# Impact of Psychological Stress and Spontaneous Tumour Regression on the Hippocampal Proteome in a Mouse Model of Breast Cancer

## TL;DR

This study shows that breast cancer and stress in mice cause lasting changes in brain proteins linked to memory and cognitive issues, even after tumors shrink.

## Contribution

The study reveals that spontaneous tumor regression in mice still causes long-term hippocampal proteomic changes, independent of treatment effects.

## Key findings

- Cancer alone reduces mitochondrial function and synaptic plasticity in the hippocampus.
- Psychological stress worsens mitochondrial dysfunction and alters lipid metabolism in tumor-bearing mice.
- Spontaneous tumor regression does not reverse hippocampal proteomic changes, suggesting lasting brain impacts.

## Abstract

Cognitive impairment is common in people diagnosed with breast cancer, but the molecular mechanisms that underlie maladaptive changes in the brain are unknown. The psychological stress of a cancer diagnosis is certainly a contributing factor. Here, we investigated alterations in the hippocampal proteome in response to both cancer and psychological stress using label‐free quantitative mass spectrometry techniques. An orthotopic syngeneic model of triple‐negative breast cancer (TNBC) was established by injecting Py230 cells into the mammary fat pads of female C57Bl/6 mice. Half of the mice were subjected to a daily restraint stress paradigm. Mice that experienced both cancer and restraint stress lost weight and displayed larger tumours compared to non‐stressed mice. Their urinary corticosterone levels were also elevated, as measured by enzyme‐linked immunosorbent assay. Non‐stressed tumour‐bearing mice displayed higher levels of TNFα in the prefrontal cortex (PFC) compared to stressed mice with cancer. Flow cytometry results suggested that the CD4+/CD8+ T cell ratios were also raised in non‐stressed tumour‐bearing mice compared to both controls and stressed mice with TNBC. Bioinformatic analysis of hippocampal proteomes indicated that cancer alone causes reduced mitochondrial respiration and ATP synthesis, as well as impaired glutamate recycling and synaptic plasticity. Moreover, daily stress in TNBC mice caused further mitochondrial dysfunction, increased oxidative phosphorylation, and altered lipid metabolism. Importantly, over half of the mammary tumours that initially developed spontaneously regressed after 7–9 weeks in these young immunocompetent mice. Tumour regression inhibited TNFα increases in the PFC. However, the hippocampal proteomes of tumour‐bearing mice were largely similar to mice in which tumours regressed, suggesting that spontaneous regression of breast cancer confers lasting physiological dysregulations that impact hippocampal protein expression. This study in mice may help to identify molecular mechanisms responsible for long‐term memory impairments in cancer survivors and reveal novel drug targets for cancer‐related cognitive impairment.

This study examines how breast cancer and psychological stress impact the hippocampal proteome, uncovering altered signalling pathways linked to mitochondrial dysfunction, impaired energy production, and altered synaptic plasticity; that is, cellular processes that are critical for memory formation and cognitive health. Using a triple‐negative breast cancer (TNBC) mouse model paired with chronic restraint stress, our findings also suggest that non‐CNS tumour regression causes enduring molecular changes in the hippocampus. By identifying these protein‐level disruptions, this work advances our understanding of cancer‐related cognitive impairment, distinct from treatment effects, and highlights the long‐lasting neurological impact of cancer itself on brain health.