Transcript Hormesis - Illinois Institute of Technology

Illinois Institute of Technology
Physics 561
Radiation Biophysics
Lecture 12: Hormesis
11 July 2014
Andrew Howard
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Lecture 12 Plans
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Biochemistry,
concluded
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Amino acids
Nucleic acids
Molecular biology
Hormesis
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Definitions
Radiation hormesis
Mechanisms
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Hormesis (concluded)
Evidence
Politics
Bystander effects,
abscopal effects
Answers for second
midterm
Biochem II; Hormesis
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Amino acid catabolism
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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
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Nucleic acid anabolism
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Pyrimidines (the simpler ones) derived from glutamine
and a few other starting materials
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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.
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Nucleic acid catabolism
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Most organisms have elaborate mechanisms for
eliminating nitrogenous waste, including brokendown 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
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DNA replication
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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
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Replication
Enzymatically catalyzed reaction;
often studied in molecular biology courses
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Involves processivity, i.e. enzymatic
complex doesn’t have to dissociate
from 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
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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
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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
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Transcription
Applies to tRNA, rRNA, sRNA as well as mRNA
Occurs when the gene product is needed, not before
In prokaryotes:
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Often directly connected to translation
multiple gene products often transcribed
through a single promoter
In eukaryotes:
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Transcription occurs in the nucleus
Initial gene product shortened in spliceosome
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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
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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
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Translation
Synthesis of protein at
ribosome using a
messenger 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
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Many proteins fold without help
Others require help through chaperonins
Many proteins undergo post-translational
modification before use or transport
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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
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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
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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.
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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
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Photo courtesy
Cato Institute
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Radiation Hormesis
We’ll define radiation hormesis as low-doseinduced 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.
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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
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Hormesis, the weak form
This might simply
suggest that LNT fails
to describe low-dose
behavior such that
the slope of the doseresponse curve is
smaller at low dose
than at high
Equivalent Dose, Sv
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How hard is it to study this?
Very
Most of the attention is paid to cancer, where:
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The background level is very high
Smoking is such a big issue that underreporting 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
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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.
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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
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Multiple mechanisms
Adaptive responses can have multiple forms
Feinendegen (2005) Brit. J. Radiology 78:3-7
suggests several, including:
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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
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Evidence for hormesis
Recent studies of cell-culture systems and animals
maintained in environments that have very low
background levels
Human epidemiological evidence, possibly
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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
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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
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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
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Leukemia in men
Thyroid cancer in women
Results specific to those exposed before age 30
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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
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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 linearquadratic 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!
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Hormesis as S(D)
Survival Fraction
1.4
1.2
α = -0.2 Gy-1
β = 0.04 Gy-2
Crossover at 5 Gy
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0.4
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Dose, Gy
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The Ramsar story
Ramsar is a city of about 31,000
people 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
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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.
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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
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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
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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.
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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
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Keeps health physicists employed!
Provides a simple quantitative framework for setting
exposure limits
Politicians can argue that they’re protecting the public
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We are judged by the
company we keep!
The world of enthusiasts for radiation hormesis
includes some folks on the fringe
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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!
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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
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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
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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
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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.
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