Medical applications of ionizing radiation (IR) have become widely used for diagnostic as well as therapeutic purposes. Emerging evidence indicates an excess risk of late occurring cardiovascular diseases, especially atherosclerosis, after IR exposure. IR induces cellular effects which may induce endothelial cell dysfunction, an early marker for atherosclerosis. In addition, intercellular communication through channels composed of transmembrane connexin (Cx) proteins, i.e. gap junctions (GJs; direct cell-cell coupling) and hemichannels (paracrine release/uptake pathway) can modulate IR-induced responses and thereforethe atherosclerotic process. However, the role of intercellular endothelial communication, particularly the role of Cx channels, in IR-induced atherosclerosis has never been described before.
Materials & methods: Telomerase immortalized human Coronary Artery/Microvascular Endothelial cells (TICAE/TIME) were exposed to X-rays (0.1, 0.5 and 5 Gy). Several biological endpoints were investigated: Cx gene expression, Cx protein levels, GJ and hemichannel function. In addition, production of reactive oxygen species, senescence, cell death and inflammatory responses were assessed with or without applying a hemichannel blocker.
Results: Exposure to IR induced acute and persistent upregulation of the pro-atherosclerotic Cx43 and downregulation of anti-atherosclerotic Cx37 and Cx40 gene and protein levels in a dose-dependent manner. In addition, IR exposure increased GJ communication and induced hemichannel opening. Moreover, IR induced a dose-dependent increase in cell death, senescence, inflammatory responses (Il-6, MCP-1 and PECAM-1) and ROS production. These effects were significantly reduced in the presence of the hemichannel blocker.
Discussion & conclusions: An increase in intercellular communication after IR exposure may alter the transfer of IR damaging signals (ROS, senescence, cell death, inflammation) between the cells, resulting in an increase in endothelial cell damage, which could be protected by blocking the hemichannels. In addition, similar alterations in Cx expression levels have been reported in the literature in endothelial cells covering atherosclerotic plaques. Therefore, these results suggest that IR may contribute to atherosclerosis progression, and blocking hemichannels might have a potential radioprotective application.