Several studies suggest an involvement of
PCBs in
breast cancer formation, but the results are ambiguous and the mechanisms not clear. We propose that local activation of
cytochrome P450 enzymes,
CYP1A1 and CYP1B1 by PCB3, may generate active metabolites which affect apoptosis and thereby promote mammary
carcinogenesis. To test this hypothesis MCF-7 human
breast cancer cells were exposed to 300 nM PCB3 and its hydroxylated metabolites, 4OH-PCB and 3,4diOH-PCB3. The
enzyme activity for
CYP1A1 was assayed using the
EROD assay, and
CYP1A1 and CYP1B1
protein expression by western blotting. PCB3 increased
CYP1A1 activity (~1.5fold) and
protein levels within 6h after exposure. No effect on CYP1B1
protein expression was observed. The effects of PCB3 and both its metabolites on
staurosporine-induced apoptosis were determined by measuring DNA fragmentation using ELISA and TUNEL assays, and by measuring
caspase-8 and
caspase-9 activity. We found that PCB3 and both of its hydroxylated metabolites had no effect on
caspase-8 and
caspase-9 activity when cells were grown in medium deprived of
estrogen, but reduced
caspase-9 activity when cells were grown in medium supplemented with serum containing
estradiol. Interestingly, a decrease of DNA fragmentation was observed upon treatment with 3,4diOH-PCB3 in both culture conditions, suggesting that 3,4diOH-PCB3 affects a
caspase-independent pathway of cell death. In summary, interactions of PCB3 and its metabolites with
estradiol by yet unknown mechanisms inhibit
caspase 9-related apoptosis and additional, other death pathways are affected by the
catechol metabolite 3,4diOH-PCB3. These anti-apoptotic effects and the change in metabolic activity may contribute to the carcinogenic effect of
PCBs.