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Illinois Institute of Technology

Illinois Institute of Technology. Physics 561 Radiation Biophysics Lecture 12: Hormesis 11 July 2014 Andrew Howard. Lecture 12 Plans. Biochemistry, concluded Amino acids N ucleic acids Molecular biology Hormesis Definitions Radiation hormesis Mechanisms. Hormesis (concluded)

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Illinois Institute of Technology

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  1. Illinois Institute of Technology Physics 561 Radiation Biophysics Lecture 12: Hormesis11 July 2014 Andrew Howard Biochem II; Hormesis

  2. Lecture 12 Plans • Biochemistry, concluded • Amino acids • Nucleic acids • Molecular biology • Hormesis • Definitions • Radiation hormesis • Mechanisms • Hormesis (concluded) • Evidence • Politics • Bystander effects, abscopal effects • Answers for second midterm Biochem II; Hormesis

  3. Amino acid catabolism • Intact proteins are broken down into oligomeric fragments and then down to individual amino acids through the action of peptidases or proteases (enzymes that cleave peptide bonds) • Amino acids are either recycled or deaminated and converted in the TCA cycle intermediates • Nitrogenous component (~ ammonia) is typically excreted; see nucleic acid catabolism, below Biochem II; Hormesis

  4. Nucleic acid anabolism • Pyrimidines (the simpler ones) derived from glutamine and a few other starting materials • Thymidine (5-methyldeoxyuridylate) is present in smallest quantities and is therefore usually the limiting reagent in DNA synthesis • ~8-step pathway to uracil; a few more for C and T • Purines: more complex; derived from glutamine, succinate, a few other starting materials • Synthesis is carefully regulated so [dC]~[dG], etc. Biochem II; Hormesis

  5. Nucleic acid catabolism • Most organisms have elaborate mechanisms for eliminating nitrogenous waste, including broken-down DNA and RNA bases • End product is urea in some organisms, urate in others, allantoin in others, ammonia in others • Large percentage of broken DNA and RNA is actually recycled and used in making more nucleotides • Disruption of these salvage pathways can be fatal or can lead to neuromuscular deterioration Biochem II; Hormesis

  6. DNA replication • Process by which a complete copy is made of double-stranded DNA • Both strands get replicated • Replication happens in both directions simultaneously under the control of enzyme complex called DNA polymerase • In prokaryotes it starts in one place on the chromosome; in eukaryotes it starts in many places, allowing replication to proceed faster Biochem II; Hormesis

  7. Replication • Enzymatically catalyzed reaction;often studied in molecular biology courses • Involves processivity, i.e. enzymaticcomplex doesn’t have to dissociatefrom the DNA molecule as it travels through it, enabling replication • Error correction occurs within the process as well as outside of replicational machinery • Moderately complicated even in bacteria • Even more complex in eukaryota • Takes place in the nucleus • Involves a multiple-protein molecular machine Biochem in Rad Bio

  8. Errors in replication • Errors occur even in the absence of ionizing radiation • Errors become more common when destabilizing chemistries are present, e.g. ionizing radiation or mutagenic chemicals • Enzymes that do surveillance and repair of replication errors are built into the DNA polymerase itself • Other surveillance & repair enzymes are external to polymerase Biochem II; Hormesis

  9. Transcription • Process by which a gene (a segment of DNA) is used as a guide for production of an RNA molecule that is complementary to that segment of DNA • A produces U, C produces G,G produces C, T produces A • In general only one of the DNA strands is used as the template for producing the RNA • Transcription is under control of RNA polymerase, another multi-protein molecular machine Biochem II; Hormesis

  10. Transcription • Applies to tRNA, rRNA, sRNA as well as mRNA • Occurs when the gene product is needed, not before • In prokaryotes: • Often directly connected to translation • multiple gene products often transcribedthrough a single promoter • In eukaryotes: • Transcription occurs in the nucleus • Initial gene product shortened in spliceosome Biochem in Rad Bio

  11. Not all RNA is mRNA! • Often the transcript is transfer RNA, ribosomal RNA, or small nuclear RNA • In fact, at any moment only ~3% of the RNA in a cell is messenger RNA; 80% is rRNA, 15% tRNA, 1% snRNA • Synthesis rate is much higher, though: 25% is mRNA, because mRNA gets degraded faster than the other types • Only mRNA is subject to spliceosomal processing Biochem II; Hormesis

  12. Fate of RNA • Ribosomal RNA (several kinds) leaves the nucleus and forms the warp and woof of the ribosome, along with some proteins • Transfer RNA (at least 20 kinds) also goes to the ribosome, where it acts by fetching and activating an amino acid so it can be attached to a growing protein • Messenger RNA leaves nucleus after spliceosomal processing and serves as template for translation • Small nuclear RNA stays in the nucleus and is involved in spliceosomal activity and other functions Biochem II; Hormesis

  13. Translation • Synthesis of protein at ribosome using amessenger RNA molecule as template • Ribosome: complex of several rRNA molecules and several protein molecules • Process similar in prokaryotes & eukaryotes • Protein partially folded as it emerges • Many proteins fold without help • Others require help through chaperonins • Many proteins undergo post-translational modification before use or transport Biochem in Rad Bio

  14. Translation • Special steps start the production of a protein in the ribosome • Then each additional amino acid is added to the growing polypeptide: • Each codon (three bases) tells the ribosome which amino acid to fetch • Appropriate amino acid is brought in, attached to its tRNA molecule • tRNA yields up the amino acid and the rRNA catalyzes the attachment process Biochem II; Hormesis

  15. How do these processes differ in eukaryotes relative to prokaryotes? • DNA polymerase has more elaborate error correction in eukaryotes • Eukaryotic promoters stimulate transcription of exactly 1 gene rather than an operon’s worth • Transcriptional and translational machines are more complex in eukaryotes • Eukaryotic mRNA gets processed extensively before it leaves the nucleus to become translated • Transcription and translation are decoupled Biochem II; Hormesis

  16. Hormesis • In its most general form:it is the principle that a substance or process that is hazardous or toxic at high doses may be beneficial at lower doses • With chemicals, this has been understood for millenia • But even with chemicals it’s often poorly recognized at the regulatory level, where a substance known to be hazardous at high doses becomes banned outright, depriving those who would benefit from low doses of that substance. Biochem II; Hormesis

  17. Toxicology and Hormesis • I was a bit surprised to realize that even the chemical toxicology community has shown skepticism about hormesis • That group, at least, has no institutional vested interest in a linear non-threshold perspective • Nonetheless Edward Calabrese (UMass School of Public Health) and others have had to champion hormetic effects for chemicals Photo courtesy Cato Institute Biochem II; Hormesis

  18. Radiation Hormesis • We’ll define radiation hormesis as low-dose-induced protection from biological harm. • Bobby R Scott, “Radiation Hormesis and the Control of Genomic Instability”, Ch. 6 in Eleanor Gloscow, ed. (2007), New Research on Genomic Instability. • This article is extensively summarized in a Google Books review, so you can get the gist of it there. Biochem II; Hormesis

  19. Hormesis, the strong form • Suggestion is that low doses can be protective against some form of injury • Perhaps from subsequent radiation doses • Perhaps from some other environmental risk Biochem II; Hormesis

  20. Hormesis, the weak form • This might simply suggest that LNT fails to describe low-dose behavior such that the slope of the dose-response curve is smaller at low dose than at high Number of cancers/105 people Equivalent Dose, Sv Biochem II; Hormesis

  21. How hard is it to study this? • Very • Most of the attention is paid to cancer, where: • The background level is very high • Smoking is such a big issue that under-reporting of smoking can, all by itself, dwarf any other influence • You can do somewhat better by focusing on cancers of specific organs for which the background levels are lower, but it’s still difficult Biochem II; Hormesis

  22. Smoking and radon • Considerable evidence that radon’s effects are significantly potentiated by smoking • Mechanism for that potentiation has been extensively discussed already • But here the relevant point is a public health one: • If there is an excess cancer burden from radon, are we better off trying to build buildings with less radon in them, or getting people to quit smoking? • Méndez et al. (2011), The impact of declining smoking on radon-related lung cancer in the United States, Amer. J. Public Health101(2): 310-314. Biochem II; Hormesis

  23. Why is hormesis plausible? • Plenty of evidence suggests that repair mechanisms, particularly enzymatic DNA repair mechanisms, are inducible • Therefore exposure may turn on protective systems that then leave the organism more capable of tolerating subsequent insult • The enzymes thus induced may be able to respond to challenges different from the one that brought forth the induction in the first place • Other mechanisms (e.g., immunological) too Biochem II; Hormesis

  24. Multiple mechanisms • Adaptive responses can have multiple forms • Feinendegen (2005) Brit. J. Radiology78:3-7 suggests several, including: • Damage prevention: rise in free glutathione, SOD • Damage repair: enhancements of DNA repair rates • Damage removal by apoptosis of pre-damaged cells and concomitant replacement of those cells with healthy cells • Stimulation of immune response • Protection & cell cycle: premature differentiation and maturation to senescence Biochem II; Hormesis

  25. Evidence for hormesis • Recent studies of cell-culture systems and animals maintained in environments that have very low background levels • Human epidemiological evidence, possibly • Exposures to irradiated steel in Taiwan • Some of the Hiroshima data? • What are we to think?The paper by Ragheb now posted on the Blackboard site is an unhysterical treatment of the subject Biochem II; Hormesis

  26. Taiwan steel • Around 1983, 180 apartment buildings were erected in Taipei for which the structural steel was contaminated with high levels of 60Co (T1/2 ~ 5.3y) • 10,000-15,000 residents received radiation doses averaging 0.4 Sv over 9-20 years • One paper (Chen et al. 2004), J.Am.Phys.Surg.9: 6) suggests significant benefits in terms of reduced rates of congenital heart malformations and cancer • These studies compared the residents of these buildings with the Taiwanese population as a whole Biochem II; Hormesis

  27. Subsequent re-examination • Hwang et al. (2006) Int.J.Radiat.Biol.12: 849 present a very different story • More exhaustive statistical analysis based on matching cohorts in terms of age, gender, smoking • Results suggest significant increases in cancer risk among the exposed population • Leukemia in men • Thyroid cancer in women • Results specific to those exposed before age 30 Biochem II; Hormesis

  28. What are we to make of this? • I’d say the jury is still out • Unarguable:any attempt to demonstrate hormesis (strong or weak) must take smoking, age, gender, biological endpoint into account • It might turn out, for example, that moderate doses are protective against certain medical endpoints for the normal population but would be detrimental for other medical endpoints • We also need to think about hypersensitive populations Biochem II; Hormesis

  29. An algebraic model that fits hormesis • The dose-effect relationships could be re-cast as survival fraction data • If you did, and you wrote down our standard linear-quadratic formula: S = exp(-αD –βD2) • Then if we take the simple step of allowing α to be negative while β is still positive, we can get the hormetic desired curve! • ln S = -αD – βD2 • S = 1 or lnS = 0 when -αD – βD2 = 0 =>Either D = 0 or βD = -α => D = -α/β …which works if α is negative and β is positive! Biochem II; Hormesis

  30. Hormesis as S(D) α = -0.2 Gy-1β = 0.04 Gy-2Crossover at 5 Gy Dose, Gy Biochem II; Hormesis

  31. The Ramsar story • Ramsar is a city of about 31,000people on the north border of Iran,opening on to the Caspian Sea,and is at an elevation of 985m. • Nearby hot springs and the rocks emanating from them are full of radium compounds • The background levels in some parts of the city are therefore around 10 mGy ~ 100 mSv / year—a factor of 100 higher than is typical elsewhere Biochem II; Hormesis

  32. So is that dangerous? • Hard to tell • Only about 2000 people live in the really high-background areas • Only a few years of monitoring have already occurred, so we’ll have to wait awhile • There might be a radioprotective effect, but there might not • If Ramsar proves to be hazardous, does that mean radiation hormesis is wrong? Not necessarily. Biochem II; Hormesis

  33. Calabrese’s analysis • Two papers from Edward J. Calabrese trace the history of the adoption of the LNT model and the rejection of threshold or hormetic models: • Archives of Toxicology (2009) 83:203-225 • Archives of Toxicology (2009) 83: 227-247 • I’ve posted both of these and encourage you to read them with a critical but open mind Biochem II; Hormesis

  34. Institutional responses to hormesis • French Academy of Sciences - National Academy of Medicine, 2005: • Using LNT to estimate carcinogenic effect @ doses < 20 mSv is unjustified in light of current radiobiologic knowledge • > 1 dose-effect relationship • Considerable evidence exists for hormesis • Summarize multiple potential mechanisms for it • Argue that LNT is only useful as a regulatory tool Biochem II; Hormesis

  35. ICRP and NCRP responses • ICRP and NCRP continue to deny the existence of reliable hormetic evidence • BEIR-VII says: in order for a dose threshold to exist, there has to be totally error-free DNA damage response and repair • Others would argue with that! • Intelligent and thoughtful professionals are involved in crafting these responses; but the final conclusions might still be seen as politically motivated rather than evidence-based. Biochem II; Hormesis

  36. Why is it difficult to overturn LNT? • We’ll talk about this next Tuesday some more • It has historically been difficult to obtain funding for studies of the effects of low doses • This can be understood through a conspiracy theory that says people in power have a vested interest in maintaining LNT • Keeps health physicists employed! • Provides a simple quantitative framework for setting exposure limits • Politicians can argue that they’re protecting the public Biochem II; Hormesis

  37. We are judged by the company we keep! • The world of enthusiasts for radiation hormesis includes some folks on the fringe • They advocate holding weakly radioactive stones on your chest and deriving healing benefits from them • Justifications from that great quantitative scientist, Carlos Castañeda • This interferes with a calm and scientific discussion of the real issues with low doses of ionizing radiation • Dogmatic adherence to LNT interferes in much the same way! Biochem II; Hormesis

  38. Bystander effects • Considerable evidence from cell-culture studies suggests that exposure of one cell to ionizing radiation can have influences on neighboring cells • More recent studies (Human & Experimental Toxicology (2004) 23: 59ff) show these effects can be observed in whole animals too • Mechanisms thought to involve migration of small molecules from one cell to another through gap junctions Biochem II; Hormesis

  39. What does this mean? • This can and sometimes is taken as evidence of an unanticipated increase in risk from low or moderate doses • However, it could work the other way:the bystander effects could include adaptive responses so that they become explanations of reduced risk from moderate doses Biochem II; Hormesis

  40. Most bystander-effect studies have been on high-LET radiation • Does that mean that bystander effects are limited to high-LET radiation? • Probably not: • It’s just that it’s a lot harder to study these effects when the radiation source itself extends its influence over many cell sizes, which is what happens with low LET Biochem II; Hormesis

  41. Abscopal effects • This refers to non-local influences of ionizing radiation • Small molecules that are directly involved in conveying damage are unlikely to travel within a large organism over length scales larger than a few cell sizes, so these effects are likely to be coming from circulating cells, most of which are immune cells like T cells. Biochem II; Hormesis

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