In the female Sprague-Dawley rat uterus
2,3,7,8-tetrachlorodibenzo-p-dioxin (
TCDD) and related compounds exhibited a broad spectrum of antioestrogenic responses. For example 2,3,7,8-TCDD inhibited the
17 beta-oestradiol-induced uterine wet weight increase,
peroxidase activity, oestrogen and
progesterone receptor levels,
epidermal growth factor (
EGF) receptor binding, and
EGF receptor and c-fos protooncogene
mRNA levels. The aryl
hydrocarbon (
Ah) receptor was identified in the rat uterus and the antioestrogenic activities of
TCDD and related compounds were structure-dependent. In parallel studies, the effects of
TCDD as an antioestrogen in MCF-7 human
breast cancer cells was also investigated.
TCDD inhibited the
17 beta-oestradiol-induced proliferation of these cells and the secretion of the 34-, 52- and 160-kDa
proteins. Treatment of MCF-7 cells with 1 nM [3H]-
17 beta-oestradiol resulted in a rapid accumulation of nuclear oestrogen receptor (ER) complexes. Pretreatment of the cells with
TCDD caused a rapid decrease in nuclear ER binding activity and immunoreactive
protein; moreover, the structure-dependent potencies of
TCDD and related compounds as antioestrogens were similar to their
Ah receptor binding affinities.
TCDD also caused a decrease in nuclear ER levels in wild-type Ah-responsive Hepa 1c1c7 cells but was inactive in Ah non-responsive mutant Hepa 1c1c7 cells. Moreover, in the wild-type cells, both
actinomycin D and
cycloheximide blocked the effects of
TCDD.
6-Methyl-1,3,8-trichlorodibenzofuran (
MCDF) has previously been characterized as a
TCDD antagonist in rodents and in transformed rodent cell lines. However, like
TCDD,
MCDF also exhibited a broad spectrum of antioestrogenic activities in both the female Sprague-Dawley rat uterus and MCF-7 cells.
MCDF is relatively non-toxic compared to
TCDD and is being investigated as a compound which may be clinically useful for the treatment of
mammary cancer.