In this large cross-sectional study based on the NHANES 1999–2016 dataset, we found novel information about the association between phthalate exposure and ER-related cancers (breast cancer, ovarian cancer, uterine cancer in women, and prostate cancer in men) in US population. The results of this study showed that individuals with higher levels of urinary phthalate metabolites MCNP had a higher prevalence of ER-related cancer than those at the lowest quantile after multivariate adjustment. When stratified by gender, the associations MCNP and breast cancer in female still remained robust. However, we did not observe a significant association between the levels of other urinary phthalate metabolites and ER-related cancers risk. To our knowledge, this study initially used a nationally-representative sample to consider the association between urinary phthalate metabolites and ER-related cancers in American adults.
ERα was mainly expressed in reproductive tissues (uterus, ovary), breast, kidney, bone, white adipose tissue and liver, while ERβ was mainly expressed in ovary, central nervous system (CNS), cardiovascular system, lung, male reproductive organs, prostate, colon, kidney and immune system. Previous studies have confirmed that phthalate metabolites are associated with ER-related cancer[27–30]. Although there is no in-depth discussion on the association between MCNP level and breast cancer risk so far. MCNP is one of the components of phthalates, and its mechanism of influencing the occurrence and development of breast cancer should be similar to the existing research about phthalates and breast cancer. Phthalates affect the occurrence and development of ER-related tumors mainly in two ways, affecting the level of endocrine hormones and their related metabolites and the expression of related genes. Phthalates as the important part of EDCs that people often come into contact with in daily life, which interfere with the production, release, transportation, binding, action and elimination of hormones. EDCs may alter epithelial growth rate, matrix composition of glands, immune response, response to endogenous hormones, existence of terminal buds and intercellular communication etc. In addition, any interruption of breast development may increases the risk of breast cancer and other abnormalities. Phthalates can also affect the microenvironment of tumor cells, which plays an important role in progression of any cancer. Among them, phthalates can induce RANTES (Regulated upon activation, normal T-cell expression and secretion), which is a cytokine, known to be associated with breast cancer progression. In addition, phthalates have been found to be estrogen analogues. Studies have shown that estrogen is a major risk factor of breast cancer. Therefore, exposure to phthalates in female may lead to hormonal dependent induction of breast cancer, as well as drug resistance in breast cancer cells.
Moreover, phthalates can also affect the progress of breast cancer from the genetic level. The pathway to induce cancer of phthalates involves the AhR-cAMP-PKA-CREB1 cascade reaction. The aryl-hydrocarbon receptor (AhR) is a ligand activated transcription factor associated with heat shock protein 90 (HSP90) and was activated by halogenated aromatic hydrocarbon, polycyclic aromatic hydrocarbons and endogenous compounds in a dose-dependent manner. Tsung-Hua Hsieh et al showed that phthalates treated cells showed elevated levels of membrane localized AhR, which is related to the increase of cAMP level and the phosphorylation of CREB1 and PKA inhibitor. López-Carrillo et al found that this cascade can further enhances the translation of HDAC6, which is known to through beta catenin-LEF/TCF4 activates c-Myc in Er negative breast cancer cells. Transcriptional activation of c-Myc is associated with increased cell proliferation, cell migration, and tumor growth. AhR also induced the transcriptional activation of CYp1b1, which is a monooxygenase that can metabolize exogenous substances. It hydroxylates estradiol to 4-hydroxyestradiol, and then further oxidizes to estrogen-3,4-semiquinone. When there are reactive oxygen species and DNA additives, it can enhance the carcinogenic effect of 4-hydroxyestradiol, which can lead to cancer or promote existing tumors. In addition, Nicolopoulou-Stamati et al also found Ahr activates COX-2 through nongenomic pathway, in which prostaglandin E2 up regulates COX-2 level, which is associated with breast cancer. What is more, Chen et al in their analysis showed that phthalates can affects P13K/AKT pathway, which has been proved in many studies to be associated with breast cancer[39, 40].
In previous studies on exposure to phthalate metabolites and breast cancer, López-Carrillo et al reported MCPP and MBzP have the inverse association with breast cancer and they also found the positive association between MECPP and breast cancer in the Mexican population. While, in Alaska native multitude, Holmes et al found the positive association between MEHP and breast cancer recently. However, no similar results were found in our study. The reasons for these differences are unclear, but may be due to racial/cultural composition of the study population (i.e., Mexican, Alaska-Native, and postmenopausal women), study design (i.e., sampling time of urine), or noncausal explanations, including random errors and biases caused by uncontrolled confusion and other sources of error. In addition, the threshold of phthalate exposure may also be one of the potential reasons for the insignificant results, which means only when the phthalate metabolites exceed the exposure threshold, the risk of breast cancer will increase.
Some limitations of this study should be noted. Firstly, the present cross-sectional designed study can not confirm the direct causal association between phthalates exposure with ER-related cancers risk. The level of phthalates in spot urine are measured once, so the variability and long-term average exposure may not be reflected in individuals. Secondly, based on the self-reported, the evaluation results of ER-related cancers cases are collected through questionnaire survey, which may lead to the misclassification of participants, thereby raising the risk of measurement bias. In addition, the sample size of ER-related cancers cases is also relatively small, resulting in statistical power in subgroup analysis. Third, although some demographic and lifestyle factors have been adjusted in the data analyses, there are still some confounding variables may affect their associations, which are not considered. Besides, many other EDCs, such as phenols and p-hydroxybenzoates may also affect the physiological function of ER. Therefore, the association between the combined exposure of phthalates and other EDCs with ER-related cancers risk should be discussed in further clinical studies.