Ionizing radiation induced signaling of DNA damage response molecules in RAW 264.7 and CD4⁺ T cells

Ionizing radiation (IR) treatment results in activation of several DNA damage response molecules, such as ataxia telangiectasia, mutated (ATM), and DNA-dependent protein kinase (DNAPK) in mammals that are increasingly recognized for their potential roles in the sensing of DNA damage and initiating the subsequent protein kinase cascade. In vitro evidence indicates that both ATM and DNA-PK are responsible for efficient repair of DNA double strand breaks in response to IR exposure. To unravel the role of ATM and DNA-PK, we studied the mRNA and protein levels of ATM, DNA-PK and their downstream substrates in two different cell types after irradiation viz. macrophage like RAW264.7 cells and CD4(+) T cells isolated from mice spleen. Our results show that despite significant increase in phosphorylation of ATM, its mRNA levels continue to remain low after IR exposure in both the cell types. Conversely, the mRNA expression of DNAPK shows a considerable increase immediately after IR exposure. Moreover, no increase in ATM mRNA levels is seen in DNAPK deficit RAW264.7 cells treated with DNAPK siRNA, indicating that ATM does not undergo any change at its transcriptional levels in response to IR treatment. However, in a similar study in CD4(+) T cells, inhibition of DNAPK by siRNA, shows a considerable increase in ATM after IR exposure. Collectively, these results suggest a discrepancy in the role of the ATM and DNA-PK pathways in the cellular response to IR at the mRNA and protein levels in two different cell types.