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Background: As two diseases with rapidly increasing incidence, the molecular linkages between obesity and breast cancer (BC) are intriguing. Overall, obesity may be a negative prognostic factor for BC.

Methods: Single-cell RNA-sequencing (scRNA-seq) was performed on tumor tissues from 6 obese and non-obese BC patients. The induced macrophages were used to evaluate the regulation role of NR1H3 on FABP4. RNA sequencing, qPCR and western blot were applied to measure the expression levels of NR1H3 and FABP4. The co-culture, cell proliferation and migration assays were adopted to explore the function of NR1H3 between macrophages and tumor cells.

Findings: With 48,033 cells analyzed, we found heterogeneous tumor epithelium and microenvironment in these obese and lean BC patients. Interestingly, the obesity-associated epithelial cells exhibited specific expression signatures which linked tumor growth and hormone metabolism in BC. Notably, one population of obesity-specific macrophage up-regulated the nuclear receptor subfamily 1 group H member 3 (NR1H3), which acted a transcription factor and increased the expression of fatty acid binding protein FABP4, and further increased the proliferation of tumor cells in BC.

Interpretation: Using single-cell signatures, our study illustrate cell diversity and transcriptomic differences in tumors from obese and non-obese BC patients, and sheds light on potential molecular link between lipid metabolism and BC.

Funding: Natural Science Foundation of Shandong Province (Grant No. ZR2022MH084), the Qingdao Science and Technology Plan Project (Grant No. 23-2-8-smjk-13-nsh) and the Flexible Talent guidance Project of Qilu Hospital (Qingdao) of Shandong University (Grant No. QDKY2021RX02) .

Declaration of Interest: The authors have declared no conflict of interest.

Ethical Approval: The primary untreated breast cancers used in this study were collected with written informed consent from all patients with approval (KYLL-KS-2020054) from the Ethics Committee of Qilu Hospital (Qingdao), Shandong University. Tumor tissues from 3 obese (BMI > 30) and 3 normal-weight BC patients (BMI < 30) were randomly selected for scRNA-seq, and tumor from 4 obese and 4 normal-weight BC patients were randomly selected for subsequent laser confocal immunofluorescence staining. Detailed clinical data were shown in Table S1. In addition, clinical data of BC patients with primary invasive ductal carcinoma were collected to analyze the correlation between BMI and tumor-related clinical parameters. These patients underwent surgical resection in Qilu Hospital (Qingdao), Shandong University from January 2015 to December 2021 with written informed consent for their clinical data.