Hormesis: What it
Means for Toxicology,
the Environment and
Public Health
Edward J. Calabrese, Ph.D
Environmental Health Sciences
School of Public Health
University of Massachusetts
Overview
• How I Became Involved with
Hormesis
• Hormesis:Toxicological Foundations
• Examples of Hormetic Responses
• Comparison with Threshold Model
• Hormesis and Risk Assessment
Hormesis
Definition:
• Dose response phenomenon characterized by a
low dose stimulation and a high dose inhibition.
• Generally similar quantitative features with
respect to amplitude and range of the
stimulatory response.
• May be directly induced or the result of
compensatory biological processes following an
initial disruption in homeostasis.
HORMESIS
Interpretation:
• Issue of beneficial/harmful effects should
not be part of the definition of hormesis.
• This assessment should be reserved for a
subsequent evaluation of the biological
and ecological context of the response.
A
Response
B
Response
Dose
(A) The most common form of the hormetic dose-response curve depicting low-dose
stimulatory and high-dose inhibitory responses, the - or inverted U-shaped curve.
(B) The hormetic dose-response curve depicting low-dose reduction and high-dose
enhancement of adverse effects, the J- or U-shaped curve.
Hormesis and Evaluative
Criteria
Assessing the Dose-Response Continuum:
• LOAEL-defining the toxic phase of the
dose response
• NOAEL (or BMD)-defining the approximate
threshold
• Below NOAEL (or BMD) doses-number
and range
• Concurrent Control
Hormesis and
Assessment Criteria
Dose Response Patterns
Statistical Significance
Replication of Findings
Evidence of Hormesis
General Summary:
• Hormesis databases: thousands of dose
responses indicative of hormesis
• Hormesis is a very general phenomenon:
independent of model, endpoint and agent
• Frequency of hormesis: far more frequent
than threshold model in fair head-to-head
comparisons
Dose Response Features
Stimulation Amplitude:
• Modest
• 30-60% Greater Than Control
• Usually Not More Than 100% Greater
Than The Control
Stimulatory Range
~75 % - Within 20-Fold of NOAEL
~20% - >20<1000-Fold of NOAEL
~<2% - > 1000-Fold of NOAEL
Maximum response
(averages 130-160% of control)
Distance to NOAEL
(averages 5-fold)
NOAEL
Control
Hormetic Zone
(averages 10- to 20-fold)
Increasing Dose
Dose-response curve depicting the quantitative features
of hormesis
Hormetic Mechanisms
Many studies have provided mechanistic
explanations to account for observed
hormesis responses;
Each mechanism is unique to the model,
tissue, endpoint and agent
Some general examples: Often existence of
opposing receptors
140
Females
Males
Longevity (% control)
120
100
80
60
40
20
0
0.00
Methanol and Fruit Fly Longevity
0.25
0.50
1.00
2.00
Methanol (%)
4.00
8.00
140
Females
Males
Incidence (% control)
120
100
80
60
40
20
Gamma Rays and Mouse Lung Adenomas
0
0
10
25
50
100
Gamma ray dose (rad)
150
200
300
140
Cell proliferation (% control)
120
100
80
60
40
Transforming Growth Factor-Beta and Human
Lung Fibroblasts
20
0
0.0
2.5
5.0
10.0
20.0
40.0
80.0
Transformng Growth Factor Beta (pg/ml)
160.0
180
160
(% control)
140
120
100
80
60
40
20
Effects of Acute Ethanol on Overall Social
Activity of Adolescent Rats Tested on
Postnatal Day 30
0
0.00
0.25
0.50
0.75
1.00
Ethanol (g/kg)
2.00
3.00
4.00
160
*
140
*
*
*
*
Root Length
(% of Control)
120
100
80
*
60
40
*
Effect of X-rays on the Root Length of
Carnation Cuttings
20
*
0
0
10
20
40
80
150
300
X-rays (R)
600 1200 2500 5000
Specific activity (% control)
160
140
120
100
80
60
40
Aluminum and
Mouse Blood Gamma-Aminolevulinic
Acid Activity
20
0
0
10
25
50
100 500 1000 2500 3500 4000 5000
Aluminum (uM)
Above ground (G)
160
Below G
*
(% Control)
140
Total Biomass
*
Stem Density
120
Max Shoot Height
100
Evap/transpir
80
60
40
Effect on Growth of Salt
Marsh Grass
20
0
0
7
14
29
57
114 171 228 342 456
Mercury Chloride (ug/L)
180
160
120
100
80
60
40
20
Comparative Dose Response Relationships for the
Pain Threshold for Vocalization
0
0.000
0.025
0.050
0.075
0.100
0.200
0.250
0.375
0.400
0.500
0.800
1.000
1.250
1.500
1.600
2.000
2.500
3.000
4.000
5.000
8.000
10.000
16.000
20.000
25.000
40.000
% Control
140
Yohimbine
Apomorphine
Promethazine
Drug Concentration (mg/kg)
00
0. 000
00 0
0. 000
00 1
0. 001
00 0
0. 100
10 0
0. 000
50 0
1. 000
00 0
3. 000
00 0
7. 000
50 0
15 00
. 0 00
20 000
.0 00
30 000
.0 00
60 000
0
.
10 000 0
0. 00
00 0
00
00
0.
% Control
150
*
125
25
*
*
100
*
*
75
*
*
50
Effect of Different Doses of Morphine
on PTZ-induced Seizure Threshold
0
Morphine (mg/kg)
Serum level (% control)
260
Testosterone
*
240
Luteinizing hormone
220
*
200
180
160
*
140
120
100
80
60
40
Alcohol and
Rat Serum Levels
20
0
0.00 0.50 0.75 1.00 1.25 1.50 2.00 2.50 3.00
Alcohol (g/kg)
150
Dry weight (% control)
*
*
*
100
50
MCPA +
OAT SHOOT GROWTH
0
0.00 0.00 0.01 0.05 0.10 0.50 1.00 2.00 5.00 10.00
4-Chloro-2-methylphenoxyacetic acid (MCPA)
(mg/pot)
260
HgCl2
MethHgCl
CdCl2
ZnCl2
240
Phagocytosis Activity (% control)
220
200
180
160
140
120
100
80
60
40
20
Effects of Metals on
Phagocytosis in the
Clam, Mya arenaria,
hemocytes
0
0.00E+00
1.00E-09
1.00E-08
1.00E-07
1.00E-06
Metal Concentration (M)
1.00E-05
1.00E-04
1.00E-03
Nitrate reductase (% control)
160
140
*
*
*
120
*
In Vitro
In Vivo
*
100
80
*
60
*
40
20
0
0.00
Cadmium and
Aquatic Plant (H. verticillata)
Nitrate Reductase Activity
0.01
0.10
1.00
**
**
10.00
Cadmium (uM)
40.00
80.00
Lymphocyte Stimulation (% control)
180
160
140
120
100
80
60
40
20
Effect of Sodium Arsenate
on PHA-treated Bovine
Lymphocytes
0
00 -05 -05 -05 -05 -05 -05 -05 -05 -05 -05 -04 -04 -04
+
E 0E 0E 0E 0E 0E 0E 0E 0E 0E 0E 0E 0E
E
0
0
0
2.0 3.0 4.0 5.5 6.0 6.5 7.0 8.0 9.0 1.0 2.5 3.0
0.0 1.
Sodium Arsenate (M)
160
*
*
140
*
*
*
120
*
Mercuric chloride
100
Methyl mercuric
chloride
80
60
Mercury and
Duckweed
Catalase Activity
40
20
Mercury (ug/L)
30
0
14
70
29
50
35
30
7
6
3
0.
7
0.
07
0.
35
0
0
Catalase activity (% control)
180
200
175
*
*
150
Days
(% of control)
*
125
100
75
50
Effect of Gamma Rays on
the Life Span of Female
House Crickets
25
*
*
*
*
0
0
500
1000
2000
4000
Gamma Rays (R)
6000
8000
10000
150
*
*
*
broods/daphnid
(% of control)
125
100
*
75
50
Effect of Acridine on the Number
of Broods per Daphnid
25
*
*
0
0.0
0.1
0.2
0.4
Acridine (mg/L)
0.8
1.6
3.2
Thymidine Uptake (% control)
280
Prostate
Prostate
Prostate
Renal
Renal
Colorectal
Colorectal
Colorectal
Gastric
Liposarcoma
240
200
160
120
80
40
Effect of Mistletoe
Lectin on Human
Tumors in Culture
0
0
1
10
Lectin Concentration (ng/ml)
50
Change in Cell Number (% control)
140
2-HE
120
2-ME
100
2-HEOL
2-MEOL
80
2-H
2-M
60
40
20
4-HE
4-ME
Effects of Ten Estradiol
A-ring Metabolites on
Endothelial Cells from Human
Umbilical Veins
0
0.00E+00 1.00E-08
1.00E-07 1.00E-06
4-HEOL
4-MEOL
1.00E-05
Ten Estradiol A-Ring Metabolites (M)
Granulocyte Phagocytosis (% control)
160
140
120
100
80
60
40
20
Effect of Plumbagin on Human Granulocyte
Phagocytosis
0
00 -09 -08 -07 -06 -05 -04 -03 -02 -01 +00
+
E 50E 50E 50E 50E 50E 50E 50E 50E 50E 0E
0
0
2.
2.
2.
2.
2.
2.
2.
2.
2.
0.
2.5
Plumbagin (ug/ml culture)
160
140
% Control
120
100
80
60
40
20
Effect of Tin (II) on MTT Conversion in C6
Glioma Cells
0
0.000
0.001
0.010
0.100
1.000
Tin (II) (ug/ml)
10.000
100.000
200
*
*
175
*
% Control
150
*
125
100
75
*
50
Number of Open Arm Entries in the Elevated
Plus Maze in Male C57BL/6 Mice Treated
with DHEA
25
*
0
0
0
0.0
1
0
0.0
6
0
0. 0
6
0.0
0
0
0
0. 6
0
0
1. 0
DHEA (mg/kg)
0
0
0
6.
0
.00
0
3
175
*
Neuronal Survival (% control)
150
*
125
100
75
*
50
The Effects of Allixin on the Survival of
Primary Cultured Hippocampal Neurons from
Embryonic (E18) Wistar Rats
25
*
0
0
1
10
100
Allixin (ng/ml)
1000
10000
150
*
*
Viability (% control)
125
*
*
100
75
*
50
The Effects of Methyl Mercury on Viability as
Measured by Mitochondrial Dehydrogenase
Activity in the D407 Cell Line
25
*
0
0.0
0.1
0.5
1.0
5.0
10.0
Methyl Mercury (µM)
25.0
500.0
200
175
% Control
150
125
100
75
50
Effects of the Disinfectant Byproduct MX on the Occurrence
of DNA Damage in the Comet Assay Using Rat Liver
Epithelial Cell Line WB-F344
25
0
0
15
30
60
120
180
240
3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX)
(mM)
15
.0
00
0
75
.0
00
0
15
0.
00
00
30
0.
00
00
7.
50
00
0.
75
00
0.
07
50
Effects of n-Hexane on DNA
Damage in Human Lymphocytes
in the Comet Assay
0.
00
75
0.
00
00
% Control
300
275
250
225
200
175
150
125
100
75
50
25
0
n -Hexane (mg/L)
300
275
250
Effects of As2O5 on Total Chromosomal
Aberrations in Human Leukocytes
225
% Control
200
175
150
125
100
75
50
25
0
0.00E+00
3.00E-07
1.20E-06
As2O5 (M)
3.60E-06
300
275
250
Effects of X-rays on Chromosomal Aberrations
(i.e., Dicentrics) in Human Lymphocytes (pooled
results of four donors and six laboratories)
225
% Control
200
175
150
125
100
75
50
25
0
0.00
3.13
5.80
9.65
19.30
X-Rays (mGy)
28.80
47.70
290.00
DDT(ppm)
0.
0
50
0.
0
10
00
00
00
0
00
0
00
0
00
0
50
0
20
0
10
.0
20
5.
2.
1.
0.
0.
0.
0
01
5
00
0
00
200
0.
0.
0.
GST-P Positive Foci (% control)
250
Effect of DDT on Liver Foci
Formation in Male F344 Rats
150
100
50
0
(% control)
240
220
200
180
160
140
120
100
80
60
40
20
0
Bladder Tumor Incidence Adjusted
for Time in ED01 Megamouse Study
0
30
35
45
60
AAF (ppm))
75
100
150
Hormetic or Threshold
Which Dose Response Is More Common?
The Threshold Model
Prediction: Random Bounce Below the
Threshold as Practically Defined by the
NOA(E)L or BMD
The Hormesis Model
• Predicts that responses to doses in
the below toxic threshold zone should
be non-randomly distributed
• The non-randomness should be
reflected in the frequency of
responses above and below the
control value and in the magnitude of
the deviation from the control
Hypothesis Evaluation
Dose-Response Evaluation Criteria
Entry Criteria:
Estimate a LO(A)EL
Estimate a NO(A)EL or BMD
One or more doses below
NO(A)EL or BMD
Testing Threshold Model
Predictions
Three Separate Database Evaluations:
• Toxicological Literature - multiple
models/endpoints - reviewed 21,000 articles with
entry criteria to yield 800 dose responses
• Yeast Cell Strains - 13 strains/2,200-57,000
dose responses-cell proliferation
• E. coli – approximately 2,000 chemicals tested
over 11 concentrations - cell proliferation
100
Cumulative Percent of Chemicals
90
Threshold Model
Predicted Mean
80
70
60
Mean
50
Prediction Interval 95%
40
30
20
10
-20
-10
0
10
30
40
50
20
Percent Difference From Control Growth
60
70
100
BMD 10.0
Cumulative Percent of Chemicals
90
BMD 7.5
80
70
BMD 5.0
BMD 2.5
60
50
40
30
20
10
0
-20
-10
0
10
20
30
40
50
Percent Difference From Control Growth
60
70
80
Threshold Model
Inconsistencies
• Below threshold responses do not provide
evidence of random bounce
• Non-random responses clearly
predominate
• The non-random responses discredit the
Threshold Dose Response Model
• Findings are consistent with the Hormetic
Dose Response Model
Why Has Toxicology
Missed Hormesis?
• Modest Response - could be normal
variation
• Emphasis on High Doses - need to define
the NOAEL and LOAEL
• Use of only few doses
Why is Hormesis
Important?
• It will change how toxicologists,
pharmacologists, risk assessors, and
physicians do their jobs
• It will change the risk communication
message
Hypothesis Testing
• Expands Dose Response Spectrum
• Creates New Categories of Questions
Study Design
• Number of Doses/Concentrations
• Spacing of Doses/Concentrations
• Temporal Features
– Key feature in recognizing the
compensatory nature of the hormetic
dose response
Implications of New
Design Considerations
Additional Costs For:
• Extra Doses
• Multiple Temporal Evaluations
• Enhanced Need for Replication
Possible Adjustments
• Less than lifetime studies/different
endpoints
• Less expensive models: cell culture,
invertebrates, fish, etc.
– increases sample size for statistical
power
Endpoint Selection
Background Incidence:
• Low Background Disease Incidence
Precludes Ability to Detect Possible
Hormetic Response
Biomathematical Modeling
Implications for Cancer Risk Assessment:
• Models: flexibility to fit observed data;
• Models: not constrained to always be
linearly decreasing at low doses;
• Low Dose Risk Characterization: include
likelihood of below background risks;
• Uncertainty Characterization: include both
upper and lower bounds.
Environmental
• Re-Defining Hazard Assessment
• Re-Defining Dose Response Default
• Re-Evaluation of Risk Assessment
Practices
• Harmonization: Cancer and NonCancer
• Cost-Benefit Re-Assessment
Therapeutics
• Cognitive
Dysfunction
• Immune
Stimulation
• Anti-Tumor
• Anti-Viral
• Anti-Bacterial
• Angiogenesis
• Cytokine/Hospital
Infections
• Hair Growth
• Molecular Designs
Life Style
• Exercise
• Alcohol Consumption
• Stress
Perspective #1
The Threshold Dose Response
Model fails to make accurate
predictions in the below threshold
zone
Perspective #2
The Threshold Dose Response
Model has been significantly outcompeted by the Hormetic Dose
Response Model in multiple,
independent comparisons
Perspective #3
There is little toxicological
justification for the continued use
of the threshold dose response to
estimate below threshold
responses
Perspective #4
Given Perspectives 1-3, there is no
basis to use the threshold dose
response model in risk
assessment practices. This has
significant implications for current
standards based on the threshold
model and future risk assessment
practices
Perspective #5
HORMESIS: a concept with
much supportive experimental
evidence that is reproducible
Perspective #6
HORMESIS: Based on
Perspective # 5 it should be
considered as a real concept
in the biological sciences
Perspective #7
HORMESIS is Generalizable
• Across Biological Models
• Across Endpoints Measured
• Across Chemical Class/Physical
Agents
Perspective #8
Based on Perspective # 7,
HORMESIS is evolutionarily
based, with broad potential
implications
Perspective #9
HORMESIS: very common in
toxicological/pharmacological
literature, making it a central
concept
Perspective #10
HORMESIS: a normal component
of the traditional dose response,
being graphically contiguous with
the NO(A)EL
Perspective #11
HORMESIS: readily definable
quantitative features, that are
broadly generalizable, making it
reasonably predictable
Perspective #12
HORMESIS: far more common
than the threshold dose response
in fair, head-to-head comparisons;
this would make the hormetic
model the most dominant in
toxicology
Perspective #13
The low dose hormetic stimulatory
response is a manifestation of
biological performance and estimates
biological plasticity in the effected
systems
Perspective #14
HORMESIS: no single specific
hormetic mechanism; there
appears to be a common
biological strategy underlying
such phenomena
Perspective #15
HORMESIS: important implications
for toxicology, risk assessment,
risk communication, cost-benefit
assessments, clinical medicine,
drug development and numerous
other areas
Perspective #16
HORMESIS: Should Become the
Default Model in Risk Assessment –
Why?
• More Common By Far Than Other Models
• Can Be Validated or Discredited with
Testing
• Generalizable by Biological Model,
Endpoint and Chemical Class
Perspective #17
HORMESIS: should become the
object of formal evaluation by
leading advisory bodies such as
the National Academy of
Sciences