The Center for Alternatives to Animal Testing is an academic center affiliated with the Division of Toxicological Sciences in the Department of Environmental Health Sciences of the Johns Hopkins University Bloomberg School of Public Health.

 

Johns Hopkins School of Public Health

Proceedings for TestSmart -- Endocrine Disruptors

Breakout Group II - Interspecies Extrapolation

Chair

John McLachlan
Tulane & Xavier Universities

Panel Members

Louis Guillette
University of Florida

William Toscano
University of Minnesota

Introduction, John McLachlan

One of the biggest problems in toxicology is deciding how to extrapolate across species, and how to apply data obtained in animal models to assessment of risks in humans. The goal of this session is to determine what we know and what we need to know about mechanistic similarities and differences between species in regards to endocrine function and signaling. There are two types of extrapolation of concern: from animal model to wildlife species of concern, and from animal model to humans.

Overview, William Toscano

  1. Evidence of endocrine disruption in wildlife species:
    1. small penis in alligator
    2. lactating male bats
    3. hermaphroditic fish
    4. depletion and deformities in frogs
    Question: are similar effects observed in the human population?
  2. What are the universal endpoints (highly conserved) that exist across species from bacteria to vertebrates?
  3. Endocrine signaling is tightly controlled, but the general paradigm of endocrine signaling is highly conversed in all organisms: Figure 1
  4. Traditional tox testing scheme: Figure 2
  5. Mice are not humans. But mice are not always mice because of genetic modifications across strains.
  6. Development of predictive tests complicated by lack of information/understanding of gene/environment interactions, effective dose vs. environmental concentrations, intergenerational effects.
  7. Historical precedence for development of predictive tests: Ames test. However, Ames test utilized DNA damage as endpoint of concern; this endpoint highly conserved across species. In contrast, not much evidence that endocrine disruptors work via genomic damage; rather evidence seems to indicate that EDs work via modulation of endocrine signaling.
  8. Steps involved in endocrine signaling: Figure 3
  9. Although there are species differences in the specifics of this signaling pathway, the general paradigm is remarkably conserved across species. Thus, may be possible to screen for effects at the level of receptor binding, using receptor-luciferase read-out systems.

General Discussion

Issues to consider regarding interspecies extrapolation:

  1. What is the endpoint of interest? If it is estrogen receptor binding, then it is reasonable to extrapolate across species; however if endpoint is disruption of neural development or immune function, then it becomes more difficult to extrapolate across species.
    But note: there is disagreement regarding the degree of similarity in estrogen and androgen receptors across species.
  2. Life stage - looking at effects in developing organism vs. mature organism?
  3. Model system - extrapolating from whole animal model system or in vitro model system?
  4. What is the target tissue - does a given hormone exert differential effects in different tissues? This is an important consideration in developing in vitro model systems for extrapolation across species.

Figure 4

What are critical mechanistic differences/similarities across species?

Signaling component Critical similarities Critical differences
Ligand/Receptor Binding Receptor structure (ER, AR, TR) 1. Tissue localization
2. Signal transduction
Metabolism Phase I enzymes Phase II enzymes
Gene exression ? ?
Hormone synthesis broadly conserved cellular context
Hormone transport some conserved
transport proteins		 species-specific transport
proteins have been identified

Application of 3Rs to interspecies testing and extrapolation?

  1. If you identify an endpoint of concern that is conserved across species, then would using less sentient species as a testing organism be considered a refinement strategy?
  2. Go back to FDA records re: interspecies extrapolation to avoid re-inventing the wheel
  3. Probably best place is in prioritization, perhaps tier I testing, e.g., as the system becomes more complex, the ability to extrapolate across species becomes more difficult, thus requiring an increased number of different test organisms.

Figure 5