Differences in sensitivity but not selectivity of xenoestrogen binding to alligator versus human estrogen receptor alpha

Abstract

Reproductive abnormalities in alligators exposed to contaminants in Lake Apopka, Florida, USA represent a clear example of endocrine disruption in wildlife. Several of these contaminants that are not able to bind to mammalian estrogen receptors (such as atrazine and cyanazine) have previously been reported to bind to the alligator estrogen receptor from oviductal tissue. Binding of known Lake Apopka contaminants to full length estrogen receptors alpha from human (hERalpha) and alligator (aERalpha) was assessed in a side-by-side comparison within the same assay system. Baculovirus-expressed recombinant hERalpha and aERalpha were used in a competitive binding assay. Atrazine and cyanazine were not able to bind to either receptor. p,p'-Dicofol was able to bind to aERalpha with a concentration inhibiting 50% of binding (IC50) of 4 microM, while only partially displacing 17beta-estradiol (E2) from hERalpha and yielding a projected IC50 of 45 microM. Chemicals that only partially displaced E2 from either receptor, including some dichlorodiphenyltrichloroethane (DDT) metabolites and trans-nonachlor, appeared to have higher affinity for aERalpha than hERalpha. p,p'-Dicofol-mediated transcriptional activation through aERalpha and hERalpha was assessed to further explore the preferential binding of p,p'-dicofol to aERalpha over hERalpha. p,p'-Dicofol was able to stimulate transcriptional activation in a similar manner with both receptors. However, the in vitro results obtained with p,p'-dicofol were not reflected in an in vivo mammalian model, where Kelthane (mixed o,p'- and p,p'-dicofol isomers) did not elicit estrogenic effects. In conclusion, although there was no evidence of exclusively species-specific estrogen receptor binders, some xenoestrogens, especially p,p'-dicofol, had a higher affinity for aERalpha than for hERalpha.