e.hormone environmental signaling epigenetics lead in the environmental
Expert Views :: Biasing Research Directions: The Importance of Precedence
Catherine R. Propper

As a graduate student in our weekly journal club, I remember a paper describing that the ovary could produce oxytocin. The general comment at the meeting was something along the lines of "What’s this hypothalamic neurohormone doing in the ovary?" It struck me as a funny – though very human – question. I also wondered if oxytocin was first discovered in the ovary, would the question be "What’s this ovarian hormone doing in the brain?" Of course, now with the discovery of steroidogenic machinery in the brain, we are asking this very question regarding estrogens and other steroids that are considered "gonadal" or "adrenal" hormones.

This anecdote illustrates that precedence (or dogma) can inhibit development of new and potentially exciting research problems. Within the arena of endocrine disruption, precedence may have played a role in limiting expansion of our research horizons.

Recently, while preparing a presentation on endocrine disruption for a symposium on the Physiology of Conservation Biology at January's Society for Integrative and Comparative Biology (SICB) Conference, I wanted to state that the field had concentrated largely on compounds that influence estrogenic function and that this bias was due to historical research precedence. Not wanting to make such a statement in a data-free environment, I conducted a literature search to test the hypothesis that most published research on endocrine disruption focused on estrogen-related topics. Using the PubMed function on Reference Manager 10, I did a key word search for several hormones from the hypothalamus through most other endocrine glands using the other key words "endocrine disruption" or "endocrine disruptor."

The search results are shown in the following table. Estradiol and estrogens represent the highest percentage of the published endocrine disruption literature (55.5% of hits and 63.2% for "endocrine disruption" and "endocrine disruptor" respectively). Androgens are next, followed by thyroid and thyroxin.


Hormone
Endocrine
Disruption
% Total
Endocrine
Disruptor
% Total

Estrogens

110

39.57

38

43.68

Estradiol

44

15.88

17

19.54

Androgen

39

14.03

9

10.34

Testosterone

19

6.86

5

5.75

Progesterone

9

3.25

2

2.30

Thyroid

18

6.50

10

11.49

Thyroxin

7

2.53

2

2.30

Glucocorticoid

6

2.17

0

0.00

Cortisol

2

0.72

0

0.00

Corticosterone

1

0.36

0

0.00

Aldosterone

1

0.36

0

0.00

Insulin

0

0.00

0

0.00

TSH

3

1.08

0

0.00

ACTH

0

0.00

0

0.00

LH

8

2.89

1

1.15

FSH

5

1.81

1

1.15

Prolactin

3

1.08

1

1.15

Growth hormone

2

0.72

0

0.00

GnRH

1

0.36

0

0.00

CRH

0

0.00

0

0.00

TRH

0

0.00

1

1.15

Total

278

100

87

100


I also felt there was a bias toward research involving reproduction. To test this assumption, I conducted a second search using keywords that represented non-hormone search terms covering many aspects of overall physiology along with the terms "endocrine disruption" or "endocrine disruptor." The results, shown in the following table, suggest the majority of the published research is in the areas of development and reproduction.


Non-hormone
Search Terms

Endocrine Disruption

% Total

Endocrine Disruptor

% Total

Development

77

35.98

35

41.18

Reproduction

78

36.45

30

35.29

Sex differentiation

9

4.21

3

3.53

Neuroendocrine

4

1.87

0

0.00

Animal behavior

23

10.75

5

5.88

Cognition

1

0.47

1

1.18

Cognitive

4

1.87

1

1.18

Pheromone

0

0.00

0

0.00

Stress

9

4.21

6

7.06

Calcium

1

0.47

3

3.53

Water balance

0

0.00

0

0.00

Immune

8

3.74

1

1.18

Digestion

0

0.00

0

0.00

Total

214

100

85

100


Because I study wastewater as a model for a complex mix of endocrine disrupting compounds, I conducted a search using a few hormones and the alternative key words "sewage" and "wastewater." The results are represented below.


Hormone

Sewage

% Total

Wastewater

% Total

Estrogen

121

86.43

38

86.36

Androgen*

18

12.86

6

13.64

Thyroid hormone

0

0.00

0

0.00

Thyroxine

1

0.71

0

0.00

Glucocorticoid

0

0.00

0

0.00

Total

140

100

44

100

* Most suggest that lowered androgen levels are due to estrogenic effects.


Again, the bias toward estrogen in the published results is very apparent. In fact, many assays are currently being developed to investigate the "endocrine disrupting capacity" of wastewater. Most of these evaluate the water for direct competition with the nuclear estrogen receptor or for gene expression or protein products downstream from the estrogen receptor.

While these results may not be a perfect representation of the literature (indeed, the search missed some of my own lab’s key papers), they do provide a sample of the literature in the field.

What do the results suggest? First, the results may indeed represent a research bias: more studies are conducted that examine estrogenic or anti-estrogenic endocrine disruption or investigate the developmental and reproductive aspects of endocrine disruption.

Alternatively, some studies may examine other endocrine endpoints or physiological systems but the results of the investigations are negative. As always, negative data are difficult to interpret, and therefore, publish. If this hypothetical interpretation has merit, then the loading of results toward estrogens, androgens, and reproduction signifies that disruption occurs mainly within these systems. In this regard, discussion and production of a public database where investigators can place a brief description of their negative results and contact information may be warranted.

The problem with precedent bias becomes especially evident within policy formation. It is easy to ignore non-existent data as signifying no effect rather than just the outcome of precedent-limiting research focused on a few specific topics. The US Environmental Protection Agency has done a remarkable job developing screening programs for endocrine disruptors, yet the focus is still limited to estrogens, androgens, and thyroid hormones or their respective hypothalamic-pituitary axes. Given the expense and complication of including other endocrine, and possibly non-endocrine, physiological endpoints into screening programs, it is not difficult to understand why precedent helps set research limits.

Finally, we may have biased our thinking in using the term "endocrine disruption." Joe Thorton made such a point in the previous e.hormone commentary The Future of the Field: Beyond Endocrine Disruption? (http://e.hormone.tulane.edu/ViewsArchives/200304-Thornton.html). First, the term "endocrine disruption" may have delayed the onset of ideas about general disruption of environmental signaling demonstrated so elegantly by Jennifer Fox and colleagues (Fox et al. 2001). Second, its use still may be limiting our thinking in terms of overall physiological disruption. Perhaps some of these compounds act by influencing non-endocrine aspects of physiology separate from toxicological outcomes. This thought struck me while I was listening to a SICB presentation by AK Green (Green et al. 2004) who found that flavonoids influence glucose transporter activity in birds in vivo, something already demonstrated in in vitro systems. The results clearly demonstrate a plant compound influencing sugar balance via a non-endocrine pathway. Perhaps such "physiological disruption" needs to be addressed with respect to environmental contaminants in a more systematic fashion.

Precedents can be helpful in setting useful limits to what appear to be limitless problems. However, when precedents in research findings act as blinders to new ideas, it can lead to limits in the development of important questions or prevent the testing of potential useful alternative hypotheses. We need to begin to understand whether the focus on reproduction, gonads, and thyroid systems in endocrine disruption comes from a precedent-based set of results or whether it is a function of endocrine disrupting compounds really acting predominantly at these already defined levels.


References

1. Fox, JE, M Starcevic, KY Kow, ME Burow, and JA McLachlan. 2001. Nitrogen fixation: Endocrine Disrupters and Flavonoid Signaling. Nature 413, 128-129.

2. Green, AK, MM Slopec, and WH Karasov. 2004. Flavonoids Decrease 3-0-Methyl D-Glucose Absorption in American Robins. Presented at the Society of Integrative and Comparative Biology, 5-9 January, New Orleans, Louisiana.