Mitochondrial-targeted rechargeable antioxidant SkQ
inhibits the senescence program

V.P. Skulachev1, M.V. Skulachev2
1Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow
2Institute of Mitoengineering, Moscow State University, Moscow

The crucial argument in favor of the concept of programmed senescence of the organism would be the switching off of senescence by a small molecule interrupting realization of such a program. As was shown by Lampert et al. [1], there is a correlation between the lifespan of 11 species of mammals and birds and production of reactive oxygen species (ROS) by energized mitochondria. However, a 12th species proved to be an exception. This was the naked mole-rat, a mouse-size rodent living almost 10 times longer than mice in spite of the fact that his mitochondria produce ROS faster than those of mice. According to Buffenstein, probability of the mole-rat's death is low and age-independent [2], and his cells in vitro could not be sent into apoptosis by adding H202 [3].These and many other pieces of indirect evidence were summarized by a hypothesis considering senescence as the last step of the ontogenetic program, mediated by mitochondrial ROS and the ROS-induced apoptosis [4,5]. If this were the case, senescence could be switched off by lowering the mitochondrial ROS level. This might be done by a mitochondrial-targeted antioxidant.
To find such an antioxidant, we organized in 2005 an international project uniting several research groups in Russia, Sweden, USA, and Germany. As a result, it was shown that the SkQ-type antioxidants composed of (i) plastoquinone, (ii) a penetrating cation with delocalized charge and (iii) decane linker, meet two major requirements for a small molecule which inhibits the senescence program, i.e. (1) they prolong the lifespan of many organisms differing greatly in their systematic position and (2) they prevent age-linked decline of numerous quite different physiological functions. In the majority of experiments, a new compound synthesized in our group was used, namely, plastoquinonyl decyltriphenylphosphonium (SkQ1). It was found that SkQ1 easily penetrates into energized mitochondria, its antioxidant (reduced) form being regenerated from the oxidized form by center i of Complex III of the mitochondrial respiratory chain. In intact cells, a fluorescent SkQ derivative (SkQR1) is shown to specifically stain mitochondria. Taking into account (i) ΔΨ values equal to 60 and 180 mV on the outer cell membrane and the inner mitochondrial membrane, respectively, and (ii) distribution coefficient in the octanol/water system of about 104, the SkQ concentration in the inner leaflet of the inner mitochondrial membrane is estimated to be about 108 times higher than in the extracellular aqueous solution [5-9]. In the following species, life-long treatment with SkQ1 resulted in an increase in lifespan: fungus Podospora anserina, crustacean Ceriodaphnia affinis, insect Drosophila melanogaster, fish Nothobranchius furzeri, and mammals (mice of various strains, dwarf hamster Phodopus campbelli and mole-vole Ellobius talpinus). In non-sterile animal houses or in outdoor cages, SkQ1 increased the lifespan of both males and females mainly due to prevention of age-linked decline of immunity [5,7,10,11]. In selected pathogen free (SPF) animal houses, the effect was specific for males [12]. Under any conditions used, both males and females showed decelerated development of many traits of senescence. Among them were osteoporosis, myeloid shift in blood cells, decline of wound healing, balding, cataract, retinopathies, some changes in behavior, appearance of β-galactosidase, phosphorylation of histone H2AX, and stimulation of apoptosis in skin fibroblasts. In females, SkQ1 prevented disappearance of estrual cycles with age [5,7,9,10,11,13]. In addition, 5 nmol SkQ1/kg per day inhibited development of lymphomas in p53-/- mice. A similar effect was produced by the conventional antioxidant N-acetyl cysteine at a million times higher concentration [5,14]. However, mammary carcinoma and some other tumors proved to be SkQ1-resistant [5,10]. This is why these cancers become the main reason for the death of SkQ1-treated mice.
In young animals, short term SkQ treatment was found to help in a number of experimental pathologies normally developing with age, such as heart attack and arrhythmia, stroke, kidney infarction, and glaucoma [5,7,13,15]. In the progeric "mutator" mice lacking proofreading activity of DNA polymerase γ, SkQ1 prolonged the healthy lifespan [12]. In OXYS rats, also showing accelerated senescence, SkQ1 inhibited accumulation of oxidized of lipids and proteins in muscles, prevented development of cataract and retinopathies, and reversed these diseases when drops of 250 nM SkQ1 were instilled into eyes of old animals. Similar effect was found in dogs, cats, and horses [13]. Clinical trials of such drops already started in February 2010 in two ophthalmological hospitals of Moscow. Stable forms of SkQ1 and SkQR1 applicable for preparation of drugs for per os administration have been developed. Clinical trials of these drugs are now in preparation. At the same time, the molecular mechanisms of the effect of SkQ are now under investigation. Special attention will be paid to its role in preventing of oxidation of cardiolipin dimers in Complex III [15,16].

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[16] W. Longo, J. Mitteldorf, V.P. Skulachev, Nat. Genet. Rev., 6, 866-872, 2005

Homo Sapiens Liberatus Workshop, Moscow State University, May 2010