ROS Production and Distribution: A New Paradigm to Explain the Differential Effects of X-ray and Carbon Ion Irradiation on Cancer Stem Cell Migration and Invasion

Abstract

Although conventional radiotherapy promotes the migration/invasion of cancer stem cells (CSCs) under normoxia, carbon ion (C-ion) irradiation actually decreases these processes. Unraveling the mechanisms of this discrepancy, particularly under the hypoxic conditions that pertain in niches where CSCs are preferentially localized, would provide a better understanding of the origins of metastases. Invasion/migration, proteins involved in epithelial-to-mesenchymal transition (EMT), and expression of MMP-2 and HIF-1α were quantified in the CSC subpopulations of two head-and-neck squamous cell carcinoma (HNSCC) cell lines irradiated with X-rays or C-ions. X-rays triggered HNSCC-CSC migration/invasion under normoxia, however this effect was significantly attenuated under hypoxia. C-ions induced fewer of these processes in both oxygenation conditions. The differential response to C-ions was associated with a lack of HIF-1α stabilization, MMP-2 expression, or activation of kinases of the main EMT signaling pathways. Furthermore,we demonstrated a major role of reactive oxygen species (ROS) in the triggering of invasion/migration in response to X-rays. Monte-Carlo simulations demonstrated that HO● radicals are quantitatively higher after C-ions than after X-rays, however they are very differently distributed within cells. We postulate that the uniform distribution of ROS after X-rays induces the mechanisms leading to invasion/migration, which ROS concentrated in C-ion tracks are unable to trigger.

Carbon Ion Radiobiology

“…In a xenograft model of human non-small cell lung cancer in BALB/c mice, irradiation with C-ions strongly reduced HIF-1α levels, a gene that can enhance survival by inducing hypoxia after exposure to X-rays [ 174 ]. Reduced HIF expression with C-ions compared to X-rays was also observed in cell experiments in vitro [ 175 , 176 , 177 ].…”

Carbon Ion Radiobiology

“…In a xenograft model of human non-small cell lung cancer in BALB/c mice, irradiation with C-ions strongly reduced HIF-1 levels, a gene that can enhance survival by inducing hypoxia after exposure to X-rays [169]. Reduced HIF expression with C-ions compared to X-rays was also observed in cell experiments in vitro [170][171][172].…”

Hadrontherapy Interactions in Molecular and Cellular Biology

“…Carbon ion RT downregulates HIF-1 signaling in lung and head and neck models [49]. HIF activation depends on ROS production, primarily HO• hydroxyl radicals, and is independent of an oxygen effect [23,50]. The dense and homogeneous distribution of ROS (HO•) following x-rays activates migration/invasion mechanisms, whereas the clustering of ROS around carbon ion trajectories can prevent HIF-1 and upstream signaling [50].…”

“…HIF activation depends on ROS production, primarily HO• hydroxyl radicals, and is independent of an oxygen effect [23,50]. The dense and homogeneous distribution of ROS (HO•) following x-rays activates migration/invasion mechanisms, whereas the clustering of ROS around carbon ion trajectories can prevent HIF-1 and upstream signaling [50]. Targeting HIF-1 or its target genes, such as survivin and erythropoietin receptor, could enhance the efficacy of photon RT and carbon RT in several tumor types [23,49,51,52].…”