The road to scientific discovery is not necessarily a straight one. In 1989, I became a "toxicologist by accident." My partner, Dr. Carlos Sonnenschein, and I had been working for almost two decades, trying to understand how cell proliferation is controlled in multicellular organisms. We had chosen sex steroids (estrogens and androgens) as models because they specifically regulate the proliferation of their target cells. Our experiments indicated that proliferation was a constitutive property of cells in multicellular organisms. This is accepted as a truism for unicellular organisms, but it was contrary to dogma for multicellular ones. Evidence collected in our lab suggested that estrogens induce cell proliferation by blocking the action of a blood-borne inhibitor, which we began to purify through a bioassay. When the inhibitor was present, the target cells were inhibited from proliferating; when we added estrogen (estradiol-17beta), they proliferated maximally. Then, one day this was suddenly no longer the case; the cells began to proliferate, regardless of the presence of estradiol. We investigated this puzzle by substituting all the components of the assay until we found that the problem was due to estrogenic activity released from the plastic tubes used to store culture media components. The manufacturers of the plastic acknowledged that the problem was due to a change in the plastic’s formulation. However, they refused to identify the chemical, adducing proprietary reasons. We later identified the estrogenic compound as nonylphenol, an antioxidant also used in the synthesis of detergents (Soto, et al., Environ. Health Perspect., 92:167-73, May 1991). This was the first of a series of non-chlorinated estrogenic chemicals found to be present in the environment, and also the first of many identified in plastics since then by others and by us.

We also realized that our assay for identifying the blood-borne inhibitor of the proliferation of estrogen target cells could be used to identify estrogenic chemicals. We called it the "E-SCREEN" assay. We used this assay to assess which of a series of about 100 chemicals that are used in large volumes and end up in the environment were estrogenic; this research produced a surprisingly large number of "true" positives. We also showed that xenoestrogens of disparate chemical structures could act additively (Soto, et al., Environ. Health Perspect.,103:113-22, Suppl. 7, October 1994; Soto, et al., Environ. Health Perspect., 102[4]:380-3, April 1994).

Later on, we combined our interest in estrogens and breast cancer with our accidental interest in xenoestrogens. We reasoned that the customary way of correlating breast cancer with exposure to a single chemical was unlikely to shed light on whether exposure to xenoestrogens is a risk factor for breast cancer. Instead, we argued, a correlation between breast cancer incidence and total xenoestrogen exposure was more realistic, since these chemicals can act additively. We developed a methodology to assess the total xenoestrogen burden in people using the E-SCREEN assay (Sonnenschein, et al., Clinical Chemistry, 41[12B]:1888-95, Suppl. S, December 1995; Soto, et al., Environ. Health Perspect., 105:647-54, Suppl. 3, April 1997). This method is being used to test the hypothesis that combined exposure to xenoestrogens is correlated with breast cancer incidence. The E-SCREEN assay is also being used to assess the presence of estrogenic activity in water.

Meanwhile, Dr. Nicolas Olea and his sister, Dr. Fatima Olea-Serrano, had returned to Granada, Spain. Nicolas, a physician-scientist from the University of Granada, was a Fullbright Scholar in our lab when the accidental discovery of nonylphenol took place. Fatima was also a visiting scientist in our lab during this time. After returning to Granada, they discovered that the synthetic estrogen bisphenol-A (BPA) leached into foods from the plastic linings of tin cans. Next, their friend Dr. Rosa Pulgar Encinas made us aware that BPA was also used in dental sealants and composites. We joined efforts and found that BPA leached from sealants into the saliva of the volunteers who participated in our study (Olea, et al., Environ. Health Perspect., 104:298-305, March 1996)

The news that nonylphenol was estrogenic resulted in my being invited to the Wingspread Conference organized by Theo Colborn in 1991. In the Wingspread Statement, the 25 scientists present at the conference concluded that "A large number of man-made chemicals that have been released into the environment, as well as a few natural ones, have the potential to disrupt the endocrine system of animals, including humans." This was the launching of the Endocrine Disruptor Hypothesis. Drs. Colborn, vom Saal, and myself decided to present this hypothesis in a paper (Colborn, et al., Environ. Health Perspect., 101[5]:378-84, October 1993) that integrated the findings discussed at Wingspread. In it, we postulated that endocrine disruptors act additively, that the developing organism is more susceptible and sensitive than the adult organism is to the effects of excess levels of hormones, and that low, environmentally relevant levels of hormonally active chemicals can alter fetal development.

Our basic scientific interest and environmental science converged again. We found non-monotonic inverted U-shaped dose-responses in the effect of androgens in cell proliferation (Sonnenschein, et al., Cancer Res., 49[13]:3474-81, 1 July 1989). At low doses, they increased cell proliferation, while at high doses, they inhibited it. We proposed that these two effects occurred through independent pathways and provided evidence for this by developing cell lines that expressed only one of the two apparently contradictory effects. Recently, we demonstrated that high physiological doses of androgens inhibit cell proliferation by means of an intracellular inhibitor (Geck, et al., Proc. Natl. Acad. Sci. USA., 97[18]:10185-90, 29 August 2000). This non-monotonic behavior of sex steroids was also apparent in work by vom Saal on the action of environmental estrogens during development of the male genital tract (vom Saal, et al., Proc. Natl. Acad. Sci. USA., 94[5]:2056-61, 4 March 1997). The existence of low-dose effects operating during development render highly plausible the notion that environmentally relevant exposures to endocrine disruptors are affecting human health.

For the last 10 years, we have continued our basic research program on the control of cell proliferation and cancer while developing this new field of environmental hormones. We isolated the inhibitor of estrogen-target cell proliferation (Sonnenschein, et al., J. Steroid Biochem. Molec. Biol., 59[2]:147-54, October 1996), and an androgen-induced intracellular inhibitor of cell proliferation (Geck, et al., Proc. Natl. Acad. Sci. USA., 97[18]:10185-90, 29 August 2000). These papers support the notion that proliferation is the default state of all cells, and therefore, cell proliferation is negatively controlled. We also published a book about what we learned during 30 years of basic research on cell proliferation and cancer (Sonnenschein and Soto, The Society of Cells: Cancer and Control of Cell Proliferation, Springer Verlag, New York, 1999).

In summary, the pursuit of knowledge is an uncharted adventure. In the end, all the effort to get funds, fights to be heard, etc., are offset by the immense pleasure of finding out that Nature does not work according to our preset anthropocentric views, and hence there are still many surprises ahead. Whether the surprises will be beautiful (like negative control) or nasty (like estrogens leaching from plastic) will probably depend on the primacy between humankind’s ability to search for an understanding of Nature on one hand, and controlling our unbridled arrogance that urges us to modify and "improve" Nature on the other.