Microglia and glioblastoma heterocellular interplay sustains tumour growth and proliferation as an off-target effect of radiotherapy

Glioblastoma (GBM), a WHO grade IV glioma, is a malignant primary brain tumour for which combination of surgery, chemotherapy and radiotherapy is the first-line approach despite adverse effects. Tumour microenvironment (TME) is characterized by an interplay of cells and soluble factors holding a critical role in neoplastic development. Significant pathophysiological changes have been found in GBM TME, such as glia activation and oxidative stress. Microglia play a crucial role in favouring GBM growth, representing target cells of immune escape mechanisms. Our study aims at analysing radiation-induced effects in modulating intercellular communication and identifying the basis of protective mechanisms in radiation-naive GBM cells. Tumour cells were treated with conditioned media (CM) derived from 0, 2 or 15 Gy irradiated GBM cells or 0, 2 or 15 Gy irradiated human microglia. We demonstrated that irradiated microglia promote an increase of GBM cell lines proliferation through paracrine signalling. On the contrary, irradiated GBM-derived CM affect viability, triggering cell death mechanisms. In addition, we investigated whether these processes involve mitochondrial mass, fitness and oxidative phosphorylation and how GBM cells respond at these induced alterations. Our study suggests that off-target radiotherapy modulates microglia to support GBM proliferation and induce metabolic modifications. Radiotherapy-induced effects, especially in whole brain irradiation, involve both tumour and surrounding healthy cells. Irradiated tumour and microglia cells create a complex network of intercellular communication. Our data reveal that off-target irradiated microglia increase GBM viability, proliferation and growth, sustaining GBM mitochondrial fitness. Irradiated microglia-mediated effects on GBM intracellular context result in a stimulation of oxidative phosphorylation mechanism and an increased protection from mitochondrial oxidative stress. Metformin administration inhibits the pro-tumoural effects mediated by microglial cells on GBM, blocking oxidative phosphorylation metabolic process. image