Single-cell RNA-seq of MMTV-PyMT mouse lungs treated with long-term systemic tamoxifen treatment

Estrogen receptor (ER)-positive breast cancer (BC) patients face constant threat of distant recurrence up to 20 years of diagnosis, reaching a total of 30% recurrence and a mortality rate of more than 25%. Here, we examined the influence of long-term tamoxifen treatment in metastatic niche formation in vivo. To test this, placebo or tamoxifen pellets were inoculateded in the 8-week old MMTV-PyMT mouse model and analyses were conducted 5-weeks after the treatment. Despite a reduction in primary tumor size compared to the placebo-treated group, lung metastasis was increased in the tamoxifen-treated mouse. At this point, scRNA-seq was also performed in mouse lungs of either placebo or tamoxifen-treated MMTV-PyMT mouse lungs, which revealed an increase in an interstitial macrophage subpopulation in the tamoxifen-treated group. This Prg4+ macrophages had enhanced lipid metabolic gene signatures and strongly interacted with fibroblasts via TGFβ1 signaling pathway, potentially contributing to lung fibrosis. This study identifies long-term tamoxifen treatment as a risk factor for promoting breast cancer cell resistance and altering the lung microenvironment, thereby facilitating lung metastasis.