6/8/2015 Biochem II; Hormesis p. 1 of 41 Illinois Institute of Technology Physics 561 Radiation...

04/18/23 Biochem II; Hormesis p. 1 of 41

Illinois Institute of Technology

Physics 561 Radiation BiophysicsLecture 12: Hormesis

11 July 2014Andrew Howard

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



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


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.


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


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

Replication Enzymatically catalyzed reaction;

often studied in molecular biology courses– Involves processivity, i.e. enzymatic

complex 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

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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


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

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 transcribed

through a single promoter In eukaryotes:

– Transcription occurs in the nucleus– Initial gene product shortened in spliceosome



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


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

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



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

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


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.



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


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.


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


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

Equivalent Dose, Sv

Num

ber

of c

ance

rs/1

05 p

eopl

e


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


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 Health 101(2): 310-314.


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


Multiple mechanisms Adaptive responses can have multiple forms Feinendegen (2005) Brit. J. Radiology 78:3-7

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


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


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


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


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

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!


Hormesis as S(D)


Dose, Gy

α = -0.2 Gy-1

β = 0.04 Gy-2

Crossover at 5 Gy

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


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.


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



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


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.

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


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!



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


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


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


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.

6/8/2015 Biochem II; Hormesis p. 1 of 41 Illinois Institute of Technology Physics 561 Radiation...

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