# Radiation-induced rescue effect on human breast carcinoma cells is regulated by macrophages

## TL;DR

Macrophages in the tumor environment can either enhance or suppress the radiation-induced rescue effect in breast cancer cells, depending on their polarization state.

## Contribution

This study is the first to demonstrate that macrophage polarization regulates the radiation-induced rescue effect in breast carcinoma.

## Key findings

- M1 macrophages suppress RIRE, while M2 and TAM macrophages enhance it in mixed cell cultures.
- Macrophages exhibit a strong basal RIRE but no active RIRE, unlike MCF7 cells.
- Macrophage polarization modulates RIRE magnitudes, impacting radiotherapy effectiveness.

## Abstract

The susceptibility of cancer cells to DNA damages is influenced by their microenvironment. For example, unirradiated neighbors of irradiated cells can produce signals that reduce DNA damages. This phenomenon, known as Radiation-Induced Rescue Effect (RIRE), has profound implications on the efficacy of radiotherapy. Using bystander cells co-cultured with mock-irradiated cells as a control, we demonstrated, for the first time, two types of RIRE. Conditioned medium from naïve by stander cells, i.e., cells not exposed to irradiated cells, could mitigate UV-induced DNA damages in human breast carcinoma MCF7 cells, as judged by phospho-H2AX and 53BP1 immunostaining. This protective effect could be further enhanced by the prior treatment of bystander cells with factors from UV-irradiated cells. We named the former effect “basal RIRE” and the latter “active RIRE” which were cell type-dependent. As bystanders, MCF7 showed a significant active RIRE, whereas THP1-derived macrophages showed a strong basal RIRE but no active RIRE. Interestingly, RIRE of macrophages could further be modulated by polarisation. The basal RIRE of macrophages was abolished by M1 polarisation, while M2 and Tumour Associated Macrophages (TAM) demonstrated pronounced basal and active RIRE. When mixtures of MCF7 cells and polarised macrophages were used as bystanders, the overall RIRE was dictated by macrophage phenotypes: RIRE was suppressed by M1 macrophages but significantly enhanced by M2 and TAM. This study shows a previously unappreciated role of the innate immune system in RIRE. Depending on polarised phenotypes, macrophages in the tumour microenvironment can interfere with the effectiveness of radiotherapy by adjusting the RIRE magnitudes.

•Macrophage polarisation regulates radiation-induced rescue effect (RIRE) in breast carcinoma.•Basal and active RIRE identified; influenced by bystander cell types and conditioning.•M1 macrophages suppress RIRE; M2 and TAM macrophages enhance RIRE in mixed cell cultures.

Macrophage polarisation regulates radiation-induced rescue effect (RIRE) in breast carcinoma.

Basal and active RIRE identified; influenced by bystander cell types and conditioning.

M1 macrophages suppress RIRE; M2 and TAM macrophages enhance RIRE in mixed cell cultures.