Acevedo Ricardo, Parnell Pamela G, Villanueva Hugo, Chapman Laura M, Gimenez Tomas, Gray Sandra L, Baldwin William S
University of Texas at El Paso, Biological Sciences, El Paso, TX 79968, USA.
J Appl Toxicol. 2005 Sep-Oct;25(5):339-53. doi: 10.1002/jat.1078.
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.
17β-雌二醇（E2）代谢的两条主要途径是2-羟化酶途径和16-羟化酶途径。研究表明，雌激素活性的16-羟基产物如雌三醇（E3）产量增加可能与乳腺癌易感性增加有关。4-壬基酚（4-NP）是一种环境雌激素，它也能激活孕烷X受体（PXR）并诱导负责E3生成的P-450酶。据推测，4-NP可能通过增加E3生成而部分作为一种环境雌激素起作用。仅基于其对雌激素受体（ER）的亲和力，4-NP在增加MMTVneu小鼠乳腺癌发病率方面可能比预期更具效力。对雌性小鼠以0、25、50或75 mg·kg⁻¹·d⁻¹的剂量经口给予4-NP处理7天，以研究急性处理后4-NP对肝脏雌激素代谢的影响。4-壬基酚以剂量依赖性方式增加肝脏中E3的生成。然而，血清E3浓度仅在25 mg·kg⁻¹·d⁻¹时升高，这可能是由于4-NP对E3生成的直接抑制作用。然后对MMTVneu小鼠以0、30或45 mg·kg⁻¹·d⁻¹的剂量给予4-NP处理32周，以确定其对乳腺癌形成和雌激素代谢的影响。4-壬基酚在45 mg·kg⁻¹·d⁻¹时增加了MMTVneu小鼠的乳腺癌形成，但在30 mg·kg⁻¹·d⁻¹时未增加。根据壬基酚和雌二醇对ERα的相对结合亲和力，用等效剂量的E2（10 μg·kg⁻¹·d⁻¹）处理的小鼠未发生乳腺癌。这表明壬基酚比基于其对雌激素受体的亲和力所预测的更具效力。然而，慢性处理后未检测到血清E3浓度或肝脏E3生成的变化。4-NP处理7天后E3生成的变化与CYP2B水平升高相关，4-NP处理32周后CYP2B和CYP3A受到抑制。对经32周处理的小鼠肝脏mRNA进行微阵列分析和定量PCR显示，PXR的异二聚体伴侣RXRα减少，这可能部分解释了所检测到的P450s转录受到抑制的原因。总之，4-NP以45 mg·kg⁻¹·d⁻¹的剂量处理32周增加了乳腺癌的形成。然而，4-NP的慢性处理并未增加肝脏E3生成或血清E3浓度。4-NP对肝脏E3生成和血清浓度的短暂诱导很可能与MMTVneu小鼠乳腺癌发病率增加无关，因为血清E3浓度仅在25 mg·kg⁻¹·d⁻¹时升高，该剂量不足以诱导乳腺肿瘤形成。尽管如此，急性暴露于4-NP时诱导的肝脏E3生成表明慢性暴露后乳腺癌发病率增加。
