Non-random peroxidation of different classes of membrane phospholipids in live cells detected by metabolically integrated cis-parinaric acid

The use of cis-parinaric acid to measure lipid peroxidation in cardiomyocytes during ischemia and reperfusion

cis-Parinaric acid (PnAc), a fluorescent, polyunsaturated fatty acid, was used to measure lipid peroxidation during simulated ischemia and reperfusion in cultured neonatal rat cardiomyocytes. PnAc was used both as free fatty acid, inserted in the membranes following cultivation of the cells, as well as constituent of the cellular complex lipids by metabolically integrating the fatty acid during growth. In the insertion experiments a pre-incubation with DL-aminocarnitine, an inhibitor of beta-oxidation, was necessary to prevent loss of fluorescent signal. Such a pre-incubation resulted in an enrichment of PnAc in the sarcolemma: In pre-treated cells 57 +/- 1.3% of total inserted PnAc is present in the sarcolemma compared to 27 +/- 5.7% in cells containing the integrated probe. Both methods to introduce PnAc into the cells were compared with respect to their sensitivity for an externally applied oxidative stress and thereafter lipid peroxidation during simulated ischemia and reperfusion was assayed. Going from normoxic to ischemic conditions lipid peroxidation did not increase and remained at a low level. When the ischemic cells were subsequently subjected to reperfusion (reintroduction of both oxygen and glucose), large scale lipid peroxidation was obvious. When, on the other hand, oxygen alone was reintroduced (reoxygenation) no increased lipid peroxidation was observed. These observations led to the conclusion that ischemia does not lead to an enhanced lipid peroxidation and that resumption of metabolic activity during reperfusion is necessary to induce lipid peroxidation.

Amphotericin B as an intracellular antioxidant: protection against 2,2'-azobis(2,4-dimethylvaleronitrile)-induced peroxidation of membrane phospholipids in rat aortic smooth muscle cells

The antifungal activity of amphotericin B (AmB) and its side-effects (e.g. nephrotoxicity and hemolytic action) are suggested to be associated with its prooxidant effects in target cells. To test this hypothesis, we have undertaken studies to examine the role of AmB in oxidative stress in cultured rat aortic smooth muscle cells (SMC) incubated in the absence or in the presence of a lipid-soluble azo-initiator of peroxyl radicals, 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). No changes in the pattern of membrane phospholipids could be detected by two-dimensional high performance thin-layer chromatography (HPTLC) after oxidative stress induced by AMVN in which the cells remained viable, as judged by trypan blue exclusion. To improve the sensitivity of detection of oxidative stress in the cells, cis-parinaric acid (PnA) was incorporated biosynthetically into the membrane phospholipids [using PnA-human serum albumin (hSA) complex]. Incubation of the cells under aerobic conditions in the presence of up to 10 microM AmB showed no significant change in the pattern of PnA-labeled phospholipids, suggesting that AmB was not affecting the oxidative state of the cells. In contrast, treatment with AMVN (0.5 mM, incubation in the dark for 2 hr at 37 degrees--conditions in which the viability of the cells was maintained) caused a significant reduction of all fluorescently labeled phospholipid fractions separated by HPLC. When PnA-labeled cells were subjected to oxidative stress by incubation with 0.5 mM AMVN in the presence of AmB, the loss of fluorescent phospholipids was reduced in a concentration-dependent manner over a concentration range of 0.25 to 10 microM. Thus, AmB does not produce any prooxidant effect but rather acts as an intracellular antioxidant.

Overexpression of metallothionein decreases sensitivity of pulmonary endothelial cells to oxidant injury

Metallothionein (MT) is a low-molecular-weight cysteine-rich protein with extensive metal binding capacity and potential nonenzymatic antioxidant activity. Despite the sensitivity of vascular endothelium to either heavy metal toxicity or oxidative stress, little is known regarding the role of MT in endothelial cells. Accordingly, we determined the sensitivity of cultured sheep pulmonary artery endothelial cells (SPAEC) that overexpressed MT to tert-butyl hydroperoxide (t-BOOH), hyperoxia, or 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN; peroxyl radical generator). Nontoxic doses of 10 microM Cd increased MT levels from 0.21 +/- 0.03 to 2.07 +/- 0.24 microg/mg and resulted in resistance to t-BOOH and hyperoxia as determined by reduction of Alamar blue or [3H]serotonin transport, respectively. SPAEC stably transfected with plasmids containing either mouse or human cDNA for MT were resistant to both t-BOOH and hyperoxia. In addition, we examined transition metal-independent, noncytotoxic AMVN-induced lipid peroxidation after metabolic incorporation of the oxidant-sensitive fluorescent fatty acid cis-parinaric acid into phospholipids and high-performance liquid chromatography separation. SPAEC that overexpressed MT after gene transfer completely inhibited peroxyl oxidation of phosphatidylserine, phosphatidylcholine, and sphingomyelin (but not phosphatidylethanolamine) noted in wild-type SPAEC. These data show for the first time that MT can 1) protect pulmonary artery endothelium against a diverse array of prooxidant stimuli and 2) directly intercept peroxyl radicals in a metal-independent fashion, thereby preventing lipid peroxidation in intact cells.

Direct evidence for antioxidant effect of Bcl-2 in PC12 rat pheochromocytoma cells

Mock-transfected PC12 rat pheochomocytoma cells and PC12 cells transfected with the bcl-2 gene, a gene associated with inhibition of apoptosis, were subjected to oxidative stress by incubation in the presence of the azo-initiator of lipid peroxyl radicals, 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). Extraction and chromatographic analysis by two-dimensional TLC of the major phospholipid classes showed no differences in the phospholipid composition between the mock- and bcl-2-transfected cell lines after incubation in the presence of 0.5 mM AMVN for 2 h at 37 degrees C. A method consisting of incorporation of cis-parinaric acid into the constituent membrane phospholipids before exposure to AMVN was developed to improve the sensitivity of detecting lipid peroxidation in PC12 cells. Analysis of the pattern of changes in parinaric acid-labeled phospholipids after exposure to 0.25 and 0.5 mM AMVN by HPLC showed significant oxidation of phosphatidylcholine (PC), phosphatidylethanolamine (PEA), phosphatidylserine (PS), phosphatidylinositol (PI), and sphingomyelin (SPH) during a 2-h incubation. The extent of oxidation of each phospholipid class was dependent on the concentration of AMVN present up to 1 mM. Based on phospholipid fractional composition, the specific rates of PnA peroxidation in phospholipid classes were estimated. In mock-transfected PC12 cells, the order of AMVN-induced oxidation effectiveness was the same for both specific rates and relative rates: PC >> PEA > PS > SPH > PI. While a dramatic decrease in both relative and specific oxidation rates was observed for all phospholipid classes in bcl-2-transfected PC12 cells, the specific oxidation rates were higher for aminophospholipids (PEA and PS) than for other phospholipids. This suggests that antioxidant protection by bcl-2-related product(s) may be phospholipid-specific and that aminophospholipids are relatively less protected than the other phospholipids. The vitamin E analogue, 2,2,5,7,8-pentamethyl-6-hydrochromane, acted as an effective antioxidant in preventing oxidation of parinaric acid-labeled membrane phospholipids during incubation in the presence of AMVN and the extent of protection was approximately the same in both cell lines. Since, unlike the agents used to generate oxidative stress in other studies, temperature-driven generation of peroxyl radicals by AMVN is not dependent on intracellular metabolism, the results presented provide proof for antioxidant protection, rather than abrogation of radical generation afforded by bcl-2 transfection of PC12 cells.

Nitric oxide prevents oxidative damage produced by tert-butyl hydroperoxide in erythroleukemia cells via nitrosylation of heme and non-heme iron. Electron paramagnetic resonance evidence

We studied protective effects of NO against tert-butylhydroperoxide (t-BuOOH)-induced oxidations in a subline of human erythroleukemia K562 cells with different intracellular hemoglobin (Hb) concentrations. t-BuOOH-induced formation of oxoferryl-Hb-derived free radical species in cells was demonstrated by low temperature EPR spectroscopy. Intensity of the signals was proportional to Hb concentrations and was correlated with cell viability. Peroxidation of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin metabolically labeled with oxidation-sensitive cis-parinaric acid was induced by t-BuOOH. An NO donor, (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]-diazen-1-iu m-1, 2-diolate], produced non-heme iron dinitrosyl complexes and hexa- and pentacoordinated Hb-nitrosyl complexes in the cells. Nitrosylation of non-heme iron centers and Hb-heme protected against t-BuOOH-induced: (a) formation of oxoferryl-Hb-derived free radical species, (b) peroxidation of cis-parinaric acid-labeled phospholipids, and (c) cytotoxicity. Since NO did not inhibit peroxidation induced by an azo-initiator of peroxyl radicals, 2, 2'-azobis(2,4-dimethylvaleronitrile), protective effects of NO were due to formation of iron-nitrosyl complexes whose redox interactions with t-BuOOH prevented generation of oxoferryl-Hb-derived free radical species.

Bcl-2 inhibits selective oxidation and externalization of phosphatidylserine during paraquat-induced apoptosis

Oxidative stress is a potential component of the final common pathway leading to apoptosis following many diverse stimuli. Here, we document that the oxidant paraquat caused apoptosis in mouse 32D cells. We examined early paraquat-induced lipid peroxidation after metabolic incorporation of the oxidant-sensitive fluorescent fatty acid cis-parinaric acid (cis-PA) into phospholipids and high-performance liquid chromatography separation of specific phospholipid classes. Paraquat induced peroxidation of cis-PA primarily in phosphatidylserine (PS) and to a lesser extent in phosphatidylinositol (PI) within 2 h. The selective oxidation of PS occurred before signs of cytotoxicity and preceded the externalization of PS as assessed by annexin V binding. Overexpression of Bcl-2 afforded significant protection against paraquat-induced apoptosis, early PS and PI oxidation, and PS externalization but not the ultimate formation of high-molecular-weight DNA fragments. Therefore, both selective phospholipid peroxidation and DNA damage occurred after paraquat exposure, but only the former was specifically associated with apoptosis. We suggest Bcl-2 may inhibit oxidant-induced apoptosis by preventing the peroxidation of specific membrane phospholipids.

Direct oxidation of polyunsaturated cis-parinaric fatty acid by phenoxyl radicals generated by peroxidase/H2O2 in model systems and in HL-60 cells

Reactivity of phenoxyl radicals towards biomolecules (proteins, nucleic acids and lipids) is essential for antioxidant (protective) versus prooxidant (cytotoxic) effects of phenolic compounds (antioxidants, phytochemicals, environmental pollutants and toxic chemicals). The present study demonstrates for the first time that phenoxyl radicals formed by peroxidase/H2O2-catalyzed oxidation of phenol can directly oxidize a natural polyunsaturated fatty acid, cis-parinaric acid (PnA) both in model systems and in membrane phospholipids of HL-60 cells. Endogenous antioxidants-ascorbate and glutathione-can act as one-electron reductants of phenoxyl radicals and provide effective protection against phenoxyl radical-induced oxidation of PnA.