Metabolomic profiling unravels DNA adducts in human breast that are formed from peroxidase mediated activation of estrogens to quinone methides

Abstract

Currently there are three major hypotheses that have been proposed for estrogen induced carcinogenicity, however exact etiology remains unknown. Based on the chemical logic, studies were undertaken to investigate if estrogens could generate quinone methides in an oxidative environment which then could cause DNA damage in humans. In presence of MnO2 estrogens were oxidized to quinone methides. Surprisingly quinone methides were found to be stable with t1/2 of 20.8 and 4.5 min respectively. Incubation of estrogens with lactoperoxidase (LPO) and H2O2 resulted in formation of respective quinone methides (E1(E2)-QM). Subsequent addition of adenine to the assay mixture lead to trapping of E1(E2)-QM, resulting in formation of adenine adducts of estrogens, E1(E2)-9-N-Ade. Targeted ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) based metabolomic analysis of the breast tissue extracts showed the presence of adenine adducts of estrogens, E1(E2)-9-N-Ade, along with other estrogen related metabolites. Identity of E1(E2)-N-Ade in LPO assay extracts and breast tissue extracts were confirmed by comparing them to pure synthesized E1(E2)-9-N-Ade standards. From these results, it is evident that peroxidase enzymes or peroxidase-like activity in human breast tissue could oxidize estrogens to electrophilic and stable quinone methides in a single step that covalently bind to DNA to form adducts. The error prone repair of the damaged DNA can result in mutation of critical genes and subsequently cancer. This article reports evidence for hitherto unknown estrogen metabolic pathway in human breast, catalyzed by peroxidase, which could initiate cancer.

Figure 1. Current three (A, B & C) major hypotheses that have been proposed for estrogen induced carcinogenicity.

D-Newly proposed pathway of estrogen metabolism that could initiate cancer in human.

Figure 2. Estrogens were oxidized by chemical means as described in Materials and method.

UV spectra and plot presenting decay of estrone quinone methide (E₁QM) (A) and estradiol quinone methide (E₂QM) (B). Observed t _1/2 for E₁QM and E₂QM was 20.8 and 4.5 min respectively.

Figure 3. MS/MS spectra of E₁QM (A) and E₂QM (B).

A plausible fragmentation of parent ions (269 and 271, M+1 ions) leading to major peaks is presented.

Figure 4. UPLC–MS/MS chromatogram of assay mixture (A) and standard E₁-9-N-Ade (B).

E₁-9-N-Ade was formed by peroxidase-catalyzed oxidation of E₁ through methide. Insets: MS/MS spectra of E₁-9-N-Ade from assay mixture (A) and standard E₁-9-N-Ade (B). A plausible fragmentation of parent ion (404, M+1 ion) leading to major peaks is presented. Arrows indicate peaks that are common to assay mixture and standard.

Figure 5. UPLC–MS/MS chromatogram of assay mixture (A) and standard E₂-9-N-Ade (B).

E₂-9-N-Ade was formed by peroxidase-catalyzed oxidation of E₂ through methide. Insets: MS/MS spectra of E₂-9-N-Ade from assay mixture and standard E₂-9-N-Ade. A plausible fragmentation of parent ion (406, M+1 ion) leading to major peaks is presented. Arrows indicate peaks that are common to standard and assay mixture.

Figure 6. Breast tissue samples were homogenized and extracted as described in Materials and Methods.

UPLC–MS/MS chromatogram of standard E₁(E₂)-9-N-Ade (A, C) and breast tissue extract (B, D).