Effects of ionizing radiation on artificial (planar) lipid membranes. I. Radiation inactivation of the ion channel gramicidin A

Dose-dependent hepatic transcriptional responses in Atlantic salmon (Salmo salar) exposed to sublethal doses of gamma radiation

Due to the production of free radicals, gamma radiation may pose a hazard to living organisms. The high-dose radiation effects have been extensively studied, whereas the ecotoxicity data on low-dose gamma radiation is still limited. The present study was therefore performed using Atlantic salmon (Salmo salar) to characterize effects of low-dose (15, 70 and 280 mGy) gamma radiation after short-term (48h) exposure. Global transcriptional changes were studied using a combination of high-density oligonucleotide microarrays and quantitative real-time reverse transcription polymerase chain reaction (qPCR). Differentially expressed genes (DEGs; in this article the phrase gene expression is taken as a synonym of gene transcription, although it is acknowledged that gene expression can also be regulated, e.g., at protein stability and translational level) were determined and linked to their biological meanings predicted using both Gene Ontology (GO) and mammalian ortholog-based functional analyses. The plasma glucose level was also measured as a general stress biomarker at the organism level. Results from the microarray analysis revealed a dose-dependent pattern of global transcriptional responses, with 222, 495 and 909 DEGs regulated by 15, 70 and 280 mGy gamma radiation, respectively. Among these DEGs, only 34 were commonly regulated by all radiation doses, whereas the majority of differences were dose-specific. No GO functions were identified at low or medium doses, but repression of DEGs associated with GO functions such as DNA replication, cell cycle regulation and response to reactive oxygen species (ROS) were observed after 280mGy gamma exposure. Ortholog-based toxicity pathway analysis further showed that 15mGy radiation affected DEGs associated with cellular signaling and immune response; 70mGy radiation affected cell cycle regulation and DNA damage repair, cellular energy production; and 280mGy radiation affected pathways related to cell cycle regulation and DNA repair, mitochondrial dysfunction and immune functions. Twelve genes representative of key pathways found in this study were verified by qPCR. Potential common MoAs of low-dose gamma radiation may include induction of oxidative stress, DNA damage and disturbance of oxidative phosphorylation (OXPHOS). Although common MoAs were proposed, a number of DEGs and pathways were still found to be dose-specific, potentially indicating multiple mechanisms of action (MOAs) of low-dose gamma radiation in fish. In addition, plasma glucose displayed an apparent increase with increasing radiation doses, although the results were not significantly different from the control. These findings suggested that sublethal doses of gamma radiation may cause dose-dependent transcriptional changes in the liver of Atlantic salmon after short-term exposure. The current study predicted multiple MoA for gamma radiation and may aid future impact assessment of environmental radioactivity in fish.

Functional consequences of oxidative membrane damage

The interaction of reactive oxygen species with biological membranes is known to produce a great variety of different functional modifications. Part of these modifications may be classified as direct effects. They are due to direct interaction of the reactive species with the molecular machinery under study with a subsequent chemical and functional modification of these molecules. An important part of the observed functional modifications are, however, indirect effects. They are the consequence of an oxidative modification of the environment of biological macromolecules. Lipid peroxidation-via its generation of chemically reactive products-contributes to the loss of cellular functions through the inactivation of membrane enzymes and even of cytoplasmic (i.e., water soluble) proteins. Oxidation of membrane lipids may, however, also increase the efficiency of membrane functions. This was observed for a series of transport systems. Lipid peroxidation was accompanied by activation of certain types of ion channels and ion carriers. The effect is due to an increase of the polarity of the membrane interior by accumulation of polar oxidation products. The concomitant change of the dielectric constant, which may be detected via the increase of the membrane capacitance, facilitates the opening of membrane channels and lowers the inner membrane barrier for the movement of ions across the membrane. The predominant effect, however, at least at a greater extent of lipid peroxidation, is the inhibition of membrane functions. The strong increase of the leak conductance contributes to the depolarization of the membrane potential, it destroys the barrier properties of the membrane and it may finally lead, via an increase of cytoplasmic Ca(2+) concentration, to cell death. The conclusions were derived from experiments performed with different systems: model systems in planar lipid membranes, native ion channels either reconstituted in lipid membranes or investigated in their natural environment by the patch-clamp method, and two important ion pumps, the Na/K-ATPase and the sarcoplasmic reticulum (SR) Ca-ATPase.

Photodynamic inactivation of gramicidin channels:a flash-photolysis study

Photosensitized inactivation of ionic channels formed by gramicidin in the planar bilayer lipid membrane (BLM) has been studied upon exposure of the BLM to single flashes of visible light in the presence of tetrasulphonated aluminium phthalocyanine. The gramicidin photoinactivation is inhibited by the addition of unsaturated phospholipids to the membrane-forming solution as well as by the addition of azide to the bathing solution, consistent with involvement of singlet oxygen. The characteristic time of the photoinactivation (tau) does not change markedly under these conditions. Moreover, tau remains nearly constant upon alteration of the flash energy and the photosensitizer concentration. The value of tau appears to be sensitive to the gramicidin concentration and to the factors affecting the open time of the gramicidin channels, namely the temperature and the solvent used in the membrane-forming solution. The photoinactivation is not observed with covalent gramicidin dimers. The equations derived from the model of Bamberg and Laeuger (J. Membrane Biol. (1973) 11, 177-194), describing the relaxation of the gramicidin-induced conductance after a sudden distortion of the dimer-monomer equilibrium, are shown to explain consistently the time course of the photoinactivation provided that the damage of the gramicidin molecules leads to deviation from the equilibrium.

The effect of free radicals on the conductance induced by alamethicin in planar lipid membranes: activation and inactivation

Exposure to ionizing radiation of planar lipid membranes doped with alamethicin gives rise to an increase and to a subsequent decrease of the membrane conductance. Both effects are due to the presence of radiation-induced free radicals of water radiolysis as was shown by addition of various radical scavengers. The increase of the conductance was found to be a consequence of free radical-initiated lipid peroxidation favouring the formation of active ion channels. The decrease of the conductance observed at larger radiation doses is due to an inactivation of alamethicin monomers. The characteristic D37 dose of inactivation was found to be about two orders of magnitude larger than in the case of gramicidin A. The comparatively high sensitivity of the latter is due to the presence of its four tryptophan residues. Inactivation of trichorzianine AIIIc, an analogue of alamethicin with a C-terminal tryptophanol residue, occurs at radiation doses two orders of magnitude lower than observed with alamethicin.

Inactivation by ionizing radiation of ion channels formed by polyene antibiotics amphotericin B and nystatin in lipid membranes: an inverse dose-rate behavior

The phenomena reported are part of a study about the effects of ionizing radiation on membrane transport. We found that the conductance of lipid membranes in the presence of the polyene-antibiotics nystatin or amphotericin B is reduced to virtually zero following irradiation. Ion channels formed by these substances seem to represent extremely sensitive structures being inactivated by radiation doses in the range of a few Centigray (1 cGy = 1 rad) at sufficiently small dose rates. Inactivation shows a so-called inverse dose-rate behavior, i.e., at constant radiation dose the effect increases with decreasing dose rate. Similar to radiation-induced lipid peroxidation the phenomenon may be understood on the basis of a radical chain mechanism initiated by free radicals of water radiolysis. The process--via peroxidation of the polyene part of the molecules--is suggested to modify the hydrophobic exterior and to destabilize the barrel-like structure of the ion channels.

Photodynamic inactivation of an ion channel: gramicidin A

The ion channel formed by the peptide gramicidin A in planar lipid membranes is inactivated by visible light in the presence of the photosensitizer Rose Bengal. This is concluded from the strong decrease of the membrane conductance by more than two orders of magnitude. Experiments performed at different oxygen concentrations, in the presence of the singlet oxygen quenchers beta-carotene or alpha-tocopherol indicate, that presumably a type I process between the dye Rose Bengal and the tryptophan residues of the gramicidin channel with a subsequent oxidation of the tryptophans is responsible for the loss of the conductance properties of the channel.

The isoquinoline sulfonamide H7 attenuates radiation-mediated protein kinase C activation and delays the onset of x-ray-induced G2 arrest

Protein kinase C activation by ionizing radiation in human tumor cell lines participates in the transcriptional activation of genes which may be associated with the phenotypic response of cells to x-rays. We gamma-irradiated cell line RIT-3 (radiation-induced human sarcoma) and quantified the phosphorylating capacity of protein kinase C. Protein kinase C activity increased rapidly and transiently in these cells. The selective protein kinase C inhibitor H7 attenuated radiation-mediated protein kinase C activation when added to cells prior to irradiation. To determine whether protein kinase C activation is associated with radiation-induced G2 arrest, we analyzed the cell cycle distribution of cells following gamma-irradiation. Following irradiation, RIT-3 cells rapidly progressed through G1 and S and subsequently underwent a dose dependent G2 arrest. At concentrations which are selective for protein kinase C inhibition, H7 delayed the onset of radiation-induced G2 arrest. However, there was no difference in the duration of G2 arrest following the addition of inhibitor as compared to cells irradiated without inhibitor. We propose that protein kinase C activation by ionizing radiation is associated with radiation-mediated cell cycle regulation.

Radiation inactivation of ion channels formed by gramicidin A. Protection by lipid double bonds and by alpha-tocopherol

The conductance induced by the channel-forming peptide gramicidin A in lipid membranes is reduced by many orders of magnitude on exposure of the membrane and its aqueous environment to ionizing radiation. This results from an interaction of free radicals of water radiolysis with the tryptophan residues of gramicidin A. The sensitivity of the ion channels towards irradiation is strongly reduced in the presence of either vitamin E or of highly unsaturated lipids. An increase of the D37 dose up to a factor of 50 was found. The phenomena are interpreted via a reduction of the effective concentration of free radicals (such as OH.) in the membrane by reaction with unsaturated fatty acid residues or with vitamin E.