Understanding the role of oestrogen metabolites

There is a complex causal relationship between hormone-sensitive sporadic breast cancer and oestrogen, unlike the dose-related relationship between oestrogen and endometrial cancer. Understanding the role of oestrogen in the pathophysiology of breast cancer will ultimately lead to an understanding of why conventional HRT (where ‘one size fits all’) should be replaced with personalised bioidentical HRT (BHRT) supported by a close monitoring of individual oestrogen metabolites and their ratios, tailored nutrition, and lifestyle changes.

The human body perceives its own oestrogen as a toxin. The metabolism of oestrogen primarily takes place in the liver through phase I (hydroxylation) and phase II (methylation, glucuronidation and sulfation) pathways, with a final excretion in the urine and faeces. Oestrogen and its metabolites show a great variation in biological activity, oestrogen receptor affinity, and carcinogenicity; therefore, the ultimate biologic effect depends on how oestrogen is metabolised. This will depend on the specific polymorphism (single nucleotide polymorphism; SNPs) of the genes involved in oestrogen detoxification in each individual, their interaction with nutrition, and the environment, and the total body oestrogen burden. It is unique to every patient.

Hydroxylation

Cytochrome P450 enzymes are responsible for the hydroxylation of oestrogen, leading to a production of different metabolites, such as 2-OH, 16α-OH and 4-OH estrones. The 2-OH metabolites — produced by SNP CYP1A1 activity — are considered good oestrogens, with a weak receptor affinity, protective against breast cancer by inducing apoptosis, and with significant antioxidant activity6. In contrast, the 16α-OH and 4-OH estrones (metabolised through CYP3A4 and CYP1B1) show persistent oestrogenic activity and promote tissue proliferation7–10. They have been associated with direct a genotoxic effect and carcinogenicity11.

It is suggested that women who metabolise a larger proportion of their endogenous oestrogen via the C16 a-hydroxylation pathway may be at a significant risk of breast cancer compared with women who metabolise proportionally more oestrogen via the C2 pathway12. As the catechol : oestrogen (Phase I) metabolite ratio is actually more important than the total oestrogen body burden, the 2-OH : 16α-OH estrone ratio is used as a predictive risk factor for hormone-sensitive breast cancers. The higher the ratio, the lower the incidence of breast cancer.

However, many conditions can lower this ratio, and physicians should be aware of these:

  • Pesticide and xenobiotic exposure
  • Hypothyroidism
  • High fat, low fibre diet
  • High omega-6 fatty acid diet
  • Oral contraceptives
  • Oestrogen dominance
  • Alcohol consumption
  • Obesity
  • Autoimmune diseases.
Figure 2 Relative risk (RR) of invasive breast cancer by type of hormone replacement therapy (HRT) and duration of exposure compared with HRT never-users

Figure 2 Relative risk (RR) of invasive breast cancer by type of hormone replacement therapy (HRT) and duration of exposure compared with HRT never-users

Another carcinogenic metabolite, 4-0H E2, can be associated with higher risk, especially when hydroxylmethylation is poor and they can readily oxidise into DNA-damaging quinones. This can happen in cases of CYP1B1 polymorphism, which is present in the general population to varying degrees, and can be detected through genetic testing. CYP1B1 is definitely an unfavourable genetic polymorphism as it can cause a double-fold increased risk of breast cancer as a result of long-term HRT and smoking. Quinone formation can be neutralised to mercapturates by GSTT1 and GSTM1 enzyme activity. GSTM1 SNP is present in a combination of high-risk genotypes for breast cancer with CYP1A1*2A and COMT V158M (Figure 3). 4-OH E2 is a carcinogenic oestrogen metabolite with binding affinity for oestrogen receptor greater than the parent E1 and E2. Reactive quinones formed from 4-OH E2 can deplete cellular thiol levels, making cells vulnerable to oxidative damage. In addition, a high oestrogen 4-/2-OH ratio appears to be a marker for the presence of neoplasms. Further metabolism to quinone/semiquinone intermediates results in oxidative damage — especially if they are not neutralised (in the case of GST genotype decreased predictive activity)13–15.

Methylation

Catechol oestrogens 2-OH and 4-OH are readily oxidised to semiquinones and quinones, which are damaging to DNA and can even promote carcinogenesis via the generation of reactive oxygen species (ROS). This oxidation can be minimised through good methylation of catechol oestrogen by the COMT (catechol-O-methyltransferase) enzyme16, 17. Evidence is accumulating to suggest that low activity of the COMT genotype leads to higher levels of depurinating oestrogen-DNA adducts that can induce mutation and initiate cancer. The COMT genotype remained the most significant determinant for breast cancer development and was associated with a four-fold increase in risk16, 17. Normal folate status appears to be of particular importance in this context, because an increasing number of COMT low-activity alleles is significantly associated with an increased breast cancer risk in women with below average levels of folate18.

Figure 3 Combination of the genotypes with an increased risk for sporadic breast cancer (COMT + GSTM1; CYP1A1 + GSTM1)

Figure 3 Combination of the genotypes with an increased risk for sporadic breast cancer (COMT + GSTM1; CYP1A1 + GSTM1)

Recent data suggest that methylation of 4-OH makes this metabolite significantly less active, while production of 2-methoxyestrone may manifest beneficial properties by inhibiting breast cancer19. As there is great variation throughout the population, identifying the presence of this SNP is vital to enhance good methylation properties in the body.

Sulfation is an additional component of endogenous oestrogen detoxification, especially as methoxy metabolites. The presence of the SULT1A1 allele combined with regular alcohol consumption, tobacco smoking, weight excess and late menopause increase the relative risk of both bronchial and breast cancer20.

Glucuronidation, as a key phase II detoxification, allows glucuronic acid to be conjugated with oestrogen and be eliminated by the body. In the case of the presence of pathogenic intestinal bacteria, which can produce β-glucuronidase, some of the oestrogen can re-enter circulation by uncoupling the bond between oestrogen and glucuronic acid, leading to an increased enterohepatic circulation and an increased incidence of breast cancer21.

Identifying oestrogen metabolism and their metabolites in each patient are the most important factors in making a decision as to whether or not HRT should be prescribed (Figure 4).