The high incidence of the mammary neoplasm is due to a number of factors, including the use of hormones, such as estrogen and exposure to chemicals, as bisphenol A (BPA). Despite the introduction of more effective treatments for mammary cancer, recurrence, tumor growth, invasion and metastasis often occur. It has been demonstrated that the cancer stem cells (CSCs) maintain tumor growth and are resistant to therapy. Studies have identified OCT4 as being responsible for self-renewal and maintenance of pluripotency of stem cells. To identify treatments that target this tumor population, the melatonin, a natural hormone, appears to have oncostatic effects. However, little is known about its therapeutic action in mammary cancer stem cells (MCSCs).

To test the hypothesis that melatonin may reduce the cancer cells, the canine mammary cancer cell line, CMT-U229, grown in 3-dimensional mammospheres, which represent canine mammary cancer stem cell (CSCs) population were treated with estradiol and BPA. The CSCs were cultured at 37°C and 5% CO₂ in MammoCult^TM culture medium. Using OCT4 expression as a marker for the CSCs, the immunofluorescence was performed with primary antibody anti-OCT4. Results demonstrated that BPA and estrogen increased the expression of OCT4 in both mammospheres. On the other hand, melatonin reduced the expression of OCT4 in estrogen and bisphenol A mammospheres. This fact suggests that melatonin is effective against CSCs and can be a possible treatment for mammary cancer.