Hypoxia and Hypoxia-Inducible Factor-1 Target Genes in Central Nervous System Radiation Injury

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Abstract
Purpose: Microvascular permeability changes and loss of blood-brain barrier integrity are important features of central nervous system (CNS) radiation injury. Expression of vascular endothelial growth factor (VEGF), an important determinant of microvascular permeability, was examined to assess its role in CNS radiation damage. Because hypoxia mediates VEGF up-regulation through hypoxia-inducible factor-1α (HIF1α) induction, we studied the relationships of hypoxia, HIF1α expression, and expression of VEGF in this damage pathway. Experimental Design: Expression of HIF1α, VEGF, and another hypoxia-responsive gene, glucose transporter-1, was assessed in the irradiated rat spinal cord using immunohistochemistry and in situ hybridization. Hypoxic areas were identified using the nitroimidazole 2-(2-nitro-1H-imidazole-l-yl)-N-(2,2,3,3,3,-pentafluoropropyl) acetamide. To determine the causal importance of VEGF expression in radiation myelopathy, we studied the response of transgenic mice with greater (VEGF-Ahi/+), reduced (VEGF-Alo/+), and wild-type VEGF activity to thoracolumbar irradiation. Results: In rat spinal cord, the number of cells expressing HIF1α and VEGF increased rapidly from 16 to 20 weeks after radiation, before white matter necrosis and forelimb paralysis. A steep dose response was observed in expression of HIF1α and VEGF. HIF1α and VEGF expressing cells were identified as astrocytes. Hypoxia was present in regions where up-regulation of VEGF and glucose transporter-1 and increased permeability was observed. VEGF-Alo/+ mice had a longer latency to development of hindlimb weakness and paralysis compared with wild-type or VEGF-Ahi/+ mice. Conclusions: VEGF expression appears to play an important role in CNS radiation injury. This focuses attention on VEGF and other genes induced in response to hypoxia as targets for therapy to reduce or prevent CNS radiation damage.