Radiobiology 7 PDF

  Generation of reactive nitrogen species Ionizing radiation can also stimulate inducible nitric oxide synthase (NOS) activity in hit cells, thereby generating large amounts of nitric oxide ( NO). Although ( NO) is chemically inert toward most cellular constituents (except for heme), it reacts w...

  Generation of reactive nitrogen species Ionizing radiation can also stimulate inducible nitric oxide synthase (NOS) activity in hit cells, thereby generating large amounts of nitric oxide ( NO). Although ( NO) is chemically inert toward most cellular constituents (except for heme), it reacts with 𝑂2∙− to form the peroxynitrite anion (ONOO−) Peroxynitrite anion (ONOO−) is highly reactive and capable of attacking a wide range of cellular targets, including lipids, thiols, proteins and DNA bases. In general, the radiolysis of water and early activation of nitric oxide synthases are a major source of ROS/RNS in irradiated cells under ambient oxygen. ROS/RNS role in the carcinogenesis process.  ◦ Endogenous and radiation-induced DNA alterations Improvements in the sensitivity of analytic methods have shown that endogenous biochemical processes greatly contribute to genome mutations. The ROS produced during normal cellular metabolic processes (mainly 𝑂2∙− and 𝐻2𝑂2 ) cause extensive depurinations and to lesser extent depyrimidinations The spectrum of ROS generated during and shortly after irradiation is similar to that produced by metabolic processes. However, differences exist in micro distribution. As a result, damage from metabolic ROS is randomly distributed in the DNA but radiation-induced DNA damage frequently occurs in clusters. The cell contains naturally occurring thiol compounds such as glutathione, cysteine, cysteamine, and metallothionein, whose sulfhydryl (SH) groups can react chemically with the free radicals to decrease their damaging effects. Other antioxidants include the vitamins C and E and intracellular manganese superoxide dismutase (MnSOD). MnSOD is an enzyme that alternately catalyzes the superoxide O−2 radical into ordinary molecular oxygen O2. 18 The intracellular levels of thiols and antioxid-ative molecules may differ between normal and tumor tissues, and their manipulation may offer a clinical strategy to protect normal tissues from radiotherapy-induced damage. One example is the use of the thiol-containing drug, Amifostine to protect against radiotherapy-induced Xerostomia (ie, dry mouth) after irradiation of salivary glands. 19 The random nature of the energy-deposition events means that radiation-induced changes can occur in any molecule in a cell. DNA is a major target of ionizing radiation, because of its biological importance to the cell. Even relatively small amounts of DNA damage can lead to cell lethality. It has been estimated that approximately 105 ionizations can occur within a diploid cell per gray of absorbed radiation dose, leading to approximately 1000 to 3000 DNA–DNA or DNA– protein crosslinks, 1000 damaged DNA bases, 500 to 1000 single-strand and 25 to 50 double-strand DNA breaks (ie, the vast majority of the ionization events do not cause DNA damage). 20 Focal areas of DNA damage can arise because of the clustering of ionizations within a few nanometers of the DNA. 21 These “local multiply damaged sites” (LMDS) include combinations of single- or double-strand breaks in the sugar- phosphate backbone of the molecule, alteration or loss of DNA bases, and formation of crosslinks (between the DNA strands or between DNA and chromosomal proteins). Most DNA lesions can be repaired by DNA repair pathways probably acting together to repair clustered LMDS- associated lesions. High LET irradiation causes an increase in both the number and complexity of DNA clustered lesions and is more difficult to repair. 22 The reactive oxygen species (ROS) induced by ionizing radiation can also interact with proteins in the cell membrane, some of which may be involved in signal transduction. This can lead to apoptosis in certain cell types (eg, endothelial cells) by activation in the membrane of a ceramide-sphingomyelin pathway (Fuks et al, 1995). 23 Furthermore, preincubation of cells with agents capable of altering either protein function or lipid peroxidation within the cell membrane, including anti-ceramide antibodies, can also modify the level of radiation-induced apoptosis (Fuks et al, 1995; Rotolo et al, 2012). Overall, the cellular response to ionizing radiation is mediated both by the direct damage to DNA and a complex interaction between proteins located within the plasma membrane, cytoplasm, and nucleus of the cell. 24  Genetic Instability, Chromosomal Damage, and Bystander Effects Many human cancers contain chromosomal rearrangements, including chromosomal translocations, deletions, and amplifications. Chromosome rearrangements can also be observed in cells after irradiation, and if nonlethal, may contribute to the carcinogenic properties of ionizing radiation. Chromosomal instability following irradiation has been demonstrated. 25 DNA double-strand breaks can lead to chromosomal rearrangements at the first mitosis after exposure to ionizing radiation and the type of aberration Fig slide 27) reflects the cell-cycle phase at the time of irradiation. Pathways for rejoining of DNA double-strand breaks in mammalian cells, include homologous recombination and non- homologous end-joining, but it is unclear which factors determine whether or not an induced break will lead to a chromosomal rearrangement. 26 27 Cell cycle G1 phase: Cell increase in size, cellura contents duplicated S phase: DNA replication, each of the 56 chromosomes(23 pairs)is replicated by the cell G2 phase: Cell prepares for cell division M phase: mitosis followed by Cytokinesis( cell separation).formation of two identical daughter cells. 28 Cell-cycle progression is controlled by a family of molecular checkpoint genes. Their function is to ensure the correct order of cell-cycle events. 29 Cyclin-dependent kinases CDK Immuno-histochemical (IHC) study of cyclin- dependent kinases Cyclin-dependent kinases (CDKs) : CDK1 is known to have a diagnostic value in esophageal and breast cancers. CDK5 is known to play a role in the lung cancer, while CDK6 is mis expressed in Ovraian cevix cancer. (CDK5) expression in non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC) 31 The effects of radiation on the progression of cells into mitosis after the treatment 32 Cells are irradiated when in different phases of the cell cycle and the mitotic delay observed is plotted as a function of radiation dose. 33 34 Survival curves for Chinese hamster cells irradiated in different phases of the cell cycle 35