Epidemiological studies and experimental analysis indicate that dietary factors influence thedevelopment of breast cancer, suggesting the role of natural products as modifying factors againstbreast cancer. Thymoquinone (TQ) (a dietary phytochemical compound) the main active ingredientof the volatile oil of black seed (Nigella sativa) has been used to be safe when administered to awide variety of normal cells. In addition, Tamoxifen (TAM), a nonsteroidal triphenylethylene derivateand selective ER modulator, has been used as a single agent in the treatment of ER positive breastcancer.In the present study, we report the possible chemo-sensitizing effect of TQ as a promisingconventional chemotherapeutic agent, using a panel of human breast cancer cell lines, MCF-7, ZR-75-1, T-47D and BT-20 respectively in vitro and in vivo.Our data revealed that TQ, TAM or combined treatments significantly inhibited cell viability of MCF-7, T-47D, ZR-75-1 but slightly of BT-20 cell lines consistently with down-regulation of signaltransducer and activator of transcription 3 (STAT 3), mTOR, cyclin D1, oncogenic β-catenin,signaling a specific transcription factor and PPAR-gamma in a dose dependent manner. Mostimportantly, combined treatment enhanced inhibition of cell viability, DNA fragmentation andapoptosis induction through Caspase-3 activation and Bcl-2 down regulation in all tested cell linesbut this effect was limited in BT-20 cells compared to controls. Moreover, our data was supported bymicroscopic examination using electron scanning microscopy that revealed many apoptoticalterations after combined treatment compared to each TQ or TAM. Furthermore, confocalfluorescence microscopy examination revealed that only combined treatment showed a significantinhibition of filopodia formation as a sign of invasion inhibition in both BT-20 and MCF-7 humanbreast cancer cell lines. In vivo, tumor xenotransplants of human ZR-75-1 cells revealed that onlycombined treatment showed a significant regression in tumor size by 53% after 72h and enhancedE-cadherin expression as analyzed by immunohistochemistry and confocal fluorescencemicroscopy.Taken together, our findings provide strong in vitro and in vivo molecular evidences in support ofour hypothesis that Thymoquinone synchronized as chemo-preventive agent with Tamoxifen to inhibithuman breast cancer cell lines proliferation, invasion and induce apoptosis that might have a potentialimplication in breast cancer prevention and treatment.

Citation Format: Islam Yahiya, Heba Helmy, Ahmed Sultan. Thymoquinone and tamoxifen co-treatment synergistically inhibit proliferation, invasion and induce apoptosis in human breast cancer cell lines in vitro and in vivo [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS19-28.