Endothelial cell death after ionizing radiation does not impair vascular structure in mouse tumor models

Abstract

The effect of radiation therapy on tumor vasculature has long been a subject of debate. Increased oxygenation and perfusion have been documented during radiation therapy. Conversely, apoptosis of endothelial cells in irradiated tumors has been proposed as a major contributor to tumor control. To examine these contradictions, we use multiphoton microscopy in two murine tumor models: MC38, a highly vascularized, and B16F10, a moderately vascularized model, grown in transgenic mice with tdTomato‐labeled endothelium before and after a single (15 Gy) or fractionated (5 × 3 Gy) dose of radiation. Unexpectedly, even these high doses lead to little structural change of the perfused vasculature. Conversely, non‐perfused vessels and blind ends are substantially impaired after radiation accompanied by apoptosis and reduced proliferation of their endothelium. RNAseq analysis of tumor endothelial cells confirms the modification of gene expression in apoptotic and cell cycle regulation pathways after irradiation. Therefore, we conclude that apoptosis of tumor endothelial cells after radiation does not impair vascular structure. image The effects of ionizing radiation on tumor vasculature are disputed. This study shows that radiation of murine tumors using either single (15 Gy) or fractionated (5 × 3 Gy) doses fails to result in significant structural changes of the perfused tumor vasculature. Because apoptosis mainly occurs in non‐perfused vessels and blind ends, the main structural network remains. Perfused and non‐perfused tumor vasculature could be simultaneously imaged using the Ve‐Cadherin‐CreERt2‐tdTomato mouse model resulting in 95% of endothelial cells labeled. Single (15 Gy) and fractionated (5 × 3 Gy) dose of irradiation eliminates smaller, poorly perfused vessels, but the more substantial, perfused vessels remain unchanged, thus not compromising vascular function. Tumor endothelial cell death after irradiation is less prominent in large, perfused vessels than in small non‐perfused vessels. Proliferation of tumor endothelial cells is comprehensively blocked and reduced as early as 12 h after single‐dose IR with longer‐term persistence of cell cycle blockade.