Significance of experimental studies for assessing adverse effects
of endocrine-disrupting chemicals
L. E. Gray, Jr. and P. M. D. Foster
2525 Highway 54, MD 72, NHEERL, Endocrinology Branch,
Reproductive
Toxicology Division, ORD, USEPA, Research Triangle Park, NC 27711, USA;
Environmental Toxicology Program, NIEHS, MD E1-06, 111 TW Alexander
Drive, Research Triangle Park, NC 27709, USA
Abstract: The U.S. Environmental Protection Agency (USEPA) is
developing an endocrine disruptor screening and testing program to detect
chemicals that alter hypothalamic-pituitary-gonadal (HPG) function,
estrogen, androgen, and thyroid (EAT) hormone synthesis or metabolism
and induce androgen (AR) and estrogen (ER) receptor-mediated effects
in mammals and other animals. The utility of this approach is based
upon the knowledge that mechanisms of endocrine-disrupting chemical
(EDC) action are highly conserved at the cellular and molecular levels
among vertebrates. Some EDC mechanisms also are shared with invertebrates.
High-priority chemicals would be evaluated in a Tier 1 screening (T1S)
battery, and chemicals that are positive in T1S would then be tested
in Tier 2 (T2). T1S includes in vitro ER and AR receptor binding and/or
gene expression, an assessment of steroidogenesis and mammalian (rat)
and nonmammalian (fish) in vivo assays. In vivo, the uterotropic assay
detects estrogens and antiestrogens, while steroidogenesis, antithyroid
activity, antiestrogenicity, and HPG function are assessed in a pubertal
female assay. Antiandrogens are detected in the Hershberger assay (weight
of androgen-dependent tissues in castrate-immature-male rats). Fish
and amphibian assays are also being developed to identify EDCs. Several
alternative mammalian in vivo assays have been proposed. Of these, a
short-term pubertal male rat assay appears most promising. T1S is designed
to be sensitive to EAT activities, but many of the effects detected
at the screening level would not be considered adverse, the dosage levels
may be high, and the route of administration used may not be the most
relevant. However, issues of adversity, dose response, and route(s)
of exposure would be resolved in the testing phase. In addition to using
an enhanced multigenerational test for Tier 2, an in utero-lactational
screening protocol is also being evaluated by USEPA for use in T2 or
T1S. For T2, the numbers of endocrine-sensitive end-points and offspring
(F1) examined in multigenerational tests need to be expanded for EDCs
in a thoughtful manner, based in part upon the results of T1S. In addition,
for some chemicals histological examination of 10 adult F1 per sex in
only the control and high-dose groups provides inadequate statistical
power to detect low-dose lesions induced during development. In these
cases, we propose that all the offspring be examined after puberty for
gross and histological reproductive abnormalities. Since EDCs, like
the phthalates and AR-antagonists, produce characteristic profiles,
or syndromes, of adverse effects, data need to be reported in a manner
that clearly identifies the proportion of animals displaying one or
more of the abnormalities in a syndrome. Consideration should be given
to tailoring T2, based on the results of T1S to assure that all of the
effects in such chemically induced developmental syndromes are included
in the study.
*Report from a SCOPE/IUPAC project: Implication of
Endocrine Active Substances for Human and Wildlife (J. Miyamoto and
J.Burger, editors). Other reports are published in this issue,
pp. 1617-2615.