Medium-mediated DNA damage response after ionizing radiation is correlated with the increase of specific cytokines in human fibroblasts

Abstract

Radiation induced bystander effects, either protective or adverse, have been identified in a variety of cells and for different endpoints. They are thought to arise from communication between cells through direct cell–cell contacts and via transmissible molecules secreted into the medium by targeted cells. We have investigated medium-mediated damage response in human dermal fibroblasts (HDF) after exposure to ionizing irradiation. We show that HDF experience an elevated level of double stranded DNA damage repair response when incubated with conditioned growth medium of irradiated cells. The magnitude of this response is much lower than observed for directly irradiated cells and is proportional to the radiation dose, as is its persistence across time. Since secretion of cytokines is one of the possible pathways linking targeted and non-targeted cells a multiplex analysis was performed. Four cytokines – IL6, IL8, MCP-1 and RANTES – were identified in the growth medium of irradiated cells after exposure to X-rays (2 Gy). These cytokines were significantly upregulated and each cytokine showed differential upregulation kinetics. Finally we performed a functional analysis to see if IL6 and MCP-1 could induce H2AX foci formation. IL6 caused a significant increase in spot occupancy compared to controls. Although only indicative MCP-1 appears to have the opposite effect as it caused a drop in spot occupancy. The combined addition of these two cytokines produced no significant response was observed. Both IL6 and MCP-1 have an effect on the H2AX spot occupancy possibly linking these cytokines to the bystander response