especially CoQ2, induced apoptosis in p53-point mutated BALL-1 cells, whereas treatment with longer isoprenoid chains did not. However, CoQ2 did not induce apoptosis in either a p53 wild-type cell line or a p53 null mutant cell line. These results indicated that the induction of apoptosis by CoQ2 was dependent on p53 protein levels. Moreover, CoQ2 induced reactive oxygen species (ROS) and the phosphorylation of p53. An antioxidant, l-ascorbic acid, inhibited CoQ2-induced p53 phosphorylation and further apoptotic stimuli. Overall, these results suggested that short tail CoQ induces ROS generation and further p53-dependent apoptosis.

Effect of coenzyme Q10 as an antioxidant in beta-thalassemia/Hb E patients

Thalassemia is a group of genetic disorders resulting from different mutations in the globin gene complex and leading to an imbalance in globin synthesis. Unmatched globin chains are less stable and susceptible to oxidation. Patients with beta-thalassemia/HbE are prone to increased oxidative stress as indicated by increased lipid peroxidation product, malondialdehyde (MDA), partly because of the presence of iron in the form of heme and hemichromes released from excess globin chains and excess iron deposition in various tissues. The level of antioxidant such as glutathione is markedly decreased while activities of antioxidant enzymes including superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSH-Px) are increased. We have recently found that the levels of coenzyme Q(10) (CoQ(10)) are also very low in thalassemia. We therefore evaluated the oxidative stress and the antioxidants in these patients before and after supplementation with 100 mg CoQ(10) daily for 6 months. The results showed that the plasma level of CoQ(10) significantly increased and the oxidative stress decreased as the level of MDA declined. The administration of CoQ(10) led to significant improvement of biochemical parameters of antioxidant enzymes. The antioxidant supplementation will be beneficial for thalassemia patients as adjunct therapy to increase their quality of life.

Sulfation and glucuronidation of phenols: implications in coenyzme Q metabolism

Phase II conjugation of phenolic compounds constitutes an important mechanism through which exogenous or endogenous toxins are detoxified and excreted. Species differences in the rates of glucuronidation or sulfation can lead to significant variation in the metabolism of this class of compounds. Conjugation of the hydroxyl groups of phenols can occur with glucuronate or sulfate. Quinone metabolism, deactivation, and detoxification are also affected by the same conjugatory systems as phenols; however, reduction of quinones to hydroquinols seems to be a prerequisite. This work reviews current knowledge on phenol conjugation and its implications on hydroquinone metabolism with special consideration for coenzyme Q metabolism.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

Coenzyme Q10 concentrations in coronary artery disease

OBJECTIVES

Ubiquinone-10 (CoQ10: the oxidized form of Coenyzme Q) is recognized as an antioxidant in the mitochondrial membrane and considered an anti-risk factor for coronary artery disease (CAD). The aim of this report is to determine the levels of plasma CoQ10 and the ratios of CoQ10 to plasma lipids in CAD patients and to compare them to a matched control group.

METHODS

Sixty-four consecutive patients with CAD were studied and compared to 34 clinically healthy individuals. In addition to conventional lipid and lipoprotein analyses, plasma CoQ10 concentrations were measured by electrochemical high pressure liquid chromatography (EC-HPLC) after extraction of plasma with hexane-ethanol.

RESULTS

Plasma CoQ10 concentrations in patients with CAD and controls were found as 0.77 and 0.41 micromol/l, respectively (P < 0.01). Also, the ratio of CoQ10 to low density lipoprotein (LDL) was found significantly lower in patients with CAD when compared to controls (P < 0.01).

CONCLUSIONS

Our results indicate that a relation exists between low plasma CoQ10 concentration and coronary artery disease, yet this correlation is not strong enough to indicate a causal relationship.