ROS production and angiogenic regulation by macrophages in response to heat therapy

Abstract

Purpose: It has been well established that inadequate blood supply combined with high metabolic rates of oxygen consumption results in areas of low oxygen tension (<1%) within malignant tumours and that elevating tumour temperatures above 39°C results in significant improvement in tumour oxygenation. Macrophages play a dual role in tumour initiation and progression having both pro-tumour and anti-tumour effects. However, the response of macrophages to heat within a hypoxic environment has not yet been clearly defined.Methods: Raw 264.7 murine macrophages were incubated under normoxia and chronic hypoxia at temperatures ranging from 37–43°C. Under normoxia at 41°C, macrophages start to release significant levels of superoxide. The combination of heat with hypoxia constitutes an additional stimulus leading to increased respiratory burst of macrophages.Results: The high levels of superoxide were found to be associated with changes in macrophage production of pro-angiogenic cytokines. While hypoxia alone (37°C) increased levels of hypoxia inducible factor-1α (HIF-1α) in macrophages, the combination of hypoxia and mild hyperthermia (39–41°C) induced a strong reduction in HIF-1α expression. The HIF-regulated vascular endothelial growth factor (VEGF) decreased simultaneously, revealing that heat inhibits both HIF-1α stabilization and transcriptional activity.Conclusion: The data suggest that temperatures which are readily achievable in the clinic (39–41°C) might be optimal for maximizing hyperthermic response. At higher temperatures, these effects are reversed, thereby limiting the therapeutic benefits of more severe hyperthermic exposure.