Mitochondria are indispensable for energy metabolism, apoptosis regulation, and cell signaling. Mitochondria in malignant cells differ structurally and functionally from those in normal cells and participate actively in metabolic reprogramming. Mitochondria in cancer cells are characterized by reactive oxygen species (ROS) overproduction, which promotes cancer development by inducing genomic instability, modifying gene expression, and participating in signaling pathways. Mitochondrial and nuclear DNA mutations caused by oxidative damage that impair the oxidative phosphorylation process will result in further mitochondrial ROS production, completing the "vicious cycle" between mitochondria, ROS, genomic instability, and cancer development. The multiple essential roles of mitochondria have been utilized for designing novel mitochondria-targeted anticancer agents. Selective drug delivery to mitochondria helps to increase specificity and reduce toxicity of these agents. In order to reduce mitochondrial ROS production, mitochondria-targeted antioxidants can specifically accumulate in mitochondria by affiliating to a lipophilic penetrating cation and prevent mitochondria from oxidative damage. In consistence with the oncogenic role of ROS, mitochondria-targeted antioxidants are found to be effective in cancer prevention and anticancer therapy. A better understanding of the role played by mitochondria in cancer development will help to reveal more therapeutic targets, and will help to increase the activity and selectivity of mitochondria-targeted anticancer drugs. In this review we summarized the impact of mitochondria on cancer and gave summary about the possibilities to target mitochondria for anticancer therapies. J. Cell. Physiol. 231: 2570-2581, 2016. © 2016 Wiley Periodicals, Inc.

Recoverin is an EF-hand Ca2؉ -binding protein that is suggested to control the activity of the G-protein-coupled receptor kinase GRK-1 or rhodopsin kinase in a Ca 2؉ -dependent manner. It undergoes a Ca 2؉ -myristoyl switch when Ca 2؉ binds to EF-hand 2 and 3. We investigated the mechanism of this switch by the use of point mutations in EF-hand 2 (E85Q) and 3 (E121Q) that impair their Ca 2؉ binding. EF-hand 2 and 3 display different properties and serve different functions. Binding of Ca 2؉ to recoverin is a sequential process, wherein EFhand 3 is occupied first followed by the filling of EFhand 2. After EF-hand 3 bound Ca 2؉ , the subsequent filling of EF-hand 2 triggers the exposition of the myristoyl group and in turn binding of recoverin to membranes. In addition, EF-hand 2 controls the mean residence time of recoverin at membranes by decreasing the dissociation rate of recoverin from membranes by 10-fold. We discuss this mechanism as one critical step for inhibition of rhodopsin kinase by recoverin.G-protein-coupled receptor kinases provide desensitization of G-protein-coupled receptors by phosphorylation of serine and threonine residues at their cytoplasmic C terminus (1). A wellknown system represents the light absorbing pigment rhodopsin and the corresponding kinase, rhodopsin kinase, or GRK-1, which phosphorylates (and thus desensitizes) photobleached rhodopsin. Arrestin then binds to phosphorylated rhodopsin and thereby stops any further activation of the G-protein transducin (2, 3). Illumination causes the decrease in the concentration of the intracellular transmitters of excitation and adaption, cGMP and cytoplasmic [Ca 2ϩ ], respectively. The decrease in cytoplasmic [Ca 2ϩ ] is sensed by Ca 2ϩ sensor proteins such as recoverin (4 -6; for a recent review, see Ref. 7). Recoverin or the amphibian orthologue S-modulin (8, 9) inhibit rhodopsin kinase at high levels of free [Ca 2ϩ ], thereby relieving inhibition when the Ca 2ϩ level decreases. The Ca 2ϩ

Mitochondria-targeted cationic plastoquinone derivative SkQ1 (10-(6'-plastoquinonyl) decyltriphenylphosphonium) has been investigated as a potential tool for treating a number of ROS-related ocular diseases. In OXYS rats suffering from a ROS-induced progeria, very small amounts of SkQ1 (50 nmol/kg per day) added to food were found to prevent development of age-induced cataract and retinopathies of the eye, lipid peroxidation and protein carbonylation in skeletal muscles, as well as a decrease in bone mineralization. Instillation of drops of 250 nM SkQ1 reversed cataract and retinopathies in 3-12-month-old (but not in 24-month-old) OXYS rats. In rabbits, experimental uveitis and glaucoma were induced by immunization with arrestin and injections of hydroxypropyl methyl cellulose to the eye anterior sector, respectively. Uveitis was found to be prevented or reversed by instillation of 250 nM SkQ1 drops (four drops per day). Development of glaucoma was retarded by drops of 5 microM SkQ1 (one drop daily). SkQ1 was tested in veterinarian practice. A totally of 271 animals (dogs, cats, and horses) suffering from retinopathies, uveitis, conjunctivitis, and cornea diseases were treated with drops of 250 nM SkQ1. In 242 cases, positive therapeutic effect was obvious. Among animals suffering from retinopathies, 89 were blind. In 67 cases, vision returned after SkQ1 treatment. In ex vivo studies of cultivated posterior retina sector, it was found that 20 nM SkQ1 strongly decreased macrophagal transformation of the retinal pigmented epithelial cells, an effect which might explain some of the above SkQ1 activities. It is concluded that low concentrations of SkQ1 are promising in treating retinopathies, cataract, uveitis, glaucoma, and some other ocular diseases.

Recoverin is a Ca2؉ -regulated signal transduction modulator expressed in the vertebrate retina that has been implicated in visual adaptation. An intriguing feature of recoverin is a cluster of charged residues at its C terminus, the functional significance of which is largely unclear. To elucidate the impact of this segment on recoverin structure and function, we have investigated a mutant lacking the C-terminal 12 amino acids. Whereas in myristoylated recoverin the truncation causes an overall decrease in Ca 2؉ sensitivity, results for the non-myristoylated mutant indicate that the truncation primarily affects the high affinity EF-hand 3. The three-dimensional structure of the mutant has been determined by x-ray crystallography. In addition to significant changes in average coordinates compared with wild-type recoverin, the structure provides strong indication of increased conformational flexibility, particularly in the C-terminal domain. Based on these observations, we propose a novel role of the C-terminal segment of recoverin as an internal modulator of Ca 2؉ sensitivity.Many biological processes are triggered or regulated by transient intracellular Ca 2ϩ signals. Because these signals elicit specific cellular responses, the precise detection of changes in cytoplasmic Ca 2ϩ concentration is a crucial step in many signaling pathways and requires sensing of Ca 2ϩ within very different concentration ranges. Ca 2ϩ -binding proteins work as intracellular Ca 2ϩ sensors and regulate their targets with high specificity and high spatial and temporal resolution. To achieve these remarkable tasks, Ca 2ϩ is recognized by specific amino acid sequence motifs, for example the C 2 domain and the EF-hand motif (1, 2). These motifs can detect subtle changes in Ca 2ϩ concentration and allow a fine tuning of Ca 2ϩ signaling. However, it remains a challenging problem to understand at a structural level how minimal changes in cytoplasmic Ca 2ϩ are reliably detected.The EF-hand superfamily of Ca 2ϩ -binding proteins includes, among others, the family of neuronal calcium sensor (NCS) 3 proteins (3), which are named because of their predominant expression in neuronal tissue. NCS proteins are grouped into five subfamilies and show a rather heterogeneous localization and function in the nervous system (4). In the photoreceptor cells of the vertebrate retina, for instance, recoverin and several isoforms of guanylate cyclase activating protein (GCAP) detect changes in Ca 2ϩ concentration during or after illumination and regulate their target proteins in Ca 2ϩ -dependent feedback loops (5).Recoverin inhibits rhodopsin kinase at high cytoplasmic Ca 2ϩ concentration (6 -9), a process that is thought to contribute to light adaptation of photoreceptor cells (9, 10). Recoverin harbors a myristoyl group at its N terminus (11), which is buried in a hydrophobic cleft in the Ca 2ϩ -free state (12). Upon Ca 2ϩ binding to the two functional EF-hands (EF-hand 2 and EFhand 3) (13) the acyl chain is exposed to the solvent. This socalled Ca 2ϩ -myrist...

NCS (neuronal Ca2+ sensor) proteins belong to a family of calmodulin-related EF-hand Ca2+-binding proteins which, in spite of a high degree of structural similarity, are able to selectively recognize and regulate individual effector enzymes in a Ca2+-dependent manner. NCS proteins vary at their C-termini, which could therefore serve as structural control elements providing specific functions such as target recognition or Ca2+ sensitivity. Recoverin, an NCS protein operating in vision, regulates the activity of rhodopsin kinase, GRK1, in a Ca2+-dependent manner. In the present study, we investigated a series of recoverin forms that were mutated at the C-terminus. Using pull-down assays, surface plasmon resonance spectroscopy and rhodopsin phosphorylation assays, we demonstrated that truncation of recoverin at the C-terminus significantly reduced the affinity of recoverin for rhodopsin kinase. Site-directed mutagenesis of single amino acids in combination with structural analysis and computational modelling of the recoverin-kinase complex provided insight into the protein-protein interface between the kinase and the C-terminus of recoverin. Based on these results we suggest that Phe3 from the N-terminal helix of rhodopsin kinase and Lys192 from the C-terminal segment of recoverin form a cation-π interaction pair which is essential for target recognition by recoverin. Taken together, the results of the present study reveal a novel rhodopsin-kinase-binding site within the C-terminal region of recoverin, and highlights its significance for target recognition and regulation.