Role of bystander communication in differential effects of low and high dose of alpha radiation in human lung cancer cells

Alpha radiation is known to be an effective way of killing cancer cells in case of targeted alpha therapy. However, this therapy of cancer needs optimization of dose and schedule of radiation, which may be further contributed with bystander communication. Hence, in present study, effect of alpha radiation was studied in human lung cancer cells (A549) in terms of clonogenic survival after low (0.013 Gy) and high (0.54 Gy) dose of alpha radiation delivered using BioAlpha. Low dose of alpha caused higher clonogenic survival, however, higher doses i.e. >0.1 Gy showed dose dependent decrease in clonogenic survival in these cells. These results were also validated by determination of cell number by counting and crystal violet assay. Such differential response of alpha radiation was not observed in normal cell lines of lung and liver origin. A549 cells were irradiated with low and high doses of alpha radiation followed by measurement of gamma-H2AX at early (30 min) and late (24 h) time points. Higher increase in percentage of gamma H2AX positive cells was observed at late time point in low dose than high dose of alpha radiation suggesting higher magnitude of bystander communication at low dose than high dose. Moreover, when alpha irradiated A549 cells were cocultured with unirradiated A549 cells, ∼80% bystander cells were gammaH2AX positive. Furthermore, A549 cells irradiated with low dose of alpha radiation showed increased levels of Connexin 43 (intracellular gap junction protein) and Phospho-Connexin 43. Expression of Caveolin-1 was down regulated, while proliferative protein survivin expression was upregulated after low dose of alpha radiation. On the contrary, high dose of alpha showed upregulation of caveolin and downregulation of survivin. These results suggest role of bystander signaling in differential survival of A549 cells after low and high dose of alpha radiation. (author)