Novel membrane-localizing TEMPO derivatives for measurement of cellular oxidative stress at the cell membrane

Synthesis and fluorescence properties of six fluorescein-nitroxide radical hybrid-compounds

Six fluorescein-nitroxide radical hybrid-compounds (2ab, 3ab, 4, and 5) were synthesized by the condensation of 5- or 6-carboxy-fluorescein and 4-amino-TEMPO (2ab), 5- or 6-aminofluorescein and 4-carboxy-TEMPO (3ab), and fluorescein and 4-carboxy-TEMPO (4), or by reaction of the 3-hydroxyl group of fluorescein with DPROXYL-3-ylmethyl methanesulfonate (5). Fluorescence intensities (around 520nm) after reduction of the radical increased to 1.43-, 1.38-, and 1.61-folds for 2a, 2b and 3b respectively; 3a alone exhibited a decrease in intensity on reduction. Since 4 was readily solvolyzed in PBS or even methanol to afford fluorescein and 4-carboxy-TEMPO, its fluorescence change could not be measured. Hybrid compound 5 containing an ether-linkage between the fluorescein phenol and 3-hydroxymethyl-DPROXYL hydroxyl centers, was stable and on reduction, showed a maximum increase (3.21-fold) in relative fluorescence intensity in PBS (pH5.0), despite its remarkably low absolute fluorescence intensity.

Development of a DNA-binding TEMPO derivative for evaluation of nuclear oxidative stress and its application in living cells

Oxidative stress in nuclei is known to induce either oxidative modification of DNA bases or single/double-strand breaks, which may lead to carcinogenesis. To evaluate the redox status in nuclei in living cells, we designed a novel nucleus-localizing redox spin probe, F-DisT, which contains a fluorescein fluorophore linked to a DNA minor-groove-binding moiety. Nuclear distribution of the probe was easily confirmed by colocalization with a nuclear stain, Hoechst 33342, in confocal microscopy. Measurement of oxidative stress with F-DisT in a murine macrophage cell line exposed to endotoxin (lipopolysaccharide) showed a remarkable increase in the ESR signal decay rate. This increase was significantly inhibited by N(ω)-nitro-l-arginine (nitric oxide synthase inhibitor) and diphenyleneiodonium chloride (NADPH oxidase inhibitor). These results indicate that nitric oxide and superoxide contribute to oxidative stress in nuclei. Similar studies in membrane or mitochondria using respective organelle-specific spin probes indicated that the redox microenvironments in these organelles are markedly different from that in nuclei. Thus, subcellular redox microenvironments show marked variability in endotoxin-stimulated living cells.

2,2,6,6-tetramethylpiperidin-1-oxyl probes for evaluating oxidative stress on the cell membrane and mitochondria

Oxidative stress is recognized to be involved in many pathological conditions, such as inflammation, arteriosclerosis, and neurodegenerative diseases. Reactive oxygen species (ROS), which induce oxidative stress responses, appear to induce different biochemical and cellular changes in each pathological condition. To elucidate the roles of ROS in these diverse pathological conditions, the measurement and evaluation of oxidative stress at subcellular levels would be very effective. We have developed EPR probes for oxidative stress in organelles, which are tagged with a fluorescent function, and assessed oxidative stress of the membrane and mitochondria using chemical probes for the corresponding organelles. These probes are localized to the expected cellular regions confirmed by confocal fluorescent microscopy, and for demonstration, the probes are employed to detect oxidative stress in RAW264.7 cells stimulated with LPS/IFN-gamma. From the radical clearance rate of the probes localized in both the membrane and mitochondria, oxidative stress has been found to be consistently more severe in the membrane region, where NADPH oxidase is known to be upregulated by LPS/IFN-gamma treatment. These specific probes have depicted each organelle under different oxidative stress conditions by a specific exogenous oxidative stimulus.

Targeting mitochondria

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are closely linked to degenerative diseases such as Alzheimer's disease, Parkinson's, neuronal death including ischemic and hemorrhagic stroke, acute and chronic degenerative cardiac myocyte death, and cancer. As a byproduct of oxidative phosphorylation, a steady stream of reactive species emerge from our cellular energy plants, the mitochondria. ROS and RNS potentially cause damage to all cellular components. Structure alteration, biomolecule fragmentation, and oxidation of side chains are trade-offs of cellular energy production. ROS and RNS escape results in the activation of cytosolic stress pathways, DNA damage, and the upregulation of JNK, p38, and p53. Incomplete scavenging of ROS and RNS particularly affects the mitochondrial lipid cardiolipin (CL), triggers the release of mitochondrial cytochrome c, and activates the intrinsic death pathway. Due to the active redox environment and the excess of NADH and ATP at the inner mitochondrial membrane, a broad range of agents including electron acceptors, electron donors, and hydride acceptors can be used to influence the biochemical pathways. The key to therapeutic value is to enrich selective redox modulators at the target sites. Our approach is based on conjugating nitroxides to segments of natural products with relatively high affinity for mitochondrial membranes. For example, a modified gramicidin S segment was successfully used for this purpose and proven to be effective in preventing superoxide production in cells and CL oxidation in mitochondria and in protecting cells against a range of pro-apoptotic triggers such as actinomycin D, radiation, and staurosporine. More importantly, these mitochondria-targeted nitroxide/gramicidin conjugates were able to protect against apoptosis in vivo by preventing CL oxidation induced by intestinal hemorrhagic shock. Optimization of nitroxide carriers could lead to a new generation of effective antiapoptotic agents acting at an early mitochondrial stage. Alternative chemistry-based approaches to targeting mitochondria include the use of proteins and peptides, as well as the attachment of payloads to lipophilic cationic compounds, sulfonylureas, anthracyclines, and other agents with proven or hypothetical affinities for mitochondria. Manganese superoxide dismutase (MnSOD), SS tetrapeptides with 2',6'-dimethyltyrosine (Dmt) residues, rhodamine, triphenylphosphonium salts, nonopioid analgesics, adriamycin, and diverse electron-rich aromatics and stilbenes were used to influence mitochondrial biochemistry and the biology of aging. Some general structural principles for effective therapeutic agents are now emerging. Among these are the presence of basic or positively charged functional groups, hydrophobic substructures, and, most promising for future selective strategies, classes of compounds that are actively shuttled into mitochondria, bind to mitochondria-specific proteins, or show preferential affinity to mitochondria-specific lipids.