# Single-cell RNA sequencing reveals the effects of mental stress on mouse mammary tumors and the tumor microenvironment

## TL;DR

This study shows how mental stress affects breast cancer growth and the tumor environment in mice using single-cell RNA sequencing.

## Contribution

The study reveals how mental stress causes tumor cell dedifferentiation and alters the tumor microenvironment in mammary tumors.

## Key findings

- Mental stress promotes tumor growth and cell cycle progression in mammary tumors.
- Stress leads to dedifferentiation of luminal tumor cells into stem cell-like basal cells.
- Stress alters immune cell composition in the tumor microenvironment, increasing neutrophils and dendritic cells while suppressing T and B cells.

## Abstract

Mental stress has been shown to negatively impact the development and progression of human cancer, including breast cancer. However, its effects on the tumor microenvironment (TME) remain unclear. In this study, we applied single-cell sequencing analysis to tumor tissues from MMTV-PyMT transgenic mice with mammary gland tumors with or without exposure to mental stress. In association with a significant promotion of the cell cycle and tumor growth induced by mental stress, we observed the dedifferentiation of luminal subtype of tumor cells into a subgroup of cancer stem cell-like basal cells, as well as enhanced cell proliferation in epithelial tumor cells, endothelial cells, and fibroblasts. In addition, stress stimulation led to an increase in tumor-associated neutrophils (TANs) and tumor-infiltrating dendritic cells (TIDCs), while suppressing immune cells such as cytotoxic T lymphocytes (CTLs), naïve T cells, B cells, and NK cells within the TME. We also observed a shift in macrophages from the M1 to the M2 phenotype. Furthermore, pathway enrichment analysis of differentially expressed genes, gene signature U score analysis, and immunofluorescence staining of the tumor tissue sections were conducted for further validation. The current study not only systematically elucidates the impact of mental stress on mammary gland tumors and the TME in vivo, but also provides insights into the mechanism underlying mental stress-induced tumor growth and progression in breast cancer.