Indolo[3,2-b]carbazole: a dietary-derived factor that exhibits both antiestrogenic and estrogenic activity.
Level 5 - mechanism / opinion, no new human data
In vitro cell culture and biochemical mechanistic study
PubMed 7966413 · doi:10.1093/jnci/86.23.1758
What was done
Researchers evaluated the induction of CYP1A1 activity (measured via ethoxyresorufin O-deethylase [EROD] activity) and antiestrogenic actions of indolo[3,2-b]carbazole (ICZ), an acid-derived condensation product of indole-3-carbinol (I3C), in MCF-7 human breast cancer cells. They assessed occupied nuclear estrogen receptor (ER) levels, 17 beta-estradiol (E2)-induced cell proliferation, [3H]thymidine uptake, secretion of the 52-kd protein, nuclear progesterone receptor (PR) levels, and chloramphenicol acetyl transferase (CAT) reporter activity using an estrogen-responsive vit-CAT plasmid. Competitive binding assays with rat cytosolic ER and gel shift assays for estrogen-responsive elements were also performed.
What was found
ICZ induced CYP1A1 at concentrations of 10 nM or greater in MCF-7 cells. ICZ inhibited E2-induced proliferation, [3H]thymidine uptake, nuclear PR binding, and vit-CAT reporter activity. In nuclear extracts, ICZ decreased ER levels and ER binding to estrogen-responsive elements; I3C similarly reduced nuclear ER binding. Antiestrogenic effects occurred at time points and concentrations where EROD activity was unchanged. ICZ also bound rat cytosolic ER with low affinity and demonstrated weak intrinsic estrogen-like activity. Exact numerical values were not reported in the abstract.
Why it matters
This study provides mechanistic evidence that dietary cruciferous vegetable derivatives can modulate estrogen signaling via aryl hydrocarbon receptor crosstalk, operating through pathways independent of cytochrome P450 hormone metabolism.
Limits
The study is restricted to in vitro assays in a single human breast cancer cell line (MCF-7) and rat cytosolic preparations. It does not measure in vivo pharmacokinetics, tissue distribution, physiological exposure levels from dietary intake, or clinical tumor outcomes in humans.
Cited by
- supports Indole-3-carbinol (I3C) and diindolylmethane (DIM) can polymerize into dimers or tetramers that structurally resemble dioxin.