Irreversible inhibition of estrogen receptor signaling and the emergence of hormonal resistance in MCF7 breast cancer cells induced by DNA damage agents

Combining chemotherapy and hormone therapy is a prevalent approach in breast cancer treatment. While the cytotoxic impact of numerous chemotherapy drugs stems from DNA damage, the exact role of these DNA alterations in modulating estrogen receptor alpha (ER alpha) machinery remains elusive. The present study aimed to analyze the impact of DNA damage agents on ER alpha signaling in breast cancer cells and assess the signaling pathways mediating the influence of DNA damage drugs on the ER alpha machinery. Cell viability was assessed using the MTT method, while the expression of signaling proteins was analyzed by immunoblotting. ER alpha activity in the cells treated with various drugs (17 beta-estradiol, tamoxifen, 5-fluorouracil) was assessed through reporter gene assays. In vitro experiments were conducted on MCF7 breast cancer cells subjected to varying durations of 5-fluorouracil (5-FU) treatment. Two distinct cell responses to 5-FU were identified based on the duration of the treatment. A singular dose of 5-FU induces pronounced DNA fragmentation, temporally suppressing ER alpha signaling while concurrently activating AKT phosphorylation. This suppression reverses upon 5-FU withdrawal, restoring normalcy within ten days. However, chronic 5-FU treatment led to the emergence of 5-FU-resistant cells with irreversible alterations in ER alpha signaling, resulting in partial hormonal resistance. These changes mirror those observed in cells subjected to UV-induced DNA damage, underscoring the pivotal role of DNA damage in shaping estrogen signaling alterations in breast cancer cells. In summary, the results of the present study suggested that the administration of DNA damage agents to cancer cells can trigger irreversible suppression of estrogen signaling, fostering the development of partial hormonal resistance. This outcome may ultimately impede the efficacy of combined or subsequent chemo- and hormone therapy strategies.