RADIATION RESPONSE OF CONNEXIN43-TRANSFECTED CELLS IN RELATION TO THECONTACT EFFECT

Some cell lines grown for only two cell doublings as multicell spheroi ds develop a form of resistance to killing by ionizing radiation that has been called the ''contact'' effect. While our previous results hav e implicated a role for higher order chromatin structure in the contac t effect, another possible explanation is the presence of intercellula r gap junctions that might facilitate communication between cells grow n as spheroids and thereby enhance the ability of cells to resist or r ecover from radiation damage. To examine the role of gap junctions in the contact effect, rat glioma C6 and mouse EMT6 cell lines were trans fected with a gene encoding the gap junctional protein connexin43. Whi le C6 glioma cells are deficient in gap junctional communication, cell s from spheroids were nonetheless more resistant than monolayers to ki lling by ionizing radiation, and the contact effect was present to a s imilar extent in the three transfected clones. For mouse EMT6 cells, r adiosensitivity was similar whether cells were grown as monolayers or spheroids. Transfection of EMT6 cells with connexin43 increased gap ju nctional communication but did not promote development of a contact ef fect. Tumor volume doubling time in SCID mice increased significantly for one transfected clone; however, doubling time in vitro was also in creased relative to the EMT6 parent. We conclude that extensive gap ju nctional communication is not a requirement for the increased radiatio n resistance observed when some cell lines are grown as spheroids. (C) 1997 Academic Press.