Investigation of the phytoestrogenic effect of oleuropein onbreast cancer

Estrogen plays a crucial role in the normal development of breast tissue and can also support the onset and progression of breast cancer. Estrogen binds to estrogen receptors (ER-α and ER-β) present in breast tissue cells to promote cell proliferation. However, high estrogen levels can increase cell division, leading to the accumulation of mutations and the initiation of carcinogenesis. Additionally, estrogen can trigger breast cancer through genotoxic metabolites produced by oxidative metabolism, independent of estrogen receptors. Since most breast cancer cases are estrogen receptor-positive [ER(+)], estrogen receptors have become significant therapeutic targets. However, patients turning to alternative methods in addition to clinical treatments can have positive or negative impacts on the course of the disease. Therefore, it is important to test the efficacy of plants or phytochemicals with phytoestrogenic activity on various tumor models.This study examined the interaction of oleuropein with estrogen receptors in breast cancer cells. The presence and expression of estrogen receptors (ERα and ER-β) at mRNA and protein levels were investigated in tumor cells obtained from Ehrlich ascites tumor (EAT)-bearing mice, which were treated with 5-fluorouracil or oleuropein, alone or in combination, for different durations. According to the results, no significant change was detected in ESR2 expression in mice given oleuropein for 6 days starting from the second day after EAT inoculation or in the group treated with 5-fluorouracil. However, when these two compounds were administered in combination, ESR2 expression increased compared to both the control and the group treated with 5FU alone. Western blot analysis results were consistent with PCR findings, showing that ER-β protein expression increased in the 5FU group but was similar to the control in groups where OL was given alone or in combination. ER-α protein expression decreased with the administration of 5FU alone or in combination with oleuropein, while oleuropein alone had an effect similar to the control. In the group created to investigate the protective effect of oleuropein against breast cancer, ER-β mRNA expression increased while ER-α and ER-β protein expression decreased. This finding suggests that long-term intake of oleuropein increases ER-β mRNA expression, but these mRNAs may not be functional, and their translation into protein might be inhibited by another mechanism. After 8 days of administration to observe oleuropein's effect on tumor initiation, ER-β expression increased at both mRNA and protein levels, while ER-α protein expression remained similar to the control. Oleuropein was observed to interact with estrogen receptors in breast cancer cells and cause changes in their expression. Especially in chemotherapy patients, the use of oleuropein may alter the efficacy of the medication, and therefore, patients are advised to be cautious with oleuropein usage.