Indoleamine 2,3-dioxygenase 1 regulates breast cancer tamoxifen resistance through interleukin-6/signal transducer and activator of transcription.

Breast cancer is the primary cause of cancer-related deaths in women. Tamoxifen (TAM) is the preferred drug for treating premenopausal luminal-type breast cancer, but TAM resistance restricts its ability to benefit patients. To date, the mechanism of this resistance remains unclear, and there is currently no effective treatment for reversing it. The expression of indoleamine 2,3-dioxygenase 1 (IDO1) has been shown to be elevated in various malignancies. Here, we aimed to investigate the role of IDO1 in TAM-resistant breast cancer. We confirmed that IDO1 is strongly expressed in TAM-resistant breast cancer, and it mediates drug-resistant cell proliferation, metastasis, and TAM resistance in vivo and in vitro through interleukin-6/signal transducer and activator of transcription 3 (IL-6/STAT3). We also found that the mechanism by which TAM upregulates IDO1 is dependent on STAT1 activation. In summary, IDO1 regulates TAM resistance and can serve as a novel target for treatment of TAM-resistant breast cancer.