Abstract: The presentation is a brief review of the oxidative damage mechanism of carcinogenesis, the anti-oxidative properties of Coenzyme Q (CoQ) and the expectation of CoQ applying to cancer treatment. The oxidative damage to DNA, lipid and protein has been suggested to contribute to initiation and progression of carcinogenesis. Nevertheless CoQ, known as the electon carrier in mitochondrial respiratory chain, has shown potent anti-oxidative activity, which is more efficient than vitamin E by about four mechanisms. Thought few researches have been reported on the pharmacology of CoQ on cancer or tumor, CoQ deficiency in human or animals bearing tumor has been long observed and CoQ administration has shown a benefit for cancer patients. But more researches should be launched to explore the relationship between CoQ and cancers, especially the dynamic relation and to illustrate the anti-carcinogenesis mechanisim of CoQ in order to enable a wider application of CoQ to cancer therapeutics.
Oxidative Damage and Cancer
Cancer is the number one killer in diverse diseases and is significant in human mortality. WHO reported that each year over nine million cancer cases occurred and five million persons died of cancer, which would soar up to twenty million and ten million by 2020, respectively. How does cancer occur? Accompanying with oxidative phosphorylation and ATP’s production, Reactive Oxygen Species (ROS) including free radical come into being. ROS is any species capable of independent existence that contains one or more unpaired electrons. In the presence of metal ions, ROS may cause an oxidative damage by reacting with macromolecules including proteins, lipids and DNA in the cells (Halliwell and Gutterage, 1989a; Halliwell, 1991), which is called as oxidative stress. Oxidative damage has been suggested to contribute to the initiation and progression of carcinogenesis (Halliwell and Gutterage, 1989b) and causes of oxidative stress including transition metals (e.g., iron and copper), ultraviolet radiation, inflammation, some chemicals or drugs (e.g., carbon tetrachloride) are all associated with carcinogenesis or tumor biology (Halliwell and Gutterage, 1989a).
Since free radicals are usually generated near membranes, lipid is the first target of free radicals and lipid peroxidation is the first reaction to occur. It has been reported that products of lipid peroxidation may cause DNA damage (Halliwell and Gutterage, 1989b; Esterbauer, 1990; Marinari et al., 1984). Lipid hydroperoxides may directly induce DNA chain breaking (Marinari et al., 1984) and lipid peroxyl and alkoxyl radicals may cause base oxidation in DNA (Cochrane, 1991). Peroxide and hydroperoxides have also demonstrated tumor promoting activity in vivo (Park, 1992). It is known that the epoxy derivative of 4-hydroxynonenal (HNE) has a tumor causing activity (Chung et al., 1993; Sodum, 1991; Wang et al., 1996). Reaction of MDA with DNA is a hot subject, which has drawn considerable attention because of its mutagenicity. An endogenous MDA-deoxyguanosine adduct that has been implicated in the induction of G to T transversions was detected by mass spectrometry in healthy human liver (Imlay et al., 1988).
Protein oxidation usually occurs at certain amino acid residues of a particular protein. Reactive oxygen radicals react with amino residues in proteins and give rise to produce of carbonyl products, changing protein structure or converting sulphydryl (thiol) groups of proteins into disulfide groups (Butterfield et al., 1998; Dean et al., 1997). Peroxynitrite generated after the reaction between superoxide radicals and nitric oxide damages protein by binding nitro groups with protein tyrosine residues to form nitrotyrosine (Kaur and Halliwell, 1994; Halliwell, 1997). Oxidation of cellular proteins might result in the inactivity of anti-oxidative enzymes and especially DNA repair enzymes.
DNA is another major object targeted by free radicals. Oxidative damage of DNA could result in strand breaks,base modifications and DNA-protein cross-links. H2O2 is known to cause DNA breaks in intact cells and purified DNA (Imlay et al., 1988; Baker and He, 1991). Though there are a variety of modified DNA bases produced by free radical reactions, only the modifications of C-8 position of guanine has been studied in many aspects because of the sensitivity detection method of 8-oxoguanine (8-oxoG) by high performance liquid chromatography and the electrochemical detector developed in 1986 (Baker and He, 1991). It has been revealed that C-8 position of guanine is hydroxylated to produce 8-oxoGs. This hydroxylation completely changes the stereographic charge mapping of the molecule to allow guanine to pair adenine as well as guanine to pair cytosine. Accordingly, 8-oxoG induces G:C to T:A transversions in DNA replication (Kuchino et al., 1987; Shibutani et al., 1991), which appears to be important in carcinogenesis and tumor biology.
In fact, oxidative stress has been founded in human or animals bearing diversity cancer or tumor (Gibanananda et al., 2000; Ilker et al., 2003). In plasma or cancer tissue, reactive oxygen species (e.g., superoxide anion, hydrogen peroxide and hydroxyl radical) and products of lipid peroxidation and protein oxidation increase, while the activities of some antioxidant enzymes activity decrease.
Under normal physiological conditions oxidation and anti-oxidation are in equilibrium. Changes in this balance in favor of free radical formation would result in oxidative stress (Rizzo et al., 1992). Anti-oxidants are enzymes and nonenzymatic agents that eliminate ROS or prevent their formation. Antioxidant enzymes include superoxide dismutase and various peroxidases such as glutathione peroxidase, catalase, thioredoxin reductase and peroxiredoxin (Holmgren and Bjornstedt, 1995; Wood et al., 2003); vitamins C and E, carotenoids, glutathione, a-lipoic acid, flavinoids, the reduced form of CoQ (CoQH2) are recognized as nonenzymatic agents of anti-oxidative activities. Antioxidants have been used as radioprotectors, antimutagens and anticarcinogens (Ames and Gold, 1990). Vitamin E (α-tocopherol) is a biological lipid antioxidant that prevents the formation of free radicals from lipid peroxidation and has proved an antimutagen or anticarcinogen in Salmonella tester strains (Tavan et al., 1997) as well as in human leucocytes in vitro (Bolkenius, 1991). Of note here is the fact that the anti-oxidative activity of CoQ is more efficient than Vitamin E (Frei et al., 1990; Shi et al., 1999), implying that CoQ might be used as antimutagen or anticarcinogen.
What Is Coenzyme Q?
In 1957, CoQ was first isolated and purified from beef hearts by Fred Crane.
Now, CoQ has been found in all cells, tissues and membranes (Dallner and Sindelar,
2000). For animals and humans, CoQ distributes in subcellular organelles, blood
plasma and serum lipoproteins. Its molecular formula is shown in Fig.
1 CoQ consists of a quinonoid head group attached to a long, hydrophobic
tail of 5-carbon isoprene units numbering from 6 to 12 in different species
(Battino et al., 1990) (e.g., 6, 8, 9, 10 in Saccharomyces cerevisiae,
Escherichia coli, rodent, human, respectively). In vivo, the quinonoid
head mainly exists alternately in three different redox states: ubiquinone,
the fully oxidized form; ubisemiquinone (●QH), the partially reduced form,
also a free radical; and ubiquinol (CoQH2), the fully reduced form
(Turrens et al., 1985; Kozlov et al., 1998). CoQ plays multiple
functional roles in living cells in many aspects. Among those functions, three
have been well characterized.
| Fig. 1: | Structure of ubiquinone, n=6,7,....,12 |
First, as an endogenous enzyme cofactor, CoQ is an essential component of the mitochondrial respiratory chain and adenosine triphosphate (ATP) production, carrying electrons from complexes I or II to complex III (Ernster and Dallner, 1995). Second, CoQ functions as pro-oxidant. Superoxide radicals are generated during the redox process associated with ●QH formation, which accounts for the major part of superoxide anion and hydrogen peroxide physiologically generated in the mitochondria (James et al., 2004). Third, as the only endogenous lipophilic antioxidant, ubiquinol exerts strong anti-oxidant power on plasma and cellular membranes (e.g. scavenging reactive oxygen species or lipid radicals and regenerating α-tocopherol from a-tocopheroxyl radical) (Crane, 2001; Hargreaves, 2003; Genova et al., 2003). Anthony and Hayden ( 2004) supposed on the base of these three roles that CoQ could regulate gene expression and metabolism.
The Anti-oxidation and Pro-oxidation of Coenzyme Q
Literature on the anti-oxidation of CoQ is exhaustive. Ubiquinol is a potent
antioxidant and its anti-oxidative activity is munch stronger than that of α-tocopherol
(a-T) (Frei et al., 1990). When phosphatidylcholine liposomes are oxidized
with a water-soluble radical initiator in the presence of ascorbate, a-T and
CoQ, the antioxidants are consumed in the order: Ascorbate-CoQ-a-T (Shi et
al., 1999); while with a lipidsoluble radical initiator, the order is CoQ-ascorbate-a-T.
It has been reported that ubiquinol inhibits lipid peroxidation in vitro using
submitochondrial particles (Mellors and Tappel, 1966; Takayanagi et al.,
1980). Simultaneously ubiquinone appears workable for anti-oxidation even without
appropriate reducing systems (Tomasetti et al., 1999). It is now well
established that CoQ prevents lipid peroxidation in most subcellular membranes
(Ernster and Dallner, 1995) and acts as antioxidant also in the circulation
(Romagnoli et al ., 1994; Alleva et al., 1995). Investigations
suggested that the endogenous content of CoQ might prevent membrane proteins
and DNA from oxidative damage mediated by lipid peroxidation (Forsmark, et
al., 1995; Forsmark and Ernster, 1994). DNA oxidation measured by 8-oxoG
formation in rat liver mitochondria and DNA strand breaks in human lymphocytes
are also prevented by CoQ administration in vitro or in vivo (Tomasetti
et al., 1999; Atroshi et al., 1997; Tomasetti et al., 2001).
CoQ supplementation increases CoQ homologues in tissues and their mitochondria,
decreases selectively protein oxidative damage andincreases anti-oxidative potential
in rat (Kwong et al., 2002). CoQ prevents the hydrazine- and chloramphenicol-induced
changes in the membrane potential of mitochondria and decreased ROS generation
rate in mitochondria. Furthermore, CoQ accelerates the repair of damage induced
by chloramphenicol in mitochondrial structure and functions (Teranishi et
al., 1999). Various studies have reported the beneficial effects of CoQ
supplementation in animal experimentation (Rauscher et al., 2001; Kwong
et al ., 2002) or human therapy on different diseases related to oxidation
damage (Geromel et al., 2002; Hodgson et al., 2002).
| Fig. 2: | The redox process of CoQ |
CoQ in vivo undergoes the redox process shown in Fig. 2. Ubiquinone is reduced by ubiquinone reductasesfirst to ●QH and then to ubiquinol, each step with an additional electron and an additional proton (Rich and Harper, 1990). Because cells have effective systems to reduce ubiquinone at all intracellular locations, CoQ mainly exists as ubiquinol andthe ratio of ubiquinol and ubiquinone varies from one tissue to another (Aberg et al., 1992). The cellular pool of ubiquinol in some tissues was reported to be elevated following the external administration of ubiquinone in order to increase the antioxidant capacity (Crane et al., 1984). Various NADH-dehydrogenases associated with cell membranes are supposed to transform ubiquinone to ubiquinol (Kishi et al., 1999). Although the antioxidant activity of CoQ in systems is mainly attributed to ubiquinol, the role of ubiquinone should not be neglected. According to reported researches, the probable mechanisms of CoQ as an anti-oxidant could be as the follows: (1) ubiquinol (CoQH2) prevents the initiation and/or propagation of free radical chain reaction. CoQH2 acts by affecting the initiation process and preventing the formation of lipid peroxyl radicals (LOO) (Ernster and Forsmark, 1993) by reaction (i). It is also possible that CoQH2 eliminated LOO directly by reactions (ii) and (iii) where a CoQH2 molecule scavenges two free radicals eventually.
| (i) |
| (ii) |
| (iii) |
(2) CoQ spares and regenerates α-tocopherol. It is established that CoQH2 regenerates α-tocopherol from a-tocopheroxyl radical. In reactions (iv) and (v) α-tocopherol was transferred to α-tocopheroxyl radical by reacting with lipid radical andin reaction(vi) CoQH2 reacts with α-tocopheroxyl radical to produce α-tocopherol. When coexisting with α-tocopherol, it is CoQH2 that is first oxidized by radical initiator, which thus spares α-tocopherol (Shi et al., 1999). This sparing effect of CoQH2 on α-tocopherol has also been observed in low-density lipoprotein (LDL) (Stocker et al., 1991; Thomas et al., 1995). Vitamin E level does not decrease, but CoQH2 is oxidized to ubiquinone at an early stage in the oxidation of human plasma (Yamamoto et al., 1991). CoQ administration enhanced α-tocopherol level in mitochondria (Kagan et al., 1990; Maguire et al., 1992; Stoyanovsky et al ., 1995) However, it is evident that the antioxidant function of CoQH2 is not dependent on the presence of α-tocopherol. An investigation suggested that submitochondria particles containing CoQ were protected against lipid peroxidation even in the absence of α-tocopherol (Forsmark et al., 1991).
| (iv) |
| (v) |
| (vi) |
(3) CoQ is a cofactor for uncoupling protein (UCP) function of performing anti-oxidative activity. UCP, situated in the inner mitochondria membrane, transfers H+ from the outside to the inside of the mitochondria. UCP could be involved in suppressing the generation of oxygen radicals. Recently Echtay et al. (2000, 2001) testified that CoQ was an obligatory cofactor for UCP function by using bacterial overexpressed UCP1, -2 and -3 in liposomes. (4) CoQ stabilizes membrane structure so as to block the propagation reaction in phospholipid bilayer (Landi et al., 1987). The mechanism suggests that such antioxidant activity is related to the localization of ubiquinone within the lipid bilayer, where CoQ prevents autocatalytic free radical reaction by stacking among phospholipid molecules and keeping the quinone ring in the nonpolar phase (Fato et al., 1986). In fact, fluorescence studies have demonstrated that ubiquinone homologues possess a strong ordering effect on the lipid bilayer, which is much higher than that produced by ubiquinol (Jemola et al., 1996). The ordering effect decreases the rate of free radicals leaking from mitochondria in the cell when they consume oxygen to generate adenosine triphosphate.
On the other hand, CoQ plays key roles in physiology as pro-oxidant. During the redox process associated with CoQ semiquinone formation as shown in Fig. 2 and Reaction ii, superoxide radicals are generated. Some superoxide radicals are converted to hydrogen peroxide (H2O2) by superoxide dismutase. More should not be re-visited here. But it should be emphasized that the pro-oxidative activity of CoQ is not evil because intact mitochondria produces only a small amount of H2O2, and this the very low level of H2O2 is crucial to both cell function and the role of H2O2 as a second messenger molecule (Gille and Nohl, 2000; St- Pierre et al., 2002).
It is emphasized that CoQ pro-oxidant and anti-oxidant roles are not mutually exclusive (Anthony and Hayden, 2004), but essential to cells. On the one hand, the action of CoQ leads to the cellular bioenergy modulation through superoxide formation and the synthesis of the mitogen H2O2, which is accompanied by the formation of ROS and the damage of some lipid, protein and DNA. On the other hand, as an anti-oxidant, CoQ contributes to alleviating the oxidative damage derived from ROS.
Coenzyme Q and Cancer
CoQ has certain relation with the occurrence and development of cancer or
tumor even though researches on this subject have not been bountiful. CoQ in
tissue or plasm is decreased in subjects with cancer (Kozlov et al.,
1998; Quiles et al., 1994). For breast cancer patients, the following
conditions were observed: CoQ concentrations in tumor tissues significantly
decreased compared with the surrounding normal tissues; higher MDA levels were
observed in tumor tissues than noncancerous tissues; The activities of MnSOD,
total SOD, GSH-Px and catalase in tumor tissues significantly increased compared
with the controls. These findings may support that reactive oxygen species increased
in malignant cells andcaused overexpression of antioxidant enzymes and the consumption
of CoQ. Increased antioxidant activities may be related to the susceptibility
of cells to carcinogenic agents and the response of tumor cells to the chemotherapeutic
agents. Hyperplastic noduli is the first stage during development of chemically
induced hepatocellular cancer in rat and during this stage the amount of CoQ
increases (Olsson et al., 1991, 1995). Interestingly, the level of CoQ
in the mitochondria is stable andthe change of CoQ concentrations is attributed
to the extra-mitochondrial compartments. During the first stage of the disease,
there is increasing oxidative stress, which has been suggested to induce an
adaptive response, resulting in the cell protecting itself by raising the concentrations
of antioxidants (Droge, 2002). Upon progression of the disease, manifest cancer
develops and the amount of CoQ decreases to only 40% in human and 76% in rat
(Eggens et al., 1989).
Because of the reasons presented above and CoQ preventing lipid, protein and DNA from oxidation damage, CoQ is considered to be feasible to treat cancer. Several small studies have shown a benefit for people with breast or prostate cancer. There are a number of reports on the use of CoQ10 in the treatment of cancer, specially breast cancer (Lockwood et al., 1994; 1995).
Additionally, Other findings have indicated that CoQ boosts the immune system (Folkers et al., 1993), possibly helping to limit the spread of cancerous tissue. Karl Folkers published that complete biochemistry relating to biosyntheses of CoQ and the DNA bases was a rationale for the therapy of cancer with CoQ (Folkers, 1996).
Conclusions
Attributed to its anti-oxidative activity and other intrinsic functions, CoQ has shown promise for cancer. Nevertheless, researches on pharmacology of CoQ on cancer are so rare that the anti-cancer mechanisms of CoQ and the dynamic relation between CoQ concentration in tissue or plasma and carcinogenesis are still unclear. More researches should be lunched to explore the anti-carcinogenesis properties and the mechanisms in order to enable CoQ to benefit human health widely.
Acknowlegement
Supportted by the Graduate Innovation Foundation of Chongqing University (Number:200504Y1A0120119)