The two major pathways for the metabolism of
estradiol-17beta (E2) are the 2- and 16-hydroxylase pathways. Research has suggested that the increased production of the estrogenically active 16-hydroxy products such as
estriol (E3) may be involved in increased susceptibility to
breast cancer.
4-Nonylphenol (4-NP) is an environmental
estrogen that also can activate the
pregnane-X receptor (PXR) and induce
P-450 enzymes responsible for the production of E3. It is hypothesized that 4-NP may act in part as an environmental
estrogen by increasing E3 production. Based on its affinity for the
estrogen receptor (ER) alone, 4-NP may be more potent than predicted at increasing
mammary cancer incidence in the MMTVneu mouse. Female mice were treated per os for 7 days at 0, 25, 50 or 75 mg kg(-1) day(-1) 4-NP to investigate the effects of 4-NP on hepatic
estrogen metabolism after an acute treatment.
4-Nonylphenol increased the hepatic formation of E3 in a dose-dependent manner. However, serum E3 concentrations were only increased at 25 mg kg(-1) day(-1) presumably due to direct inhibition of E3 formation by 4-NP. MMTVneu mice were then treated for 32 weeks at 0, 30 or 45 mg kg(-1) day(-1) 4-NP to determine its effects on
mammary cancer formation and
estrogen metabolism.
4-Nonylphenol increased
mammary cancer formation in the MMTVneu mice at 45 mg kg(-1) day(-1) but not at 30 mg kg(-1) day(-1). Mice treated with an equipotent dose of E2, 10 microg kg(-1) day(-1), based on the relative binding affinities of
nonylphenol and
estradiol for ER alpha, did not develop
mammary cancer. This suggests that
nonylphenol is more potent than predicted based on its affinity for the
estrogen receptor. However, no changes in serum E3 concentrations or hepatic E3 production were measured after the chronic treatment. Changes in E3 formation were correlated with increased CYP2B levels after the 7 day 4-NP treatment, and repression of CYP2B and
CYP3A after 32 weeks of 4-NP treatment. Microarray analysis and Q-PCR of liver
mRNA from the mice treated for 32 weeks demonstrated a decrease in RXR alpha, the heterodimeric partner of the PXR, which may in part explain the repressed transcription of the P450s measured. In conclusion, 4-NP treatment for 32 weeks increased
mammary cancer formation at a dose of 45 mg kg(-1) day(-1). However, chronic treatment with 4-NP did not increase hepatic E3 formation or serum E3 concentrations. The transient induction by 4-NP of hepatic E3 formation and serum concentrations is most likely not involved in the increased incidence of
mammary cancer in MMTVneu mice since E3 serum concentrations were only increased at 25 mg kg(-1) day(-1), a dose that was not sufficient to induce mammary
tumor formation. Nevertheless, the induced hepatic E3 production in the acute exposures to 4-NP was indicative of an increase in
mammary cancer incidence after the chronic exposure.