| Recent observations of bystander effects in unirradiated cell populations have focused attention on cell-cell communication, particularly gap junction intercellular communication (GJIC), as a means through which the bystander effect may be transmitted. The bystander expression of CDKN1A in unirradiated AG1522 human fibroblast cells observed in another laboratory was verified. The dose response of the bystander effect in the AG1522 cells showed that the effect had reached its maximum at the lowest α-particle fluence tested, 0.013 α/nuclei. To test potential mechanisms for communication to bystander cells, the fluorescence recovery after photobleaching technique was used. Only the rat liver epithelial cell line (Clone 9) exhibited GJIC based upon a fluorescence recovery after photobleaching assay, and there was no change in the rate constant for GJIC following exposure to low LET or high LET radiation. The fibroblast cell lines (AG1521, AG1522, and GM5758) showed no evidence of GJIC in three separate assays including immunohistochemistry.; Lindane, an inhibitor of GJIC, eliminated the bystander expression of CDKN1A in AG1522 cells while octanol, another inhibitor of GJIC, did not change the bystander expression of the protein. The two chemicals act in different ways to disrupt GJIC and each one may alter other functions as well, so the elimination of the bystander effect by lindane apparently indicates that lindane is interfering with a bystander signaling mechanism that is not mediated by gap junctions. The lack of connexin localization in the cell membrane of the fibroblast cell lines and the elimination of the bystander expression by lindane, but not octanol, indicates that the bystander effect must be mediated by a non-GJIC mechanism. The experimental evidence suggests that the mediator of the bystander expression of CDKN1A in human diploid fibroblasts is most likely an extracellular signal, such as a cytokine, that acts in a calcium-dependent signal transduction pathway. |
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