Dihydronicotinamide riboside (NRH) quinone oxidoreductase 2 (NQO2) is involved in quinone metabolism reducing quinone to hydroquinone. Quinones are products of oestrogen metabolism and are responsible for the oestrogen-initiated breast carcinogenesis. It has been demonstrated that oestrogen quinones are endogenous biological substrates of NQO2 which acting as a detoxification enzyme catalyses the reduction of oestrogen quinones to hydroquinone. Hydroquinone can then be removed by conjugation to glutathione or glucuronic acid. In this study, the oestrogen dependent and oestrogen independent effects of NQO2 in a variety of networks implicated in breast tumorigenesis were investigated aiming to understand the potential role of NQO2 overexpression in mammary carcinomas. The use of NRH as a cofactor for NQO2 is being studied in parallel with the Î2-oestradiol and tamoxifen treatments. The MCF-7, T47D, MDA-MB-231 and MDA-MB-468 breast cancer cells were transfected with increasing amounts of NQO2 and its biological activity in regulating ERα transcriptional activity, reactive oxygen species (ROS) generation, cell cycle control, mitochondrial membrane potential and antioxidant activities including catalase activity, glutathione (GSH) levels and glutathione peroxidase (GPx) activity were studied. NQO2 overexpression in MDA-MB-231 and T47D cells reduced ROS generation. Increasing amounts of transfected NQO2 induced the ERα transcriptional activity in Î2-oestradiol treated MCF-7 and T47D cells and decreased cyclin D1 protein levels in these cells treated with Î2-oestradiol compared to untransfected cells. Reduction of catalase activity was detected in tamoxifen treated T47D cells overexpressing NQO2, an effect that was not evident in Î2-oestradiol treated cells, whereas NQO2 mediated reduction of GSH levels was detected in these cells treated with Î2-oestradiol but not with tamoxifen. Finally, NQO2 affected mitochondrial membrane depolarization in Î2-oestradiol treated MDA-MB-231 cells. Given the fact that NRH is not physiologically synthesized in humans, the results presented in this study are valuable from the fundamental science point of view indicating the existence of a potential link between NQO2 and estrogens affecting a number of biological pathways important for breast carcinogenesis and as such from the clinical angle it could be assumed that NQO2 effects could impact the design of personalised breast cancer treatment of oestrogen receptor positive and negative breast cancers.

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