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.

Response

Response

Dose

A

B

(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 FeaturesStimulation 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)

Hormetic Zone(averages 10- to 20-fold)

NOAEL

Control

Dose-response curve depicting the quantitative features of hormesis

Increasing Dose

Hormetic MechanismsMany 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

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Methanol (%)

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Methanol and Fruit Fly Longevity

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Gamma Rays and Mouse Lung Adenomas

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Transformng Growth Factor Beta (pg/ml)

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Transforming Growth Factor-Beta and Human Lung Fibroblasts

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Ethanol (g/kg)

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Effects of Acute Ethanol on Overall Social Activity of Adolescent Rats Tested on Postnatal Day 30

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X-rays (R)

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Effect of X-rays on the Root Length of Carnation Cuttings

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Aluminum (uM)

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Aluminum andMouse Blood Gamma-AminolevulinicAcid Activity

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Mercury Chloride (ug/L)

(%

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Below G

Total Biomass

Stem Density

Max Shoot Height

Evap/transpir

Effect on Growth of Salt Marsh Grass

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Drug Concentration (mg/kg)

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YohimbineApomorphinePromethazine

Comparative Dose Response Relationships for the Pain Threshold for Vocalization

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Effect of Different Doses of Morphine on PTZ-induced Seizure Threshold

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Alcohol (g/kg)

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% c

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TestosteroneLuteinizing hormone

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Alcohol andRat Serum Levels

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4-Chloro-2-methylphenoxyacetic acid (MCPA) (mg/pot)

Dry

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MCPA +OAT SHOOT GROWTH

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Metal Concentration (M)

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% c

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HgCl2

MethHgClCdCl2

ZnCl2

Effects of Metals on Phagocytosis in the Clam, Mya arenaria, hemocytes

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Cadmium (uM)

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Cadmium andAquatic Plant (H. verticillata)Nitrate Reductase Activity

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Sodium Arsenate (M)

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Effect of Sodium Arsenate on PHA-treated Bovine Lymphocytes

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Mercury (ug/L)

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Methyl mercuric chloride

Mercuric chloride

Mercury and Duckweed

Catalase Activity

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Gamma Rays (R)

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Effect of Gamma Rays on the Life Span of Female House Crickets

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Acridine (mg/L)

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Effect of Acridine on the Number of Broods per Daphnid

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Lectin Concentration (ng/ml)

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ProstateProstateProstateRenalRenalColorectalColorectalColorectalGastricLiposarcoma

Effect of Mistletoe Lectin on Human Tumors in Culture

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Ten Estradiol A-Ring Metabolites (M)

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Effects of Ten Estradiol A-ring Metabolites on Endothelial Cells from Human Umbilical Veins

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Plumbagin (ug/ml culture)

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Effect of Plumbagin on Human Granulocyte Phagocytosis

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Tin (II) (ug/ml)

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Effect of Tin (II) on MTT Conversion in C6 Glioma Cells

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DHEA (mg/kg)

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Number of Open Arm Entries in the Elevated Plus Maze in Male C57BL/6 Mice Treated with DHEA

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Allixin (ng/ml)

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The Effects of Allixin on the Survival of Primary Cultured Hippocampal Neurons from Embryonic (E18) Wistar Rats

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Methyl Mercury (µM)

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The Effects of Methyl Mercury on Viability as Measured by Mitochondrial Dehydrogenase Activity in the D407 Cell Line

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3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) (mM)

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Effects of the Disinfectant Byproduct MX on the Occurrence of DNA Damage in the Comet Assay Using Rat Liver Epithelial Cell Line WB-F344

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Effects of n-Hexane on DNA Damage in Human Lymphocytes in the Comet Assay

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As2O5 (M)

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Effects of As2O5 on Total Chromosomal Aberrations in Human Leukocytes

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X-Rays (mGy)

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Effects of X-rays on Chromosomal Aberrations (i.e., Dicentrics) in Human Lymphocytes (pooled results of four donors and six laboratories)

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Hormetic or Threshold

Which Dose Response Is More Common?

The Threshold ModelPrediction: 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 CriteriaEntry 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

Percent Difference From Control Growth

Cum

ulat

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Per

cent

of

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mic

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Mean

Prediction Interval 95%

Threshold Model Predicted Mean

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Percent Difference From Control Growth

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 ModelingImplications 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 Non-

Cancer• 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 out-competed 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