Phosphatidylinositol 3-kinase mediates activation of ATM by high NaCl and by ionizing radiation: Role in osmoprotective transcriptional regulation

High NaCl causes DNA double-strand breaks and activates the transcription factor, TonEBP/OREBP, resulting in increased transcription of several protective genes, including those involved in accumulation of compatible organic osmolytes. Several kinases are known to contribute to signaling activation of TonEBP/OREBP, including ATM, which is a member of the phosphatidylinositol 3-kinase (PI3K)-like kinase family and is activated by DNA double-strand breaks. The purpose of the present studies was to investigate a possible role of PI3K Class 1A (PI3K-1A). We found that high NaCl increases PI3K-1A lipid kinase activity. Inhibiting PI3K-1A either by expressing a dominant negative of its regulatory subunit, p85, or by small interfering RNA-mediated knockdown of its catalytic subunit, p110 alpha, reduces high NaCl-induced increases in TonEBP/ OREBP transcriptional activity and transactivation, but not nuclear translocation of TonEBIP/OREBP, or increases in its abundance. Further, suppression of PI3K-1A inhibits the activation of ATM that is caused by either ionizing radiation or high NaCl. High NaCl-induced increase in TonEBP/OREBP activity is reduced equally by inhibition of ATM or PI3K-1A, and the effects are not additive. The conclusions are as follows: (i) PI3K-1A activity is necessary for both high NaCl- and ionizing radiation-induced activation of ATM and (h) high NaCl activates PI3K-1A, which, in turn, contributes to full activation of TonEBP/OREBP via ATM.